feasibility study
TRANSCRIPT
2010
Faculty of Engineering
University of Bristol
10/28/2010
Feasibility study of Irfon Valley
Dam Scheme
Group 19
14501
15052
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i Feasibility study of Irfon Valley Dam Scheme
Contents
Executive Summary
1. Introduction 1
1.1 Context 1
1.2 Aim 1
1.3 Objectives 1
2. Initial Site Choices 2
2.1 Site 1 2
2.2 Site 2 3
2.3 Site 3 4
2.4 Site 4 5
2.5 Conclusion 6
3. Final Site Choice 7
4. Hydrological Analysis 9
5. Flood Routing 12
5.1 Introduction 12
5.2 Unit hydrograph 13
5.3 Design Storm 13
5.4 Flood routing 15
5.5 Key Data 16
6. Geological Analysis 16
6.1 Introduction 16
6.2 General Overview 16
6.3 Geology at specific site 16
6.4 Foundation compressive strength 17
6.5 Valley 17
6.6 Materials 18
7. Scheme Design 20
7.1 Dam design 20
7.2 Spillway design 25
7.3 Diversion works 27
7.4 Aqueduct design 28
7.5 Diverted services and dam maintenance 32
8. Environmental Assessment 33
8.1 Introduction 33
8.2 Considerations 34
8.3 Summary 36
9. Health and Safety 37
9.1 Introduction 37
ii Feasibility study of Irfon Valley Dam Scheme
9.2 Site Practice 37
9.3 Site access 38
10. Costing 38
10.1 Preconstruction costs 38
10.2 Scheme costs 40
10.3 Other costs 42
10.4 Total costs 42
11. Conclusion 43
iii Feasibility study of Irfon Valley Dam Scheme
Executive Summary
The following report details a feasibility study into the viability of the construction of a
water supply reservoir in the Irfon Valley, Powys, Wales. The reservoir will supply water to a
town of 200,000 people approximately 10km to the east of the valley.
Several sites were chosen along the length of the River Irfon and were compared with
respect the shape of the valley, the environmental concerns, and the availability of building
materials and the ease of construction.
We decided to further analyse Site 2. The overall decision for choosing Site 2 was the result
of a combination of reasons. One of the overriding criteria was the amount of properties
and land that was going to be flooded, and consequently how much compensation would
have to be paid. The dam site at Site 2 creates the smallest flooded area, floods the fewest
properties, has good site access and lies in close proximity to local building materials.
We decided that the site would suit a rockfill embankment dam, due to the wideness of the
valley and the shallowness of its slopes. This provides good access for the large machinery
that will be necessary for this type of structure and there is a good selection of rock and clay
nearby.
Hydrological analysis of the site has confirmed that the site is suitable for dam construction.
The water supply far exceeds the expected demand. The dam requires a height of 40m and
will have a useful storage of 14.81 million cubic metres.
The chosen site overlies an area of mudrocks, overlain by a thin layer of alluvium, a weak,
loose soil. The underlying mudrock will provide solid foundations, but the alluvium layer
should be removed.
The dam will be constructed of a clay core, sourced from within the flooded area, and will
be stabilised using a rockfill arrangement using locally available mudstones as the fill. The
slope sides will be at 30˚, at a height of 40m, crest width of 400m and a length of 138m. The
core is a tapered design and is 5m wide at the base. The crest of the dam will have a 3 m
wide access road for use during construction and for maintenance.
The spillway of the dam has been designed for a 1 in 10,000 year flood event. The spillway is
16m wide, 460m long to cope with a peak outflow of 158.41 cubic metres.
While the dam construction is ongoing, the current river channel will have to be diverted.
Due to the shape of the valley, the best solution is a cofferdam and culvert design. The two
pipes, encased in concrete, are 2.25m in diameter and are 183m long. The cofferdam will be
7m tall. The combined cost of the culvert and cofferdams will be £454,900.
iv Feasibility study of Irfon Valley Dam Scheme
The aqueduct will carry from the dam site to a service reservoir near to the planned town.
The planned route will be 14.5km long and the pipe will be 1m in diameter. The total cost of
the aqueduct scheme will be £1,386,000. This is the cheapest of the five options considered
and will require the least tunnelling and will not require pumping.
All existing services that run along the valley will be relocated to a new roadway to the west
of the site. This road will use an existing logging track as its base and will run along the top
of the ridge to the town of Abergwesyn. The new roadway will be 12km long and is
projected to cost around £1m. A service road will also connect the new road to the crest of
the dam.
An environmental assessment of the scheme highlighted some of the threatened species in
the valley. Animals such as the otter and Atlantic salmon are rare due to loss of habitat and
plants such as the globeflower only grow in the soil similar to what is provided in the valley.
The reservoir created as a result of building the dam will cover a site of special scientific
importance (SSSI) containing these plants. In addition, standing stones, erected during the
late Bronze Age will also be lost. Large swathes of woodland will be covered and will most
likely have to be cut down prior to the creation of the reservoir.
The overall cost of the dam is expected to be £7.55 million. The total cost for the scheme is
expected to be £22.49 million including a contingency for costs of 15%.
In conclusion, the study has shown that a scheme such as this in the location we have
provided is feasible. Further study must be taken into the exact ground conditions in the
immediate site area and to the reaction of local people and the authorities.
1 Feasibility Study of Irfon Valley Dam Scheme
1.0 Introduction
1.1 Context
A new town of Hywelfynydd is being considered for construction in near Ty-mawr on the
upper reaches of the River Wye in Mid-Wales. It is expected to have a population of
between 150,000 and 200,000, with an anticipated water usage of approximately 300 litres
of treated water per person per day; in addition to currently unknown industrial usage. A
feasibility study is required to determine whether the required water supply can be
obtained from the River Irfon catchment above Llanwrtyd Wells, and how it may be done.
1.2 Aim
The aim of this report is to provide a feasibility study into the creation of a dam across the
Irfon valley to provide water to the new town. The design, cost and environmental impacts
must all be considered and the analysis leading to the recommendations outlined. As such
we have come up with a number of objectives for the report.
1.3 Objectives
1. Find a suitable location for a direct supply reservoir In the Irfon Valley to provide the
town of Hywelfynydd with water.
2. Determine whether this location is suitable using hydrological analysis.
3. Determine whether the local geology can support a dam in this location.
4. Chose a dam type based on research and valley profile.
5. Complete a scheme design for the proposed works.
6. Carry out an environmental impact assessment.
7. Produce initial costing for the scheme and assess cost effectiveness of options provided.
2 Feasibility Study of Irfon Valley Dam Scheme
2.0 Initial Site Choices
An investigation into a series of factors was carried out in order to locate possible dam sites.
These features cover the topographical characteristics, environmental concerns and dam
specific aspects.
2.1 Site 1
2.1.1 Topography
This site has a large flat
area at the base of the
valley approximately 80m
across. The river runs
through the centre of the
site and is lined by trees.
Approximately 20m from
the proposed dam site
there is a tributary to the main river. There is a church, graveyard and a bridge downstream
of the dam. There is also a phone line and two minor roads that would need to be relocated.
Site 1 also lies on a fault line, which although dormant may be an indicator of disturbed rock
and subterranean features that we may not be aware of. This will require further and more
detailed site exploration which will be costly.
2.1.2 Environmental Concerns
This site is in close proximity to a church and
graveyard. While we need to pay close attention to it,
as our dam is sufficiently upstream so that we do not
need to relocate these buildings. The site is used as
grazing land for farm animals, which would have to be
relocated. There could also be an issue of noise for
the surrounding residents.
2.1.3 Materials
Due to the profile of the valley at this point and its proximity to the quarry the site lends
itself to a rockfill or earthfill dam. We would procure the clay core from one of the areas of
glacial till shown on the map in appendix 2, however as it is a rocky clay we would need to
provide a wider base for the core. As this is some distance we need to consider the
transportation of the material.
Fig.1 Cross section of valley at site 1
Fig.2 Showing plan of valley at site 1
3 Feasibility Study of Irfon Valley Dam Scheme
2.1.4 Dam Construction
Due to this site’s locality to the base of the valley it has good access for large plant meaning
a rockfill or earthfill dam could be constructed quickly and efficiently, with minimum
disruption to the surrounding roads and logging industry. The spillway will be constructed
on the eastern side to avoid relocating the church and graveyard.
2.2 Site 2
2.2.1 Topography
The site has a large flat area at
the base of the valley
approximately 50m across.
There is no evidence of slips on
the surrounding hillsides;
however the river has
previously taken a different
route as shown by a cutting
scar in the bank. There are two minor roads, a power line and a phone line that will all need
to be moved to allow the construction of the dam.
2.2.1 Environmental Concerns
This site is used as grazing land for farm animals,
which will need to be relocated. In addition we will
also need to clear some of the forest. There will be a
substantial amount of disruption to the residents of
the valley who do not need to move and we need to
be aware of the possibility of vibration damage to
people’s properties. There will also need to be a
number of structures moved due the flooded area.
2.2.3 Materials
The profile of the valley at this point again lends itself to a rockfill or earthfill dam. This site
is situated on mudrock with some silt laminations but should be a good source of material
for a rockfill dam. The clay core can be sourced from the borrow pit, or one of the other
glacial till locations.
Fig.3 Cross section of valley at site 2
Fig.4 Showing plan view of valley at site 2
4 Feasibility Study of Irfon Valley Dam Scheme
2.2.4 Dam Construction
Site 2 has good access to the dam location, the valley is still relatively wide at this point and
we do not envisage any problems in accessing it with construction equipment, as indicated
by the number of logging vehicles passing through the area. We would place the spillway on
the Eastern bank of the river to avoid disturbance to the houses situated downstream.
2.3 Site 3
2.3.1 Topography
The site is situated in a steep sided
valley, with the base measuring
approximately 20 – 30m across. The
valley sides are densely vegetated
with many large trees and
shrubbery. The river channel runs
through the centre of the valley with
a minor road located to the west.
There is also a telephone line which
follows the road, crossing at various
points, which would need to be relocated. There is a property about 100m downstream
which would need consideration during the dam construction phase. There is a picnic area
and Forestry Commission area just upstream which would be affected by the flooded area
and would need consideration.
2.3.2 Environmental Concerns
There is an abundance of large trees in the area, some of
which are used for the logging industry. These could be
removed and sold in preparation for the construction of
the dam. Several upstream buildings, including the
Abergwesyn community, lie within the projected dam
storage area. The hillsides are currently uninhabited by
livestock, however there is Site of Special Scientific Interest
located 200m downstream from the proposed dam site.
Fig.5 Cross section of valley at site 3
Fig.6 Showing plan view of valley at site 3
5 Feasibility Study of Irfon Valley Dam Scheme
2.3.3 Materials
A concrete dam can be considered due to the narrow nature of the valley. The foundations
should be strong enough to support a concrete gravity dam, although further investigation
will be needed. Aggregate can be sourced from the local quarry reducing transportation of
materials.
2.3.4 Dam Construction
The dam would preferably be made of concrete due to the valley’s topography. The spillway
would be incorporated within the dam structure, saving space. The riverside road would be
useful for delivering resources to the site. Deforestation would be needed in order to
facilitate the structure, although this may weaken the immediate strata.
2.4 Site 4
2.4.1 Topography
Site 4 has a much narrower base and
steeper sides than at sites 1 and 2. The
Eastern bank is much steeper than the
west, and rocky outcrops can be seen.
These have been subject to much
weathering and as such may be
unstable. The topography of this site
lends itself to a rockfill or earthfill dam.
2.4.2 Environmental concerns
The flood area associated with site 4 extends to
Abergwesyn which would involve not only the
relocation of many residents but also the road junction,
and hence three roads. There is also an area used for
forestry, and a Site of Specific Scientific Interest that
will be affected by the dam as well as a power line, a
telephone line and a bridal path.
2.4.3 Materials
Site 4 is located in close proximity to one of the smaller glacial till deposits, which would be
ideal for the core of a rockfill or earthfill dam. Once again it is mainly situated on mudstone,
which would provide adequate foundation for the aforementioned dam types.
Fig.8 Showing plan view of valley at site 4
Fig.7 Showing section view of valley at site 4
6 Feasibility Study of Irfon Valley Dam Scheme
2.4.4 Dam Construction
At Site 4, the valley starts to narrow and access for large construction materials may
become more difficult. There is also a large area that would need to be deforested, possibly
weakening the surrounding soil.
Fig. 9 Table comparing displaced infrastructure for flooded areas.
2.5 Conclusion
Following our investigation of the valley we have
decided that site 2 will be most suitable site to dam the
river. We have made this decision based on a number of
criteria such as the effect on infrastructure, access and
proximity to construction materials. Site 2 will require
the relocation of the least number of structures as it has
the smallest flooded footprint. This has the added
benefit of requiring less land to be purchased. In
addition, it has good site access, which is necessary for
the construction of a rockfill dam to allow plant on
site. The site is close to a quarry, in addition to
sources of glacial till for the core. Finally, this site requires much less deforestation than
upstream sites, therefore lowering the environmental impact further.
Fig. 10 Flooded area map for site 2
7 Feasibility Study of Irfon Valley Dam Scheme
3.0 Final Site Choice
Fig.11 Map overlaying all four potential sites
Our final site choice is site number two. After investigating many possible locations up and
down the valley, we decided that this site struck a perfect balance between hydrology,
geology, social and environmental impacts as well as having some advantages from a
construction standpoint.
Fig.12 Map showing the reservoir (green) and maximum flood (red)
All through the site selection process we were keen not to flood the town of Abergwesyn,
and as soon as we had the storage volumes required for each site from our hydrological
analysis we began to optimise our site location. Site two sits just far enough upstream from
a fault line as to not suffer from excessive leakage and is downstream enough that even in a
maximum flood situation where the water would rise from 35m to 38m above the base of
the valley, while the reservoir will be very close, it will not flood the small town.
As well as being upstream of a fault line, it is also just upstream of a church with a graveyard
which would have presented a major ethical challenge to the project. Obviously we were
not keen to interfere with the church of graveyard in any way so we are pleased to be able
to have the dam just upstream of its location.
8 Feasibility Study of Irfon Valley Dam Scheme
Fig.13 Cross section of the valley at site 2
The width of the valley lends itself to either an earth fill or rock fill dam, and having a very
wide and flat valley bottom means that we can get large machinery into the valley to place
material and compact it. This should have the positive effect of reducing construction time
even though more material is being placed. In turn this will also reduce our overheads and
should mean that the cost of the project is lowered.
The downstream location of our site is also useful for access. The large machinery required
to build our dam would likely not have been able to be transported up the small valley road
if our site was any further upstream without building separate access roads which would in
turn increase the cost of the build.
As with all the sites there are some negative implications of damming the river at this
location. We will flood the smaller of two sites of special scientific interest (SSSIs) at this
location. While this is unfortunate, we feel that going further upstream would cause more
problems as we would flood the small town of Abergwesyn, we feel this is more significant
than flooding a 0.3 acre SSSI.
There are also some standing stones that will be flooded. These ancient monuments require
special permission to be removed before work would start which we would obviously have
to apply for, but as above we feel that this is the best option to take as the alternative
would be to go further upstream and flood the graveyard in the town of Abergwesyn. Given
that this is a rural community, it is likely that the descendants of those buried in that
graveyard still live in the area, and would be more upset by the graveyard being flooded
than the standing stones.
After considering each site from many standpoints, we believe that site two provides the
best location to create a dam. We feel that the implications of damming further upstream or
downstream of this location would cause more problems for the local people and the
environment.
9 Feasibility Study of Irfon Valley Dam Scheme
4.0 Hydrological Analysis
4.1 Introduction
Hydrological analysis is the first step in the determination of whether the dam site is
suitable for supplying the town of Hywelfynydd. Through the process of hydrological
analysis we are able to work out the storage volume and height of the dam.
4.2 Demand
The first step in hydrological analysis is calculating the demand that the dam needs to
satisfy. This is important as it tells us the rate of extraction that the dam has to deal with.
For this project we assumed that the population of Hywelfynydd will be 200000 people, this
was combined with a water consumption of 300 litres/day/person, a compensation flow of
0.15 m3/s and a safety factor of 1.2. This then allowed us to calculate the demand to be
1.013 m3/s using equation 1 in appendix 1. The average flow rate of any catchment must be
at least greater than this in order to satisfy the demand and make a reservoir feasible.
4.3 Flow Rate
There is flow data present at the nearby Cilmery station. There are some pieces of missing
data, which can be reproduced using the Thiessen Polygon Method. The application of this
method is described in more detail in appendix 1. Once the missing flow data has been
reconstructed; then by measuring the catchment area for each of our four sites we were
able to work out the actual flow rate that would enter the dam at each site by using
Equation 3 in appendix 1. The average flow rate for each site can be calculated allowing us
to check the initial feasibility of each dam site. Due to initial calculations that we performed
with regard to the catchment made sure that all of our sites satisfied the initial demand as
shown below.
0
2
4
6
8
0 50 100 150 200 250 300
Flo
w m
3/s
Month
Site one Flow data
10 Feasibility Study of Irfon Valley Dam Scheme
Fig. 14 Flow data for the four sites.
4.4 Reservoir Storage Estimation
The design criteria required by the client for this dam is the ability to continue to supply the
town in the event of a one in one hundred year drought. The method used in this project is
the synthetic minimum flow. This involves using the existing flow records available selecting
the lowest monthly runoff for each year and ranking them, which in turn allows a probability
to be assigned to them. This data can then be extrapolated to allow the estimation of the
0
2
4
6
8
0 50 100 150 200 250 300
Flo
w m
3/s
Month
Site Two Flow Data
0
2
4
6
0 50 100 150 200 250 300
Flo
w m
3/s
Month
Site Three Flow Data
0
2
4
6
8
0 50 100 150 200 250 300
Flo
w m
3/s
Month
Site Four Flow Data
11 Feasibility Study of Irfon Valley Dam Scheme
one hundred year return period drought or 1% chance drought. This is process is repeated
for consecutive month droughts from two months two eleven months. This is done by
selecting the lowest flow rate in each year for that number of consecutive months. This is
then plotted in figure four in appendix 1. This data is then used to plot the minimum runoff
diagram for the one hundred year return period.
The minimum runoff diagram for site two is
shown in figure 2. This indicates that the
minimum storage required from the Site
Two reservoir, to meet the demand during
droughts, is 14.81 Million m3. This
information is then used in conjunction
with the terrain modelling software which
allows us to model the storage elevation
data for each particular site. This in turn
allowed us to look at the flooded area
present for each site, which allowed us to
look at the impact that placing the dam in each site would have. The flooded area diagrams
are in appendix 5.
4.5 Dam Height
Once the storage elevation curve could be constructed the height required by the storage
can be calculated. This is important information that is used within the selection of the final
dam site.
Dam Height Volume (less
dead storage) Flooded Area
Site # m Million m3 km
2
Site One 28 14.7 1.06
Site Two 36 14.81 1.00
Site Three 25 15.2 1.41
Site Four 32.5 15.1 1.46
Fig. 16 Summary of key hydrological data for the four sites.
4.6 Hydrological Analysis Conclusion
Hydrological analysis has an important role in this project as the relative storage capacities
of each site are an important factor in the final site choice. In addition they are also vital in
Fig. 15 Minimum run- off diagram for Site 2.
12 Feasibility Study of Irfon Valley Dam Scheme
the smooth running and fulfilment of purpose of the dam. However it is important to
remember the design criteria that have been applied to the design as they are key to
understanding the limitations of the design. The first that needs to be considered is that we
are estimating the one hundred year return period for the dam from twenty year data and
while the probability analysis underpinning the design are correct and following a design
guide, there is the possibility that this data set will fail to reflect the actual weather
conditions that the dam will experience during operation. In addition the design criteria
imply that a drought event of lower than 1% chance could cause the dam dry. These are
important caveats when considering the design and performance during the use of the dam.
Key information
Design Criteria
Population 200000 people
Water Consumption 300 litres/day/person
Compensation Flow 0.15 m3/s
Safety Factor 1.2
Return Period 100 years
Dimensions for Dam at Site 2
Demand 1.013 m3/s
Average Flow 1.93 m3/s
Storage 14.81 x106 m
3/s
Dam Height (without
freeboard) 36 m
Fig. 17 Hydrological analysis conclusions
5.0 Flood Routing
5.1 Introduction
Flood routing is the technique by which we determined the flow going flow our spillway as a
result of the design storm applied to our catchment. There are a number of design criteria
that have been applied to this stage in the process. They are detailed below.
In this project we will be designing the hydrograph to withstand a one in ten thousand year
chance or 0.01% percent chance storm.
13 Feasibility Study of Irfon Valley Dam Scheme
In this project we have been following a simplified version of the design process of the Flood
Studies Report (NERC 1975).
We will also neglect base flow in the calculation of the flow rates for the design storm, this is
due its relative size to the flow generated and as such the base flow is of relatively minor
importance in flood estimation.
5.2 Unit Hydrograph
Statistical techniques are used to determine the shape of our unit hydrograph; these are
described in detail in part 5 in appendix 1. They rely on four key constants (figure 18)
derived from the terrain and region of the dam site. These result in unit hydrograph in figure
20. This hydrograph shows the response of the flow rate from site two’s catchment for each
input of ten millimetre of rain.
5.3 Design Storm
In order to determine the rainfall depth of the design storm first the length of our storm
must be determined using equation 6 in appendix. Duration of our storm is 9 hours.
s1085 23.94812 m/km SAAR 1713.067 mm
Urban 0 MSL 14.17 km
Tp(T) 3.52 hrs TB 8.88 hrs Qp 62.41 m3/s/10mm
Fig. 18 Key topographical constants for hydrograph
0
5
10
15
20
25
30
35
0 2 4 6 8 10Flo
w Q
cu
bic
me
tre
s/se
con
d/1
0m
m
Time Hours
Unit Hydrograph
Fig. 20 Unit
hydrograph
Fig.19 Key characteristics of the hydrograph
14 Feasibility Study of Irfon Valley Dam Scheme
In order to estimate the depth of the design storm a number of ratios are applied to
measured storm data to grow the measured data (a two day long storm with a five year
design criteria). These are taken from a number of different tables and charts with the key
figures presented in figure 21 and the equations to apply them are in entry 7 Appendix 1.
The percentage runoff is then calculated to determine the proportion of rain that will
become runoff. This process is detailed in appendix 8. Rainfall over catchment, P, can then
be calculated.
In order to apply the storm to the unit hydrograph we need know how the storm how the
rainfall is distributed during the storm. For this project we have used a summer storm as the
model for the rainfall distribution. An assumption within this model is that there is no snow
melt is included in the flow rates. The storm profiles assume a symmetrical shape with the
peak intensities in the middle of the storm. The percentage distributions are calculated from
the summer storm profile from Flood Studies Report (NERC 1975 vol. 2 pg 44). The
percentage distributions for each hour of the storm are in entry 9 appendix 1. The storm
rainfall is distributed to each hour of the storm; it is now convoluted with the hydrograph to
give flow rates resulting from the rainfall over the duration of the storm. This data is present
in entry 10 appendix 1. Figure 22 shows the flow input for each hour with hour one
representing the inflow for the first hour of the storm.
Hour 0 1 2 3 4 5 6 7 8
Qin m3/s 0 0.76 2.78 7.84 23.43 106.17 196.55 278.55 297.95
Hour 9 10 11 12 13 14 15 16 17
Qin m3/s 249.52 189.54 126.90 65.18 14.56 4.77 1.67 0.44 0.00
r 22 X 42
ARF 0.95 Growth Factor 4.86
M5-2 93 mm ARF = 0.95
M5-DHr 39 mm
M10000-DHr 189 mm
P 180 mm
Fig. 21 Key data relating to storm profile
Fig. 22
15 Feasibility Study of Irfon Valley Dam Scheme
5.4 Flood Routing
The general purpose of a flood routing model is to determine the downstream flows from
the input flow into the reservoir. This is important to note if we are looking to protect an
area against flooding. The essential theory behind flood routing is the continuity equation.
Change in reservoir volume = inflow – outflow. This can also be written in terms of discrete
time intervals as shown in entry 11 appendix 1. Both these equations rely on the evaluation
of the reservoir surface, which we assume is level at all times for the purpose of these
calculations.
The reservoir level will be taken from the storage elevation curve for the reservoir location
we have chosen. This is then processed using a flood routing model applying the general
principles outlined above. In addition it uses a constant C derived from the laboratory
experiment of 1.95. Details of the laboratory experiment can be seen in entry 12 appendix
1. A process of trial and error is then used to determine the spillway width that keeps the
flow height of the water below the design criteria which in this project is 3m.This gives a
flood routing graph displayed in figure 23.
Fig.23 Showing the flood routing graph for our reservoir
This gives a number of clues about the information with regard to our reservoir the first is
that while there is a substantial difference between the upstream flow and the downstream
flow it is not as large as in other locations. This is because as the water level rises in our dam
the storage does not increase as quickly as in other locations. This is because the sides of
the valley at site are steeper, the closer the slope is to vertical the closer the increase in
volume will be to linear, where as if the slopes are less steep then the relative increase in
volume will be greater as the flooded area will increase. This ties in with the previous data
0
50
100
150
200
250
300
350
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
Flo
w m
3/s
Time (h)
Flood Routing Graph
Inflow
Outflow
16 Feasibility Study of Irfon Valley Dam Scheme
about our site in that it has the smallest flooded area of the four trialled, which implies it
has steepest valley sides.
5.6 Key Data
Peak outflow 158.41 m3/s
Peak height (Hd) 2.95 m
Peak Inflow 298.0 m3/s
Peak difference 139.54 m3/s
Spillway Width 16 m
6.0 Geological Analysis
6.1 Introduction
Geological analysis is crucial to ensure a safe and efficient design for the dam.
6.2 General overview
The Irfon valley is mainly underlain by rocks typical of the Ordovician period; rocks typical of
this era are approximately 440 million years old. Geological strata tend to run in a North –
Easterly to South – Westerly direction, with an overall dip of between 200 and 40⁰ to the
North West. Geological maps show that the region mainly consists of turbiditite mudrocks
and some shales. (1) The valley was originally thought to have been shaped by structural
folding, and was made steeper due to glacial activity in the Quaternary period. (2)
6.3 Geology at specific site
The Geology at our chosen site, site 2 was considered suitable for construction of a dam at
this point. The bedrock was mostly comprised of mudrock, which dipped in a North West
direction at approximately 30⁰.
There is an overlying layer of alluvium and weathered rock which would need to be
removed before construction of the dam could begin. Although, in some cases dams can be
built over the material, the thinness of the layer means that it would be preferable to
excavate this material prior to construction to improve stability.
Fig. 24 Flood routing key data
17 Feasibility Study of Irfon Valley Dam Scheme
Fig.25 Some overlying alluvium and weathered rock would need to be removed prior to
construction.
6.4 Foundation compressive strength
Foundation material is one of the most important considerations in dam design, and can be
seen to be a common cause of dam failure. An example where this can be seen is in the
collapse of the Teton dam in 1976, where inadequate geological investigation and lack of
proper sealing of the joints in the rocks are both sited as contributing causes to the failure of
the dam.(3)
Fig.26 Teton dam failure 1976
Therefore it is vital to ensure the foundations are suitable for the design.
The mudrock underlying the site has an estimated bearing capacity of 5Mpa (4) and an
estimated compressive strength of 18,700Kpa to 56,400Kpa (5)
6.5 Valley
6.5.1 Topography and stability
The Valley profile at the chosen dam location is relatively shallow and ‘U- shaped’. There is
a flat bottom with relatively gentle sides. The relatively shallow angle of dip of the mudrock
18 Feasibility Study of Irfon Valley Dam Scheme
means that we would not anticipate landslides to be a problem. However, the stability of
the valley should be monitored during excavation and Tensar reinforcing anchors may need
to be considered to stabilise the valley sides. The valley topography chart is included in
Appendix 4.
6.6.2 Seepage and Permeability
Whilst the mudrock itself can be considered to be relatively impermeable, the folded nature
of the valley may mean that there could be a high rate of secondary permeability.
Secondary permeability occurs when cracks allow water to enter the rock layer and hence
pass through it. It is therefore necessary to consider the angle and direction of strata dip,
and whether it will be necessary to grout underneath the dam. As the strata dips
northwest, the majority of the seepage should occur into the reservoir area and should not
affect the stability of the system. There is a nearby valley to the west of the area, and
seepage from the reservoir into this valley is a potential concern. As such borehole tests
should be carried out to monitor pore water pressures and any potential seepage.
The presence of faults needs to be considered when assessing the suitability of the site.
There are no faults at the specific site 2, however there are two fairly major faults, one
upstream (the Abergwesyn fault) and one downstream (the Llanwrtyd fault) (1). The upper
of these faults will be submerged by the reservoir area, however it is not anticipated that
the fault will affect the stability of the valley slopes.
6.6 Materials
Selecting the appropriate materials for dam construction is very important. Inadequate clay
core material has also been cited as one of the causes of failure in the Teton dam (3).
6.6.1 Mudrock
The river runs along the mudrock itself, suggesting the material is both strong enough and
impermeable enough to be used as rockfill for the dam. The strength of the mudrock was
also suitable for use in the construction of the dam at Lynn Brianne, a further indication of
its suitability for the rockfill of an embankment dam. Using local materials will then also
reduce transportation costs associated with outsourcing materials.
The Mudrock was tested on site and would not be hard enough to be used as aggregate for
concrete, so alternative sources of aggregate will need to be found.
6.6.2 Quarry
There is a nearby quarry situated to the south east of the site, approximately 1.25 km away
from the proposed dam location. The quarry contains mudrock and Kilsby Tuff, a hard
igneous rock which is much stronger than mudstone and could be used for aggregate. The
band of Kilsby Tuff in the quarry area is approximately 2km long, 100m wide and 40m deep.
19 Feasibility Study of Irfon Valley Dam Scheme
Fig. 27 Showing exposed Kilsby Tuff in the nearby quarry. Some weathered rock can be
seen on the top right which would need to be removed before investigation.
The access to the quarry is along the bottom of the
valley, although the access is currently limited and a
new road and conveyor system would need to be put
in place. The environmental impact assessments
associated with the reopening of the quarry will need
to be very detailed as there are a number of
stakeholders and authorities to consider. The time
and associated cost of these would mean that it may
be more economical to source materials from Builth
Wells, especially if constructing an embankment dam
which will not require much concrete.
Fig. 28 Weathered rock (scree) can
be seen at the base of the slope.
20 Feasibility Study of Irfon Valley Dam Scheme
6.6.3. Boulder Clay
There are also several areas of glacial till. The largest of these was used as the borrow pit
for the construction of the dam at Lynn Brianne. The more easterly of the two smaller areas
shown in appendix 2 will be flooded by the reservoir area, and although further
geographical investigation will need to be carried out, geological maps suggest there should
be enough boulder clay here for the impermeable core of an embankment dam. Sources of
materials can be seen Appendix 2.
6.6.4 Alluvium
As has already been mentioned, there is a thin layer of alluvium overlying the rock. This will
not be suitable for the clay core of the dam, and so should be removed before construction
of the dam begins.
7.0 Scheme Design
7.1 Dam design
7.1.1 Dam choice
In designing the dam, we first need to determine the most suitable dam type for the
location. There are several different types of dam, which have been divided into two
categories, embankment dams and concrete structures.
7.1.1.1 Embankment dams
Embankment dams are normally constructed of earth or rock materials that can be sourced
close to the site location. An embankment dam would normally be used where the
foundations and abutment conditions were unsuitable for a concrete dam and where
suitable materials for the embankment were present at or close to the site (6) Most
embankment dams are non-homogenous and contain ‘zones’ of different materials. Usual
practice is to place a low-permeability ‘core’ layer at the centre of the dam, with areas of
other suitable material at either side. The upstream face needs to be protected from
erosion by wave action, either by the placing of concrete or rip-rap. Some internal drainage
is also needed to reduce problems associated with the build up of internal pore pressures
and seepage.
21 Feasibility Study of Irfon Valley Dam Scheme
7.1.1.2 Concrete structures
Concrete gravity structures consist of large volumes of concrete, and use self weight of the
block to hold back the water. An advantage of concrete gravity dams is that the spillway can
be built into the crest of the dam, so the cost of an additional spillway is avoided. Concrete
dams require a large amount of aggregate and good quality water, which would need to be
sourced prior to construction. The construction period for a concrete dam is longer, more
labour intensive and is also affected by weather conditions.
There are also other types of concrete dams, such as arch or buttress but these require
ground strengths in excess of those found at our site and as such have not been considered
for this project.
7.1.1.3 Final choice of dam type
The flat nature of our valley means that an embankment dam would be most suitable for
the dam location. The valley is fairly wide at its base (approximately 240 m) at this point
and as such a large volume of material is required. The high cost of transporting large
amounts of aggregate to the site means that a concrete dam would not be suitable for our
chosen dam location.
The availability of local materials has meant that we have chosen a rockfilled embankment
dam for our site. As previously mentioned in the ‘Geological analysis’ section of this report,
the local mudrock strata are suitable for use for the rockfill of the dam, whilst the glacial till
could be used to construct the impermeable clay core. Both of the materials have been
successfully used in the construction of the embankment dam at Lynn Brianne, shown
below.
7.1.2 Dam structure
The initial design of the dam has been based on the geological maps and site visit data, as
well as previous examples of dam construction. Detailed geological investigation such as
boreholes as well as further shear strength and bearing capacity tests will need to be carried
out in the area.
The dimensions of the dam can be seen on the cross-section plan shown overleaf.
22 Feasibility Study of Irfon Valley Dam Scheme
Fig. 29 Dam through section.
23 Feasibility Study of Irfon Valley Dam Scheme
7.1.2.1 General Dimensions
The dam was found to be required to be 40m in height, including the 1m of freeboard.
Details of this height calculation can be seen in Appendix 2. The dam is 400m across the
crest and 138m deep. A 3m wide asphalt road is to be constructed along the crest of the
dam which will be used for maintenance and construction vehicles. The construction
sequence for the dam is included in Appendix 12.
7.1.2.2 Clay Core
The dam incorporates a sloping vertical clay core to provide stability and reduce seepage
through the dam. The clay core will be constructed from the local boulder clay as previously
discussed earlier in this section. The clay core is 3m at the crest and 8m at the base of the
dam. Calculations for these dimensions can be seen in Appendix 2. Construction of the clay
core should be performed in layers, with the clay being placed and compacted slightly wet
of optimum to produce lowest possible permeability. (7) Clay should avoid being placed in
the wetter months to avoid ‘shrinkage cracking’ associated with over-wetted clay (7).
7.1.2.3. Rockfill
The main purpose of the rockfill is to provide stability to the dam. The material being used
as rock fill is predominately mudrock with some siltstone laminations. The mudrock has a
typical ф value of 37° (8). Further laboratory tests should be carried out to check that this is
true for our strata. A Coulomb ‘two- part wedge analysis’ was carried out to ensure stability
and design the optimum slope sides. The optimum slope angle was found to be 30° and the
results for this analysis can be seen in Appendix 11. By changing the location and inclination
of the wedges, it is possible to determine the Factor of Safety at the most critical location.
This was found to be 1.68. As the minimum requirement was 1.5, the dam design can be
considered safe.
7.1.2.4 Foundations
It is necessary to assess whether the foundations were adequate for the embankment dam.
By considering a 1m by 1m cross section block, we can determine the force applied to the
foundations by the dam. A 1m x1m x40m block with a unit weight of 22000N/m3 exerts a
force of 880 Kpa on the ground. As the foundations can withstand a bearing pressure of.5
Mpa this is greater than the force exerted and so the foundations are adequate.
7.1.2.5 Concrete facing
There is a 0.3m thick concrete layer on the upstream face to prevent the rock from being
eroded by wave action. (9)
24 Feasibility Study of Irfon Valley Dam Scheme
7.1.2.6 Transition zone
This is a layer of graded rock which is designed to protect the clay core on the upstream
face. Further investigation will need to be carried out on the exact properties of the boulder
clay in order to determine the size and grading of material needed.
7.1.2.7 Filter
The purpose of the filter is to protect the integrity of the clay core itself. The filter allows
water to seep freely from the core, whilst preventing any soil particles from passing (10).
The filter should contain finer material close to the core, and progressively coarser material
towards the rockfill zone. Further investigation will again need to be carried out on the clay
material, as the design of the filter is based on the size of the clay particles.
7.1.2.8 Chimney and blanket zones
The Chimney and blanket drains work in conjunction with the filter to reduce potential pore
pressure build up on the downstream face of the dam. Seepage through the downstream
face of the dam can damage the dams’ structural integrity and can cause failure. It is
therefore important to provide adequate drainage. An example of dam failure due to
inadequate drainage can be seen in the collapse of the Stava Tailings Dam, Italy in 1985. (11)
Fig. 30 The destruction in the valley caused by the failure of the Stava Tailings dam in 1985
The chimney and blanket drains allow water that has passed through the filter to safely run
out through the downstream toe of the dam. The chimney and blanket drains were
designed as 3m thick, in accordance with construction requirements (12) although the
integrity will need to be verified by carrying out further seepage modelling.
7.1.2.9 Grout cut-off curtain
There is an additional grout cut-off curtain provided under the clay core, to reduce seepage
under the dam. The foundations of the dam will be grouted prior to construction to reduce
the secondary permeability of the mudrock.
25 Feasibility Study of Irfon Valley D
7.2 Spillway Design
7.2.1 Introduction
One of the most common caus
mitigated by the design and co
dam at a quickened rate when
perform its important second f
living further down the valley t
7.2.3 Design Criteria
There are three stages to the
the catchment area will respo
which will need to be applied
mitigate against. These steps h
report. The third step is to ap
spillway; this will be covered in
7.2.4 Spillway Design
Once the peak outflow and wid
in the spillway. The type of spil
7.2.5 Laboratory experiment
In order to determine dimens
calculate dimensions which can
Fig. 31 Dimensions of the og
ley Dam Scheme
auses by which a dam can fail is overtopping. T
d construction of a spillway which will allow wate
hen the storage height is exceeded. The spillway
nd function of flood control, which provides ben
ley that could otherwise be affected by high volu
the design of a spillway for the dam the first is
espond to rainfall. The second is to find the a
lied to it in order to represent the level of risk w
ps have been covered in detail in the Flood rout
o apply the results of this analysis to the phys
d in this section.
width is known the weir, chute and stilling basin
spillway that is being designed is an overflow sp
ensions of our weir a lab experiment is perfo
can then be scaled up using comparison factors
by the lab experiment
deriving these values
detail in entry 12 app
values are Hmd=0.044m
Hmd is then compared
growth factor S of 67
allows the model weir t
life size for our dam. T
the weir are shown in fi
The next aspect of th
designed is the chute. T
simple open channel hy
solved using the equatio
13 appendix 1. For this
e ogee weir.
. The risk of this is
water to leave the
ay also lets the dam
benefit to people
volumes of rain.
st is to work out how
e amount of rainfall
isk we are looking to
routing section of the
hysical design of the
asin can be designed
w spillway.
erformed in order to
ctors also determined
ent. The process of
ues is described in
appendix 1. The key
44m and Cd=0.625m.
red to Hd to get a
f 67.045. This then
eir to be scaled up to
. The dimensions of
in figure 31.
f the spillway to be
te. This is a relatively
el hydraulics problem
uations listed in entry
this project it was felt
26 Feasibility Study of Irfon Valley Dam Scheme
that the most economical option with regard to the terrain and the dam design is to build a
chute that roughly shadows the slope of the underlying terrain. However as the flow in the
chute must remain supercritical, we had to calculate the critical slope to ensure this flow
regime. The key data is provided in figure 32. A plan of spillway is include in the appendix.
yc 2.15 m
Sc 0.00150319 m/m
Terrain slope 0.2 m/m
Height of Chute Water 0.46 m
Height of Wall 1.25 m
Fig. 32 Key data of the spillway chute
The final aspect of the spillway that needs to be designed is the stilling basin. The stilling
basin is a hydraulic structure which serves the purpose of dissipating the energy that is
generated from the spillway and as such prevents erosion of the river bed. The design of the
stilling basin uses a number of equations which are detailed in entry 14 appendix 1. The
downstream depth is checked using the manning equation. The wall should be taller than
the downstream but not be much if this is the case then wall and stilling basin is sunk into
the ground. In our case this is not required.
Wall Height 1.67m
Stilling Basin Length 24.54m
Height of Water after Basin 0.171m
Fig. 33 Key Stilling basin data
27 Feasibility Study of Irfon Valley Dam Scheme
7.3 Diversion Works
7.3.1 Introduction
One of the key parts of the scheme is the diversion works. The works will dictate how the
remaining parts of the project will be timetabled and how they must be completed. The two
options for diverting the water while dam construction goes ahead is as follows: Tunnelling
in the valley wall or by building a culvert under the dam site. Due to site topography, mainly
the mild slopes, a culverted solution was sought.
7.3.2 Diversion Works Design
The culvert will consist of a pair of pipes encased in concrete. The design flood for the
temporary works is highlighted as a 1 in 5 year flood event. This is equivalent to an inflow of
61.79 cumecs. As a cost saving measure the culvert will be paired with a cofferdam system
that will eventually be included in the final dam structure.
The height of the cofferdam has been optimised with respect to the cost of the culvert. The
technical specification of the diversion works are highlighted in the table below.
Diameter of Pipes: 2.25m
Height of Cofferdam: 7m
Length of Culvert: 183m
Cost of Culvert: £44,800
Cost of Cofferdam: £410,100
Total Cost of Diversion Works: £454,900
Fig 34. Technical specification of diversion work.
A graph showing the relationship between the total cost, height of cofferdam and size of
culvert is included in Appendix 9. The costing of the culvert has been made on the
assumption that the concrete is ready mixed and is supplied from the Cemex Ltd. Works in
Builth Wells. The supplier is located around 25km (15.7 miles) from site.
28 Feasibility Study of Irfon Valley Dam Scheme
7.4 Aqueduct Design
7.4.1 Introduction
The aqueduct will carry the water from the dam site to the service reservoir at Hywelfynydd.
The straight-line distance between the two is 10.64km, although as the region has many
undulations the route must be carefully considered. The route has been chosen as a result
of careful consideration of the economic, social and construction factors.
The aqueduct will be formed of a single pressurised pipe, capable of carrying the required
amount of water to the service reservoir every day.
7.4.2 Water Demand
No. of Residents = 200,000
Demand per Resident = 300litres/day
Demand = 60,000 m3/day
= 0.69 cumecs
7.4.3 Intake Tower
The intake tower is being designed by an outside company and has been costed at £50,000.
The tower will be connected to one of the culvert pipes used in the temporary work in order
to start the aqueduct route. This is the most cost effective option to start the scheme.
7.4.4 Basic Costs
Operation Cost
Tunnelling £320/m
Trenching £80/m
River Crossing £32,000/crossing
Intake Tower £50,000
Fig. 35 Details of the cost data of the aqueduct
29 Feasibility Study of Irfon Valley D
7.4.5 Aqueduct Routes
Route 1:
Route 3:
Fig. 36 Routes considered for a
The initial route is the most dir
local topography, the route req
The second route bypassed the
route 1 and there was an impro
Unfortunately, the extra distan
South of the site there is a train
shallowest contours in the area
that. Route 3 shows an enormo
On further investigation we dis
least 50m away from the train
further north from the track. A
contours of the land and tried t
a small hill near to the dam site
outside the valley, the tunnellin
ley Dam Scheme
Route 2:
Route 4:
for aqueduct.
t direct between the sites. Due to the mountaino
required a lot of tunnelling and as a result was
the worst of the mountainous areas that were e
provement in the amount of tunnelling that wa
stance made the new route even more expensiv
train line. We believed that the train line would
area and that in order to get the best route we s
rmous saving over previous routes by taking this
discovered that for safety reasons the pipe sho
ain line embankment. Therefore, for route 4 we
k. Additionally, for this route we paid special att
ied to avoid steep slopes. We also tested runnin
site. Although there was a saving on the route o
elling that was required to by-pass the hill was n
ainous nature of the
as very expensive.
ere encountered in
t was required.
nsive.
uld follow the
we should follow
this approach.
should remain at
we moved the line
l attention to the
ning the pipe around
te once the pipe was
as not cost-effective.
The final choice of site, shown
mindful of the contours as well
of the dam.
Route 5:
Fig. 37 Final aqueduct route.
7.4.6 Terrain (Route 5)
Fig. 38 Terrain profile and hyd
0
50
100
150
200
250
0 2000 40
Me
tre
s a
bo
ve
Ord
ina
nce
Da
tum
Terrai
Feasibility Study of Irfon Va
wn below, takes the best from routes 3 and 4. W
well as choosing the best route through the valle
hydraulic gradient of the chosen route.
4000 6000 8000 10000 12000
Horizontal Distance (m)
rrain vs. Hydraulic Gradient
30 n Valley Dam Scheme
4. We have been
valley downstream
14000 16000
31 Feasibility Study of Irfon Valley Dam Scheme
7.4.7 Route Costs
Route Basic Route
Cost
Cost of Intake
Structure
No. River
Crossings
Cost of River
Crossings Total Cost
Route 1 £2,810,151.74 £50,000.00 8 £256,000.00 £3,116,151.74
Route 2 £3,184,916.63 £50,000.00 9 £288,000.00 £3,522,916.63
Route 3 £1,462,640.00 £50,000.00 6 £192,000.00 £1,704,640.00
Route 4 £1,979,009.17 £50,000.00 6 £192,000.00 £2,221,009.17
Route 5 £1,175,940.13 £50,000.00 5 £160,000.00 £1,385,940.13
Fig. 39 Cost comparison of aqueduct options.
7.4.8 Diameter of Aqueduct Pipe
The next consideration is to look at how large the service pipe to the reservoir is. The pipe
diameter directly affects how much water can be delivered and how much energy is lost on
the way.
Using the formula �� �����
�� the head loss was found for the length of pipe in the solution
chosen. As stated above the pipe flow is expected to be 0.69 cumecs and route 5 is 14.5km
long. The available head for this system is 25m. The spreadsheet that calculates the head
loss is included in Appendix 8. The results are given below:
Pipe Diameter (m) Head Loss (m) Available Pipe Length (for 25m
head loss (km))
0.75 48.1 7.5
0.85 25.7 14.1
1 11.4 31.8
1.5 1.5 241.3
Fig. 40 Pipe dimensions and associated head losses
7.4.9 Conclusion
The route chosen for the final design will be route 5. The figure 40 highlights the financial
differences between the different options and the maps show the different routes. The
chosen route is the cheapest option of the five potential pipelines. It is one of the longest at
14.5km but it requires the least tunnelling of any option as well has having the fewest river
crossings. Additionally, the contact the pipeline has to local residents is minimised by
diverting the pipeline mainly through the countryside.
32 Feasibility Study of Irfon Valley Dam Scheme
We believe that this is the best and most cost effective route, which will not include the
need for pumping. With the above considerations taken into account, the route should also
be the quickest to construct. Additionally, using the culvert as the outlet for the dam will
save on cost and time for the construction team. The pipe will have a 1m diameter. A
connection detail will be added where it connects to the culvert to facilitate the changing of
the pipe diameter.
7.5 Diverted Services and Dam Maintenance
The existing road which connects Abergwesyn to Llanwrtyd Wells has to be relocated due to
the extent of flooding caused by the new reservoir. The proposed alignment follows an
existing forest track to the west of the flooded area. Utilising an existing route reduces the
need for new infrastructure and minimises the budget. However, the road will have to be
suitably improved in order to accommodate heavy goods vehicles, logging trucks and plant.
Excessive vertical grades will be avoided by meandering up steep hillsides. The proposed
alignment will tie into the existing roads to the west of Abergwesyn and north of Llanwrtyd
Wells.
Other services such as telephone lines and power lines will be repositioned to follow the
alignment of the new road. The total length of the proposed road is 12km with projected
costs of £1,037,000. Details of these costs are in appendix.
Fig. 41 Planned diversion works
33 Feasibility Study of Irfon Valley Dam Scheme
There will also be a service road (figure 42) which will connect to a 3m wide asphalt road
running along the 400m wide dam crest. This will provide access to the dam for
maintenance and service inspections. We will implement annual checks to ensure that there
is no damage to the dam which would ultimately cause a catastrophic failure.
Fig. 42 Plan view of the service road
8. Environmental Impact Assessment
8.1 Introduction
It is inevitable that the proposed dam and reservoir will affect the local environment due to
the scale and nature of the project. Therefore it is necessary to consider an Environmental
Impact Assessment to investigate the short-term and permanent changes within the valley.
The undertaking of an environmental assessment is necessary if the structure comes under
schedule 1 in the environmental impact assessment guidelines. The building of a dam comes
under schedule 2 ‘ 10(f) a dam or other installation designed to hold water or store it on a
long term basis’, which states that an environmental assessment only need be carried out if
there is significant reason to do so.(13)
The valley of the River Irfon is home to many protected and rare species and flora and is
location of many Sites of Special Scientific Interest (SSSI). In addition, the site is in a very
rural area. It is clear that the scheme is larger than something that would only benefit the
immediate population. It is therefore imperative that an Environmental Assessment is
carried out. The Assessment guidelines state that the impacts of the new development are
to be fully understood and all possible options to be considered. It is also a way to identify
possible issues early on in order to reduce delays at later stages of the project. Our company
values the need for environmental considerations and engineering design to interact
throughout the project.
Figure 43 shows the sensitive features in the local environment which require special
consideration.
34 Feasibility Study of Irfon Valley Dam Scheme
Fig. 43 Sensitive features in the local environment that require special attention
8.2 Considerations
The first thing we considered was how the construction of the dam would affect the current
workings of the valley. Currently the valley is used for a combination of agriculture and
logging. The base of the valley contains fields for grazing animals and the hillsides are
heavily wooded. The construction of the dam would mean that neither of these industries
could occur in the vicinity of the construction site or in the flooded area.
In addition, the construction would require large amounts of deforestation in the area. The
current woodland is sustainably farmed by the loggers and represents a significant
investment on their part. Compensation would have to be given to the loggers, although the
money gained from selling the timber from the deforested area could go some way to
offsetting this cost.
The construction of the dam will require local materials and a large amount of fill is required
from the surrounding area. Much of the high quality fill and core required will not be
available from the flooded area and will have to be sourced from elsewhere. This will leave
scars on the landscape.
Moving quantities of materials will inevitably require the use of large plant. Moving these
through a small valley such as this one will create problems, especially with the logging
industry that is in operation. Tension can be reduced with the fellow valley users and
residents by the creation of slipways, alternative routes and limiting the use of heavy plant
to certain times of the day.
35 Feasibility Study of Irfon Valley Dam Scheme
Using any vehicle on site can lead to an increase in dust pollution and the construction team
should go to the correct lengths to reduce this. Dust suppression techniques such as wetting
the surfaces where vehicles will drive are a good way to do this, especially with the constant
availability of water in the area. Furthermore, a rock fill dam may require the use of an on-
site rock crusher and the general site works will incur vibrations in the valley. The
construction team must be aware that some areas, such as near to animals, are sensitive to
these vibrations and should be avoided, where possible. It is also worth considering that all
work that creates vibration and noise must be kept within social hours to avoid conflict with
other valley users.
The next major considerations are the people that will be affected by the creation on the
reservoir. The flooded area created by the dam will cover thirteen properties, of various
purposes. These include residences, farms and farmland and holiday cottages. At least a
kilometre square of grazing farmland will also be covered by the water. Compensation will
have to be paid to the owners of these properties although a compulsory purchase order
may have to be obtained for the entire area in the event of residents being unwilling to
move.
Other utilities that will have to be moved are electricity and phone lines. These are owned
and maintained by Western Power Distribution and BT respectively. Each of these services
will have to be rerouted past the dam to the properties and businesses on the other side of
the lake. This is expected to be rerouted in line with the new road that will be built along the
western side of the reservoir.
The construction and operation of the dam carries a certain amount of risk for the workers
on the site and the people living in the vicinity. A dam collapse would be catastrophic for
everyone and the environment. In order to mitigate this risk, the site team will ensure best
practice is used at all times and that the appropriate checks and tests are carried out when
necessary. It is also the contractor’s responsibility that everyone on their team is fully
trained or appropriately supervised while the work is in operation.
The valley has a rich cultural heritage and has been in use for
at least 800 years. The valley contains some historical
standing stones, which have been set aside as a scheduled
ancient monument (SAM). The stones are estimated to be
from the Bronze Age (2100-750BC) and will require special
permissions to move or affect in any way from the Royal
Commission on the Ancient and Historical Monuments of
Wales. (15) As well as the stones the valley contains two
churches with adjoining graveyards. In order to respect the
wishes of the families of the deceased the two sites should remain unaffected by the dam’s
construction.
The valley is home to a variety of different aquatic species and plant life that thrive in the
unique conditions that the valley offers. The large amount of woodland increases the acidity
of the soil, conditions that the rare globeflower thrives on. There is a site of special scientific
Fig. 44 Globe-flower (14)
36 Feasibility Study of Irfon Valley Dam Scheme
interest (SSSI) near to the northern edge of our flooded area that contains a very rare
example of globeflowers in a meadow environment. We have taken special care to avoid
damaging all SSSI’s in the area, including the Nant Irfon national parkland to the north of
Abergwesyn and the Cae Pwll-y-Bo meadow mentioned above.
The river itself has been designated an SSSI by the Countryside
Council for Wales. This is due to its populations of Atlantic salmon
and otter, both of which are threatened species in the UK. It is also
the home to various submerged aquatic plants, such as the Water
Crowfoot, and fish species, such as lamprey and bullhead. Steps
should be taken during construction in order to minimise the
pollution going into the river. The use of cofferdams on both sides
of the site should prevent water contamination but the contractor
needs to ensure that proper waste disposal techniques are used.
The other reason that the river has been graded as an SSSI is
because of its mesotrophic (16) and oligo-mesotrophic types. This
describes the bacterial activity within the water. These particular types are highly suited to
drinking water due to their relatively low nutrients, and therefore low algal content.
Additionally, the water has a high oxygen level, which is good for wildlife.
We plan to reduce the impact on the environment by identifying risks and implementing
preventative measures. For example, we plan to encourage staff to promote recycling in
order to reduce waste from food packaging and consumables. Staff will be trained to use
plant and other machinery efficiently to reduce the consumption of fossil fuels. A detailed
site investigation will be carried out to identify undefined materials and ancient
archaeology. Natural habitats will also be relocated to reduce the environmental impact.
Construction will not be carried out during unsociable hours to reduce the extent of light,
noise and air pollution for the locals. Vibrations will inevitably be created due to the nature
of works, however we plan to spread the onerous works out along the valley and avoid
sensitive areas. (See Appendix 6)
8.3 Summary
In summary, the environmental impact incurred by the project has been investigated and
assessed. Key features have been identified and preventative measures have been put in
place where possible. Particular sensitive areas such as the standing stones have been
focused on. The concluding statement of the assessment is that the dam and reservoir will
have a large impact to the environment, however the scheme is believed to be viable due to
the measures implemented allowing harmony between the design and conservation of the
environment and contained ecosystems.
Fig. 45 Otter (17)
37 Feasibility Study of Irfon Valley Dam Scheme
9.0 Health and Safety
9.1 Introduction
Health and safety is an important aspect for every engineering job. Recent schemes such as
‘Zero Harm’ (18) aim to eliminate site accidents altogether and the number of man hours
without incident achieved by different companies can be assessed by potential clients. (18)
Dam construction is considered high risk with concerns to site safety due to the large
volumes of water and construction material required. It is important to highlight as many of
these as possible at the planning stage in order to minimise risk later on.
9.2 Site Practice
Keeping a safe working site will be challenging on such a large project, using a relatively
confined site. The necessity of large plant for the duration of the project will require
stringent safety management systems in order to ensure the safety of everyone on the site
team. To mitigate the risk of collisions and accidents on site, detailed planning of works will
be carried out before the project starts. This should ensure that workers walking around on
site will not come into contact with large machinery.
The site will have closed pedestrian walkways for the workers, which will be separated from
routes the plant uses. Full Personal Protection Equipment (PPE) will also be compulsory in all
parts of the working site, excluding the canteen and inside the site management huts. All
site staff, including visitors will be briefed on site safety and will be refreshed on a six month
basis. All visitors must be supervised by a member of the site team for the duration of their
visit. Groups should be limited to ten per member of supervising staff and no group larger
than twenty should be shown round site at any one time.
In parts of the site where noise and vibration are an issue, additional protective clothing will
be provided. Staff using this equipment will also be briefed on considerate use, to mitigate
the disruption to the local residents.
All staff that will be in charge of heavy plant and machinery must possess the relevant
qualifications and/or licenses in order to use these vehicles. The users of heavy plant must
be briefed on how they can alleviate their environmental impact by driving efficiently and
keeping to predefined routes.
Dust on any site can be problematic and can be a risk to the site staff as well as local valley
users and the environment. Routes used by plant and vehicles will be regularly wetted to
avoid excess dust being kicked up. The water will be sourced from the river due to the
unimportance of water quality.
In the unlikely event of an accident, the site management team will set up checkpoints
around the site which will have basic first aid equipment and a communication link to the
main site office. A fully trained
as well as a central first aider in
In the event of adverse weath
procedures, as set out by the c
event of extreme rainfall event
The site should be well lit, es
workers while moving around
residents and valley users in or
9.3 Site Access
The site will have a large wor
consideration will have to be t
in an efficient and safe manner
It is currently planned to set up
workers can live or leave their
transport the workers. The tow
system via the A483 and has a
Workers should adopt safe wo
traffic laws and respecting the
10.0 Costing
The costing for this scheme ha
These are the preconstruction
length of the project or at spec
10.1 Preconstruction Costs
Fig. 45 C
Feasibility Study of Irfon Va
ined first aider will be on duty in each area of th
er in the site office.
eather conditions, site workers should be awar
he contractor. All compaction works should be
vents and during very cold weather.
, especially during the winter months to avoid
nd site. Hours of acceptable lighting will be dis
n order to minimise impact and disruption.
workforce and the site is small and the acces
be taken on how to transport the workforce to
ner.
t up a compound near to the town of Llanwrtyd
heir vehicles. The site will then offer a shuttle bu
town of Llanwrtyd Wells is well connected to t
s a train station, which has a direct link to Swan
e working practice when travelling to and fro
the local residence while the works are ongoing.
e has been split into three stages over the lifesp
tion costs, the scheme costs and other costs tha
specific times during it. First we will look at preco
osts
Chart of preconstruction costs.
Farmland
Forest
Houses
Service relocation
Site clearing
38 n Valley Dam Scheme
f the site at all times
ware of the relevant
be suspended in the
void accidents to the
discussed with local
ccess is poor. Special
to and from the site
rtyd Wells where the
le bus into the site to
to the road transport
wansea. (2) (3)
m site, obeying all
ing.
fespan of the project.
that appear over the
reconstruction costs.
39 Feasibility Study of Irfon Valley Dam Scheme
10.1.1 Land Acquisition
Before construction of the dam and surrounding infrastructure can begin, the land that is to
be built upon and flooded must be purchased, and cleared of obstacles. Some infrastructure
that is removed will also have to be rerouted elsewhere. With any large engineering project,
even in the countryside these are always going to be large costs. To ensure we had accurate
costs for this vital area of the scheme, we did a lot of research. The land acquisitions were
split into three areas; farmland, forest and housing. After contacting Brigtwells land agents
in the nearby town of Builth Wells we established that in the current market in the Irfon
valley, farmland would sell for between £3,000 and £10,000 an acre. An acre costing £3,000
would likely be very steep land which only a few animals could graze on whereas a £10,000
acre would be the same quality as a paddock. Using our knowledge of the land from the site
visit, we made the conservative estimate that we would budget for each acre of farmland to
cost £7,000. At close to 200 acres of farmland on our site we would pay close to £1.4M to
acquire it. The land agents also informed us that forested land would sell for between
£1,500 and £3,000 per acre depending on the demand for the land. As we saw active logging
while we were in the valley we assumed that demand for forest land would be high and so
budgeted to spend £3,000 an acre on the 13 acres of forest that we would need to flood.
This is by far the lowest cost at just £37,200.
The hardest area for us to budget in terms of land acquisition is domestic housing as once it
becomes public knowledge that the dam could be built, house prices would likely rise
quickly. to assess the market in its current state, we checked the land registry and averaged
the prices of all detached houses sold in the local area in the last 12 months, we then
applied this price to every building that we would need to acquire regardless of whether it
was a house or not to allow us to raise offers on inhabited properties if necessary. Then a
15% contingency was added to the total housing budget in the event that we would have to
follow a compulsory purchase route. The total cost for land acquisition came to £5,600,000.
10.1.2 Land Clearing
The land must then be cleared to make way for the dam and reservoir. There is no need to
demolish the houses as they can have their gas and water cut off and then be flooded. The
forest and farmland however must be cleared as debris could block pipes in the draw off
tower or culvert. Figures from the Spon’s Civil Engineering and Highway Works Price book
2008 suggest that it will cost £239,000 to clear the 200 acres of farmland and 13 acres of
forest that we will be using in this project.
10.1.3 Service Relocation
Relocating services will also be a large cost in the preconstruction phase. Using pricing from
the same book a table was generated to cost the relocation of power lines, telephone lines,
sewerage and water and gas mains.
Service
Electric cable
Sewerage/Drainage
Telephone likes
Water mains
Gas mains
Fig. 46 Breakdown of ser
The total preconstruction cost
10.2 Scheme Costs
Fig. 47 Chart of
10.2.1 Dam
As expected, the dam is the la
largely due to the fact that we
we avoided flooding many prop
dam itself will be 400m wide. T
cubic metres of rock. The che
however the quality of rock in
blast significantly more rock th
We looked at getting higher q
away, but once the transporta
per Km thereafter per cubic m
Feasibility Study of Irfon Va
Price (m) Length to be relocated (m) Co
37 4000 £1
118 4000 £4
28 4000 £1
88 4000 £3
115 4000 £4
£1,
f service relocation costs
ost of this project is expected to be £7,401,000
t of scheme costs.
largest individual cost of the project at just ov
we have a very wide valley to dam. Our choice o
properties and larger areas of farmland but it do
de. To construct the rock fill dam, we require ju
cheapest way to get rock is to blast it from th
k in the quarry is not ideal for our project and w
k than we would use to construct the dam.
er quality rock from a RMC owned quarry in B
ortation costs were factored in (£0.95 for the fi
ic metre) it was more economically viable to
Dam
Road
Diver
Aque
40 n Valley Dam Scheme
Cost
£148,000
£472,000
£112,000
£352,000
£460,000
£1,544,000
00.
st over £7.5M. This is
ice of site means that
it does mean that the
e just over 1,000,000
the quarry on site,
nd we would have to
in Builth Wells 25Km
e first Km and £0.45
to quarry twice the
am
oad building
iversion works
queduct
41 Feasibility Study of Irfon Valley Dam Scheme
amount of rock needed and filter the higher quality rock from the quarry on site. Drilling,
blasting and transporting 2,000,000 cubic metres of rock will cost £5,100,000. Filling and
compaction of the dam materials was the second largest cost of the dam at £2,375,000.
Obviously this is a very labour intensive process so a high cost is to be expected even though
the work in question is not priced as skilled labour.
The weir and spillway require a large amount of concrete, and we looked into a few options
to minimise the cost of over 3000 cubic metres of concrete that need to be poured. The
easiest way to get concrete is to have it delivered premixed and ready to pour into the
formwork, however we investigated other avenues to find a cheaper alternative. Using
ratios provided to us, a spreadsheet was generated to cost the large amounts of concrete
we would require. We priced sand, graded rock and cement as being delivered from the
RMC quarry 25km away, and after investigating different ways of getting suitable water (19)
to mix the concrete (including adding agents to reduce the acidity of the river water and
hiring in a purification system) we found that taking water from the mains at an industrial
rate was significantly cheaper at under £1.25 per cubic metre. After including the costs of
crushing, batching and placing the concrete in formwork on site, the total cost of concrete
came to £173,000. This is a saving or approximately £25,000 over having ready mixed
concrete delivered to the site, at which point it would then have to be placed at extra cost
(20).
10.2.2 Aqueduct
Part of the scheme is to construct an aqueduct to pipe water to the storage reservoir for the
new town. We analysed five potential routes trying to find a balance between the length of
the aqueduct and the amount of tunnelling required to prevent the water from travelling
above the line of hydraulic looses over the run of the aqueduct. The route that we decided
upon requires almost no tunnelling and with a 1.5m diameter tube will cost £1,386,000. This
price includes £50,000 for a draw off tower in the reservoir.
10.2.3 Diversion Works
We plan to use a one stage diversion process in order to construct our dam. This will involve
constructing a cofferdam upstream of our eventual dam site with a culvert running through
it. The culvert will contain two 2.5m diameter pipes that will divert water away from the
dam site while construction is in progress. As the elevation change over the dam site is not
that large, we will construct a second cofferdam downstream to prevent water from flowing
back up and inundating the construction site. Both of these cofferdams will eventually form
part of the dam itself.
There is a cost efficiency process to determine the lowest price for the diversion works. As
the height of the cofferdam increases, its cost of construction also goes up however the
culvert doesn’t need to be as large as the cofferdam can handle a larger surge were a large
rainfall event to happen. A height of cofferdam vs. Cost of diversion works graph was
plotted which showed that the ideal height for the cofferdam is 7m with the total diversion
works totalling £450,000.
10.2.4 Road Building
Given that we will be flooding
devised. In an effort to reduce
road begin and end as close as
road. For this reason it was dec
Due to the elevation changes t
the 4Km that the existing road
points so we think this is the
concrete road has been budget
We will also budget for Km of t
site. This road will use the lowe
already budgeted for in the sc
using similar rock that is not o
enough to be used in roads. Ho
in the budget and look at this s
a cost to be incurred.
10.3 Other Costs
Other costs that do not apply w
onsite staff who aren’t doing t
to the Spon book referenced e
costs and consultancy fees am
at £869,000 and 521,000 respe
10.4 Total Costs
Fig. 48 Graph of all project cos
Preconstruction costs
Scheme costs
Other costs
Feasibility Study of Irfon Va
ing a road to create the reservoir, an alternativ
uce the impact on local people, it was decided
e as possible to the beginning and end of the f
decided to build a road over the hill on the wes
es the new road will be approximately 12Km lo
road travels. This is still the shortest way of co
the best solution for the local residents. The
dgeted at £1,038,000.
of temporary roads to transport materials from
lower grade blasted rock that cannot be used as
e scheme costs. It is possible that the other roa
ot of high enough quality to be used in the d
. However we will retain the cost of purchasing
his situation as a potential saving that could be
ply within the two sections covered so far includ
ng the work budgeted for so far and consultanc
d earlier, on site staff should cost approximatel
amount to 3% of capital expenditure. This wou
spectively.
costs
0 5,000,000 10,000,000 15,000,0
sts
sts
sts
42 n Valley Dam Scheme
ative route had to be
ded to make the new
he flooded section of
west side of the dam.
m long as opposed to
f connecting the two
The cost of a 12Km
rom the quarry to the
d as rock fill and so is
r roads could be built
e dam yet still good
ing higher grade rock
be made rather than
clude a provision for
tancy fees. According
ately 5% of all capital
would place the costs
0,000
43 Feasibility Study of Irfon Valley Dam Scheme
The total cost of the project comes to £22,490,000. This includes a 15% contingency to cater
for unforeseen circumstances that may arise during the construction process. In addition to
this figure, the continual running costs of the site should be considered when assessing the
long term viability of the scheme.
A full breakdown of costing can be found in Appendix 7.
11.0 Conclusion
At the beginning of the project we identified a number of objectives for the successful
completion of the feasibility study about the construction of a reservoir in the Irfon Valley.
These were:-
1. Find a suitable location for a direct supply reservoir in the Irfon valley to provide the
town of Hywelfynydd with water.
2. Determine whether this location is suitable using hydrological analysis.
3. Determine whether the local geology can support a dam in this location.
4. Choose a dam type based on research and valley profile.
5. Complete a scheme design for the proposed works
6. Carry out an environmental impact assessment.
7. Produce initial costing for the scheme and assess cost effectiveness of the options
provided.
The location we have chosen for the dam site is suitable for a number of reasons. The first is
that, as outlined earlier in the report, a dam in this site displaces the fewest number of
houses and requires the relocation of the second least length of services. This was an
important consideration for us as we identified early on the project that large numbers of
displaced population and services would add a substantial amount of compensation to the
project. In addition, we avoid flooding the largest SSSI in the area and avoid flooding any
sites that would cause serious problems of relocation, such as a graveyard.
Through the use of hydrological analysis we have shown that our dam site is more than
capable of servicing the demand placed upon it. This is evidenced by an average flow into
the dam of 1.93 Cumecs compared to the demand of 1.013 Cumecs. Additionally, through
the use of linear regression we have been able to design our dam for a one hundred year
return period or 1% chance drought. Our storage required to weather such a drought is
14.81Mm3. This storage also does not result in a flooded area that displaces large numbers
of people or services.
Through the studying of the various geological maps, assessment of the sites during our visit
and through looking at previous dam projects we have determined that there is readily
available material (mudstone from the valley sides for the embankment and glacial till for
the core) which we can use to construct the different parts of our dam. We know that the
foundations will support the pressure exerted by our dam and that it is not resting on any
44 Feasibility Study of Irfon Valley Dam Scheme
major fault lines. However, before a scheme could go much further a site investigation will
need to be carried out to determine the exact nature of the geology and the site conditions.
We chose the rockfill embankment dam type as the valley profile was wide and flat. We felt
that this type of dam suited our site geology, site topography and the local availability for
materials as we were unsure of the viability of reopening the quarry to provide concrete
aggregate. This has been discussed in more detail in the geology section (6.6) of the report.
Our scheme design has taken into consideration the quality of local materials through the
widening of the core and has used wedge analysis to check the slope stability. The width of
138m includes allowances for a wider core to account for lower quality clay. The final height
of the dam of 40m, including a 1m freeboard, is sufficient to account for the lifetime and
purpose of the dam. However there is scope for additional height to be added at a later
date to allow for changing weather conditions as the foundations are only currently being
used at around 20% capacity. In addition, the spillway has been designed to cope with a
10,000 year flood event, which is in accordance with the design guidance on flood
protection.
Our environmental impact assessment has made us aware of the diverse local wildlife, the
industrial importance of the valley, the number of locations of significant historical
importance and the local residents who will be affected by our scheme. We have designed
our scheme to minimise the impact on these stakeholders and have made a number of
recommendations to mitigate any further affect our scheme will have. The most notable is
our focus on minimising the displacement caused by the flooded area of our dam and
reducing the additional works that need to be completed. We have done this by making use
of currently available facilities such as the logging track through the woods, which we plan
to widen and use as our road diversion. This is further evidence of the ethos that we have
tried to apply throughout the project of presenting a holistic solution that is acceptable to
all stakeholders.
The total scheme cost of £22,490,000 and the dam construction cost of £7,550,000 are the
result of a detailed cost analysis where we have assessed each part of the dams design and
construction for cost effectiveness and fitness for purpose. An example of this process is the
choice of aqueduct route. Through detailed analysis of the surrounding terrain and careful
avoidance of tunnelling and pumping we have been able to choose a route that gives a final
aqueduct cost of £1.386 Million, a greater than 50% reduction in cost over all other routes
considered.
The project has succeeded in fulfilling the objectives that were set out at the beginning of
the project. The completion of these objectives is evidence of the comprehensive nature of
the solution and for the feasibility of the project as a whole. The conclusion of this report is
that a water supply scheme at the aforementioned location is feasible and should be
considered for construction.
45 Feasibility Study of Irfon Valley Dam Scheme
Appendix 1 - Hydrological analysis
1. Water demand = safety x (abstraction rate +Compensation flow)
Abstraction Rate = population x water consumption
2. Thiessen Polygon Method
Using the rain data from the different stations a weighted average was created for each year. This was
created using the relative areas found by constructing a thiessen polygon. This was then plotted against
the flow data for those months and from that we could work out the flow data for the months where it
was missing using the rainfall during the time period for the flow that was missing. The linear Regression
graph is shown below.
3. Flowrateat damsitei = Cilmery flowrate XDamsitei catchment area
Cilmerycatchment area
where i = 1,2,3 Cilmery catchment =240km2
y = 0.0779x + 2.8651
0
5
10
15
20
25
0 50 100 150 200 250 300
Flo
w D
ata
m3/
s
Rain Fall mm
46 Feasibility Study of Irfon Valley Dam Scheme
4.
5. Unit hydrograph
Tp(0)=283.0*S1085-0.33
*(1+URBAN)-2.2
*SAAR-0.54
*MSL0.23
(hours)
S1085 is the stream slope between 10% and 85% of its length in m/km
URBAN is the proportion of built up area in the catchment (=0 here)
SAAR is the standard average annual rainfall
MSL is the main stream length in km
(needs diagram to show the representation on hydrograph.)
6. D = (1+SAAR/1000).Tp then rounded to the nearest odd number.
7. r = M5-60/M5-2 day Rainfalls, X = M5-D/M5-2 day percentage where D is design storm duration.
8. PRRural = SPR + DPRCWI +DPRRAIN
SPR = 10*S1 +30*S2 + 37*S3 + 47*S4 + 53*S5
DPRCWI = 0.25*(CWI-125)
DPRRAIN = 0.45*(P-40)0.7
for p greater than 40 for p less than 40 =0
P= ARFx(MT-D hour) mm
9.
y = 6.0098x-0.767
y = 20.761x-0.805
y = 39.461x-0.748
y = 61.568x-0.656
y = 84.563x-0.585
y = 110.09x-0.51
y = 143.66x-0.429
y = 176.92x-0.35
y = 211.36x-0.292y = 259.82x-0.272
y = 312x-0.269
0.1
1
10
100
1000
1 10 100 1000
Ru
no
ff m
etr
es
Cu
be
d 1
0^
6
Return Period
S1 0
S2 0.05
S3 0
S4 0
S5 0.95
CWI 128
hour 1 2 3 4 5 6 7 8 9
47 Feasibility Study of Irfon Valley
10.
11. S2 − S1 = (I2 − I1
2)δt − (
O2 −O1
2)δ
12.
Measure the discharge using the fl
Measure the head over the top of
Assess whether pressure over the
pressure is negative bubbles will be
Measure the depth of flow over th
Create a hydraulic jump by lowerin
(after jump).
Calculate theoretical values of y1 a
13. V = R2
3 S0 / n and yc
3
2 =Q
b. g the
14.
φ is usually taken as 0.9
depth before the jump
the sequent d
the wall at the
percent 0.75 1.25
V1 = φ 2gh1
y1 =Qmax
bV1
y2 =y1
2( 1+ 8F 2r1 −1)
Q = 1.705bh1.5
∆z = y2 − h
lb = 5y2
ley Dam Scheme
δt S2 − S1 = (I2 − I1
2)δt − (
O2 −O1
2)δt where dt
he flow gauge attached to the pipe in the inlet ta
of the spillway.
the top of the weir is positive or negative using a
ill be observed.)
r the vee weir notch on the outlet tank.
ering the sluice gate. Measure water depths y1
y1 and y2 using the formula �2 � 0.5�1 �1
he second equation utilising the fact that fr =1
s 0.9 to 0.95
nt depth
t the end is treated as a weir hence we ignore en
3 11 68 11
e dt is the time interval.
et tank.
ing a vertical tube. (If
s y1 (before jump) and y2
8� � � 1� .
=1 for critical flow.
e energy loss.
3 1.25 0.75
48 Feasibility Study of Irfon Valley Dam Scheme
Height of wall
Appendix 2 - Geological analysis
Fig. ... Geological map showing the locations of glacial till deposits and the Kilsby tuff quarry
Appendix 3 – Dam Design
Regular storage in dam = 36m
Maximum flood water level= 39m (from flood routing#0
Freeboard calculations
� � �� � �
Where:
d- freeboard
e- water level rise near dam due to wind= 0.3m
ha- wave climbing height= 0.2m
A- Extra height = 0.5m
(Values taken from water resources project hand out)
Glaci
al Till
Quarry
49 Feasibility Study of Irfon Valley Dam Scheme
Total freeboard= 1m
Clay core dimensions
Width of clay core at base= 20% of flood water level (1)
40*0.2 = 8m
Width of clay core at crest should not be less than 3m
Appendix 4 - Site 2 Valley Profile
Appendix 5 – Flooded area Diagrams
Site 1 Flooded Area Site 2 Flooded area
0
20
40
60
80
100
120
0 100 200 300 400 500 600
Va
lle
y H
eig
ht
Valley Width
Site 2 Valley Profile
50 Feasibility Study of Irfon Valley Dam Scheme
Site 3 Flooded Area Site 4 Flooded area
Appendix 6 – EA Checklist
Welfare faci l i tiesWaste from leftovers , food
packaging and consumables .5 6 30
Promote recycl ing to reduce food
packagi ng. Compost heap for disposable
Changes in des ignIncrease i n environmental
impact from construction.3 3 9
Ensure des igns have long been in place
before construction commences .
Discovery of
undefined materials
Gas/as bestos/ancient
archaeology.3 7 21
Ensure a s i te investigation is carried out
and extra time is al located in the project
Noise generationNoise dis ruption to loca l
communities /fauna.5 5 25 Avoid working unsociable hours .
Dust generationAir pol lution for loca l
res idents /fauna.5 6 30 Empl oy dust suppress i on techni ques .
Vi bration generationVibrati ons to surrounding
environment.5 6 30
Spread the vibration works throughout the
va l ley. Avoid works near s ens i ti ve areas .
Discharge to water
(river/groundwater)
Potential contamination of clean
water.4 6 24
The flow of water should be l imited
through the cons truction s i te due to use of
Use of l ightingLight pol lution to s urroundi ng
community/environment.2 3 6 Avoid working unsociable hours .
Fuel use for pl ant
operation.
Air pol lution, us e of non-
renewabl e resources .5 6 30 Train s ta ff to operate plant efficiently.
Adverse weather
conditions .
Eros ion of materia l s tockpi les
caus ing contamination.5 6 30
Poor weather conditions are expected in
Wales , so avoid s tockpi le of materials
Waste generationPotential pol lution al ong
transport routes .3 5 15
Provide tra ining on waste management,
promote recycl ing.
Habi tat damageLos s of ecosystems from s i te
work, impact to loca l flora.5 6 30
Thorough s i te investigation and ensure
ti me for habitat relocation.
Ris
k L
ev
el
(Lo
w,
Me
diu
m,
Hig
h)
Preventative MeasuresDetails of Hazard Risk/Impact
Lik
eli
ho
od
of
Occ
urr
en
ce (
1-5
)
Se
ve
rity
of
Ris
k/I
mp
act
(1
-10
)
Ris
k S
core
(1
-50
)
Environmental Impact
No
ise
& v
ibra
tio
n
Vis
ua
l im
pa
ct/l
igh
t e
mis
sio
ns
Wa
ste
(so
lid
&li
qu
id)
Em
issi
on
s to
wa
ter
Em
issi
on
s to
air
(d
ust
/od
ou
r)
Em
issi
on
s to
la
nd
Flo
ra a
nd
fa
un
a
No
ise
& v
ibra
tio
n
Re
sou
rce
use
(e
ne
rgy
/ma
teri
als
)
51 Feasibility Study of Irfon Valley Dam Scheme
Appendix 7 - Costings
Preconstruction costs
Land/Property
Type Quantity Cost Total
Farmland 197.7 acres £1,384,000
Forest 12.4 acres £38,000
Houses 13 x 1.15 £4,196,000
Total cost of acquisition £5,618,000
Service
relocation
Service Cost/m Metres Total
Electric cable 37 4000 £148,000
Sewerge/Drainage 118 4000 £472,000
Telephone likes 28 4000 £112,000
Water mains 88 4000 £352,000
Gas mains 115 4000 £460,000
Site Clearing
Total cost of service relocation £1,544,000
Type of land Cost/acre Acres Total
Farmland 197.7 1010 200000
Forest 12.4 3112 39000
Total cost of site clearing £239,000
Total preconstruction costs £7,401,000
Excavation, Transportation & Placement
Material Volume Transport cost Placement Material cost
Sand Filter 8200 m3 £0 5000 £40,353
Topsoil 13800 m3 £0 £33,948
Stiff Clay 104000 m3 £426,400 107000 £312,000
Broken Rock 27600 m3 £0 1340000 £193,200
Drilling & Blasting 2000000 m3 £1,900,000 £3,200,000
Total cost of Excavation, Transportation & Placement £7,558,000
Road building
Type of Road length of road Cost
Permanent 12 Km £1,050,768
Service 2 Km £145,128
Total cost of road building £1,196,000
52 Feasibility Study of Irfon Valley Dam Scheme
Concrete
Concrete required 3400 m3 distance from pit Delivery cost Material cost
Materials
Sand 1478.105 m3 26 £5,055 £13,820
Graded Rock 2458.625 m3 26 £7,081 £20,898
Cement 374.000 m3 26 £1,077 £26,341
Water 448.800 m3 £555
Production
Crushing 2458.625 m3 £5,901
Batching 3400.000 m3 £12,189
Placing 3400.000 m3 £81,260
Total cost of concrete £175,000
Appendix 8 – Head Loss in Pipe
D = 1 m 0.75 m 1.5 m 0.85 m
V = 0.878535 Q/A 1.561841 Q/A 0.39046 Q/A 1.215966 Q/A
Q = 0.69 m^3/s
A = 0.785398 m^2 0.441786 m^3 1.767146 m^4 0.56745 m^5
hf = 11.40821 48.07409 1.502315 25.71123
Available
L = 31775.37 m 7540.444 m 241294.2 m 14098.9 m
= 31.77537 km 7.540444 km 241.2942 km 14.0989 km
Appendix 9 – Diversion Works
£0.00
£500,000.00
£1,000,000.00
£1,500,000.00
£2,000,000.00
£2,500,000.00
£3,000,000.00
0 5 10 15 20 25 30
Height of Cofferdam
Cost(culvert) v Hc
Cost(coffdam) v Hc
Total Cost v Hc
53 Feasibility Study of Irfon Valley Dam Scheme
Appendix 10 – Technical Drawings
54 Feasibility Study of Irfon Valley Dam Scheme
55 Feasibility Study of Irfon Valley
Appendix 11 – Coulomb Wedge A
X θ1 θ2 δ
60 70 10 20
60 60 20 20
55 70 10 20
55 60 10 20
50 60 20 20
40 45 20 20
60 55 25 20
25 50 5 20
60 50 5 20
60 40 20 20
50 40 20 20
50 50 20 20
ley Dam Scheme
ge Analysis
FOS using β= 30
0
20 3.32
20 1.86
20 4.27
20 2.64
20 2.89
20 2.17
20 1.76
20 3.61
20 2.65
20 1.7
20 1.68
20 1.9
56
F
ea
sib
ilit
y S
tud
y o
f Ir
fon
Va
lle
y D
am
Sch
em
e
Ap
pe
nd
ix 1
2 -
Co
ns
tru
cti
on
Se
qu
en
ce
En
vir
on
me
nta
l a
ss
es
sm
en
t
Lan
d a
cq
uis
itio
n
Sit
e c
lea
ran
ce
Serv
ice
relo
ca
tio
n
Ro
ad
bu
ild
ing
Div
ers
ion
wo
rks
Fo
un
da
tio
n w
ork
s
Em
ba
nk
me
nt
co
ns
tru
cti
on
Sp
illw
ay
co
ns
tru
cti
on
Dra
w o
ff t
ow
er
& A
qu
ed
uc
t c
on
str
uc
tio
n
Res
erv
oir
fil
lin
g
Co
nti
ng
en
cy
Ye
ar
1Y
ea
r 2
Ye
ar
3Ye
ar
4
57 Feasibility Study of Irfon Valley Dam Scheme
References
1. University of Bristol, Geology map.
2. Nash, D, University of Bristol 2010
3. Bureau of Reclamation, "The Failiure of Teton Dam".
http://www.usbr.gov/pn/about/Teton.html. Retrieved 2010-10-25
4. Hawkins, A.B, ‘Weathering and rocks’ lectures notes 2008
5. Alberto Foyo, ‘A proposal for a Secondary Permeablity index obtained from water pressure tests
in dam foundations, s.I : Engineering Geology 77, 2005
6. Craig, R.F, ‘Craig’s soil mechanics’ 7th edition, page 364
7. ‘Soil Compaction’ notes, Martin Fahey, University of western Australia (slide 25)
8. Wilson, G.Ward et al, ‘The Influence of Shear Strength Properties on the Stability of Rock Piles’
University of Bristish Columbia geoinfo.nmt.edu/staff/mclemore/documents/gsa_wilson.doc .
Retrieved 2010-10-25
9. University of Bristol, Water resources project handbook 2010
10. Fahey, M, ‘Embankment dams: some geotechnical aspects of embankment dams’ lecture notes.
University of Western Australia. 2009
11. ‘Foundation Stava 1985’ http://www.stava1985.it/ Retrieved 2010-10-16
12. Fahey, M, ‘Embankment dams: some geotechnical aspects of embankment dams’ lecture notes.
University of Western Australia. 2009
13. The landscape institute, ‘Environmental impact assessment: a guide to procedures, Part 1: 2000
Guidelines for Landscape and Visual Assessment Guidelines,’ 1995, ‘http://www.environment-
agency.gov.uk/static/documents/Research/eia.pdf
Retrieved 2010-10-25
14. Picture of Globeflower, wikiImages – BernrdH, Retreived 2010-10-21
15. Royal commission on the ancient and historical monuments of Wales, ‘Cwm Irfon
StandingStones’
http://www.coflein.gov.uk/en/site/102/details/CWM+IRFON+STANDING+STONES/
Retrieved 2010-10-27
16. Michigan Water Research Centre, Accessed online at:
http://mwrc.bio.cmich.edu/glossary.htm, Retrieved 2010-10-27
17. Picture of Otter, http://www.mtuk.org/, Retrieived 2010-10-21
18. Balfour Beatty Civil Engineering Ltd., http://www.bbcel.co.uk/zeroharm, Retrieved 2010-10-
25
19. Charges for Industrial Water Use, Welsh Water,
http://www.dwrcymru.co.uk/English/library/publications/Scheme%20Of%20Charges/Englis
h.pdf, Retrieved 2010-10-21
20. Cemex Ltd., Local Concrete Supplier, www.cemex.co.uk, Retrieved 2010-10-21
58 Feasibility Study of Irfon Valley Dam Scheme
Bibliography
1. Blyth, FGH& de Freitas, MH, 1984, ‘A geology for engineers’ (7th edition)
2. Chadwick, A et al,1998, ‘Hydraulics in Civil and Environmental Engineering’ (3rd edition)
3. DTLR/ National assembly for Wales, 2000. Environmental Impact Assessment: a guide to the procedures.
4. http://maps.google.co.uk
5. Maps of SSSI sites, Countryside Council for Wales. Accessed online at:
• Country countryside for Wales interactive maps, ‘Cae-pwll-y-bo’
http://www.ccw.gov.uk/interactive-maps/official-maps-search/official-
maps.aspx?sitetype=SSSI&sitecode=0272, Retrieved 2010-10-27
• Country countryside for Wales interactive maps, ‘Afon Irfon’
http://www.ccw.gov.uk/interactive-maps/official-maps-search/official-
maps.aspx?sitetype=SSSI&sitecode=0777, Retrieved 2010-10-27
6. Shaw, E. 1993 ‘Hydrology in practice’ (3rd edition)
7. http://www.thetrainline.com/buytickets/
8. United States Department of the Interior, Bureau of Reclamation, 1987. ‘Design of Small Dams’ (3rd edition).
Accessed online at www.usbr.gov/pmts/hydraulics_lab/pubs/manuals/SmallDams.pdf
59 Feasibility Study of Irfon Valley Dam Scheme
Previously Submitted Documents
Spillway design Lab report
1. Objective
To calculate the Cd value of the spillway profile in the lab at the design discharge. By obtaining the Cd
value of the spillway profile in the Lab, we can then scale up the model in order to design the spillway
for our chosen dam site. We need to obtain the Cd value at Hd, that is the total head over the spillway
which correlates to the ‘design discharge’, which we are taking to be the ‘maximum probable flood’.
This is necessary to make sure that the dam can cope with the maximum probable flood, and will not
cause damage downstream.
2. Procedure
� Measure the discharge using the flow gauge attached to the pipe in the inlet tank.
� Measure the head over the top of the spillway.
� Assess whether pressure over the top of the weir is positive or negative using a vertical tube. (If
pressure is negative bubbles will be observed.)
� Measure the depth of flow over the vee weir notch on the outlet tank.
� Create a hydraulic jump by lowering the sluice gate. Measure water depths y1 (before jump) and y2
(after jump).
� Calculate theoretical values of y1 and y2 using the formula �2 � 0.5�1 ��1 8� � � 1�
3. Experimental apparatus
Fig.1. Plan view of experimental apparatus
Fig.2 Cross section of experimental apparatus
Inlet
tank Outlet
tank
Stillin
g
statio
Spillway Stilling
60 Feasibility Study of Irfon Valley Dam Scheme
4, Results
Qin
(m3/h) Qin(m3/s)
H(spillway)
(m) H(weir)(m) Q(vee)(m3/s) Pressure Cd
8 0.002222222 0.026 0.0753 0.002131611 p 0.554872
10 0.002777778 0.031 0.0869 0.003049791 p 0.532747
12 0.003333333 0.034 0.0932 0.003632958 p 0.556578
15 0.004166667 0.0375 0.1013 0.004474497 p 0.600631
17 0.004722222 0.04 0.1061 0.00502353 p 0.617906
19 0.005277778 0.0425 0.1108 0.005598478 p 0.63057
20 0.005555556 0.0445 0.1137 0.005972027 p 0.619517
22 0.006111111 0.046 0.118 0.00655278 n 0.648409
25 0.006944444 0.051 0.1243 0.007462741 n 0.631172
Fig. 3 Table showing experimental results
4. Analysis of results
It can be seen that there is an approximate 5% difference between the flow measurement from the
inlet gauge and that calculated from the head over the vee weir notch. This can be attributed to a
number of factors.
1. The fact that the equation used to calculate Qvee is an empirical relationship and is therefore not always
accurate.
2. The input gauge was quite high and there may have been some human error in reading the level
especially as it was not always at eye height.
3. Frictional losses along the spillway.
The correlation between the coefficient of discharge (Cd) and the head over the spillway (H) can be
seen below:
Fig.2. Graph to show coefficient of discharge against design head (m)
From the graph, we can see that the coefficient of discharge needed to design the spillway, Cd =
0.625, with a corresponding design head of Hd= 0.044m. There was one anomalous data point on the
0
0.01
0.02
0.03
0.04
0.05
0.06
0.45 0.5 0.55 0.6 0.65 0.7
He
ad
ov
er
spil
lwa
y (
m)
Coefficient of discharge
H(spillway) (m)
Linear (H(spillway) (m))
Positive weir pressure
Negative weir pressure
Anomalous
data point
61 Feasibility Study of Irfon Valley Dam Scheme
graph, shown above. This corresponded to the flow at 19m3/hr, and although shown on the graph has
been discounted in analysis.
Hydraulic Jump
Y1 Y2 V Fr 8Fr2
y2
(Calc)
0.0145 0.0675 1.57 4.163 138.7 0.07568
From the result shown above it can be seen that there is a difference between the measured height of
the water after the jump and the calculated height of water after the jump. As the Fr number >1, the
flow was supercritical and it was correct to use the formula as outlined in the procedure above.
However the difference could be predominately due to the fact that the edge of the sluice gate was not
sealed properly, creating turbulence, and making it hard to take accurate measurements.
5. Summary
The Cd and Hd values to be used are 0.625 and 0.044m respectively. These will enable us to scale up
the model in order to design the spillway in conjunction with the data obtained from the flood routing.
Site Report Group 19
A site visit was conducted in the Irfon Valley to assess the viability of damming the river. Four sites were identified prior to
the visit and investigated to determine their suitability.
General Geology
The main rock type found in the valley is a mudstone from the Silurian strata. The strata tend to dip to the northwest, with
a dip angle of between 20 – 40 degrees. There are large amounts of alluvium at the base of the valley which will have to be
excavated. The whole valley has been carved out by glacial activity which has several geological implications, such as the
variedly sized, angular rocks. As a result, they have been less well sorted due to their transportation in ice. This has
resulted in an anisotropic soil.
General Environmental Concerns
Special consideration for rare populations of animals such as the otter, atlantic salmon and various other fish must be
taken. In addition the river itself is an SSSI due mesotrophic and oligo-mesotrophic river types which include communities
of submerged water plants. In addition the high acidity level caused by the high proportion of coniferous forestry is a
concern, with our dam looking to avoid destabilising the PH any further.
General Material
The quarry site is in the region of the LLANWRTYD VOLCANIC FORMATION, and is close to all of the potential dam sites.
At the bottom of the quarry there is a slatey mudstone, which is probably indicative of the bedrock in much of the valley.
Whilst it is not soft, it is not hard enough for aggregate but may be good for rockfill. Hard volcanic rock (Kilsbury tuff) can
be used for aggregate. However it may be very difficult to negotiate access to the quarry as there are many different
agencies involved, in addition it may be more economical to source stone from the mine in Builth Wells. If we were to
source our stone/rock from the quarry an environmental impact assessment would have to be produced. If building an
earth or rock filled dam, we will need fine grained, impermeable material for the clay core. Given the local geology of the
area, we have identified 3 main locations of glacial till which although not ideal, would be suitable for the clay core. The
largest of these is the borrow pit, which was used in the constrution of the Lynn Brianne dam located in the neighbouring
valley, although there are two smaller deposits located closer to our dam sites.
Site 1
Topography
This site has a large flat area at the base of the
valley approximately 80m across. The river runs
through the centre of the site and is lined by trees.
62 Feasibility Study of Irfon Valley Dam Scheme
Approximately 20m from the proposed dam site there is a tributary to the main river. There is a church and graveyard
downstream of the dam, which should not be affected by the dam structure, consideration needs to be taken for any
vibrations caused during construction. In addition there is also a bridge downstream. There is also a phone line and two
minor roads that would need to be relocated. There were some minor slips present indicated by exposed rock on the
eastern face. Site 1 also lies on a fault line, which although dormant may be an indicator of disturbed rock and subterrain
features we may not be aware of. This will require further and more detailed site exploration which will be costly.
Environmental Concerns
This site is in close proximity to a church and graveyard. While we
need to pay close attention to it, as our dam is sufficiently upstream
we do not need to relocate. The site is used for grazing land for farm
animals, which would have to be relocated. There could be an issue
of noise for the surrounding residents. There are a number of
structures that will need to be moved due to the flooded area.
Materials
Due to the profile of the valley at this point and its proximity to the
quarry the site lends itself to a rockfill or earthfill dam. We would
procure the clay core from the borrow pit, however as it is a rocky clay we would need to provide a wider base for the
core. As this is some distance we need to consider the transportation of the material.
Dam Construction
Due to this sites locality to the base of the valley it has good access for large plant meaning a rockfill or earthfill dam could
be constructed quickly and efficiently, with minimum disruption to the surrounding roads and logging industry. The
spillway will be constructed on the eastern side to avoid relocating the church and graveyard. We will need to source
aggregate for the constuction of the spillway, however more research will need to be undertaken as to the most ideal
loation for extraction. The western side is heavily forested and as such will need to be cleared in order for us to construct
the dam.
Site 2
Topography
The site has a large flat area at the base of the valley
approximately 50m across. There is no evidence of slips on
the surrounding hill sides, however the river has previously
taken a different route as shown by a cutting scar in the
bank. The presence of wetland grasses indicates the large
amount of water flowing into the area making a suitable dam
location. There are houses downstream which although
unaffected will need to considered during construction.
In addition there are two minor roads, a power line and a phone line that will all need to be moved to allow the
construction of the dam.
Environmental Concerns
This site is used as grazing land for farm animals, which will need to be
relocated. In addition we will also need to clear some of the forest.
There will be a substantial amount of disruption to the residents of the
valley who do not need to move and we to be aware of possible
vibration damage to people’s properties. There will also need to be a
number of structures moved due the flooded area.
Materials
63 Feasibility Study of Irfon Valley Dam Scheme
The profile of the valley at this point again lends itself to a rockfill or earthfill dam. This site is situated on mudrock with
some silt laminations but should be a good source of material for a rockfill dam. The clay core can be sourced from the
borrow pit, or one of the other glacial till locations.
Dam Construction
Site 2 has good access to the dam location, the valley is still relatively wide at this point and we do not envisage any
problems in access for construction equipment, as indicated by the number of logging vehicles passing through the area.
We would place the spillway on the Eastern bank of the river, so as to avoid disturbance to the houses situated
downstream. Aggregate for the spillway will be sourced from locations as discussed for site 1.
Site 3
Topography
The site is situated in a steep sided valley, with the base
measuring around 20 – 30m across. The valley sides are
densely vegetated with many large trees and shrubbery. The
river channel runs through the centre of the valley with a
minor road located to the west. There is also a telephone line
which follows the road crossing at various points which
would also need to be relocated. There is a property about
100m downstream which would need consideration
during the dam construction phase. There is a picnic area and
Forestry Commission area just upstream which would be
affected by the flooded area and need consideration.
Environmental Concerns
There is an abundance of large trees in the area, some of which are used for the
logging industry. These could be removed and sold in preparation for the
construction of the dam. Several upstream buildings, including the Abergwesyn
community, will be affected by the dam storage area. The hillsides are
currently uninhabited by livestock, however there is Site of Special Scientific
Interest located 200m downstream from the proposed dam site.
Materials
A concrete dam can be considered due to the narrow nature of the valley. The stable
hillsides would provide established abutments for the concrete arch. Aggregate can be sourced from the local quarry
reducing transportation of materials.
Dam Construction
The dam would preferably be made of concrete due to the valley’s topography. The spillway would be incorporated within
the dam structure saving space. The riverside road would be useful for delivering resources to the site. Deforestation
would be needed in order to facilitate the structure, although this may weaken the immediate strata.
Site 4
Topography
Site 4 has a much narrower base and steeper sides
than previously seen at sites 1 and 2. The Eastern bank
is much steeper than the west, and rocky outcrops can
be seen. These have been subject to much weathering
and as such may be unstable. The topography of
this site lends itself to a rockfill or earthfill dam.
Environmental concerns
The flood area associated with site 4 extends to
Abergwesyn which would involve not only the relocation of many
residents but also the road junction, and hence three roads. As
this is considerably more than at the other sites it may not be the
64 Feasibility Study of Irfon Valley Dam Scheme
most viable option. There is also an area used for forestry, and a Site of Specific Scientific Interest that will be affected by
the dam as well as a power line, a telephone line and a bridal path.
Materials
Site 4 is located in close proximity to one of the smaller glacial till deposits, which would be ideal for the core of a rockfill
or earthfill dam. Once again it is mainly situated on mudstone, which would provide adequate foundation for a rockfill or
earthfill dam.
Dam Construction
At Site 4, the valley starts to narrow and access for large construction materials may become more difficult. There is also a
large area that would need to be deforested, and may lead to weakening of the surrounding soil. If we were to construct
an earthfill or rockfilled dam, we would place the spillway on the eastern bank of the river due to the valley topography.
Figure 1. Table
comparing displaced
infrastructure for
flooded areas.
Conclusion
Following our investigation of the valley we have decided that site 2 will be most
suitable site to dam the river. We have made this decision based on a number of
criteria such as the effect on infrastructure, access and proximity to
construction materials. Site 2 will require the relocation of the least number of
structures as it has the smallest flooded footprint. This has the added benefit of
requiring less land to be purchased. In addition it has good site access, which is
necessary for the construction of a rock fill dam to allow plant on site. It is also
close to the quarry and sources of glacial till for the core. Finally this site
requires much less deforestation lowering the environmental impact
further.
Figure 2. Flooded area map for site 2
AM
PM
AM
PM
AM
PM
AM
PM
AM
PM
AM
PM
AM
PM
AM
PM
AM
PM
AM
PM
AM
PM
AM
PM
AM
PM
AM
PM
All
Meeting
All
Site Choice
JE, SW
Analyse Sites
All
Identify Potential Sites
GC, FW, DW
Area Geology
GC, FW, DW
Site Geology
GC, FW, DW
Environmental Status & Regs
All
Areas To Be Flooded & Implications
GC, FW, DW
Impact Assessment For Final Site
All
Site R
esearch
General Site Research
All
Prepare For Site V
isit
TBC
Labs
All
Location Of Spillway
FW
Spillway Design
All
Flood Routing
GC
Diversion
All
Aqueduct To Service Reservoir
All
Position Of Draw-off Tower
FW
Cost Of Spillway
All
Finalise Dam Type
GC, DW
Produce Site Plans
GC, FW, DW
Design Dam and Foundations
GC, FW, DW
Calculate Volume Of Materials
GC, FW, DW
Dam Costing
All
Sources/Cost/Transport of Materials
JE, SW
Calculate Product Costs
All
Identify Health And Safety
All
Construction Sequence
All
Individual Writing
All
Compiling
JEForm
at/Proof Reading
GC, FW, DW
Gantt Chart
5pm
JE, SW
Site Data
1pm
All
Site Visit Report & Final Site Choice
5pm
JE, SW
Routed Flood Hydrograph
5pm
FW
Spillway Dimensions
5pm
TBC
Lab Report On Spillway Design
5pm
GC, FW, DW
Spillway Section And Plan
1pm
GC, DW
Plans Of Dam Site
1pm
All
Summary Sheet
5pm
All
Presentation
All
Individual Report
5pm
All
Group Report
5pm
Environmenta
l Analysis
Geological Analysis
Hydrological Analysis
Report
Deliverables (hand in
date
s)
Unknown
Around Lectures
S i t e V i s i t
Research
Responsibility
Design A
nd Construction
Management
Structu
ral and
Foundation D
esign
Hydraulic D
esign
15-Oct
14-Oct
13-Oct
12-Oct
11-Oct
25-Oct
22-Oct
21-Oct
Thr
28-Oct
27-Oct
26-Oct
Mon
Tue
Wed
Thr
Fri
Mon
20-Oct
19-Oct
18-Oct
Tue
Wed
Mon
Tue
Wed
Thr
Fri
5pm
5pm