drainage structures' sizing guidelines

TABLE OF CONTENTS

LIST OF DEFINITIONS .......................................................................................................................... A

1.0 INTRODUCTION ..........................................................................................................................1

2.0 RAINFALL ZONES .......................................................................................................................2

3.0 INTENSITY - DURATION - FREQUENCY CURVES ................................................................6

4.0 DRAINAGE DESIGN PARAMETERS .........................................................................................7

5.0 DESIGN FLOOD DETERMINATION FOR BRIDGES, BOX CULVERTS, ETC USING

THE RATIONAL METHOD ....................................................................................................................8

5.1 GUIDELINES FOR DRAINAGE STRUCTURE PROVISION .................................................... 11

6.0 EROSION CONTROL MEASURES ........................................................................................... 18

6.1 PROTECTION MEASURES ...................................................................................................... 18

7.0 CONSTRUCTION GUIDELINES FOR DRAINAGE STRUCTURES ...................................... 20

7.1 REQUIREMENTS OF VARIOUS DRAINAGE STRUCTURES ................................................. 20 7.1.1 Mitre Drains ....................................................................................................................... 20 7.1.2 Catch-water Drains ............................................................................................................. 21 7.1.3 Scour Checks ...................................................................................................................... 21 7.1.4 Grass Planting .................................................................................................................... 22 7.1.5 Turfing ................................................................................................................................ 22 7.1.6 Lined Side Drains................................................................................................................ 22 7.1.7 Drainage for Roads with a 'Sunken' Profile.......................................................................... 23

8.0 COST EFFECTIVE DRAINAGE DESIGN ................................................................................ 25

A

List of Definitions

Term Definition

Catchment area Total area contributing runoff to the inlet of drainage structure. This area is

obtained from a topographical map by connecting the high and low points on the

map to delineate the area contributing runoff to the inlet of the drainage structure

being designed.

Runoff Coefficient (C) This is a factor that is applied in the flood discharge equation which is an

integrated value representing many factors that influence the runoff relationship

i.e. topography, soil permeability, vegetation cover and land usage.

Return Period Refers to the time interval during which a given rainfall depth/intensity is likely

to be equaled or exceeded once.

Rainfall Intensity Refers to the depth of rainfall over a given period of time.

Intensity-Duration-

Frequency (IDF)

relationships

This is a representation of frequency data for storms of various durations. The

Return Period usually denotes this frequency.

Areal Reduction Factor

(ARF)

This is a factor that is multiplied by a point rainfall (as derived from the IDF

curves) to represent or distribute it over the catchment area.

Mainstream The major stream collecting runoff in the catchment leading to inlet of drainage

structure.

Catchment Mainstream

Slope (S)

This is the gradient (%) of the Mainstream measured by the ratio: level difference

between the highest and lowest point divided by the mainstream length.

Time of Concentration

(Tc)

The time of concentration of a catchment is the time it takes for water from the

hydraulically most remote portion of the catchment to reach the point where flow

is being estimate i.e. drainage structure inlet. For small catchments (A < 10 Km2),

Tc = 10min while for larger catchments (A > 10 Km2), Tc is calculated using the

widely accepted Kirpich formula.

B

Design Storm This is the Maximum Storm depth/intensity that is likely to be equaled or

exceeded once or a little more (rarely) during the design period.

Design Discharge This is the Maximum Discharge that is likely to be equaled or exceeded once or a

little more (rarely) during the design period.

Contour Rate Number of ground level contours per kilometer.

1

1.0 INTRODUCTION

Drainage structures and associated works, such as scour protection, account for a

considerable part of the total cost of road works mainly because of the purpose they

serve to protect the investment in roads. To this end, the factors associated with

drainage design must receive due attention.

The design of drainage structures is based on the worst expected flood situation at the

drainage structure's proposed location. The area of land draining to the structure site is

the catchment and the drainage structure is located at the catchment exit.

When rain falls on a drainage catchment, some of the water may be prevented from

reaching the catchment exit, while some may be delayed en route. Other precipitation

losses may also arise from infiltration, evaporation, storage in surface depressions and

interception by vegetation cover. The excess precipitation then travels by the

hydraulically shortest route to the catchment exit.

The determination of the volume of this runoff and the rate at which it arrives at the

catchment exit is the prime objective of this manual. The factors affecting flood peaks

and volumes may be conveniently grouped as those affecting rainfall and runoff, which

are the Area Reduction Factor (ARF) and Runoff Coefficient (C). The principal factor

used in this manual to link rainfall and runoff is the time taken for the catchment to

respond to the rainfall input i.e. Time of Concentration (Tc). The time of concentration

has been adopted as the measure of the catchment response time i.e. time for surface

runoff from the hydraulically most remote part of the catchment area to reach the

catchment exit point under consideration. This remotest point is not necessarily the

most distant point in the drainage area.

The design flow is established by selecting the appropriate combinations of rainfall and

runoff characteristics that can reasonably be expected to occur. This is calculated in

consideration of a selected design return period. The design criterion is usually the

maximum flow carried by the drainage structure with no flooding or limited amount of

flooding to be exceeded on the average of once during the design return period.

2

However, selection of a proper design storm does not preclude the possibility of a

larger storm destroying the drainage structure immediately after it is built since the

selection is based on statistical probabilities.

The accuracy with which flood estimates can be made depends on the amount and

quality of relevant information available. Practical experience under local conditions

and the application of sound judgment are particularly important in determining the

data needed for the design storm intensity.

2.0 RAINFALL ZONES

Because of the variability of rainfall in Uganda, delineation rainfall zones have been

adopted in this study using results of the study for Design of a Regional Minimum rain

gauge network. The method used was based on the Principal Component Analysis

(PCA). The principal components were rotated using the principal of orthogonal

varimax.

The spatial patterns of the dominant principal components were used to classify

Uganda into 14 homogeneous zones as shown in figure 1 below. The zones were

delineated following the analysis of monthly rainfall records at 102 rain gauges for the

period 1940-75. Within the delineated zones, rainfall characteristics are similar.

3

Figure A: Rainfall Zone Map

4

Table 1: Description of the Rainfall Zones

ZONAL AVERAGE RAINFALL, STD AND EVAPORATION ANALYSIS

Zone Districts, 2000 boundaries Annual Rainfall and its zonal

variability

Main rainy seasons Main dry seasons Evaporation verses rainfall

NORTHEASTERN TO NORTH CENTRAL AREAS

G Moroto, Kotido and

Northeastern Kitgum

Average of 745 mm, STD 145 mm.

High variability, from ~ 600 over the

north and northeastern parts to ~ 1000

mm over the southern and western

parts.

One rainy season of about 5½

months, from April to early

September with the main peak

in July/August and a secondary

peak in May.

One long dry season of

about 6 months, October to

March. Driest months

December to February.

Evaporation > rainfall by a factor of over 10

during the driest months, December to

February. During the rainy season

evaporation is slightly > rainfall.

H Kitgum, Eastern Lira, South Kotido, Western Moroto and

Katakwi

Average of 1197 mm, STD 169 mm. Moderate variability, from ~ 1000 over

the north and northeastern parts to ~

1300 mm over western and southern

parts

One rainy season of about 7 months, April to late October

with the main peak in

July/August and a secondary

peak in May.

One long dry season of about 4 months, mid-

November to late March.

Driest months December

to February.

Evaporation > rainfall by a factor of over 10 during the driest months, December to

February. During the rainy months, May;

July and August rainfall is slightly >

evaporation.

I Adjumani, Gulu, Apac,

Western Lira and Eastern

Masindi


Moderate variability, from ~ 1200 over

northwestern and western parts to ~

1500 mm over the southern parts.

One rainy season, about 7½

months, April to about mid

November with the main peak

in August to mid October and a

secondary peak in April/May.


about 4 months, mid-

November to late March.


to February.

Evaporation > rainfall by a factor of up to 10


February. During the rainy months of May,

August and September rainfall >

evaporation.

NORTHWESTERN TO CENTRAL WESTERN AREAS

J Moyo and Arua Average of 1371 mm, STD 185 mm.

Moderate variability, from ~ 1200 over the eastern parts and highest ~ 1500 mm

over the western parts.

One rainy season of about 7½

months, April to about mid November with the main peak

August to October and a



about 4 months, late November to late March.


to February.

Evaporation > rainfall by a factor of ~ 10

during the dry months, December to March. During the rainy season, July to October,

rainfall > evaporation.

K Nebbi, Southwestern Gulu and

Western Masindi


High variability, from ~ 800 within the

Lake Albert basin to ~ 1500 mm over

the western parts

Mainly one rainy season of

about 8 months, late March to

late November with the main

peak August to October and a



about 3½ months,

December to about mid

March. Driest months

December to February.



March. During the rainy season, July to

October, evaporation > rainfall.

L Hoima, Kiboga, Western

Luwero, Kibale, North

Kabarole and Bundibugyo


High variability, from ~ 800 over

eastern L. Albert parts to ~ 1400mm

over the western parts.

Two rainy seasons, main season

August to November with peak

in October and secondary

season March to May with peak

in April.

Main dry season December

to about mid March,

secondary dry season is

June to July.


during the dry months, December to March.

During the rainy months, March and August

to November rainfall > evaporation.

5

ZONAL AVERAGE RAINFALL, STD AND EVAPORATION ANALYSIS

Zone Districts, 2000 boundaries Annual and its zonal variability Main rainy seasons Main dry seasons Evaporation verses rainfall

CENTRAL WESTERN AREAS TO CENTRAL REGION

MW Kabarole, Kasese, Northern Rukungiri, Bushenyi and

Mbarara

Average of 1223 mm. High variability, lowest ~ 800 mm Kasese

Rift Valley, highest over slopes of

Rwenzori mountains, over 1500mm.

Two rainy seasons, main season August to November with peak in

September to November and

secondary season March to May with

peak in April.

Main dry season December to late March, secondary

dry season is June to July.

Evaporation > rainfall by a factor of ~ 5 during the dry months, December to March.

During the rainy months, March, and August

to November rainfall > evaporation.

ME Mubende, West Mpigi,

Sembabule, and Northern

Rakai

Average of 1021 mm. Two rainy seasons, main season

March to May with peak in April and

secondary season September to

December with a modest peak in

November.

Main dry season June to

August, secondary dry

season is January to

February.


during the dry months, June to August.

During the main rainy months, April and

May rainfall ~ evaporation.

B Luwero, Mukono, Kampala,

and Mpigi.



secondary season August to

November with a modest peak in October/November.

Main dry season December

to February, secondary dry

season is June to July.


during the dry months, December to

February. During the peak of the rainy

seasons rainfall is greater and or equal to evaporation.

SOUTH WESTERN AREAS TO WESTERN SHORES OF LAKE VICTORIA BASIN

CW Kisoro, Kabale, Ntugamo,

Southern Rukungiri

Bushenyi and Mbarara


September to December with peak in

October/November and secondary

season March to May with a peak in

April.



season is January and

February.



During the rainy seasons rainfall is greater

and or equal to evaporation.

CE Rakai, West Masaka, and

East Mbarara



secondary season September to

December with a peak in

October/November.



season is January and

February.



During the main rainy season rainfall is

greater and or equal to evaporation.

A1_

W

Western shores of Lake

Victoria and Western

Masaka.



secondary season October to December with a peak in November.


September, secondary dry

season is January and February.



During the main rainy season rainfall is greater and or equal to evaporation.

These climatological zones are very useful for the presentation and analysis of features of the hydro-climatic regime and derivation of the

Intensity - Duration - Frequency relationships in an area.

6

3.0 INTENSITY - DURATION - FREQUENCY CURVES

In the design process, two important characteristics of the 'design' storm are considered:

-

- The duration, and

- Intensity of rainfall.

To assist in arriving at the 'intensity of rainfall' for the design storm duration, Intensity-

Duration-Frequency relationships have been derived using the 'Watkins and Fiddes'

approach which uses the following relationship: -

Equation 1

n

T

tbt

ai

Where a, b and n are coefficients,

T

tt is the intensity (in mm/hr) of duration t hours of rainfall and return period T years.

The Intensity-Duration-Frequency (IDF) graphs obtained for the zones of interest to

this study i.e. g, h & i, are as shown in annex 1 of this report. For Designers to

determine the intensity of rainfall to be used for design purposes as derived from the

time of concentration or otherwise they will use these IDF curves.

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4.0 DRAINAGE DESIGN PARAMETERS

The table below provides a guide towards the design of rural transport drainage

structures in terms of choice of return period and duration of design storm.

Table 2: Drainage Structure Design Return Periods

Drainage

feature

Return

Period (Yrs)

Design Storm

duration

(min)

The intensity of rainfall (mm/hr) is

equivalent to the indicated value or is

to be determined from the indicated

graph

All Demarcated Zones

Side drainage

and relief

culverts.

5 10 Read off the 5-year Return period 10min

storm from the Intensity-Duration-

Frequency Curves in Annex 1

Drifts 10 10 (Tc) IDF IDF IDF

Bridges 25 or 50 Tc IDF IDF IDF

The parenthesis provide for alternative approaches to design when catchments are

mapped out for Drift crossings and bridges.

8

5.0 DESIGN FLOOD DETERMINATION FOR BRIDGES, BOX

CULVERTS, ETC USING THE RATIONAL METHOD

The following step-by-step approach will be used by designers intending to determine

the design discharge from a catchment of interest as delineated from a topographical

map. Most of the parameters to be used are explained earlier.

Design Assumptions

The main assumptions inherent to this method are:

(i) The design storm produces a uniform rainfall intensity over the entire catchment

(ii) The relationship between rainfall intensity and rate of runoff is a constant for a

particular catchment.

(iii) Time of concentration (Tc) is the time taken for rainwater to flow from the

hydraulically must remote point to the catchment exit.

(iv) The flood peak at the catchment exit occurs at the time of concentration (Tc).

(v) The coefficient of runoff (C) is constant and independent of rainfall intensity.

Step 1: Determine the catchment area i.e. area that contributes runoff to the inlet

of drainage structure (A).

Step 2: Determine the runoff coefficient (C). The runoff coefficient can be

estimated by use of the table 3 below:

9

Table 3: Runoff Coefficient Parameters

Runoff coefficient (C) = Cs + Ck + Cv

Cs (topography) Ck (soils) Cv (vegetation)

Very flat < 1% 0.03

Sand &

gravel

0.04

Forest

0.04

Undulating 1 – 10% 0.08 Sandy clay 0.08 Farmland 0.11

Hilly 10 – 20% 0.16 Clay & loam 0.16 Grassland 0.21

Mountainous > 20% 0.26 Sheet rock 0.26 No

vegetable

0.28

Step 3: Estimate the time of concentration (Tc)

For larger catchments:

- Determine the Length (L) of the mainstream,

- Estimate the Slope (S) of the main stream,

- Using the formula given in equation 2 below, calculate Tc as

follows:

Equation 2

HoursS

LTC

385.02

1000

87.0

Where L = Length of main steam (Km)

S = Average slope of main stream (m/m)

10

Step 4: Determine the corresponding rainfall intensity (I)

In order to determine the corresponding design rainfall intensity, the

following has to be done:

- Determine the drainage feature return period from table 2 above,

- Using the return period above determine the intensity of rainfall from

the Intensity – Duration – frequency curves (figures B - D) at the

time Tc. This is the rainfall duration expected to yield a maximum

flood at the drainage structure entry point.

NB. It should be noted at this stage that figures B, C and D provide IDF

curves for zones I, G and H respectively. Therefore, the particular zone

for which the drainage structure is being design should be taken note of

at this stage so that the correct intensity is obtained from the right IDF

curve.

Step 5: Estimate the Area Reduction Factor (ARF)

The following formula for ARF developed by Fiddes for East Africa

should be used to convert the maximum intensity (I) to an average

rainfall intensity covering the whole catchment:

Equation 3

275.0044.01 AARF

Step 5: Calculate the Design peak flow (Q) by applying equation 4 below:

Equation 4

)/(6.3

3 smAIARFC

Q

Where Q = The Maximum Design Flood

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5.1 GUIDELINES FOR DRAINAGE STRUCTURE PROVISION

1. For road sections with Unlined side drains in situations where only Turnouts or

Mitre drains can be provided for the longitudinal drainage system the table 4 below

provides guidelines on the maximum frequency of turning off water using mitre

drains or turnouts.

Table 4: Mitre Drain spacing for Unlined Side Drainage Channels

Contour Rate

(5m ground

contours)

Slope or gradient

(%)

MAXIMUM Spacing of Mitre drains (m)

2 1 200

4 2 150

6 3 100

>6 >3 Line with stone masonry or concrete lining or

apply scour checks

It is uneconomical to provide turnouts for side drainage gradients in excess of 3%

because then the required frequency of turnouts is very high (<= 20m).

NB. It should be noted that the frequency of turnouts for unlined drains is

more dependant on how much the erosive velocity threshold for bare

ground is exceeded and not the capacity of the drainage ditch. The

erosive threshold for unlined drains is approximately 0.9m/s and flow

velocity beyond this cause erosion damage to the road infrastructure.

The location of the mitre drains/offshoots shall be established in

cooperation and agreement with the Land Owners.

2. It is recommended that for all road alignments with grades > 3%, the side drainage

should be lined to avoid the consequences of severe erosion. For road sections

where some form of Lining (stone pitching or concrete lining) has been provided,

then the frequency of providing Offshoots/Turnouts/Mitre drains are as indicated in

the table 5 below.

12

Table 5: Mitre Drain spacing for Lined Side Drainage Channels

Contour Rate

(5m ground

contours)

Slope or gradient

(%)

MAXIMUM

Spacing of Mitre

drains (m)

MAXIMUM

Spacing of Mitre

drains (m)

Type of Lining Stone Pitching Concrete Lining

2 1 450 950

4 2 650 1350

6 3 750 1700

8 4 900 1950

10 5 1000 2150

12 6 1100 2400

14 7 1200 2600

16 8 1300 2750

18 9 1350 2950

20 10 1450 3100

3. For special situations whereby both Relief Culverts OR Turnouts and 200mm

Scour Checks can be provided on the side drainage system, then following

guidelines in the table 6 apply to all alignments. The maximum spacing between

relief structures i.e. Culverts/ Mitre drains/offshoots will be 350m.

13

Table 6: Considerations for Provision of Scour Checks (table Dwg No. WWP 001

Sheet 1/1)

Contour Rate

(5m ground

contours)

Slope or

gradient (%)

Level difference between

Sour Checks (mm)

MAXIMUM

Spacing of Scour Checks

(m)

2 1 60 25

4 2 30 12

6 3 15 7

8 4 10 5

10 5 10 4

>10 5 Line side drainage system

Other considerations need to be taken into account when providing for culverts

in this case as opposed to turnouts: -

- The majority of relief culvert locations require a single 600mm

diameter barrel with larger or multiple openings reserved for

permanent water crossings or larger catchments,

- Provision of culverts should only be considered for situations when

it is not possible to provide turnouts, and

- Stop end Drop inlet structures should be constructed as substitutes

for turnouts to channel water through the culvert structure from the

uphill side of the road to the lower side.

3. Culverts should be provided at the bottom or sag of any two straights with a

minimum diameter of 900mm.

14

4. For the Design of single barrel culverts table 7 shows guide discharge

values and corresponding culvert diameters that can convey the discharge.

The assumption made is that 95% efficiency of discharge is achieved due to

inlet and outlet friction and other losses. For the design of multiple barrel

culverts the table 7 shows guide discharge values and corresponding culvert

diameters that can convey the discharge. The assumption made is that 80%

efficiency of discharge is achieved due to inlet and outlet friction and other

losses

Table 7: Capacity and Inlet Velocities for Piped Culverts

Diameter

of Culvert

(mm)

Number of Pipe Barrels

1 2 3 4 5

V

(m/s)

Q

(m3/s)

V

(m/s)

Q

(m3/s)

V

(m/s)

Q

(m3/s)

V

(m/s)

Q

(m3/s)

V

(m/s)

Q

(m3/s)

600 1.51 0.4275 1.272 0.72 1.272 1.08 1.272 1.44 1.272 1.80

900 1.793 1.140 1.509 1.92 1.509 2.88 1.509 3.84 1.509 4.80

1000 1.997 1.568 1.682 1.32 1.682 3.96 1.682 5.28 1.682 6.60

1200 2.100 2.375 1.768 4.00 1.768 6.00 1.768 8.00 1.768 10.00

1500 2.312 4.085 1.947 6.88 1.947 10.32 1.947 13.76 1.947 17.20

*Main assumptions include that the pipes flow full at the inlet and the Headwater to

Culvert Diameter ratio is 1.2.

The Engineer will have to use his judgment for discharge flows that lie

between those indicated in the table above to interpolate and establish the

number of barrels needed to carry a given discharge. More so, for situations

demanding more than 5 barrels of a given culvert diameter given site cover

limitations or otherwise, the Engineer will have to extrapolate the

discharges for multiple culverts to arrive at the required number of barrels.

15

5. For the design of both vented and normal drift crossings will be based on

the over-flow peak discharge as determined from the contributing catchment

during the wet weather. The drift will be defined by the DIP (h) designed to

suit the overflow discharge and also prevent the flood flows spreading.

Figure E below provides a graphical guide for determining the Plan length

(L) of the approach slab for different DIP (h) values of 300, 500 and 700

mm and also deck slab lengths (B). The Engineer will have to use his

judgment of the maximum clearance acceptable for the drift crossing

depending of the traffic using that particular route. The maximum DIP (h) =

700mm can be used for routes where the most common traffic type are

Heavy trucks, Lorries and other Heavy Goods Vehicles which have high

chassis clearances. The normal DIP (h) value should be 300mm for small

design flood discharges with high flood design discharge taking on DIP (h)

values of 500mm. Therefore, in the design for drifts, the main variables will

be three i.e. DIP (h), deck slab length and slope of the approach slab.

The design of openings for vented drifts will be based on the dry weather

flow. Since culverts will form most openings, then the guide for single and

multiple barrel culverts can be used to determine the number of openings

required to convey the discharge.

Gabion mattresses will usually be required on the downstream side of the

drifts for erosion protection especially when high overflow velocities are

anticipated.

16

Figure C: Variation of Discharge (Q) with Aproach Slab Length (L) for Drift DIP value = 500mm

Figure B: Variation of Discharge (Q) with Approach Slab Plan Length (L) for Drift DIP value =

300mm

17

Figure D: Variation of Discharge (Q) with Aproach Slab Length (L) for Drift DIP value = 700mm

18

6.0 EROSION CONTROL MEASURES

Roads interrupt the internal drainage of an area by concentrating water discharge

through culverts and drains often leading to soil erosion if the drainage is not carefully

planned and constructed.

Good erosion control should preferably start from the top of the rainfall catchment with

the objective of reducing water run out towards the road.

Along the road, sufficient numbers of drifts, vented drifts, culverts and mitre drains

must be installed to avoid large concentrations of water discharging through the

structures. The best approach to date is “Land husbandry” using good land management

practices especially biological control measures

The most important soil erosion control measure is the careful selection of sites for

structures and mitre drains. A guiding principle should be the discharge of water “little

and often”, to avoid potentially harmful concentrations of flow.

6.1 PROTECTION MEASURES

a) Slopes and embankments

- Plant grass or grass turfs

The type of grass to be used should be strong, fast growing and develop

good cover (e.g. napier grass). Grass removed by grubbing can often be

used if collected and stored properly for reuse.

- Stone pitching

Especially for dry areas or very sleep slopes.

19

b) Drains & waterways

- Grass

Should be established as soon as possible on the sides and inverts of new

drains and waterways.

- Scour checks

Such as wooden pegs, stones or grass sods to assist establish vegetation (low

growing creeping grasses are must suitable)

- “At-level” scour checks

For gently sloping channels in erodible soils.

c) Gullies

- Establish vegetation

This cover is to aid in the resistance to erosion

- Other structures made from stones or wooden materials

For places where grass and other vegetation – cover is not expected on it’s

own to resist erosion.

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7.0 CONSTRUCTION GUIDELINES FOR DRAINAGE

STRUCTURES

Various drainage measures are needed to satisfactorily deal with rainwater falling on or

near the road. Rainwater is the main cause of damage to district roads and as such a

good drainage system will significantly reduce rainwater damage and in the long run

minimize maintenance requirements.

Water damages district roads in two principal ways:

- Weakening road materials hence reducing traffic heavy bearing capacity.

- Erosion and silting which damages and reduces effectiveness of drainage

system.

An efficient drainage system must therefore collect all rainwater and dispose of it

quickly to minimize road damage. This enables the road materials to rapidly dry out

after the rains and regain traffic bearing strength.

The major components of the drainage system are the following:-

- Road surface camber: - sheds water from road surface

- Side drains: - collect water from road surface and adjoining land.

- Mitre drains: - lead water out of the side drains safely to adjoining land

- Catchwater drains: - intercept surface water flowing towards the road from

adjacent land leading it away.

- Scour check: - prevent erosion in side drains by slowing down water.

- Culverts/drifts/bridges:- allow water pass from one side of the road to the

other.

7.1 REQUIREMENTS OF VARIOUS DRAINAGE STRUCTURES

7.1.1 Mitre Drains

- Must be constructed in a manner to avoid erosion at discharge point,

- Must be provided as often as possible to avoid build up of water volume in

drain,

- Tables 4, 5 and 6 provide guidelines for the frequency of providing mitre

drains,

21

- Discharge should be channeled to garden/shamba/field boundaries and not

into farmland to course nuisance or damage,

- Minimum width of mitre drains should be 0.60m and x-section should have

at least same capacity as side drain,

- Some excavated soil should be used to block the downhill side of drain to

ensure water flows into mitre drain.

7.1.2 Catch-water Drains

- To be provided only for roads situated on hillsides with significant amount

of rainwater flowing from hill towards road,

- Catch-water drain should be constructed to intercept this surface water and

carry it to a safe discharge point usually a natural water course,

- Catch-water drain should have a satisfactory gradient throughout its length

(>2%),

- Catch-water drain should not be so close to the cut face because that will

increase the danger of a land slip,

- If steep gradients are unavoidable then scour checks should be provided,

- Excavated drain material should be placed on the downhill side to form a

bund,

- Vegetation cover should be established as soon as possible in the invert and

sloping sides of catch-water drain to resist erosion,

- Catch-water drains should normally be 0.6m wide, 0.4m deep with side

slope of 3:1

7.1.3 Scour Checks

- To be provided for longitudinal drain gradient steeper than 4% for erodible

soils,

- Should be constructed in natural stones or with wooden stakes,

- Scour check level should be a minimum of 0.2m below edge of

Carriageway,

- Scour checks should not be constructed on roads with grade < 4%,

- An apron should be constructed immediately downstream of scour check

using either stones or grass turfs pinned to the ditch invert with wooden

pegs,

22

- Grass sods should be placed against the upstream face of scour check to

prevent water seeping through scour check and to encourage silting behind

scour check,

- Table 6 provides guidelines for the frequency of providing scour checks.

7.1.4 Grass Planting

- To be used for effective prevention of erosion,

- Should be planted on all slopes where scouring is likely to occur,

- Grass type should be strong, fast growing and provide good coverage.

7.1.5 Turfing

- Excavating an area of live grass and lifting the grass complete with about

50mm of topsoil and roots still attached forms a grass turf. The turfs are

then replanted in another location,

- Grass turfs give a faster and more effective protection to slopes than planted

grass. They can be cut in the grubbing activity,

- The size of the turfs should not be smaller than 0.20x0.20m. Wooden pegs

may be required to secure them on steep slopes. They will require watering

to re-establish themselves.

7.1.6 Lined Side Drains

- If a side drain is more than 200m long without a mitre drain or relief culvert,

and its gradient is greater than 3%, there will be a serious risk of erosion. In

this situation consideration should be given to lining the ditch invert and

lower sides with hand packed stones. These should be well bedded and

wedged into place with smaller stones and soil,

- Side drain lining may be necessary for sections of road with “sunken”

profile,

- It is also effective for short steep sections of road where drains have a

gradient of more than 8% and there is an erosion risk.

23

7.1.7 Drainage for Roads with a 'Sunken' Profile

Roads with a “sunken profile” refers to roads that have been trafficked for many years,

subjected to poor grading practices or suffering from severe erosion such that they are

situated below the surrounding ground level for a considerable length.

This situation presents serious drainage problems as even after improvement

operations, they can still be impossible to drain. They will simply act as channels in

wet weather creating continuous maintenance problems.

Where a road with a sunken profile exceeds 200m in length without any possibility to

take away water to surrounding ground, the following drainage options should be

considered: -

a) Raise the level of the road, at least in some locations so that it may drain

to the adjoining land,

b) Where option (a) is difficult to achieve or where the earthworks

involved would be excessive and the road has a noticeable longitudinal

gradient, then ditch lining should be considered with a possibility of

increasing ditch cross-section area,

c) In some locations constructing additional drains parallel to the road

several metres offset from the side drains may relieve the volume of

water in the side drains. These should be 1m wide and excavated to a

level just below the side drains. Water should be channeled from the

side drain to the parallel drains by constructed mitre drains between

them at least every 20m. This option should be carefully considered as

it creates considerable additional drainage maintenance. Deep parallel

drains are difficult to desilt, can rapidly become overgrown and shelter

wildlife as a hazard to maintenance workers,

d) If the soil adjacent to the road is free draining e.g. sandy, mitre drains

can be constructed to soakaway ponds. These ponds may be constructed

of suggested dimensions 5m x 5m x 1m deep for example every 50m

along both sides of the road. This capacity would be able to hold water

falling on the road from a storm of at least 100mm of rain. If infiltration

to the surrounding ground is high, the spacing would be increased

24

correspondingly. The soakaway ponds need to be desilted in the dry

season. Smaller but deeper ponds filed with rocks and larger stones may

be more appropriate in some situations. Soakaway pond should be

located at least 10m from the side drains.

25

8.0 COST EFFECTIVE DRAINAGE DESIGN

The design and appraisal of rural transport infrastructure drainage interventions is field

that is not always properly articulated in most rural transport manuals. Whereas the

poor condition of rural roads will hinder poverty reduction efforts and stifle economic

growth, the poor condition of rural transport drainage systems often precludes

development altogether. The concept of low cost structures has often been misused as

we seek to spend as little as possible on rural transport network infrastructure. It is in

this regard that we should adopt the new notion of "Least Life Cycle Cost" which

means the option that will cost least in the life of the infrastructure taking into account

construction, maintenance, all-weather operability and access.

It has often been the norm in Uganda to associate the provision of cross drainage

infrastructure with only culverts, bridges or box culverts even where it is inappropriate.

It should be noted at this stage that other drainage infrastructure exists which is even

more amenable to the notion of Least Life Cycle Cost options by providing reliable and

efficient all-weather access. Therefore, for cost effective design options such as drifts

and vented drifts need to be considered because they provide more reliable all-weather

access and operability for least maintenance of rural networks compared to the

traditional options.

Various studies have provided evidence that poverty is more pervasive in areas with no

or unreliable motorized access (often referred to as unconnected areas). It should also

be noted that the poor drainage design is a major contributor to these problems by

creating isolated trouble spots on the network, which are often impassable during

inclement weather conditions. Research in this area has revealed that the long-term

improvement of trouble spots using submersible structures such as vented drifts and

drifts can yield considerable benefits by having roads that are open all year round to

bicycles, animal drawn carts and motorized transport. These structures provide the key

to keeping all previously unconnected areas open and accessible hence complementing

Government's poverty reduction strategies by way of increased access to economic

opportunities and social services. Without adequate rural transport networks, rural

communities lack the necessary physical access for domestic responsibilities,

agricultural activities, social and economic services, and job opportunities. Without

reliable access to markets and productive areas, economic development stagnates, and

26

poverty reduction cannot be sustained. Therefore, spot drainage improvements to the

rural network using long-term drainage solutions especially drift and vented drifts

crossings are a viable alternative available to the District Engineers and should be taken

into account.

Effective transport as a complementary input to nearly every aspect of rural activity is

an essential element of poverty reduction. The removal of surface water is crucial for

the success of rural networks because weather causes more damage that does traffic.

This means that adequate side drains and carefully designed cross drainage structures

are required. Usually, stone or concrete drifts are viable alternatives and substitutes for

culverts. It is always essential to remember that very limited resources will be available

for maintenance. As such, the use of structures that can be overtopped without damage

e.g. drifts or vented drifts, at minimal maintenance in the place of culverts, will most

likely be economically justified especially for areas prone to flooding. For small rivers

and streams with wet-weather flow only, a simple drift is usually adequate to secure

vehicle access. However, for continuous flows, vented drifts can be designed to pass

normal discharge, only submerging during floods.

To sum up from the above discussion, it is quite important to explore all available cross

-drainage options in view 'least life cycle cost' before any decision is taken on building

one. The notion of low-cost should be avoided for it leads to poor designs. It is

therefore very important that all drainage options be considered in light of maintenance

& construction costs and all-weather accessibility or passability.

Annex 1 Intensity – Duration – Frequency Curves for all the Demarcated Rainfall Zones

I

INTENSITY - DURATION - FREQUENCY CURVES (ZONE J)

-

50

100

150

200

250

0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

2 yr

5 yr

10 yr

25 yr

50 yr


II

INTENSITY - DURATION - FREQUENCY CURVES (ZONE I)

-

50

100

150

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350

0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

2 yr

5 yr

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25 yr

50 yr


III

INTENSITY - DURATION - FREQUENCY CURVES (ZONE G)

-

50

100

150

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250

300

0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

2 yr

5 yr

10 yr

25 yr

50 yr


IV

INTENSITY - DURATION - FREQUENCY CURVES (ZONE K)

-

50

100

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350

0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

2 yr

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10 yr

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50 yr


V

INTENSITY - DURATION - FREQUENCY CURVES (ZONE H)

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50

100

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250

0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

2 yr

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50 yr


VI

INTENSITY - DURATION - FREQUENCY CURVES (ZONE E)

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50

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300

0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

2 yr

5 yr

10 yr

25 yr

50 yr


VII

INTENSITY - DURATION - FREQUENCY CURVES (ZONE F)

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50

100

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250

0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

2 yr

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50 yr


VIII

INTENSITY - DURATION - FREQUENCY CURVES (ZONE L)

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0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

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Y (

mm

/hr)

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50 yr


IX

INTENSITY - DURATION - FREQUENCY CURVES (ZONE MW)

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180

0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

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5 yr

10 yr

25 yr

50 yr


X

INTENSITY - DURATION - FREQUENCY CURVES (ZONE ME)

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0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

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25 yr

50 yr


XI

INTENSITY - DURATION - FREQUENCY CURVES (ZONE B)

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50

100

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300

0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

2 yr

5 yr

10 yr

25 yr

50 yr


XII

INTENSITY - DURATION - FREQUENCY CURVES (ZONE A1)

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100

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600

0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

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50 yr


XIII

INTENSITY - DURATION - FREQUENCY CURVES (ZONE A2)

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50

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0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

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50 yr


XIV

INTENSITY - DURATION - FREQUENCY CURVES (ZONE D)

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50

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0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

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SIT

Y (

mm

/hr)

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50 yr


XV

INTENSITY - DURATION - FREQUENCY CURVES (ZONE CE)

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0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

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SIT

Y (

mm

/hr)

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50 yr


XVI

INTENSITY - DURATION - FREQUENCY CURVES (ZONE CW)

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50

100

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0 10 20 30 40 50 60 70 80 90 100 110 120

DURATION (min)

INT

EN

SIT

Y (

mm

/hr)

2 yr

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10 yr

25 yr

50 yr

drainage structures' sizing guidelines

Documents