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FLOODS
Hydrology and Water Resources RG 744Institute of Space TechnologyDecember 18 2013
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Flood: Peak in a hydrograph
Response of river to precipitation An unusually high stage of a river May fill up the stream up to its banks and
often spills over to the adjoining flood plain
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Movement of Flood Wave
Flood may be considered as a wave that propagates downwards
In an ideal channel with frictionless fluid, flood wave may be considered traveling with no change from its point of origin
In natural channel energy is lost due to frictional forces
As a result magnitude of flood wave reduced or attenuated as it travels downstream
But discharge may also increase in downstream reaches due to increase in watershed area
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Flood Estimates
Estimate of extreme flood flow is required for the design of hydraulic structures
Proper selection of design flood value is of great importance A higher value results in an increase in the cost of
hydraulic structures,
An under-estimated value is likely to place the structure and population involved at some risk
Magnitude of flood may be estimated in accordance with the importance of the structure
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Design Flood
Design flood may be defined as
The maximum flood that any structure can safely pass
The flood considered for the design of a structure corresponding to a maximum tolerable risk
The flood which a project (involving a hydraulic structure) can sustain without any substantial damage, either to the objects which it protects or to its own structures
The largest flood that may be selected for design as safety evaluation of a structure
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Routing
Routing is the process of predicting temporal and spatial variation of a flood wave as it travels through a river (or channel) reach or reservoir
There are Hydrologic and Hydraulic Routings – here we will study Hydrologic Routing
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Flood/Channel Routing
Once overland flow arrives at a stream it becomes channel flow
Routing
To know how outflow from a reservoir/stream is related to its inflow?
What the downstream hydrograph (outflow) will be if upstream hydrograph (inflow) is known?
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Hydrologic Routing
Known parameters: you have a hydrograph at one location
(I) you have river characteristics
Need: a hydrograph at different location (O)
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Flood Routing
2
4
1
3
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Hydrograph computed at outlet of each subarea
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Hydrographs routed to the outlet of the watershed
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Hydrographs routed thru one reach of the watershed
I – Q = dS/dt
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Routing Impact: Inflow, Outflow Hydrographs
Assume no seepage, leakage, evaporation or inflow other than main inflow
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Hydrologic Routing
In hydrologic routing techniques, equation of continuity and an analytical/empirical relationship between storage and outflow in a river or reservoir are used
Continuity Equation:
I – O = ΔS/ΔtWhere:I = Inflow rateO = outflow rateΔS/Δt = rate of change of storage
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Application of Flood Routing
Flood prediction and flood warning
Design of hydraulic structures (dams, spillways, etc.)
Evaluation of flood control measures
Etc.
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Two Flood Routing Problems
Reservoir Routing (storage routing):
Study of effect of a flood wave entering a reservoir
River Routing (channel or stream flow routing):
change in shape of hydrograph as it travels down a channel is studied
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River vs. Reservoir Routing
During the advance of a flood wave, inflow exceeds outflow producing a wedge of storage
During the recession, outflow exceeds inflow, resulting in a negative wedge
Also, there is a prism of storage which is formed by a volume of constant cross-section along the length of prismatic channel
Assumption: X-sectional area of the flood flow is directly proportional to the discharge at that section S = KQ
Level pool reservoir
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Flood Routing in River
Predicting temporal and spatial variation of a flood wave as it travel through a river reach
Flood waves passing down a river have their peaks attenuated due to storage characteristics of the stream reach if no lateral inflow is added
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Muskingum Method
Estimates the transformation of flood wave as it moves through a river channel
S = KO + KX(I-O)S = storageO = OutflowI = inflowX = weighting factor that varies between 0
and 0.5K = storage time constant (T)
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Muskingum Method
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Muskingum Method
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Muskingum Method
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Muskingum Method
Homework: show derivation of the above simplified equations
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Estimation of K, X and Δt
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Example:
tp = 4 hr L = 2 miles Vavg = 2.5 ft/s K = ? X = ? Δt = ? Co =? C1 =? C2 =?
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Example:
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Flood Routing in Reservoir
Reservoirs are important in flood control because of large storage capacity
Downstream hydrograph peaks are smaller in magnitude and delayed in time
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Peak flow at the upstream side of the reservoir is controlled in such a way that the flow at the downstream side is reduced to safe discharge
In figure below upstream hydrograph has higher peak with shorter base
Flood waves passing through a reservoir have their peaks attenuated and time base enlarge due to storageDownstream hydrograph with lower peak and broader base achieved by detaining flood water for some time by closing the spillways and then gradually releasing it
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Flood routing involves
Fixation of maximum reservoir level up to which the structure is completely safe
Implementation of outflow pattern from the reservoir so that it may not create any danger in the downstream side
Inflow = outflow + change in storage
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Zones of Storage and Levels
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Reservoir Routing
Two relationships specific for reservoir
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Reservoir Routing
Data required Inflow hydrograph Starting elevation above spillway
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Storage-Indication Method Consider a reservoir having an ungated
spillway (weir, outlet discharge pipe) or gated spillway with fixed position
Use relationship between outflow (Q) and elevation head (H) for a sharp crested rectangular weir
Q = CLH3/2
Q = Discharge at the outlet (cfs) C = Discharge coefficient of weir (cfs) L = Length of crest (ft) H = Depth above spillway
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Reservoir Routing
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Reservoir Routing
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Reservoir Routing
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Routing Steps
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Reservoir Routing
Estimation of Δt Δt < tp/5
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Example
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Example
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Example
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Flood Estimation
Rainfall-runoff modeling Flood Frequency Analysis (Statistical
methods)
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Review: Flood Frequency Analysis
There may be floods exceeding the design specification of a structure
What is a probability of those floods occurring in any given year
The probability of a flood exceeding or equaling a given magnitude?
Highest or peak discharges (floods) in each year used for calculation
Procedure for frequency calculation: Refer lecture 3 under “ Recurrence Interval of a Storm”
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Estimate the 100 year Flood?
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Flood: Over topping of banks
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History of Floods
Floods occurring along rivers, streams and in coastal area are natural events that have always been occurring throughout the history
Due to excessive rain runoff increases and streams or rivers overflow
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Natural Floodplain Features
Floodplain: Normally dry land area adjoining rivers, streams, lakes, bays or ocean that is undated during flood events
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Flood Losses and Damages
Lots of damages and death due to river flooding each year throughout the world
Damages caused by floods are sometimes aggravated by manmade factors e.g.
Increasing urbanization
Deforestation
Uncontrolled development restricting waterways
Floodplains used for agriculture or settlement purposes
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Flood Mitigation/Control
Flood control is an important issue throughout the world
Measures to reduce or alleviate the negative consequences of flooding
Range of options to be considered in flood protection schemes
Both structural and non-structural approaches
These approaches must be weighted in terms of costs and benefits
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Structural Approaches to Flood Control
Levees (dikes or flood embankments) Detention/retention Ponds, Dams/reservoirs Diversions Channel widening/modification
Due consideration to be given to the design of
hydraulic structures to prevent from collapsing
Collapsing may also cause further damage by the force
of water released from behind the structures
Involves huge sums of capital investment
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Levees
Earthen banks along the river course
To confine river into a limited cross-sectional width
Heights of levees are higher than the design flood level with sufficient free board
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Flood Control Operation of a Reservoir
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Non-Structural Approaches to Flood Control
Floodplain zoning and Management Preservation of natural wetlands Flood forecasting and warning
system
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Floodplain Management
Corrective and preventive measures for reducing flood damages.
Some of the measures are Floodplain management program Emergency preparedness plan Flood control works Floodplain management regulations
Defines flood hazard area Investigates problems arisen in developed
areas and potential problems due to future development
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Floodplain Zoning
Zoning features of a regulated floodplainThe flood hazard area is generally defined
at the 100-year floodplain.
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Development and Flooding
Increasing urbanization leads to increased overbank flows (floods)
More runoff and more stream runoff
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Floods of Pakistan
Flood season 15th June- 15th October
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Ref: Flood Frequency Analysis
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Notes
1. Most modern dams are designed so that they can afford to lose some storage capacity without their performance being impaired – the part of a reservoir known as "dead storage" which lies beneath the elevation of the dam’s lowest outlet. However sediments do not build up evenly along a horizontal plane, so that some "live storage" is usually lost long before the dead storage is filled. At Tarbela Reservoir in Pakistan, for example, 12 per cent of the live storage had been lost by 1992 (after 18 years of operation) while 55 per cent of the dead storage was still empty of sediment.
2. Absolute control over flood is rarely feasible either practically or economically. What we seek to do is to reduce flood damage to a minimum consistent with the cost involved.
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River Management
Barriers: levees, floodwalls, storage basins, riprap
Adjustment: floodplain regulation (i.e. zoning)
Redesign: channelization (e.g. Trinity River)
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Riprap
To add erosion-resistant material to a stream bank
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Detention Ponds
Provide flood control Treat urban runoff Recreational spots
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Dam Effect on Streams
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Weir