CE323-T
FLOOD ROUTING DEFINITION
Hydrology
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Flood Routing Flood Routing
It is a procedure whereby the shape of a flood As the discharge in a stream due to flood increases,
hydrograph at a particular location on the stream the stage also increases and hence the volume of
is determined from the known or assumed flood water in temporary storage in the channel increases.
hydrograph at some other location upstream.
USES OF FLOOD ROUTING
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Flood Routing Flood Routing
It is used in: d. Predicting the behavior of river after a change
a. Establishing the flood peak at a downstream has been done in the channel conditions
location
b. Establishing the effect of construction of
e. Determining the adequacy of spillway
BASIC TECHNIQUES USED IN
reservoir on flood
c. Determining the required level height for
FLOOD ROUTING
flood protection
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Distributed Routing Lumped/Hydrologic Routing Lumped/Hydrologic Routing
In distributed routing, In lumped routing, There are two types of routing used:
a. The discharge is a function of time and a. The discharge is a function of time at any a. Storage routing or reservoir routing
space given x location. - In storage routing, we assume level pool
b. We use continuity and momentum equation b. We use continuity equation routing concept i.e. the water level will be
assumed horizontal
b. Channel routing
Where: - Calculates the outflow from a stream for
V is the velocity of flow
S0 is the bed slope which inflow and channel characteristics
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Sf is the energy line slope 11 12
are known or assumed.
Storage Routing Storage Routing
The storage will be a function of elevation only: In level routing, the inflow hydrograph is known
S = S(h) and at the same time the outflow-elevation and
If we assumed the varied flow concept with water storage elevation curve can be established.
STORAGE ROUTING level, then the storage will be a function of
inflow and outflow It is important to find out outflow discharge vs
time or elevation vs time or storage vs time
curves.
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Storage Elevation Curve Storage Elevation Curve Example
The topographical survey of a proposed reservoir site
The outflow elevation and The outflow-storage relation yielded the following data:
storage elevation relationship ship is shown:
will be established as shown.
The volume of water can be
obtained using: There are two circular sluices with diameter of 2.5 m and
with their centres at an elevation 470 m. A spillway with
an effective crest length of 20 m is also provided with its
Where Cd1 is coefficient of discharge of spillway, Cd2 is coefficient of sill at 480 m. The Cd for sluices may be taken as 0.8 and
discharge of sluices, Hs is sill level of spillway, Hsl is the centre of
for spillway C = 2.25. Prepare the storage-discharge
sluice height, A is the area of sluices, L is the length of spillway
curve for the reservoir. Use trapezoidal rule
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FLOOD CONTROL FLOOD CONTROL
I. Structural Methods I. Structural Methods
a. Storage Reservoirs a. Storage Reservoirs
- To temporarily store a portion of the flood - To temporarily store a portion of the flood
FLOOD CONTROL so that the flood peaks are flattened out
- Ideally situated immediately upstream of
so that the flood peaks are flattened out
- Ideally situated immediately upstream of
the area to be protected and the water the area to be protected and the water
discharge in the channel downstream at its discharge in the channel downstream at its
safe capacity safe capacity
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FLOOD CONTROL FLOOD CONTROL FLOOD CONTROL
I. Structural Methods I. Structural Methods I. Structural Methods
b. Detention Reservoirs c. Levees d. Floodways
- Consists of an obstruction to a river with an - Also known as dikes or flood embankments - Are natural or manmade channels into
uncontrolled outlet - Are earthen banks constructed parallel to which a part of the flood will be diverted
- Are essentially small structures and operate the course of the river to confine it to a during high stages
to reduce the flood peak by providing fixed course and limited cross-sectional
temporary storage and by restriction of the width
outflow rate - Heights of levees will be higher than the
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design flood with sufficient freeboard 24
FLOOD CONTROL FLOOD CONTROL FLOOD CONTROL
I. Structural Methods I. Structural Methods II. Non-Structural Methods
e. Channel Improvement f. Soil Conservation a. Flood plain zoning
The works under this category involve: - Soil conservation measures lead to b. Flood warning
- Widening or deepening of the channel to increased infiltration, greater c. Flood evacuation
increase the cross-sectional area evapotranspiration, and reduced soil d. Flood relocation
- Reduction of the channel roughness by clearing erosion.
of vegetation from the channel perimeter - Small and medium floods are reduced by
- Short circuiting of meander loops by cutoff soil-conservation measures
channels, leading to increased slopes.
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