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CC606-HYROLOGY
CHAP 5- FLOOD ROUTING
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FLOOD ROUTING
Evaluating the risk of
flood requires
hydrologic studies.
- Risk evaluation under
current landuse
- Prediction of the
impact of landuse
change
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DEFINITION
FLOODROUTINGISTHEPROCESSDETERMINING
THETIMINGANDSHAPEOFAFLOODWAVE.
In channel
The flood were observedat successive points
along the river .
Reservoir and channel
are the two types of
routing that will be
considered
In reservoir
It is done by accounting
for the storage available in
the reservoir
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STORAGEANDTRANSMISSIONOFFLOODWATER
INCHANNEL
Floodwater moves
downstream along a
channel.
Flood occurs when the
channel storage
capacity is exceeded.
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FLOODPREDICTION
Important considerations.
(a) Volume of storm runoff
(b) Peak flood discharge
(c) Flood height (d) Time distribution of storm hydrograph
(e) Area of inundation
(f) Velocity of flow across the valley bottom
Practical procedures.
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STREAM ROUTING USING
MUSKINGUM METHOD
STEP 1
The Muskingum and
the Lag and K routing
techniques estimate
storage within a
channel by using a
series of containers
shaped as prisms and
wedges.
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STEP 2
The prism part of the
channel storage is a
regularly shaped
volume where inflow
and outflow are equal
for a particular reach.
MUSKINGUM METHOD
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STEP 3
The wedge storage
represents the positive
or negative storage
occurring during the
passing of a flood
wave.
MUSKINGUM METHOD
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STEP 3
A reach has increased
wedge (or positive)storage because it is
on the rising limb (or
curve) of the
hydrograph. The
upstream reachcontains the peak of
the flood wave.
MUSKINGUM
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STEP 4
On the receding side of
the flood wave the
reach storage
decreases as indicated
by the negative wedge
storage. The outflow is
greater than the inflow.
MUSKINGUM
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Assume that storage canbe approximated as;
S = K[xI + (1 - x)O]
K [s] is a constantk is travel time through the
reach .
x is a weighting factorrepresents the degree ofattenuation (FLOODWAVE SPREAD OUT )
Weighting factor for
volume distributionbetween wedge andprism = x
x = 0. This is the case ofa reservoir
x = 0.5. In this case, S =K(I/2 + O/2)
X =0.1
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EXAMPLE 1
The Muskingum parameters for the channel AB are
K= 60 minutes andX= 0.12. The subarea
characteristics are tabulated below.
Determine the total runoff hydrograph at the
watershed outlet that would result from the rainfall
tabulated below.
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watershed is comprised of two
subareas as shown in the
figure below. Point A is the
lowest point in subarea 1, andthe runoff from this subarea
flows to point A.
Runoff from subarea 2 flows to
point B, which is the lowestpoint in subarea 2 as well as
the whole watershed. In other
words point B is the watershed
outlet. The runoff reaching
point A from subarea 1 flowsthrough the channel AB to
reach the watershed outlet, B.
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Inflow
cfs
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Use the Muskingum method. First calculate the
weighting coefficients by usingX= 0.12, K= 1 hour,
t= 0.5 hr, and
The coefficients are obtained as C0= 0.115, C1 =
0.327, and C2= 0.558. Then perform the routing by
using
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THERESULTSARETABULATEDBELOW
max
outflow
=374cfs
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EXERCISE 1
Given s runoff
hydrograph for a
stream with a K value =
1.5 days and x = 0.25.
Calculate the streamby Using Muskingham
Method and create
outflow hydrograph,
assume that thestarting outflow as
31m3/s
0 31
6 50
12 86
18 123
24 145
30 150
36 144
42 128
48 11354 95
60 79
66 65
72 55
78 46
84 40
90 35
96 31
102 27
