FLOOD ROUTING Flood Routing Techniques Siti Kamariah Md Sa’at PPK Bioprocess..2010.
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Transcript of FLOOD ROUTING Flood Routing Techniques Siti Kamariah Md Sa’at PPK Bioprocess..2010.
FLOOD ROUTING
Flood Routing Techniques
Siti Kamariah Md Sa’atPPK Bioprocess..2010
Flow Routing
Procedure to determine the flow hydrograph at a point on a watershed from a known hydrograph upstream
As the hydrograph travels, it attenuates gets delayed
Q
t
Q
t
Q
t
Q
t
Why route flows?
Account for changes in flow hydrograph as a flood wave passes downstream
This helps in Accounting for storages Studying the attenuation of flood peaks
Q
t
Types of flow routing Lumped/hydrologic
Flow is calculated as a function of time alone at a particular location
Governed by continuity equation and flow/storage relationship
Distributed/hydraulic Flow is calculated as a function of space and
time throughout the system Governed by continuity and momentum
equations
Lumped flow routing Three types
1. Level pool method (Modified Puls) Storage is nonlinear function of Q
2. Muskingum method Storage is linear function of I and Q
3. Series of reservoir models Storage is linear function of Q and its time
derivatives
S and Q relationships
Level pool routing Procedure for calculating outflow hydrograph
Q(t) from a reservoir with horizontal water surface, given its inflow hydrograph I(t) and storage-outflow relationship
Wedge and Prism Storage
• Positive wedge I > Q
• Maximum S when I = Q
• Negative wedge I < Q
Hydrologic river routing (Muskingum Method)
Wedge storage in reach
IQ
QI
AdvancingFloodWaveI > Q
II
IQ
I Q
RecedingFloodWaveQ > I
KQS Prism
)(Wedge QIKXS
K = travel time of peak through the reachX = weight on inflow versus outflow (0 ≤ X ≤ 0.5)X = 0 Reservoir, storage depends on outflow, no wedgeX = 0.0 - 0.3 Natural stream
)( QIKXKQS
])1([ QXXIKS
Muskingum Equations
• Continuity Equation I - Q = dS / dt
• S = K [xI + (1-x)Q]
• Parameters are x = weighting and K = travel time
- x ranges from 0.2 to about 0.5
where C’s are functions of x, K, t and sum to 1.0
Q2 C0I2 C1I1 C2Q1
Muskingum Equations
C0 = (– Kx + 0.5t) / D
C1 = (Kx + 0.5t) / D
C2 = (K – Kx – 0.5t) / D
Where D = (K – Kx + 0.5t)
Repeat for Q3, Q4, Q5 and so on.
Q2 C0I2 C1I1 C2Q1
Reservoir Routing
• Reservoir acts to store water and release through control structure later. • Inflow hydrograph• Outflow hydrograph• S - Q Relationship• Outflow peaks are reduced• Outflow timing is delayed
Max Storage
Inflow and Outflow
I Q dSdt
Inflow and Outflow
I1 + I2 – Q1 + Q2 S2 – S1
2 t2=
= change in storage / time
Re Repeat for each day in progression
Inflow & Outflow Day 3
I2 I3 / 2 Q2 Q3 / 2 S3 S2
dt
Determining Storage• Evaluate surface area at several different depths
• Use available topographic maps or GIS based DEM sources (digital elevation map)
• Outflow Q can be computed as function of depth for either pipes, orifices, or weirs or combinations
Q CA 2gH for orifice flow
Q CLH 3/2 for weir flow
Typical Storage -Outflow• Plot of Storage in vs. Outflow in Storage is largely a function of topography
• Outflows can be computed as function of elevation for either pipes or weirs
S
Q
Combined
Pipe
Comparisons:River vs. ReservoirRouting
Level pool reservoir
River Reach
Example 3:Level Pool Routing
Example 4:Resevoir Routing
Example 5:Flow Routing (Muskingum) Route the following flood hydrograph through
a river reach for which K=12.0hr and X=0.20. At the start of the inflow flood, the outflow flood, the outflow discharge is 10 m3/s.
Time (hr)
0 6 12 18 24 30 36 42 48 54
Inflow (m3/s)
10 20 50 60 55 45 35 27 20 15