FLOOD ROUTING Flood Routing Techniques Siti Kamariah Md Sa’at PPK 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 Advancing Flood Wave I > Q Receding Flood Wave Q > I K = travel time of peak through the reach X = weight on inflow versus outflow (0 ≤ X ≤ 0.5) X = 0  Reservoir, storage depends on outflow, no wedge X =  Natural stream

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

Muskingum Equations C 0 = (– Kx  t) / D C 1 = (Kx  t) / D C 2 = (K – Kx – 0.5  t) / D WhereD = (K – Kx  t) Repeat for Q 3, Q 4, Q 5 and so on.

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 1 + I 2 – Q 1 + Q 2 S 2 – S 1 2 tt 2 =

= change in storage / time ReRepeat for each day in progression Inflow & Outflow Day 3

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

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. Reservoir Routing 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 m 3 /s. Time (hr) Inflow (m 3 /s)