FLOOD ROUTING Siti Kamariah Md Sa’at School of Bioprocess Engineering, UniMAP

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 Calculate 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 Flood Routing Basic Equation

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

Continuity Equation in Difference Form Referring to figure, the continuity equation in difference form can be expressed as 2 ) 2 O 1 (O 2 ) 2 I 1 (I _ O _ I 1 t 2 t 1 S 2 S t S         

Routing Derivation of Muskingum Routing Equation By Muskingum Model, at t = t 2, S 2 = K [X I 2 + (1 - X)O 2 ] at t = t 1, S 1 = K [X I 1 + (1 - X)O 1 ] Substituting S 1, S 2 into the continuity equation and after some algebraic manipulations, one has O 2 = C o I 2 + C 1 I 1 + C 2 O 1 Replacing subscript 2 by t +1 and 1 by t, the Muskingum routing equation is O t+1 = C o I t+1 + C 1 I t + C 2 O t, for t = 1, 2, … where ; ; C 2 = 1 – C o – C 1 Note: K and  t must have the same unit.

Muskingum Routing Equation where C’s are functions of x, K,  t and sum to 1.0

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

Estimating Muskingum Parameters, K and x Graphical Method: Referring to the Muskingum Model, find X such that the plot of XI t + (1-X)O t (m 3 /s) vs S t (m 3 /s.h) behaves almost nearly as a single value curve. The assume value of x lies between 0 and 0.3. The corresponding slope is K.

Example 8.4: Estimating the value of x and K. Try and error to get the nearly straight line graph.

Muskingum Routing Procedure Given (knowns): O 1 ; I 1, I 2, … ;  t; K; X Find (unknowns): O 2, O 3, O 4, … Procedure: (a) Calculate C o, C 1, and C 2 (b) Apply O t+1 = C o I t+1 + C 1 I t + C 2 O t starting from t=1, 2, … recursively.

Example 8.5 Given K and x. Initial outflow, Q also given. Solution: Calculate C o, C 1, and C 2 C 0 = (– Kx + 0.5Dt)/ D C 1 = (Kx + 0.5Dt)/ D C 2 = (K – Kx – 0.5Dt)/ D D = (K – Kx + 0.5Dt)

Solution: 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)

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 Repeat 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

Flood Control Structural Measures Non-structural Methods

Structural Measures Storage and detention reservoir Flood ways (new channel) Levees (flood embankment) Channel Improvement

Reservoirs Reservoirs reduce flooding by temporarily storing flood waters behind dams or in storage or detention basins. Reservoirs lower flood heights by holding back, or detaining, runoff before it can flow downstream. Flood waters are detained until the flood has subsided, then the water in the reservoir or detention basin is released or pumped out slowly at a rate that the river can accommodate downstream.

Timah Tasoh Dam

Reservoirs Flood control reservoirs are most commonly built for one of two purposes. Large reservoirs are constructed to protect property from existing flood problems. Smaller reservoirs, or detention basins are built to protect property from the impacts of new development (i.e., more runoff).

Many dams have a function for flood management, but in some cases dams actually make floods worse! Think!

Flood way diversion A diversion is a new channel that sends floodwaters to a different location, thereby reducing flooding along an existing watercourse. Diversions can be surface channels, overflow weirs, or tunnels. During normal flows, the water stays in the old channel. During flood flows, the floodwaters spill over to the diversion channel or tunnel, which carries the excess water to a receiving lake or river.

Flood levees Also known as dikes or flood embankments. Probably the best known flood control measure is a barrier of earth (levee) or concrete (floodwall) erected between the watercourse and the property to be protected. Levees and floodwalls confine water to the stream channel by raising its banks. They must be well designed to account for large floods, underground seepage, pumping of internal drainage, and erosion and scour.

Flood levees

Channel/Drainage Improvement There are three types of drainage improvements that are usually pursued to reduce storm water flooding: Channelization- straightening, deepening and/or widening a ditch or drainage way to remedy local drainage or flooding problems. removing obstructions caused by stream crossings, such as culverts and bridges with small openings - constricts flows and causes localized backwater flooding. Drainage system maintenance to clean out blockages caused by debris, sediment or vegetation and repair stream bank erosion

Clearing the drainage/channel

Non-structural measures Flood plain zoning Normal level, alert level and danger level Flood forecast/warning Flood forecasting system by DID and MMD, Malaysia. Flood warning is meaningful if given in sufficient time. Evacuation and relocation Evacuation of communities along their livestocks and other valuables in the flood affected areas and relocate them to the safer locations. Flood insurance Provide a mechanism for spreading the loss over large numbers of individuals and modifies the impact of loss burden.

Flood control in Malaysia.

In 5 month interval, 2 flood event happen at Perlis in From your opinion, how to control the flood from happen again. Write your proposal. Submit next week