Hydrology and Water Resources RG 744 Institute of Space Technology December

 Response of river to precipitation  An unusually high stage of a river  May fill up the stream up to its banks and often spills over to the adjoining flood plain

 Flood may be considered as a wave that propagates downwards  In an ideal channel with frictionless fluid, flood wave may be considered traveling with no change from its point of origin  In natural channel energy is lost due to frictional forces  As a result magnitude of flood wave reduced or attenuated as it travels downstream  But discharge may also increase in downstream reaches due to increase in watershed area

 Estimate of extreme flood flow is required for the design of hydraulic structures  Proper selection of design flood value is of great importance  A higher value results in an increase in the cost of hydraulic structures,  An under-estimated value is likely to place the structure and population involved at some risk  Magnitude of flood may be estimated in accordance with the importance of the structure

 Design flood may be defined as  The maximum flood that any structure can safely pass  The flood considered for the design of a structure corresponding to a maximum tolerable risk  The flood which a project (involving a hydraulic structure) can sustain without any substantial damage, either to the objects which it protects or to its own structures  The largest flood that may be selected for design as safety evaluation of a structure

Routing is the process of predicting temporal and spatial variation of a flood wave as it travels through a river (or channel) reach or reservoir There are Hydrologic and Hydraulic Routings – here we will study Hydrologic Routing

 Once overland flow arrives at a stream it becomes channel flow  Routing  To know how outflow from a reservoir/stream is related to its inflow?  What the downstream hydrograph (outflow) will be if upstream hydrograph (inflow) is known?

 Known parameters:  you have a hydrograph at one location (I)  you have river characteristics  Need:  a hydrograph at different location (O)

Hydrograph computed at outlet of each subarea

Hydrographs routed to the outlet of the watershed

Hydrographs routed thru one reach of the watershed I – Q = dS/dt

Assume no seepage, leakage, evaporation or inflow other than main inflow

 In hydrologic routing techniques, equation of continuity and an analytical/empirical relationship between storage and outflow in a river or reservoir are used  Continuity Equation: I – O = ΔS/Δt Where: I = Inflow rate O = outflow rate ΔS/Δt = rate of change of storage

 Flood prediction and flood warning  Design of hydraulic structures (dams, spillways, etc.)  Evaluation of flood control measures  Etc.

 Reservoir Routing (storage routing):  Study of effect of a flood wave entering a reservoir  River Routing (channel or stream flow routing):  change in shape of hydrograph as it travels down a channel is studied

 During the advance of a flood wave, inflow exceeds outflow producing a wedge of storage  During the recession, outflow exceeds inflow, resulting in a negative wedge  Also, there is a prism of storage which is formed by a volume of constant cross-section along the length of prismatic channel  Assumption: X-sectional area of the flood flow is directly proportional to the discharge at that section S = KQ Level pool reservoir

 Predicting temporal and spatial variation of a flood wave as it travel through a river reach  Flood waves passing down a river have their peaks attenuated due to storage characteristics of the stream reach if no lateral inflow is added

Estimates the transformation of flood wave as it moves through a river channel S = KO + KX(I-O) S = storage O = Outflow I = inflow X = weighting factor that varies between 0 and 0.5 K = storage time constant (T)

Homework: show derivation of the above simplified equations

 tp = 4 hr  L = 2 miles  V avg = 2.5 ft/s  K = ?  X = ?  Δt = ?  Co =?  C1 =?  C2 =?

 Reservoirs are important in flood control because of large storage capacity  Downstream hydrograph peaks are smaller in magnitude and delayed in time

Peak flow at the upstream side of the reservoir is controlled in such a way that the flow at the downstream side is reduced to safe discharge In figure below upstream hydrograph has higher peak with shorter base Flood waves passing through a reservoir have their peaks attenuated and time base enlarge due to storage Downstream hydrograph with lower peak and broader base achieved by detaining flood water for some time by closing the spillways and then gradually releasing it

 Fixation of maximum reservoir level up to which the structure is completely safe  Implementation of outflow pattern from the reservoir so that it may not create any danger in the downstream side Inflow = outflow + change in storage

 Two relationships specific for reservoir

 Data required  Inflow hydrograph  Starting elevation above spillway

 Consider a reservoir having an ungated spillway (weir, outlet discharge pipe) or gated spillway with fixed position  Use relationship between outflow (Q) and elevation head (H) for a sharp crested rectangular weir Q = CLH 3/2  Q = Discharge at the outlet (cfs)  C = Discharge coefficient of weir (cfs)  L = Length of crest (ft)  H = Depth above spillway

 Estimation of Δt  Δt < tp/5

 Rainfall-runoff modeling  Flood Frequency Analysis (Statistical methods)

 There may be floods exceeding the design specification of a structure  What is a probability of those floods occurring in any given year  The probability of a flood exceeding or equaling a given magnitude?  Highest or peak discharges (floods) in each year used for calculation  Procedure for frequency calculation: Refer lecture 3 under “ Recurrence Interval of a Storm”

 Floods occurring along rivers, streams and in coastal area are natural events that have always been occurring throughout the history  Due to excessive rain runoff increases and streams or rivers overflow

Floodplain: Normally dry land area adjoining rivers, streams, lakes, bays or ocean that is undated during flood events

 Lots of damages and death due to river flooding each year throughout the world  Damages caused by floods are sometimes aggravated by manmade factors e.g.  Increasing urbanization  Deforestation  Uncontrolled development restricting waterways  Floodplains used for agriculture or settlement purposes

 Flood control is an important issue throughout the world  Measures to reduce or alleviate the negative consequences of flooding  Range of options to be considered in flood protection schemes  Both structural and non-structural approaches  These approaches must be weighted in terms of costs and benefits

 Levees (dikes or flood embankments)  Detention/retention Ponds, Dams/reservoirs  Diversions  Channel widening/modification  Due consideration to be given to the design of hydraulic structures to prevent from collapsing  Collapsing may also cause further damage by the force of water released from behind the structures  Involves huge sums of capital investment

 Earthen banks along the river course  To confine river into a limited cross-sectional width  Heights of levees are higher than the design flood level with sufficient free board

 Floodplain zoning and Management  Preservation of natural wetlands  Flood forecasting and warning system

 Corrective and preventive measures for reducing flood damages.  Some of the measures are  Floodplain management program  Emergency preparedness plan  Flood control works  Floodplain management regulations  Defines flood hazard area  Investigates problems arisen in developed areas and potential problems due to future development

Zoning features of a regulated floodplain The flood hazard area is generally defined at the 100-year floodplain.

 Increasing urbanization leads to increased overbank flows (floods)  More runoff and more stream runoff

 Flood season 15th June- 15th October

1. Most modern dams are designed so that they can afford to lose some storage capacity without their performance being impaired – the part of a reservoir known as "dead storage" which lies beneath the elevation of the dam’s lowest outlet. However sediments do not build up evenly along a horizontal plane, so that some "live storage" is usually lost long before the dead storage is filled. At Tarbela Reservoir in Pakistan, for example, 12 per cent of the live storage had been lost by 1992 (after 18 years of operation) while 55 per cent of the dead storage was still empty of sediment. 2. Absolute control over flood is rarely feasible either practically or economically. What we seek to do is to reduce flood damage to a minimum consistent with the cost involved.

 Barriers: levees, floodwalls, storage basins, riprap  Adjustment: floodplain regulation (i.e. zoning)  Redesign: channelization (e.g. Trinity River)

 To add erosion-resistant material to a stream bank

 Provide flood control  Treat urban runoff  Recreational spots