FLOOD CONTROL Flood is a relatively high flow. The floods cause overtopping of natural as well as artificial banks of the streams. The flood is caused by run off from rainfall and/ or melting snow, too large to be confined in the banks of streams. When banks are overtopped the flood water spreads over the adjoining area and causes heavy damage to crops and property. It is not possible to prevent occurrence of floods but it is possible to prevent or reduce the damage due to floods by controlling the floods. This flood control or flood management may be defined as the prevention or reduction of the flood damage. In India about 40 million hectares of area has been identified as flood prone. Out of this about 12 million hectares have been provided with flood protections measures by March 1982. Govt. of India set up National Flood Commission in July…

FLOOD ABSORPTION OR FLOOD ROUTING Flood routing is a process with the help of which characteristics of hydrograph of a flood, entering the reservoir are completely change when it emerges out of the reservoir. The change in flood hydrograph characteristics takes place because certain volume of flood is store in the reservoir temporarily. The base of the modified hydrograph, which emerges from the reservoirs becomes wider and its penk gets reduced. The extent by which the inflow hydrograph gets changed due to reservoir storage can be computed by a process. It is an important technique with the help of which solution of a flood control problem is arrived at. In flood routing problem one has to calculate water levels in the reservoir, the storage quantities and outflow rates. It is used to determine the level upto which water may rise during floods and also the rate of the discharge in…

SILT CONTROL OF RESERVOIRS Entrance of silt into the reservoir may be controlled by adopting following preventive measures. Reservoir site should be properly selected. If nature of catchment soil contributing water to the reservoir is very soft, the rate of silting will he more, as soft sand will be easily carried by run-off. If slope of the catchment is steep, it will also cause more of silting, as steepness would cause increased velocity of flow of run-off and thus more erosion of the catchment soil. The tributaries carrying more of silt load to the reservoir should be provided with check bunds some where upstream of the tributaries. Check bunds would cause sedimentation of silt load upstream and comparatively clear water will reach the main reservoir. The small reservoirs created at the back of the check bunds also supply stored water to the main reservoir when supplies in the tributaries have…

RESERVOIR LOSSES Reservoir losses may be classified under following three heads. Evaporation losses Absorption losses Reservoir leakage or percolation losses. 1. Evaporation Losses. These are the major losses from a reservoir. This loss is affecte by expos surface area of the reservoir, wind velocity, temperature, relative humidity etc. This loss is express in cm of water depth and varies from place to place depending upon the local conditions such temperatures, relative humidity, wind etc. Average values of losses in cm for North and South India for various methods of the year are given as follows. 2. Absorption Losses. This loss of water is considerable in the beginning, but falls to very small values after some time when pores get saturat. These losses depend upon the soil forming the reservoir. These losses are not consider while planning a reservoir. 3. Reservoir Leakage or Percolation Losses. Reservoirs being very large, their banks…

RESERVOIR SEDIMENTATION All the waters entering the reservoir carry some amount of silt. The amount of silt in the water coming into the reservoir depends upon the topography, nature of the soil, vegetation cover over the catchment, and also upon the intensity of rainfall. If soil in the catchment is soft there is always possibility of sheet erosion and amount of silt in the water coming to reservoir will be more. If catchment has large slope, it will cause rain water to flow with greater velocity and possibilities of increas amount of slit in the water will occur. Similarly higher intensities of rain fall also cause greater run off and greater erosion and consequently increas amount of silt in water approaching the reservoir. If catchment has good cover of vegetation, the amount of silt in water would decrease. The silt load held in river water approaching the reservoir may be…

LIFE OF RESERVOIR Any reservoir cannot last for ever. Ultimately all the reservoirs get silted up. Silting of the reservoir starts from the day it is created. When reservoirs are created some of its capacity is left unused. This is the capacity of the reservoir lying below the crest level of the bottom most under sluices. This storage capacity which remains unused is know dead storage. This dead storage capacity is used to accommodate deposited silt so that effective storage of the reservoir is not affected. So long as dead storage capacity of the reservoir is not silted completely, effective storage or useful storage capacity is not affected. The process of silting continues even after complete silting of dead storage. The further silting affects the effective storage of the reservoir and the reservoir does not have enough water to fully fulfil its obligations. Generally useful life of the reservoir is…

SILTING POWER OF RESERVOIRS For reservoirs construct only for power generation, silting is not as important  for the reservoirs construct for irrigation, and water supply. This is due to the fact that for running turbines a minimum head of water is required. So long  this head is available the working of power reservoir is not affected. Obviously, if the silting has affected the minimum capacity of the reservoir required for generation of power, the generation. Hence while silting of the reservoir is not affecting the head of water over the turbines, it is certainly affect.

ECONOMIC HEIGHT OF THE DAM It is that height of the dam, corresponding to which the cost of the dam per unit of storage is minimum. This requires estimates of construction cost for several heights of dam and also the corresponding storages in the reservoir. For each dam height, the reservoir storage is know. Draw the curve between dam heights and their corresponding costs of construction Now work out the construction cost per unit storage for all the dam heights and plot a curve The lowest point on this curve gives the height of dam for which the cost per unit of storage is minimum and hence adopt the most economical.