ABSTRACT
On determining the maximum runoff and flood in a river following procedures outlined in earlier chapters, an economic study has to be made regarding the relative merits of the various methods of controlling the floods. Measures of flood control essentially consist of storing water in reservoirs and regulating the flow so that the channel downstream can safely accommodate and carry it. Of all the methods, use of natural reservoirs for detention of flood is most economical. By providing control works for the regulation of outflow and increase of capacities by construction of embankments along those which increases the depth of storage the utility of such reservoirs is greatly increased. Planning of reservoirs is closely related to their operation and both problems should be critically considered. No definite rules can be laid dawn for planning the size of multipurpose reservoirs as much depends on circumstances. However, normally it is possible to reduce the reservoir storage requirements for improved operative technique and more reliable prediction of runoff. The main purposes of the reservoirs are for the following: (i) flood control, and (ii) conservation. The aim of flood control storage is to hold over some of the flood water of a river when the discharge rate reaches a stage likely to cause damage to the valley downstream and to release the flood water gradually at a safe rate when the flood recedes. Conservation storage is meant to store the surplus water brought down by a river during periods when the
natural flow exceeds current demand, and to use this stored water during periods when demand exceeds the natural flow. Conservation may be done for any one or more of the following purposes: (i) irrigation, (ii) hydropower, (iii) regulation of low water flow for navigation, (iv) public and industrial water supply, and (v) recreation, pisciculture, etc. The reservoir may be a single-purpose conservation reservoir or a single-purpose flood control reservoir or a multipurpose reservoir. Multipurpose reservoirs are designed for two or more purposes. The common type of such reservoirs usually have three main objectives to serve, i.e., irrigation, power generation and flood control. The purpose of live storage in a reservoir is to guarantee a certain quantity of water usually called safe (confirm) yield with a predetermined reliability. Though sediment is distributed to some extent in the space for live storage, the capacity of live storage is generally taken as the useful storage between the full reservoir level and the minimum drawdown level in the case of power projects, and dead storage in the case of irrigation projects. Flood storage depends on the height at which the maximum water level (MWL), is fixed above the normal conservation level (NCL). The determination of the MWL involves the routing of the design flood through the reservoir and spillway. When the spillway capacity provided is low the flood storage required for moderating a particular flood will be large and vice versa. A higher MWL means greater submergence and hence this aspect has also to be kept in view while fixing the MWL and the flood storage capacity of the reservoir. Apart from above by providing additional storage volume in the reservoir for sediment accumulation over and above the live storage it is ensured that the live storage although it contains sediment will function at full efficiency for an assigned number of years. This volume of storage is referred to as dead storage and is equivalent to the volume of sediment expected to be deposited in the reservoir during the designed life of the structures.
