The capacity of reservoirs is governed by a number of factors which are covered in IS: 5477 (Parts 1 to 4). From the point of view of sediment deposition, the following points may be given due consideration:
The sediment yield which depends on the topographical, geological and geo- morphological set up,meteorological factors, land use/land cover, intercepting tanks, etc
Sediment delivery characteristics of the channel system
The efficiency of the reservoir as sediment trap
The ratio of capacity of reservoir to the inflow
Configuration of reservoir
Method of operation of reservoir
Provisions for silt exclusion.
The rate of sediment delivery increases with the volume of discharge. The percentage of sediment trapped by a reservoir with a given drainage area increases with the capacity. In some cases an increased capacity will however, result in greater loss of water due to evaporation. However, with the progress of sedimentation, there is decrease of storage capacity which in turn lowers the trap efficiency of the reservoir.
The capacity of the reservoir and the size and characteristics of the reservoir and its drainage area are the most important factors governing the annual rate of accumulation of sediment. Periodical reservoir sedimentation surveys provide guidance on the rate of sedimentation. In the absence of observed data for the reservoir concerned, data from other reservoirs of similar capacity and catchment characteristics may be adopted.
Silting takes place not only in the dead storage but also in the live storage space in the reservoir. The practice for design of reservoir is to use the observed suspended sediment data available from key hydroIogica1 networks and also the data available from hydrographic surveys of other reservoirs in the same region. This data be used to simulate sedimentation status over a period of reservoir life as mentioned in IS 12182: 1987.
Control of Sediment Inflow
There are many methods for controlling sediment inflows and they can be divided as under:
Watershed management/soil conservation measures to check production and transport of sediment in the catchment area.
Preventive measures to check inflow of sediment into the reservoir.
The soil conservation measures are further sub-divided as:
The engineering methods include:
Use of check dams formed by building small barriers or dykes across stream channels.
Contour bounding and trenching;
The agronomic measures include establishment of vegetative screen, contour farming, strip cropping and crop rotation.
Forestry measures include forest conservancy, control on grazing, lumbering, operations and forest fires along with management and protection of forest plantations.
Preventive measures to check inflow of sediment into the reservoir include construction of bypass channels or conduits.