Sediment Load Measurements
Periodic samples front the stream should be taken at various discharges along with the stream gauging observations and the suspended sediment concentration should be measured as detailed in IS 4890: 1968. A sediment rating curve which is a plot of sediment concentration against the discharge is then prepared and is used in conjunction with stage duration curve (or flow duration) based on uniformly spaced daily or shorter time units data in case of smaller river basins to assess sediment load. For convenience, the correlation between sediment concentration against discharge, may be altered to the relation of sediment load against run-off for calculating sediment yield. Where observed stage/flow data is available for only shorter periods, these have to be suitably extended with the help of longer data on rainfall. The sediment discharge rating curves may also be prepared from hydraulic considerations using sediment load formula, that is, modified Einstein‟s procedure.
The bed load measurement is preferable. How- ever, where it is not possible, it may be estimated using analytical methods based on sampled data or as a percentage of suspended load (generally ranging from 10 to 20 percent). This should be added to the suspended load to get the total sediment load.
Fixing of Live Storage
The storage of reservoir includes the Active Storage (or Conservation Storage) and the Buffer Storage.
Active or conservation storage assures the supply of water from the reservoir to meet the actual demand of the project whether it is for power, irrigation, or any other demand water supply.
The active or conservation storage in a project should be sufficient to ensure success in demand satisfaction, say 75 percent of the simulation period for irrigation projects, whereas for power and water supply projects success rates should be 90 percent and 100 percent respectively. These percentages may be relaxed in case of projects in drought prone areas. The simulation period is the feasible service period, but in no case be less than 40 years. Storage is also provided to satisfy demands for maintaining draft for navigation and also maintaining water quality for recreation purpose as envisaged in design.
Live storage capacity of a reservoir is provided to impound excess waters during periods of high flow, for use during periods of low flow. It helps the usage of water at uniform or nearly uniform rate which is greater than the minimum flow live storage has to guarantee a certain quantity of water usually called safe (or firm) 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 draw-down level in the case of power projects and dead storage in the case of irrigation projects.
The design of the line storage include certain factors, of which the most important in the availability of flow, since, without an adequate flow, it is not possible to cope up with the demand at all periods and seasons throughout the year. When adequate flow is available, there may still be certain problems like the possible maximum reservoir capacity from physical considerations may be limited and then this becomes the governing criteria. Even if an adequate reservoir capacity may be possible to be built, the governing factor may have to be based on the demand.
For fixing the live storage capacity, the following data should be made use of:
Stream flow data for a sufficiently long period at the site;
Evaporation losses from the water-spread area of the reservoir and seepage losses and also recharge into reservoir when the reservoir is depleting;
The contemplated irrigation, power or water supply demand;
The storage capacity curve at the site.
Stream flow records are required at proposed reservoir site. In the absence of such records the records from a station located upstream or downstream of the site on the stream or .on a nearby stream should be adjusted to the reservoir site. The run off records are often too short to include a critical drought period. In such a case the records should be extended by comparison with longer stream flow records in the vicinity or by the use of rainfall run off relationship.
The total evaporation losses during a period are generally worked out roughly as the reduction in the depth of storage multiplied by the mean water-spread area between the full reservoir level and the minimum draw-down level. For accurate estimation, monthly working tables should be prepared and the mean exposed area during the month is found out and the losses should be then worked out on the basis of this mean exposed area, and the evaporation data from pan evaporimeter at the reservoir site. The details are expected to be covered in the draft „Indian Standard criteria for determination of seepage and evaporation losses including the code for minimizing them. In the absence of‟ actual data these may be estimated from the records of an existing reservoir with similar characteristics, like elevation, size, etc., in the neighbourhood.
Of the various methods available for fixing the live storage capacity, the Working Table method may be used which is prepared on the basis of preceding long term data on discharge observation at the site of the proposed reservoir, inclusive of at least one drought period. A typical format for carrying out the working table computation, is given in the following table:
The working table calculations may be represented graphically by plotting the cumulative net reservoir inflow exclusive of upstream abstraction as ordinate against time as abscissa. This procedure is commonly called the Mass Curve Technique, where the ordinate may be denoted by depth in centimetres or in hectare meters or in any other unit of volume. Discharge, with units of 10 days or a month may be used culmination in the mass curve. A segment of the mass curve is shown in Figure 3.
The difference in the ordinate at the end of a segment of the mass curve gives the inflow volume during that time interval. Lines parallel to the lines of uniform rate of demand are drawn at the points b and c of the mass curve. At d,
The following inferences can be made:
The inflow rate between a to b is more than the demand rate and the reservoir is full.
Reservoir is just full as the inflow rate is equal to the demand rate.
Reservoir storage is being drawn down between b and c since the demand rate exceeds the inflow rate.
Draw down, S, is maximum at c due to demand rate being equal to inflow rate.
Reservoir is filling or in other words draw down is decreasing from c to d as the inflow rate is more than the demand rate.
Reservoir is full at d and from d to b again the reservoir is over flowing because the inflow rate exceeds the demand rate. The greatest vertical distance, S at c is the storage required to make up the proposed demand.
The withdrawals from the reservoir to meet the irrigation demand are generally variable and in such cases the demand line becomes a curve instead of a straight line. The demand mass curve should be super-imposed on the inflow mass curve on the same time scale. When the inflow and demand mass curves intersect, the reservoir may be assumed to be full. For emptying conditions of the reservoir the demand curve would be above the inflow curve and the maximum ordinate between the two would indicate the live storage capacity required.