REDUCTION IN RESERVOIR CAPACITY

11.2 REDUCTION IN RESERVOIR CAPACITY

The useful life of a reservoir gets reduced due to sediment deposition causing a decrease in its storage capacity.The factors affecting the pattern of sediment deposition in reservoirs are:

(i) sediment load (i.e., sediment inflow rate) (ii) sediment size (i.e., gradation of silt) (iii) compaction of sediment

(iv) river inflow pattern

(v) river valley slope

(vi) shape of reservoir

(vii) capacity of reservoir (its size and storage period)

RESERVOIR SEDIMENTATION

(viii) vegetal growth at the head (ix) outlets in the dam (their types, location and size) (x) reservoir operation (xi) upstream reservoirs, if any. It has been found by experience that a low sediment inflow rate, large fraction of fine

particles, steep slope, no vegetation at head of reservoir, low flow detention time in the reservoir (by operation of outlets of suitable size at different levels), possibly series of upper tanks or reservoir upstream (where deposition occurs) do not favour sediment deposition and compaction. The silt carried in the rainy season may be excluded from the reservoir by means of scouring sluices slightly above the deep river-bed, which discharge the heavily silt-laden water at high velocity. The percent of the inflowing sediment, which is retained in a reservoir is called the trap efficiency and it is a function of the ratio of reservoir capacity to total annual sediment inflow, since a small reservoir on a large stream passes most of its inflow quickly (giving no time for the silt to settle) while a large reservoir allows more detention time for the suspended silt to settle. The relation between trap efficiency of reservoir vs. capacity-inflow ratio is shown in Fig. 11.3 (Brune, 1953), on the basis of data from surveys of existing reservoirs. The rate at which the capacity of a reservoir is reduced by sediment deposition depends on

(i) the rate of sediment inflow, i.e., sediment load. (ii) the percentage of the sediment inflow trapped in the reservoir, i.e., trap efficiency. (iii) the density of the deposited sediment.

90 Semi-log paper Semi-log paper

Median curve for normal Median curve for normal

ponded reservoirs ponded reservoirs

% h ap tr 60 ,( 50

efficiency 40 rap T

Capacity–Inflow: C- ratio) I

Fig. 11.3 Reservoir trap efficiency vs. capacity-inflow ratio (after Brune, 1953)

In estimating the useful life of a reservoir, the correct prediction of the density of the deposited sediment is an important factor. Lane and Koelzer (1943) gave the equation for the dry specific weight γ t after time t years as

...(11.2) where γ i = initial specific weight K = a constant for the rate of compaction

γ t =γ i + K log 10 t

HYDROLOGY

If the deposited sediment consists of a mixture of materials like sand, silt and clay, a weighted average specific weight is calculated as (Lane and Koelzer). γ t =γ 1 x 1 + (γ 2 +K 2 log t) x 2 + (γ 3 +K 3 log t) x 3 ...(11.2 a)

where γ t = average specific weight of reservoir sediment after t years γ 1, 2, 3 = specific weight of sand, silt and clay after t years K 1, 2, 3 = constant for the rate of compaction of sand, silt and clay, respectively (K 1 ≈ 0, for

sand) x 1, 2, 3 = fractional part of total sediment, of sand, silt and clay, respectively t = time in years (≥1 yr) The useful capacity of reservoir lost each year by sediment deposition is

...(11.3) where V s = volume of useful capacity of reservoir lost each year Q s = annual sediment inflow into the reservoir η trap = trap efficiency of the reservoir while allocating space for the dead storage in the reservoirs (i.e., to provide space for sediment

V s =Q s η trap

deposition during the life of the project) the trap efficiency is taken as at least 95% and rarely below 90%. Sediment deposits in the upper end of the reservoirs generally become covered by vegetation resulting in heavy evapotranspiration loss of the available water, which is more critical in arid regions.

A proposed reservoir has a capacity of 400 ha-m. The catchment area is 130 km 2 and the annual stream flow averages 12.31 cm of runoff. If the annual sediment production is

Example 11.1

0.03 ha-m /km 2 , what is the probable life of the reservoir before its capacity is reduced to 20% of its initial capacity by sediment deposition. The relation between trap efficiency and capacity- inflow ratio is given below .

Trap C

efficiency, I

η trap (%)

η trap (%)

RESERVOIR SEDIMENTATION

Solution The useful life may be computed by determining the number of years required for each incremental loss of reservoir capacity (i.e., for the decreasing values of capacity-inflow ratios) upto the critical storage volume of 400 × 0.20 = 80 ha-m as tabulated below:

Loss of No. of years Capacity

Trap efficiency

Capacity*

reservoir for the C inflow

η trap (%)

Annual **

capacity capacity (ha-m)

Ave. for

trapped

∆C loss

I I ratio

*increment

V s =Q s × η trap (ha-m) ∆C ÷V s

Total = 92.6 say, 93 yr

*Average annual inflow, I =

For reservoir capacity C = 400 ha-m,

I 1600

**Annual sediment inflow into the reservoir

Q s = 0.03 × 130 = 3.9 ha-m

Note: If the average annual sediment inflow Q s is given in tons, say Q s = 43600 tons and for η trap = 93.5% (for the first incremental loss), assuming a specific gravity of 1.12 for the sediment depos- its, annual sediment trapped W s = 43600 × 0.935 = 40750 tons.

40750 × 1000 kg

m 3 = 3.64 ha-m.

1.12 × 1000 kg/m 3 1.12

Usually the specific gravity of sediments deposits ranges from 1 to 1.4.