Worked Example 4.2
The inflow to a reservoir has an average sediment concentration of 800 ppm.
If the volume of the reservoir is 100 106 m^3 and the annual flow of the river
is 900 106 m^3 , determine the approximate ‘half-life’ of the reservoir. Assume
that the average porosity of the settled sediment over this period is 0.4.Solution
For the empty reservoir the ratio of capacity and annual inflow is
100/9000.111; for the half-full reservoir this ratio is 50/9000.055.
From Fig. 4.2 the trap efficiency of the full reservoir is about 89%, and
for the half-full reservoir 71%. The average trap efficiency is thus about 80%.
The annual sediment discharge isQs
900
1
1
0
0
(^6)
(^6)
s
800
^ (m^3 year^1 ).
The annual bulk discharge, assuming 103 and (^) s2650 kg m^3 , is
Qb452 830 m^3 year^1.
The half-life of the reservoir is
138 years.
Worked Example 4.3
The overfall spillway in example 4.1 has been designed for a head of
2.80 m. What will the discharge be for heads of 0.20 m and 1.50 m, and what
is the maximum discharge that can be passed over this spillway (assuming
the dam freeboard to be high enough and the spillway to be well con-
structed) without cavitation?
Solution
At the design head
Q2/30.75 2
g1/2 200 2.83/2590.660.752.83/22075 m^3 s^1.
ForH0.20 m, H/Hd0.20/2.800.071; thus Cd0.58 and
Q590.660.580.23/231 m^3 s^1.
50 106
452 8300.8900 106 800
103 2650(1 0.4)