Page 47 - Water Engineering Hydraulics, Distribution and Treatment

Reservoir drawn down
Depletion
of
of
storage
storage
600
full
500
Cumulative runoff and draft (MG) 700 period at point Reservoir full 1. Draw cumulative draft Σ D Reservoir empty Replenishment Reservoir 2.4 Design Storage 25
Must intersect
parallel to rate of draft A
runoff curve,
and tangent to curve B
400
if reservoir
is to refill
3. Measure maximum
Curve B.
Start of dry
deficiency cumulative
Cumulative
ΣD–ΣQ = 124 MG
300
runoff, ΣQ
of tangency
Storage, S
End of dry
200 ΣQ–ΣD = 124 MG period at point
of tangency
2. Draw parallel to line A
and tangent to curve B
100 Line A, slope = rate of draft
Must intersect runoff curve,
if reservoir is to be full
at start of dry period
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Order of months
Figure 2.7 Mass diagram method for the determination of storage required in impounding reservoirs.
2
2
(A constant draft of 750,000 gpd/mi = 23 MG/mi for a month of 30.4 days is assumed.) Conversion
2
2
2
factors: 1 MG = 1,000,000 gal = 3.785 ML = 3,785,000 L; 1 MG/mi = 1.461 ML/km ; 1 gpd/mi =
2
1.461 L/d/km .
intervals of time—from monthly to daily values, for exam- generally suffices in the well-watered regions of the United
ple. The additional storage identified by such a shift may be States. Water is plentiful, stream flows do not vary greatly
as much as 10 days of draft. from year to year, reservoirs generally refill within the annual
Assuming that inflow and drafts are repeated cyclically, hydrologic cycle, and it does not pay to go in for advanced
in successive sets of T years, a procedure called sequent or complete development of catchment areas. In semiarid
peak was developed for determining minimum storage for no regions, on the other hand, water is scarce, stream flows fluc-
shortage in draft based on two needed cycles. Example 2.1 tuate widely from year to year, runoff of wet years must be
illustrates the procedure. conserved for use during dry years, and it pays to store a large
For variable drafts and inclusion of varying allowances proportion of the mean annual flow. In these circumstances,
for evaporation from the water surface created by the operational records of adequate length become important,
impoundage, the analytical method possesses distinct along with computational aids.
advantages over the graphical method. The principal value Given a series of storage values for the flows observed or
of the mass diagram method, indeed, is not for the estimation generated statistically, the engineer must decide which value
of storage requirements, but for determining the yield of he will use. Will it be the highest on record, or the second,
catchment areas on which storage reservoirs are already third, or fourth highest? Obviously, the choice depends on the
established. degree of protection to be afforded against water shortage.
This must also be considered in terms of drought experience,
which is a function of the length of record examined. To
2.4 DESIGN STORAGE
arrive at a reasonable answer and an economically justifiable
Except for occasional series of dry years and very high den- storage design, the engineer may resort to (a) a statistical
sity housing and industrial developments, seasonal storage analysis of the arrayed storage values and (b) estimates of