Page 77 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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40 Chapter 2 Water Sources: Surface Water
Individual factors vary within the annual hydrologic cycle and from year to year. They
can be measured. Exact calculations, however, are commonly handicapped by inadequate
data on evaporation. Required hydrological information should come from local or nearby
observation stations, areas of water surface being determined from contour maps of the
reservoir site. The mean annual water surface as a fraction of the reservoir area at the spill-
way, f, is normally about 0.90 or 90%.
For convenience, the water-surface response is expressed in one of the following
ways:

1. Revised runoff: Q Q (Q E R)(fa>A) (2.2)
r
2. Equivalent draft: D (Q E R)(fa>A) (2.3)
e
3. Effective catchment area: A A fa[l (R E)>Q] (2.4)
e
4. Adjusted flow line F Q E R (2.5)
5. Effective draft D D md D (A) (2.6)
e
ed
where Q revised runoff, in./year or cm/year; Q mean annual runoff, in./year or
r
cm/year; R mean annual runoff, in./year or cm/year; E mean annual evaporation,
2
2
2
2
in./year or cm/year; a reservoir area, mi or km ; A catchment area, mi or km ;
f 90% 0.9 effective factor for reservoir area; D equivalent draft, in./year or
e
cm/year; F adjusted flow line at spillway level, in. or cm; A effective catchment
e
2
2
area, mi or km ; D effective draft, MGD or MLD; and D md mean draft, MGD
ed
or MLD.
The values of A (total catchment area) and a (reservoir surface area) obtained are used
as follows in recalculating storage requirements: Q replaces Q; D D replaces D; and A e
r
e
replaces A. A fourth allowance calls for raising the flow line of the reservoir by Q E R,
expressed in units of length yearly. In rough approximation, the spillway level is raised by
a foot or two in the eastern United States.
EXAMPLE 2.3 CALCULATIONS OF MEAN ANNUAL RUNOFF AND DRAFT
A mean draft of 30.0 MGD (113.6 MLD) is to be developed from a catchment area of 40.0 mi 2
2
2
(103.6 km ). First calculations ask for a reservoir area of 1,500 acres (6.07 km ) at flow line. The
mean annual rainfall is 47.0 in./year (119.4 cm/year), the mean annual runoff is 27.0 in./year
(68.6 cm/year), and the mean annual evaporation is 40.0 in./year (101.6 cm/year). Find the
following:
1. The revised mean annual runoff
2. The equivalent mean draft
3. The equivalent land area
4. The adjusted flow line

Solution 1 (U.S. Customary System):
1. By Eq. 2.2, the revised annual runoff is:
Q r Q (Q E R)(fa>A)
Q r 27.0 (27.0 40.0 47.0)[(0.9 1,500>640)>(40.0)] 27.0 1.1
25.9 in./year
2
Here, 1 mi 640 acres.