Page 78 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch02_029-060.qxd 8/2/10 9:14 PM Page 41

2.5 Loss by Evaporation, Seepage, and Silting 41

2. By Eq. 2.3, the equivalent mean draft is:
D e (Q E R)(fa>A)
D e (27.0 40.0 47.0)[(0.9 1,500>640)>(40.0)] 1.1 in./year
2
D e 1.1 in./year 52,360 gpd/mi 0.052 MGD/mi 2
and the effective draft D ed is
2
2
D ed D md D e (A) 30.0 MGD (0.052 MGD/mi )(40.0 mi ) 32.1 MGD
3. By Eq. 2.4, the equivalent land area is:
A e A fa[l (R E)>Q]
A e 40.0 (0.9 1,500>640)[1 (47.0 40.0)>27.0] 40.0 1.6 38.4 mi 2
4. By Eq. 2.5, the adjusted flow line is:
F Q E – R
F 27.0 40.0 47.0 20 in, equaling 20 0.9 18 in. at spillway level
Solution 2 (SI System):
5. By Eq. 2.2, the revised annual runoff is:
Q r Q (Q E R)(fa>A)
Q r 68.58 (68.58 101.60 119.38)[(0.9 6.07)>(103.6)]
68.58 2.68 65.9 cm/year
6. By Eq. 2.3, the equivalent mean draft is:
D e (Q E R)(fa>A)
D e (68.58 101.60 119.38)[(0.9 6.07)>(103.6)] 2.68 cm/year
2
D e 2.68 cm/year 0.07337 MLD/km and the effective draft D ed is
2
2
D ed D md D e (A) 113.55 MLD (0.07337 MLD/km )(103.6 km ) 121.2 MLD.
Here, 1 cm/year 0.0273793 MLD/km 2
7. By Eq. 2.4, the equivalent land area is:
A e A fa[l (R E)>Q]
A e 103.6 (0.9 6.07)[1 (119.38 101.60)>68.58] 103.6 4.47 99.13 km 2
8. By Eq. 2.5, the adjusted flow line is:
F Q E – R
F 68.58 101.60 119.38 50.8 cm, equaling (50.8 cm) 0.9
45.7 cm at spillway level

2.5.2 Seepage
If the valley enclosing a reservoir is underlain by porous strata, water may be lost by seep-
age. Subsurface exploration alone can foretell how much. Seepage is not necessarily con-
fined to the dam site. It may occur wherever the sides and bottom of the reservoir are suf-
ficiently permeable to permit water to escape through the surrounding hills.

2.5.3 Silting
Soil erosion on the watershed causes reservoir silting. Both are undesirable. Erosion de-
stroys arable lands. Silting destroys useful storage (see Fig. 2.9). How bad conditions are
in a given catchment area depends principally on soil and rock types, ground-surface
slopes, vegetal cover, methods of cultivation, and storm-rainfall intensities.