Civil Engineering Reference
In-Depth Information
120,000
100,000
80,000
Image Not Available
60,000
40,000
20,000
0
Fig. 8-4.
Mass curves of water utilization and weekly flow (From Davis, C. V.,
Handbook of
Hydraulics,
5th ed. Copyright
1973 by the McGraw-Hill Companies. Reproduced by permission
of the McGraw-Hill Companies)
4
January 31, 74,000 acre-feet (91.28 Mm
3
). The reservoir was full again on May 4,
when the two curves intersected.
The mass curve is plotted from the hydrograph of reservoir inflow. In most cases,
this is taken as the hydrograph at the dam site.
Area-Volume Relationships
The shape of the reservoir site, the width of the valley floor, and the steepness of the
adjoining hills will determine how much storage can be provided and how much
reservoir surface area will be created for a given volume of storage. Through analysis
of the contour lines on topographic maps, it is possible to develop the relationships
between reservoir surface elevation, reservoir area, and volume of storage for a given
site. These relationships are normally presented in the form of tables, but can also be
presented graphically as in Figure 8-5. The arrangement shown in Figure 8-5, with
two lines plotted in opposite directions, is used so that the curves do not overlap and
can be easily read.
It is usually desirable that the reservoir have as little surface area as possible in
relation to the volume of storage because land costs and natural evaporation losses
will be proportioned to the surface acreage. Alternative sites can be compared by
plotting their respective area-versus-capacity curves on the same graph and selecting
the one that offers the most storage per unit of area.
