Civil Engineering Reference
In-Depth Information
need to be determined on a case-by-case basis before designing a storage reservoir.
As discussed earlier, these criteria apply where source(s) of supply are capable of
meeting the maximum day demand so that storage reservoirs are refilled each day
during daily low-demand periods.
NUMBER AND LOCATION OF RESERVOIRS
Under most conditions, the capital cost to construct a single reservoir to store a given
volume of water is less than the cost to build several smaller reservoirs to store the
same volume. However, the use of several smaller reservoirs to serve different parts
of the system may make it possible to use smaller pipelines, and reduce pumping
heads. In these cases, the overall system cost may be less with several smaller, well-
placed reservoirs rather than a single large reservoir at a central location. Factors to
consider in selecting the location of storage reservoirs are:
Ground elevation
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Effects of location on pressure variations
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Location with respect to the sources of supply and the center of water demands
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Location with respect to pumping stations
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Other issues, such as foundation conditions, visual impacts, and other environ-
mental impacts
•
The location with respect to water demands and pumping stations may influence the
size of an area to be provided with two-directional flow during times of peak water
demands. Meeting peak demands from opposite directions will increase the overall
capacity of the distribution system. Two-directional flow in the main transmission lines
can also increase the volume of water that they can deliver during peak demands.
Developing a two-directional flow in a transmission main generally requires that the
main source of supply pumping station be located at the opposite side of the center
of the water system demand from primary storage facilities. Unfortunately, this ar-
rangement can result in considerable differences in system pressures when source of
supply pumps are on or off. Therefore many primary system reservoirs are located
closer to the center of water demand. This more central location serves to minimize
undesirable pressure variations, but also limits the area capable of receiving flow from
two directions.
The hydraulic grade line necessary to provide service pressures to the area or zone
being served determines the range of elevations of the reservoir. If the topography is
such that the storage reservoir can be located on a hill at the necessary elevation,
considerable savings in construction cost can be realized by reducing the height of the
structure.
Water systems that receive part or all of their supply from a water treatment plant
usually provide part of the necessary system storage at the treatment plant in a clear-
well. If space is available and hydraulics and system hydraulics are favorable, the
treatment plant is often also used for siting a significant storage reservoir.
