1921.] 
Mead.—Impounding-dams for Multiple Functions. 
165 
THE USE OF IMPOUNDING-DAMS FOR MULTIPLE 
FUNCTIONS. 
Bv Arthur D. Mead. B.E. 
A dam may be constructed across a stream for any of several purposes, 
such as city water-supply, improvement of navigation, hydraulic power, 
irrigation, or flood-prevention. Dams are usually costly works, and 
advantage should be taken of favourable conditions, when they occur, for 
more than one function to be fulfilled by the same structure. The present 
discussion does not apply to projects for the diversion of the water (city 
supply, irrigation), but to cases in which the water, after passing through 
the reservoir and attendant works, returns to the stream (power, naviga¬ 
tion, or flood-control schemes). If suitably located and operated, a dam 
may serve no less than five functions. By virtue of its height the dam 
provides—(1) the head, either wholly, or in part if combined with a natural 
fall of the stream, for the power plant ; (2) a pool of deep waiter suitable 
for navigation, specially useful if it can be connected by a flight of locks 
with the lower navigable course ; isolated reaches are not of much value 
for water traffic. Its storage capacity may be utilized to impound water 
during flood and release it in the dry season, thus providing—(3) additional 
power ; (4) improved navigation of the down-stream course in cases where 
trouble arises from shoals in dry w'eather ; (5) reduction of flooding below 
the dam. 
This fivefold effect cannot be always realized, but there are occasions 
when two or more functions might profitably be combined, in which at 
present, owing to the lack of co-operation between various river activities, 
dams are constructed which can serve only a single purpose. The view is 
here advanced that all river-works of magnitude, especially if undertaken 
by the State, should be studied from the wddest viewpoint, and not designed 
for one purpose merely ( e.g ., hydro-electric power). 
In discussing the regulating action of reservoirs on the stream-flow the 
following constants, &c., are useful. Capacity of reservoir is conveniently 
stated in equivalent depth of w r ater in inches on the catchment area, thus 
preserving the relation between watershed area, rainfall, and storage 
capacity. Volume in cubic feet may be obtained by—Capacity (inches) 
X catchment area (square miles) X 2,323,200. Stream-gaugings are usually 
stated in cubic feet per second (cusecs). A useful unit of quantity of water 
passing down a stream is one day’s flow at the rate of one eusec, w'hich is 
86,400 cubic feet. Each inch of rain per annum running off a square mile 
of catchment produces a theoretical flow of 0-0736 cusec at 100 per cent, 
run-off. 
At any given site in a valley of ordinary shape the length and wfidth 
both of the reservoir and of the dam are roughly proportional to the height 
of the dam, so that the volume of the reservoir and of the masonry in the 
dam (considered as an ordinary gravity structure) are proportional to the 
cube of the height. Topography naturally produces variation in individual 
cases, but the rule may be taken as sufficiently close for theoretical dis¬ 
cussion. It may be stated : Volume of water in reservoir = KH 3 , volume 
of masonry in dam £H 3 , where K and h are constants applicable to a 
