New-York tribune., 1918-07-28
81 The Great Workshop of Science q
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voirs are Of two classes: storage reservoirs,
in which the water is held for some later
use such as power, municipal supply, irri-
cation and the like, food control being
rather an incidental than a main purposes
and detention, or retsrdng, reservoirs, with
outlets always open, so that the water de-
tanned during a storm is promptly released
afterward, food-control being in this case
the sole consideration. In the storage
ervor the land occupied is permanently lost
to other use. In the detention reservoir it
remains available for agricultural use.
with the storage reservoir there is gen.
orally conflict Of purpose between food con-
troll and other uses. inasmuch as the quantity
of rainfall in the wet season, when water is
being collected, can never be forecold, it be-
comes important, for storage purposes, tn
fill the reservoirs soon as possible, so
to be sure of supply; while, for food
control it is important to reserve ample
space until the season of storms is safely
past. The conflict can best be harmonized
by building the reservoir so large that il
can safely store a maximum run-off; but
this may greatly increase the cost and may
often be impracticable for lack of site. In
the majority of cases both purposes cannot
be completely satisfied.
"The influence of reservoirs upon flood
tions of the streams which lie Immediately
below the dams. The protection there af-
forded may be absolute. But the effect
diminishes rapidly downstream as addi-
tional tributaries come in, and in large part
disappears on the lower courses of great
rivers like those of the Mississippi basin
This is due in part to the impracticability
OF building reservoirs all the tributaries
but mainly to the impossibility of so manip-
lot tAo,,,
on the main stream. Such floods are the
result Of combinations of floods from many
tributaries. Reservoirs particularly of the
detention type, might release their stored
waters at the wrong time, so far as the
combination below is concerned There can
be no doubt that this would often be the
case, and for that reason any system of
reservoirs which could not hold back the
flood water above them until the season of
Hoods was safely past would be of doubtful
value. The enormous development of tho
system necessary to make it at all effectual
and the difficulty Of so manipulating it as to
reduce tributary combination cause engi-
neers to look with misgiving upon the
scheme as means of controlling the floods
on the lower courses of great rivers like the
Mississippi.
"Flood-protection. This is distinguished
from flood-prevention in that it does not
seek to hold back the run-off, but rather 20
expedite Its progress to the sea, and TO
make the channels carry it without over
flow. The whole purpose is to increase
channel capacity. This is accomplished by
one of the following measures or by a com
olnatlon OF two or more of them:
"(a) By cutting off bends, thus shorten
ing the channel and increasing its slope.
streams may be made to carry more water
Tor the same dimensions. Cut-offs are 4
very important resource on small streams
and even on some large rivers, like those OF
the Hungarian plain. Official sentiment Is
strong against them in the case of the Mis-
slss!ppi, but for other reasons than those of
food-control.
"(b) Enlargement of the channel ITSELF
may be effected by widening (Which is least
desirable because it costs more for right
ofway, requires much excavation and re-
stricts the area of bottom land available for
occupancy; or by deepening, which in
wolves expensive excavation and is likely to
have to be repeated more than once; or.
nnaliy, by enlarging the channel upward
through the building of dikes along the
banks. This is the levee system - the
cheapest, most effective and most univcr-
sally used of all methods of channel en-
largement. .
"(c) On some rivers it seems impossible
ever to confine the greatest floods to the
main channel. In such cases auxiliary food
channels, or by passes, have to be provided
The Sacramento is the most prominent ex-
ample. 1t is considered desirable TO avoid
this method whenever it is possible to do
so, and it has been definitely rejected on
the Mississippi, although it has always had
its advocates there.
"(d) bypass, strictly speaking, returns
to the main stream. An outlet differs from
1T IN leading to the by its own channel.
All deltaic rivers have outlets near their
mouths. From the point of view of naviqa-
tlon they are looked upon with disfavor be-
cause they tend to disspate the energy OF
the current and lead to the deterioration of
the main channel. On the lower Mississipni
one such natural outlet the Atchafalava
has been maintained; all the others have
been closed Surveys have been made for
another in the vicinity of New Orleans. but
official sentiment is generally against them
for that stream"