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data of Bulletin D are only indicative, and not comprehensive or
conclusive.

This fact is illustrated further by the record of an excessive rain-
fall at Ealeigh, N. C, on Tuesday, July 14th, 1914, which has been
plotted on Fig. 9. It should be remembered, of course, that records
of excessive rainfall such as that at Raleigh are not of controlling
importance in the present case, because they are not of sufficient
duration to tax the equalizing capacity of the reservoir.

On Fig. 9 there has also been plotted a curve (6), derived from
the graph shown on Fig. 12, on the assumptions that during the 90
min. of greatest run-off, in the case of the Cane Creek flood, the rain-
fall likewise was at its maximum, and that during such period the
run-off was equal to the rainfall, variations being simultaneous.
These assumptions, of course, are not strictly true, as the maximum
rate of run-off could not have been equal to the maximum rate of
rainfall. Necessarily, there must be some equalization, and it is evi-
dent from an inspection of the Cane Creek flood curve (6) of Fig.



EECONSTKUCTION" OF THE STOXY RIVER DAM



929



9 that in this instance there must have been a considerable difference
between the maximum rates of run-off and rainfall.

Yet, even allowing for a reasonable equalization or smoothing off
of the peak of the rainfall, as compared with the peak of the run-off,



DURATION OF MAXIMUM INTENSITIES OF
RAINFALL IN UNITED STATES

CaTve(a) For authority see p. 5S, Bulletin D,
U.S.W.B. 1897. Records probably
cover only years 1S93-96, inclusive.
Curve (^S.) Cased on assumption that maximum
rainfall was equal to maximum
run-off.

Curve fC^ Assumed maximum flood for Stony
River, W. Va. Drainage area
representing conditions 50 per cent
more severe than those of (b)

Curve CdMeported through U.S.W.B. office
at Pittsburgh, Pa.




10 15 20 25



30 35 40 45 50 55 60
Duration of rainfall, in minutes.

Fig. 9.



it is apparent that the rainfall which caused the Cane Creek flood
was by no means as great as may be expected in either the Cane Creek
or Stony Eiver localities. This conclusion is confirmed by a study of
the total rainfall in 24 hours.



930 EECONSTRUCTION OF THE STONY RIVER DAM

The total run-off, as shown in Fig. 12, for the 24-hour period begin-
ning at 7 p. M., May 19th, 1901, is equivalent to a depth of 7.48 in.
over the whole Cane Creek drainage area of 22 sq. miles. For the
same period it is reported that records at precipitation stations rea-
sonably near the Cane Creek water-shed show a total of 8 in. of rainfall.
(The fact that these two figures check so closely indicates that the
estimate of the Cane Creek flood discharge is reasonably accurate.)

As compared with this 24-hour rainfall, there were available 24-hour
rainfall records at other points in the United States east of the Missis-
sippi River which were instructive with reference to the problem, as
follows :

Carlisle, Pa 9.35 in. August 26th-27th, 1899.

Newton, Ala 10.29 " March 22d, 1897.

Wheeler, Ohio 10.47 " May 16th-17th, 1893.

Horse Cove, K C. 11.00 " October 3d-4th, 1898.

Morgan, Ga 11.52 " March 21st-22d, 1897.

Manning, S. C 13.22 " August 27th-28th, 1893.

Falkland, N. C 13.55 " August 3d-4th, 1894.

Jewell, Md 14.75* " July 26th-27th, 1897.

Sebastian, Fla 19.08 " October 2d-3d, 1899.

The foregoing records are by no means complete, inasmuch as
they refer only to the period, 1891-1899, inclusive. On the basis
of these records, and taking the geographical position and topography
into account, it appeared proper to assume that the greatest flood
reasonably to be provided for at Stony River would be the result
of a total rainfall of about 12 in. in 24 hours on the drainage area
above the dam site.f

Spillway Capacity Provided in the Reconstruction. — The foregoing
considerations having made it evident that additional spillway capacity
must be provided, the volume of such additional capacity, and the

* Reported actually to have occurred within 18 hours.

t Since the reconstruction, the following pertinent data have come to the atten-
tion of the writer :

(1) A run-off having a rate between 2 000 and 2 333 sec-ft. per sq. mile is
reported to have occurred over a drainage area of about 1 Va sq. miles near
Le Roy, N. Y., in May, 1916. Engineei-ing Neics, June 24th, 1916, p. 842.

(2) For July 8th, 1916, Professor Alfred J. Henry, U. S. Weather Bureau,
reports that tlie area (Alabama and Georgia) covered by a 24-hour rain-
fall of 8 in. and more was about 4 945 sq. miles. Monthly Weather Review,
August, 1916, p. 467.

(3) A 24-hour rainfall of 22.22 in. is reported to have occurred on July 15th-
16th, 1916, at Altapass, Mitchell County, N. C. (Elevation 2 625). Ibid,
p. 467.

(4) It is reported that rain fell at the rate of 5.21 in. per hour for 25 min. at
Mobile, Ala., on July 8th, 1916. Ibid, p. 468.



PLATE X.
TRANS. AM. SOC. CIV. ENORS.
VOL. LXXXI, No. 1397.
SCHEIDENHELM ON
RECONSTRUCTION OF
STONY RIVER DAM.



TYPICAL SECTION

OF

NEW SPILLWAY



/ Minimum deptb of concrete et xtai

portion of BpfUway bucket -= 2 ft.




CoDStruction joint where neceBBaxf



Footings have minimum depth of 2 ft.



.xect .ov



RECONSTKUCTION OF THE STONY RIVER DAM 931

means of providing it, remained to be determined. ISTo attempt
will be made in this paper to discuss the alternatives considered for
increasing the original spillway capacity of the dam, for such alter-
natives differed essentially only as to the extent to which the several
available factors were utilized. Instead, the conditions caused by the
maximum flood within the limits of reason will be applied to the
means finally adopted for increasing the spillway capacity, in order
to test the adequacy of these means. The adopted spillway pro-
vision, which will be understood by reference to Plate IX, was obtained
by the following means :

1. — Reconstructing that portion of the original bulkhead section
of the dam between Buttresses 10 and 19 to form a second
spillway. The transverse cross-section of this new spillway
is shown in Plate X. It was provided with a suitable
reinforced concrete channel mat extending about 85 ft. down
stream from the bucket of the spillway apron.

2. — Adding 3 ft. 6 in. to the height of the original bulkhead por-
tions of the dam outside of the spillways, viz., east of the
old spillway and west of the new spillway. This was accom-
plished by adding a parapet, of the form shown in outline in
Plate XII, with its top at Elevation 142.5.

3. — Increasing the height of that portion of the original bulk-
head section of the dam between Buttresses 19 and A (viz.,
between the old and new spillways) by the addition of a
parapet 1 ft. high. This forms an intermediate spillway with
its crest at Elevation 140, as shown in the typical sections
of Plate XII.

4. — Utilizing the horizontal upper member of the new anchoring
wall at the toe of the dam, between the old and new spill-
ways, to form a reinforced concrete mat or "tumbling hearth"
to receive the impact of the sheet of water falling well-nigh
vertically from the intermediate spillway. This horizontal
mat, together with an inclined, reinforced concrete, protec-
tion mat (section B-B of Plate XII), forms a channel sloping
downward from the ends of the intermediate spillway section
toward, and discharging through, a new outlet channel which
extends down stream from Gate-bays 30 and 31.



932 EEC0NSTKUCTI03ST OF THE STONY EIVER DAM

5. — Improving the shape of the crest of the original or "old"
spillway. This warranted the assumption of a somewhat
higher coefficient of discharge, as applied in the Francis
formula.

Under the original conditions, disregarding wave action, over-
topping of the dam would have occurred when the water level in
the reservoir had reached Elevation 139; whereas, under the condi-
tions obtaining after reconstruction, overtopping cannot occur until
the water level reaches Elevation 142.5. It is probable that over-
topping due solely to wave action would be of minor consequence.
However, as will presently be shown, the water level due to the great-
est flood reasonably to be provided for would probably leave a margin
of about 6 in. below Elevation 142.5.

Maximum Flood Within Limits of Reason. — For the purpose of
studying the effect of such a flood, in the case of the Stony River
Dam and Reservoir, let it be assumed that it would have a graph
similar to that shown in Fig. 13. This graph was derived by increas-
ing the rates of run-off shown in Fig. 12 by 50%, thus corresponding
to the ratio of the estimated maximum 24-hour precipitation to the
8-in. precipitation reported to have occurred within a like period in
the case of the Cane Creek flood. It is true, of course, that no two
rain storms or floods are exactly alike; yet, for the purpose in mind,
the foregoing ass^umption appeared to afford a reasonable basis for
design.

For purposes of comparison, there has been plotted on Fig. 9 a
curve (c) showing the rainfall for the critical 90-min. period of such
assumed greatest possible flood discharge, the curve being based, as
before, on the assumption (not strictly true) that the rate of rain-
fall would be equal to the rate of run-off. Even allowing for the
evident equalization, or smoothing-off of the peak of the rainfall, it
is apparent that a rainfall causing a flood such as that represented
by the curve (c) would not equal certain actual records of rainfall.

Equalization Ejfect of Reservoir. — In studying the results of such
floods as those plotted on Figs. 12 and 13, when applied to the Stony
River drainage area and spillway provision, it may reasonably be
assumed that in each instance there would be, initially, one point of
time when, for all practical purposes, a state of equilibrium would exist.



EECONSTRUCTTON OF THE STOXY RIVER DAM 933

At such a time the total ruii-oii from the drainage area and the dis-
charge over the spillway would be equal. Thereafter, as the run-off
increased, a certain quantity of water would be stored over the whole
reservoir area for each foot of rise in water level. The extent of such
equalization or absorption would depend on the spillway capacity and
the reservoir area.

The capacity of the reservoir above the elevation of the lower
spillway crests is shown on Fig. 10. On Fig. 11 is plotted the spill-
way capacity (after reconstruction), derived from the data and
assumptions there shown.

On the basis of these diagrams, the equalization effect of the reser-
voir has been studied with reference to a number of flood conditions,
as follows :

I. — Before considering the greatest flood reasonably to be antici-
pated, it appears proper to study the equalization effect of the reser-
voir on a flood exactly similar, and equal in intensity, to that reported
for Cane Creek on May 19th-20th, 1901. The result of such study
is shown in Fig. 12. The spillway crests are assumed to be without
flash-boards.

The method of deriving the curve showing the discharge over the
spillways is as follows: Referring to Fig. 12, it is assumed that at
7 A. M. the total run-off from the drainage area and the discharge
over the spillways are equal. The graph of the flood then shows that
at this time the run-off would be approximately 100 cu. ft. per sec,
which (from Fig. 11) would require the reservoir water level to be
at Elevation 136.2. The effect of the storage capacity of the reser-
voir at increasing elevations of head-water is then determined at arbi-
trary intervals, depending on the refinement desired. Selecting a
0.8-ft. rise in water level as the first interval, it is found from the
reservoir capacity curve of Fig. 10 that for this interval the reservoir
will absorb approximately 39 000 000 gal. This quantity, when con-
verted into the proper units and scale, is represented in Fig. 12 by
the area, a-h-c, viz. : The point, 6, of course, must have an ordinate
representing the spillway discharge at Elevation 136.5, approximately
365 cu. ft. per sec. (from Fig. 11). The position of the line, h-c, is
then determined by trial, the required area, a-b-c, having been deter-
mined as above stated.



934



KECONSTRUCTION OF THE STONY RIVER DAM



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KECONSTRUCTION OF THE STONY EIVER DAM



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Online LibraryAmerican Society of Civil EngineersTransactions of the American Society of Civil Engineers (Volume 81) → online text (page 76 of 167)