ML19312C356

Forwards Summary of Hydrologic Engineering Studies Pertaining to Facility Development

ML19312C356
Person / Time
Site:	Oconee
Issue date:	11/15/1972
From:	Cochran A LYLES, BISSETT, CARLISLE & WOLFF
To:	Hulman L US ATOMIC ENERGY COMMISSION (AEC)
References
CON-AT(49-24)-0008, CON-AT(49-24)-8 NUDOCS 7912130886
Download: ML19312C356 (7)

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50-270 50-287 Lf1C&W ASSOCIATES OF SOUTH CAROLINA a 1800 GERVAIS STREET, COLUMBIA. SOUTH CAROLINA 292 November 15, 1972 5$'f Mr. l.. G Hulman /Y ,

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/s Senior Hydraulic Engineer '

Site Analysis Bronch ['

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Deor Mr. Hulman

1. In accordance with contract i AT(49-24)-008, dated 31 October 1972, and j ossociated understandings, I have made hydrologic engineering studies pertaining to the Keowee-Toxoway development of the Duke Power Company's Oconee Nuclear Station in Oconee County, South Carolino. A concise summary of my findings and conclusions is presented herein. Supporting technical details in documented form are available for your review if desired,

2. The assignment related specially to the following:

l o. Computation of probable maximum flood (PMF) hydrogrcphs of inflow into Jocassee and Keowee Reservoin, respectively;

b. Routing of the PMF hydrographs through Jocessee and Keowce Reservoin under critical ossumptions, to determine hydrographs of reservoir stages and outflow rates;

c. Computation of wave chorouteristics and vertical heights of runup on em-bankments of the Jocassee, Keowee and Little River Doms, respectively, that might coin-cide with maximum reservoir stages during the probable maximum floods; '

d. A summary of conclusions regarding estimates referred to above, particularly as they relate to the safety of the dams egoinst failure during^ extreme floods.-

3. The policy concepts, methods, hydrometeorological criterio, and basic flood routing ossumptions adhered to in the assignment conform essentially with those adopted by the Atomic Energy Commission to govem the determination of spillway copocities and freeboard requirements for very large doms, generally as summarized in the February 1972 draft,' Standard Format and Contents of Safety Analysis Reports for Nuclear Power Plo,t.:.

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Mr. L. G. Hulman Page 2 November 15, 1972 i i

PERTINENT DATA

4. Following is a brief summary of physical features that relate to the studies:

a. Keowee Lake, controlling 439 se,oore miles, is formed by a 150-foot high earthfill dom on the Keowee River and a l'0-foot high earthfill dom on Little River; o

" connecting canal" joins lakes formed by thue two doms, and is large enough to allow the two lakes to act as one. The Keowee and Little River Doms each have top eleva-tions of 815' msl. On each dom the upstream embankment is surfaced with dumped rock for erosion protection above elevation 772' mst, which has a slope at l' V to 2.0' H.

The downstream embankment has a slope of l' V to 2.5' H.

b. The 385-foot high rockfill Jocessee Dom, located about 12 miles upstream from Keowee Dam, controls 148 square miles of the total 439 square mile basin above the Keowee-Little River Doms. The top elevation of the embankment is 1125' msl.

Upstream and downstream rockfill embankments each have a slope of l' V to 1.75' horizontal in the upper 100 feet of elevation, which includes the zone considered in i estimating potential wave runup effects.

c. Keowee Lake has a gross storage capacity of 956,000 acre-feet at a normal full-pool elevation of 800' msl, with a surface creo of 18,372 acres (29 square miles).

The Jocassee Reservoir has a gross storage capacity of 1,160,000 acre-feet at a normal full-pool elevation of 1,110 feet msi, and a surface creo of 7565 acres (12 square miles).

d. The Keowee Dom spillway consists of four tainter gates, 38' W x 35' H, with a crest elevation of 765' mst and a total discharge capacity of 106,000 cubic feet per second at a normal full pool level of 800' msl. The Jocessee spillway has two tainter gates, 38' W x 33' H, crest elevation 1077' msl, with a combined capacity of 46,200 cfs at a normal full-pool level of 1,110 msl.

e. In addition to spillways, the Keowee and Jocessee Doms have power tur-bines ccpoble of discharging substantial quantities of water. Operating plans for Jocassee Dom provide for releases up to 15,000 cfs through power turbines to augment spillway discharges during floods, if needed. However, such turbine operations could be precluded by interruptions in power loadings or for other reasons under emergency conditions asso-ciated with extreme floods. Accordingly, in the studies reported herein, releases through power turbines were assumed as zero in routing the Probable Maximum Flood through the reservoirs.

i November 15,1972 Mr. L. G. Hulman Page 3 CRITICAL FLOOD ESTIMATES

5. Probable Maximum Precipitation (PMP) estimates for the 439 square mile basin above  !

Keowee Lake, and relevent sub-division thereof, were obtained from Hydrometeorological l Report No. 33 of the U. S. Weather Bureau (now NOAA). Alternative areal distributions of j PMP quantities were tested to develop critical flood producing relations. The Probable Maximum Flood from the 148 square mile basin above Jocassee Dom would result when the heaviest PMP concentration occurred over this area; the critical PMF hydrograph at Keowee Dom would result with the heaviest PMP omounts concentroted over the 291 square mile intermediate creo between Jocessee and the dams forming Keowee Lake. PMF hydrographs were computed by application of synthetic unit hydrographs to estimates of PMP rainfall-excess assuming on infiltration index of .05 inch per hour. The synthetic unit hydrographs were derived for component drainage creas tributary to full reservoirs to account for the accelerating effects of unusually lcrge water surfaces of lake Keowee and Lake Jocessee at normal full pool elevations, which represent more than 9 percent of the total 439 square mile drainage area. The selection of coefficients used in developing synthetic unit hydregraphs were based on studies cf unit hydrographs derived from analyses of major floods.

6. The computed PMF hydrograph of inflow into Jocessee Lake had a peak of 245,000 cfs, and a runoff volume of 210,000 acre-feet (26.6 inches runoff from 148 sq. mi.).

Assuming the reservoir would be filled to elevativ i 1,110' nsi at the beginning of the PMF, and all releases made through the two spillway gates, a peak reservoir stage of 1,122.5' msl was computed; stages exceeding 1,119.7' msl would prevail for 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. The peak rate of reservoir outflow through the spillway would be 72,000 cfs.

7. The computed PMF hydrograph of inflow into Lake Keowee had a peak of 450,000 cfs, and a runoff volume of 550,000 acre-feet (23.5 inches runoff from 439 square miles).

Assuming the reservoir would be initially filled to elevation 800' mst, and all releases made through the four spillway gates, o peak reservoir level of 809.8' msl was computed; levels exceeding 806.6' mst would prevail for 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. These values are predicated on the )

assumption that all concurrent releases from Jocassee Ressivoir are made through the spill- l l

way, without flows through power turbines.

FREEBOARD FOR WAVE ACTION

8. Following is a review of the apparent odequacy of existing dams to safely accom-modate wave action in Lake Jocassee and Lake Keowee in the event high winds blowing toward the dams should prevail for several hours while reservoir levels are equal to or near the maximum elevations indicated for PMF conditions. Relevant procedures and computo-tional aids contained in publications by the Army Corps of Engineers (EC 1110-2-27, and ETL 1110-2-8, dated 1 August 1966) were used in the analyses.

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Mr. l.. ' G. Hulman Page 4 November 15, 1972

9. Wind records and analyses show that wind velocities as high as 35 to 40 miles per hour over-land, for durations of 1 to 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, may occur infrequently in the vicinity of Lake Jocos:ce and Lake Keowee. Research studies indicate that over-land wind velocities of 35 to 40 mph would accelerate approximately 25 percent over the open water surfaces near the dams - that is to over-water velocitics of 44 to 50 mph. Whether or not such winds raight coincide with peak reservoir levels during the PMF, and be oriented over the

" effective fetch" in a critical direction toward the respective doms, is largely a matter of conjecture, in general, studies show that wind directions tend to change substantially over local orcos os storm centers move, even though velocities in various directions may persist at high rates for several hours.

In general recognition of the improbability of the most critical wind velocity-duration-direction relations coinciding with maximum reservoir levels, the publications cited above provide for adoption of " design wind" criteria that are con-sidered reasonable on the basis of avoilable dato and design objectives involved. Estimates bmod on these criteria are used as aids to over all judgement of possible wave effects on ,

project features. In the instant case, o " design wind" corresponding to on over-land '

velocity of 25 mph for a period of 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> wcs considered reasonable in estimating heights of runup that might be expected during the PMF. However, the possible effects of wave action that could conceivably result from winds equal to 40 mph over-land were also considered to ossure that hozords from possible breaching of the dams from wave erosion would not exist even during extreme conditions. (Wind velocities cited horcin refer to "over-land" rates; however, corrections for velocity increases over water have been accounted for in wave 1 computations).

10. The Jocassee Dom top elevation of 1125' msl provides a freeboard of 2.5' above the computed maximum reservoir level during the PMF elevation 1122.5' msl. Computations indicate that a sustained wind velocity equivalent to 25 mph over-land, acting on a 2.5 mile

" effective fetch," could result in "significant waves" (hs) 1.9 feet high, which would break and run up the face of Jocassee Dom to a vertical height of opproximately 2.5 feet above reservoir leve'ls prevailing during the PMF; a negligible amount of wave splash or over-wash  !

from waves exceeding hs might pass over the crest of the dom. For corresponding conditions o sustained wind velocity equal to 40 mph over-land would produce runup upproximately 1.5 feet higher, and moderate amounts of wove-splash and wave over-wash might pass over the crest of the dom foro period of a few hours, ,

in view of the chorocteristics of the rockfill  !

embankments of Jocessee Dom, it is concluded that this wave action would not be sufficient to represent any risk of breaching of the embankment of Jocassee Dom.

11. The_ Keowee Dum top elevation of 815' mst providcsa freeboord of 5.2 feet above the computed maximum rese voir level during the PMF elevation 809.8' msl. A sustained wind velocity comparabic to 40 mph over-land, blowing toward the dum over on effective fetch of 2.2 miles would produce significant waves 3.2'. high, capable of running up '

4.0 feet on the riprop embankment (slope 1:2); the maximum wave in a spectrum of 100 waves would run~ up about one foot higher. Accordingly, computations indicate that Keowee Dom is high enough to prevent wave over-wash under the most cirtical PMF conditions. The some i

conclusion is applicable to Little River Dom, where the effective fetch _(1.9 miles) is less than for the'Keowee Dom.

4 Page 5 November 15, 1972 Mr. L. G. Hulman

CONCLUSIONS  !

12. Procedures and Criteria. The policy concepts, methods, hydrometeorological criteria, and basic flood routing assumptions used in the subject studies are consistent with sound engineering practices associated with the design of very large dams in the 11nited i States; they foster a safe degree of conseivatism in evaluations pertaining to the projects covered by this report.

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13. Jocassee Dam and Reservoir.

a. The Probable Maximum Flood hydrograph of inflow into take Jocassee would l

have a peak discharge of approximately 245,000 cfs, and a runoff volume of 210,000 acre-feet.

b. A maximum reservoir level of 1,122.5' mst c.ould be attained in Lake Jocassee during the PMF under the most adverse circum tances considered reasonably possible.

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c. The top clevation of 1,125' mst of Jocassee Dam provides a freeboard allowance for possible wave runup on the rockfill embankment equal to 2.5 feet above the peak PMF l j reservoir level (1,122.5' msl) estimated herein. It is remotely possible that sustained wind velocities (equal to 25 to 40 mph over land), blowing toward the dam could cause wave l j runup and some wave over-wash of the Jocassee embankment for a few hours during the PM However, the rockfill composisjon of the dam embankment is such as to preclude breaching of l the embankment from wave wash of thogeneral magnitude indicated. j 1

14. Keowee and Little River Doms. I c.

The Probable Maximum Flood Hydrograph of inflow into Lake Keowee would i have a peak discharge of approximately 450,000 cfs and a runoff volume of 550,000 acre-feet.

b. A maximum reseivoir level of approximately 810.' mst could be attained in Lake Keowee during the PMF under the most critical circumstances considered reasonably possible.

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.. .o Mr. L. G. Hulman Page 6 November 15, 1972

c. It is remotely possible that sustained wind velocities (40 mph or less over land),

blowing toward the dams during the PMF could cause wave runup on the riprop covered face of each dam opproaching crest elevation 815' msl. '

ff ALBERT L. COCHRAN d ^

Director of Special Projeurs Lyles, Bissett, Carlisle & Wolff ALC:ad

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