ML19323F222

Forwards Response to NRC 800505 Request for Addl Info Re Floodplain Mgt at Facility

ML19323F222
Person / Time
Site:	North Anna
Issue date:	05/23/1980
From:	Brown S VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	Harold Denton, Youngblood B Office of Nuclear Reactor Regulation
References
422A-050580, 422A-50580, NUDOCS 8005280669
Download: ML19323F222 (9)

Text

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} Um mh uOpO vina,y,,, c ccynic aun ro.rn econ sv. nic ve~o. s in aisi a ;3:s-Mdy 23, 1980 Mr. Harold R. Denton, Director Serial flo. 422A/050580 Of fice of fluclear Reactor Regulation PSE&C/CES: bat Attn:

Mr. B. J. Youngblood, Chief I.icensing Branch 1 Docket No. 50-339 Division of Licensing U. S. Nuclear Regulatory Commission License No. NPF-?

Washington, D.C. 20555

Dear Mr. Denton:

We have received Mr. Youngblood's letter, dated May 5,1980 requesting additional information concerning floodplain management at the North Anna Power Sta tion. Enclosed, please find a report titled " Response to f1RC Request for Addi-tional Information (Dated May 5,1980) for the Review of the fiorth Anna Power Sta-tion Unit 2 Operating License Application -

Subject:

E.0. 11988 Floodplain Manage-ment." The responses contained in the report are listed by number and correspond to the numbered items in the request submitted with the May 5,1980 letter.

If you have any ~ estions relative to the information contained in the report or if additional information is required, please contact us.

VerytruYyours,

/$yh tem SamC./

pv Brjwn, Jr.

Senior Vice President - Power Station Engineering & Construction 80052805 6 a

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RESPONSE TO NRC Request for Additional Information I

(dated May 5, 1980)

For the Review of the North Anna Power Station, Unit. 2 Operating License Application

SUBJECT:

E.O. 11988 Floodplain Management 4

Prepared for a

VIRGINIA ELECTRIC AND POWER CO.

i RICHMOND, VA Prepared by i

STONE & WEBSTER ENGINEERING CORP.

BOSTON, MASS.

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1 RESPONSE TO NRC REQUEST FOR ADDITIONAL INFORMATION DATED MAY 5, 1980 Introduction i

Lake Anna is a man-made lake formed in 1972 by impounding the North Anna River at a location approximately five miles downstream of the North Anna Power Station.

The lake is utilized by the North Anna Power Station as a cooling pond for condenser circulating water.

In order to improve the thermal performance of the lake, it has been divided by a series of dikes and canals into two parts. The larger, referred to as the reservoir, has a surface area of approximately 9,600 acres and receives rainfall runoff from approximately 297 sq mi as well as the inflow due to rain falling directly onto its surface. The smaller part, called the Waste Heat Treatment Facility (WHTF), has a surface area of approximately 3,400 acres and receives rainfall runoff from approximately 26 sq mi as well as inflows due to rain falling directly on its surface. The power station withdraws circulating water from the reservoir and discharges it to the WHTF.

Water passes from the WHTF to the reservoir through submerged orifices in one of four dikes between the two bodies of water. The crests of the dividing dikes are at elevation 260 ft above mean sea level, with the exception of the most downstream dike (the dike with the orifices) which is constructed with a saddle at elevation 253.5 ft to allow the passage of flood flows.

Discharges from the North Anna reservoir are regulated by a concrete gravity spillway structure utilizing two eight foot wide overflow skimmer gates and three forty-foot wide radial gates.

During non-flood flow conditions the level of the reservoir is held at elevation 250.0 ft.

With the plant n

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operating, the WHTF will reach a maximum water surface elevation of 251.5 ft (also under non-flood flow conditions).

The following is offered in response to the NRC request for additional information concerning the floodplain of the one percent chance (100-year recurrence interval) flood for Lake Anna. The responses are listed by number and correspond to the numbered items in the NRC request.

1.

The floodplain of Lake Anna and the North Anna River above the site varies from gently to steeply sloping with wooded as well as cleared land. Within or adjacent to the site, the floodplain consists of mildly sloping wooded and cleared areas with much of the area within the site protected by gravel or riprap cover.

The land area flooded by the 100 yr storm is quite narrow in the site area as shown in Attachment A.

This figure, which is an enlargement of a USGS quadrangle sheet, shows the floodplain for the estimated peak elevation of the one percent chance flood of 253.4 ft for the reservoir and 254.9 ft for the WHTF. The approximate limit of the floodplain is shown as a dashed line on the attachment.

The one percent chance flood flow and water elevation were determined by performing a hydrologic analysis similar to the analysis performed for the Probable Maximum Flood (PMF) for Lake Anna.

Details of the PMF analysis including a complete description of the computer model developed for Lake Anna can be found in Appendix J to the North Anna Power Station FSAR. A brief summary of the one percent chance flood analysis is provided below:

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In response to project requirements in the past, flood levels in Lake Anna were determined for floods of various recurrence intervals.

The analysis utilized two computer models: The U.S. Army Corps of Engineers Flood Hydrograph computer program HEC-1 and a slightly 4

modified version of the computer model of Lake Anna presented in Appendix J to the FSAR.

Precipitation quantities covering a range from the 2-year to the 500-year (1/2 the Probable Maximum Precipitation) recurrence interval were used as input to HEC-1.

This program generated inflows to Lake Anna by combining the precipitation with unit hydrographs for the drainage basin for the lake. The values of input parameter constants for the ifEC-1 program for the different precipitations were based on the reaction of the lake and drainage basin during historical storms as well as previous experience with the model.

The inflows to Lake Anna from the precipitation quantities were routed through the lake using the Lake Anna computer model. The operating rule curve for the dam was the same in all cases and conforms to the rule curve used for the PMF.

Each recurrence interval precipitation produced a corresponding peak water elevation in the lake. These elevations were plotted against the percent chance of the particular precipitation occurring in any given year. The peak water surface elevation of 253.4 ft for the one-percent chance flood is taken from a curve drawn through all plotted points. The value of 253.4 ft is for the entire lake, including the WHTF, assuming the plant is not operating.

Since the WHTF will be up to 1.5 ft higher than the lake during 3

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i plant operation, this value has been added to ti.e above elevation to produce an estimated peak water surface elevation for the 100-year flood in the WHTF of 254.9 ft.

F 2.

All structures within or adjacent to the site and which lie within the floodplain for the one-percent chance flood are identified 4

and located on Attachment A.

The structures are described below:

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Transmission Towers - No description necessary, i

Dike IV - One of the dikes separating the reservoir from the WHTF.

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Boat Docks - Private structures for recreational craft.

Intake and Discharge Structures - House various types of equipment for the condenser circulating water system for the plant.

The applicant owns all land surrounding the reservoir to Eleva-tion 255.0 ft and all land surrounding the WHTF to Elevation 256.5 ft.

Thus, the construction of any significant structure within the floodplain for Lake Anna is regulated by the Applicant.

3.

The dam is the only hydrologically significant control during the 100-year flood.

It has been designed for floods much greater i

4 than the one percent chance flood and is fully capable of controlling j

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i the water Icvel during the 100-year flood.

The dam has been fully considered in the 100-year flood analysis.

At the present time, no changes are planned that would increase I

flood flows and levels within or adjacent to the site. A peak i

water elevation for the 100-year flood greater than that computed by the present analysis could only occur if heavy urban development takes place within the drainage basin during the lifetime of the plant. A substantial increase in urban development and the associated increase in runoff during a rainstorm is not considered likely.

Debris accumulation on structures within the site area will have no effect on the floodplain. The intake structure, which is the only onsite structure downstream of any potential source of debris, is located at a point on the shore where Lake Anna is quite wide and deep. Due to its location, debris could not possibly accumulate on the intake structure in sufficient quantity to block t'ae flow in the lake.

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Since most of the site is constructed at El. 271 ft, well above i

the maximum expected water 1cvel, it is highly unlikely that any A

f significant debris would be generated from the site during the one-percent chance flood.

Some light debris could be released from rainfall runoff. This debris could only impinge and accumulate i

in a few locations of any importance,' the canals connecting the I

different segments of the W11TF, the orifices under the most i

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downstream dike, and the dam.

Due to the size of these structures, and the size of the debris that can possibly be expected, the potential effect of accumulation is limited. The canals are over 30 ft deep and more than 150 ft wide.

The discharge orifices are submerged more than 20 ft below the water surface and thus not subject to jamming by the light floating debris postulated.

Each tainter gate at the dam is 40 ft wide.

Any postulated debris from the site approaching and entering these structures would pass right through.

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