BEAVER PA F/G NATIONAL DAM SAFETY PROGRAM. … · ad-ai03 718 baker (michael) jr inc beaver pa f/g 13/13 national dam safety program. thelma pitts oam (inventory number--etc(u) apr

AD-AI03 718 BAKER (MICHAEL) JR INC BEAVER PA F/G 13/13NATIONAL DAM SAFETY PROGRAM. THELMA PITTS OAM (INVENTORY NUMBER--ETC(U)APR 81 J A WALSH DACW65-GO000032

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YORK RIVER BASIN

Name of Dam: Thelma Pitts DamLocation: Caroline County, State of VirginiaInventory Number: VA 03343

PHASE I INSPECTION REPORTNATIONAL DAM SAFETY PROGRAM

D1 DTICEECTE

SP31981

PREPARED FOR ANORFOLK DISTRICT CORPS OF ENGINEERS

803 FRONT STREET )NORFOLK, VIRGINIA 23510 )

This document h been cappm] -

.'. "PREPARED BY for publc relee and aso.; its

MICHAEL BAKER, JR., INC. I'"°" 1 uinliited.

BEAVER, PENNSYLVANIA 15009

April 13 081 9 O3.0O51

I ~~SECL:RITY CCASSIFICATION OF THIS PAGE (Whae Data EnteZ*'%%__________________

REPOT DOUMETATIN P~E ~READ INSTRUCTIONSREPOT DOUMETATIN PAE 4ORE COMPLETING FORM

IREPORT NUMBER 2. GOVT ACCESSION NO. 3. RCIPIE *S CATALOG NUMBER

VA 03343

*4. TITLE (and Subtitle) 5 Wta r OEE

Thelma~~~ ~ ~ ~ ~ ~ s Pi t ar 4.7vr- O .' N " 6. P P NG ORG. REPORT NUMBER

S. PERFORMING ORGANIZA. tO. PROGRAM ELEMENT, PROJECT, TASK

11. CONTROLLING OFFICE NAME AND ADDRESSApJS 8

14 MONIORIN fro Contrllin Offce __._ECRIY _LAS._ofthseprt

17. DISTRIBUTION STATEMENT (of the abetract entered In Block 20, If different from Report)

16. SUPPLEMENTARY NOTES

Copies are obtainable from National Technical Information Service,.4 Springfield, Virginia 22151

It. KEY WORDS (Continue an reveree aids it nac*aaaaY end Identify by block number)

Dams - VAt National Dam Safety Program Phase I

Dam SafetyDam Inspection

Z& ABBTIRACr (Cm0dw- - sevaesna N~D neeeamy en identify by block num~bw)

(See reverse side)

kD I 3Qw 3 m-now oF NO 65v as onLETIESECulrTy CLASSIPICATtON Of T"12 PA44E (11110 D410 fttere)

.9SECURITY CLASSIFICATION -OF THIS PAE Dat

20. Abstract

Pursuant to Public Law 92-367, Phase I Inspection Reports are preparedunder guidance contained in the recommended guidelines for safetyinspection of dams, published by the Office of Chief of Engineers,Washington, D. C. 20314. The purpose of a Phase I Inspection is toidentify expeditiously those dams which may pose hazards to human lifeor property. The assessment of the gereral conditions of the dam isbased upon available data and visual inspection. Detailed investigationand analyses involving topographic mapping, subsurface investigations,testing, and detailed computational evaluations are beyond the scope of aPhase I investigation; however, the investigation is intended to idenifyany need for such studies.

Based upon the field conditions at the time of the field inspection andall available engineering data, the Phase I report addresses thehydraulic, hydrologic, geologic, geotechnic, and structural aspects ofthe dam. The engineering techniques employed give a reasonably accurateassessment of the conditions of the dam. It should be realized thatcertain engineering aspects cannot be fully analyzed during a Phase Iinspection. Assessment and remedial measures in the report include therequirements of additional indepth study when necessary.

Phase I reports include project information of the dam appurtenances, allexisting engineering data, operational procedures, hydraulic/hydrologicdata of the watershed, dam stability, visual inspection report and anassessment including required remedial measures.

SECURITY CLAUIFICATtON OF THIS PAGE4Whe Dale Nnt'..

, i i i I I I -i I 4 9" ' ,

4{ - ,1

PREFACE

This report is prepared under guidance contained in theRecommended Guidelines for Safety Inspection of Dams, forPhase I Investigations. Copies of these guidelines may beobtained from the Office of the Chief of Engineers,Washington, D.C. 20314. The purpose of a Phase I investi-gation is to identify expeditiously those dams which maypose hazards to human life or property. The assessment ofthe general condition of the dam is based upon availabledata and visual inspections. Detailed investigation andanalyses involving topographic mapping, subsurface investi-gations testing, and detailed computational evaluations arebeyond the scope of a Phase I investigation; however, theinvestigation is intended to identify any need for suchstudies.

In reviewing this report, it should be realized that thereported condition of the dam is based on observations offield conditions at the time of inspection along with dataavailable to the inspection team. In cases where the reser-voir was lowered or drained prior to inspection, such action,while improving the stability and safety of the dam, removesthe normal load on the structure and may obscure certainconditions which might otherwise be detectable if inspectedunder the normal operating environment of the structure.

It is important to note that the condition of a dam dependson numerous and constantly changing internal and externalconditions, and is evolutionary in nature. It would beincorrect to assume that the present condition of the damwill continue to represent the condition of the dam at somepoint in the future. Only through continued care and inspec-tion can there be any chance that unsafe conditions bedetected.

Phase I inspections are not intended to provide detailedhydrologic and hydraulic analyses. In accordance with theestablished guidelines, the spillway design flood is basedon the estimated "Probable Maximum Flood" for the region

4 . (flood discharges that may be expected from the most severecombination of critical meteorologic and hydrologic con-ditions that are reasonably possible), or fractions thereof.Because of the magnitude and rarity of such a storm event, afinding that a spillway will not pass the design floodshould not be interpreted as necessarily posing a highlyinadequate condition. The design flood provides a measureof relative spillway capacity and serves as an aid in deter-mining the need for more detailed hydrologic and hydraulicstudies, considering the size of the dam, its general con-dition, and the downstream damage potential.

S -

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-4


CONTENTS

Page

Preface ........ ....................... ...

Brief Assessment of Dam ....... ............... 1Overall View of Dam ........ ................. 7Section 1: Project Information ..... ........... 9Section 2: Engineering Data ...... ............. 15Section 3: Visual Inspection .... ............ .. 17Section 4: Operational Procedures .. .......... ... 21Section 5: Hydraulic/Hydrologic Data .. ........ 23Section 6: Dam Stability .............. 27Section 7: Assessment/Remedial Measures. ......... ... 31

Appendices

I. PlatesII. PhotographsIII. Visual Inspection Check ListIV. General References

AccesSion For

V'TIS GRA&I•TIC TAB El

U-enuoticed El" J ;istifi*cat ion

Distribution/

Availability CadesAvail and/or

Dist I Special

NAME OF DAM: THELMA PITTS DAM

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Name of Dam: Thelma Pitts DamState: Commonwealth of VirginiaCounty: CarolineUSGS 7.5 Minute Quadrangles: Sparta, Virginia and

Penola, VirginiaStream: Unnamed Tributary to the Maracossic CreekDate of Inspection: 12 January 1981

BRIEF ASSESSMENT OF DAM

Thelma Pitts Dam is an earthfill embankment approximately18.6 feet high' and 250 feet long. The principal spillwayis a concrete drop inlet riser located near the left abut-ment2. An emergency spillway is located on each abutment.The dam, located about 0.9 miles west of Sparta, Virginia,is used for recreation. The dam is owned by Thelma Pitts,Sparta, Virginia 22552. Thelma Pitts Dam is a "small"size - "significant" hazard structure as defined by theRecommended Guidelines for Safety Inspection of Dams. Thedam and appurtenant structures were in good overall condi-tion at the time of inspection. Maintenance of the dam isconsidered to be inadequate. A stability check of the damis not required.

Using the Corps of Engineers' screening criteria for initialreview of spillway adequacy, the 100-year flood was selected

* as the spillway design flood (SDF). The spillway is capableof passing up to 45 percent of the SDF or 8 percent of theProbable Maximum Flood (PMF) without overtopping the non-overflow section of the dam. The spillway is adjudged asinadequate but not seriously inadequate.,.--

The areas of seepage near the center of the dam and on theleft abutment should be examined at regular intervals andafter periods of heavy rain for turbidity and/or increase inflow which may indicate the potential for piping of embank-ment material.

2Measured from the streambed at the downstream toe to theembankment crest.

2Facing downstream.

II


£Q4u1., , , ..- .

Regular inspections should be made of the dam and appurtenantstructures. A thorough check list should be compiled foruse by the owner's representative as a guide for the inspec-tions. Maintenance items should be completed annually.

A formal warning system and emergency action plan should bedeveloped and implemented as soon as possible.

The following repair items should be accomplished as part ofthe general maintenance of the dam:

1) Remove all trees and brush growing on the embank-ment by cutting them off at ground level. Treeswith a trunk diameter of greater than 3 inchesshould also have their root systems removed and

4 the resultant holes backfilled, compacted, regradedll and seeded. If turbidity and/or increased flowsare noted, a qualified geotechnical engineeringfirm should be retained to perform a stabilitycheck of the dam.

2) Place riprap on the upstream face to minimizeerosion due to wave action.

3) Backfill, compact and seed the erosion ditch atthe junction of the left abutment and the down-stream embankment, and the erosion ditch on theleft abutment in the spillway discharge channel.

4) Remove logs lodged in the principal spillway andinstall a trash rack on the principal spillway.

5) Backfill, compact, and riprap the eroded areasbeneath and beside the apron at the principalspillway outlet.

6) Line the stilling basin with riprap to prevent

erosion and scouring of the pool.

7) Remove the sections of concrete pipe partially4obstructing the outlet of the stilling basin.

8) Install a staff gage to monitor reservoir levelsabove normal pool.

NAME OF DAM: * THELMA PITTS DAM

PA= IuhIII nu1] ... .. .... ... ... . ,3

original Bigzaod by

MICHAEL BAKER, JR., INC. SUBMITTED: JAMES A. WALSH

James A. Walsh, P.E.Chief, Design Branch

Original signed by

Michael Baker, II , P.E. RECOMMENDED:Chairman of the Board and Jack G. Starr, P.E.Chief Executive Officer Chief, Engineering

Original signed by:

Douglas L. Hailer* ~~~~~~~APPROVED: _____________,E Douglas L. Haller

Colonel, Corps of EngineersDistrict Engineer

~~MICHAEL '

SBAKER IIINO.3176 Date: APR 2 1 1981

"I.

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NAME OF DAM: THELMA PITTS DAM ID# VA 03343

SECTION 1 PROJECT INFORMATION

1.1 General

1.1.1 Authority: Public Law 92-367, 8 August 1972,authorized the Secretary of the Army, throughthe Corps of Engineers, to initiate a nationalprogram of safety inspections of dams through-out the United States. The Norfolk Districthas been assigned the responsibility ofsupervising the inspection of dams in theCommonwealth of Virginia.

1.1.2 Purpose of Inspection: The purpose is toconduct a Phase I inspection according to theRecommended Guidelines for Safety Inspectionof Dams (Reference 12, Appendix IV). Themain responsibility is to expeditiouslyidentify those dams which may be a potentialhazard to human life or property.

1.2 Description of Project

1.2.1 Description of Dam and Appurtenances: ThelmaPitts Dam is an earthfill embankment approxi-mately 18.6 feet high' and 250 feet long.The crest of the dam is about 12 feet wideand the minimum elevation of the crest is1002.6 feet Temporary Bench Mark (T.B.M.)2 .The slope of the upstream embankment isapproximately 2.1H:1V (Horizontal to Vertical)and the slope of the downstream embankmentvaries between 1.9H:1V to 2.3H:lV. There isno information available on any possiblezoning of the embankment. No evidence of aninternal drainage system or slope protectionfor the dam was found.

' 1Measured from the streambed at the downstream toe to theembankment crest.2 A1 elevations are referenced to a Temporary Bench Marklocated on top of the concrete intake riser. The assumedelevation is 1000.0 feet.


9 1.i PAW XNKRNO? 1SD

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The principal spillway is a square concreteriser with an internal opening of four feetby four feet. It is located near the leftabutment approximately 20 feet from the

,, embankment crest. The riser has a crestelevation of 1000.0 feet T.B.M. Three of thefour sides are 10 inch concrete walls. Theupstream side of the riser contains 2 inch by8 inch boards acting as stop logs. Thesestop logs make up the top 4 feet of theintake riser and can be removed to lower thereservoir level 4 feet. Water passing intothe intake riser is transmitted through thedam by a 30 inch diameter concrete pipe thatextends 63 feet to the toe of the dam. Thepipe discharges onto a 6 foot long concreteapron before dropping into a stilling pool atthe toe of the dam.

There is an emergency spillway located oneach abutment. The emergency spillway locatedon the left abutment is a trapezoidal shaped,partially vegetated channel. It has a bottomwidth of about 36 feet and a top width of 57feet. The spillway has a crest elevation of1001.4 feet. There is an eleven foot widebituminous access road partially lining thisemergency spillway. The spillway dischargechannel follows the bituminous road for about30 feet past the embankment; it then turnsright and follows a poorly defined channelthrough a wooded area and discharges into thedownstream end of the stilling pool.

The emergency spillway on the right abutmenthas a crest elevation of 1001.6 feet T.B.M.This grass- and tree-lined trapezoidal channelis poorly defined. It has a bottom width ofabout 10 feet and gently rising side slopeswith a top width of about 40 feet. Immediatelydownstream from the spillway is a small bermrunning perpendicular to the embankment toprevent discharge from flowing toward the toeof the dam.

The reservoir has a 1.37 square mile drainagearea to the west of the dam. A second dam islocated about 2900 feet upstream from theThelma Pitts Dam and about 800 feet from the


S10__A u _n__|

back water of the Thelma Pitts Dam. Thewatershed consists primarily of woodland withonly 10 percent of the drainage area beingopen and unforested land. Present developmentis limited to a few houses and churches inthe upper reaches of the watershed.

1.2.2 Location: Thelma Pitts Dam is located inCaroline County, Virginia on an unnamedtributary of the Maracossic Creek, approximately0.9 miles west of Sparta, Virginia. A LocationPlan is included with this report in Appendix~I.

1.2.3 Size Classification: The height of the damis 18.6 feet; the reservoir storage capacityat the crest of the dam (elevation 1002.6feet T.B.M.) is 78 acre-feet. Therefore, thedam is in the "small" size category as definedby the Recommended Guidelines for SafetyInspections of Dams.

1.2.4 Hazard Classification: Virginia Route 654crosses the downstream channel over a fourfoot diameter concrete pipe approximately 200feet downstream from the embankment. Theapparently unmanned Sparta Volunteer FireCompany house is located within the floodplain approximately 300 yards downstream. Afour foot diameter concrete pipe carries theflow from the downstream channel beneath theaccess road to the volunteer fire house.Loss of human life in the event of a damfailure is not considered highly probable.However, economic losses due to damage toVirginia Route 654, the access road to .thefire house, and the fire house are likely inthe event of dam failure. Thelma Pitts Damis therefore considered to be in the "signifi-cant" hazard category as defined by theRecommended Guidelines for Safety Inspectionsof Dams. The hazard classification used tocategorize dams is a function of locationonly and is not related to stability orprobability of failure.

1.2.5 Ownership: The dam and reservoir are ownedby Thelma Pitts, Sparta, Virginia 22552.


i1i

41.2.6 Purpose of Dam: The reservoir is used forrecreational purposes.

1.2.7 Design and Construction History: Accordingto the owner, a dam was first built on thissite in 1950. This first dam failed due tothe burrowing of small animals and was rebuiltin 1960. No other information on the designor construction history was available for usein this report.

1.2.8 Normal Operating Procedures: The reservoirlevel is maintained automatically by thecrest of the principal spillway (elevation1000.0 feet T.B.M.). No formal operatingprocedures are followed for this structure.

1.3 Pertinent Data

1.3.1 Drainage Area: The total drainage areatributary to Thelma Pitts Dam is 1.37 squaremiles. However, a second dam located approxi-mately 2900 feet upstream from the ThelmaPitts Dam controls 1.14 square miles of thedrainage area tributary to Thelma Pitts Dam.

1.3.2 Discharge at Dam Site: The maximum dischargefrom the reservoir is unknown.

SiPool level at minimum top of dam:

Principal Spillway 86 c.f.s.Right Emergency Spillway 40 c.f.s.Left Emergency Spillway 125 c.f.s.

1.3.3 Dam and Reservoir Data: Pertinent data onrothe dam and reservoir are provided in the

following table:


12

tie.Hwvr eoddmlctdapoi

TABLE 1.1 DAM AND RESERVOIR DATA

ReservoirCapacity

Elevation Area Acre- Watershed LengthItem (feet T.B.M.) (acres) feet (inches) (feet)

Top of dam 1002.6 15.9 78 1.1 2900Emergency 1001.6 13.9 63 0.9 2660spillway Rt.

Emergency 1001.4 13.1 60 0.8 2610spillway Lt.

Principal 1000.0 11.2 43 0.6 2280spillway crest

Streaxnbed at 984.0 - - - -

toe

.4


* 13

SECTION 2 - ENGINEERING DATA

2.1 Design: Design plans, specifications, and boring logswere not available for use in preparing this report.No stability analyses or hydrologic and hydraulic datawere available for review.

2.2 Construction: Construction records, as-built plans,

and inspection logs were not available for review.

2.3 Evaluation: No design or construction records were

available for use in assessing the condition of thedam. All evaluations and assessments in this reportwere based upon field observations, conversations withthe owner, and office analyses.

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II III IIII BI r r i -7 IU

SECTION 3 - VISUAL INSPECTION

3.1 Findings

3.1.1 General: The field inspection was conductedon 12 January 1981. At the time of theinspection, the pool elevation was at 999.7feet T.B.M., the tailwater elevation was984.3 T.B.M., and the weather was clear witha temperature of 12 degrees Fahrenheit. Theground surface on the embankment and theabutments was generally frozen. The dam andappurtenant structures were found to be infair overall condition at the time of inspec-

4 tion. Deficiencies found during the inspec-tion will require remedial treatment. Thefollowing are brief summaries of thesedeficiencies. A Field Sketch of conditionsfound during the inspection is presented asPlate 1 in Appendix I. The complete visualinspection check list is provided in AppendixIII. No record was found of any previousinspections.

3.1.2 Dam: The embankment was found to be ingenerally fair condition with no surfacecracks or slides observed. The upstreamembankment is well vegetated with grass andsome small brush near the water surface.There was erosion due to wave action on the

*upstream embankment. The downstream embankmentis heavily vegetated with numerous trees.Erosion was observed at the junction of theleft abutment and the downstream embankment.There was also an erosion channel about 40feet downstream from the embankment on theleft abutment, due to runoff from VirginiaRoute 721 and the access road.

Some areas of slight seepage were detectedbeyond the toe near the center of the dam anda small seep was detected on the left downstreamabutment. No evidence of an internal drainagesystem was found during the inspection.

3.1.3 Appurtenant Structures: The principal spillway(as described in Section 1.2.1) was found tobe in fair condition. No signs of crackingor spalling of the concrete were observed onthe inlet structure. However, there is no


17_ _ _ _ _M__M_ PB Oi.ei Ua W I

trash rack on the inlet and several logs werelodged within the inlet structure. Theoutlet area was in fair condition. The 6foot long concrete apron contained severalsmall cracks. The area around the outlet and

" the banks of the stilling pool were eroded.Sections of a 30-inch diameter concreteconduit were located in the outlet channelabout 140 feet downstream from the outlet.These pipes lie in a haphazard manner andpartially obstruct the stilling pool whereit discharges into the downstream channel.Bulging was observed in places on the down-stream slope of the embankment. This bulgingis probably the result of either trees grow-ing on the dam or downhill creep.

The approach area to the emergency spillwayon the left abutment is broad and unobstructed.The spillway is fairly well vegetated and ispartially lined with a bituminous accessroad. No significant erosion or sloughinghas occurred upstream from the crest. Thedischarge channel is not well defined downstreamof the crest. A small gully has formed inthe discharge channel due to runoff fromVirginia Route 721 and the access road.

The approach area to the secondary emergencyspillway on the right abutment is broad andcontains many trees and bushes. The channelis not well defined. No signs of erosion orsloughing were observed. A small earth bermdiverts flow from the spillway away from thetoe of the dam. The downstream channel iswooded with many six to ten inch diametertrees.

3.1.4 Reservoir Area: The area surrounding thereservoir is moderately sloping. The slopesare wooded and in good condition with noevidence of erosion. Soundings taken at thetime of inspection show the water to be 11.5feet at a distance of 30 feet from shore nearthe center of the embankment.

3.1.5 Downstream Channel: The downstream channelarea is overgrown with brush and trees. Thestream passes under Virginia Route 654


18

through a 4 foot diameter concrete pipe about200 feet downstream from the embankment. Thestream also passes through another 4 footdiameter concrete pipe about 1000 feet down-stream at the access road to the SpartaVolunteer Fire Company. The channel has aslope of about I percent and enters theMaracossic Creek about 1100 feet downstream.

3.1.6 Instrumentation: There was no instrumentationpresent at the dam.

3.2 Evaluation: In general, the dam and appurtenant struc-tures were in good condition. The downstream embankmentshould be cleared of all trees and brush by cuttingthem off at ground level. Trees with a trunk diameterof greater than three inches should also have theirroot systems removed. The resultant holes should thenbe backfilled, compacted, regraded, and seeded. Riprap

-* should be placed on the upstream face to minimize waveerosion. The erosion gullies on the left abutmentshould also be backfilled, compacted, and seeded. Thelogs lodged inside the principal spillway intake risershould be removed and a trash rack installed to preventdebris from clogging the principal spillway. The areasof erosion beneath and around the sides of the spillwayoutlet apron should be backfilled, compacted, andriprapped to prevent erosion of the toe of the dam.The discharge basin should be lined with riprap toprevent erosion and scouring of the pool. The sectionsof concrete pipe partially obstructing the stillingbasin outlet should be removed.

The seep areas near the center of the dam and on theleft abutment should be examined at regular intervalsand after periods of heavy rain for turbidity and/orincrease in flow, which may indicate the potential forpiping of embankment material. If turbidity and/orincreased flows are noted, a qualified geotechnicalfirm should be retained to perform a stability check ofthe dam.

A staff gage should be installed to monitor reservoirlevels above normal pool.


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SECTION 4 - OPERATIONAL PROCEDURES

4.1 Procedures: Operation of the dam is an automaticfunction controlled by the principal spillway and thetwo emergency spillways. Water entering the reservoirflows into the principal spillway at elevation 1000.0feet T.B.M. When inflow is sufficient to cause thereservoir level to rise above elevation 1001.4 feetT.B.M., discharge takes place through the emergencyspillway on the left abutment. When the reservoir levelrises above elevation 1001.6 feet T.B.M., water dis-charges through the emergency spillway on the rightabutment.

4.2 Maintenance of Dam: Maintenance of the dam is theresponsibility of the owner. An inspection or maintenanceschedule has not been instituted.

4.3 Maintenance of Operating Facilities: The only operating.1 facilities at the dam are the stop logs in the riser of

the principal spillway. These logs can be used tocontrol the reservoir level and dewater the reservoir 4feet. The maintenance of this facility is the responsi-bility of the owner. A formal inspection and maintenanceschedule has not been instituted.

4.4 Warning System: At the time of inspection, there wasno warning system or emergency action plan in operation.

4.5 Evaluation: Maintenance of the dam in the past hasbeen inadequate. Regular inspections of the dam andappurtenant structures should be made and documented.A thorough check list should be compiled for use by theowner's representative as a guide for the inspections.Maintenance items should be corrected annually. Awarning system and emergency action plan should bedeveloped and implemented as soon as possible.


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SECTION 5 - HYDRAULIC/HYDROLOGIC DATA

5.1 Design: No design data were available for use inpreparing this report.

5.2 Hydrologic Information: No rainfall, stream gage or

reservoir stage records are maintained for this dam.

5.3 Flood Experience: No records were available.

5.4 Flood Potential: The Probable Maximum Flood (PMF), 1/2Probable Maximum Flood (1/2 PMF), and 100-year floodwere developed and routed through the reservoir by useof the HEC-l DB computer program (Reference 9, AppendixIV) and appropriate unit hydrograph, precipitation andstorage-outflow data. Clark's Tc and R coefficientsfor the local drainage areas were estimated from basincharacteristics. The rainfall applied to the unithydrograph was taken from publications by the U.S.Weather Bureau and the National Oceanic and AtmosphericAdministration (References 16 and 17, Appendix IV).Rainfall losses for the PMF and 1/2 PMF were estimatedat an initial loss of 1.0 inches and a constant lossrate of 0.05 inches per hour thereafter. Rainfalllosses for the 100-year flood were estimated at aninitial loss of 1.5 inches and a constant loss rate of0.15 inches per hour thereafter.

5.5 Reservoir Regulation: Pertinent dam and reservoir dataare provided in Table 1.1, Paragraph 1.3.3.

Regulation of flow from the reservoir is primarilyautomatic. Normal flows are maintained by the crest ofthe principal spillway at elevation 1000.0 feet T.B.M.Some manual regulation of flow is possible by theremoval of the wooden stop logs from the upstream sideof the concrete crest riser. By removing these stoplogs, the spillway crest can be lowered to elevation

4996.0 feet T.B.M. Water may also discharge through theemergency spillway on the left abutment when the reservoirrises above an elevation of 1001.4 feet T.B.M. Inaddition, there is an emergency spillway on the right-abutment with a crest elevation of 1001.6 feet T.B.M.

Outlet discharge capacity was computed by hand. Reservoirarea was estimated from the Sparta, Virginia andPenola, Virginia, 7.5 minute USGS quadrangles, andstorage capacity curves above normal pools were computed


j 23 ,oMMM PAMi uARANE-aio IMZm

by the HEC-l DB program. All flood routings were begun

with the reservoir at normal pool. Flow through thespillways was included in the routings.

5.6 Overtopping Potential: The probable rise of the reservoirand other pertinent information on reservoir performanceare shown in the following table:

TABLE 5.1 RESERVOIR PERFORMANCE

Hydrographs1/2

Item Normal' 100-Year PMF PMF2

Peak flow, c.f.s.Inflow 1 1272 3742 7573Outflow 1 1248 3715 7541

Peak elev., ft. T.B.M. 1000.0 1003.9 1005.1 1006.5Non-overflow section2(elev. 1002.6 ft. T.B.M.)Depth of flow, ft. - 1.3 2.5 3.9Average velocity, f.p.s. - 5.3 7.3 9.2Total duration of over-topping, hrs. 4.4 12.3 21.1

Tailwater elev., ft. T.B.M. 984.3 - - -

'Conditions at time of inspection.2The PMF is an estimate of flood discharges that may beexpected from the most severe combination of criticalmeterologic and hydrologic conditions that are reasonablypossible in a region.'Velocity estimates were based on critical depth at controlsection.

5.7 Reservoir Emptying Potential: The reservoir can bedrawn down 4 feet by removal of the wooden stop logsfrom the upstream side of the intake riser. Neglectinginflow, the reservoir can be drawn down 4 feet fromnormal pool in approximately 30 hours. This is equiva-lent to an approximate drawdown rate of about 3.2 feetper day, based on the hydraulic height measured fromnormal pool divided by the time to lower the reservoir4 feet.

5.8 Evaluation: Thelma Pitts Dam is a "small" size -"significant" hazard dam requiring evaluation for aspillway design flood (SDF) in the range between the


24

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100-year flood and the 1/2 PMF. Due to the risk involved,the 100-year flood was selected as the SDF. The 100-yearflood was routed through the reservoir and found toovertop the dam by a maximum depth of 1.3 feet with anaverage critical velocity of 5.3 feet per second (f.p.s.).Total duration of dam overtopping would be 4.4 hours.The spillways are capable of passing up to 8 percent ofthe PMF or 45 percent of the SDF without overtoppingthe dam.

Conclusions pertain to present day conditions and theeffect of future development on the hydrology has notbeen considered.

III

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25

SECTION 6 - DAM STABILITY

6.1 Foundation and Abutments: No information is availableon foundation conditions other than observations madeat the time of the inspection. The dam is located inthe Atlantic Coastal Plain geologic region, and thepredominant deposit in the area of the dam is Tertiary-age Calvert formation consisting of interbedded sandand clay. The Geologic Map of Virginia also showsQuaternary-age upland gravel and sand existing at thesurface in the area. An eroded soil exposure adjacentto the outlet conduit discharge in the left downstreamabutment contained reddish-brown clayey sand with thinlenses of light gray clayey silt. A slight amount ofclear seepage was observed from the abutment at thatpoint. Small seeps and wet areas were also observedbelow the toe of the downstream embankment. Smallamounts of iron precipitate were present in some ofthese wet areas. However, the seepage did not appearturbid at the time of inspection, and no evidence ofpiping of foundation or abutment material was observedat any of these locations. Based on the visual inspec-tion, it is believed that the dam has no internaldrainage system. No information was available concerningthe keying of the embankment into the foundation.

6.2 Embankment

6.2.1 Materials: No information is available onthe nature of the embankment materials otherthan observations made at the time of inspec-tion. Embankment material at the surface ofthe downstream face was observed to consistof reddish-brown clayey silt with fine sand.The plasticity of this soil was judged to below.

6.2.2 Stability: Design plans or the results of aprevious stability analysis were not availablefor use in this investigation. The embankmentis assumed to be a homogeneous type consistingof fine-grained silty soils of low plasticity(classified as ML according to the USCSsystem). The dam is 18.6 feet high with acrest width of 12 feet. The downstreamembankment was observed to have a 1.9H:IVslope at Station 0+96 where the outlet conduitpasses through the dam, and a 2.3H:lV slope


27 I I "I I ! a ","- --

i'7

at Station 2+00 near the center of the embank-ment. The upstream embankment slopes at2.lH:lV at Station 2+00. In the event of anemergency, the reservoir can be drawn down atotal of four feet below the top of theconcrete drop inlet by removing the wooden

"= stop logs from the upstream side of theinlet. According to calculations made aspart of this investigation, the dam is subjectto rapid drawdown (3.2 feet per day), eventhough the total drawdown is limited to fourfeet. This rate is greater than the criticalrate for earth dams of 0.5 feet per day.

According to guidelines outlined in Design ofSmall Dams by the U.S. Department of Interior,Bureau of Reclamation, the upstream slope ofa small homogeneous dam constructed of MLsoils, with a stable foundation, should be3.5H:1V if subject to rapid drawdown. Therecommended downstream slope is 2.5H:lV. Acrest width of 13.7 feet is recommended,considering the height of the dam. Based onthese guidelines, both the upstream anddownstream slopes and the crest width areinadequate.

Signs of instability such as tension cracks,misalignment of the crest, or unusual movementat the downstream toe were not observed.However, slight bulging of the downstreamslope of the dam was observed in severalplaces, particularly on the left half of theembankment. This bulging may be the resultof the many trees taking root in the embankment,or the result of slow downhill creep of theembankment soil, or both. Erosion of theembankment was very slight except for thearea of the downstream embankment and abutmentadjacent to the outlet discharge. The erosionhad cut slightly into the toe of the slope oneach side of the outlet conduit. A shallowerosion gully was also observed at the junctionof the downstream face and the left abutment.No seepage was observed on the face of theembankment, but the downstream slope wascovered with brush and trees up to one footin diameter.


28

6.2.3 Seismic Stability: The dam is located inSeismic Zone 1, which presents no hazard fromearthquakes according to the RecommendedGuidelines for Safety Inspection of Dams bythe Department of the Army, Office of theChief of Engineers. This determination iscontingent on the requirements that staticstability conditions are satisfactory andconventional safety margins exist.

6.3 Evaluation: The results of a previous stability analysiswere not available for review. Both the upstream anddownstream slopes are steeper than recommended in theBureau of Reclamation guidelines. The crest width isless than the recommended width of 13.7 feet. However,other than the slight bulging of the downstream facewhich is not considered to be serious at this time, nomajor signs of instability were observed during theinspection. A stability check is not considered to benecessary.

As discussed in Section 5 of this report, the dam wouldbe overtopped by the design flood. The SDF wouldovertop the dam by a maximum depth of 1.3 feet with anaverage critical velocity of 5.3 feet per second (f.p.s.).The total duration of overtopping would be about 4.4hours. Since the critical velocity is less than theeffective eroding velocity for a vegetated earth embank-ment, and the depth and duration of overtopping aresmall, the overtopping is not considered detrimental tothe embankment.


29

SECTION 7- ASSESSMENT/REMEDIAL MEASURES

7.1 Dam Assessment: There were no engineering data availablefor use in preparing this report. Deficiencies dis-covered during the field inspection and office analysesrequire remedial treatment. The dam and appurtenantstructures are generally in fair overall condition.Maintenance of the dam is considered inadequate. Astability check of the dam is not required.

Using the Corps of Engineers' screening criteria forinitial review of spillway adequacy, the 100-year floodwas selected as the SDF for the "small" size - "signifi-cant" hazard classification of Thelma Pitts Dam. Thespillways are capable of passing up to 45 percent ofthe SDF or 8 percent of the PMF without overtopping thenon-overflow section of the dam. The SDF was found toovertop the dam by a maximum depth of 1.3 feet with anaverage critical velocity of 5.3 feet per second (f.p.s.).A dam failure from overtopping would significantlyincrease the hazard to loss of life downstream from thedam over that which would exist just before overtoppingfailure. However, overtopping flows resulting from theSDF are not considered detrimental. Therefore, thespillway is adjudged as inadequate but not seriouslyinadequate.

There is no warning system or emergency action plancurrently in operation.

7.2 Recommended Remedial Measures: Areas of seepage nearthe center of the dam and on the left abutment should

vbe examined at regular intervals and after periods ofheavy rain for turbidity and/or increase in flow whichmay indicate the potential for piping of embankmentmaterial. If turbidity and/or increased flows arenoted, a qualified geotechnical firm should be retainedto perform a stability check of the dam.

Regular inspections should be made of the dam andappurtenant structures. A thorough check list shouldbe compiled for use by the owner's representative as aguide for the inspections. Maintenance items should becompleted annually.

A formal warning system and emergency action planshould be developed and implemented as soon as possible.


31 RmmG PA= awa-m0T naML

The following repair items should be accomplished aspart of the general maintenance of the dam:

1) Remove all trees and brush growing on the embank-ment by cutting them off at ground level. Treeswith a trunk diameter of greater than 3 inchesshould also have their root systems removed andthe resultant holes backfilled, compacted, regradedand seeded.

2) Place riprap on the upstream face to minimizeerosion due to wave action.

3) Backfill, compact and seed the erosion ditch atthe junction of the left abutment and the down-stream embankment and the erosion ditch on the

4left abutment in the spillway discharge channel.

4) Remove logs lodged in the principal spillway andi install a trash rack on the principal spillway.

5) Backfill, compact and riprap the eroded areasbeneath and beside the apron at the principalspillway outlet.

6) Line the stilling basin with riprap to preventerosion and scouring of the pool.

7) Remove the sections of concrete pipe partiallyobstructing the outlet of the stilling basin.

8) Install a staff gage to monitor reservoir levelsabove normal pool.

S


32

APPENDIX I

PLATES

CONTENTS

Location Plan

Plate 1: Field Sketch

Plate 2: Top of Dam Profile

Plate 3: Typical Cross Sections

I

NAME OF DAM." THELMA PITTS DAN

I ?

- | l i * .*II I [" r ' " •-*

THELMA PITTSDAM ~-

VATi H-.JT M A PIT SS/ DAM

VA.3

1- -Ri

.44

~b~6

SCALE 0'26?000 1 IWLS

S ~ I RI 1~S50 000 400 sm m T"s mn

REFERENCES:LOAINP N1. U.S.G.S.7.5'SPARTA,VALOAONPN

QUADRANB .E .96 THELMA PITTS DAM2. U.S.G.S.7.5 PENOLA,VA.

QUADRANGLE. 1969

RESER VOIR 0

I PRIMARY SPILLWAY/ 4'x 4'CONCRETE

RISER WITH WOODUPTEMSTOP LOGS- N

Z SLOPE APPROX.W zZ.HI RODED

m BRUSH ON UPSTREAM FACET.M AE~I~(NO RIP RAP -

0 APRX APPROX.K

SLOPE MI

W :6' CONCRETE APRON z.E~E

-J SEEPAGE -,5gPM CER

IRONSTIIGIO

STAININGNG

*SECTIONS OFEOSO0CONCRETEGULCONDUIT

NATURALNOTE: THE DOWNSTREAM EMBANKMENT STREAM

IS COVERED WITH MANY TREESjj CANEAND B3RUSH VARYING FROM 1/21INCH TO NEARLY A FOOT INDIAMETER.

FIELD SKETCH

THELMA PITTS DAMMICHAEL BAKER,JR. INC.

12 JANUARY 1981'NO SCALE PLATE I

MICHAEL BAKER, JR., INC. Subject V ~JJf A . .. N._____

THE BAKER ENGINEERS f'hZINm Pitts 1) -am Shoet No.-.. f....

Box 80 7e -o - -*, ProileDrowing No.

Beaver, Pa. 15009 Computed by A K Chocked by Date

4uCl~

AU- . 0 .... .

- .. - - -& S.

ID go4Z.

cc -..-.-

7... . .7............ . . .. .. . . . . .. . . . . .. . -,-- . ... .

cc ... ..

m- m -. .. . . .

oIi ma- I --.... . . . .

MICHAEL BAKER, JR., INC. subject VIK QA DAM 57 S.0. No._____

THE BAKER ENGINEERS 7- lm- Pifts 0D-& Shoet Nio.-.f......

Cross Sec-tlons Drawing No. _____

Box 280Beaver, Pa. ISM0 Computed by AP Checked by Date

DAM C R055 - SECTION' AT -OUTLET -WOR~KS--

C~R~Es WvIDTHl12 FEET - . .-..- ...........

171 .9 N IV SLOPE ~.-

- ~30"' COACTE PIPE.

-- - ThqLwATER.

0 20 40 s0 o1LO - -

DISrgqAXE yr7~

-47- -- 1

DAM CROSS- SECTIOW ST9 4O.:

CREST WIDTH11 F Et -717

- 1000 - - - -----. -- - -. .-- .

U-

o +oIf0 (10 so t ATE0ISTXJVCE -(PTr)PLT

I

APENI I

'I __

POTOGRAPHS

•~~~ PHTORAw

-I

CONTENTS

Photo 1: Principal Spillway Inlet

Photo 2: Outlet of Principal Spillway and Stilling Basin

Photo 3: Erosion Gully at Junction of Left Abutmentand Downstream Embankment

Photo 4: Erosion at Stilling Basin of Principal Spillway

Photo 5: View of Emergency Spillway on Left Abutment

Photo 6: View of Emergency Spillway Area on Right Abut-ment

Photo 7: Crest of Dam (Looking Left) Showing Trees Growingon Downstream Embankment

Photo 8: Seepage Area at Center of Dam Beyond the Toe

Note: Photographs were taken on 12 January 1981.

tf4

£

; NAME OF DAM:" THELMA PITTS DAM

.:,1

THELMA PITTS DAM

PHOTO 1. Principal Spillway Inlet

PHOTO L. Outet of Principal Spillway a&W Silling Bauin

THELMA PITTS DAM

j!

PHOTO 3. Erosion Gully at Junction of Left Abutment andDownstream Embankment

PHOTO 4. Erosion at Btilling Basin of Principal Spillway

THELMA PITTS DAM

PHOTO 5. View of Emergency SpilNay on Left Abutment

AW-

PHOTO 6. View of Emergency Spilliwy Area on Right Abutment

-I

APPENDIX III

VISUAL INSPECTION CHECK LIST

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APPENDIX IV

GENERAL REFERENCES

I

.- a

!~

GENERAL REFERENCES

1. Bureau of Reclamation, U.S. Department of the Interior,Design of Small Dams, A Water Resources TechnicalPublication, Revised Reprint, 1977.

2. Chow, Ven Te, Handbook of Applied Hydrology, McGraw -Hill Book Company, New York, 1964.

3. Chow, Ven Te, Open Channel Hydraulics, McGraw - HillBook Company, New York, First Edition, 1959.

4. Commonwealth of Virginia, "Geologic Map of Virginia,"Department of Conservation and Economic Development,and Division of Mineral Resources, 1963.

5. HR 33, "Seasonal Variations of Probable Maximum Precipita-tion, East of the 105th Meridian for Areas 10 to 1000Square Miles and Durations of 6 to 48 Hours," (1956).

6. King, Horace Williams and Brater, Ernest F., Handbookof Hydraulics, Fifth Edition, McGraw - Hill Book Company,New York, 1963.

7. Soil Conservation Service, "National Engineering Handbook -Section 4, Hydrology," U.S. Department of Agriculture,1964.

8. Soil Conservation Service, "National Engineering Handbook -Section 5, Hydraulics," U.S. Department of Agriculture.

9. U.S. Army, Hydrologic Engineering Center, "Flood HydrographPackage (HEC-1), Dam Safety Investigations, UsersManual," Corps of Engineers, Davis, California, September1978.

10. U.S. Army, Hydrologic Engineering Center, "HEC-2 WaterSurface Profiles, Users Manual," Corps of Engineers,Davis, California, October 1973.

11. U.S. Army, "Inventory of United States Dams," Corps ofEngineers, 9 September 1978.

12. U.S. Army, Office of the Chief of Engineers, "Appendix D,Recommended Guidelines for Safety Inspection of Dams,"

*National Program of Inspection of Dams, Volume 1, Corpsof Engineers, Washington, D.C., May 1975.


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13. U.S. Army, Office of the Chief of Engineers, EngineeringCircular EC-1110-2-163 (Draft Engineering Manual),"Spillway and Freeboard Requirements for Dams, Appendix C,Hydrometeorological Criteria and Hyetograph Estimates,"(August 1975).

14. U.S. Army, Office of the Chief of Engineers, EngineeringCircular EC-1110-2-188, "Engineering and Design, NationalProgram of Inspection of Non-Federal Dams," Corps ofEngineers, Washington, D.C., 30 December 1977.

15. U.S. Army, Office of the Chief of Engineers, EngineerTechnical Letter No. ETL 1110-2-234, "Engineering andDesign, National Program of Inspection of Non-FederalDams, Review of Spillway Adequacy," Corps of Engineers,Washington, D.C., 10 May 1978.

16. U.S. Department of Commerce, "Technical Paper No. 40,Rainfall Frequency Atlas of the United States for Dura-tions from 30 Minutes to 24 Hours and Return Periodsfrom 1 to 100 Years," Weather Bureau, Washington, D.C.,May 1961.

17. U.S. Department of Commerce, National Oceanic andAtmospheric Administration, "Hydrometeorological ReportNo. 51, Probable Maximum Precipitation Estimates,United States East of the 105th Meridian," Washington,D.C., June 1978.


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BEAVER PA F/G NATIONAL DAM SAFETY PROGRAM. … · ad-ai03 718 baker (michael) jr inc beaver pa f/g 13/13 national dam safety program. thelma pitts oam (inventory number--etc(u) apr