AD-AI09 971 TIPPETTS-AAAETT-MCCARTHY-STRATTON NEW YORK F/6 13/13NATIONAL DAM SAFETY PROGRAM. LAKE MAHOPAC DAM (INVENTORY NUMBER--ETC(U)SEP Al E O'BRIEN DACW5l1-1C-0008
I UNCLASSIFIED NLllllKElElhlhEIO
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MICROCOPY RESOLUJIION TEST CHART
SECURITY CLASSeF.CATVD4 OF THIS PGS W-S 0 ct 1-..d
I. ~ P aT REPORT DOCUMENTATICH PAGE -%-,L BiN5TCO M LCh& fO-N
TOTLC (and SblIti.-S.*rp TYP O T PFPtOO COVEREDase iInspection Reor Phase I Inspection Report
ake Mahopa Ic Dam National Damn Safety Prograao wer Hudson River Basin, Putnam County, NY EOA
nY ventory No. 13296-ER0F"-GR. .4AhmiPA
7.AUh~( ~8. C0T7#,r. OA GAAT NUt*&3R(*>EUGENE O'Brien .DC5-1coo
0 ~.P PtiORMING OAGANiZA-1ON NAME AND ADDRSS 15- PpoccA.A T Ai.4S Tippetta-Abbett-McCartily-Stratton'
fria TAM4S Building;55 Thid Az-np. ew Ynrk 1()(172_________
COXTOJLINGd OFFICE NAME ASI ADDRESS IZ. RR-
Departmen~t of the ArmyZIA,
26 Federal Plaza V'ew York District, CofE IL MUz M ~OFPAGESNewj York. Nev York 10287 _____ ________
2 Dpart=en:- of 1tbe A---lay26 Fedaral Plaza 'News York District, CafE[ LGtSD
1-3. OSTP13UJTO0 ST ATE-qENT 4vi V13R .!
Appr-vedifo pu'-' c reiea~a; Distribution nlir'ited
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13- SUPPLNOT ZS-
1.KEY WOAO0S (COMdItan OS'turao .40 it n.ra an d ld~tjjn1-7 ~ bloC.k nw ke Mha a
Dam Safetpy
National Damn Safety-Prog-.zm Lk ao aVisual Tnspaction' Putnam County
!~ydolog, Sruc~ral ta~±i~yLower Hudson River Basin
U-This report provides info~iat--on and analypis c-.'.iic n:n as a; the report date, lnc-ci and on.>:"d ~ vis--aI
41nspection or :ha dam by tiae parEa7-ming orga.-V I C~~)The evAT~in*rv% rsf Arvimv- d the~ visual inspec-
tion of Lake Mahopac Dam and its appurtenant structures didnot reveal conditions which constitute an immediate hazard
Ato human life and property._
iD I 4/ -r h/-. ra-op o 1'
FCUH2TY CLA-' SIFICATI-
# OF THIS PAGr(W?'.n t)ba- rnlrred) , _
Using Corps of Engineers screening criteria, it hasbeen determined that the dam would be overtopped for allstorms exceeding approximately 56 percent of Probable
-Maximum Flood (PMF). The maximum spillway discharge capacity* is 7.9 percent of the PMF peak outflow. The spillway is,
thereforq adjudged as "inadequate".
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SECUfilTY CL A-
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ICA i|ON OF 1lrHI , PAGr(rI47T~. I'dJ /:'n 4d
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I LOWER HUDSON RIVER BASIN
LAKE MAHOPAC DAM
I PUTNAM COUNTY, NEW YORKI INVENTORY NO. N.Y. 1329
IPHASE I INSPECTION REPORTI NATIONAL DAM SAFETY PROGRAM
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INEW YORK DISTRICT CORPS OF ENGINEERS
SEPTEMBER 1981II
IP EFACE
This report is prepared under guidance contained in theReconmended Guidelines for Safety Inspection of Dams, for
* " Phase I Investigations. Copies of these guidelines may be ob-tained from the Office of Chief of Engineers, Washington, D. C.,20314. The purpose of a Phase I Investigation is to identify
• expeditiously those dams which may pose hazards to human lifeor property. The assessment of the general condition of the damis based upon available data and visual inspections. Detailedinvestigations, and analyses involving topographic mapping, sub-surface investigations, testing, and detailed computational eval-uations are beyond the scope of a Phase I Investigation; however,the investigation is intended to identify any need for such studies.
In reviewing this report, it should be realized that thereported condition of the dam is based on observations of fieldconditions at the time of inspection along with data availableto the inspection team. In cases where the reservoir was loweredor drained prior to inspection, such action, while improving thestability and safety of the dam, removes the normal load on thestructure and may obscure certain conditions which might other-wise be detectable if inspected under the normal operating environ-ment of the structure.
It is important to note that the condition of a dam dependson numerous and constantly changing internal and external con-ditions, and is evolutionary in nature. It would be incorrectto assume that the present condition of the dam will continue torepresent the condition of the dam at some point in the future.Only through frequent inspections can unsafe conditions be de-tected and only through continued care and maintenance can theseconditions be prevented or corrected.
Phase I inspections are not intended to provide detailedhydrologic and hydraulic analyses. In accordance with theestablished Guidelines, the Spillway Test flood is based on theestimated "Probable Maximum Flood" for the region (greatest rea-sonably possible storm runoff), or fractions thereof. Becauseof the magnitude and rarity of such a storm event, a findingthat a spillway will not pass the test flood should not be in-terpreted as necessarily posing a highly inadequate condition.The test flood provides a measure of relative spillway capacityand serves as an aid in determining the need for more detailedhydrologic and hydraulic studies, considering the size of thedam, its general condition and the downstream damage potential.L
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IPHASE I INSPECTION REPORT
NATIONAL DAM SAFETY PROGRAMI LAKE MAHOPAC DAMI.D. NO. N.Y. 1329
D.E.C. NO. 213D-4462LOWER HUDSON RIVER BASIN
PUTNAM COUNTY, N.Y.
CONTENTS
I PageNo.
I -. ASSESSMENTOVERVIEW PHOTOGRAPH
jI 1. PROJECT INFORMATION 11.1 GENERAL 1
I a. Authority 1b. Purpose of Inspection 1
1.2 DESCRIPTION OF THE PROJECT 1a. Description of Dam and Appurtenant
Structures 1b. Location 2c. Size Classification 2d. Hazard Classification 2e. Ownership 2f. Purpose of Dam 2g. Design and Construction History 2h. Normal Operating Procedures 3
1.3 PERTINENT DATAa. Drainage Area 3b. Discharge at Damsite 3C. Elevation 3
I d. Reservoir 3e. Storage 3f. Dam 3g. Spillway 3h. Reservoir Drain 3
2. ENGINEERING DATA 4
2.1 GEOLOGY 4
I 2.2 SUBSURFACE INVESTIGATIONS 4iAl
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IPageI No.
2.3 DAM AND APPURTENANT STRUCTURES 4
2.4 CONSTRUCTION RECORDS 4
2.5 OPERATION RECORDS 4
2.6 EVALUATION DATA 4
3 VISUAL OBSERVATION 5
3.1 FINDINGS 5
a. General 5b. Dam 5c. Spillway-Reservoir Drain 5d. Abutments 5
4 e. Downstream Channel 6f. Reservoir Area 6
1 3.2 EVALUATION OF OBSERVATIONS 64 OPERATION AND MAINTENANCE PROCEDURES 7
4.1 PROCEDURES 7
I 4.2 MAINTENANCE OF THE DAM 74.3 MAINTENANCE OF OPERATING EQUIPMENT 7
I 4.4 WARNING SYSTEM IN EFFECT 7
4.5 EVALUATION 7
1 5 HYDROLOGIC/HYDRAULIC 8j 5.1 DRAINAGE AREA CHARACTERISTICS 8
5.2 ANALYSIS CRITERIA 8
I 5.3 SPILLWAY CAPACITY 8
5.4 RESERVOIR CAPACITY 9
I 5.5 FLOODS OF RECORD 95.6 OVERTOPPING POTENTIAL 9
5.7 EVALUATION 9II -ii-
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!PageNo.
6 STRUCTURAL STABILITY 10
6.1 EVALUATION OF STRUCTURAL STABILITY 10
a. Visual Observation 10b. Design and Construction Data 10c. Operating Records 10d. Post Construction Changes 10e. Seismic Stability 10
6.2 STRUCTURAL STABILITY ANALYSIS 10
7 ASSESSMENT/RECOMMENDATIONS 11
7.1 ASSESSMENT 11
a. Safety 11b. Adequacy of Information 11c. Need for Additional Investigations 11d. Urgency 11
7.2 RECOMMENDED MEASURES 11
APPENDICES
A. DRAWINGS
a. Vicinity Mapb. Topographic Mapc. Mahopac Lake Dam-Pland. Mahopac Lake Dam-Spillway
Reservoir Drain Structure, Plane. Mahopac Lake Dam-Spillway
Reservoir Drain Structure, Section
B. PHOTOGRAPHS
C. VISUAL INSPECTION CHECKLIST
D. HYDROLOGIC DATA AND COMPUTATIONS
E. REFERENCES
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PHASE I INSPECTION REPORTNATIONAL DAM SAFETY PROGRAM!
NAME OF DAM: Lake Mahopac Dam, N.Y. 1329
STATE LOCATED: New York
COUNTY LOCATED: Putnam
STREAM: Tributary of Muscoot River
BASIN: Lower Hudson River
DATE OF INSPECTION 6 May 1981
ASSESSMENT
The examination of documents and the visual inspec-tion of Lake Mahopac Dam and its appurtenant structures didnot reveal conditions which constitute an immediate hazardto human life and property.
-Using Corps of Engineers screening criteria, it hasbeen determined that the dam would be overtopped for allstorms exceeding approximately 56 percent of ProbableMaximum Flood (PMF). The maximum spillway discharge capacityis 7.9 percent of the PMF peak outflow. The spillway is,therefore adjudged as "inadequate".
'The following remedial measures should be performedwithin 12 months from notification:
" i Missing portions of the upstream stone masonrywall should be repaired.,
21 Brush and shrubbery should be removed from theembankment, spillway-reservoir drain structure and down-stream channel. Provide a program of periodic cutting andor mowing of the dam surfaces, spillway-reservoir drainstructure and the downstream channel.
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(3)2The collected debris at the spillway-reservoirdrain structure and in the downstream channel should beremoved and hauled away from the dam.
1 )"(41A program of periodic inspections and maintenanceof the dam and appurtenances should be provided, includingyearly operation and lubrication of the reservoir drain andits control facilities. This information should be docu-mented for future reference.
An emergency action plan for the project shouldbe developed.
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Now York No. 92
Approved by: 4~Ab Col. W. M. Smith, Jr.New York District Engineer
I Date: __
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IPHASE I INSPECTION REPORTNATIONAL DAM SAFETY PROGRAM
LAKE MAHOPAC DAMI.D. NO. N.Y. 1329
D.E.C. NO. 213D-4462LOWER HUDSON RIVER BASINPUTNAM COUNTY, N.Y.
SECTION 1 - PROJECT INFORMATION
1.1 GENERAL
a. AuthorityThe Phase I inspection reported herein was
authorized by the Department of the Army, New York District,Corps of Engineers by Contract No. DACW 51-81-C-0008 dated14 December 1980 in fulfillment of the requirements of thLNational Dam Inspection Act, Public Law 92-367, 8 August 1972.
b. Purpose of InspectionThe inspection was conducted to evaluate the exist-
ing conditions of the dam, to identify deficiencies and haz-ardous conditions, to determine if these deficiencies consti-tute hazards to life and property, and to recommend remedialmeasures where required.
1.2 DESCRIPTION OF THE PROJECT
a. Description of Dam and Appurtenant StructuresLake Mahopac Dam consists of an earth embankment
about 190 feet long and has a maximum height of about 9 feet.The width of the crest varies from 25 feet to 50 feet. Thevisible portion of the upstream face of the embankment isvertical and is retained by a 2 foot wide stone masonry wall.The downstream slope of the embankmentto the right of aspillway-reservoir drain structure, is vertical and protectedby a 2 foot wide stone masonry wall; to the left of the spill-way-reservoir drain structure it is 1 vertical to 4 horizontalnear the crest and vertically retained by one of the stonemasonry walls of the spillway-reservoir drain downstreamchannel.
A stone masonry/concrete structure, which servesas a spillway and a reservoir drain is located about 25 feetfrom the right abutment. The crest of the spillway is about3 feet from the top of the dam and is 8 feet wide. Two footwide stone masonry approach and downstream training wallsflank the structure.
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The rese-voir drain is located at the bottom ofthe structure. The discharge through the reservoir drainis controlled by a sluice gate which can be operated fromthe top of the structure. The gate is about 12 feet down-stream from the spillway crest. The discharges over thecrest and through the reservoir drain flow over a slopingconcrete apron, then into the downstream channel. The down-stream channel is stone masonry lined, about 5 feet wide and6 feet high. The channel invert fors the toe of the em-bankment. Its alignment makes a 120 bend about 100 feetfrom the spillway-reservoir drain structure; continues alongPeeksville Road for 1500 feet where it joins the downstreamchannel of Kirk Lake.
b. LocationThe dam is located on a tributary of Muscoot
River and Hudson River; approximately 800 feet from theintersection of Peeksville Road and West Lake Blvd. in theVillage of Mahopac Falls.
c. Size ClassificationThe dam is about 9 feet high and impounds approxi-
mately 1300 acre-feet. Therefore, the dam is in the "inter-mediate" size category (between 1000 acre-feet and 50,000acre-feet).
d. Hazard ClassificationThe dam is classified as high hazard due to
several homes, a highway and commercial properties located0.25 miles downstream from the dam.
e. OwnershipLake Mahopac is owned by the Town of Carmel. The
person to contact is Town Supervisor, Town Hall, Town ofCarmel, McAlpin Avenue, Mahopac, New York, 10541, TelephoneNo. (914) 628-1470. At present the town supervisor is Mr.Dean Barrett. The maintenance of the dam is the responsibilityof the Highway Department of the Town.
f. Purpose of DamThe impoundment provided by the dam is mainly for
recreation. The lake also supplies water to developmentsaround the lake.
g. Design and Construction HistoryOriginal design and construction records are not
available. It is reported that the dam was designed andconstructed in 1940. The names of the designer and the con-tractor are unknown.
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h. Normal Operating ProceduresThere are no normal operating procedures.
Lake level is maintained at the spillway crest in springand summer. In fall the lake level is lowered by 1 to1.5 feet to allow maintenance of docks and prevent damageto docks due to ice in winter.
1.3 PERTINENT DATA
a. Drainage Area, Square Miles 2.7
b. Discharge at Damsite, cfsUngated spillway capacity atEl 660 108.8
c. Elevation (Feet above MSL)Top of Dam 660.00Spillway Crest 657.00Reservoir Drain (at Sluice Gate) 653.7
d. ReservoirLength of Normal Pool (Miles) 0.8Surface Area at El 657 (Acres) 608Surface Area at El 660 (Acres) 625
e. Storage (Acre-Feet)Top of Spillway Crest (El 657) 1300Top of Dam (El 660.0) 3080
f. DamType Earth EmbankmentLength (Feet) 190Height (Feet) 9Crest Width (Feet) Varies frcm 25 to 50 feetSiede Slopes:Upstream Vertical, Stone masonry wallDownstream, Right of Spill-way-Reservoir Drain Stbucture Vertical, Stone masonry wall
Downstream, Left of Spill-way-Reservoir Drain Structure 1 ( :4 (H) and vertical stone
masonry wallg. Spillway
Type Sharp -crested, StoneMasonry-concrete
Length 8 ftCrest Elevation 657.0
h. Reservoir DrainType 15-inch diameter sluice wayElevation (Invert) Intake 653.7
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SECTION1 2 - ENGINEERING DATA
2.1 GEOLOGY
The records of the owner contain no data on sitegeology. However, there is data available in the liter-ature on the general geology of the area. Mahopac LakeDam is located in the Hudson Highlands section of theNew England Uplands physiographic province. The provinceis characterized by a low, but rugged mountain range con-sisting primarily of igneous and metamorphic rock. Therock underlying the area of the dam is Precambrian biotite-quartz-plagioclase paragneiss with subordinate biotitegranitic gneiss, amphibolite and calcilicate rock.
2.2 SUBSURFACE INVESTIGATIONS
There are no records of subsurface investigationsavailable. It is known that the surficial soils in thevicinity of the dam and reservoir are coarse grainedglacial till material.
2.3 DAM AND APPURTENANT STRUCTURES
There are no design data, construction drawings ordesign memoranda available for the project features.
2.4 CONSTRUCTION RECORDS
There are no records of the original constructionof the dam available. The name(s) of the contractor is (are)unknown.
2.5 OPERATION RECORDS
There are no records of operation of the dam. Thereservoir drain sluice gate is reportedly operated duringfall to lower the lake level by 1 foot to 1.5 feet so thatmaintenance of the docks may be carried out and to preventdamage to the docks by ice. There is no formal operationand maintenance manual for the project. No records ofreservoir levels and rainfall are kept.
2.6 EVALUATION DATA
Information was made available by the New York StateDepartment of Environmental Conservation and the Town ofCarmel, New York.
The information obtained from the available data,personal interviews and the visual inspection are considered
adequate for this Phase I inspection and evaluation.
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SECTION 3 - VISUAL OBSERVATION
3.1 FINDINGS
a. GeneralThe visual observation of the Mahopac Dam was
made on 6 May 1981. The weathe6 was sunny with the temper-ature ranging between 60 and 70 F. At the time of inspec-tion, the reservoir level was about El 657.2, 2 inchesabove the crest of the spillway-reservoir drain structure.The reservoir drain was closed.
b. DamThe dam, which consists of earth embankment
appears to be in generally good condition. There are novisible signs of distress or movement. The horizontal andvertical alignment of the crest are also good.
The visible portion of the upstream slope whichis protected by a vertical stone masonry wall, appears tobe in good condition except in the vicinity of the leftabutment, where about 50 feet of the wall is missing.There is minor vegetation growing through the wall jointsparticularly near the spillway.
The downstream slopes as well as the stonemasonry wall of the channel appear to be in good conditionexcept for minor vegetation such as bushes and overgrowngrass. The slope does not exhibit any erosion, sloughingor signs of trespassing. There are no signs of seepage atthe downstream toe.
c. Spillway-Reservoir DrainThe spillway and reservoir drain which are
located about 25 feet from the left abutment are part of astone masonry-concrete structure. The exposed spillwayportion of the structure and the downstream concrete apronappear to be in good condition except for debris collectedat and inside the structure.
The reservoir drain and the control are inoperating condition.
A detailed sketch showing the configuration ofthe spillway-reservoir drain structure as was determinedfrom field measurements is given in Plates 4 and 5.
d. AbutmentsThere are no signs of seepage or other unusual
conditions at both abutments.
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e. Downstream ChannelThe channel downstream of the spillway-
reservoir drain structure is stone masonry lined rectan-gular in section and appears to be in generally good con-dition except for minor vegetation growing through thejoints and collected debris.
f. Reservoir AreaIn the vicinity of the dam there is no evidence
of sloughing, potentially unstable slopes or other unusualconditions which would adversely affect the dam. No evi-dence of excessive sedimentation was observed. The lakewater was relatively clean.
3.2 EVALUATION OF OBSERVATIONS
Visual observations made during the course of theinspection did not reveal serious problems which wouldadversely affect the adequacy of the dam and the appurte-problem areas, in order of importance with the recommended
remedial action:
1. The missing portion of the upstream stonemasonry wall should be repaired.
2. The brush and shrubs should be removed from theembankment, spillway-reservoir drain structure and down-stream channel. A program of periodic cutting and or mow-ing of the dam surfaces, spillway-reservoir drain structureand the downstream channel should be provided.
3. The collected debris at the spillway-reservoirdrain structure and in the downstream channel should beremoved and hauled away from the dam.
4. An emergency action plan for the project shouldbe developed.
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SECTION 4 - OPERATION AND MAINTENANCE PROCEDURES
4.1 PROCEDURES
There are no operating procedures for regulating
the discharges. The reservoir drain sluice gate is keptclosed during the spring and sumer so that the lake levelis maintained at the spillway crest. In fall the lakelevel is maintained 1 to 1.5 feet below the spillway crestby discharging through the reservoir drain (gates keptopen). It is reported that lowering the lake is done toallow for dock maintenance and to prevent ice damage todocks in the winter.
4.2 MAINTENANCE OF THE DAM
There is no operating and maintenance manual for thedam. There is no regular maintenance done at the dam, butis done as needed. The presence of the debris in the spill-way and discharge channel, overgrown bushes and shrubs andtrees is considered unacceptable.
4.3 MAINTENANCE OF OPERATING EQUIPMENT
There is no regular maintenance of the sl.iee gateof the reservoir drain. During the inspection the sluicegate was operated and appears to be in good conditionexcept for minor rusting of the stem. The operating mech-anism for the gate is kept at the Highway Department,Town of Carmel.
4.4 WARNING SYSTEM IN EFFECT
There is no warning system in effect or in preparation.
4.5 EVALUATION
The operation and maintenance of Lake Mahopac Damis considered inadequate in the following areas:
1. Maintenance of the upstream stone masonry wall.
2. Control of vegetation on the embankment and inthe spillway-reservoir drain structure and its channel.
3. Debris in and at the spillway-reservoir drainstructure.
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SECTION 5 - HYDROLOGIC/HYDRAULIC
5.1 DRAINAGE AREA CHARACTERISTICS
Lake Mahopac is located in Carmel Township, PutnamCounty, New York (Hydrologic Unit Code No. 02030101), andis the headwater of the Muscoot River, with a drainagearea of 2.7 square miles. The lake surface area at eleva-tion 657 is 0.95 square miles or 35 percent of the drainagearea. The basin consists of a northern sub-area of 0.94square miles, relatively undeveloped with some surfacestorage in Wixon Pond and a small wet-land area. Theurbanized southern sub-area has little storage in its 0.83square miles and no defined river channels. The basin isextremely narrow on the eastern and western sides of thelake.
5.2 ANALYSIS CRITERIA
The analysis of the adequacy of the spillway wasperformed by developing a design flood using the unithydrograph method and the Probable Maximum Precipitation(PMP). The all season 200 square mile 24 hour PMP forthe Lake Mahopac basin of 22 inches was obtained fromHydro-meteorological Report No. 33, and distributed accord-ing to Standard Project Flood Determination, EM 1110-2-1411,Army Corps of Engineers.
The unit hydrograph for each sub-area was computedusing Snyder's method with coefficients of 2 and 0.625 forCT and Cp) respectively. Rainfall loss parameters of1.0 inch initial loss and 0.1 inch per hour constant losswere selected as representative for the PMP event. Inaccordance with the recommended guidelines for SafetyInspection of Dams, the adequacy of the spillway wasanalyzed using the Probable Maximum Flood (PMF). A multi-plan analysis was performed for 50%, 75% and 100% PMF.
5.3 SPILLWAY CAPACITY
The maximum discharge of the unconventionalstructured spillway, was computed to be 108.8 cfs with thelake surface at El 660, and the sluice gate closed. Alow coefficient of discharge was selected in order torepresent the obstruction to flow caused by the structurebuilt in the spillway channel.
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5.4 RESERVOIR CAPACITY
The normal capacity of the reservoir is listed as1300 acre-feet. The computed surcharge storage betweenspillway crest (El 657) and the top of the dam (El 660)is 1780 acre-feet, which is equivalent to approximately12.4 inches of runoff over the entire drainage area.
5.5 FLOODS OF RECORD
There are no records of floods or maximum reservoirelevations at the dam.
5.6 OVERTOPPING POTENTIAL
The potential of the dam being overtopped was investi-gated on the basis of the spillway discharge capacity andthe available surcharge storage to meet the selected designflood inflows.
The HEC-lDB analysis was performed assuming that thewater surface of the lake was at spillway crest elevationat the start of the flood event. The basin was dividedinto two sub-areas and the combined hydrographs routedthrough the lake.
The results of the multi-ratio analysis are asfollows:
RATIO OF PMF PEAK INFLLM PEAK CUrFLOW OV'FRPPING
1.00 7797 cfs 1372 cfs 1.57 ft.
0.75 5848 cfs 540 cfs 0.74 ft.
0.50 3899 cfs 99 cfs 0.00 ft.
The dam would be overtopped by all floods exceeding56 percent of the PMF. The maximum spillway dischargecapacity is 7.9 percent of the peak PMF outflow.
5.7 EVALUATION
The dam does not have sufficient capacity to passPMF, however, it has capacity to pass one-half PMF. Usingthe Corps of Engineers screening criteria, the spillway is jtherefore assessed as being "inadequate".
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SECTION 6 - STRUCTURAL STABILITY
6.1 EVALUATION OF STRUCTURAL STABILITY
a. Visual ObservationVisual observations did not indicate either
existing or potential problems with the dam and spillway-reservoir drain structure. The observed missing wall atthe upstream face is not detrimental to the dam's stabilityor safety at the present time, however, condition shouldbe repaired before it worsens.
b. Design and Construction DataThere exists no design computations or other
data regarding the structural stability of the dam or spill-way-reservoir drain structure.
c. Operating RecordsThere are no available records of the reservoir
ele -,ons and gate operations. No major operational pro-le- which would affect the stability of the dam were
d. Post Construction ChangesThere are no records of post construction changes
available.
e. Seismic StabilityAccording to the recommended Corps of Engineers
guidelines, the dam is located in Seismic Zone No. 1.However, based on past earthquake history the New York StateGeological Survey considers the site to be in Zone 2. Basedon this assessment the dam is considered in the Seismic Zone 2.
6.2 STRUCTURAL STABILITY ANALYSIS
Analyses to determine the stability of the spillway-reservoir drain structure to withstand sliding or overturningwere not performed. Considering the relatively small exter-nal loadings acting upon the relatively large, heavy concretestructure, it has been concluded on engineering judgment thatthe stability criteria (overturning and sliding) as set forthby U.S. Army Engineering Regulations ER 1110-2-106 Appendix B,jwill be met for all required loading cases.
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SECTION 7 - ASSESSMENT/RECOMMENDATIONS
7.1 ASSESSMENT
a. SafetyExamination of the available documents and the
visual inspection of Lake Mahopac Dam did not reveal anyconditions which constitute an immediate hazard to lifeor property.
Using the Corps of Engineers screening criteriafor examination of spillway adequacy, it has been deter-mined that the dam would be overtopped for all storms ex-ceeding approximately 56 percent of Probable Maximum Flood(PMF). The maximum spillway discharge capacity is 7.9percent of the PMF peak outflow. The spillway is, there-fore adjudged as "inadequate".
b. Adequacy of InformationThis report and its conclusions are based on the
visual inspection, interviews and hydrologic/hydraulicstudies. This information and data are adequate for aPhase I inspection.
c. Need for Additional InformationNo additional investigations are required for
the project.
d. UrgencyAll remedial actions described below should be
completed within one year of notification to the owner.
7.2 RECOMMENDED MEASURES
The recommended measures are as follows:
1. The missing portion of the upstream stonemasonry wall should be repaired.
2. Brush and shrubs should be removed from theembankment, spillway-reservoir drain structure and down-stream channel. A program of periodic cutting and ormowing of the dam surfaces, spillway-reservoir drainstructure and the downstream channel should be provided.
3. The collected debris at the spillway-reservoirdrain structure and in the downstream channel should beremoved and hauled away from the dam.
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4. A program of periodic inspections and main-tenance of the dam and appurtenances including yearlyoperation and lubrication of the reservoir drain and itscontrol facilities should be provided. This informationshould be documented for future reference.
5. An emergency action plan for the projectshould be developed.
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DRAW INGS
APPENDIX A
CCor
PLAT NO2
10541WiI
OSCAWANA LAKE. N.Y. QUADAJECARMEL, N.Y. QUAD
\ ~~A2 "--L C' CROTON FALLS, N.Y. QUADS'D Rock ll -
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N ) 75 '~ecr Corners.Pbnd-
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LAKE MAHOPAC
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I PHOTOGRAPHS
APPENDIX B
-
71
DRAWINGS
APPENDIX A
1op~sConer
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o i 23 tleES -
LAK LakePA DAMp
i3PLAT NOS
-~~~~~~~ a -- .--
t8CAWANA LAKE. N.Y. QUAD
- '&~ (2 *iAJE CARMEL, N.Y. QUADA* ; 7 -CROTON FALLS, N.Y. QUAD4 ./\ * J~ii MOHEGAN LAKE, N.Y. QUAD4 ,-----Cenr.
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PHOTOGRAPHS
APPENDIX B
2. UPSTREAM VIEW OF DAM. (NOTE: FloatingDebris in Reservoir)
3. UPSTREAM VI EW AT SPTILLWT\Y-RESERVOIIRI DRAIN STRUCTURE.
I.e
4. VIEW OF CREST FROM SPILLNAY-RESERVOIRDRAIN STRUCTURE LOOKING LEFT. (NOTE: TPrees)
5. VIEW OF ('PE'ST 110KI NG RI GUT. I'IOTOC-RM' IT£ ~ ~~~~~TAKEN AlBOUT 70 PT ' ROTAiM''A 2IETlACON TACT'.
JJ
6. VIEW OF SPILLWAY-RESERVOIR DRATN STRUCTURESILL. (NOTE: Collected Debris)
7. VIEW OF SPILLWAY-PIESERVOTP. DRAIN STPJICTUREi AT DRAIN OUT.ILE"T. (NOTE: Collected Debris),
- -'IVr
-" °-A -.
tU-.-
*8. VIEW OF DOWNSTREAM FACE OF SPILLWAY-
V 4 RESERVOIR DRAIN STRUCTURE. (NOTE: Vege-j tation and Collected Debris)
II:
1 9. VIEW OF' tPSTRRA!. FACE. (NOTE: Upstrom~nStone Wall. Condition)
10. VIEW OF DOWNSTREAM CHANNEL -LOOK
VISUAL INSPECTION CHECKLIST
APPENDIX C
VISUAI. I N p irt iro ("llif.1-!,t'r
Basic Data
a. Ceneral
qae of Dam LA%-g tAAVAOPAC
Fed. I.D. q 4%.'. 1'Zg DEC Dam No. 213 - -.-
River Basin 14t. '2 50A ?',v /3FASv/
Loc.ation: Town MA4oeYc3 VL ; .County ' "Rt'% ..4tA
Stream Name U S _ 0a) V_.
Tributary of- rLP$OA1 .i k-
Latitude (N) 41/ 22,S Longitude (W) 73 -5s, 3 0
Type of Dan _-_ ___ _ _
)hazard Category 4 (>Date(s) of Inspection /1 6
Weather Conditions '.QW44kL-702 F.
R ~eservoir Level at Time of inspection L.~ 2b. -Inspection Personnel IA1V EY P-E: Lb Mr% PM7 QT
c. Persons Contacted (Including Address & ho21e No.)
TOWAJ ,1b91 -0 A 01 F Cit eME4.
______A,'AL PIAL A ,'
MA No PA r AV Y lo"4I
i-d. llistory: *
D~ate Constructed 1940 Date!(s) heconstructe]________
Designer 1ptkOIJtP
Constructed By nown4)r)
Owner 7r,&'4/ OFP C,'U MEZL (/ddre ';--as. ahpre).Y1[ .Sheet I
a. Characteristics
f (1) Dmbankmen t Material lCr+ iU C/sc~oni o/ .e~r1-A
(2) Cutoff Type knowi
I (3) Impervious Core P/nknrotan9
(4) Internal Drainage System ank/ ownflO
-(5) Miscellaneous A/on2e
j. Crest
(1) Vertical, Alignment 600d
.2) Horizontal Aliginent QC'.
(3) Surface Cracks /One Observed
(4) liscellaneous brUSheS and tS vral /5rie $ Vres;Sevenm/
..a . On,,al burw
c." Upstream Slope
* (1) Slope (Estimate) (V:11) VEP-riCAL - MKA. 6" LL.
(2) Undesirable Growth or Debris, Animal Burrows
"I - (3) Sloughing, Subsidence or Depressions ANer /e. "b/f4Cf
aboo* 30 ' o( Ala/I d,/sft/ced od Sirnes
ISheot 2
(4I) Slope2 Pr.otection -~ ~~ vi
(5) Surface Cracks or Movcment at Toe Un oherlblc - 6io w wkfle ve
d. Domstream Slope
(1) Slope (Estimate- V V:') 10ries9 rerbh / -1o /6r) 41 H)
(2) Undesirable Growth or Debris, Animal Burrows _ .,____
/ad -an,,. AurrvA.S
(3) Sloughing, Subsidence or Depressions _ t/0o AslerA'e.
(I) Surface Cracks oi Mov'ement at Toe A/oe o.5ser-rca'
.(5) Seepage A/ne oier-e
(6) External Drainage System (Ditches, Trenches; Blanket) 1/o n
(7) Condition Around Outlet Structure
(8) Seepage Beyond Toe "4/o"2 obsrc-jed
e. Abutments - Embankment Contact
1.J
Sheet 3
* ".
(1) Erosion at Contact , , n-e obk f--e
(2) Seepage Along Contact ban e e-v-cd•
3) Drainage Svtcm
a. Description of System &P ar/0&-ro e
b. Condition of System
0. Discharge from Drainage System ------ _
) Instur.icntatiop (Momumentaton/Surveys ", Observation Wells, Weirs,Piezometers, Etc.)
Se• Sheet 4
- • -
a.S3opes 4LAI SLotes tlle. -
b. Sedimentation
c. Unusual Conditions Which Affect Daim _______- ___
0) Area Downstream of Dam
a. Downstream Hazard (No. of Homes, 15-.ghwziys, etc.) ,Several hODle'S And
_UceIf brys ,4es cmd ".___ees_c." Evidence of Movement Beyond Toe of Da n-Okne Ohsei--d.
d. Condition of Downstream Channel se6 qkll/. c/ ,?n/(7d) __,___Spi.wav(s) (Includfir., Di.scharre Conveyance Channel)
&r eiarc also -",hs /qW liel OMu/d.
a. Grneral -9;. sS/ne . e3,.cbre aIv ./ I/48 eve/
S • t-- ,. 00 / / - .
b. .Condition of Fervice Spillway
•I. /Ic,_ . ,,' a .
Sh.. 5
EMO~. 1
C. Condition of Auxiliary Spillway A/,o
d. Condition of Discharge Corrveyaneec Channel .~ AM7 .& ckma 1wdso 4c ka o /"goo .
) Reservoir Dain/Outiet ""
Type: Pipe _ __Conduit ______other eIrCC4/a(..4 1/e 6oncrek
Material: Coficrete G,_ _ metal Other
50: / Length
Invert Elevations: Entrance Exit
Physical Condition (Describe): / . Unobservable
Material:. ------
Joints: . Alignment
Structural Integrity:
Ilydraulic Capability:
Means of Control: Gate -_ Valve . .. Uncontrolled
Operation: Operable _V_ Inoperable -.------- Other
Present Condition (Describe): MO/ .,0era, cl- Afi, rf
rv/ ma JeI.n b
/oW/n9Sheet6S.fe 6
II . • • "
a. Concrete Surfacecs 7_P,_______4tt'X dr--O
- b. Structural Cracking O :
c. Movement - Horizontal & Vertical Alignment (Settlemmnt) -
to ., r ved
d. Junctions with Abutments or Dhnm.t ~t
e. DLains - :Foundation, .Jo.Lnt, race v sijbl-e I,
f. Wlater Passagcs, Conduits, Sluices A
g- Seepage or Leakage _ _ _'_ _ _._ _
S e
*1
h. Joints Constr'uction, etc. a/1 AL_ L s /jP.
ti F~oundation to~
II
I j- Abutmients o
k. - Control Gates ______ _________
I.- Approach & Outlet Channels __________________j 1
: C0
m'. Energy Dissipators (Plunge '?ool, etc.)1\o.
n. Intake Structures _
3 o. Stability L%0 1J Aa A-~AJL. V JJII
P. Miscellaneous
• . .. ..h e e t 8
10) Appurtenant Structures (Powerhouse,Lock,Gatehouse,Other)
a. Description and Condition 62n/V ,h;fIL--
-A
I'
[ .See
HYDROLOGIC DATA AND COMPUTATIONS '
APPE~NDIX. D
CIR1CK LIsTr FOR, DAMSHYDROLOGIC A1!0) ItYI)IKAULIC
ENCIE~R1G DTA
AREA-CAPACITY DATA:
Elevation Surface Area Storasge Capnacity(f.)(acres)__ (acre-ft.)
1)Top of Dam _ _ __ _ _ __ _ _ _ _
2) Design High WJater(Max. Design Pool)-
3) Auxiliary SpillwpyC rest Nvove ~ ______ _____
4) Pool Level withFlashboards________________
5) Service Spillway /300Crest
DISCHtAPRGEVolume(cis)
1) Average Daily (jJjJO AJ A)
2) Spillway Q Maximurrt HN'-h Water 42
3) Spillway @ Desigyn High Wat( r V4 J~'rj b"-"-
4) Spillway @ Auxiliary Spillway Crest Elevation______
5) Low Level Outlet (Rg4se~nlvt - 9(r"lri'
6) Total (of all facilities) @ maximum High Water______
7) Maximum Known Flood rjbwr.)
8) At TimeC of Inspection
IZ
CREST: ELEVATION;: _ __ _
Type: 5-a r__
Width: VAri.s 4 _67;) Length:. /9¢ FT
Location* ArbJ z6' .r.Ap". d L 1.'h
SP I LLWAY:
SERVICE AUXILIARY - //e4,
( _67 Elevation_._.,5/r r - C,1rceL Type
Width
T.ype of Control
Uncontrolled
Controlled:_Type
(Flashboards; gate)
_Number
Size/Length,
Invert Material-
Anticipated Lengthof operating service
____ ___ ___ ___ ___ ___ ___Chute Length _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
__ _ __ _ _ Height Between Spillway Crest& Approach Channel Invert
(Weir Flow)
i
I'..
II1YDROMFTIROLOGICAL GAGES: 0 nu
Ty pe: _ _ ___ _ _ _ _ _ _ _ _ _ _ _ _ _
Location. _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Records:
Date-
flax. Reading-
r LOOD WATER CONTROL SYSTEM:
Warning System: __ _ _ _ _ _ _ __ _ _ _ _ _ _ __ _ _ _ _ _ _ _
* Method of Controlled Releases (mechanisms):
. whlwli
4
DRAIVAGE AREA: -7 A ___.__,_I___
DRAINAGE BASIN RUNOFF CHARACTERISTICS:Land Use - Type: -, 6o * rt V rb .La e\ 4 oC-A, r
Terrain -Relief: L. i it,. lA,( 9 S"L A000r.Surface - Soil: A- i W "i4
Runoff Potential .(existing or planned extensive alterations to existing'(surface or subsurface conditions)
Potential Sedimentation problem areas (natural or man-made; present or future)
K~b~X-ik. v~r'ij 4 LO{C
Potential Backwater problem areas for levels at maximum storage capacityincluding surcharge storage:
Dikes - Floodwalls (overflow & non-overflow ) - Low reaches along the
Reservoir perimeter:
Location:
Elevation:
Reservol r:
Length @ Maximum Pool (__ _ _ (Miles)
Length of Shoreline (@ Spillway Crest) 5" 8 (Miles)
f
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REFERENCES
APPENDIX E
'REFERENCES
3. "Flood Hydrograph Package (HEC-I) Users Manual forDam Safety Investigations", U. S. Army Corps of Engineers,Hydrologic Engineering Center, September 1979.
2. "Seasonal Variation of the Probable Maximum Precipitation,East of the 105th Meridian for Areas from 10 to 1,000Square Miles, and Durations of 6, 12, 24 and 48 flours",Hydrometeorological Report No. 33. Weather Bureau, U.S.Department of Commerce, April 1956.
3. "Recommended Guidelines for Safety Inspection of Dams",
Department of the Army, Office of the Chief of Engineers,Appendix S.
4. The University of the State of New York, The StateEducation Department State Museum and Science ServiceGeological Survey - MAP and Chart Serves No. 5, GeologicMAP of New York 1961, Lower Hudson Sheet.
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