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AD-AO66 182 NEW YORK STATE DEPT OF ENVIRONMENTAL CONSERVATION ALBANY F/S 13/13NATIONAL DAM SAFETY PROGRAN. LAKE DE FOREST DAM (INVENTORY NUIPS-ETCtU)JUNd 76 C NH6(1*4 DACW51-79-C-oool
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IIIII::I II
HACKENSACK RIVER BASIN
ROCKLAND COUNTY, NEW YORK
CD INVENTORY NO. 95
PHASE I INSPECTION REPORT
NATIONAL DAM SAFETY PROGRAM
LAKE DE FOREST DAM
MnDTICf t SE-ECTE n
Prepared by: NEW YORK DISTRICT CORPS OF ENGINEERS
For: THE STATE OF NEW YORK
Date: 14 JANUARY 1978 PLCfl=TIS t T I'S BYST
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HACKENSACK RIVER BASIN - LAKE DE FOREST DAM
INVENTORY NO. 95
TABLE OF CONTENTS
Paragraph Description Page
Assessment i
Topographic Map -
Lake de Forest Dam and Reservoir ii
PHOTOS iii to v
I PROJECT INFORMATION 1
1.1 GENERAL I
1.la Authority 1
1.lb Purpose of Inspection 1
1.2 DESCRIPTION OF PROJECT 1
1.3 PERTINENT DATA 2
1.3a Drainage Area 2
1.3b Discharge at Damsite 2
1.3c Elevation 2
1.3d Reservoir 2
1.3e Storage 2
1.3f Reservoir Surface 2
1.3g Dam 3
1.3h Diversion and Regulating Tunnel 3
1.3i Spillway 3
1.3j Regulating Outlets 3
2 ENGINEERING DATA 4
2.1 DESIGN 4
3 VISUAL INSPECTION 5
3.1 FINDINGS 5
3.1a Embankment 5
3.1b Concrete 5
3.1c Downstream Channel 5
3.1d Spillway/Gates/Valves 5
3.le Reservoir Area 5
3.2 EVALUATION 5
4 OPERATIONAL AND MAINTENANCE PROCEDURES 7
4.1 PROCEDURES 7
4.2 Maintenance of Dam 7
4.3 Maintenance of Operating Facilities 7
4.4 Evaluation 7
5 HYDROLOGIC EVALUATION OF FEATURES 8
5.1 DESIGN DATA 8
5.2 EXPERIENCE DATA 9
5.3 VISUAL OBSERVATIONS 9
5.4 OVERTOPPING POTENTIAL 9
6 STRUCTURAL STABILITY 10
4
T A B L E O F CONTENT S (Cont'd)
Paragraph Description Page
6.1 EVALUATION OF STRUCTURAL STABILITY 106.1a Visual Observations 106.1b Design and Construction Data 106.lc Operating Records 106.1d Post Construction Changes 106.le Seismic Stability 10
7 ASSESSMENT: RECOMMENDATIONS/REMEDIAL MEASURES 117.1 SAFETY 117.2 RECOMMENDATIONS/REMEDIAL MEASURES 11
LIST OF DRAWINGS
Drawing No. Description
400.15-1 LOCATION PLAN400.15-2 to 4 BORING LOCATIONS PLAN400.15-5 GENERAL PLAN400.15-6 DAM ELEVATIONS400.15-7 ABUTMENT SECTION AND DETAILS400.15-8 SPILLWAY SECTIONS AND DETAILS400.15-9 EARTH FILL SECTION AND DETAILS400.15-10 TOE DRAIN-PROFILE AND DETAILS400.15-11 INTAKE STRUCTURE
LIST OF APPENDICIES
Appendix Description
I Geology of Lake De Forest Dam
II Hydraulic/Hydrologic ComputationsIII Check List - VISUAL INSPECTIONIV Check List - ENGINEERING DATA
I
PHASE I REPORTNATIONAL DAM SAFETY PROGRAM
Name of Dam: LAKE DE FOREST DAMState Located: NEW YORK STATECounty Located: ROCKLAND COUNTYStream: HACKENSACK RIVERDate of Inspection: 14 DECEMBER 1977
ASSESSMENT
The Lake de Forest Dam is well maintained and operated. Nothing wasdetermined to render the dam unsafe. No remedial measures are nec-essary. However, to monitor actual behavior, instrumentation in theform of weirs and piezometers should be installed within a reasonableperiod of time and read and evaluated periodically under the super-vision of a licensed professional engineer.
Approved by:CLARK H. BENNColonel, Corps of EngineersDistrict Engineeer
aTIs GRA&I
Uhannounced
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PHOTO
LOOKING ALONG DAM ALIGNMENT(FROM EAST TO WEST)
PHOTO2
DOWNSTREAM TOE OF SLOPE(LOOKING WEST)
iii
PHOTO3
OVERFLOW SPILLWAY AND ABUTMENTS
PHOTO4
DOWNSTREAM SLOPE OF DAM
(LOOKING WEST)
iv
PHOTO5
CREST OF DAM (LOOKING WEST)NOTE GROWTH ON UPSTREAM FACE
PHOTO .
OUTLET CHANNEL LOOKING DOWNSTREAMTOWARD RELOCATED OLD MILL ROAD BRIDGE
v
ai 'Or
PHASE I INSPECTION REPORTNATIONAL DAM SAFETY PROGRAM
LAKE DE FOREST DAM I.D. NO. 95
SECTION 1 - PROJECT INFORMATION
1.1 GENERAL:
a. Authority. National Dam Inspection Act, Public Law 92-367,August 1972.
b. Purpose of Inspection. The purpose of this inspection and reportis to investigate and evaluate the existing conditions of subject dam inorder to: (1) identify deficiencies and hazardous conditions; (2) deter-mine if they constitute hazards to human life or property; (3) and notifythe State of New York of these results along with recommendations for reme-dial measures where necessary.
1.2 DESCRIPTION OF PROJECT:
a. The Lake de Forest Dam, designated hazard category I as per the"National Inventory of Dams", is owned by the Spring Valley Water Works andSupply Company, with main offices located in West Nyack, New York. Con-structed in 1956, it is located along the Hackensack River within the Hack-ensack Basin, approximately on-half mile north of West Nyack in RocklandCounty, New York. It is a 40-foot high (max.) compacted homogeneous earthfill section dam approximately 1330 feet long, with a uniform embankmentslope and combination manhole and toe drain system to collect subsurfaceseepage on the downstream toe-of-slope. The dam empounds the HackensackRiver for distribution to residents of the Town of Clarkstown, Orangetownand Haverstraw and portions of the Towns of Ramapo and Stony Point in Roclk-land County, New York. The outlet works consist of two Howell-Bunger valves,Z4-inches and 12-inches in diameter, located adjacent to the west side ofthe spillway, Water is accepted on the rip-rap upstream face of the damthrough a semi-circular intake via a 60-inch diameter pipe and dischargingdownstream of the spillway. A valve control room and chamber facilitatesregulation of the outflow. Adjacent to the outlet valve system is a 100-footwide ogee-section concrete spillway regulated by two 5-foot high in its raisedposition, 50-foot long bascule gates and associated gate control rooms allow-ing for overfow into the unpaved downstream channel. Diversion to the treat-ment plant (not shown on plans and constructed subsequently to this project)is accomplished through a 24-inch diameter conduit. The downstream slope ofthe dam is grassed. A 20-foot wide run-of-bank gravel service road extendsalong the top of the dam. The relocation of Old Mill Road and constructionof a steel service bridge spanning the east and west abutments extending overthe spillway was also included in this project.
b. Normal Operational Procedure - During normal operations, the two bas-cule gates are in the raised position (elevation 85 feet) which will maintaina fulL normal pool elevation which is desirable for the purpose of water supply.Above elevation 85 feet, water flows over the spillway. The hydraulic pressure4 from the water surcharge above 85 feet causes the bascule gates to lower toelevation 80 feet. This procedure goes into effect automatically. A hydrau-lically controlled system is provided in the control structures to lower or
raise the bascule gates at will. The manually-operated 12-inch outlet andthe electrically-operated 24-inch outlet are generally kept closed duringnormal operation. Thv dam, bascule gate Control Area, and intake structureare inspected hourly as part of the normal patrolling and operating proce-dures. A continuous reservoir stage record is kept and read every hour.The dam is inspected annually by the Chief Engineer.
1.3 PERTINENT DATA:
a. Drainage Area - 26.6 sq. miles
b. Discharge at Darrsite -Maximum known tlood at damsite 80-85 MGPWarm water outlet 88 CFSDiv. tunnel low pool outlet 139 CFSDiversion tunr. outlet 591 CFSGated spillway at elevation 85 feet 3,578 CFSGated spillway at max. pool elevation 10,118 CFSUngated spillway at max. pool 10,118 CFSTotal spillway at max. pool 10,118 CFS
c. Elevations (above MSL -
Top of dam 100 FTMax. pool 90 FTSurcharge 10 FTFull flood control pool 85 FTRecreation pool 85 FTSpillway crest (bascule gates Up) 85 FTSpillway crest (bascule gates Down) 80 FTUpstream portal invert dlv. tunnel 57 FTDownstream portal invert div. tunnel 56 FTStreambed at centerline of dam 51 FTMaximum tailwater 64.5 FT
d. Reservoir -Length of maximum pool 5.02 MILESLength of recreation pool 4.39 MILESLength of flood control pool 4.39 MILES
e. Storage -Recreation pool 17,250 ACRE FEETFlood control pool 17,250 ACRE FEETDesign surcharge 19,110 ACRE FEETTop of dam 36,362 ACRE FEET
f. Reservoir Surface -Top of dam 1,675 ACRESMaximum pool 1,130 ACRESFlood control pool 1,025 ACRESRecreation pool 1,025 ACRESSpillway crest 900 ACRES
2
g Dam-Type: Homogeneous compacted earth fill (gLacial till)Length: 1,330 feetHeight: 40 feet (maximum)Top width: 2' feetSide Slopes: 2 H on 1V (upstream and downstream)Zoning: NoneImpervious core: NoneGrout Curtain: No grout used on project.
h. Diversion and Regulating Tunnel -Regulating Facilities - A portion of the outflow from De Forest Lake
is diverted through a 24-inch diameter conduit at elevation 59.75 feet to
the 6pcing Valley Treatment Plant where it is treated and dispensed to
local consumers.
i. Spillway -Type: OgeeLength of weir: 100 feetCrest elevation (gates down): 80 feetCrest elevation (gates up): 85 feetGates: Two 50 feet hydraulically-operated bascule gates. Each gate
is 5 feet high, raising spillway crest to 85 feet (above msl).Downstream Channel: Reinforced concrete spillway discharges into
small stilling basin.
J. Regulating Outlets -There are three sluice gates in the intake structure, each 36 inch
x 36 inch in size, at elevation 60.00, 67.50, and 75.00 feet msl respectively.These are used to control the total flow from the dam into the treatment plantand thence into the Hackensack River. The Howell-Bunger valves work similarlyto control the outflow from the dam. The 24-inch Howell-Bunger valve is elec-trically operated by a manual switch. The.12-inch Howell-Bunger valve is manu-ally operated. The bascule gates allow the storage of 5 feet of additionalwater. These are automically operated by an air operated hydraulic cylinderenergized by a float mechanism responding to the elevation of the water in thereservoir pool.
3
I;4
SECTION 2: ENGINEERING DATA
2.]1 DESIGN:
The design of the damn and related structure was made by the engineeringfirm of Buck, Seifert, and Jost of Englewood Cliffs, New Jersey. Per-tinent sections of the following have been utilized in this report andcopies retained in the New York District's file:
a. Invitation and Instructions to Bidders - Specifications, Pro-posal, Contract, and Bond for the Construction of Lake de Forest Damn,August 1954.
b. Report on a Geophysical Survey at the Proposed Northern ReservoirDarn Site of the Spring Valley Water Works and Supply Company December 1956January 1951.
C. Engineering field book utilized during construction (sheets 32through 51) providing steel sheet piling cut-off information.
d. Spillway design computations for Lake de Forest Dam.
e. Structural stability and analysis computations for overflow section.No as-built drawings exist for this project, however, contract drawings wercutilized for analysis purposes. Portions of the design analysis have beeninspected and are available for further review at the offices of Buck, SeiterL,and Jost.
4
SECTION 3: VISUAL INSPECTION
3.1 Findings:
a. Embankment. The dam which is approximately twenty two (22) yearsold showed no signs of sloughing or erosion of the embankment slopes. Damembankment/abutment interfaces showed no visible signs of distress and/ormovement. Neither the vertical nor horizontal alignment of the crest wasaltered with no unusual movement or cracks observed including riprap fail-ures or dislocations. Visual inspection of the toe drain pipe collectionsystem, which collects toe drainage prior to being conveyed to the junctionchamber for discharge out through the west trainig wall, was made throughthe manhole at Sta. 11+13. Observation of discharge at the manhole, reflec-tive of toe drainage only, shoed only slight flow. On the upstream riprappedface, brush and small trees were growing uncontrolled over the area.
b. Concrete. Visual inspection of the concrete structures includedchecking for surface cracks, structural cracking, vertical and horizontalalignment, monolith and construction joints, the spillway, the valve con-trol room, chamber facilities, the intake tower, and associated concrete wallsand appurtenant structures. In general, the concrete appeared to be in excel-lent condition. The design of the abutment section calls for a joint drainat each vertical joint discharging into a washed gravel and crushed stone rockdrain which in turn discharges through a 12-inch pipe to a weir chamber.Joint drainage through the spillway joint system likewise flows into this weir.Visual observation indicated slight seepage through this sytem into the weirchamber.
C. Downstream Channel. No visual evidence of excessive scour wasobserved. Minor brush exists on the channel slopes.
d. Spillway/Gates/Valves. The heaters in the spillway pier and the up-stream spillway approach were functioning and preventing ice floes from block-ing the spillway. The bascule gates were raised (elevation 85 feet). Onegate was temporarily locked for the purpose of maintenance. The Howell-Bungervalves exhibited a very slight leak leading to a negligible discharge fromthe outlets. The 24-inch outlet was opened showing a clear discharge withoutany visible sign of sedimentation or debris. Approximately three inches ofwater was discharging over the spillway. The flow was smooth and uniform.
e. Reservoir Area. At the time of the inspection, the reservoir poolelevation was at 85.2 feet (msl). Although small ice floes were spotted onthe reservoir surface, the spillway area was clear of ice and any otherobs truc tions.
3.2 Evaluation:
Visual observation revealed that the dam and attendant structures are struc-turally sound and no immnediate actions to remedy any observed deficienciesshould be taken.
5
a. The slight flow previously mentioned as occurring and collectingthrough the toe drainage system is indicative of the minimal through andunderseepage resultant from the combination of the compacted glacial tillembankment material and the steel sheet pile cutoff extending from groundsurface along the centerline of the dam down to rock.
b. Although it is aesthetically unpleasing, the brush and growth onthe upstream riprapped face does not cause any structural problem.
c. The observed discharge through the joint drains in the abutmentsand spillway section is not considered excessive, but should be monitoredfor changes in quantity.
6
e - -
SECTION 4: OPERATIONAL AND MAINTENANCE PROCEDURES
4.1 PROCEDURES:
By New York State Law, the Spring Valley Water Company is required to dis-charge a minimum of 9.75 MGD from the dam. This discharge is releasedprimarily through the Howell-Bunger valves. De Forest Lake is the firstof a series of water supply reservoirs along the Hackensack River. Afterleaving de Forest Lake, the Hackensack River discharges into Lake Tappan(capacity: 3,378,000,000 gal.), another water supply facility, then pro-ceeds downstream to Oradell Reservoir (capacity: 2,850,000,000 gal.), theterminus of the water supply system. The Hackensack Water Supply Systemserves consumers in Rockland Company, New York and Bergen Company, NewJersey.
4.2 Maintenance of Dam: The dam and associated structures are very wellmaintained with a regular program of lawn cutting and general maintenancein effect.
4.3 Maintenance of Operating Facilities: At the time of this inspection,repair of the west bascule gate hydraulic cylinder was under way with thegate locked in its vertical position. Repair was scheduled to be completedshortly. This repair is not considered to have any effect on the safetyof the dam. Furthermore, although there was no warning system in effectat the time of the inspection other than a regular surveillance patrol, itwas learned that the owner was presently contracting for an electronic sur-veillance system to guard against unauthorized entry into restricted areas.
4.4 Evaluation. The Lake de Forest Dam, which is approximately twenty two(22) years old is exceptionally well maintained and operated. The operatinggates and valves are in good operating condition (except the bascule gatementioned above). The dam was not designed for rapid drawdown, and there-fore could not effectively be accomplished with the existing outlet worksdue to the relatively small discharge through the two valves as compared tothe larger impoundment in the reservoir.
7
|'I
SECTION 5: HYDROLOGIC EVALUATION OF FEATURES
5.1 DESIGN DATA:
a. DeForest Lake Dam was designed to discharge a peak flood of 10,000cfs. At that time, the spillway length of 100 feet and spillway elevationof 80 feet with bascule gates lowered were set.
DeForest Lake Dam was designed to function primarily as a water supply facil-ity, not a flood detention structure. However, for the purpose of this in-vestigation, the design features were analyzed with respect to their floodcontrol potential. This potential was investigated through the applicationof the Probable Maximum Flood (PMF) to the contributing basin. The PMF isthat hypothetical flow induced by the most critical combination of precip-itation, minimum infiltration losses, and concentration of run-off at a spc-cific location, that is considered reasonably possible for a particular drain-age area.
The drainage area of DeForest Lake Dam is 26.6 square miles. Hydrologically,the basin characteristics are similar to those of Pascack Brook at Westwood(Drainage Area - 29.4 sq. mi.). Clark unit hydrograph parameters (Tc and R)were derived from the ratio of DeForest Lake Drainage Area to Pascack BrookDrainage Area (See Appendix II). Although a U.S.G.S. stream gage is situatedon the Hackensack River one mile downstream of the dam, Clark parametersderived from the gage values are not representative of the basin because ofthe regulating effect of the dam as a diversion. In light of recent guide-lines for determining the Probable Maximum Precipitation (HydrometeorologicalReport No. 51" - Sept. 1976), the PMP index rainfall was determined to be24.6 inches for a 24 hour duration, 200 sq. mi. basin. The percentages of theindex rainfall applied to other durations were interpolated from the plot ofdrainage area versus percent of 24 hour, 200 sq. mi. (See Appendix II). Thecomputed PMF peak flow was 23,497 cfs, or more than twice the design peak flowof 10,000 cfs. A plot of the PMF inflow hydrograph is included in the aboveappendix.
b. Assumptions made concerning the discharge - storage (capacity) of thereservoir were:
(1) That the initial storage of the reservoir prior to the PMF was12,448 acre-feet, based on spillway crest at 80 feet.
(2) That the two Howell-Bunger valves are closed for the durationo; the PMF. Only the 100 feet spillway is effective in discharging theattenuated flood. (The discharge capacity of the two valves is negligiblecompared to the capactiy of the spillway.).
(3) That the bascule gates are lowered to elevation 80 feet to dis-Lharge the surcharge of five feet at the beginning of the PMF.
c. The PMF was routed through the reservoir yielding a reduction of peakflow from 23,500 cfs to 15,610 cfs. The reservoir stage corresponding to theattenuated PMF was determined to be 93.5 feet (from the spillway rating curvein Appendix II). Since the top of the dam is at elevation 100 feet, theattenuated PMF was contained, leaving 6.5 feet of freeboard to the top of the
I
| i n nn m8
dam. An attenuated peak of nearly 28,650 cfs would correspond to the topof the dam.
5.2 EXPERIENCE DATA:
DeForest Lake Dam has been operational since 1956. During this time, ithas safely discharged the floods which have occurred in the Rockland CountyWatershed. The maximum pool elevation was 86 feet. The maximum dischargeas recorded by the U.S.G.S. gage at West Nyack was 1,550 cfs on 3 February1973. The drought years of the mid 1960's produced the maximum drawdown ele-vation of 70 feet MSL.
5.3 VISUAL OBSERVATIONS:
At the time of the on-site inspection of DeForest Lake Dam, no deviationsfrom the design plans were observed.
5.4 OVERTOPPING POTENTIAL:
No actual storm event has had sufficient magnitude and duration to approachovertopping of the dam during the 20 year period of record. The rating curvesdeveloped for this evaluation indicate that overtopping of the dam occurs ata discharge of 28,650 cfs (See Appendix II). For the hypothetical design PMFof 15,610 cfs, the maximum elevation is 93.5 or 6.5 feet below the top of thedam.
, i I9
SLJTION 6: STRUCTURAL STABILITY
6.1 EVALUATION OF STRUCTURAL STABILITY:
a. Visual Observations. Visual observations did not indicate anyexisting structural problems. Both embankment and concrete sectionsshowed no evidence of distress or deterioration. Joint drainage throughthe spillway joint system is well below the tolerable limit and poses nostructural problem at this time.
b. Design and Construction Data. Available stability and structuralcalculations for the spillway have been checked using the guidelines inOCE "Recommended guidelines for safety inspection of dams, Appendix D" andEM 1110-2-2200 and were found to be satisfactory. The calculations ana-lyze the structure for overturning, sliding, concrete stresses, and pres-sure on rock at four loading conditions.
Condition #1 - Reservoir empty.
Condition #2 - Reservoir with three foot thick ice and with topof ice at crest of spillway.
Condition #3 - Reservoir at flood condition with water 10 feetabove crest of spillway.
Condition #4 - Reservoir at flood condition with water 10 feetabove crest of spillway and bascule gate in raisedposition.
Using the above loading conditions the structure was checked at seven hori-zontal planes through the structure and at a horizontal plane through thebase. The structure was found safe for overtuning and sliding. The con-crete stresses and pressure on rock were acceptable. Furthermore, based oncalculations contained in Appendix 11, the spillway is adequate to carrythe computed PMF. Although no embankment slope stability analyses wereperformed, embankment material is of a suitable nature to be stable at thedesigned and constructed slopes. The combination of compacted glacial tillembankment material and the steel sheet pile cut-off extending down a suf-ficient depth to refusal or rock (records of sheet pile location and rockelevations available in District file) have resulted in the absence of bothcontrolled and especially uncontrolled seepage contributing to a structurallysafe dam.
c. Operating Records. Operational records of the dam are availablefrom the Spring Valley Water Works and Supply Company. No major operationalproblems were reported since its completion. As the DeForest Lake Dam wasdesigned and constructed as a water supply dam and has been subjected to afull head of water almost continuously since its construction, its stabilityis considered to be adequate based on performance with a factor of safety ofat least one.
d. Post-Construction Changes'. Since completion of this project, theSpring Valley Water Works and Supply Company has added a treatment plant tothis facility. The treatment plant was not inspected as part of this study.
e. Seismic Stability. The dam is located in Seismic Zone No. 1, there-fore a seismic analysis is not warranted.
10_ _ _
SECTION 7: ASSESSMENT: RECOMMENDATIONS/REMEDIAL
MEASURES71 SAFETY.The Lake de Forest Dam and the related structures of this project are in
such condition that they are not unsafe. Specific recommendationsbelow are suggested at t .Athis
e a re iof the design PMF was checked with the results thate ° med t Mn putlined
this time. As part of this investigation, the validit .flow was noticeably lower than the design value and that actual storm events
have not been of sufficient magnitude and duration to approach the PMF (PNF
calculations are contained in Appendix 11. these determinations
were made
in part by the information available through the various sources listed pre-
Vously in this report and the visual and engineering analysis m adeinspection group. a th te
7.2 RECOMMENDATIONS/REMEDIAL MEASURES.
No remedial measures are necessary. However, it is recommended that thefolliwng be affected:
(1) As no means of quantitatively measuring toe drainage in the
drain system has been provided (although this quantity is not considered
significant as mentioned Previously), a Pipe weir should be installed in the
8 -inch toe drain pipe at the manhole located at Station 11+13. Flow meas-
urements should be made periodically to monitor changes in discharge. The
fiow should be related to reservoir stages.(2) Cut and remove brush and small trees at the ground line on
the upstream riprapped face with disposal off site. These trees should not
be uprooted or bent over so as to create "cavities" on the embankment sur-
face which could introduce "zones of weakness" along the face of the dam.(3) Joint drainage in the abutments and spillway section should be
monitored and measured by the installation of a V-notch weir in lieu of the
flat weir Plate presently being utilized, Periodic measurements should be
recorded and related to reservoir stage.(4) As field observations
revealed no monitoring systems in the
dam, it is recommended that a system of three piezometers be installed in
the vicinity of the weir chamber (approx. Station 11+60). The piezometers
should be set at different elevations to monitor pore pressures within the
dam shell, the foundation material, and at a point intermediate betweenthese locations. Pore pressures should be related to reservoir stage.
11
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rfe, *.wwl DA Dy SPRING VALLEY WATER WORKS AND SUPPLY COMANsiY
p LAKE do FORESTDAM
S. .*EARTH FILL SECTION AND DETAILS
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PROFILE AND DETAILS
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INTAKE STRUCTUREAFA~w/.4 0.. iu-,_ -- ~7PLANS AND SECTION
SECTION~~~wo A-Ae' owos Jost~.O~8 bn~8d.
0. afsul, E a'eo-cl Ne P.y -15
GEOLOGY OF LAKE DE FOREST DAM
I - PHYSIOGRAPHY OF ROCtTLAND COUNTY
'[he Piedmont province and the New England province are sharply defined topo-graphically in Rockland County. The northwestern or highland part of thecounty is underlain by crystalline rocks of the Reading Prong extension ofthe New England province. The part of the highland near the New York-NewJersey boundary is the Ramapo Mountains and the part near the Hudson Riveris the Hudson Highlands. The surface of the upland is rolling and has lowrelief. The eastern face of the upland is a steep escarpment that overlooks.i broad lowland to the east. The bedrock consists chiefly of gent!y-dippingLcds of relatively soft sedimentary rocks that have been eroded to form aseries of low, northerly-trending ridges separated by narrow valleys. Theeastern slopes of the ridges are steeper than the western slopes. A ridgeof diabase rises above the lowland in eastern Rockland County and follows thet-cnd of the Hudson River to Haverstraw where it curves west and terminateseveral miles from the river. The western slope of the ridge is gentle at
some places and steep at others. The streams in Rockland County are tributary:to the Hudson,Hackensack, and Passaic Rivers. The HackensacK River drains anarea of about 48 square miles in eastern Rockland County. The discharge fromt he northern part of the watershed drains into Lake de Forest Reservoir U. -chis controlled by the Lake, de Forest Dam at West Nyack. The reservoir is,out four miles long, 0.25 to 0.5 mileswide, and has an area of about 1,020
,,.ire6. The storage capacity is about 5.6 billion gallons at a water surfaceof 85 feet above mean sea level. The maximum depth of water in the reservoirat full capacity is about 20 feet. The N-w York State Water Power and ControlQommission requires release of sufficient water at the dam to maintain a dis-charge equivalent to 9.75 MGD in the stream inmmediately above the intake worksof the village of Nyack at West Nyack.
II - GENERAL GEOLOGY
The Lake de Forest area is underlain by sedimentary and igneous rocks of Tri-assic age (the Newark group) which are covered by deposits of the Pleistoceneand recent epochs. The eastern boundary of the Newark group terminates be-queath the Hudson River in Westchester County. The constituent shale and silt-stonu form lowlands in the eastern part of the basin. In some places sand-stone and shale occur in alternating beds from about 2 to 20 feet thick. Thestrike of the beds of the Newark group ranges from north to N.450 E. and thetfip is westerly, ranging from 40 to 200. Faults have been reported in theNewark group at Upper Nyack and at Stony Point. Cross faults in the Palisadediabase extend for some distance into the adjoining rocks of the Newark,roup. The Newark group is the principal source of ground water in the countyit is well-cemented generally and most of the water occurs in openings alongbedding planes, joints, and irregular fractures. Such openings are as muchs 0.5 to 1.5 feet wide.
APPENDIX I
-- 7- !o-
--- .. . . .. . ....-
A) GEOLOGIC HISTORY
Near the end of the Triassic boundary, the Palisade diabase was intrudeiinto the sedimentary rocks in eastern Rockland County. At the (lost ofthe period, the Triassic rocks probably were broken into a series ot biuK
by northeast-trending faults. During the Pleistocene epoch, glaciers lte.t,
south across the county several times. The ice sheets eroded the surface It
the bedrock and during melting stages, they deposited an unsorted mixture ut
clay, sand, gravel, and boulders (till). Streams flowing from the ice shttdeposited stratified material in the larger valleys. During the mtltin..stage of the last ice sheet large lakes occupied parts of the HackensackHudson River valleys. Thick deposits of clay and silt were laid doun iiithese lakes along with layers of sand and gravel. Among the valleys th,.were deepened by glacial streams were those of the Hackensack and HudonRivers. The deepest point on the bedrock surface in the Hackensack valitis about 55 feet below sea level. Several abandoned channels have been tili,with these glacial deposits.
B) STRUCTURAL GEOLOGY
iri bedrock of Rockland County has been subjected to folding, faultinb, anc..,nous intrusions. The crystalline rocks of Precambrian age and the adjeil-: g belt of Paleuzoic rocks are highly deformed and so closely folded that:fs of the folds are nearly vertical in many places. The general trend oi
,oLk is northeasterly. Normal and reverse faults are conmmon. Differ-.itiai erosion in the Triassic basin has produced a series of parallel stli
-dges and valleys on the surface of the northeast-trending beds. Changes
Jn the strike of the beds in a few places imply the presence of faults. Veastern part of the county the Palisade diabase cuts across the Trias;i
L~d- as dikes or sills with prominent joints perpendicular to their moraint
'A its northern limit the trend of Lhe diabase ridge is almost due west an'
iily perpendicular to the regional strike of the sedimentary rocks. Thtdiabase ridge is cut by a series of faults, mostly of the normal type, andare seen as notches. The shattered condition of the diabase and the irregulaiconfiguration of its outcrop near Nyack suggest faults in the diabase. The
>tLcp back slope of the diabase ridge in several places is due to longitudii .:!,t ng.
C) UNCONSOLIDATED DEPOSITS
i bedrock is covered by unconsolidated deposits of Pleistocene and Recent.tge except in scattered areas chiefly on hills. The deposits of Pleistocene
wcre laid down chiefly during the last glaciation (the Wisconsin stage),aind are classified as till and stratified drift. The deposits of Recent ag(
:av. generally low permeability. Till (or hardpan) is generally the upper-r.io,t unconsolidated deposit. In the eastern part of the county till deposite0
ou Lot Newark group contains much locally-derived sandstone and shale and isr(,u-by-n. Generally, the deposits are less than 25 feet thick. Most of the
:.a'fied drift was deposited during the retreat of the ice when isolatedFiA1,s of ice occupied large depressions such as the Hudson, Hackensack, and
aq... o valleys. Deposits of reddish-brown varved clay and silt in the Hack-.%-,.k River valley are as much as 30 feet thick.
-r -
III - VISUAL DESCRIPTION OF THE PROPOSED DAM SITEFROM REPORT MADE IN 1951 BY KELLY GEOPHYSICAL SERVICES
The west abutment at the proposed darn site consists of a small hill lyingbetween the swampy land through which the Hackensack River runs on the east,and a higher and slightly less swampy area on the west. The hill has asteep easterly face and a gentle westerly slope. At its summit there areoutcrops of Palisades diabas;e which occur as sills injected between the bedsof shale. The gentle, westerly slope of this hill is close to the dip slopeof the underlying rock; the steep, easterly scarp would therefore cut acrossthe sill and bedding of the underlying shales. Geophysical observationssuggest that a dam built at the proposed location would probably have shalebedrock underlying it on the east abutment. The west abutment would beagainst igneous material and metamorphosed shales. The dike would run formost of its length on the igneous and metamorphic material, but for a shortdistance after it swings westerly towards the Strawtown Road, the bedrockwould be predominantly shale. Approaching the rise in ground towards theStrawtown Road, the bedrock again becomes shales, and igneous close to theroad. This igneous rock rock may be at a stratigraphically higher levelthan that capping the hill to the northeast. The spillway would be inigneous and metamorphosed material at the higher elevation, and in shaleymaterial at the lower elevation, near the point of confluence with the river.The underlying shales may carry minor intrusions and fingers of igneous rock.The overburden is mainly an alluvial deposit. On the higher ground, in theorchard and field west of the Hackensack River, it is silty and does not con-tain much sand. in the swamp east of the hill, through which the HackensackRiver meanders, the overburden is mostly clay. Sandy material may be en-countered in the overburden under the alignment of the dike. In some areas,a minor, buried channel may underlie part of the dike and because of thesandy filling, furnish a passageway for infiltrating water.
--w
HYDRAULIC/HYDROLOGICCOMPUTATIONS
APPENDIX 11I '
H4ACJ<EAI5Aqck R. AT W. NACK. D.A. =29.4 .Sc;.Mf.
F LOW. IREG.ULAThVD 65Y DE F09EST LAC-.
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I' 1 J.*LIST
EMGI7ERINC W.T.1
DRAINIAGE AREA C1UPOCTrlISTICS: 2 5 SQ. A4ILES
ELEVATION TOP MfRMtL FOOL (STar.AGF CAPACITY)- 17 ~ 50 A CA,= -,C7-
ELEVATION TO? PLOOD ==0'.L PCOL (STC7ACECAPA crci-T' : 7, ?60 4AC -Fr
MLVATION t~~lMDESIGN POOL:- go
ELEVATION TOP W01 : 100'
CREST:
a. Elevation 80(AE OWA/J) 5'A ( rES UP)
-b. Type .i D OIAET PLW '
c. Width ___________________________________
-d. Le n g th I/i L--AJ6TH' -0 YCOoce . Location Spillover 1 6,C7-E.D OF 'AAAf. rumber and Type of Gates 7 Vp 5ol ffAsCLE 6A7-r--S
OUTLET VORES:
,, y , 2 o10 4 . - 8VA1156 V',LveD COAJD(JI7rsb. Locatio. W,47' COA17ROL 5 rR ur.-r f,t
* C. Entrancc in-verts .3 5Sc//IC '647LS F_,Lem-o#.l5 bo', 47-x',-A. Exit inverts Z VLvES 04r E1.VATr#J 5
a. Eiargency draindown facilities 2WPv'V6 A1V /,Z 24$uW(
- IYROMETEOROLOGICAL. CAGES:a. Type S1L5 TREA P4 GAF - (t'Ano(5r QAIJi6/z ZX 41,ie,
-b. Location W IEDW TFI4,r~ EST HYAC4e
4c. Records A%/A ILA SL&
.!=XIUV11 ON-DAIMAGfl'G DISCErUrCE* 5085 h4D
