long pond dam engineering assessment report table …

GEA Engineering, P.C. - 1 -

LONG POND DAM

ENGINEERING ASSESSMENT REPORT

TABLE OF CONTENTS

Page

EXECUTIVE SUMMARY..................................................................................................5

1.0 INTRODUCTION........................................................................................................8

1.1 Location

1.2 Owner/Operator

1.3 Purpose of the Dam

1.4 Description of the Dam and Appurtenances

1.5 NYSDEC Dam Inventory Data on Long Pond Dam

2.0 RECORDS REVIEW.................................................................................................13

2.1 Summary of USACE Phase I Report 1981

2.2 Summary of Response to Phase I Dam Safety Requirements 2.3 Summary of NYSDEC Regulations, Letters, Inspections, Notice of

Violation 2.4 Summary of Kellard Sessions-Map, Plan and Report for Dam

Maintenance District 6/2013 and 3/2014

3.0 HAZARD CLASSIFICATION EVALUATION.............................................................26

3.1 Dam Reclassification

4.0 DAM INSPECTIONS................................................................................................28

4.1 Safety Inspection

4.2 Diver Inspection

4.3 Low Level Outlet Investigation

4.4 TV Inspection of Dam Drawdown Pipe

5.0 HYDROLOGY ASSESSMENT.................................................................................32

5.1 Hydrologic Model

5.2 Existing Hydrologic Conditions


Page

6.0 HYDRAULIC ASSESSMENT...................................................................................37

6.1 Hydraulic Model and Flow Regime

6.2 Site Survey

6.3 HEC-RAS Modeling 6.4 Existing Hydraulic Conditions for 100-yr Storm and "150% of the

100-yr" Storm

6.5 Dam Break Existing Conditions

6.6 Existing Dam Drain with Gate Valve

6.7 Proposed Hydraulic Conditions

6.8 Proposed Dam Break Conditions

7.0 STRUCTURAL STABILITY.......................................................................................49

7.1 Introduction

7.2 Geotechnical Investigation

7.3 Stability Analysis for Existing Conditions

7.4 Stability Analysis for Proposed Conditions

8.0 EMERGENCY ACTION PLAN REVIEW..................................................................60

9.0 CONFORMANCE TO CURRENT DAM SAFETY REGULATIONS..........................61

10.0 CONCLUSIONS/SUMMARY OF ACTIONS NEEDED...........................................65

REFERENCES...............................................................................................................68


TABLES Table 1 - Dam Deficiencies and Proposed Remediation Table 2 - Curve Number Table Table 3 - Hydrocad Results Summary Table 4 - Dam Break Analysis Summary Table 5 - Unified Soil Classification Table Table 6 - Existing Conditions Stability Analysis Table 7 - Proposed Conditions Stability Analysis

FIGURES Figure 1 - Location Map Figure 2 - Location Plan of Test Pits and Borings Figure 3 - Cross Section of Long Pond Dam Figure 4 - Long Pond Dam Profile Figure 5 - Inundation Map Figure 6 - Long Pond Watershed Area Figure 7 - Mianus River Watershed Area Figure 8 - Graphical Representation of the Resultant Force Direction Figure 9 - Load Configuration Diagram Figure 10 - Proposed Tree Removal Plan Figure 11 - Proposed Project Schedule


APPENDICES Appendix A - Previous Reports and NYS DEC Documents

• NYS District Corps of Engineers Phase I Inspection Report. September 1981

• Satterthwaite Associates Response to Phase I Dam Safety Requirements. January 1984

• NYSDEC Letter from Alon Dominitz to Supervisor Lombardi. April 16, 2003

• NYSDEC Inspection. March 22, 2007

• NYSDEC Notice of Violation from Donald E. Canestrari, P.E. April 29, 2015

• Kellard Sessions, Map, Plan and Report for Dam Maintenance District. June 2013 and March 2014

• NYS Inventory of Dams. Date of Data Update August 17, 2015

• NYSDEC, GEA Engineering, Town of North Castle Meeting. December 4, 2015

• Underwater Investigation Field Report. September 24, 2015 Appendix B – Safety Inspection Results Appendix C - Field Photographs Appendix D – HydroCad Model Results Appendix E – HEC-RAS Model Output Appendix F – Test Pit and Boring Logs, and Soil Test Results Appendix G – Emergency Action Plan Appendix H – Inspection and Maintenance Plan


Executive Summary

The Long Pond Dam is an earthen embankment gravity, hazard class B dam

constructed in the 1930's in the Town of North Castle. The purpose of this Engineering

Assessment is to comply with the NYSDEC Dam Safety Regulations Part 673 and

secure a permit. The dam is owned by the Town of North Castle and three property

owners. In February 2016 the town formed the Long Pond Dam taxing district to

manage the dam's expenses.

The dam surface area is 9 acres and impounds 83 acre-feet of water. The watershed

tributary to the dam consists of mostly residential and forest areas and includes two

dams: North Lake Dam and Windmill Dam. The dam discharges to a tributary of the

Mianus River which flows in the eastern direction.

The dam crest has a width 12-25 feet averaging about 16 feet. The height of the dam

ranges from 9.5 feet to 38.0 feet and averages about 25 feet. An existing spillway

consists of a 4 feet by 6 feet box culvert which discharges to a channel and flows to

Duck Pond.

The dam was constructed of low permeability soil, a silty sand (SM) above bedrock. It is

lined with a clayey sand (SC) liner on the pond side embankment.

Hydrologic and Hydraulic Analyses were conducted using a Watershed Model TR-20

(HydroCAD) and HEC-RAS Model Version 5.0 to assess flooding downstream. The

results of the analyses indicated for existing conditions:

• The dam overtops for a 100 year storm event

• Middle Patent Road, a Principal Arterial Other-NYSDOT designated road, is

flooded during the 100 year storm by 1.37 feet.

• Middle Patent Road is flooded by 2.52 feet for the 150% of the 100 year spillway

design flood.


The Engineering Assessment included the Dam Break Analyses utilizing HEC-RAS for

the following conditions with the results indicated:

• Sunny Day Dam Break (no rainfall)-Middle Patent Road was flooded by 2.10 feet.

• The 100 year storm event and Dam Break-Middle Patent Road was flooded by

4.87 feet.

• The 150% spillway design flood and Dam Break-Middle Patent Road was flooded

by 4.01 feet.

Middle Patent Road is used as critical infrastructure that could be impacted by a flood

hazard outlined above.

Raising the dam to provide additional storage and improving the spillway will lessen the

downstream flooding impacts.

Dam Stability Analyses were conducted for a range of conditions in accordance with the

NYSDEC Dam Safety Regulations Part 673. A summary of the Stability Analyses

shows:

• The dam is stable under all scenarios

• The improved dam has higher factors of safety and this is even more stable than

existing conditions.

This Engineering Assessment Report addresses improvements to the dam needed to

rehabilitate and comply with the NYSDEC Dam Safety Regulations Part 673. The

improvements will include:

• Additional spillway capacity.

• Improvement to the spillway discharge channel.

• Raising the height of the dam about 1.5 feet.

• Removal of trees at the toe area of the dam, trees on the water side of the dam,

trees that are 6 inch diameter or less, unhealthy trees and have an arborist

conduct an annual inspection of the condition of the trees and roots.

• The existing 12" diameter drawdown drain will be filled with cement or plugged

with a mechanical plug.


• The gazebo gate valve will be abandoned

• An agreement between the Armonk Fire District and Town of North Castle for the

Armonk Fire District to provide pump truck and hoses to pump down the Long

Pond Dam.

Following the review of this Report and associated Plans, GEA will need to prepare the

following documents:

o Engineering Plans and Specifications for Dam Repair

o NYSDEC Part 673 Dam Permit Application

o NYSDEC Protection of Waters Permit Application

Upon review and approval by NYSDEC, the Town/District may proceed with

construction. Tree removal is proposed to be conducted in November 2016.

Improvements are scheduled to be constructed in 2017. A detailed schedule is

provided.


SECTION 1.0

INTRODUCTION

The Town of North Castle secured the services of GEA Engineering to prepare an

Engineering Assessment Report for the Long Pond Dam, ID # 232-1094. This document

is prepared by GEA Engineering for the Town of North Castle in accordance with the

New York Code of Rule and Regulations, Part 673, for the Long Pond Dam.

The Long Pond Dam Engineering Assessment is required by the DEC to determine the

safety of the existing structure and to secure a permit.

In order to complete this Engineering Assessment Report, the following tasks were

completed:

• A review of records kept by the Town of North Castle

• On-site engineering inspection and investigation

• Engineering review, calculations and conclusions

• Engineering Assessment Report

• File report with the DEC and provide a copy for the Town of North

Castle

1.1 Location

Figure 1 presents a Location Map of the Long Pond Dam. The Long Pond Dam is

located in Westchester County in the Town of North Castle, New York, about 250 feet

northwest of Long Pond Road near the Long Pond Court cul-de-sac. Access is available

from Long Pond Court.

1.2 Owner/Operator

The properties surrounding the Long Pond which have a direct beneficial use of the lake

have formed the Long Pond Dam District. The Long Pond Dam District is a special

taxing district in accordance with NYS law, created to administer, manage, and fund the

construction, regulatory compliance, and annual maintenance costs for the Long Pond

Dam. The Long Pond Dam District includes twenty (20) parcels and the Town of North

Castle. The dam owners/members of the Long Pond Dam District are as follows:


Tax Lot Designation Owner Address

1/04/10.-45.BB Asif Ahmed 5 Oakridge

1/04/10.-45.C Stephen and Amy Berman 7 Oakridge

1/04/10.-600 June B. Greenspan 6 Oakridge

1/04/10.-601 Robert M. and Susan P. Klein 14 Pond Lane

1/04/10.-602 Susan R. and Zachary Shimer 16 Pond Lane

1/04/10.-603 Murray Leffler and Kay Trus 18 Pond Lane

1/04/10.-604 Joseph and April Paresi 20 Pond Lane

1/04/10.-605 Jeannette H. Loring 22 Pond Lane

1/04/10.-606 Daniel J. and Bertha McKee 24 Pond Lane

1/04/10.-72 Steven and Deborah L. Lipman 43 Windmill Road

1/04/10.-73 Peter J. Weiller 45 Windmill Road

1/04/10.-74 Robert and Ruth Shwab 47 Windmill Road

1/04/10.-202 Kenneth and Teresa Gleicher 4 Long Pond Court

1/04/10.-201 Amy and Joel Dworetzky 43 Long Pond Road

1/04/10.-200 Arturo and Victoria Maceira 41 Long Pond Road

1/04/10.-207 Aaron and Jennifer Katz 38 Long Pond Road

1/04/10.-208 Adam and Stephanie Gershuny 36 Long pond Road

1/04/10.-209 Russell and Melissa Katz 34 Long Pond Road

1/04/10.-210 Jeffrey N. and Elaine Allen 32 Long Pond Road

1/04/10.PK Town of North Castle Long Pond Road

The owners’ representative is the Town of North Castle’s Town Administrator under the

authority of the Town Supervisor.

1.3 Purpose of the Dam

The dam was constructed for recreation and aesthetic purposes. The dam dates from

about the 1930’s and was thought to be designed by the famous engineer Elwyn E.

Seelye. Construction design plans by Elwyn E. Seelye do not match the existing

conditions of the dam. The dam contains a spillway and a low level drain line. The dam

impounds water and stores it prior to releasing the excess to Duck Pond.


1.4 Description of the Dam and Appurtenances

The Long Pond Dam is an earthen dam that was constructed circa 1936. The dam

impounds approximately 83 acre-feet of water to form the Long Pond on the dam’s

western side. The pond has a maximum impoundment volume of approximately 117

acre-feet. The impoundment volume was estimated based on GEA's limited bathymetric

survey of the pond.

The Pond has a maximum length (north to south) of approximately 2000 feet and a

variable width ranging from 400 feet at its widest to about 150 feet at its shortest width.

The area of the pond is approximately 9 acres based on Westchester County GIS. The

Pond is surrounded by dense vegetation, trees and steep slopes.

The dam system is comprised of an earthen fill embankment with a 6 feet wide by 4 feet

high concrete box spillway which discharges into a rip-rap channel draining to Duck

Pond.

The height of the dam ranges from 9.5 feet to 38.0 feet as measured from the crest

(Elev. 474.5±) to the native material of the down stream embankment. The overall

length is approximately 600 feet. The crest of the dam is approximately 12-25 feet ±

wide and is comprised of fill soils covered with grass, trees and other vegetation. The

dam is considered an Earthen Fill Gravity Dam. Figures 2, 3, and 4 show the Plan,

Section and Profile of the dam respectively.

The condition of the dam at the time of inspection on August 26, 2015 was assessed as

follows:

• Functional

• Not in compliance

• Stable

• Deficiencies apparent

The concrete spillway is deteriorated and the dam has excessive tree growth which is

unacceptable by Dam Safety Standards.


1.5 NYSDEC Dam Inventory Data on Long Pond Dam

Name of Dam Long Pond Dam

State ID 232-1094

Hazard Code B

Year Completed 1936

Most Recent Inspection 3/31/2015

Location Information

Westchester County Town of North Castle Mianus River

Latitude: Longitude:

41.1463888889 73.6708333333

Type of Construction RE-Earth

Purpose Recreation

Technical Information

Federal ID Number NY00115

Dam Length (feet) 600

Dam Height (feet) 401

Spillway Width (feet) 6

Maximum Discharge (CFS) 2222

Maximum Storage (Acre-feet) 1743

Normal Storage (Acre-feet) 1154

Reservoir Surface Area (Acres) 115

Drainage Area (Square Miles) 0.466

Basin Long Island

Date of Data Update 8/17/2015

Last Condition Rating Not Rated

Date information obtained from NYSDEC: 3/5/16

1Field surveys and soil borings show that the actual dam height is 38 feet.

2 Hydrologic and Hydraulic Engineering Analyses determine that the

Maximum Discharge is 126.30 CFS.

3 The

Westchester County GIS, and Field and bathymetric surveys determined that the

Maximum Storage is 117

acre-feet.

4 The


Normal Storage is 83 acre-

feet.


5 The


Reservoir Surface Area is 9

acres.

6 The


Drainage Area is 0.63 square

miles.


SECTION 2.0

RECORDS REVIEW

A review of records maintained by the Town of North Castle and the NYSDEC included

the following documents:

• Dam Design Drawings by Seelye 1936

• NYS District Corps of Engineers Phase I Inspection Report, 9/1981

• Satterthwaite—Response to Phase I Dam Safety Requirements, 1/1984

• NYSDEC Regulations

• NYSDEC Inspection on 4/4/02 and Letter from Alon Dominitz to Supervisor

Lombardi, 4/16/03

• NYSDEC Inspection, 3/22/07

• NYSDEC Inspection on 3/31/15 and Notice of Violation from Donald E.

Canestrari, P.E., 4/29/15

• NYS Inventory of Dams, 3/5/16

• Kellard – Map, plan and report for Dam Maintenance District, 6/2013 and

3/2014

• Annual Certification, Emergency Action Plan (Appendix G), Inspection and

Maintenance Plan(Appendix H), Schedule

• NYSDEC, GEA Engineering, Town of North Castle Meeting, 12/4/15

The above records are included in Appendix A unless noted otherwise. Relevant information obtained from the records review includes:

• A Phase I Inspection Report for the dam which includes a description of the dam, the

purpose of the dam, the hazard classification, and the date the dam was constructed.

• A Response to Phase I Dam Safety Requirements which includes a seepage

analysis and an outlet works investigation.

• A NYSDEC inspection which identified dam seepage, undesirable tree and shrub

growth, and maintenance deficiencies.

• The Kellard Sessions Report which identified existing conditions of the dam including

maintenance deficiencies, tree overgrowth, and seepage. Proposed improvements

included tree removals and spillway repair.


• A NYSDEC “Notice of Violation” regarding lack of maintenance and safety issues

with the Long Pond Dam.

The NYS District Corps of Engineers Phase I Inspection Report and the Satterthwaite

Response to Phase I Dam Safety Requirements are both summarized below.

2.1 Summary of USACE Phase I Report 1981

The United States Army Corps of Engineers (USACE) completed a Phase I

Investigation to assess the general condition of the dam. The investigation was intended

to identify any need for detailed studies.

The USACE Phase I Report dated September 1981 contained the following remedial

measures that were required to be completed within 1 year of the report:

• Trees growing on dam

o All small trees, dead large trees and larger trees located near the crest

should be removed

o All large trees on the downstream slope should be inventoried

• The spillway capacity is inadequate

• Concrete spillway walls are spalling and deteriorating and should be repaired

• Downstream spillway bottom slab is undermined and should be repaired

• The low level drain valve condition is unknown

• The low level drain outlet could not be located

• Two seepages located at downstream embankment

o Seepage located at “Central toe” (area near large boulders)

o Seepage located at South abutment

• Capacity of the rock lined spillway channel should be evaluated

• Seepage area at the south abutment should be blanketed with a properly

filtered drainage blanket

• The low lying area upstream of Duck Pond should be backfilled to prevent

backwater encroachment on downstream slope

• An inspection and maintenance plan should be developed

• The Emergency Action Plan should be maintained and updated

The Phase I Report stated that the dam was structurally stable based on a visual

inspection and did not require a stability analysis to be done.


2.2 Summary of Response to Phase I Dam Safety Requirements

The Satterthwaite-Response to Phase I Dam Safety Requirements was prepared in

response to the USACE Phase I Report remedial measures.

The Response to Phase I Dam Safety Requirements was written by Walter B.

Satterthwaite Associates, Inc. dated January 1984. The report contains engineering

analyses and recommended actions needed to address the Long Pond Dam safety

concerns from the Phase I Dam Safety Inspection and Report dated September 1981.

The following are the engineering analyses completed:

• Hydrologic and Hydraulic Analysis

• Seepage Analysis

The report also included several recommended actions in response to the Phase I Dam

Safety Requirements.

2.2.1 Engineering Analysis— Hydrologic and Hydraulic Analysis

The report included a Hydrologic and Hydraulic analysis in response to the Phase I

report. The spillway design flood (SDF) for Long Pond Dam (Class B) is 150 percent of

the 100 year flood. The hydrologic analysis utilized a 100 year rainfall event of 7.5

inches in a 24 hour period. The runoff hydrograph was simulated and routed using the

USACOE HEC-I software package. The SDF hydrologic and hydraulic analysis resulted

in the following:

Peak Inflow

Rate (CFS)

Peak

Overflow

Rate (CFS)

Maximum Depth of

Embankment Overtopping

(in.)

Maximum Velocity of

Overtopping Flow

(ft/s)

484 271 5.3 2.0

The SDF analysis concluded that the spillway had inadequate capacity and the dam

was overtopped. However the dam being overtopped was considered safe due to the

minimal depth over the embankment and maximum velocity of overtopping flow was

safe for a grass lined channel.


The analysis was also simulated with the dam crest raised 10 inches (average across

crest). This raised dam crest resulted in the dam being overtopped by 1 inch

(considered negligible). The raised dam crest analysis concluded that adequate spillway

capacity is achievable without altering the spillway.

The current 100 year rainfall event is greater than the previous rainfall event used in the

Satterthwaite Response to Dam Safety Requirements Report and will also result in the

dam being overtopped.

2.2.2 Engineering Analysis—Seepage Analysis

Two seepages were observed at the right abutment (looking downstream) and central

toe (the area near the large boulders).

The report indicates that a 12 inch cast iron outlet drain was located on the downstream

face 34 feet below the crest of the dam at the right abutment at the seepage location.

A Seepage analysis was performed to determine the source of water discharging from

the right abutment and central toe (near large boulders) of the dam. Water samples

were collected from the two (2) seepage locations, Long Pond and local ground water

wells. The chemical properties of the water samples were analyzed to determine the

sources of the seepages. The following parameters were analyzed:

• Maturity of Groundwater (Silica, Calcium, Magnesium, and Chloride

concentrations)

• Iron concentrations (sensitive to Oxygen content)

• Copper, Zinc and Barium concentrations (trace constitutes of

groundwater)

• Temperature, pH

The report concludes that the right abutment seepage is due to groundwater infiltrating

the 12 inch cast iron outlet drain through its joints. It is also concluded that the seepage

from the central toe of the dam is a “spring” fed by the by Long Pond. Recommended


actions included: installation of a filter blanket and drain at the “central toe”, rock lined

channels to convey water from the seepage locations to Duck Pond and program for

routine monitoring of the seepages. The recommended actions were not implemented.

The two seepage locations have been recently observed.

2.2.3 Recommended Action—Modification of Dam to Achieve Spillway Capacity

The report recommended the crest of the dam to be raised an average of 10 inches for

a length of 300 feet to provide adequate spillway capacity.

2.2.4 RECOMMENDED ACTION—PROVISION OF WORKING RESERVOIR DRAIN

The report recommended the following procedure for investigating the condition of the

drain:

• The drain outlet should be excavated and examined for damage.

• The gate at the gazebo should be verified that it is in satisfactory

condition and in the fully closed position.

• Once the main gate is closed the drain pipe can be unblocked.

• Once the pipe is unblocked the main gate should be operated.

• If the drain valve was found to be in satisfactory condition, it could

be left in service

• If the drain and valve was found to be in poor condition and

inoperable, it was recommended to abandon the drain and valve by

installing a concrete plug at the upstream drain pipe.

2.2.5 Recommended Action—Repair of Spillway Structure

The report recommended that the undermined spillway foundation be repaired by

pouring concrete for a firm foundation. The spillway areas that were spalling and

deteriorating were recommended to be repaired with concrete patchwork.

2.2.6 Recommended Action—Control Wet Conditions on the Embankment

The wet conditions on the embankment caused by the seepages included several

recommended actions.


The recommended action for right abutment seepage by the drain outlet was to provide

a rock lined channel to safely convey seepage to the Duck Pond.

The recommended action for the “central toe” seepage by the large boulders included

the following:

• Removal of the boulders covering the discharge

• Excavation of the area over which the spring developed

• Placement of a sand bed and filter fabric

• Placement of drain pipe

• Backfill excavation with coarse gravel

• Provide rock lined channel to safely convey flow to the Duck Pond

For both seepages, it was recommended to implement a program to monitor the

discharges.

2.2.7 Recommended Action—Selective Removal of Trees

A professional forester examined and inventoried the trees growing on the dam. The

following were the recommended actions:

• All trees and brush occupying the upstream slope and crest should

be removed and seeded with grass.

• Unsound, unhealthy, unwanted and dead trees should be removed

from the downstream embankment.

• Routine evaluation of the trees on the dam should be incorporated

into the Operations and Maintenance Plan.

2.2.8 Recommended Action—Backfill Marshy Area

The report stated that the marshy area near the toe of the dam at Duck Pond was not

hazardous to the dam and no action was recommended.

2.2.9 Recommended Action—Place Upstream Protection

The report recommended rip-rap be placed on the upstream slope for a distance of 300

feet between the spillway and a cluster of boulders located at the southern extent of the


embankment. It included placing twelve (12) inch diameter rip-rap from the crest to at

least two (2) feet below the normal pool level.

There is no evidence that the recommended actions were completed.

2.2.10 Remedial Work

The report concluded with the following primary remedial work items:

• Provision of a working reservoir drain

• Selective Clearing of trees from the embankment

• Provision of positive means to control and monitor discharges on

the embankment

• Raising of the dam crest to achieve spillway capacity

2.3 Summary of NYSDEC Regulations, Letters , Inspections, Notice of Violation

The purpose of the NYSDEC’s Division of Water Dam Safety Section includes: safety

inspection of dams; technical review of proposed dam construction or modification;

monitoring of remedial work for compliance with dam safety criteria; and emergency

preparedness. The NYSDEC Division of Water Dam Safety Section have composed

several letters, conducted inspections, and issued a notice of violation in regards to

Long Pond Dam. These documents are summarized below.

2.3.1 Summary of NYSDEC Regulations

The NYS DEC Dam Safety Regulations were revised and revisions became effective on

August 19, 2009 including 6 NYCRR Part 673 ("Dam Safety"). The revisions required

the owners of all Class C and Class B dams to:

• Operate and maintain the dam and all appurtenant structures in a safe condition

• Maintain in good order all available records regarding the dam, and provide those

records to any new owner and;

Owners of Class B Dams - Intermediate Hazard Dams are required to:

• Submit an Annual Certification to DEC by January 31, 2010, and annually

thereafter; Develop and submit to DEC an Emergency Action Plan (EAP) by

August 19, 2011. Submit annual updates to DEC thereafter. (Have some or all of

the EAP prepared by a Professional Engineer only upon request of DEC);


• Develop and implement an Inspection and Maintenance Plan by August 19, 2010

(do not submit the plant to DEC unless requested); Have an Engineering

Assessment (EA) conducted by a Professional Engineer and submit the Report

to DEC every 10 years. The first EA for a Class B dam is due by August 19,

2015;

• Have a Safety Inspection Conducted by a Professional Engineer on a regular

schedule as defined in the Inspection and Maintenance Plan. Do not submit the

Safety Inspection report to DEC unless requested;

• Report flows in erodible spillways to DEC within 5 days.

The NYS DEC Guidelines for Design of Dams also outlines specific design guidelines.

The Guidelines for Design of Dams for Hazard Class B Dams that apply to Long Pond

Dam include the following but are not limited to:

• A permit is required to construct, reconstruct, repair, breach, or remove without a

permit issued pursuant to 6 CRR-NY Part 608.3 Dams.

o All permit applications must include an engineering design report, plans

and specifications. Design must be stamped by a NYS registered

Professional Engineer.

o After completion of construction NYS DEC Dam Safety Section requires

� A certification from the construction engineer that the project was

constructed in accordance with approved plans

� As-built documents for the project

• Hydrologic Design Criteria for Existing Dams

o For Existing Hazard Class B Dams the Spillway Design Flood is 150% of

100 year

• Hydraulic Design Criteria

• Minimum Freeboard for Small Hazard Class B Dams is 1 foot (Freeboard is

measured from the top of the dam to the maximum water surface elevation)

• Single Spillway should have capacity and durability to handle extreme floods and

be non-erodible

• A low level outlet or drain is required to discharge 90% of the storage below the

lowest spillway crest within 14 days, assuming no inflow into the reservoir.


• Seepage Control

• Geometries of Earth Dams

• Vegetation Control - Trees and Brush are not permitted on earth dams

• Structural Stability Criteria including Geotechnical Investigations

• Existing Dams: Rehabilitation and Modification

• Prepare Inspection and Maintenance Plan, and Emergency Action Plan

2.3.2 Summary of DEC Letter From Alon Dominitz to Supervisor Lombardi, 4/16/03

The letter was written by Alon Dominitz, P.E. dated April 16, 2003 to the Town of North

Castle’s Supervisor Lombardi regarding a NYSDEC inspection on April 4, 2002. Mr.

Dominitz noted the following adverse conditions during the visual inspection:

• The concrete spillway structure has cracks and spalling.

• There are shrubs planted at the upstream and downstream edges of the dam

crest

• There are many mature trees on the upstream and downstream slopes of the

dam.

• The crest of the dam was well mowed, but had lawn furniture and yard waste on

it.

• There is a seep at the tow of the dam which may be associated with a leaking

and inoperable impoundment drain.

• There is no known mechanism for draining the impoundment incase of

emergency or for maintenance.

Mr. Dominitz also notes that these conditions are similar to those observed during

previous inspections. The NYSDEC recommended that a registered professional

engineer be retained by the Town to evaluate and report upon the dam and to propose

a design to bring the dam to conformance with current safety criteria.

2.3.3 Summary of NYSDEC Inspection, 3/22/07

The NYSDEC conducted a visual inspection of Long Pond Dam on March 22, 2007.

The NYSDEC inspection noted deficiencies such as Seepage, Maintenance and

Undesirable Growth and included several comments:


• The exact extent of the dam crest and downstream embankment should be

reviewed for hazard classification

• Dam Spillway

o At the time of inspection logs appeared to be in the outlet channel.

o At the time of inspection there was an unsecured hinged chain link fence

in spillway

o Surficial deterioration, spalling and chipping on sides of spillway

o Voids/concrete deterioration, and exposed reinforcing rods

• Upstream Dam Face

o Juniper bushes (near spillway) and mature trees on embankment (right

side)

• Dam Crest

o Mature trees on dam crest

• Downstream Face

o Seepage observed at center of dam- where crest makes a turn creating a

valley

o Rust colored seepage (2-5 GPM) from 3 large rocks- possible location of

drain pipe outlet (leaking valve?)

o Yard waste dumped on embankment

o Mature trees along entire length of embankment. Shrubs on far left end.

o House located at far right end-near right groin

2.3.4 Summary of NYSDEC Notice Of Violation, 4/29/15

The NYSDEC issued a Notice of Violation written by Donald E, Canestrari, P.E. to the

Long Pond Dam owners(The town of North Castle, Teresa and Kenneth Gleicher, Amy

and Joel Dworetzky, and Victoria and Arturo Maceira) on April 29, 2015 after conducting

an inspection on March 31, 2015. The NYSDEC stated that the dam owners have

neglected the dam, not performed inspections, recommended maintenance and not

monitored identified deficiencies at the dam. The NYSDEC issued the following

violations of the Department’s Dam Safety Regulation (6 NYCRR Part 673):


Citation Requirement Status

673.6 Develop and Implement an Inspection and

Maintenance Plan. Due August 19, 2010

VIOLATION—Not

developed/ implemented

673.7 Develop and distribute Emergency Action Plan

(EAP). EAP must be reviewed on annual basis.

Due August 19, 2011

VIOLATION—Not

submitted

673.8 Submit Annual Certification—must be submitted

by January 31 of each year, beginning in 2010.

VIOLATION—Not

submitted

673.13 Complete and submit Engineering Assessment

(EA) by August 19, 2015

Due August 19, 2015

The NYSDEC acknowledged that the Town was pursuing establishing a taxing district.

The NYSDEC recommended that the impoundment be drained until a responsive

owner(s) is identified. The NYSDEC stated “By June 12,, 2015, provide a written

response that includes a detailed plan and schedule to bring the Long Pond Dam into

conformance with Dam Safety requirements, or your plan to lower and/or breach the

structure.”

2.3.5 Summary of Meeting Between NYSDEC, Town of North Castle and GEA,

12/4/15

On December 4, 2015 a meeting was held to discuss permit issues and alternatives for

Long Pond Dam between:

• John Kellard P.E., Town of North Castle Engineer

• Alon Dominitz, P.E.-NYSDEC Chief, Dam Safety

• Syed Alam P.E.- NYSDEC

• Mark E. Lewis-NYSDEC

• Steven Gamelsky, P.E., President GEA Engineering

Re-classifying the dam’s hazard classification was discussed in order to reduce the size

of the spillway. A lesser class (from a “B” to an “A”) would result in a smaller spillway

size requirement. The NYSDEC said that it would be difficult to change the classification

and was generally not receptive to it. However, the NYSDEC was receptive to a lesser

spillway design criteria from 150% to 100% of the 100 year storm event flow, if the


Town can demonstrate no additional impact from flooding at Middle Patent Road. In this

regard, NYSDEC needed the Town to evaluate these scenarios:

• 100 yr. flow

• 150% of 100 yr flow

• Sunny day dam break failure

• 100 yr flow with dam break

• 150% flow with dam break

The NYSDEC said that they are adopting the Draft NOAA Atlas 14 Precipitation Data

and requested the data be used in the models.

The dam drain down was also discussed. The discussion first focused on the possibility

of eliminating the use of the 12-inch drain pipe (and not having to repair the gatehouse-

valve box structure). The alternatives discussed included:

• Private Contractor on-call for pumping down the dam. Need 1500 GPM.

NYSDEC was not convinced that during a major storm event a private

contractor would absolutely live up to the terms of the contract

particularly in terms of response times.

• Use of Town’s fire trucks dedicated for emergency dam pump down.

Most fire trucks have pumps rated at 1000 – 1500 GPM. This will

require a minimum of one truck or possibly two. JK to check with the

Town to see what are their fire truck pumping capacities. Also, if they

could check the pump’s suction-lift and if their pumps are self-priming.

The depth of the pond is about 24 – 26 feet and the pumps would have

to provide a suction lift of about 30 ft.

• Provide a stand alone generator with pumps. This would be a fully

enclosed unit mounted on a pad with a small shed for hoses.

• Provide a permanent or portable siphon. This will require a pump to

prime the siphon and some new pipe construction on the water side of

the pond with attachments for hose connections.


The NYSDEC also needed the Town to demonstrate:

• Integrity and condition of the existing 12" drain pipe.

• Conduct cleaning and TV inspection of the 12" dia. drain.

The NYSDEC was receptive to modifying the plan for complete tree removal as follows:

• Remove trees at the toe of the dam area – dry embankment side (fill

area). • Remove trees on the water side of the dam. • Remove trees that are 6" dia. or less. • Remove unhealthy trees • Conduct an annual inspection of the condition of trees, roots. • Implement a Maintenance Plan.

2.4 Summary of Kellard Sessions—Map, Plan, and Report for Dam Maintenance

District, 6/2013 and 3/2014

The Kellard Sessions Consulting, P.C. Map, Plan and Report for Dam Maintenance

District provided information on the Dam, existing conditions of the dam and the need to

create a Dam Maintenance District to bring the dam into compliance with NYSDEC Dam

Safety Regulations. The report proposes the Dam Maintenance District to include 20

properties surrounding the pond which have direct benefits of the pond. The report also

confirms that the future management of the dam must proceed under an Inspection and

Maintenance Plan, an Emergency Action Plan must be developed, records need to be

kept and annual certifications must be provided to the department.


SECTION 3.0

HAZARD CLASSIFICATION EVALUATION

Long Pond Dam is located within a Residential-Rural area, and the associated

downstream hazards include residences, utilities as well as rural/county roadways. It

appears that a structural or a hydraulic failure of the embankment could result in

flooding and damage to homes and downstream structures as shown on the Inundation

Map (Figure 5). There are however, no houses downstream of the dam that would be

inundated by a dam failure. The inundation map also includes Long Pond Road at two

points where the roadway is overtopped and one point along Middle Patent Road.

According to NYSDOT Functional Class Road Viewer, Middle Patent Road is classified

as a Principal Arterial Other. A ‘Principal Arterial Other’ roadway has the following

characteristics:

• Serve corridor movement having trip length and travel density characteristics

indicative of substantial statewide or interstate travel.

• Provide an integrated network without stub connections except where unusual

geographic or traffic flow conditions dictate otherwise

Based on current available studies presented later in this report, Middle Patent Road

also overtops during a 100 year flood event.

Therefore, In accordance with NYSDEC Dam Safety Regulations Part 673, Long Pond

Dam is, in our opinion, a Class B hazard structure.

Class B dams are considered to be Intermediate Hazard Dams whereby the

downstream impact due to a failure of the dam could cause damage to homes and

serious economic loss.

3.1 Dam Reclassification

In order to reduce the impact of flooding at Middle Patent road, a larger spillway than

the existing 6’ wide x 4’ high is required at the Long Pond Dam. The larger the spillway,

the more difficult and expensive it would be to construct. A reduction in the dam’s

hazard classification from a class B to class A would require a smaller spillway. During

discussions between the NYSDEC, the Town of North Castle, and GEA Engineering,


the NYDEC was not receptive to a dam reclassification. The NYSDEC was however

receptive to using a lesser spillway design criteria from 150% to 100% of the 100-yr

storm event flow if it can be demonstrated that no additional significant impact from

flooding at Middle Patent Road would occur. The NYSDEC therefore wants the

following scenarios evaluated:

• 100-yr flow

• 150% of 100-yr flow

• Sunny day dam break failure

• 100-yr flow with dam break

• 150% flow with dam break

The scenarios were evaluated and are presented in Section 6.0 Hydraulic Assessment.


SECTION 4.0

DAM INSPECTIONS

4.1 Safety Inspection

The safety inspection for the Long Pond Dam was performed on August 26, 2015 in

conformance with 6 CRR-NY 673.12. The safety inspection included the following:

• Document review (Section 2.0)

• Review of data

• Visual Safety Inspection

• Corrective Action Assessment

The Visual Safety Inspection Results for the Long Pond Dam are included in Appendix

B. The items inspected in the field are as follows: upstream dam face, downstream dam

face, dam crest, spillway, downstream area, and access roads. The condition of the

dam is satisfactory. The main concerns are the trees and brush located on the crest,

upstream slope, and downstream slope. All trees and brush located on the dam

embankment are required to be removed in accordance with NYSDEC Guidance for the

Design of Dams Section 9.4 Vegetation Control – Trees and Brush (9.4.1). Roots can

cause additional seepage paths, fallen trees can leave large holes in the embankment,

and brush can limit visual inspection, retard grass growth and provide a haven for

burrowing animals. A list of the dam deficiencies and proposed remediation measures is

presented in Table 1.

The records review indicated that the dam has a history of seepage. Seepage is still

occurring in two areas:

1. The south abutment (area at the low level outlet pipe)

2. Central Toe (An area under the large rocks/boulders)

Both seepages are not of immediate concern but they should be monitored and

inspected regularly. The records review also indicated that these two seepages date

back to September 1981. The seepage at the central toe was measured in the field by


confining the seepage flow and collecting in a 1000 ml beaker. Six (6) trials were

performed with an average measured seepage flow rate of 0.78 GPM at the central toe.

4.2 Diver Investigation

On September 24, 2015 GEA supervised a diver inspection of the drain down valve box.

The divers were provided by McLaren Engineering. The diver investigation of the dam

included underwater visual/video inspection of the gate valve and gate valve shaft

structure to determine operability and notable defects. Visual/video inspection was also

used to determine the existence and location of the outlet drain pipe. In addition three

soil core samples were obtained from the pond embankment (pond-side) to determine

whether a pond liner exists and also to ascertain the thickness of the liner. See Section

7.2.7 for soil core sample results from the pond embankment. A copy of the McLaren

Underwater Investigation Field Report is included in Appendix A.

The diver inspection revealed the following:

- The Gazebo in the middle of the pond is a concrete inlet structure for the

low level outlet.

- The concrete inlet structure has a trash rack inlet box located at the

bottom of the pond.

- Trash Rack measured 5 feet long x 3 feet wide and consists of 24 vertical

bars spaced 2 inches on center.

- The concrete inlet structure has deteriorating timber stop log guides, the

timber stop logs would have been used to close the concrete inlet

structure to access the gate valve via the manhole located on the Gazebo

floor.

- The gate valve operating handwheel, valve stem, stem guides, hardware,

anchor brackets and fasteners are in poor condition.

- The handwheel is seized and inoperable.

- The upper section of the valve stem between the handwheel and the first

coupler is missing.

- The upper stem guide bracket is corroded and damaged.

- Gate valve could not be inspected due to 3 ft to 6 ft high debris buildup in

the bottom of the gate valve shaft.


- The low level drain outlet pipe within the pond was not observed due to silt

buildup

4.3 Low Level Outlet Investigation

The low level outlet on the downstream embankment was inspected by GEA on May 16,

2016 with the use of a Cat 302.5 Mini Excavator provided by Woodward Organic

Recycling. The outlet was located 75 feet downstream of the crest of the dam at the

south abutment in line with the gazebo. The low level outlet pipe is a 12" cast iron pipe.

Upon uncovering the pipe, a large amount of stored water emptied from the pipe for

approximately 30 seconds. After the initial volume of water emptied from the pipe water

flowed out of the pipe at a "trickle". Water also appears to flow around the pipe, as there

is evidence of water flowing around and on top of the exposed pipe. Seepage from

within and around the drain outlet was measured in the field by confining the seepage

flow and collecting flow in a one (1) gallon bucket. Five (5) trials were performed with an

average measured seepage flow rate of 0.5 GPM at the outlet pipe. Photographs of the

outlet pipe can be seen in Appendix C - Field Photographs 1-3.

4.4 TV Inspection of Dam Drawdown Pipe

The dam drawdown pipe TV inspection was conducted by GEA on June 2, 2016 with

Contractor Fred A. Cook Jr. Inc.

Before the TV inspection it was noted that the water pooling at the outlet of the pipe

showed signs of rust-colored iron bacteria. This indicated that the water flowing out of

the outlet pipe was most probably groundwater.

The TV Inspection started at the exposed outlet on the downstream embankment of the

dam. The TV Inspection went upstream of the outlet approximately 176 feet +/-.

The pipe was not pre-cleaned prior to the TV Inspection.

During the inspection there were small obstructions that did not impede the inspection.

The following were the obstructions noted:

• 3-4 inch diameter rock obstruction 10 ft upstream from the outlet


• Small debris (apparent piece of pipe) at 62.7 and 68.9 feet upstream from the

outlet (No breaks in the pipe were found that matched debris)

One (1) crack in the pipe was located 32 feet upstream of the outlet, located on the pipe

in 10 to 12 o'clock position.

Overall the pipe joints appear to be in good condition. Sections of the cast iron pipe are

approximately twelve (12) feet long estimated by the distance between joints. There

was seepage at one (1) pipe joint located at 82 feet upstream of the outlet, located on

the pipe in the 3 to 5 o'clock position. This was categorized as a running seep (Seeps

categorized as Dripping, Running or Gushing).

At the end of the pipeline there appears to be a closed gate valve. The valve appears to

be slightly crooked. Pictures of the TV Inspection are located in Appendix C Field

Photographs.


SECTION 5.0

HYDROLOGIC ASSESSMENT

5.1 Hydrologic Model

The Long Pond receives stormwater runoff from surrounding properties including the

Windmill Lake, the North Lake, and the South Windmill Lake. The primary land use is

residential properties.

Hydrologic Models were prepared for this Engineering Assessment Report based on the

latest hydrologic guidance criteria, including precipitation data from NOAA Atlas 14.

An Existing Conditions watershed hydrological model was developed using Hydrocad

version 7.0 (TR-20, TR-55) computer program to estimate the 10-yr, 25-yr, and 100-yr

peak discharge flow rates from the Long Pond. The Long Pond discharges to the

smaller Duck Pond. The duck pond discharges via a spillway into a tributary to the

Mianus river that crosses under Long Pond road and meets with the upstream reaches

of the Mianus river at a confluence point approximately 1150 LF downstream of the

Banksville Road culvert.

In addition, the Hydrocad model was used to estimate the peak discharge flow rates

and Hydrographs along the Mianus River. The model includes flows from tributary

watershed areas up to the Limit of Study indicated in the FEMA FIS report dated

September 2007 (2713.2 ft downstream of Brookwood Road). The flows calculated

were used as inputs to the HEC-RAS Model (Section 6) using hydrographs generated

by the Hydrocad model.

Inputs to the Hydrocad Models include the following:

1. Watershed Areas

A. Long Pond

The Long Pond watershed area was delineated into drainage sub-areas with the

use of Westchester County GIS as shown on the Long Pond Watershed Map -

Figure 6. The watershed area includes residential properties with a combination of

driveways, rooftops, and grass, roads, woodland, and lakes.


The Long Pond watershed area includes North Lake, Windmill Lake, South Windmill

Lake, and their respective tributary areas. Files were created in Hydrocad for each

lake and linked to the Long Pond. The total area of the Long Pond watershed is

405.5 acres.

B. Mianus River (FEMA FIS Study Limit)

The watershed area contributing flow to the FEMA FIS limit includes the Mianus

river watershed area starting from the New York/Connecticut border upstream and

extending downstream to the study limit located approximately 3000 ft downstream

of Brookwood Road as shown on the Mianus River Watershed Map Figure 7. The

watershed area also includes the Long Pond and its tributary areas, and the Middle

Patent Road culverts and their tributary areas.

The Middle Patent road culverts are double HDPE culverts along the main channel

of the Mianus River and a single CMP culvert located approximately 1000 ft due

east of the double culverts along Middle Patent road. The single culvert conveys

flow from a tributary branch of the Mianus River east of the main river channel.

Sub-areas contributing runoff to the Mianus river include a combination of storage

areas, stream reaches, culvert crossings, residential areas, woodlands, roadways,

and industrial/commercial areas which were all factored into the runoff time of

concentration for the development of the Hydrocad model.

The storage areas include low-lying swamps, wetlands, and ponding areas. The

stream reaches include relatively flat meandering overland waterways as well as

defined streams. Culvert crossings are represented at locations where the river

meets roadways. Residential areas include a combination of driveways, rooftops,

and grass areas which are all accounted for in the curve numbers used in the

creation of the model. Woodlands also affect the curve numbers used and were

accounted for in the model based on percentage of cover within each sub-area.

Roadways and Industrial/Commercial areas which included parking lots and

rooftops were factored in the curve number calculations as impervious areas.


Runoff from Sub-areas 9 and 10 were routed to the east culvert at Middle Patent

Road via tributary streams and culverts.

Runoff from sub-areas 1, 2, 3, 4, 5, 6, 7, 8, 20, and the Long Pond tributary were all

routed to the main channel double culverts at Middle Patent Road.

Runoff from sub-areas 11, 12, 13, 14, 15, 16, 17, 18, and 19 were routed to the

Mianus River downstream of Middle Patent road.

The total area tributary to the FEMA Study Limit is 2942.4 acres.

2. Time of Concentration (Tc)

The time of concentration for each sub-area was determined by entering sheet flow,

shallow concentrated flow, channel flow lengths and slopes, and curve numbers (CN)

into the Hydrocad model. Flow lengths and slopes were obtained from the latest version

of the topographic maps for the watershed area. The Hydrocad model calculated the Tc.

3. Runoff Curve Number (CN)

A Weighted CN for each sub-area was calculated using Soil Conservation Service

(SCS) Technical Release 55 (TR-55). The Weighted CN for each sub-area was

determined based on land use combinations (surface cover ratios) and NRCS

Hydrologic Soil Group (HSG) mapping combinations for each sub-area. The Weighted

CN data were entered in the Hydrocad model to determine runoff. Table 2 presents the

Weighted CN’s for the watershed subareas along the Mianus river that contribute flow

up to the study limit.

4. Rainfall

Rainfall data used for the watershed areas was SCS Type III, 24 hr. obtained from the

latest NOAA Atlas 14 Volume 10 Version II 24-hr precipitation. The rainfall data is

summarized below:

Storm Precipitation* (in)

10-yr storm 5.3

25-yr storm 6.45

100-yr storm 8.23


*Taken from the Bedford Hills NOAA station, Latitude 41.2333°, Longitude -73.7167°.

Bedford Hills NOAA station was selected due to its close geographic proximity and

similar weather to Long Pond Dam.

5.2 Existing Hydrologic Conditions

The existing long pond dam has a crest length of approximately 600 ft and an elevation

of 474.46’ at its lowest point. The spillway is located at the north end of the pond

adjacent to long pond court. The spillway is 6 ft wide by 4 ft high. The invert of the

spillway is 471.02’. The crest elevation directly above the spillway is 477.5’. The

spillway discharges to a stone-lined channel that outlets to the Duck Pond located due

east of the long pond dam. The Existing Conditions Hydrocad Model was created to

assess the hydraulic capacity of the dam.

The Hydrocad Model for the existing conditions indicates that the spillway is capable of

passing storms up to a 25-yr storm without overtopping the dam crest. The peak water

surface elevation during a 25-yr storm is 474.21 and is below the elevation of the low

point of the crest (474.46’). Storms greater than a 25-yr storm will overtop the dam

crest. Overtopping is undesirable and could lead to dam failure.

The peak elevation during a 100-yr storm is 474.83. This will lead to dam overtopping

and possible failure.

Table 3 Hydrocad Results Summary, presents flows and peak water surface elevations

for the 10, 25, and 100-yr storms.

In accordance with the NYSDEC Guidelines for Design of Dams Section 5.3, existing

dams that are being rehabilitated shall have adequate spillway capacity to pass 150 %

of a 100-yr design flood for Hazard Class B dams without overtopping. The existing

spillway capacity is 126.30 CFS from a 100-yr storm. The 100-yr total flood outflow rate

from the Long Pond is 315.86 CFS. The existing spillway is therefore undersized and

incapable of passing a 100-yr storm. In addition, the existing spillway is not flowing full


under 100-yr storm conditions because there are multiple locations along the dam crest

where the surface elevation is lower than the elevation (475.02) of the top of the existing

spillway. HydroCad model results for the existing and proposed conditions are

presented in Appendix D.


SECTION 6.0

HYDRAULIC ASSESSMENT

This hydraulic assessment was completed based on the New York State DEC

Guidelines for Design of Dams for Hazard Class “B” to determine the capacity of the

existing spillway, to propose improvements, and to assess downstream flooding under

multiple storm conditions. The stage vs. storage data was obtained from Westchester

County GIS Topographic Data, 2004 with 2-foot contour intervals. The spillway

dimensions and elevations were obtained from on-site survey and data collection

conducted by GEA Engineering.

6.1 Hydraulic Model and Flow Regime

In order to assess flooding caused by the 100-yr and the 150% (of the 100-yr) storms,

the U.S. Army Corp of Engineers, HEC-RAS, River Analysis System Computer Model

Program, Version 5.0, February 2016, was used to estimate water surface elevations at

Middle Patent Road and at selected station cross-sections along the Mianus River.

The HEC-RAS program is intended for calculating water surface profiles for unsteady

flow conditions in natural or man-made channels. The effects of various obstructions

such as culverts, weirs, and structures in the river are considered in the computations.

The computational procedure is based on the solution of the one-dimensional energy

equation with energy loss due to friction evaluated with Manning's equation. The

computational procedure is generally known as the standard step method. The

program also assesses the effects of channel improvements on water surface profiles.

GEA utilized the Unsteady Flow Option in HEC-RAS based on its ability to solve the full

dynamic, Saint-Venant equations using the implicit finite difference method. Under

unsteady flow, discharge hydrographs from Hydrocad are input into the HEC-RAS

model. At the upstream boundary, a discharge hydrograph is inputted and at the

downstream boundary, a stage vs. discharge rating curve is inputted. Along the brook,

subarea hydrographs generated from Hydrocad are also inputted. Using the unsteady

flow regime, the use of the dynamic wave approach allows for the attenuation of the


water as it moves downstream. This type of analysis is better suited for the Mianus

River watershed with its confluence of streams, lakes and impoundments. It will also

serve as a more effective means to assess alternatives and improvements.

6.2 Site Survey In order to produce a hydraulic model, GEA Engineering conducted an on-site survey to

determine the elevations of the roadway above the Mianus River crossing at Middle

Patent Road using a Total Station surveying instrument and a Data Collector. The water

surface elevation (WSEL) of the Long Pond was determined by using a known elevation

(CB rim) at Long Pond Court. GEA Engineering then setup the instrument along Long

Pond Road by the south end of Long Pond and used the known WSEL obtained from

the previous setup to establish turning points (elevations) along Banksville Rd and Long

Pond Rd. The turning points were then used as known elevations to determine the

culvert inverts and road surface elevations at Banksville Rd and Long Pond Rd.

GEA then established turning point elevations and traversed along Windmill Rd,

Windmill Pl, Thornwood Rd, and Middle Patent Rd. Culvert sizes, materials, inverts, and

road surface elevations were then recorded at Middle Patent Rd at the Mianus River

crossing.

During GEA’s survey, the Middle Patent Road culverts were measured to be 30” and

36” diameter respectively. There were no headwalls on the inlet or the outlet side of the

culverts.

The Existing HEC-RAS model represents all culverts in clean and unblocked conditions

and functional at full capacity.

The elevations of the roadway and culvert inverts at Brookwood Road were surveyed

using an assumed road centerline elevation. The road centerline elevation was later

determined using Westchester County GIS Topographic information and the culvert

inverts adjusted accordingly.

6.3 HEC-RAS Modeling

Data inputs include flows for the 10-year, 25-year, and 100-year storms under existing

and proposed storm flow conditions respectively. Station cross-section geometry along

the Mianus River, manning’s roughness coefficient (n-value) of the stream and culverts,


dimensions of stream and culverts, and road elevations were also used as data inputs

to the HEC-RAS model. All hydraulic structures and road crossings along the Mianus

River from Banksville Road to Middle Patent Road were incorporated into the model.

The Stream Network Model was developed as follows:

• Delineation of the stream section using topographic maps including:

- Adjusted Westchester County GIS Topographic Map, NAVD 88

- GEA Engineering, P.C. field survey data

• Dimensions of critical control structures such as culverts and in-stream channel

elevations were obtained from the following:

- FEMA FIS report dated September 2007

- GEA Engineering, P.C. field survey data, April 2016

The input to the HEC-RAS model included the Mianus River cross-sections,

hydrographs, and flows at selected river stations obtained from the Hydrocad model.

Cross-sections were obtained from the Westchester County GIS Topography and the

FEMA FIS report.

The elevations of the river basin obtained from the GIS maps were compared with the

inverts obtained from the FEMA Study Flood Profiles at the corresponding cross-

sections and the differences in elevations were used as the depth of the stream at that

particular cross section since both elevations are in the same (NAVD 88) datum.

A channel bottom width was determined along with a 3:1 side slope, based on field

observations.

Figure 7 – Mianus River Watershed Map presents the location of cross-sections taken

along the Mianus River and utilized in the model for the 100-yr storm event. Appendix E

HEC-RAS Model Output presents the Water Surface Profiles, Cross Sections at Middle

Patent Road, and Hydraulic Results Tables for Existing and Proposed conditions along

the Mianus River.

Boundary Conditions

Boundary conditions are necessary to run an unsteady flow HEC-RAS model. The

model includes:


1. Downstream boundary conditions

2. Upstream boundary conditions, and

3. Internal boundary conditions

The Downstream and Upstream boundary conditions are required to run an unsteady

flow model while Internal boundary conditions are used as a means of simulating the

incoming flows along the stream.

The “Normal Depth” method was used as a downstream boundary condition in which

the channel slope was obtained from the FEMA channel profile. In addition, the culvert

at Brookwood Road acts as a control structure for the stream flows.

The “Flow Hydrograph” method was used as an upstream boundary condition. The flow

hydrographs were obtained from the Hydrocad model.

The “Lateral Inflow Hydrograph” method was used as an internal boundary condition.

Similar to the flow hydrograph case, lateral flow hydrographs are obtained from the

Hydrocad model.

Hydrographs obtained from the Hydrocad Program contained 241-data entries with 6-

min increments for a duration period of 24-hours.

6.4 Existing Hydraulic Conditions for 100-yr Storm and '150% of the 100-yr' Storm

Both the 100-yr storm and the ‘150% of the 100-yr’ storm indicate an overtopping of the

roadway at Middle Patent road under existing conditions. The elevation of the lowest

road elevation above the culverts at Middle Patent road is 377.9’.

The WSEL at Middle Patent Road during a 100-yr storm was modeled as 379.27’ with a

flood depth of 1.37’. The WSEL at Middle Patent Road for '150% of 100-yr' storm was

380.12' for a flood depth of 2.22'.

River Section 6963.14 indicates the extent of the flood plain for the 100-yr storm event

and 150% of the 100-yr storm event at Middle Patent Road.


The HEC-RAS model results are included in Appendix E and include flows (CFS),

minimum channel elevation, WSEL’s, E.G elevation and slope, channel velocity (ft/s),

channel flow area and top width at selected cross-sections for all hydraulic models.

Also presented in Appendix E, are the River Station Cross-sections. Cross-sections are

shown looking downstream as follows:

RS = River Station in feet upstream of the Limit of Study boundary. Numbers across Top of X-Axis are Manning's “n” values as shown below:

Number Shown Manning “n” value

.085 0.085

.

0

3

5

0.035

Numbers across Bottom of X-Axis are the horizontal cross section stations from left to right looking downstream.

Storm events that produce significant flooding and the indicated flood widths include:

Middle Patent Road, 268 LF – 100-yr

Middle Patent Road, 272 LF – 150% of 100-yr

6.5 Dam Break Existing Conditions

The unsteady HEC-RAS model was also used to analyze dam break for existing

conditions. The dam break analysis consisted of three scenarios:

• Sunny day dam break

• Rainy day dam break with 100-yr storm

• Rainy day dam break with 100-yr storm with 150% Long Pond storm inflow

The sunny day dam break scenario simulates a "piping" failure with the starting water

surface at the normal pool/spillway crest elevation of 471.02 feet. Piping dam failures

are also known as internal erosion. Piping occurs when water flows through a crack or


defect within the dam at a high velocity causing soil from within the dam embankment to

erode. The piping will continue to expand the crack or defect until the flow path is large

enough to empty the pond. The piping dam failure occurs at time zero with no inflow to

the pond.

The rainy day dam break scenario simulates an "overtopping" failure with the starting

water surface corresponding to the maximum water surface resulting from the

appropriate spillway design flood at elevation 474.90 feet. Overtopping failures for

earthen dams begin by water overtopping the crest and eroding the toe first, then

eroding the embankment upstream to the crest. Once the pond water flows to the

breach, erosion of the embankment occurs until the breach forms its final dimensions.

The dam breach geometry is trapezoidal. The dam failure is set to occur when the water

surface elevation at the dam crest reaches 474.90. The rainy dam break with 100-year

storm was modeled with 100-year storm for the Long Pond and Mianus River. The rainy

day dam break for 150% was modeled with the Mianus River 100-year storm and only

the Long Pond inflow was amplified was increased to 150% (of 100-yr). This is

consistent with HEC-RAS software requirements.

The consequences of the dam failure were assessed by comparing the difference in

flood (water surface) elevations between the respective dam-failure and non-failure

scenarios at Middle Patent Road. The NYS DEC DOW TOGS 3.1.5-Guidance for Dam

Hazard Classification was used to determine if the dam failure scenario is significant

and states that:

In general, the consequences of failure are considered not significant

when the difference in flood elevations between the respective dam-failure

and non-failure scenarios is approximately two feet, or less. However, the

two-foot increment is not an absolute decision-making point. Engineering

judgment must be applied in making a final determination.

The breach dimensions and development times were estimated for the failure scenarios

using the four different methods:

• MacDonald and Langridge-Monopolis

• Froehlich(1995)


• Froehlich (2008)

• Von Thun and Gillette

The above methods were developed from data obtained from documented dam failures

that are representative of the Long Pond Dam and are incorporated into the HEC-RAS

model. The estimated breach parameters(geometry of failure) are shown in Table 4 for

each scenario. Each set of the estimated breach parameters results in different outflow

hydrographs. The outflow hydrographs are then routed downstream. The outflow

hydrographs generated by each method converge near Middle Patent Road as the flood

wave moves downstream because the volume of water impounded by the dam is the

same for each scenario. Each outflow hydrograph was routed approximately 6,600 feet

downstream of Middle Patent Road. The water surface elevation errors at Middle Patent

Road for each method are all less than the 1.0 foot acceptable error as outlined in the

HEC-RAS user manual for unsteady flow modeling.

The hydraulic results at Middle Patent Road for the existing conditions dam breach

scenarios modeled are shown in Table 4. Appendix E presents the HEC-RAS model

output cross sections at Middle Patent Road crossing along with the profiles of the

model results. Standard deviations for the rainy day dam break for a 100 year storm

event and the rainy day dam break for 150% of 100 year storm event are small (e.g.,

0.15, 0.59) indicating that the calculated water surface elevations for the different

methods at Middle Patent Road are very similar and exhibit a high level of confidence in

the results.

For the sunny day dam failure (piping) the breach bottom elevations varied for each

method. Model results are accurate if and only if model stability is achieved. Breach

Bottom elevations that are too low can cause model instability. The breach bottom

elevations used in the model are the lowest achievable with model stability. These

"lowest" breach bottom elevations are conservative as they produce the highest volume

of discharge.

The average velocities over Middle Patent Road are also presented in Table 4. The

average velocity is computed with the flow over the road divided by the cross sectional

flow area over the road. For each scenario the average velocities are all less than 2 feet


per second, less than the permissible velocities for channels with lined vegetation.

Since the velocities are less than permissible velocities for channels lined with

vegetation, the flow over Middle Patent Road will not cause a washout or significant

erosion during the dam break scenarios modeled.

The consequence of dam failure is only significant for the 100-year storm event

because the difference between the WSEL at Middle Patent Road for the 100-year

storm non-failure event and 100-year storm dam failure event is 2.64 feet. As per the

NYSDEC DOW TOGS 3.1.5-Guidance for Dam Hazard Classification, the 100-year

storm is a significant dam failure scenario.

The consequence of dam failure is insignificant for the '150% 100-yr storm' event

because the difference between the WSEL at Middle Patent Road for the non-failure

and failure events is 1.49 feet (less than 2 feet).

Since only the 100-year storm dam break scenario is significant, it is

recommended that the spillway be designed for the 100 year storm.

Also, the average WSELs at Middle Patent Road for the dam break under both the

100 year storm event and '150% of the 100 year' storm event are similar, it is

recommended that the spillway be designed for the 100 year flood.

6.6 Existing Dam Drain with Gate Valve

The Long Pond dam is equipped with a gate valve and a low level drain pipe installed

under the dam. The Gate valve is currently inoperable and in the closed position

thereby preventing the outflow of water from the pond. The low level 12” diameter cast

iron pipe was assessed to determine the drawdown time. Drawdown time was

calculated assuming discharge under a falling head with no inflow to the Long Pond.

The drawdown time was calculated to be 7.5 days based on an operational gate valve

and drain pipe. This meets the NYSDEC Guidelines for Design of Dams 90% drawdown

requirement of 14 days in the event the gate valve were operational.


6.7 Proposed Conditions

In order to meet the current dam safety requirements and to reduce the 100-yr peak

flow from the pond, improvements of the dam and spillway are required. In order to

satisfy the requirements for hazard class B dams stated in Section 6.1 of the NYSDEC

Guidelines for Design of Dams , the spillway capacity needs to be increased The

current 100-yr storm flow rate is 315.86 CFS which includes the 128.35 CFS flow

through the current spillway and 187.51 CFS over the dam crest.

Proposed dam improvements are as follows:

A. Raising Dam Crest

The dam crest will be raised to elevation 476.02’ to increase the storage capacity of the

pond while allowing a reduced combined outflow from the pond. This dam crest must

maintain a freeboard of at least 1.0 ft (maximum WSEL of 475.02') The existing spillway

will remain.

B. Pond Outlet Structure

A pond hydraulic outlet structure is proposed to supplement the existing spillway and

pass the 100-yr flow without overtopping the dam crest while taking advantage of the

storage capacity of the pond. A concrete outlet structure is proposed with four 2.5’x1’

inlet orifice openings. Two of the orifice opening inverts will be set at elevation 471.02’

and the other two set at elevation 473.52’. The outlet from the structure will be a 36”

diameter pipe to the existing spillway outlet channel. A Hydrocad model was developed

to simulate the proposed improvements for a 100-yr storm flow.

An iterative calculation process involving the adjustment of the orifice sizes and inverts

was used within the Hydrocad model to generate a 100-yr proposed outlet hydrograph.

The goal of the process was to increase storage capacity while reducing total pond

outflow. The outlet hydrograph was then used within the HEC-RAS model for the

proposed 100-yr flow conditions along the Mianus river.

An additional Hydrocad model was developed to simulate 150% of the 100-yr flow. An

iterative process similar to the 100-yr flow was undertaken to generate the 150% flow.

This additional flow would require a hydraulic structure with two 5.5’x1’ outlet orifice


openings with inverts set at elevation 471.02’ and two 5.5’x1’ openings with inverts set

at elevation 473.52’.

The proposed 100-yr and the 150% flows and WSEL’s are presented below:

Proposed Pond Outflows and WSEL’s

STORM

FREQUENCY

PEAK OUTFLOW

(CFS)

LONG POND

PEAK WSEL

(ft)

100-yr 200.80 474.97

150% of 100-yr 306.00 475.01

C. Spillway Discharge Channel

The stone-lined spillway discharge channel will be modified to facilitate the combined

100-yr flows from the spillway and the pond outlet structure.

A proposed conditions Hydrocad model was developed to demonstrate the hydraulic

improvements including the proposed outlet structure, the existing spillway, and the

raised dam crest.

D. Pond Drain-down Pipe

GEA recommends the following alternative improvements to the existing pond drain-

down system outlet pipe:

Alternative 1: Grout the Drain-down pipe

Fill the entire 176’ length of pipe with low-strength concrete mixed with

bentonite (Approximately 5 C.Y.) for water-tightness to prevent any pond

discharge and groundwater infiltration via this pipe.

Alternative 2: Plug the drain-down pipe at the outlet end

This alternative involves the plugging of the outlet pipe with a permanent

rubberized expandable insert.


Both alternatives require a portable standby diesel pump to be onsite complete with

hoses and appurtenances for the purpose of draining down the pond if the need arises.

6.8 Proposed Hydraulic Conditions for 100-yr Storm and '150% of the 100-yr'

Storm

The dam proposed conditions designs for the 100-yr storm and '150% of the 100-yr'

storm was modeled. The proposed conditions models utilized the existing conditions

models with additional spillway capacity at the dam. The two different proposed

additional spillway capacity designs for the 100-yr storm and the '150% of the 100-yr'

storm events maintain 1 foot of freeboard with a maximum WSEL at the Long Pond

Dam crest of 475.02' with the raised dam crest at 476.02'. Appendix E presents the

proposed HEC-RAS model output for the proposed conditions models.

The WSEL at Middle Patent Road during a 100-yr storm event with 100-yr storm

proposed conditions was modeled as 379.1' with a flood depth of 1.2'. The proposed

conditions designed for 100-yr storm event reduces flooding by 0.17'.

The WSEL at Middle Patent Road during a '150% 100-yr' storm event with '150% 100-yr

storm' proposed conditions was modeled as 379.59' with a flood depth of 1.69'. The

proposed conditions designed for the '150% 100-yr' storm event does reduces flooding

at Middle Patent Road by 0.83'.

6.9 Proposed Dam Break Conditions

Dam break was also analyzed for the proposed conditions. The proposed sunny day

dam break is the same as the existing conditions. The proposed dam break analysis

included the following scenarios:

• Rainy day dam break with 100-yr flood

• Rainy day dam break with 100-yr flood with150% Long Pond flood Inflow

The rainy day dam break scenario for the proposed conditions simulated an

"overtopping" failure with the starting water surface corresponding to the maximum

water surface at the top from the spillway design flood. The dam failure is set to occur


when the water surface elevation at the dam crest reaches elevation 476.02 feet. In

order for the water surface elevation to reach the dam crest for the proposed conditions,

there was no spillways/outflows from the Long Pond Dam modeled (because proposed

conditions WSEL does not reach crest of dam).

The breach dimensions and development times were estimated for the failure scenarios

using the MacDonald and Langridge-Monopolis method. Other methods were not used

because they caused model instability. The hydraulic results at Middle Patent Road

from the proposed conditions dam breach scenarios modeled are shown in Table 4.

Also attached are the HEC-RAS model output cross sections at Middle Patent Road

crossing and the water surface profiles of the models in Appendix E.

The rainy day dam break with 100-yr storm and rainy day dam break with 100-yr storm

with 150% Long Pond inflow caused Middle Patent Road to be overtopped by 2.22 and

2.40 feet respectively. This is an improvement from the existing dam breaks analyzed,

water surface elevations decreased by 1.79 and 1.61 feet respectively. The difference in

water surface elevations for the proposed non-failure and dam failure situations also

show that the dam failures are insignificant (differences between non-failure and failure

scenarios are less than 2 feet).

The average velocities over Middle Patent Road for proposed dam break analysis are

also presented in Table 4. The velocities computed are also less than the permissible

velocities for a vegetated lined channel. The velocities computed would not cause

significant damage to the roadway.

Analyzing the proposed conditions shows that the dam breaks will be insignificant

downstream at Middle Patent Road and that the spillway designed for the 100-year

storm will be sufficient.


SECTION 7.0

STRUCTURAL STABILITY

7.1 Introduction

The structural stability of the dam was determined from geotechnical investigations

which include test pits, test borings, and laboratory analyses in addition to onsite visual

inspection, and subsequent analytical determination of stability based on NYSDEC

current standards.

7.2 Geotechnical Investigation

A Geotechnical Investigation is essential in an Engineering Assessment Report to

establish subsurface conditions and the properties of the dam’s foundation materials.

The Geotechnical Investigation was conducted by GEA Engineering and included test

pits, soil borings, laboratory analyses, and soil classification. A diver investigation was

conducted which included sampling of the soils on the slope on the pond side of the

dam and inspection of the low level drain down pipe. Soil samples were analyzed in a

laboratory to determine the properties.

7.2.1 Test Pits

The initial investigation involved test pits which were conducted on July 17, 2015.

Four (4) test pits were excavated on the dam crest using a backhoe to determine

the subsurface conditions. Figure 2 presents a location plan of the test pits. A

test pit log is presented in the Appendix F. Subsurface conditions from Individual

test pits are presented as follows:

Test Pit No.1 was excavated to the following dimensions: 10 ft wide x 10 ft

long x 10 ft deep. The soil strata revealed loose to medium dense brown silty

sand from a depth of 6” to 10 ft. Large boulders or bedrock was encountered

at a depth of 10 ft.

Test Pit No.2 was excavated to the following dimensions: 6 ft wide x 8 ft long

x 8 ft deep. Loose to medium dense brown silty sand was observed from a

depth of 6” to 5 ft and loose gray silty sand from a depth of 5 ft to 8 ft.


Test Pit No.3 was excavated to the following dimensions: 6 ft wide x 8 ft long

x 10 ft deep. Loose to medium dense brown silty sand was observed from a

depth of 6” to 6 ft - 3 in and loose gray silty sand from a depth of 6 ft – 3 in to

10 ft.

Test Pit No.4 was excavated to the following dimensions: 4 ft wide x 10 ft

long x 11 ft – 6 in deep. Loose to medium dense brown silty sand was

observed from a depth of 6” to 6 ft and loose gray silty sand from a depth of 6

ft to 11 ft – 6 in.

7.2.2 Test Borings

Subsequent to the test pits, test borings were performed on July 22, 2015. Four

(4) test borings were conducted by SOILTESTING, INC. using a 4-1/4” hollow

stem auger, a 1-3/8” split spoon sampler, and a 140 lb hammer to advance

and sample soils along the dam crest to auger refusal. A rock coring bit was used

to obtain a 5 ft core sample of the bedrock. A test pit and test boring plan is

presented on Figure 2. Test boring logs are presented in Appendix F.

Boring B-1 was advanced to auger refusal at a depth of 21 ft – 6in. Soil

strata indicates topsoil from 0 - 6 in, brown silty sand from 6 in – 13 ft,

brown silty sand with rock fragments from 13 ft to 15 ft, brown silty sand

from 15 ft to 17 ft, brown silty sand with rock fragments from 17 ft to 21 ft-

6 in. Bedrock was encountered at 21 ft – 6 in. A 5 ft rock core sample was

obtained between 21 ft – 6 in and 26 ft – 6 in. The soil was observed to be

wet at 15 ft.

Boring B-2 was advanced to a depth of 9 ft – 6 in before auger refusal.

The Soil type was observed to be brown silty sand.

Boring B-2A was advanced from a depth of 10 ft to 14 ft – 9 in before

auger refusal. The soil type was observed to be brown silty sand with

some gravel.


Boring B-3 was advanced to auger refusal at a depth of 38 ft. Soil strata

indicated Brown silty sand from 5 ft to 38 ft. Brown silty sand with rock

fragments were observed from 30 ft to 32 ft. a combination of weathered

bedrock and silty sand were observed from 35 ft to 37 ft, solid bedrock

was encountered at 38 ft. The soil was observed to be wet at a depth of

20 ft. Figure 5 indicates a cross-section of the dam showing the soil strata.

7.2.3 Soil Sampling

Soil samples were obtained from the test pits and the test borings for Laboratory

Analysis and presented in Appendix F.

7.2.4 Laboratory Analysis

Laboratory Analysis was conducted on selected representative soil samples to

evaluate the engineering properties of the soil used to construct the dam. The

Laboratory Analysis consisted of the following:

1. Particle size analyses by sieve and hydrometer methods.

2. Gradation (sieve analysis)

3. An unconfined compressive strength test on an intact soil cylinder from the

split spoon sampler in accordance with ASTM D422, ASTM D4318 and ASTM

D2166.

4. An Atterberg Limits test on a representative soil sample to determine the

plasticity of the soil.

The test results are attached in Appendix F.

7.2.5 Soil Classification

The Laboratory test results were used to classify the soil used to construct the

dam. The test results from the Sieve and Hydrometer Tests (Appendix) and the

Unified Soil Classification Table (Table 5) were used to classify the soils as

follows:

- It was determined that since more that half (69.6 %) of the material was

larger than #200 sieve the soil is considered coarse grained.


- Since more than half (79.9%) of the soil is smaller than the No.4 sieve

size, the soil was determined to be a sand.

- It was further determined that the sand had an appreciable amount

(30.4%) of fines (i.e. < #200 sieve size). This narrowed the classification to

either a silty sand (SM) or a clayey sand (SC).

- The atterberg limits test results indicate that the soil is non-plastic. This

further classified the soil as a silty sand, sand-silt mixture or SM

- Using the Laboratory Classification Criteria (Table 3) the amount of fines

is more than 12%. This further proves that the soil is SM.

Based on the results of the Laboratory Analyses of soil samples obtained from

test pits and soil borings, it is determined that the soils that constitute the center

of the dam are non-plastic and are classified under the Unified Soil Classification

System (USCS) as predominantly SM – silty sand, sand-silt mixtures.

7.2.6 Rock Quality Designation (RQD)

The rock core sample obtained in boring B-1 was measured and used to

calculate the rock quality based on the following formula:

RQD = [Sum (Length of core pieces > 4 in)/ Total core length (in)] x 100%

= [(4 ½” + 6” + 4” + 6 ½”) / 60] x 100% = 35%

The RQD was calculated to be 35%. An RQD between 25-50% is described as

“poor quality”.

Ref: Deere, D.U. and Deer, D.W., “The Rock Quality Designation (RQD) Index in

Practice”, Rock Classification Systems for Engineering Purposes, ASTM STP

984, Louie Kirkaldie, Ed., American Society for Testing and Materials,

Philadelphia, 1988, pp 91-101.

7.2.7 Pond Liner Soil Sampling

During the diver investigation of the Long Pond Dam on September 24, 2015

three soil core samples were obtained from the pond embankment (pond-side) to


determine whether a pond liner exists and also to ascertain the thickness of the

liner.

(A) Soil Sampling

Soil core samples were extracted at the locations indicated on Figure 4 (Test

Borings and Test Pit Plan). The cores were extracted at depths of 6 ft, 7 ft, and

11 ft respectively below the water surface.

Soil samples obtained from the cores were analyzed using sieve analysis,

hydrometer, and atterburg limits testing. The test results are attached in

Appendix F.

(B) Soil Classification

The Laboratory test results for the soil core samples were used to classify the

soil on the pond-side of the dam embankment. The soils were classified as

follows:

- It was determined that since more that half (68.2 %) of the material was

larger than #200 sieve the soil is considered coarse grained.

- Since more than half (100%) of the soil is smaller than the No.4 sieve size,

the soil was determined to be a sand.

- It was further determined that the sand had an appreciable amount

(31.8%) of fines (i.e. < #200 sieve size). This narrowed the classification to

either a silty sand (SM) or a clayey sand (SC).

- The Atterburg limits are as follows: PL = 25, LL = 15, PI = 10

Using the Plasticity Chart shown on Table 3, the atterburg limits are above

the A-line and the plasticity index (PI) is greater than 7 therefore the

soil is classified as a clayey sand – SC.

- The liner thickness varied from 1 to about 2 feet thickness averaging

about 1.5 feet.

7.2.8 Subsurface Conditions

The results of our subsurface investigation indicate that the dam is underlain by a

combination of fill and silty sand. There were variations in the color and texture of

the strata, however based on the Laboratory analyses it was concluded that the

underlying soils were silty sand.


Bedrock was observed in borings B-1, B-2, B-2A, and B-3 at depths of 21’-6”, 9’-

6”, 14’-9”, and 38’-0” below grade respectively as indicated on the profile drawing

Figure 5.

Groundwater was observed in borings B-1and B-3 at depths of 15’-0” and 20’-0”

below grade respectively. The long pond dam profile (Figure 4) indicates the soil

layers, the bedrock profile, and the groundwater elevation.

7.3 Stability Analysis for Existing Conditions

Based on the NYSDEC requirements outlined in the Guidelines for Design of Dams

(Jan.,1989), Section 10.5 (Loading Conditions) and Section 10.7 (Stability Analysis for

Existing Dams), GEA Engineering conducted a Stability Analysis for the long pond

dam.

As part of the structural stability investigation, a Subsurface (Geotechnical)

Investigation, as described in Section 7.2 of this report, was conducted to determine the

materials of the dam and its foundation. Samples were collected and analyzed to

determine material properties.

A Stability Analysis table (Table 6) was created to calculate factors of safety based on

various scenarios (Case 1-4) using formulas obtained from the following references:

- “Stability of Earth and Rock-Fill Dams”, EM 1110-2-1902; April 1970.

Bureau of Reclamation; U.S. Department of the Interior.

- “Design of Small Dams”, 1977 revised reprint

- “Design of Gravity Dams”, 1976

In conducting the stability analysis, “Overturning” and “Sliding” conditions presented in

Sections 10.7.2 and 10.7.4 respectively of the NYSDEC Guidelines for Design of Dams

were analyzed under the loading conditions outlined in Section 10.5 of the NYSDEC

Guidelines for Design of Dams.


7.3.1 Overturning Analysis

In order for existing dams to be considered stable, the following conditions must

be met:

• The resultant force from an overturning analysis should be in the middle

third of the base for normal loading conditions (Case 1)

• The resultant force from an overturning analysis should be within the

middle half of the base for the ice loading condition (Case 2)

• The resultant force from an overturning analysis should be within the

middle half of the base for the spillway design flood loading condition

(Case 3).

• For the seismic loading condition (Case 4), the resultant force should fall

within the limits of the base.

Maximum hydrostatic loading conditions; is the maximum differential head

between headwater and tailwater levels as determined by storms smaller in

magnitude than the spillway design flood. This condition is considered when the

dam is submerged under Case 3 and is represented as loading condition (Case

3A). Figure 8 presents the graphical representation of the resultant force

direction for cases 1, 2, 3, 3A, and 4 respectively.

7.3.2 Sliding Analysis

The sliding safety factors were computed using the shear-friction method of

analysis based on the internal angle of friction of the soils obtained from the

subsurface investigations addressed in Section 7.2. This method should result in

a minimum safety factor of 2.0 for Case 1 and 2, 1.5 for Case 3, and 1.25 for

Case 4.

Loading Conditions

The loading conditions analyzed include the following:

• Case 1 – Normal loading condition; water surface at normal reservoir level.

• Case 2 – Normal loading condition; water surface at normal reservoir level

plus an ice load of 5000 pounds per linear foot, where ice load is applicable.

Dams located in more northerly climates may require a greater ice load.


• Case 3 – Design loading condition; water surface at spillway design flood

level.

• Case 3A – Maximum hydrostatic loading condition; maximum differential

head between headwater and tailwater levels as determined by storms

smaller in magnitude than the spillway design flood. This loading condition will

only be considered when the dam is submerged under Case 3 loading

condition.

• Case 4 – Seismic loading condition; water surface at normal reservoir level

plus a seismic coefficient applicable to the location.

Table 6 is comprised of data entries for load condition cases 1-4 and dam stability

assessments summary results.

Figure 9 presents a Load Configuration Diagram. It shows all loads with directions

and distances to the overturning point of the dam. These loads are represented in

Table 6.

Data entries are as follows:

1. Geometry:

a. Areas of the load wedges

b. Distances d(i) of the load wedge’s center of gravity to the dam toe

(overturning point)

c. Dam base length “L”

2. Units weights of:

a. Various soil layers (based on field investigations)

b. Water

3. Ice load

4. Coefficients of:

a. Seismic acceleration

b. Sliding

c. Cohesion intercept (neglected as a conservative method)


5. Required Safety factors per NYSDEC “Guideline for Design of Dams” for

each load condition case

Dam Stability assessments summary results are based on:

1. Engineering formulas pertinent to recognized dam stability analysis design

manuals:

a. Overturning as a ratio of:

i. Resisting moments (earth dam and water gravity) versus

overturning moment (water pressure, uplift, ice or water and

earth seismic component)

ii. Distance of resultant force direction intersecting the base of the

dam versus allowed distance (middle third, middle half or base)

to the toe of the dam

b. Sliding as a ratio of:

i. Resisting force versus active sliding force(s) (water pressure,

ice or water and earth seismic components.

2. Calculated safety factors in comparison with the required NYSDEC “Guideline

for Design of Dams” for each load condition.

Input entry includes applicable formulas utilized for calculation of various loads and

moments versus the toe of the dam (point “O” in the “Load Configuration Diagram”).

7.3.3 Stability Analysis Results (Existing Conditions)

Overturning

Based on the analyses performed and presented in Table 6, the overturning analysis

results under existing conditions indicate that the location of the resultant force

under cases 1, 2, 3, 3A, and 4 respectively satisfied the conditions required for the

stability of the dam. The conditions of stability are as presented in Section 7.3.1

above and as shown on Figure 8.

Sliding

The factors of safety for cases 1, 2, 3, 3A, and 4 are 8.89, 7.51, 7.48, 7.43, and 2.33

respectively. In each case the factor of safety exceeds the minimum required safety


factors of 2.0 for Case 1 and 2, 1.5 for Case 3 and 3A, and 1.25 for Case 4. These

results indicate that the dam is stable under existing conditions.

7.4 Stability Analysis for Proposed Conditions

The Stability Analysis for Overturning and Sliding were performed for the Proposed

Conditions. The loading conditions analyzed include the following:

• Case 1 – Normal loading condition; water surface at normal reservoir level.

• Case 2 – Normal loading condition; water surface at normal reservoir level plus

an ice load of 5000 pounds per linear foot, where ice load is applicable. Dams

located in more northerly climates may require a greater ice load.

• Case 3 – Design loading condition; water surface at spillway design flood level.

• Case 4 – Seismic loading condition; water surface at normal reservoir level plus

a seismic coefficient applicable to the location.

Case 3A was not considered since under the proposed condition, the dam is never

submerged.

Table 7 presents the results of the stability analysis based on proposed conditions.

Under proposed conditions, the spillway crest elevation is increased thereby increasing

the area of segment 1 shown on Figure 9.

Additionally, the water pressure (Pw) is reduced under case 3 (proposed conditions).

Stability Analysis Results (Proposed Conditions)

Overturning

Based on the analysis and results presented in Table 7, the location of the resultant

forces satisfied the conditions for dam stability under proposed conditions for cases 1,

2, 3, and 4 respectively.

Sliding

The factors of safety for cases 1, 2 3, and 4 were determined to be 8.95, 7.57, 7.31, and

2.33 respectively. In each case, the factor of safety exceeds the minimum required


safety factors of 2.0 for case 1 and 2, 1.5 for case 3, and 1.25 for case 4 for dam

stability.

The factors of safety calculated for the existing and proposed conditions in each case

exceeded the required factors of safety significantly. This result indicates that the dam is

stable under both existing and proposed conditions.


SECTION 8.0

EMERGENCY ACTION PLAN REVIEW

The Long Pond Dam Emergency Action Plan was prepared by GEA Engineering P.C.

on August 14, 2015 in conformance with 6 CRR-NY 673.7. The Emergency Action plan

is currently under review by the NYSDEC.

The Emergency Action Plan was reviewed and revised as part of this Engineering

Assessment. The revised Emergency Action Plan is attached in Appendix G. Revisions

include the removal of the house located at 43 Long Pond Road from the flooded area

on the inundation map after review of the topography and current dam break analysis.

The current dam break analysis was also used to determine if the house located at 45

Long Pond Road was inundated The lowest elevations for house number 43 and 45

Long Pond Road are 450 and 440 feet respectively. The houses are not inundated

because the highest WSEL for the dam break scenarios modeled near these houses is

approximately 436 feet.


SECTION 9.0

CONFORMANCE TO CURRENT DAM REGULATIONS AND SAFETY

The current dam safety regulations are part of the Official Compilation of Codes, Rules

and Regulations of the State of New York, Title 6 CRR-NY Part 673. The dam safety

regulations give authority to the NYSDEC, and responsibilities to dam owners. The

sections of the current regulations that are applicable to this report for the Long Pond

Dam are listed, in a paraphrased form, below:

• [673.3a] Any owner of a dam shall at all times operate and maintain the dam and

all appurtenant works in a safe condition.

• [673.3c] Upon reasonable notice, the owner of a dam shall furnish any available

information which is reasonably necessary for the department’s inspection or

investigation of the dam and appurtenant works, and the assessment of the

safety thereof, including, without limitation, the records and reports required to be

maintained.

• [673.6] Develop and implement an Inspection and Maintenance Plan, the plan

shall be retained by the dam owner, kept in good order, and updated as

necessary to reflect changes in current conditions. Completed attached in

Appendix H.

• [673.7] Develop and implement an Emergency Action Plan and annual updates

thereof. Completed attached in Appendix G.

• [673.8] Submit to the Dam Safety Section an Annual Certification, in a form

prescribed by the department, by January 31 of each year. Previously submitted.

• [673.9] Notify the NYSDEC in writing, in a format acceptable to the department,

within five calendar days of any flow in an erodible auxiliary spillway.


• [673.11] Provide Notices of Property Transfer

• [673.13] Perform Engineering Assessments by a professional engineer at a

minimum every 10 years on behalf of the owner. Engineering Assessment Report

June 2016.

• [673.12] Perform Safety Inspections on a regular basis by a professional

engineer on behalf of the owner as part of the Inspection and Maintenance Plan.

Completed August 26, 2015.

Long Pond Dam is not in conformance with the current Dam Safety Regulations. The

dam owners received a Notice of Violation (NOV) on April 29, 2015 by the NYSDEC

indicating the following violations:

• Owners have not performed the recommended maintenance nor have they

monitored identified deficiencies of the dam.

• Owners have not developed and implemented an Inspection and Maintenance

Plan. Has been remedied, submitted and included in Appendix H.

• Owners have not developed and distributed an Emergency Action Plan. Has

been remedied, submitted and included in Appendix G.

• Owners have not submitted an Annual Certification by January 31 of each year

beginning in 2010. Was submitted August 14, 2015.

• Owners did not complete and submit an Engineering Assessment. Completed

June 2016.

The dam owners are working to address all violations of the NYSDEC Dam Safety

Regulations. Currently the dam owners have prepared an Emergency Action Plan, an

Inspection and Maintenance Plan, Annual Certification and schedules of work that were


submitted to the NYSDEC on August 14, 2015. These have all been updated with this

submittal.

GEA’s Engineering Assessment found the following deficiencies:

Hydraulics/Spillway

The spillway capacity of the dam is not sufficient. The dam is overtopped for the 25

year, 100 year and 150% of the 100 year storm.

Crest and Embankments

There is mature woody vegetation covering crest and embankments

NYSDEC Guidelines for Design of Dams Revised January 1989 Section 9.4.1.

states that trees and brush are not permitted on earth dams

Section 9..4.1 also states that stumps should be removed from existing dams either

by pulling or machine grinding and all woody material should be removed to about 6

inches below the ground surface. The cavity created by stump removal, should be

filled with well compacted soil and with grass vegetation established.

Select trees and vegetation will be removed as per the meeting held on 12/4/15

between NYSDEC, GEA Engineering, and the Town of North Castle.

There are two seepages located on downstream embankment. One of the seepages

is located at the "central toe" (area near large boulders) and the other is located at

the low level drain outlet.

Spillway

The spillway has inadequate capacity. The concrete spillway has spalled,

deteriorated and exposed reinforcing rods. Also a section of one of the concrete

discharge wingwalls is broken off.

Discharge Channel

The spillway discharge channel has inadequate capacity to convey the spillway

design flood to the Duck Pond.


Low Level Drain

The low level drain is not operable. The valve inside the gazebo located in the

middle of the pond is not accessible or operable. The valve is in the closed position.

The 12" cast iron discharge pipe currently has water infiltrating at a joint located

within the dam. Water also appeared to be seeping around the outside of the pipe.


SECTION 10.0

CONCLUSIONS AND SUMMARY OF ACTIONS NEEDED

The Long Pond Dam is a NYSDEC regulated dam, and essential repairs are

recommended in order to maintain a structurally stable dam, embankment, and crest

and also to control the flow of water to prevent downstream flooding.

Recommended Repairs

Removal of Trees

The removal of trees should be conducted as follows:

• Remove trees at the toe of the dam area - dry embankment side (fill area).

• Remove tress on the water side of the dam.

• Remove trees that are 6 inch diameter or less.

• Remove unhealthy trees

• Conduct an annual inspection of the condition of trees and roots.

Figure 10 presents the proposed tree removal plan.

Spillway Reconstruction

The concrete spillway needs to be repaired. The surficial spalling, deterioration, and

exposed reinforcing rods should be repaired with concrete patch work. The spillway

discharge apron voids should be repaired (filled) with concrete. The downstream left

wingwall should also be repaired with concrete. The overgrown vegetation around the

spillway should also be removed.

Add Additional Spillway Capacity

The existing concrete spillway capacity should be supplemented by additional spillway

capacity to service the 100-yr storm. The additional spillway capacity will be sufficient to

pass the 100 year storm. The new spillway structure shall be constructed as a

4'x4'x12.5' (LxWxH) concrete box founded on concrete and stone fill. For this size there

will be four openings, two (2) 2.5 feet by 1 foot at elevation 473.52 and two (2) 2.5 feet

by 1 foot at elevation 471.02. A 3 feet diameter pipe will be required to convey water

from the structure through the dam to the existing spillway discharge channel.


Increase Capacity of Spillway Discharge Channel

The capacity of the spillway discharge channel should also be increased to pass the

100 year storm. The channel should be widened and lined with rip-rap to prevent

erosion. The proposed dimensions of the improved channel will be: 16 to 20 feet top

width, 2 feet depth and 8 to 12 feet bottom width. The required rip rap is to be 6 inch d50

size. the rip rap thickness will be 12 inches plus 4 inches of stone bedding overlaying

geotextile material.

Raise Dam Crest

The dam crest should be raised to elevation 476.02 feet. Raising the dam crest to

476.02 feet provides 1 foot of freeboard within the pond for the 100 year storm event.

Providing 1 foot of freeboard increases the structural stability of the dam. The fill will

consist of silty sand, SM with a minimum of 4 inches of top soil and grass cover.

Abandon Outlet Pipe

After the investigation of the outlet pipe it is recommended that the outlet pipe be

abandoned. The outlet pipe can be abandoned by pumping a low strength concrete with

bentonite into the entire length of pipe or installing a mechanical plug at the outlet. Both

options would require a graded filter blanket placed around the outlet to prevent erosion

around the pipe, due to seepage flowing outside the pipe.

Dam Drawdown

To provide a means to drawdown the dam, it is recommended that an agreement

between the Armonk Fire District and Town of North Castle be negotiated with the

Armonk Fire District to provide a pump truck and hoses to pump down the Long Pond

Dam.

Dam Seepage at "Central Toe" (area near large boulders)

GEA recommends leaving the large boulders in place and monitoring the seepage. The

seepage monitoring should occur quarterly (four times per year) and after significant

storm events.


Following the review of this Report and associated Plans, GEA will need to prepare the

following documents:

o Engineering Plans and Specifications for Dam Repair

o NYSDEC Part 673 Dam Permit Application

o NYSDEC Protection of Waters Permit Application

Upon review and approval by NYSDEC, the Town/District may proceed with

construction. Tree removal is proposed to be conducted in November 2016. Figure 11

provides a detailed schedule of the proposed improvements.


REFERENCES

“DOW 3.1.3 – Emergency Action Plans for Dams,” NYS DEC, June 2010, http://www.dec.ny.gov/lands/4991.html. "DOW 3.1.4 – Guidance for Dam Engineering Assessment Reports" NYS DEC http://www.dec.ny.gov/docs/water_pdf/damfengarpt.pdf "DOW 3.1.5 – Guidance for Dam Hazard Classification" NYS DEC http://www.dec.ny.gov/docs/water_pdf/togs315.pdf "Design of Small Dams" United States Department of The Interior, Bureau of Reclamation, A Water Resources Technical Publication.1977 revised reprint “Design of Gravity Dams” United States Department of The Interior, Bureau of Reclamation, A Water Resources Technical Publication. Denver, Colorado 1976 Environmental Conservation Law 15-0507: Structures impounding waters; structures in waters; responsibility of owner; inspection Environmental Conservation Law 15-0503: Protection of water bodies; permit “Guidelines for Design of Dams,” NYS DEC, 1989. http://www.dec.ny.gov/lands/4991.html New York State Regulations Title 6 of the Official Compilation of Codes, Rules and Regulations of the State of New York (6 NYCRR): Part 608 – Protection of Waters Part 621 – Uniform Procedures (includes Emergency Authorization) Part 673 – Dam Safety NOAA Atlas 14 Precipitation-Frequency Atlas of the United States Volume 10 Version 2.0: Northeastern States (Connecticut, Maine, Massachusetts, New Hampshire, New York, Rhode Island, Vermont) (Perica, et al., 2015) “Owners Guidance for the Inspection and Maintenance of Dams in New York” NYS DEC, June 1987, http://www.dec.ny.gov/lands/4991.html. “Stability of Earth and Rock-Fill Dams”, EM 1110-2-1902; April 1970. Bureau of Reclamation; U.S. Department of the Interior. State of Colorado Dam Safety Branch,. (2010). Guidelines for Dam Breach Analysis. United States Army Corps of Engineers,. (2014). Using HEC-RAS for Dam Break Studies. TD-39.