S01_1 Conneautville Final Design Report.pdf

FINAL DESIGN REPORT

Conneautville Dam NRCS Task Order AG-SPEC-D-16-0006 Conneautville, Pennsylvania

Schnabel Reference 15715008.00 June 9, 2020

Prepared For:

U.S. Department of Agriculture – Natural Resources Conservation Service

FINAL DESIGN REPORT

Conneautville Dam NRCS Task Order AG-SPEC-D-16-0006 Conneautville, Pennsylvania

Schnabel Reference 15715008.00 June 9, 2020

Prepared For:

U.S. Department of Agriculture – Natural Resources Conservation Service

Jeremy R. Young, PE PA Professional Engineer PE073291

GSFW Engineering JV 3 Dickinson Drive, Suite 200, Chadds Ford, PA 19317 Phone (610) 696-6066; Fax (610) 696-7771

PA NRCS

Conneautville Dam – Final Design Report

June 9, 2020 Page i GSFW Engineering JV Project 15715008.00 ©2020 All Rights Reserved

FINAL DESIGN REPORT

CONNEAUTVILLE DAM

CONNEAUTVILLE, PENNSYLVANIA

TABLE OF CONTENTS

1.0 INTRODUCTION

1.1 Authority

1.2 Purpose

1.3 Referenced Reports

1.4 Project Control and Nomenclature

2.0 PROJECT BACKGROUND

2.1 Site Description and Project History

2.2 Identified Dam Safety Deficiencies

2.3 Proposed Rehabilitation Design

3.0 GEOTECHNICAL DESIGN

3.1 Introduction

3.2 Lime Treatment of Dispersive Soils

3.3 Drain Reconfiguration and Filter Diaphragm

3.4 Sheetpile Wall

4.0 HYDROLOGIC AND HYDRAULIC DESIGN

4.1 Introduction

4.2 Watershed and Hydrologic Parameters

4.3 Existing Topographic and Structure Data

4.4 Soil Material Properties

4.5 Precipitation Data

4.6 Hydrologic Analysis

4.7 Auxiliary Spillway Stability and Integrity Analysis

4.8 Hydraulics Analysis

4.9 Proposed Spillway Modifications

4.10 Proposed Replacement Trash Rack

4.11 Staging Area Flood Routing Analysis

5.0 PLANS, SPECIFICATIONS, AND OTHER PROJECT DOCUMENTS

5.1 Plans

5.2 Technical Specifications

5.3 Other Project Deliverables

5.4 Opinion of Probable Construction Cost

5.5 Estimated Construction Schedule

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6.0 PERMITTING

6.1 Anticipated Permits

6.2 Land Rights

7.0 REFERENCES

LIST OF TABLES

Table 2.1: Pertinent Existing Site Elevations (NAVD88) Table 2.2: Principal Spillway Riser Cover Elevations Table 4.1: Project Information Table 4.2: Existing Project Elevation and Reservoir Storage (NAVD88) Table 4.3: Auxiliary Spillway Soil Depths and Properties Table 4.4: Existing Condition SITES Results Table 4.5: Proposed Condition SITES Results Table 4.6: 1- through 100-Year Frequency Floods Table 5.1: AACE Accuracy Matrix for Estimating Classes Table 5.2: AACE Class 1 Estimate Description

APPENDICES

Appendix A: Design Calculations Appendix B: Opinion of Probable Construction Cost Appendix C: Estimated Construction Schedule

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1.0 INTRODUCTION

1.1 Authority

Our Plan of Work, dated March 25, 2016, and the Pennsylvania Specifications for Engineering Services, dated March 6, 2015, define the scope of services for this project.

1.2 Purpose

This Final Design Report presents the results of analysis and design conducted by GSFW ENGINEERING JV (GSFW) for the Conneautville Dam Rehabilitation in accordance with guidance from the Natural Resources Conservation Service (NRCS) and the Pennsylvania Department of Environmental Protection – Division of Dam Safety (PADEP).

In 2012, NRCS completed a Watershed Project Plan-Environmental Evaluation (Plan-EE), which provided recommendations for a variety of upgrading measures to bring the dam into compliance with NRCS and PADEP dam safety criteria. The purpose of the dam rehabilitation presented herein is to address the items of concern identified by GSFW and the NRCS through our file review, development of the Plan of Work, and preliminary design.

1.3 Referenced Reports

The Field Survey Map, Existing Structural Condition Investigation Report, Final Geotechnical Report, Hydrology Report, and Hydraulic Design Report are referenced throughout this report, and have been submitted individually by GSFW to the NRCS under separate covers. We updated the 50% Design Report, dated April 24, 2017, to reflect design advancements and refinements completed since the 50% Design Submittal. We made very minor editorial changes to the Detail Design Report (Revision 1), dated February 28, 2018, for compilation with the Final Design Submission (i.e., no technical changes have been made). Appendices to this report include design calculations, an Opinion of Probable Construction Cost, and an estimated construction schedule.

1.4 Project Control and Nomenclature

All elevations contained in this report are referenced to the NAVD88, but the As-Built Drawings had used the National Geodetic Vertical Datum of 1929 (NGVD29). Based on the NOAA North American Vertical Datum Conversion (VERTCON), the datum shift is -0.564 ft from the NGVD29 to NAVD88. References to “left” and “right” within this report assume one is looking downstream.

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2.0 PROJECT BACKGROUND

2.1 Site Description and Project History

Conneautville Dam is located in the Borough of Conneautville (Borough), Crawford County, Pennsylvania.

Conneautville Dam is situated across Thatcher Run, about 2,400 ft upstream of the confluence with the West Branch of the Conneaut Creek.

The dam is an earthen embankment with a maximum height of about 31 ft. It is owned, operated, and maintained by the Borough as a flood protection structure. The principal spillway consists of a standard NRCS covered riser structure and 36-inch diameter concrete pressure conduit constructed through the dam. The reservoir is drained through an 18-inch sluice gate installed at the base of the riser structure.

A vegetated auxiliary spillway was constructed on the left abutment. Seepage intercepted by the internal embankment drainage system is collected through a series of 8-inch diameter PVC drain pipes, which outlet downstream of the dam: one pipe outlets through the left bank (looking downstream) of the outlet channel, and two pipes outlet through the sidewalls of the principal spillway impact basin. The dam is currently operated as a dry pond based on PADEP’s request in October 2004, following an evaluation completed by Gannett Fleming.

A brief summary of the construction, performance, and inspection of Conneautville Dam is provided below:

• Dam construction began in 1982 by the Soil Conservation Service (presently known as NRCS).

• PADEP Construction Inspection documentation indicates that the principal spillway outlet conduit was set on concrete blocks over earthfill. Reportedly, the blocks settled approximately 0.6 ft and were reset. Settlement of approximately 0.1 ft was reported after resetting the pipe and block.

• In December 1982, PADEP notes that the site was not covered with 4 to 5 ft of soil as previously planned, and consequently, the outlet conduit and subgrade were left exposed during the winter of 1982–1983.

• On July 8, 1983, PADEP Construction Inspection documents indicate that the outlet conduit cradle was constructed and correctly aligned with the pipe joints.

• On July 15, 1983, PADEP Construction Inspection records indicate that the outlet conduit was backfilled to original ground surface.

• On August 25, 1983, NRCS observed distress, sloughing, and cracking in the embankment.

Movement of outlet conduit joints was also noted at this time. Joint movement up to 1¾ inches was recorded at Joint No. 4. A maximum of about 21 ft of fill had been placed above the pipe invert before fill placement was halted to conduct a field and laboratory investigation. Fill placement was reportedly completed without incident.

• Dam construction was completed in 1984.

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• An instrumentation program was initiated in 1986 to establish regular monitoring of the outlet conduit joints, and many regular measurements were taken of the pipe joints.

• An NRCS memo dated December 5, 1996, notes that annual measurements of the outlet conduit joints were discontinued in 1992, and that subsequent readings should be obtained at least every 5 years.

• Documents from the Borough dated July 26, 2002, note that it has been at least 10 years since outlet conduit joints were last measured.

• The NRCS Survey Report dated May 11, 2004, indicates that outlet conduit joint openings were within tolerances and no leakage was observed.

• PADEP Dam Inspection Report, dated May 20, 2004, notes that debris and mud blocked the low-level intake at the riser structure. The pool was drained at this time. The right embankment drain in the impact basin is noted as having “considerable flow.”

• Epoxy repairs were performed on outlet conduit Joint Nos. 3, 4, and 5 in 2005.

• Jugholes and other erosion features were locally repaired on the downstream slope with lime-treated earthfill in 2005. Since 2005, additional erosion features, such as jug holes, cracks, and tunnels have been observed at the site, although at a much lesser degree than what had been observed prior to 2005.

• Outlet conduit joint measurements and an invert survey were conducted by Insight Pipe Contracting for Bioengineering Group on November 16, 2010.

• Outlet conduit joint measurements were conducted by NRCS on August 17, 2011.

• As part of the Existing Structural Condition Investigation, GSFW conducted outlet conduit joint measurements on June 1, 2016.

An Operation and Maintenance Plan was developed in 1981. The Emergency Action Plan was most recently updated in July 2004.

There are no reports of flow through the auxiliary spillway since the dam was constructed.

The dam currently is classified by NRCS and PADEP as a High Hazard Potential Dam. Both agencies require that the spillways safely pass the Probable Maximum Flood (PMF) without overtopping the dam.

According to the As-Built Drawings, the top of dam was constructed 31 ft above the downstream toe of the embankment with an allowance of 0.6 ft of settlement, for a settled height of 30.4 ft, which generally agrees with the 2016 survey. Table 2.1 summarizes pertinent existing site elevations based on the 2016 survey.

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Table 2.1: Pertinent Existing Site Elevations (NAVD88)

Elevation (ft)

Drawdown Conduit Inlet Invert 972.6

Orifice Invert 982.2

Principal Spillway Crest 995.5

Top of Dam (settled) 1001.1

Auxiliary Spillway Crest 996.0

Principal Spillway Outlet Invert 970.37

2.2 Identified Dam Safety Deficiencies

In 2012, NRCS completed a Plan-EE for rehabilitation of the dam in order to comply with NRCS and PADEP dam safety criteria. The following dam safety deficiencies were identified as part of the Plan-EE, previous evaluations by others, and/or recent evaluations performed by GSFW to support the ongoing rehabilitation design:

• Geotechnical analyses indicate the presence of dispersive soils in the embankment and auxiliary spillway that could impact dam stability and integrity.

• The layout of the auxiliary spillway does not meet current criteria in TR-60 due to a curved level section. Also, the training dike does not extend a sufficient distance downstream to protect the embankment toe and principal spillway outlet from erosion during spillway operation.

• An eroded channel, about 15 ft deep along the left edge of the exit channel, may cause a headcut to form in the exit channel and progress through the spillway.

• The existing drainage system does not include a filter diaphragm below the principal spillway conduit, which is required under current NRCS criteria.

• The current configuration of the embankment drain outlets through the principal spillway impact basin headwall prevents access for video inspections.

In addition to addressing the above mentioned dam safety deficiencies, NRCS requested that the rehabilitation design improve the functionality of the existing trash rack installed on the inlet structure.

GSFW also analyzed other potential areas of concern to evaluate the need for repairs, including the following items:

• Principal Spillway Riser Settlement/Displacement: The surveyed elevations of the four corners of the top of the principal spillway riser cover were compared to those elevations reported on the As-Built Drawings and are summarized in Table 2.2. The differences in elevation are relatively consistent and considered negligible. The differences are likely due to different reference datum or benchmark elevations, and not indicative of riser movement.

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Table 2.2: Principal Spillway Riser Cover Elevations

Point Location

Survey Elevation

As-Built Survey

Elevation(1)

Elevation Difference

(ft) 1 Upstream Right 1000.08 1000.03 0.05 2 Downstream Right 999.95 999.93 0.02 3 Upstream Left 1000.07 1000.02 0.05 4 Downstream Left 999.94 999.90 0.04

(1) Elevations adjusted to NAVD88.

• Principal Spillway Outlet Conduit Joints: The condition of the existing principal spillway conduit joints was evaluated through visual inspection in 2016 and a review of previously collected monitoring data. The principal spillway conduit appears to be stable and no repairs are recommended at this time as indicated in the Existing Structural Condition Investigation Report.

• Embankment Slope Stability: Embankment slope stability was analyzed as part of our geotechnical evaluation. The results of that analysis are presented in the Final Geotechnical Report and indicate that the computed factors of safety meet current NRCS and PADEP criteria for steady state conditions. Seismic performance is considered adequate based on our seismic deformation analysis described in the Final Geotechnical Report.

Also, previous evaluations by others raised concerns over the potential for varved soil deposits in the foundation. However, varved or thin interbedded soil layers were not observed in samples collected from the foundation soils during the 2016 investigation and are not considered a concern at Conneautville Dam.

2.3 Proposed Rehabilitation Design

The rehabilitation design presented herein generally includes the following measures to address the identified dam safety deficiencies:

• Realignment of auxiliary spillway channel and expansion of the training dike to comply with TR-60 criteria.

• Installation of a sheetpile wall to prevent auxiliary spillway flow from entering the gully adjacent to the left edge (looking downstream) of the exit channel.

• Lime treatment of surficial dispersive soils on the embankment, auxiliary spillway, and training dike to reduce the potential for development of jug holes and irregularities.

• Reconfiguration of the 8-inch diameter embankment drain pipes in the vicinity of the principal spillway impact basin to enable a modular, long-range video internal pipe inspection.

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• Construction of a filter diaphragm below the existing principal spillway outlet conduit.

• Replacement of the existing trash rack installed on the inlet structure to improve functionality.

The following sections of this report summarize or reference previously completed analyses to support the design.

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3.0 GEOTECHNICAL DESIGN

3.1 Introduction

This section summarizes the geotechnical analyses performed to support the rehabilitation design.

Details related to these analyses are presented in the Final Geotechnical Report, dated March 8, 2017.

We have noted in the following sections where previous analyses have been refined or updated.

3.2 Lime Treatment of Dispersive Soils

As discussed in the Final Geotechnical Report, the presence of dispersive soils in the embankment and auxiliary spillway has resulted in the formation of surficial erosion features such as jug holes, tunnels, and cracks. The goal of the proposed rehabilitation is to address dispersive soils, such that frequent maintenance or filling of erosion features is not required.

The proposed remediation includes removing 0.5 ft of topsoil and 2 ft of surficial soil from the existing embankment, auxiliary spillway, and training dike; treating the excavated topsoil and soil with hydrated lime; and replacing and compacting the lime-treated soil in lifts. The existing topsoil will also be lime-treated and replaced for permanent turf establishment. As an alternative to lime treating the existing topsoil, the Contractor may use imported non-dispersive topsoil. The on-site soils will be treated with hydrated lime at a rate of two percent by dry weight. The augmentation rate is proposed based on the results of pinhole dispersion testing presented in the Final Geotechnical Report, and on our experience with soils that are treated with dry fine-grained materials such as lime, cement, or dry powdered bentonite. Hydrated lime has been selected over unhydrated or “quick” lime to avoid potential issues associated with heat of hydration generated by mixing quick lime with moist soils.

Lime treatment is proposed for the surface of the entire embankment, including the crest and the upstream and downstream slopes. The treatment area will extend to the edge of the existing rock gutter and/or 5 ft beyond the toe (in areas where no gutter exists). The treatment area will also extend through the auxiliary spillway, including the training dike and side slopes.

The Contractor will be required to mix lime and soil with a pugmill continuous mixing plant. This process provides more accurate measurement of base soils and additives, reduces the likelihood of dust, and requires a smaller staging area, as compared to other mixing methods. Excavation on the upstream and downstream embankment slopes will be benched to facilitate replacement of the lime-treated earthfill and to promote a good bond at the interface between the lime-treated and foundation soils. Horizontal lifts will also reduce the risk of developing a failure plane along the slope.

3.3 Drain Reconfiguration and Filter Diaphragm

The existing embankment drain pipes near the principal spillway impact basin contain 90-degree bends, restricting access for inspections and cleaning. Each of the drain pipes will be reconfigured to have an additional outlet through the headwall of the impact basin. A threaded cap will be placed over the outlet, so flow will typically exit through the existing outlet at the sidewalls. The threaded cap can be removed to facilitate inspection and cleaning.

Excavation behind the headwall is required for reconfiguration of the drain pipes. Since excavation is required upstream of the headwall for the drainpipe reconfiguration, the design includes increasing the

June 9, 2020 Page 8 GSFW Engineering JV depth and lateral extent of the excavation locally to allow a filter diaphragm to be installed below the existing principal spillway conduit. The excavation geometry for the filter diaphragm (depth and lateral extent) beyond the excavation needed for the drain pipe realignment was selected considering the following factors:

• The excavation for the filter diaphragm shall extend below the pipe conduit and allow personnel to enter the excavation without extensive shoring systems.

• More difficult construction dewatering would be needed for a deeper filter diaphragm excavation, and therefore construction risk would increase if a failure of the dewatering system was to occur.

• The embankment has an existing chimney filter that would serve the combined purpose of a filter diaphragm above the conduit, in accordance with Chapter 45 of the National Engineering Handbook (NEH) Part 628, Dams (1997).

• The dam is dry, and concentrated seepage/leakage has not been observed near the conduit outlet.

• The original construction of the dam was reported to have been challenging due to a wet foundation and difficulty in adequately dewatering the foundation. The foundation soils encountered during the original dam construction were saturated silts and clays, and were very difficult to drain. They reportedly exhibited lateral squeezing or flow due to vibrations from site construction equipment.

Given the above factors, the local excavation proposed will allow the existing chimney drain to better function as a filter diaphragm around the principal spillway conduit. The excavation also was designed to extend 4 ft below the existing conduit cradle, and 3 ft laterally beyond the limits of the excavation required to realign the embankment drain piping. The proposed filter diaphragm geometry will allow potential concentrated leakage below the conduit to be filtered, and also enhance the filtering capability of the chimney drain along the edges of the excavation required for the realigned drain pipe.

TR-60 requires the filter diaphragm to extend 5.5 ft below the conduit. The proposed geometry only extends 4 ft below the concrete cradle (4.75 ft below the conduit). The depth was limited to 4 ft to balance construction risk and construction cost while still providing a functional filter diaphragm. In the opinion of GSFW, the proposed filter diaphragm design meets the intent of TR-60 to intercept potential cracks in the embankment, poorly compacted soil zones, or other discontinuities associated with the principal spillway conduit or its installation.

We expect that some local dewatering by pumps and sumps will be needed to control groundwater and seepage that could be encountered from the existing blanket. Dewatering wells were not considered due to the fine-grained soils encountered in the dam and foundation, and the short duration that the excavation will be open. Dewatering wells are ineffective in clays and silts and have a slow response time. The proposed design requires that sumps be installed prior to performing the final portion of the excavation (below groundwater). The sumps will be run sufficiently in advance of excavation below the groundwater to ensure that the soils are pre-drained and all excavation work is performed in the dry. We also anticipate that the lower portion of the filter diaphragm excavation (bottom 4 ft) would be excavated

June 9, 2020 Page 9 GSFW Engineering JV and backfilled in one work shift. The excavation and dewatering requirements are defined in the construction specifications.

3.4 Sheetpile Wall

A sheetpile wall is proposed along the left side of the proposed auxiliary spillway outlet to prevent spillway flow from entering into a gully located downstream of the dam. The height of the proposed sheetpile wall above the auxiliary spillway ground surface is 4.5 ft to match the height of the training dike, which was designed to contain the PMF with 3 ft of freeboard. The design calculations considered that the gully would erode 2 ft lower than the lowest existing gully elevation, and that the sideslopes of the gully would erode toward the sheetpile wall. This would result in the sideslope steepening from the existing 2H:1V slope to an estimated 1H:1V slope. Because the gully has been observed by NRCS to be relatively stable with no significant erosion for many years, the assumed level of erosion is considered conservative.

The sheetpile wall was designed using Shoring Suite Version 8.12 d. Three critical sections were considered in the design of the sheetpile wall, located at Sta 3+00, Sta 4+25, and Sta 6+00. These sections were chosen based on the location of the critical wave load, and the proximity and depth of the gully. The three load cases considered were as follows:

• Dynamic wave load

• Hydrostatic load at the top of wall (conservative approximation of the FBH load)

• Normal conditions (no flow)

Based on the geometry of the auxiliary spillway channel, the dynamic wave load was calculated at Sta 3+00, where the force would make contact with the wall. The hydrostatic load case was considered at Sta 4+25 and Sta 6+00. Normal conditions were considered at each section, but did not control. The minimum computed sheetpile lengths are 13.3 ft from Sta 2+75 to Sta 5+00 (based on Sta 4+25), 29.5 ft from Sta 5+00 to Sta 5+50 to provide a transition zone, and 45.6 ft from Sta 5+50 to Sta 7+00 (based on Sta 6+00). The top 15 ft of each sheetpile will be coated with zinc-rich primer and coal tar epoxy for corrosion protection. Calculations meet NRCS and USACE criteria, and are included in Appendix A.1.

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4.0 HYDROLOGIC AND HYDRAULIC DESIGN

4.1 Introduction

This section summarizes the hydrologic and hydraulic analyses performed to support the rehabilitation design. Details related to these analyses are presented in the Hydrology Report, dated September 27, 2016, and the Hydraulic Design Report, dated June 16, 2017. We have noted in the following sections where previous analyses have been refined or updated as needed.

4.2 Watershed and Hydrologic Parameters

The computed drainage area of 261 acres (0.4 sq mi) varies in elevation from about EL 1214 ft to EL 970 ft at the toe of the dam. The drainage area was delineated and measured using ArcGIS and available LiDAR topography. The computed drainage area is consistent with the area used for the original design of the dam.

As discussed in the Hydrology Report, the computed curve number (CN) of 80 was based on current land cover and SSURGO data revised in 2015. Similarly, the Time of Concentration (Tc) of 1.53 hours was computed using the NRCS Velocity Method, as described in the NEH Part 630 Hydrology (revised in 2010).

4.3 Existing Topographic and Structure Data

Critical dimensions of Conneautville Dam are summarized in Table 4.1. Project elevations and reservoir storage are summarized in Table 4.2. The elevations are based upon the field survey performed in 2016.

Reservoir storage is based on the field survey, augmented with LiDAR data above EL 979.0.

Table 4.1: Project Information

Embankment Length 660 ft

Maximum Height(1) 31 ft

Inside Dimensions of Riser 3 ft x 9 ft

Pool Drawdown Conduit Diameter 18 inches

Size of Orifice 7 inches x 12 inches

Principal Spillway Conduit Diameter 36 inches

Length of Conduit 150 ft

Principal Spillway Weir Length 18 ft

Auxiliary Spillway Width 100 ft

(1) Based on settled design height to channel bottom at the downstream toe of the dam.

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Table 4.2: Existing Project Elevation and Reservoir Storage (NAVD88)

Elevation (ft)

Storage (ac-ft)

Drawdown Conduit Inlet Invert 972.6 0.0

Normal Pool(1) 982.2 5.6

Principal Spillway Riser Crest 995.5 66

Auxiliary Spillway Crest 996.0 70

Settled Top of Dam 1001.1 119

(1) Reservoir is operated nearly empty. Elevation represents low-stage orifice in the riser.

4.4 Soil Material Properties

Auxiliary spillway soil material properties and layout were summarized in the Hydraulic Design Report, and were revised from the Hydrology Report based on field sampling and analysis outlined in the Final Geotechnical Report.

The soil properties of the proposed auxiliary spillway in SITES were selected to reflect the findings outlined in the Final Geotechnical Report and are presented in Table 4.3. The surficial 2 ft of the spillway profile will be lime-augmented clay soils, underlain by a clay layer and with sandy soils at the valley floor (EL 970 ft). The headcut erodibility index (Kh) values provided in the Final Geotechnical Report were calculated using procedures outlined in Chapter 52 of the NEH Part 628, Dams (1997).

Table 4.3: Auxiliary Spillway Soil Depths and Properties

Soil Class

Depth (ft)

Plasticity Index

Dry Density (lb/cu ft)

Headcut Index

Clay

Representative Diameter (inches)

Clay(1) 0-2 27 109.4 0.02 47.7 0.00026

Clay(2) 2-25 14 101.1 0.05 59.3 0.0006

SP/SM >25 9 107 0.04 50.9 0.0008

(1) Plasticity Index, dry density, percent clay, and representative diameter were calculated by averaging values from 2016 Test Pits TP-17, TP-19, TP-20, TP-21, and TP-22.

(2) Percent clay and representative diameter were calculated by averaging values from Plan-EE documentation, and Jar Samples B-1, B-2, and B-3 (JLT Laboratories, 2011).

Note: Headcut index calculations can be found in Appendix C of the Final Geotechnical Report. All other values not mentioned were obtained from the Hydraulic Design Report, Table 2.3.

4.5 Precipitation Data

As described in the Hydrology Report, NOAA precipitation-frequency and Probable Maximum Precipitation (PMP) Data presented in the Plan-EE were used as the basis for this project’s hydrologic analysis. Rainfall was distributed using the NRCS standard distributions within SITES for Principal Spillway Hydrograph (PSH), the 6-hour Stability Design Hydrograph (SDH), and the 6-hour Freeboard Hydrograph (FBH); and a 5-point rainfall distribution for the 24-hour FBH.

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4.6 Hydrologic Analysis

Hydrologic analysis was performed using the USDA-NRCS SITES computer program following NRCS procedures in accordance with TR-60. The analysis is described in the Hydrology Report and indicates that the existing dam and spillways meet NRCS criteria for the PSH, SDH, and FBH, assuming a dry pond and a condition where the reservoir is refilled to the original normal pool level. As described below, a realigned auxiliary spillway is proposed to address stability and integrity concerns. The hydrologic analysis was updated to reflect the proposed spillway alignment and is described in the Hydraulic Design Report. The results indicate that the proposed rehabilitation meets TR-60 capacity criteria.

Table 4.4: Existing Condition SITES Results

Storm 6-Hour Duration 24-Hour Duration 10-Day Duration

Outflow (cfs)

Peak Stage

(ft) Outflow

(cfs) Peak Stage

(ft) Outflow

(cfs) Peak Stage

(ft)

PSH N/A N/A N/A N/A 396 995.7(1)

SDH 479 997.6 403 997.5 N/A N/A

FBH 2322 1000.1 1092 998.6 N/A N/A

(1) This elevation is the peak elevation of the PSH, without consideration of drawdown requirements. The 10-day drawdown requirement was met, so this elevation is the minimum elevation for the auxiliary spillway.

Table 4.5: Proposed Condition SITES Results

Storm 6-Hour Duration 24-Hour Duration

Outflow (cfs)

Peak Elevation

(ft) Outflow

(cfs) Peak

Elevation (ft)

SDH 450 997.9 400 997.8

FBH 2,310 1000.4 1,090 999.0

The PADEP approved dam breach analysis was performed in 1997, and served as a basis for the breach inundation map included in the current EAP. This analysis conservatively assumed a starting pool elevation at 1001.4 ft, NGVD29 (1000.8 ft, NAVD88).

4.7 Auxiliary Spillway Stability and Integrity Analysis

As described in the Hydrology Report, we evaluated the potential for erosion of the existing auxiliary spillway using SITES, and the dam was found to meet both auxiliary spillway stability and integrity criteria, assuming a uniform cross-section of the auxiliary spillway and no curvature in the level section or outlet;

however, the existing conditions do not meet these assumptions and do not comply with TR-60. With the eroding channel on the left edge of the outlet channel, a headcut is likely to progress much more quickly and farther than predicted by SITES. The level section and spillway outlet are curved, and the training dike does not extend far enough downstream to protect the embankment toe and principal spillway outlet;

therefore, a realigned auxiliary spillway is proposed.

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The Hydraulic Design Report summarizes the stability and integrity analysis that was updated for the proposed spillway alignment. The results indicate that the proposed rehabilitation meets TR-60 stability and integrity criteria.

4.8 Hydraulics Analysis

Flow conditions through the proposed auxiliary spillway alignment, and near the principal spillway impact basin and the downstream toe of the dam, were simulated for the 6-hour SDH and FBH using HEC-RAS

5.0.3 as described in the Hydraulic Design Report. This version of HEC-RAS includes a 2D computational fluid dynamics (CFD) model, which was used to estimate hydraulic conditions through proposed auxiliary spillway. The proposed auxiliary spillway alignment was designed to comply with TR- 60 criteria and to provide favorable performance as indicated by the hydraulic model results.

4.9 Proposed Spillway Modifications

Based on the results of the hydrologic and hydraulic analysis, the following measures are proposed to bring the auxiliary spillway into compliance with NRCS and PADEP criteria.

4.9.1 Realigned Auxiliary Spillway

The proposed auxiliary spillway has been designed to maintain the same width as the existing spillway.

The level section was straightened and shortened to 30 ft, the elevation of the control section was raised

0.5 ft (EL 996.5), and the elevation of the approach was lowered by at least 0.5 ft to provide a 1-ft elevation difference just upstream of the control section with a 1 percent adverse grade. TR-60 recommends that the approach be depressed below the level section to reduce velocities. Additionally, the raised control section provides an increased level of flood protection. The 100-year flood is contained in the reservoir, and the 100-year pool elevation is 5.2 ft below the proposed auxiliary spillway elevation.

The inlet channel will be curved, with the level control section downstream of the curve and approximately in line with the crest of the dam. A transition with a 10 percent slope connects the curved inlet with the level section. The outlet channel is in line with the level section and rotated clockwise from its existing location to prevent flow from entering the gully located on the left edge of the spillway outlet. A sheetpile wall extends along the left side of the outlet channel to contain flow. The spillway outlet channel will be graded at about a 4 percent slope to EL 990.0, where the channel ties into the existing grade.

4.9.2 Training Dike

The training dike will be rotated and extended downstream about 220 ft from the downstream end of the control section on the right side of the auxiliary spillway to contain the PMF along the new alignment. The training dike will have a 12-ft crest width and 3H:1V sideslopes. The top of the training dike will be equal to the top of dam elevation at and upstream of the level section of the spillway. Downstream of the level section, the height of the training dike will be 4.5 ft to contain the PMF with flow depths of about 1.5 ft and standing waves that are expected to occur. The top of the sheetpile wall will match the elevation of the training dike.

Riprap armoring is proposed on the right side slope of the auxiliary spillway (looking downstream) to protect against erosion of the embankment and training dike. Riprap armoring is also proposed at the contact point between the new training dike and the existing dam embankment to control surface erosion

June 9, 2020 Page 14 GSFW Engineering JV where concentrated flow is anticipated. The sizing calculation for the proposed riprap armoring is included in Appendix A.3.

4.10 Proposed Replacement Trash Rack

The existing concrete inlet structure to the reservoir drain has two bars that are intended to function as a trash rack; NRCS has indicated a desire to improve its functionality. The dam will continue to function as a dry dam following construction of the proposed rehabilitation, and the reservoir drain will provide primary inlet control during normal conditions. The proposed trash rack improves functionality and has better maintenance characteristics. The existing principal spillway riser meets TR-60 requirements that either a trash rack or ported concrete riser extends above the anticipated sediment elevation for the design life of the dam. The proposed replacement trash rack at the inlet structure was designed to meet the velocity requirements in both paragraphs of TR-60: (1) 2.5 ft/sec under the full range of stage and discharge, and (2) 2 ft/sec when the water level is 5 ft above the trash rack. The hydraulic calculation for the proposed replacement trash rack is included in Appendix A.2.

4.11 Staging Area Flood Routing Analysis

A proposed staging area for stockpiling and curing lime-treated soils was identified at the east side of the site, and is referred to as Borrow Area B in the As-Built Drawings. The elevation of the east staging area generally ranges from EL 990 to EL 1008. We have performed flood routing analyses to estimate the depth of flooding that may be expected in the proposed staging area during various frequency precipitation events. Table 4.6 summarizes the results of the flood routing analyses, considering the existing dam with a drawdown conduit that is open or blocked (clogged). We anticipate providing this information to bidders in a data report so that they can evaluate the risk of flooding during construction.

Table 4.6: 1- through 100-Year Frequency Floods

24-Hour Storm Frequency

Rainfall (inches)

Water Surface Elevation – Open

Drawdown Conduit (ft)

Water Surface Elevation – Blocked Drawdown Conduit

(ft) 1-year 2.10 982.3 985.8

2-year 2.51 983.5 987.0

5-year 3.10 985.3 988.9

10-year 3.57 986.6 990.4

25-year 4.25 988.5 992.4

50-year 4.80 989.9 993.9

100-year 5.39 991.3 995.4

June 9, 2020 Page 15 GSFW Engineering JV

5.0 PLANS, SPECIFICATIONS, AND OTHER PROJECT DOCUMENTS

5.1 Plans

Plans have been developed to detail the designs for site layout of the auxiliary spillway, lime treatment, embankment drain reconfiguration and filter diaphragm, trash rack details, and erosion and sediment control measures.

The following drawings have been prepared as part of the Final Design Submission:

• CS-1: Cover Sheet

• G-1: General Notes

• G-2: Existing Plan

• G-3: Test Boring Location Plan

• G-4: Geologic Sections

• G-5: Contract Limits and Staging

Area Plan

• G-6: Project Layout and Control

• C-1: Proposed Plan

• C-2: Lime Treatment

• C-3: ASW Typical Sections

• C-4: ASW Multiple Sections

• C-5: Sheetpile Wall Profile

• C-6: Drainage Plan and Sections

• C-7: Trash Rack Details

• ES-1: ESC Plan – Extent

• ES-2: ESC Plan – Main Dam

• ES-3: ESC Plan – Staging Areas

• ES-4: ESC Details

• ES-5: ESC Notes

5.2 Technical Specifications

Standard NRCS construction and material specifications included in NEH Part 642 will be used for this project, with references to Pennsylvania Department of Transportation and Crawford County standards where applicable. These specifications were developed in accordance with instructions contained in NEH Part 642. The following is a list of final specifications that we prepared for the Final Design Submission and are submitting under separate cover:

• Construction Specifications o 1 – Clearing o 2 – Clearing and Grubbing o 3 – Structure Removal o 5 – Pollution Control o 6 – Seeding, Sprigging, and

Mulching o 7 – Construction Surveys o 8 – Mobilization and Demobilization o 11 – Removal of Water o 13 – Piling o 21 – Excavation o 23 – Earthfill o 24 – Drainfill o 26 – Topsoiling o 28 – Lime-Treated Earthfill o 45 – Plastic Pipe o 61 – Rock Riprap o 81 – Metal Fabrication and Installation o 94 – Contractor Quality Control o 95 – Geotextile o 96 – Field Office

June 9, 2020 Page 16 GSFW Engineering JV Project 15715008.00 ©2020 All Rights Reserved

• Material Specifications o 511: Steel Piles o 521: Aggregates for Drainfill and

Filters o 523: Rock for Riprap o 547: Plastic Pipe o 581: Metal o 592: Geotextile o 593: Lime

5.3 Other Project Deliverables

As part of the Final Design Submission, we have also prepared the following documents, which are being submitted under separate cover:

• Final Quality Assurance and Staffing Plan

• Final Operation and Maintenance Manual (Revision 1)

5.4 Opinion of Probable Construction Cost

Our opinion of probable construction cost (OPCC) for this project is __________. The detailed estimate is included in Appendix B. Quantities were derived from the Final Design drawings and are consistent with pay items identified in standard NRCS construction specifications. We developed unit costs for the pay items based on our experience with recently completed construction projects of similar magnitude and complexity in Pennsylvania and published construction cost data. This OPCC is considered a “Class 1” estimate according to the categories of cost estimates defined by the Association for the Advancement of Cost Engineering (AACE) International, which are described in Tables 5.1 and 5.2.

Table 5.1: AACE Accuracy Matrix for Estimating Classes

ESTIMATE

CLASS

Primary Characteristic Secondary Characteristic

LEVEL OF

PROJECT

DEFINITION

Expressed as % of Complete Definition

END USE

Typical Purpose of Estimate

METHODOLOGY

Typical Estimating

Method

EXPECTED

ACCURACY

RANGE

Typical Variation in

Low and High Ranges

PREPARATION

EFFORT

Typical Degree of Effort Relative to

Least Cost Index of 1

Class 5 0% to 2% Concept Screening

Capacity Factored, Parametric Models, Judgment or Analogy

Low: -20% to -50% High: +30% to +100% 1

Class 4 1% to 15% Study or Feasibility

Equipment Factored or Parametric Models

Low: -15% to -30% High: +20% to +50% 2 to 4

Class 3 10% to 40% Budget

Authorization, or Control

Semi-Detailed Unit Costs with Assembly

Level Line Items

Low: -10% to -20% High: +10% to +30% 3 to 10

Class 2 30% to 70% Control or Bid/ Tender

Detailed Unit Cost with Forced Detailed Take-

Off

Low: -5% to -15% H: +5% to +20% 4 to 20

Class 1 50% to 100% Check Estimate or Bid/Tender

Detailed Unit Cost with Detailed Take-Off

Low: -3% to -10% High: +3% to +15% 5 to 100

June 9, 2020 Page 17 GSFW Engineering JV

Table 5.2: AACE Class 1 Estimate Description

CLASS 1 ESTIMATE

Description:

Class 1 estimates are generally prepared for discrete parts or sections of the total project rather than generating this level of detail for the entire project. The parts of the project estimated at this level of detail will typically be used by subcontractors for bids, or by owners for check estimates. The updated estimate is often referred to as the current control estimate and becomes the new baseline for cost/schedule control of the project. Class 1 estimates may be prepared for parts of the project to comprise a fair price estimate, or bid check estimate to compare against a contractor’s bid estimate, or to evaluate/dispute claims. Typically, engineering is from 50% to 100% complete, and would comprise virtually all engineering and design documentation of the project, as well as complete project execution and commissioning plans.

Level of Project Definition Required:

50% to 100% of full project definition.

End Usage:

Class 1 estimates are typically prepared to form a current control estimate to be used as the final control baseline against which all actual costs and resources will now be monitored for variations to the budget, and form a part of the change/variation control program. They may be used to evaluate bid checking, to support vendor/contractor negotiations, or for claim evaluations and dispute resolution.

Estimating Methods Used:

Class 1 estimates involve the highest degree of deterministic estimating methods, and require a great amount of effort.

Class 1 estimates are prepared in great detail, and thus are usually performed on only the most important or critical areas of the project. All items in the estimate are usually unit cost line items based on actual design quantities.

Expected Accuracy Range:

Typical accuracy ranges for Class 1 estimates are -3% to 10% on the low side, and +3% to +15% on the high side, depending on the technological complexity of the project, appropriate reference information, and the inclusion of an appropriate contingency determination. Ranges could exceed those shown in unusual circumstances.

Effort to Prepare (for US$20MM project):

Class 1 estimates require the most effort to create, and as such are generally developed for only selected areas of the project, or for bidding purposes. A complete Class 1 estimate may involve as little as 600 hours or less, to perhaps more than 6,000 hours, depending on the project and the estimating methodology used. Bid estimates typically require more effort than estimates used for funding or control purposes.

ANSI Standard Reference Z94.2 Name:

Definitive estimate (typically -5% to + 15%).

Alternate Estimate Names, Terms, Expressions, Synonyms:

Full detail, release, fall-out, tender, firm price, bottoms-up, final, detailed control, forced detail, execution phase, master control, fair price, definitive, change order estimate.

5.5 Estimated Construction Schedule

We prepared an Estimated Construction Schedule based on the final design, which is included as Appendix C. Total construction duration is estimated to be about seven months. Assuming NRCS will issue a construction notice to proceed in February 2021, the estimated construction schedule considers a completion in September 2021.

June 9, 2020 Page 18 GSFW Engineering JV

6.0 PERMITTING

6.1 Anticipated Permits

We have developed the following list of anticipated permits and potential studies that may be required for this project.

• Delineate wetlands/waters using the USACE 1987 Wetland Delineation Manual within the limits of proposed disturbance. The Wetland Delineation Report was submitted on June 30, 2017.

• Preliminary Jurisdictional Determination from the USACE to verify the delineated wetland boundary. The Jurisdictional Determination Request Form was submitted to the USACE on July 25, 2017, and accepted by letter dated August 22, 2017.

• Pennsylvania Historic and Museum Commission Cultural Resource Notice to determine presence of cultural resources (completed December 2012).

• Pennsylvania Natural Heritage Program’s Natural Diversity Index Search was performed on May 1, 2017, and updated on March 18, 2020, to determine if protected species are within the project vicinity. The Pennsylvania Fish and Boat Commission (PFBC) responded to each search indicating that no adverse impacts are expected.

• The request for PADEP Letter of Amendment was submitted in September 2018 and was issued on February 20, 2020.

• Environmental Assessment Form was submitted in August 2018 with the request for Letter of Amendment.

• Erosion and Sediment Control Plan was submitted to Crawford County Conservation District in September 2018 and was approved May 12, 2020.

• NPDES Phase II Plan (for more than 1 acre of disturbance to land areas and/or wetlands) was submitted to Crawford County Conservation District in September 2018 and was approved May 12, 2020.

• PADEP Encroachment Permit (Chapter 105/Section 404, Federal Clean Water Act Permit), which can be jointly issued through the PA State Programmatic General Permit process because impacts are less than 1 acre. This was submitted in September 2018 and issued on February 20, 2020.

• Mitigation Plan (if wetland impacts are greater than 0.05 acre). USACE issued a jurisdictional determination confirming our wetland delineation. Based on the wetland boundary and our proposed impacts, mitigation will not be required because there are no permanent wetland/water impacts. There may be temporary wetland impacts to access the staging area.

June 9, 2020 Page 19 GSFW Engineering JV

6.2 Land Rights

Based on cursory evaluation of property lines, easement boundaries, and the proposed limits of disturbance, we do not anticipate that additional land rights will be required for this project.

June 9, 2020 Page 20 GSFW Engineering JV

7.0 REFERENCES

The following project-specific information was obtained and reviewed or developed as part of our design:

Bioengineering Group (2010). “Attachment 17 – Subsurface Explorations and Laboratory Test Data.”

Unknown Report Title.

Bioengineering Group (July 27, 2011). “Technical Memorandum.”

Bioengineering Group (October 2012). “Report of Findings of the Topographic and Sedimentation

Evaluation for the Conneautville Dam (PA-RCD-112).”

Gannett Fleming, Inc. (October 2004). Conneautville Dam Technical Review. Harrisburg, PA.

Gannett Fleming, Inc. (June 2005). Conneautville Dam Stability and Compatibility Evaluation.

Harrisburg, PA.

GSFW Engineering JV (March 3, 2016). Revised Plan of Work.

GSFW Engineering JV (March 8, 2017). Final Geotechnical Report.

GSFW Engineering JV (April 24, 2017). 50% Design Report.

GSFW Engineering JV (February 28, 2018). Detail Design Report (Revision 1).

GSFW Engineering JV (June 16, 2017). Hydraulic Design Report.

GSFW Engineering JV (June 30, 2017). Wetland Delineation Report.

GSFW Engineering JV (May 6, 2016). Final Geotechnical Investigation and Outlet Inspection Plan.

GSFW Engineering JV (September 2, 2016). Laboratory Investigation Plan.

GSFW Engineering JV (September 15, 2016). Existing Structural Condition Investigation Report.

GSFW Engineering JV (September 27, 2016). Hydrology Report.

NRCS (August 30, 1979). “EN – Soil Mechanics 40-22 – Pennsylvania RC&D; Conneautville PA-RCD-

113.” Midwest TSC, Lincoln, NE.

NRCS (October 23, 1979). “EN 40-22 – Soil Mechanics – Pennsylvania WF-11; Conneautville Site PA-

RCD-112.” Midwest TSC, Lincoln, NE.

NRCS (November 21, 1979). “EN-40-22-Soil Mechanics; Conneautville Site PA-RCD-112.” Northeast

TSC, Broomall, PA.

NRCS (1981). Conneautville Flood Prevention Project As-Built Drawings.

June 9, 2020 Page 21 GSFW Engineering JV Project 15715008.00 ©2020 All Rights Reserved

NRCS (December 2012). Conneautville Site PA-RCD-112 Watershed Project Plan-Environmental Evaluation. Harrisburg, PA.

The following technical guidelines, codes, and criteria were referenced as part of our design:

American Society for Testing and Materials (ASTM). Standard Specifications, Standard Test Methods, Standard Classifications, and Standard Guides.

ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures.” (July 2006).

Bonnin, G.M., Martin, D., Lin, B., Parzybok, T., Yekta, M., and Riley, D. (2004). “Precipitation-Frequency Atlas of the United States.” NOAA Atlas 14, Volume 2, Version 3, NOAA, National Weather Service, Silver Spring, MD.

NRCS (December 1980). NEH 6-4, “Structural Design – Concrete.”

NRCS (July 2005). TR-60, “Earth Dams and Reservoirs.”

NRCS (November 2006). TR-48, “SITES Water Resource Site Analysis Computer Program User’s Guide.”

NRCS (March 6, 2015). “Pennsylvania Specifications for Engineering Services – Dam Rehabilitation Design.”

NRCS. NEH Part 628, “Dams,” Chapters 50, 51, and 52.

NRCS. NEH Part 630, “Hydrology.”

NRCS. NEH Section 5, “Hydraulics.”

NRCS. NEH Part 642, “Specifications for Construction Contracts.”

NRCS. Standard Detail Drawing ES3230-2020R, Standard Baffle Top Riser.

Temple et al. (1987). “Stability Design of Grass-Lined Open Channels.” Agriculture Handbook Number

667. USDA ARS.

United States Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service (1978). “Probable Maximum Precipitation Estimates, United States East of the 105th Meridian.” NOAA Hydrometeorological Report HMR-51.

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