Lake Erin Dam 100_ Deliverable Basis of Desig.pdf

Lake Erin Dam Rehabilitation

Basis of Design Report

NID ID: GA01324

City of Tucker Dekalb County, Georgia

Project number: 60727041

September 13, 2024

Lake Erin Dam Rehabilitation Basis of Design Report

PreparedFor: City of Tucker

AECOM

Table of Contents

1. Introduction

1.1 Scope of the Report

1.2 Statement of Limitations

1.3 Site Description

1.4 Previous Work

1.4.1 Lake Erin Dam Hydrologic and Hydraulic Analysis (AECOM, 2021)

1.4.2 Spillway Alternatives Analysis (AECOM, 2022a)

1.4.3 Lake Erin Dam Geotechnical Analyses (AECOM, 2022b)

2. Scope of Repairs

2.1 Demolish and Remove Existing Features

2.2 Install New Principal Spillway

2.3 Raise Embankment Crest

2.4 Flatten the Upstream Slope

2.5 Regrade and Armor Outlet Channel Downstream

2.6 Filter Drain, Toe Drain, and Filter Diaphragm

2.6.1 Filter Drain (West Toe Drain)

2.6.2 East Toe Drain

2.6.3 Filter Diaphragm

2.7 Piezometers

3. Proposed Conditions Geotechnical Analysis

3.1 Filter Diaphragm Design

3.1.1 Filter Compatibility

3.1.2 Proposed Conditions Slope Stability Analysis and Seepage Modeling

3.2 Bearing Capacity

3.3 Settlement Analysis

4. Proposed Conditions Hydrologic and Hydraulic Analysis

4.1 Proposed Conditions Model

4.2 Low-Level Outlet Sizing

4.3 Impact Basin Sizing

4.4 Outlet Channel and Riprap Apron

4.5 Wave Height Analysis

5. Proposed Conditions Structural Analysis

5.1 Overview

5.2 Industry Codes and Standards References

5.3 Material Properties

5.4 Loads and Load Combinations

5.5 Load Conditions

5.6 Evaluation Criteria

5.7 Method of Analysis

5.8 Analysis Results

6. Construction Considerations

6.1 Water Management

6.2 Dam Safety

6.3 Property Impacts

6.4 Underground Utility Impacts

7. References

Appendices

Appendix A – Previous AECOM Investigations Appendix B – Geotechnical Analysis Calculations Appendix C – Hydrologic and Hydraulic Analysis Calculations Appendix D – Structural Calculations Appendix E – Previous Geotechnical Reports

Figures

Figure 1: Location and Context Map Figure 2: Cross section locations for seepage and stability analyses Figure 3: Maximum Embankment Cross Section Figure 4: 100ft Offset Cross Section Figure 5: Elevation-Storage Rating for Proposed Conditions Figure 6: Elevation-Discharge Ratings for Existing and Proposed Principal Spillway Figure 7: Proposed Drawdown Rating Curve

Tables

Table 1. Dam and Reservoir Data Table 2: Proposed Conditions Table 3. Drain Filter Compatibility Analysis Results Table 4: Soil Hydraulic Conductivity and Anisotropy Material Properties Table 5: Soil Strength Material Properties Table 6. Proposed Conditions Critical Slope Stability Factors of Safety Deep Cross Section Table 7. Proposed Conditions Critical Slope Stability Factors of Safety Offset Cross Section Table 8. Cut Slope Analysis Table 9. Comparative HEC-HMS Model Results Table 10. Summary of Material Properties for Structural Analysis Table 11. Summary of Individual Loads Table 12. Summary of Stability Criteria for Sliding, Overturning, Flotation, and Bearing Table 13. LRFD Load and Reduction Factors Table 14. Impact Basin Stability Analysis Results – Sliding and Flotation Table 15. Impact Basin Stability Analysis Results – Overturning and Bearing Table 16. Intake Tower Stability Analysis Results – Sliding and Flotation Table 17. Intake Tower Stability Analysis Results – Overturning and Bearing

1. Introduction

1.1 Scope of the Report

This report documents the basis of design for repairs to the Lake Erin Dam. The basis of design includes the supporting analyses, engineering assumptions, input data, modeling results, and design calculations that support the design of each item in the scope of the repairs.

1.2 Statement of Limitations

Interpretation of general subsurface conditions presented herein is based on the soil, rock, and groundwater conditions encountered in the limited number of soil borings. Although representative portions of the samples taken were tested, subsurface conditions may vary outside of the exploration locations. This report does not reflect any variations that may occur across the site in areas not sampled.

This report has been prepared for the specific application to the project discussed and has been prepared in accordance reasonable and accepted engineering practices and standard of care based on the information available to AECOM Technical Services, Inc. (AECOM) at the time of performance of the work.

No warranty or guarantee, either written or implied, is applicable to this work.

1.3 Site Description

The Lake Erin Dam (State ID number 044-004-00033 and National Inventory of Dams – NID identification number GA01324) is located on an unnamed tributary to the North Fork Peachtree Creek. Lake Erin Dam is an earthen dam located in Henderson Park in the City of Tucker, DeKalb County, Georgia. The dam was previously owned and maintained by the DeKalb County Department of Public Works, Roads and Drainage

Division, but its ownership was recently transferred to the City of Tucker. The primary function of the dam is to serve as a recreational amenity for Henderson Park. Lake Erin Dam is regulated by the Georgia

Department of Natural Resources, Environmental Protection Division, Safe Dams Program (SDP). Based on the current characteristics for Lake Erin Dam, SDP has categorized the dam as a Category I, Medium

Dam. Figure 1 below shows the location of the dam and its watershed in context of the surrounding vicinity.

Figure 1: Location and Context Map

The dam is a 32-foot-high earth embankment dam and impounds a 5-acre reservoir. The outlet works of the dam is a principal spillway system consisting of a 30-inch Corrugated Metal Pipe (CMP) intake conduit leading to a riser structure that was constructed of a 30-inch CMP conduit. Immediately downstream of the riser structure is a 30-inch Reinforced Concrete Pipe (RCP) conduit that discharges into the auxiliary spillway channel. The outlet structure for the principal spillway conduit is a concrete end wall that is abutted by the vertical stone masonry training walls of the auxiliary spillway. The auxiliary spillway is an open channel spillway with vertical stone masonry training walls. The auxiliary spillway is founded on bedrock and has a bottom width of approximately 12 feet. There is currently no low-level outlet system at Lake Erin

Dam. There is a 9-inch steel standpipe located on the upstream slope that is assumed to be the historic dewatering system but no longer functions. According to the NID, the dam was constructed in 1960.

However, the lake appears on the 1956 USGS Stone Mountain Quadrangle sheet, indicating the dam was built before the aerial photograph was taken in 1955.

Table 1, provided on the following page, presents pertinent information about the dam.

Table 1. Dam and Reservoir Data

Description Value

General Data

Year of Original Construction Before 1956 (estimated)

Purpose Recreation

Current Hazard Classification Category I

Drainage Area 0.5 square miles

Dam Height at Maximum Section 32 feet

Crest Length 360 feet

Crest Width Varies from 13 to 20 feet

Upstream Slope 2.5H:1V

Downstream Slope 3H:1V

Critical Elevations1

Normal Pool (Service Spillway Invert) 955.7 feet

Dam Crest Varies from 968.5 to 969.4 feet

Intake Riser Invert 962.1 feet

Auxiliary Spillway Invert 967.0 feet

Natural Channel Bottom Elevation 936.2 feet

Storage Capacities

Normal Pool (Elevation 955.7 feet) 30 acre-feet

Max Storage (Elevation 968.5 feet) 159 acre-feet

Pool Surface Areas

Normal Pool (Elevation 955.7 feet) 5 acres

Dam Crest (Elevation 968.5 feet) 15 acres

Outlet Features

Principal Spillway Intake Conduit 30-inch CMP conduit

Principal Spillway Intake Structure 30-inch CMP riser structure

1 All elevations are reported in the North American Vertical Datum of 1988 (NAVD88).

Description Value

Principal Spillway 30-inch RCP conduit

Principal Spillway Outlet Structure Concrete Endwall

Auxiliary Spillway 12-foot-wide masonry spillway founded on bedrock

1.4 Previous Work

Prior to the development of design of repairs to the dam, several efforts were completed to investigate the issues related to the Lake Erin Dam. These efforts are described in this section. Previous work completed by AECOM for the rehabilitation of Lake Erin Dam is included in Appendix A of this report.

1.4.1 Lake Erin Dam Hydrologic and Hydraulic Analysis (AECOM, 2021)

AECOM completed a hydrologic and hydraulic (H&H) analysis for the Lake Erin Dam and compared the results against the requirements set by SDP for this category of dam for compliance. Recommendations were also given in this report to correct any non-compliance issues. This work yielded the following findings:

• The design storm for Lake Erin Dam (Criterion 1.1.4.2: Medium Dam) is 33.3% of the Probable

Maximum Precipitation (PMP) event.

• No energy dissipator exists at the downstream end of the principal and auxiliary spillway outlets.

• The auxiliary channel is not activated more frequently than during the 2% annual exceedance probability (AEP - 50-year) storm event.

• A freeboard analysis considering the maximum wave height was prepared and indicated a recommended freeboard value of 0.8 feet.

• There is no low-level outlet system at the reservoir; therefore, there is no way to lower the reservoir below the normal pool elevation of 955.7 feet.

• The low point of the crest (elevation 968.5 feet) is overtopped by 0.7 feet during the Spillway Design

Flood (SDF) event.

The general recommendations made in this report to address the non-compliance deficiencies identified are:

• Criterion 5.3.4 – Design a sufficient energy dissipation device at the outlet of the spillway(s) such that it is capable of sufficiently dissipating energy from the design peak discharge to meet this criterion.

• Criterion 5.5 – Modify the dam, outlet works, or spillways to increase freeboard to meet the required amount. This could be done by increasing spillway capacity to lower peak water surface elevation for the SDF, raising the crest of the dam, or combinations thereof. A spillway erosion analysis using the

SITES software application may be required if an earth-cut auxiliary spillway is pursued as an alternative.

• Criterion 5.7 – Design a gated pipe structure or siphon system to drain two-thirds of the volume at normal pool of the reservoir within ten days or an alternative time frame if approved by SDP.

Three potential rehabilitation alternatives were developed to address the deficiencies identified in this report:

• Rehabilitation Alternative 1 – Widen and lower the auxiliary spillway at the left abutment, raise dam crest low point to elevation 969.0 feet, install a low-level outlet siphon system, and install an energy dissipator at the outlet of the existing masonry spillway.

• Rehabilitation Alternative 2 – Construct an earth-cut auxiliary spillway located at the right abutment, block off the crest of the existing auxiliary spillway at the left abutment, install a low-level outlet siphon system, and install an energy dissipator at the outlet of the existing masonry spillway and new auxiliary spillway at the right abutment.

• Rehabilitation Alternative 3 – Abandon the existing stone masonry spillway, remove the existing principal spillway pipe, construct a new riser structure with controls for a gated low-level outlet and twin 48-inch pipes, and an energy dissipator at the outlet of the pipes.

1.4.2 Spillway Alternatives Analysis (AECOM, 2022a)

AECOM completed a comparative analysis of the three alternative rehabilitation designs presented in the report described in Section 1.4.1 above. This report describes the non-compliance deficiencies previously identified and goes into further detail of the alternatives originally described in the H&H report. The recommendations in the Spillway Alternatives Analysis memo were also updated with minor revisions from the original alternatives, including a fourth alternative which was a variation of the original Rehabilitation

Alternative 3.

• Rehabilitation Alternative 3A – A less expensive medication of Alternative 3 would be to re-locate the new principal spillway near the left abutment immediately right of the existing spillway conduit. The relocation would allow the spillway conduit to be shorted and located higher up in the embankment thus reducing costs.

The client selected Alternative 3 as the preferred solution.

1.4.3 Lake Erin Dam Geotechnical Analyses (AECOM, 2022b)

AECOM completed a review of historic geotechnical information and performed two-dimensional slope stability and seepage analyses of the embankment for Lake Erin Dam. The geotechnical analysis Technical

Memo (AECOM, 2022) presented the analysis and parameters used to conduct modeling of the existing conditions at Lake Erin Dam. The cross section analyzed was located along the centerline of the existing

6-inch toe drain outlet, which is at the approximate maximum height section of the dam. The subsurface investigation performed by Accura Engineering & Consulting, Inc (Accura) in 2021 was used to determine soil and rock engineering properties for the slope stability and seepage analyses. There were also borings drilled in 2013 by Willmer Engineering Inc. (Wilmer) that were used to estimate engineering parameters of soils.

The results of the slope stability and seepage analyses showed that the existing embankment does not meet the minimum required factor of safety required by Georgia Rules and Regulations 391-3-8 Rules for

Dam Safety (Rule 391-3-8-0.9) or U.S. Army Corps of Engineers (USACE) guidelines for upstream rapid drawdown and seismic loading conditions. Analysis with the reservoir pool at the approximate top of dam

(elevation 968) indicated that the downstream slope does not meet the required factor of safety at that pool level.

2. Scope of Repairs

The scope of the repairs to the dam and its appurtenant works includes the following measures:

• Demolish and remove existing features as identified on the Drawings;

• Install a new principal spillway system that includes an intake riser, a low-level and principal spillway conduit, and an impact basin;

• Raise the crest elevation to provide sufficient freeboard above the peak water surface elevation for the spillway design flood event that includes wave action and runup for a fetch length and minimum wind velocity of 50 miles per hour (MPH);

• Flatten the upstream slope to 3H:1V for stability and maintenance;

• Regrade and armor approximately 50 linear feet (LF) of the outlet channel downstream of the impact basin;

• Armor the upstream embankment slope from elevation 952.0 to 958.0 feet.

• Install a filter diaphragm around the principal spillway conduit;

• Install a drain at the downstream toe of the embankment, between the principal spillway conduit and the right abutment;

• Install a filter drain on the bedrock of the existing auxiliary spillway along the left abutment of the downstream face of the embankment to collect any seepage along the auxiliary spillway base and backfill spillway; and

• Install a new row of four (4) open well piezometers to monitor and measure the performance of the dam.

• Repair, by regrading and armoring, the downstream section of an eroded lateral channel that ties into the outlet channel of the principal spillway system.

The basis of design for each measure and its resulting effects on previously identified dam safety concerns are documented in subsequent sections of this report. Related calculations, model results, and other supporting data, where applicable, are provided in the appendices to this report. Table 2 below shows the proposed conditions for Lake Erin Dam.

Table 2: Proposed Conditions

Description Value

General Data

Year of Original Construction Before 1956 (estimated)

Purpose Recreation

Current Hazard Classification Category I

Drainage Area 0.5 square miles

Dam Height at Maximum Section 34 feet

Crest Length 360 feet

Crest Width 15 feet

Upstream Slope 3H:1V

Downstream Slope 3H:1V

Critical Elevations

Normal Pool 955.7 feet

Description Value

Dam Crest 970.0 feet

Intake Tower Low Stage Weir 955.7 feet

Intake Tower High Stage Weir 961.0 feet

Low Level Outlet Invert 941.0 feet

Natural Channel Bottom Elevation 936.33 feet

Storage Capacities

Normal Pool (Elevation 955.7 feet) 30 acre-feet

Max Storage (Elevation 970.0 feet) 181 acre-feet

Pool Surface Areas

Normal Pool (Elevation 955.7 feet) 5 acres

Dam Crest (Elevation 970.0 feet) 16 acres

Outlet Features

Principal Spillway Intake Structure Reinforced Concrete Intake Tower

Principal Spillway Conduit 48-inch RCP

Principal Spillway Outlet Structure USBR Type VI Impact Basin

Low-Level Outlet Intake Structure GDOT Standard 1001-B, U Type Wing Headwall

Low-Level Outlet Intake Conduit 15-inch RCP

2.1 Demolish and Remove Existing Features

There are existing features at Lake Erin Dam that must be removed to install the proposed features. The existing features to be demolished includes trees, vertical stone masonry training walls of the auxiliary spillway, principal spillway system (intake conduit, intake riser, spillway conduit, concrete endwall), standpipe, toe drains, piezometers, etc. The demolished features will need to be disposed of off-site in accordance with local, state, and federal regulations and laws.

2.2 Install New Principal Spillway

To replace the demolished existing principal and auxiliary spillway, a new principal spillway system will be installed. The principal spillway system will include an intake riser structure, a conduit, and an impact basin at the downstream end. The principal spillway conduit will be a 48-inch diameter ASTM C361 Reinforced

Concrete Pipe that consists of a cage or cages of steel reinforcing bars or wire; an encasing wall of concrete;

and a preformed gasket of rubber to provide the joint seal between adjacent pipes. To provide a stable foundation for the principal spillway conduit, the conduit will be installed in a reinforced concrete encasement. The principal spillway conduit will run at a slope of approximately 0.77% and outlet directly to a United States Bureau of Reclamation (USBR) Type VI impact basin. A rating curve relating water surface elevation in the reservoir to discharge through the spillway system was developed for the riser structure.

The capacity of the system at the proposed dam crest elevation of 970.0 feet is approximately 330 cubic feet per second (cfs). The intake structure for the system will be a similar design to a standard Natural

Resources Conservation Service (NRCS) style riser structure that is constructed of reinforced concrete.

This riser structure will have two weir elevations, one to maintain the normal pool elevation and discharge flows for flood events up to the 50-year frequency event, and one to discharge the SDF flows. The low-stage weir will be placed at the existing normal pool elevation of 955.7 feet to maintain the existing pool level. The high-stage weir will be placed at elevation 961.0 feet. During the SDF event, the intake riser will be submerged, which is accommodated by the NRCS design.

The low-level outlet system will consist of a 15-inch diameter ASTM C361 Reinforced Concrete Pipe that consists of the same material specifications as identified above for the principal spillway conduit. To provide a stable foundation for the low-level conduit, the conduit will be installed on a reinforced concrete cradle up to the springline. The low-level outlet system will be controlled by a gate in the intake structure that will be operated from the top of the riser structure. The gate will have a hand activated actuator that only needs one person to operate. At the upstream end of the low-level outlet system, a reinforced concrete headwall will be installed. The headwall will be a modification of the Georgia Department of Transportation (GDOT)

Number 1001-B, Standard Pipe Culvert Concrete Headwall U Type Wing for 15-inch circular conduits. The modification specifically is to expand the footing of the wall upstream to the ends of the wings to accommodate a trash rack. The trash rack will be installed over the openings to the conduits for two reasons:

• To prevent debris from clogging the conduits; and

• To prevent unauthorized access to the upstream end of the conduits.

The USBR Type VI impact basin is designed for the principal spillway conduit’s size and hydraulics anticipated during the SDF event. Due to the velocities (26 feet per second) anticipated from this spillway system, other energy dissipation structures such as scour pools and stilling basins were ruled out as insufficient to adequately dissipate the energy from the spillway discharge to non-erosive levels. The USBR

Type VI impact basin’s minimum geometry was designed in accordance with guidance provided in the

FHWA’s Hydraulic Engineering Circular Number 14, Hydraulic Design of Energy Dissipators for Culverts and Channels, 3rd Edition (FHWA, 2006) and is primarily based on the Froude number for the design discharge entering the basin. The minimum geometry was augmented to ensure structural stability and the ability to retain the surrounding embankment fill, resulting in a basin measuring approximately 25 feet long by 20 feet wide by 15 feet high. Approximately 50 feet of riprap (D50 = 12 inches) will be placed in the stream channel immediately downstream of the impact basin’s end sill to further dissipate energy.

A safety fence will be installed around the impact basin on the upstream end and sides of the structure to prevent unauthorized access to the structure.

2.3 Raise Embankment Crest

The crest of the embankment will be raised to establish a consistent elevation of 970.0 feet along the downstream top of crest. A two percent cross slope will be provided along the crest from the downstream to upstream, draining runoff towards the reservoir. New earth fill will be placed and compacted in lifts using material excavated from other work in the dam embankment or imported from an approved source as necessary. The purpose of this improvement is to ensure that, in the unlikely event of dam overtopping, water flows evenly over the dam at a consistent depth without concentrating in low areas. This helps reduce the risk of dam failure as it lessens the erosive forces of the overtopping discharge. The proposed conditions hydrologic and hydraulic modeling indicate that a crest at this elevation will provide approximately 1.1 feet of freeboard above the peak water surface elevation (968.9 feet) for the SDF event. This exceeds the recommended freeboard that was determined in the previously completed H&H report (AECOM, 2021).

2.4 Flatten the Upstream Slope

As part of the crest raise work described above, the upstream slope will be flattened to a consistent 3H:1V.

New earth fill will be placed and compacted in lifts using material excavated from other work in the dam embankment. The purpose of this repair is to address slope stability and maintenance concerns. The existing typical upstream slope is approximately 2.5H:1V. The upstream slope stability for existing conditions does not meet the required factor of safety. It is also difficult to maintain the vegetation on the upstream slope due to the steep slope. Flattening the slope would extend the toe of the upstream slope approximately 40 feet upstream of its current location. To protect the upstream slope from erosion due to wave action, riprap slope protection (D50 = 12 inches) will be installed along the upstream slope from elevation 952.0 feet to elevation 958.0 feet. The riprap will be founded on a layer of bedding stone and sand.

2.5 Regrade and Armor Outlet Channel Downstream

The existing outlet channel will be regraded to allow for the installation of the proposed principal spillway system and convey reservoir discharges with an alignment that is hydraulically efficient. The proposed channel grade will tie into existing ground and the channel will be armored with riprap (D50 = 12 inches) for approximately 50 feet downstream of the impact basin. The purpose of this repair is to provide a well-defined channel downstream of the impact basin with a consistent slope that will reduce the flow to a non-erodible velocity downstream of the impact basin. There is an existing localized high point downstream of the toe of the dam that would impede the flow discharging from the impact basin. This high point will be graded out and removed. The outlet channel will have varying channel sections to maximize the space available to convey the SDF at a non-erosive velocity and to limit the amount of fill that is placed within the existing stream channel. At the outlet of the impact basin, there will be a well-defined channel that has a bottom width of 6 feet, a depth of 6 feet, a channel slope of 0.35%, and 2H:1V side slopes. Approximately

12 feet downstream of the initial channel section, the left overbank widens out approximately 40 feet towards the invert of the old auxiliary spillway while the right side slope is maintained at 2H:1V. The channel slope will be consistent throughout both of the channel sections described above. After the widening, the flow will constrict back to an existing well-defined channel that continues beyond the site. Using the sizing parameters in HEC-14 (FHWA, 2006), a riprap apron that is approximately 50 feet long will reduce the velocity to non-erosive conditions.

2.6 Filter Drain, Toe Drain, and Filter Diaphragm

Given the existing conditions and proposed installation of a principal spillway conduit, the following drainage systems are proposed:

• Filter drain (West Toe Drain) along the bottom of the downstream left abutment at the existing stone masonry auxiliary spillway

• Filter Diaphragm around the principal spillway conduit

• Toe Drain along the right downstream groin of the embankment (East Toe Drain)

The proposed drainage systems are designed to capture flow within and adjacent to the embankment and convey flow downstream via drainpipe conduits. The drainage systems will be constructed with coarse aggregate, which primarily acts as a drain material to convey seepage to the drainpipes, and a fine aggregate, which primarily functions as a filter to prevent particle movement. This system will provide filtered seepage collection and discharge to reduce the internal erosion of the embankment and natural soils.

Several benefits of the drainage systems include:

• The filtering capability of the material in the filter diaphragm will work to prevent preferential seepage paths within the embankment around the principal spillway conduit from carrying fine soil particles out of the dam and will reduce the potential for internal erosion issues;

• Collecting seepage will result in a lower phreatic surface in the embankment, decreasing the porewater pressure in the embankment and improving the slope stability; and

• The drain conduit outlets will provide a way to measure and monitor seepage from the embankment, creating an early detection system for potential future seepage issues.

2.6.1 Filter Drain (West Toe Drain)

As part of the proposed rehabilitation for Lake Erin Dam, the existing channel lining of the stone masonry auxiliary spillway is to be abandoned-in-place and backfilled. A filter drain, hereby referred to as the West

Toe Drain and comprised of a 6-inch diameter High-Density Polyethylene (HDPE) drainage pipe, is proposed to be placed near the bottom of the existing auxiliary spillway to capture any flow along the base or through fractures of the bedrock. The West Toe Drain filter will extend the width of the existing auxiliary spillway and the 6-inch HDPE drainpipe will outlet through a precast concrete endwall to the riprap lined outlet channel downstream.

2.6.2 East Toe Drain

Along the toe of the downstream embankment slope is an existing toe drain; the precise location of which is unknown at this time. This drainpipe will be removed during rehabilitation and a new 6-inch HDPE drainpipe, embedded in filter materials, is proposed to be installed. This design prevents sediment transport or blockage of the new 6-inch HDPR drainpipe, facilitates the collection and transport of seepage along the toe to the downstream to the outlet channel, and limits the potential for surficial seepage at or immediately downstream of the toe of the dam. The East Toe Drain will be installed along the right toe and the 6-inch

HDPE drainpipe will outlet through a precast concrete endwall to the riprap lined outlet channel downstream.

2.6.3 Filter Diaphragm

Current standards of practice recommend that a filter diaphragm or chimney filter be installed on embankment dams. Therefore, a filter diaphragm for the 48-inch RCP principal conduit is designed to meet the requirement of the National Engineering Handbook, Part 628, Dams, Chapter 45, Filter Diaphragms

(NRCS, 2007). The filter diagram will intercept water that can flow through cracks in the surrounding fill or along the interface of the conduit, and prevent sediment transport along the conduits, which could lead to internal erosion.

2.7 Piezometers

A row of four open well piezometers will be installed parallel and to the right of the proposed principal spillway. One piezometer will be installed just upstream of the embankment centerline. The second piezometer will be placed approximately mid-slope on the downstream embankment. The third piezometer will be placed near the toe of the embankment, approximately in line with the impact basin end sill. The fourth piezometer will be installed beyond the toe of the dam between the spillway outlet channel and the lateral drainage swale along the right groin. The piezometers will be installed using 2-inch diameter schedule 40 Polyvinyl Chloride (PVC) pipe. All piezometers will have a well cap encased in concrete to protect from damage due vandalism or maintenance. The vaults should be installed flush with the proposed grade so that regular mowing activities do not damage the vault or piezometer. For the two piezometers installed on the downstream embankment, the vaults will need to be installed flush to the 3H:1V slope.

3. Proposed Conditions Geotechnical Analysis

3.1 Filter Diaphragm Design

As discussed in Section 2.6, a filter diaphragm was designed for the 48-inch RCP principal conduit to meet the requirements of chapter 45, Part 628 of the National Engineering Handbook (NRCS, 2007) to prevent particle movement adjacent to the principal spillway conduit, which could initiate internal erosion.

The filter diaphragm is comprised of fine aggregate meeting either ASTM International Standard C-33 Fine

Aggregate or Georgia Department of Transportation (GDOT) 10 NS sand. The required thickness of the filter diaphragm is 3.0 feet times the height of the conduit in the horizontal and vertical direction, and a minimum of 2.0 feet below the bottom of the conduit (NRCS 2007). The filter diaphragm will contain a drainage layer of coarse aggregate and a slotted 6-inch HDPE pipe convey the collected seepage to the toe drain. Design calculations for the filter and drain features are provided in Appendix B of this report.

A primary drainage system was designed to outlet flow from the filter diaphragm to the downstream embankment toe. The drainage system consists of a slotted 6-inch HDPE pipe within the filter diaphragm and embedded in ASTM #8 or similar coarse aggregate, transitioning to a solid wall 6-inch HDPE drainpipe running adjacent to the RCP principal spillway conduit along the right side. The 6-inch HDPE pipe will be the primary drain of the filter diaphragm and will discharge adjacent to the impact basin, which will enable the measurement of seepage collected by the filter diaphragm.

On the left side of the RCP principal spillway conduit, a two-stage strip drain consisting of ASTM C33 fine aggregate and ASTM #8 coarse aggregate was designed as a secondary drainage system. The design of the strip drain system was based on NRCS guidelines (NRCS, 2007). The base (fine aggregate section) of the strip drain is set at 9-inches above the invert of the filter diaphragm outlet drainpipe so that accumulated seepage in the filter diaphragm will pass through the drainpipe during normal operation of the dam. The strip drain is designed as a secondary system and to activate only if the 6-inch HDPE drainpipe became unable to function properly and hydrostatic pressure were to build up within the filter diaphragm. Results from the geotechnical seepage model and NRCS (2007) design guidelines were used to estimate the required discharge capacity of the filter diaphragm and drain conduits.

Capacity calculations of the 6-inch slotted filter diagram drainpipe sizing showed that the proposed 6-inch diameter drain conduit is sufficient to convey the estimated seepage discharge from the filter diaphragm, which was calculated assuming an embankment fill permeability 100 times the estimated design permeability value. The 6-inch filter diagram pipe slots were designed to retain the adjacent coarse drain materials. Pipe stiffness was also checked to ensure that the proposed pipe will withstand overburden stresses and construction loads. Calculations for the filter diaphragm, strip drain design, and pipe design are provided in Appendix B of this report.

3.1.1 Filter Compatibility

Filter compatibility analysis was performed based on USACE methodology detailed in EM 1110-2-2300

General Design and Construction Considerations for Earth and Rock-Fill Dams (2004). Filter compatibility analysis is based on grain size distribution of a base and filter material to determine if a candidate filter material is of sufficient size to adequately allow seepage to free flow while maintaining the integrity of the base soil. The proposed filter material is ASTM C33 Fine Aggregate which will be utilized to filter seepage from Embankment Fill and Alluvial Soils. Analysis was also performed to evaluate compatibility between

ASTM C33 Fine Aggregate and ASTM No. 8 Coarse Aggregate. ASTM No. 8 Coarse Aggregate will be utilized as a drain material to transmit captured seepage once passed through the filter material. In addition, Georgia department of Transportation 10 NS Sand was analyzed as an alternate for the ASTM C33 Fine

Aggregate. For this analysis, United States Army Corps of Engineers methodology was used as detailed in

EM 1110‐2‐2300 (2004).

Filter compatibility for the riprap base layer was analyzed using ASTM No. 3 coarse aggregate. This material will be utilized as a transition material between the embankment and foundations soils and the riprap.

Gradations for the Embankment Fill were obtained from laboratory testing on samples collected from subsurface investigation performed in 2021 and detailed in the Geotechnical Data Report for Erin Lake

Dam (2021). Results from borings AB‐2 and AB‐3 were utilized as they are representative of the soils adjacent to the filters.

Results of the compatibility analyses are summarized in Table 3.

Table 3. Drain Filter Compatibility Analysis Results

Dam Material Compatible For:

Filtration Drainage

Embankment Fill (base) to ASTM C33 Fine Aggregate (filter)

Yes Yes

Alluvium (base) to ASTMC33 Fine Aggregate (filter) Yes Yes

ASTM C33 Sand (base) to ASTM C33 No. 8 Stone (filter)

Yes Yes

Embankment Fill (base) to GDOT 10 NS Sand (filter) Yes Yes

Alluvium (base) to GDOT 10 NS Sand (filter) Yes Yes

ASTM C33 No. 8 Stone (base) ASTM No. 3 Coarse Aggregate (filter)

Yes Yes

The results of the analysis show the ASTM C33 Fine Aggregate and GDOT NS 10 Sand are compatible with the existing Embankment Fill and Alluvial soil at Erin Lake Dam. In addition, the ASTM C33 Fine Aggregate is compatible with ASTM No. 8 Coarse Aggregate while the ASTM No. 8 coarse aggregate is compatible with ASTM No. 3 coarse aggregate.

3.1.2 Proposed Conditions Slope Stability Analysis and Seepage Modeling

Slope stability analysis and seepage modeling for proposed conditions were performed in a similar manner for existing conditions using GeoStudio 2020 SEEP/W and SLOPE/W computer modelling software. The seepage analyses were performed in general accordance with USACE EM 1110-2-1901 Engineering and

Design “Seepage Analysis and Control for Dams” (USACE, 1993).

The stability evaluation utilizes section geometry with input parameters including soil and rock parameters and loading conditions. The slope stability analyses of the embankment were performed using Spencer’s method of slices. Spencer’s method of slices satisfies all conditions of static equilibrium for horizontal and vertical force equilibrium and moment equilibrium. Factors of safety for slope stability analysis were analyzed using a minimum slip surface depth of two feet. Optimization was utilized for all slope stability analyses.

The embankment slope stability analysis was conducted using required minimum factors of safety from the

Georgia Rules and Regulations 391-3-8 Rules for Dam Safety (Rule 391-3-8-.09) for the stability of earth embankment structures and USACE EM 110-2-1902 Slope Stability (USACE, 2003). These two guidance documents provide minimum factor of safety values for steady state seepage (normal and maximum storage pool), maximum surcharge pool, rapid drawdown (upstream), earthquake (seismic) loading, and end-of-construction conditions.

End-of-construction conditions were analyzed using drained strength parameters in free-draining materials and undrained strength parameters for materials that assumed to drain slowly. The end-of-construction case evaluates the condition that non-free draining materials may not drain sufficiently as loading conditions are applied, such as during or directly following placement of fill material. The end-of construction loading condition was analyzed with the reservoir at drawn down and normal pool conditions. For this analysis, the

Embankment Fill, Alluvium, and Residual soils were analyzed with undrained material strength parameters for the maximum height cross section. The offset cross section was not analyzed for this condition as it is outside the excavated cut slope for the principal spillway installation and therefore not subject to end of construction conditions.

In addition to end of construction, a stability analysis was performed on the excavated slope section for the principal spillway construction. While the construction is anticipated to be performed under drained conditions, both effective and total strength conditions were analyzed. The excavated slope section was analyzed along the crest of the dam which is the maximum height of the excavation. The maximum allowable excavation slopes are 2H:1V. As both excavated slopes are similar, the interior excavated slope was analyzed.

Georgia Department of Natural Resources “Engineer Guidelines” (2015) details that in limited instances, rapid drawdown due to submergence of the downstream toe may be a consideration. Rapid drawdown on the downstream toe of the dam was not analyzed as the downstream toe is not anticipated to become submerged beyond the designed riprap protection at the principal spillway outlet.

Based on the proposed design, the crest of the dam will be increased to 970.0 feet. The normal pool elevation was analyzed to be 955.7 feet with maximum surcharge pool elevation at 969.0 feet.

Slope stability analysis and seepage modeling for proposed conditions were analyzed for two cross-sections: 1) adjacent to the proposed principal spillway conduit which represents the maximum embankment section of the dam outside of the principal spillway conduit profile, and 2) an offset cross section approximately 100 feet to the right of the principal spillway conduit which is outside of the proposed excavated section. The geometry of the maximum embankment section contains the filter diaphragm with drainpipe to represent phreatic surface conditions. The locations of the cross-sections analyzed are shown in Figure 2 below.

Figure 2: Cross section locations for seepage and stability analyses

Soil material properties for alluvium, residual soil, bedrock, and existing embankment fill used in the analysis were the same as the evaluation for existing conditions detailed in the Lake Erin Dam Geotechnical

Analyses (AECOM, 2022b). Soil strength properties for the Proposed Embankment Fill were based on empirical values obtained from literature and engineering judgement. The Proposed Embankment Fill is anticipated to have increased strength properties from the existing embankment material when compacted during construction, given the low blow counts observed during subsurface investigation of the existing embankment.

Filter drain hydraulic conductivity was estimated using Hazen’s equation, which is based on D10 (10 percent size) gradation. Soil strength material properties for the filter material were estimated based on USACE

ERDC/GSL TR-0802 Mechanical and Physical Properties of ASTM C33 Sand (2008). Material properties used in the analysis are provided in Table 4 and Table 5.

Table 4: Soil Hydraulic Conductivity and Anisotropy Material Properties

Material Description

Range of Typical Hydraulic Conductivity Values (cm/sec)

(NRCS)

Laboratory Tested

Values (kv) (cm/sec)

Selected Horizontal Hydraulic

Conductivity, kh

(cm/sec)

Selected Anisotropy

(kv/kh) maximum minimum

Embankment Fill

1.00E-03 (SM)

1.00E-06 (SC)

1.00E-06 (SM)

1.00E-08 (SC)

2.7E-07 1.08E-06 0.25

Alluvium

1.00E-03 (SM)

1.00E-06 (SC)

1.00E-06 (SM)

1.00E-08 (SC)

- 1.10E-06 0.67

Residual Soil 1.00E-03 (SM) 1.00E-06 (SM) - 2.50E-06 0.5

Bedrock

1.2E-08 unfractured igneous and metamorphic rock

1.2E-12 unfractured igneous and metamorphic rock

- 1.00E-09 1

Proposed Embankment

Fill

- - - 1.08E-06 0.25

Drain Fill - - - 3.5E-02 1

Table 5: Soil Strength Material Properties

Material Description Saturated Unit Weight (pcf)

Shear Strength Parameters

Total Stress Effective Stress

Φ (deg) c (psf) Φ’ (deg) c’ (psf)

Embankment Fill 125 15 301 28 33

Alluvium 129 15 252 27 11

Residual Soil 132 18 250 31 0

Bedrock 165 0 225,000 0 225,000

Proposed Embankment Fill

128 0 1000 31 33

Drain Fill 130 35 0 35 0

An schematic of the maximum cross section and offset cross section are shown in Figure 3 and Figure 4.

Figure 3: Maximum Embankment Cross Section

Figure 4: 100ft Offset Cross Section

For the modeled proposed embankment cross sections, the critical (lowest) slope stability factors of safety and the respective USACE, and Georgia State regulations minimum required factors of safety are presented in Table 6 for the cross section adjacent to the principal spillway conduit and in Table 7 for the offset cross section. The results of the seepage and slope stability analyses are provided in Appendix B of this report.

Table 6. Proposed Conditions Critical Slope Stability Factors of Safety Deep Cross Section

Analysis Condition Required Minimum

Factor of Safety

(USACE, 2003)

Required Minimum Factor of Safety (Georgia State Regulations)

Calculated Minimum Factor of Safety (Deep)

Steady State Seepage – Normal Pool

1.5 1.5 1.8

Steady State Seepage – Maximum Surcharge Pool

1.4 N/A 1.7

Steady State Seepage with Seismic Loading – Normal Pool

N/A1 1.1 1.1

Rapid Drawdown (Upstream, Normal Pool to (942 ft)

1.3 1.3 1.4

Rapid Drawdown (Upstream, Maximum Pool to Normal Pool)

1.1 - 1.6

End of Construction 1.3 1.3 1.7

No required value provided by USACE EM 1110-2-1902 Slope Stability, 2003

Table 7. Proposed Conditions Critical Slope Stability Factors of Safety Offset Cross Section

Required Minimum

Factor of Safety

(USACE, 2003)

Required Minimum Factor of Safety (Georgia State Regulations)

Calculated Minimum Factor of Safety (Offset)

Steady State Seepage – Normal Pool

1.5 1.5 2.0

Steady State Seepage – Maximum Surcharge Pool

1.4 N/A 1.5

Steady State Seepage with Seismic Loading – Normal Pool

1.11 1.1 1.1

Rapid Drawdown (Upstream, Normal Pool to (942 ft)

1.3 1.3 1.6

Rapid Drawdown (Upstream, Maximum Pool to Normal Pool)

1.1 - 1.5

Table 8. Cut Slope Analysis

Required Minimum

Factor of Safety

(USACE, 2003)

Required Minimum Factor of Safety (Georgia State Regulations)

Calculated Minimum Factor of Safety (Offset)

During Construction 1.3 1.3* 1.3

*Assumed based on end of construction requirements

The results presented above indicate that the proposed rehabilitation design will meet the minimum required factors of safety for the analyzed load conditions.

3.2 Bearing Capacity

The allowable bearing capacity for the intake riser and impact basin was estimated based on guidance from

EM 1110-1-1905 Bearing Capacity of Soils (USACE, 1992) and Principles of Geotechnical Engineering, 5th ed (Das, 2003). Based on previous subsurface investigation, the intake riser and impact basin will be founded on Alluvium or weathered rock/Residual Soil. For conservatism, bearing capacity analysis assumed Alluvial materials. Based on reference guidance and engineering judgement, the recommended bearing capacity for the intake riser is 3924 psf and 5390 psf for the impact basin. Analysis of the foundation materials for bearing capacity are provided in Appendix B of this report.

3.3 Settlement Analysis

At Erin Lake Dam, material is to be excavated along the alignment of the 48” principal spillway conduit.

Once construction of the conduit is complete, the embankment will be reconstructed, raised to a crest elevation of 970 ft, and the slopes regraded. Settlement analysis was therefore performed to determine the anticipated total settlement above the excavated area. Analysis was performed using Rocscience Settle3 software (Version 5.022) which analyzes immediate settlement, primary consolidation, and secondary settlement.

The results of the analysis show an expected total settlement of less than 1-inch. Given the sandy nature of the existing foundation material, the majority of the settlement is expected to occur during construction, Therefore, it is recommended to overbuild the crest of the dam by 1-inch to ensure the crest maintains a minimum of 970 ft elevation. The results of the Settlement analysis are provided in Appendix B of this report.

4. Proposed Conditions Hydrologic and Hydraulic Analysis

To confirm that the proposed repairs address the dam safety issues relating to the inadequate hydrologic and hydraulic performance of the dam, a hydrologic and hydraulic model of the proposed conditions of the dam was developed. Along with the proposed conditions model, calculations were performed for the sizing of the low-level outlet, principal spillway, impact basin, and the riprap outlet protection.

4.1 Proposed Conditions Model

A proposed conditions model was developed to represent the proposed conditions for this rehabilitation project. The model developed in the previous H&H analysis (AECOM, 2021) was used as a starting point.

The elevation discharge curve developed for the proposed intake structure was input into the hydrologic model. This rating curve was developed empirically using equations for the prevailing hydraulic regimes that are referenced in Appendix C of this report. During the SDF event, the 48-inch principial spillway conduit will control the discharge from the reservoir and the riser structure will be submerged. The hydrologic inputs and the precipitation inputs are not expected to change and therefore, were not revised.

The elevation storage rating for the reservoir was updated to reflect the proposed conditions. The elevation storage rating for proposed conditions did not impact the results of the SDF event but did have a small impact on the resulting peak water surface elevation for 4%, 2%, and 1% AEP events. The proposed stage storage rating for the reservoir is included in Appendix C of this report. Figure 5 shows stage-storage rating for the proposed reservoir conditions. There was one error found with the previous H&H analysis where the curve number for the land cover types of developed (medium intensity) and forest were swapped.

Since the Lake Erin watershed has a larger area of developed (medium intensity) than forested area, this error resulted in the curve number being underestimated. The calculations were revised to assign the correct curve number to both land cover types, which resulted in the composite curve number increasing by 3. The revised curve number was used in the existing and proposed conditions model. Figure 6 shows a comparison of the elevation-discharge ratings for the existing and new principal spillways.

Figure 5: Elevation-Storage Rating for Proposed Conditions

Figure 6: Elevation-Discharge Ratings for Existing and Proposed Principal Spillway

The proposed conditions were modeled using the same hydrologic model that was prepared for the existing conditions, with the revisions noted above. The model was run using the USACE Hydrologic Engineering

Center, Hydrologic Modeling System (HEC-HMS), version 4.11 software application. The 4%, 2%, and 1%

AEP storm events were modeled along with the SDF. Under proposed conditions, the dam provides adequate freeboard during the modeled storm events. Table 9 compares results for each of the modeled events under existing and proposed conditions. Supporting model output is provided in Appendix C of this report.

Table 9. Comparative HEC-HMS Model Results

Flood Event Peak

Inflow (cfs)

Existing Conditions Proposed Conditions

Peak Reservoir Elevation (feet)

Peak Outflow

(cfs) Freeboard

(feet) Peak Reservoir Elevation (feet)

Peak Outflow

(cfs) Freeboard

(feet)

4% AEP

(25-Year) 240 959.5 34 9.0 959.7 30 10.3

2% AEP

(50-Year) 314 960.3 44 8.2 960.6 40 9.4

1% AEP

(100-Year) 397 961.3 50 7.2 961.4 64 8.6

SDF (1/3

PMP)

1,588 969.8 320 -1.3 968.9 322 1.1

4.2 Low-Level Outlet Sizing

Georgia Safe Dams requires that a dam has sufficient capacity to drain two thirds of the normal pool volume in 10 days. At the normal pool elevation of 955.7 feet, the total volume in the reservoir is 30 acre-feet. This means that the proposed low-level outlet system will need to be required to discharge 20 acre-feet, or

864,409 cubic feet, of water in 10 days. The target elevation for this drawdown is approximately 950.2 feet.

The proposed low-level outlet conduit is a 15-inch ASTM C361 Reinforced Concrete Pipe conduit that is approximately 20 feet long and embedded within a concrete cradle. A discharge rating curve was developed for the low-level outlet system and was developed using the guidance from United Stated Bureau of

Reclamation Design of Small Dams (USBR, 1987) Chapter 10 Outlet Works. Based on the guidance in

Chapter 10, it is estimated that the proposed low-level outlet system could…