B09_Att_17_Title_I_Dam_Assessment_Report_0002.pdf

20090469.D10

Title 1 Assessment and Treatments Recommendation Report to

Address Dam Safety Issues at Colonial National Historical Park

National Park Service Colonial National Historical Park

Williamsburg, Virginia

April 2014

146 Hartford Rd.

Manchester, CT 06040

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Table of Contents

Title 1 Assessment and Treatments Recommendation Report to Address Dam Safety Issues at the Colonial National Historical Park National Park Service

1 Project Study and Assessments Overview

1.1 Introduction

1.2 Dam Location and Summary

1.3 Dam Hazard and Safety Classification

1.3.1 Downstream Hazard Potential Classification

1.3.2 Dam Safety Condition Code

1.4 Dam Condition Assessments and Site Investigations

1.4.1 Visual Assessment

1.4.2 Topographic Survey

1.4.3 Wetland Resource Field Assessments

1.4.4 Structural Dive Inspection

1.4.5 Geotechnical Investigation

2 Field Investigation Program Results

2.1 Sediment Analytical Test Results

2.2 Geotechnical Investigation Results

2.2.1 Deep Boring Program

2.2.2 Shallow Boring Program

2.2.3 Initial Assessment of Boring Program Results

3 Initial Analysis of Potential Dam Failure Modes

3.1 Subsurface Erosion or Piping Due to Seepage Flows at an Embankment

Dam

3.2 Overtopping of an Embankment Dam

3.3 Earthquake-related Failure of an Embankment Dam Due to Slope

Instability

3.4 Earthquake-related Failure of an Embankment Dam Due to Seepage

Erosion Failure

4 Evaluation of Remedial Alternatives

4.1 Recommended Supplemental Investigations and Evaluations

4.1.1 Field Investigations and Evaluations

4.1.2 Screening Level Risk Assessment

4.2 Consideration of Repair/ Maintenance Alternatives

4.2.1 Dredging at Low-Level Pipe Inlet

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Title 1 Assessment and Treatments Recommendation Report to Address Dam Safety Issues at the Colonial National Historical Park National Park Service

4.2.2 Repair/Replace Low-Level Gate Valve

4.2.3 Repair Settled Concrete Slabs

4.2.4 Slope Stabilization Alternatives

4.2.4.1 Alternatives Maintaining Current Embankment Footprint

4.2.5 Slope Regrading Alternative

4.2.6 Discharge Channel Bank Stabilization Alternatives

4.2.6.1 Gabions

4.2.6.2 Vegetated Articulating Concrete Block Mats

4.2.6.3 Vegetated Stone Armor Scour Protection

4.2.6.4 Bioengineered Stabilization Practices

4.2.7 Embankment Drains

4.2.7.1 Toe Drains

4.2.7.2 Blanket Drains

4.2.7.3 Curtain Drains

4.2.7.4 Slope Grading

4.2.8 Tree Removal

4.2.9 Energy Dissipator Alternatives

4.2.9.1 Level Spreader Alternatives

4.3 Selected Alternatives

4.4 Order-of Magnitude Opinion of Construction Costs

5 Permitting and Regulatory Compliance Requirements

5.1.1 Environmental Permitting

5.1.2 National Historic Preservation Act

5.1.3 National Environmental Policy Act

Tables Page 1 Jones Mill Pond Dam Data Summary 2 2 USGS Flood-peak Discharge Regression Equations for Rural Virginia 18 3 Potential Sediment Disposal Facilities 26 4 Selected Repair Alternatives/Approaches 44 5 Order-of-Magnitude Opinion of Construction Costs 48

Figures Page 1 Jones Mill Pond Dam Existing Conditions Plan End of Report 2 Jones Mill Pond Dam Conceptual Repair Plan Alternative No. 1 End of Report

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Title 1 Assessment and Treatments Recommendation Report to Address Dam Safety Issues at the Colonial National Historical Park National Park Service

Figures Page 3 Jones Mill Pond Dam Conceptual Repair Plan Alternative No. 2 End of Report 4 Jones Mill Pond Dam Conceptual Recommended Repair Plan End of Report 5 Possible Misalignment of Timber Guardrail Due to Slope/Shoulder Movement 15 6 Portable Hydraulic Operator, Control Panel and Submerged Hydraulic Actuator 28 7 Vegetated Soil-Filled Articulating Concrete Mat Slope Protection 32 8 Example Anchor Trench Detail 32 9 Example Duckbill Anchor Detail 33 10 Vegetated Soil-Filled Stone Armor Scour Protection 34 11 Pyramat High Performance TRM and RoLanka BioD-Mat 35 12 Gabions on a Stream Bank Slope 36 13 Bioengineered Streambank Stabilization Practices 38 14 Conceptual Toe Drain Configuration 39 15 Conceptual Blanket Drain Configuration 39 16 Conceptual Curtain Drain Configuration 40 17 GeoWeb® Retaining Wall 41 18 Conceptual Slope Regrading and Retaining Structure Configuration 41

Appendices End of Report A Visual Inspection Summary and Photographs B Archive Research Drawings, Photographs and Records C Wetlands Field Assessment Data Sheets and Whorled Pegonia Survey Report D Dive Inspection Report E Impoundment Sediment Laboratory Analysis Summary Table and Report F Subsurface Boring Laboratory Analysis Report

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1 Project Study and Assessments Overview

1.1 Introduction

A dam safety evaluation study is being conducted by the National Park Service (NPS) to assess the condition of Jones Mill Pond Dam in the Colonial National Historical Park in Williamsburg, Virginia.

This study includes the following elements.

Visual inspection of the dam.

Diving inspection of the low-level intake pipe and control valve, including an assessment of sediment quantity/quality.

Topographic and bathymetric survey of the dam and adjacent areas.

Wetland resource field survey and mapping of the dam and adjacent areas.

Subsurface boring program on the embankment crest and slopes.

Evaluation of state/federal permitting and National Environmental Policy Act (NEPA)/Section 106 National Historic Preservation Act (NHPA) regulatory requirements.

Assessment of repair/maintenance activity alternatives with order-of-magnitude opinions of cost.

Fuss & O’Neill, Inc. (Fuss & O’Neill) has conducted this study under Contract No. C4526101001 as a subconsultant to Einhorn Yaffee Prescott architects (EYP). Field activities were undertaken with the support of NPS staff in the Colonial National Historical Park.

1.2 Dam Location and Summary

The Jones Mill Pond Dam is an earthen embankment structure located approximately three miles east of Williamsburg, Virginia, on the Colonial National Historic Parkway (Colonial Parkway) and west of the U.S. Naval Supply Center, Cheatham Annex. The dam impounds the Cub Creek watercourse, and discharges through a tidal channel to Queens Creek. The following studies and maintenance/repair activities have been undertaken at the site since its construction to its current configuration in 1931-32, based on available records provided to Fuss & O’Neill.

1935: Timber pile revetment constructed along upstream slope to address instability.

1937-38: Repairs to unstable downstream slopes.

1989: Upstream slope failure during storm; repair competed.

1995: Dam safety evaluation completed by U.S. Bureau of Reclamation; inspection, stability analysis and hydrologic/hydraulic analysis.

1998: Dive inspection completed to evaluate low-level outlet structure (valve chamber not accessed). Historic engineering documentary report completed.

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2001: Trees removed from embankment slopes.

2005: Condition assessment report completed.

2010: Settlement of concrete pavement slabs observed; bituminous pavement patches placed.

A summary of pertinent data for Jones Mill Pond Dam is provided in Table 1 below.

Table 1 Jones Mill Pond Dam Data Summary

COLO ID Number 5 NID ID Number VA19905 Latitude/ Longitude 37.28317, -76.64204 County/ State York, Virginia Crest Length +/- 600 ft.

Crest Width +/- 38 ft.

Structural Height 27 ft.

Hydraulic Height 18 ft.

Spillway Length 21 ft.

Discharge Capacity 2,000 cfs Low-Level Outlet 24” dia. ductile iron pipe with inline gate valve Hazard Classification Low Impoundment Size (Acres) 65 Drainage Area (Acres) 1280

Data taken from 2004 dam inspection report entitled “Colonial National Historical Park, Virginia: 2004 joint examinations by National Park Service and Bureau of Reclamation” and “Evaluation Report, Jones Mill Pond Dam, Colonial National Historic Park, U.S. Department of the Interior, Bureau of Reclamation, March 15, 1995.”

1.3 Dam Hazard and Safety

Classification

1.3.1 Downstream Hazard Potential

Classification

As documented in a May 2005 report entitled “Colonial National Historical Park, Virginia – 2004 Joint Examinations by National Park Service and Bureau of Reclamation,” the Bureau of Reclamation (BOR) and National Park Service Washington Office (WASO) classified Jones Mill Pond Dam as a Low Downstream Hazard Potential structure. This assessment was also reflected in a spreadsheet table file dated Nov. 19, 2013 provided by NPS. The Low Downstream Hazard Potential Classification indicates that no loss of life is expected should failure occur (no permanent facilities, campsites or visitor use areas are located downstream) and minimal economic loss is expected should failure occur (only occasional facilities or agricultural facilities are located downstream and no significant park resources are located downstream). It is noted that the United States Army Corps of Engineers categorizes low hazard

G:\P2009\0469\D10\Deliverables\Report\COLO\100% Report\mkf_NPSCOLOReport_20140522.docx 3 potential dams as those where loss of life is not expected and minimal economic loss is expected should failure occur.

This determination in the May 2005 report noted that the dam was classified at the time of the report’s inspection (2004) as a Significant Hazard structure because loss of the dam would have a major impact on motorist and public using the Colonial Parkway. It stated its assessment that the dam’s hazard classification should be based on potential loss of life or property damage downstream from the dam and not include loss of the dam itself or impacts from losing the reservoir, as opposed to the prior assessment by Reclamation in 1999. The report noted that Department of Interior has adopted FEMA 333, Hazard Potential Classification for Dams, under Part 753, Chapter 2 of the Departmental Manual in supporting this interpretation, and based on the recommendation from this report the hazard classification was subsequently changed to Low Hazard.

For purposes of reference, language defining Low and Significant Hazard classifications in FEMA 333 is provided below.

Low Hazard Potential: Dams assigned the low hazard potential classification are those where failure or misoperation results in no probable loss of human life and low economic and/or environmental losses.

Significant Hazard Potential: Dams assigned the significant hazard potential classification are those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environmental damage, disruption of lifeline facilities, or can impact other concerns.

While FEMA 333 makes references to downstream impacts in other sections of describing its classification system and bases for determinations, it is recommended that potential impacts to the Colonial Parkway on the dam crest should not be discounted in any assessment of the structure’s hazard potential, particularly since this roadway is a significant arterial with an annual average daily traffic volume (AADT) of 31,000 according to a the Virginia Department of Transportation Traffic Engineering Division’s 2012 traffic data report. Beyond the direct hazard potential to motorists using the parkway, and indirect impacts to emergency response routes, loss of the roadway would result in some economic loss due to traffic detours to parkway users (additional transit mileage and time) and impacts to park visitation. Further loss of the impoundment would likely entail discharge of significant soil and sediment volumes to the downstream tidal marsh, resulting in environmental damage to this coastal resource.

Beyond reference to FEMA 333, the National Park Service Dam Safety Program’s Reference Manual 40 (RM-40) dated December 2013, states that a significant hazard dam is one that “is not expected that failure would result in loss of life, but loss of life is plausible (not remote): or failure of the dam would cause flooding that would result in appreciable and/or long-term impacts to a park’s visitation/resources/mission…” Additional criteria provided in RM-40 supporting a reviewer’s consideration of whether a dam falls within the a Significant Hazard Potential classification are provided below:

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The flood failure flows could wash out a well-traveled road, but it is unlikely (but plausible) that someone would lose their life.

The dam and reservoir are highly visited. Loss of the dam would result in major impacts to park visitation and resource access.

Repair of the dam and re-establishment of the reservoir would be a major undertaking.

Should the model or other analyses indicate no probable loss of life, but would cause significant damage to park or off-park resources or infrastructure, the dam would be classified as “Significant.”

Based on the above considerations, it is recommended that the hazard potential classification for Jones Mill Pond Dam be returned to Significant. Should failure occur, loss of life for motorists utilizing the Parkway is plausible, appreciable direct economic loss could occur to the roadway and embankment, and environmental impacts to downstream tidal resources would be likely. Failure would result in significant damage to Colonial National Historic Parkway which is a very significant cultural element within the park.

Increasing the Downstream Hazard Potential Classification will enhance the dam’s ability to compete for Dam Safety funding and will require the following, per RM-40:

A updated and modified Emergency Action Plan to coordinate and facilitate local, state and federal response to Dam Safety events (it is understood that one may currently be in place);

A checklist form documenting monthly monitoring;

Condition Assessments (every two years) performed by the Bureau of Reclamation or consulted dam safety engineers;

Additional analysis of the hydrologic capacity and spillway limitations; and An updated Operation and Maintenance Manual.

1.3.2 Dam Safety Condition Code

As documented in a 2005 Dam Inventory Report Completed by the BOR and WASO, the Jones Mill Pond Dam has a “Satisfactory” Dam Safety Condition Code. This condition assessment indicated that “no existing or potential dam safety deficiencies are recognized,” and that the structure “will fulfill intended purpose but maintenance, operations or minor repair is needed” under the Operation and Maintenance Condition Code designation, which also indicates that “all essential elements can be expected to function under all conditions, including such events as the Maximum Probable Flood (MPF) or Maximum Credible Earthquake (MCE), which have only a remote chance of occurring.”

Based on the field assessment described in this report below, it is recommended that the Dam Safety Condition Code be revised to Fair due to the deficiencies noted, particularly the slab settlement within the parkway, but also due to the fact that the spillway can only pass 42% of the Maximum Probably Flood event which would result in significant overtopping with no scour protection. After the proposed repairs are made it is expected that the Dam Safety Condition Code would be restored to Satisfactory.

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1.4 Dam Condition Assessments and

Site Investigations

1.4.1 Visual Assessment

Fuss & O'Neill completed visual assessments on August 8, August 29 and December 12, 2013 to document existing conditions at the dam and identify deficiencies. Note that this report reflects conditions at the time of the examination, and does not include conditions that may have changed since the last inspection in December 2013. Also note that references within this report “left” and “right” are made from the perspective of a person facing in the downstream direction.

In general, the following primary conditions and deficiencies were observed at the dam. A more detailed inspection form and photographs are provided in Appendix A, and the location of certain features/deficiencies are depicted on Figure 1 attached at the end of this report. Copies of construction drawings, historic photographs and other documents obtained during a review of NPS historical archives are provided in Appendix B.

Embankment Crest: Concrete pavement roadway with grassed earthen shoulders and timber guardrails.

o Two concrete pavement slabs have settled up to approximately 0.2 feet and were patched with asphalt pavement.

o Grassed shoulders were soft due to saturated soils following heavy rain.

Downstream Embankment Slope: Earthen slope varying from 1.6H:1V to 2.5H:1V.

o Seepage was observed at right embankment toe of slope.

o Several sinkholes/scarps were observed on left embankment slope, resulting from decomposing tree stumps/roots or animal burrows.

o Trees and brush were observed on the lower portion of left downstream slope and within 25 feet of toe of slope and abutments.

o Runoff erosion was noted adjacent to the endwall structure and discharge channel (from parkway surface) and below toe of slope (resulting from discharge from a concrete drainage swale near the right abutment groin).

o Apparent settlement areas on embankment face, with soft areas/incipient cracking along top of slope adjacent to timber guardrail.

Upstream Embankment Slope: Earthen slope varying from 1.3H:1V to 1.5H:1V.

o Significant runoff erosion was noted near left abutment groin (from parkway surface).

o Minor depressions (decomposing stumps or burrows) were observed on the embankment face.

o Some evidence of runoff erosion was observed on the embankment faces and adjacent to spillway training walls.

G:\P2009\0469\D10\Deliverables\Report\COLO\100% Report\mkf_NPSCOLOReport_20140522.docx 6 o Trees and brush were observed within 25 feet of abutments.

Low-Level Outlet Structure: 24-inch diameter ductile iron pipe with inline gate valve.

o Low-level outlet is inoperable, due to the inlet pipe in impoundment being covered with accumulated sediment.

o Gate valve cannot be exercised to determine if it can open/close, due to accumulated sediment.

Discharge Channel: Tidally influenced channel through forested area.

o Significant scour erosion has formed scarps on both sides of discharge channel immediately below outlet from spillway culvert.

o Root mass on left channel bank exhibited voids extending up to 30-inches into streambank.

o Dense tree/brush growth was noted within 25 feet of toe of slope.

A dive inspection performed to evaluate the location of the low-level intake pipe and its gate valve was performed to supplement the visual observations, as described in the following section.

1.4.2 Topographic Survey

A topographic ground survey of the dam and immediately adjacent areas was completed by AES Consulting Engineers (Williamsburg, VA) on December 12 and 13, 2013. This survey effort also measured bathymetry in the impoundment adjacent to the embankment. Wetland flags placed by a qualified biologist were field located, and trees with diameters greater than approximately 9-inches on and immediately adjacent to the embankment were also located.

Topographic mapping was compiled with 2007 LiDAR mapping obtained from York County’s Geographic Information System administrator and parcel boundary mapping provided by NPS. The existing conditions survey plan prepared through this work effort is provided as Figure 1 at the end of this report.

1.4.3 Wetland Resource Field

Assessments

Wetland resource assessments were completed at the site by Kerr Environmental (Hampton, VA) on July 14, 2013 and December 6, 2013. The purpose of the initial site visit was to assess the presence of small whorled pogonia (Isotria medeoloides), which is a threatened plant species (orchid family) in Virginia.

The second site visit was completed to flag wetland resources at the site, which were subsequently located by the surveyor as noted in the previous section.

The small whorled pogonia field assessment determined that while this plant species is not present at the site, suitable habitat does exist and a future assessment may identify an occurrence. It is noted that the results of this survey are valid until July 2015, for purposes of securing permitting approvals for proposed repair/modification activities.

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Wetland Determination Data Forms supporting maps were prepared and submitted to the Army Corps of Engineers Norfolk District Regulatory Branch on February 11, 2014 for review and acceptance.

The small whorled pogonia letter of finding and Army Corps of Engineers wetland resource submittal are provided as Appendix C.

1.4.4 Structural Dive Inspection

Fuss & O’Neill and Crofton Diving Corporation (Portsmouth, VA) completed a dive inspection of the low-level outlet structure on August 29, 2013. The primary goals of this inspection were to complete the following:

Investigate the condition of the gate valve and its configuration in the valve chamber immediately below the spillway crest.

Locate the intake of the low-level pipe within the impoundment upstream of the spillway; assess depths/types of sediment over and adjacent to the intake pipe; and collect sediment samples for laboratory analysis.

A crane was mobilized to the site to remove the valve chamber’s concrete cover. Once removed, a wet dive inspection was completed to take clearance measurements and collect photographs of the valve body and flanges. From this inspection the valve appeared to be in good condition with no indications of damage or other conditions that would inhibit operation. The divers noted that approximately one inch of sediment was observed on the chamber floor, and measured approximately six inches of clearance from the bottom of the valve to the chamber floor. Once the inspection was completed, the concrete cover was returned to its position and sealed in place.

In order to locate the intake pipe, a pump was mobilized to the site to discharge impoundment water through a steel probe with a nozzle. This arrangement successfully cleared sediment from over the top of the intake pipe in the impoundment. Findings from this investigation are listed below.

The end of the intake pipe was located approximately 12 feet below the spillway crest elevation.

The specific measurement was used to determine the intake pipes invert elevation shown on Figure 1.

The intake pipe was found to be buried below approximately 7 to 9 feet of sediment, comprised of approximately 6 feet of organic sediment and aquatic plant matter overlying approximately one to three feet of uniform fine sand (noted as “sugar sand” by the divers). It was noted that numerous logs and branches are located in sediment over/around the intake pipe.

The dive inspection determined that the end of the intake pipe is located approximately 17.3 feet upstream of the spillway’s exterior wall face. The dive team observed sheeting left in place from the dam’s construction approximately 12 feet from the spillway’s exterior wall face, flanking both sides of the intake pipe.

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Based on probing at the end of the pipe (could not be completely uncovered due to unstable side slopes where removed), it does not appear to have a debris screen or headwall structure at the pipe’s inlet.

A report documenting dive activities and observations is provided as Appendix D. Photographs taken during the dive are included in this report, as well as in the visual inspection report provided as Appendix A.

Representative sediment samples were collected and relinquished to a certified laboratory for chemical analysis. The purpose of this sampling/testing was to assess the presence and concentrations of potential contaminants that may determine permitting, handling and disposal requirements to for maintenance dredging to restore the low-level intake’s original configuration and allow proper future operation. Sediment sample results are discussed further in Section 2.1 below.

1.4.5 Geotechnical Investigation

Fuss & O’Neill and ESC Mid-Atlantic, LLC (Williamsburg, VA) completed subsurface boring investigations at the site on December 11 and 12, 2013. Boring locations are depicted on Figure 1 provided at the end of this report.

The purpose these investigations was to observe, sample and test subsurface soils below the settled concrete pavement slabs for deficient soil conditions allowing settlement to occur (e.g., voids, weak soils), and below embankment slopes to evaluate soil conditions that may contribute to potential embankment failure modes. Samples were collected from within embankment and foundation soils in support of future geotechnical assessments of the validity of soil parameters assumed in previous embankment stability analyses, such that an updated analysis can be performed using soil properties field-verified by this current exploratory program.

This program consisted of the following elements:

Two 8-inch diameter concrete cores were advanced with rotary equipment through the settled concrete pavement slabs by Diamond Equipment, Inc. (Virginia Beach, VA). These core holes provided access for two soil borings advanced by an ATV-mounted auger drill rig utilizing split spoon and hollow stem auger techniques. Drilling services were provided by Fishburne Drilling, Inc (Chesapeake, VA). The two borings were advanced to 40 and 50 feet below the ground surface, respectively.

- Representative samples were obtained from the soil borings collected at two foot intervals to a depth of ten feet below surface grade (bsg) and at five-foot intervals thereafter, using split-barrel sampling procedures in accordance with ASTM Specification D 1586-99, (Standard Test Method for Penetration Test and Split-Barrel Sampling of Soils).

- In this procedure, a 2 inch O.D., split barrel sampler is driven into the soil a distance of 24 inches by a 140-pound automatic hammer falling 30 inches. The first 6-inch depth increment is considered the seating interval. The number of blows required to drive

G:\P2009\0469\D10\Deliverables\Report\COLO\100% Report\mkf_NPSCOLOReport_20140522.docx 9 the sampler through the next two 6-inch intervals is designated the Standard Penetration Test (SPT) N-value and is indicated for each sample on the boring logs.

- A field log of the soils encountered in the soil test borings was maintained by ECS personnel and the drill crew. Samples were visually classified on the basis of texture and plasticity in accordance with the Unified Soil Classification System (USCS) and ASTM D 2488-00 (Description and Identification of Soils-Visual/Manual Procedures).

Respective soil types were grouped into the major zones as noted on the boring logs.

- Representative portions of each sample were removed from the sampler and placed in sealed glass jars. The samples were taken to ECS’s laboratory for classification in accordance with ASTM D 2488-00 (Standard Practice for Description and Identification of Soil (Visual-Manual Procedure)) and laboratory testing.

Six hand auger borings with Dynamic Cone Penetrometer (DCP) testing were performed to depths between 6 and 10 feet below the ground surface on both downstream and upstream embankment slopes. Three borings were conducted on the left downstream slope, and one boring each was completed on the left upstream and right downstream embankment slopes.

Photographs documenting drilling and auger activities are provided in Appendix A. Further details on the investigation program and a discussion of the results are provided in Section 2.2 below.

2 Field Investigation Program Results A review of field data and laboratory analytical results obtained from sediment sampling and soil boring activities is provided in the sections below. These results are intended to support future assessments of site conditions, such that appropriate controls and practices can be determined for future maintenance dredging around the low-level intake pipe, and geotechnical seepage/stability analyses can be completed based on the soil properties confirmed by this boring program. The results of these analyses will support the evaluation and ultimate selection of remedial alternatives to address seepage, settlement and slope instability issues observed at the site, as well as subsequent final design of the selected practices.

It is noted that additional supplemental field investigations may be required to augment data collected to date to adequately inform these future analyses and design; the specific scope of supplemental investigations will be determined through the review and selection of recommended alternatives to address respective deficiencies, as discussed in Section 4 below.

2.1 Sediment Analytical Test Results

Two sediment samples were collected by Fuss & O’Neill during the August 29, 2013 dive inspection and relinquished to Reed & Associates Laboratory (Newport News, VA) for chemical analysis testing.

Representative samples were collected from the upper organic layer of sediment and the underlying finer sand/silt layer, respectively.

Note that the sampling and analytical proceedures used at this phase of investigation are sufficient for regulatory agency review and permitting (through Title II), however additional confirmatory samples and testing will be required for the contractor in the project’s construction phase for the purposes of

G:\P2009\0469\D10\Deliverables\Report\COLO\100% Report\mkf_NPSCOLOReport_20140522.docx 10 disposal characterization. This testing will determine the disposal category and tipping fee and will be in compliance with RCRA guidelines. Samples were tested by the following analytical methods:

RCRA 8 Metals (USEPA Methods 6010/7470/7471)

Chlorinated pesticides (USEPA Method 8081)

Nonhalogenated Organics by GC (USEPA Method 8015)

Chlorinated Herbicides by GC (USEPA Method 8151)

Volatile/Semivolatile Organic Compounds by GC/MS (USEPA Methods 8260/8270)

Corrosivity (USEPA Method 9045D)

Flashpoint (USEPA Method 1010A)

Cyanide (USEPA Method 9012B)

Sulfide (USEPA Method 9034)

Primary results from this testing are summarized below:

The concentration of cadmium exceeded the EPA Ecological Risk Assessment Freshwater Sediment Screening Benchmark in the sample of upper organic sediment.

The concentration of chromium exceeded the DEQ Tier II Screening Concentrations for Residential and Commercial/Industrial criteria in both samples.

Other parameters tested did not exceed evaluation criteria or otherwise indicate hazardous properties.

A summary table listing detected analytical parameters in comparison to respective evaluation criteria is provided in Appendix E, along with the full analytical laboratory report. An assessment of potential handling/disposal issues and alternatives is provided in Section 4.2.1 below.

2.2 Geotechnical Investigation

Results

Field observations and test results for the two borings through the parkway’s concrete pavement slabs are summarized below. Refer to Figure 1 for a depiction of boring and hand auger locations.

2.2.1 Deep Boring Program

Boring logs, laboratory analytical reports and a tabular summary of test results are provided in Appendix F. It is noted that the stratification lines designating the interfaces between earth materials on the boring logs are approximate and that in situ transitions are gradual and can vary significantly at points further from the borehole locations.

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Embankment Fill Materials

Concrete pavement slabs were cored (7 inch depth) and subsurface soils were noted to be in firm contact with the bottom of the slab; no void space was observed below the slab at the location of the boring.

A 12-inch thick layer of gravel with some fines was observed immediately below the slabs.

Embankment fill materials were observed to approximately 18 feet below ground surface (bgs) in Boring B-2 and approximately 23 feet bgs in Boring B-1. The fill materials were cohesive and granular in nature and consisted of clayey sand (SC), silty sand (SM) and sandy lean clay (CL).

The Standard penetration N-Values for the granular materials encountered within the embankment crest ranged from 1 to 9 blows-per-foot (bpf), indicating a very loose to loose consistency.

Standard penetration N-values through the embankment soils ranged from 3 to 12 bpf, indicating a soft to stiff consistency. That for the granular materials encountered within the embankment crest borings (B-1 and B-2) indicated a very loose to loose consistency.

Testing of soil from 8-10 feet bgs in Boring B-2 indicated a silty clay soil (SC) with 41.5% passing the #200 sieve and a moisture content of 20.6%.

Testing of soil from 13-15 feet bgs in Boring B-2 indicated a sandy lean clay soil (CL) with 54.9% passing the #200 sieve and a moisture content of 29.5%.

Testing of soil from 13-15 feet bgs in Boring B-1 indicated a silty clay soil (SC) with 31.6% passing the #200 sieve, a moisture content of 20.1% and plasticity index of 10.

Groundwater was encountered at a depth of 18 feet bgs in Boring B-1. The observed groundwater depth in Boring B-2 of 35 feet bgs is assessed to be the result of an unequilibriated condition in the borehole, due to the soil’s relative impermeability and the short duration of the program.

Foundation Materials

Natural subsurface soils below embankment fill were generally comprised of interbedded layers of sandy lean clay, organic clay and silty sand with trace marine shell fragments to the boring termination depths of 40 and 50 feet bgs.

Standard penetration N-values through the cohesive foundation materials encountered ranged from 2 to 7 bpf, indicating a very soft to medium stiff consistency.

Standard penetration N-values for granular materials encountered in the foundation ranged from “weight-of-hammer” to 2 bpf, indicating a very loose relative density

2.2.2 Shallow Boring Program

Field observations and test results for the six hand augers on the embankment slopes are summarized below.

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Right Downstream Slope

Hand auger H-1 was completed on the embankment’s right downstream slope, immediately above the seep area on the downstream slope.

- Six (6) inches of topsoil were observed over clayey sand and sandy lean clay extending to a depth of 6 feet bgs. The auger could not be advanced further due to borehole instability/collapsing.

- A sample collected from 3.5 feet bgs was classified as clayey sand with 39.9% passing the #200 sieve and 30% moisture content.

- Standing water was encountered in the borehole at 2 feet bgs.

- Dynamic cone penetrometer (DCP) test values (number of blows to advance the cone 3 inches) were 5-6-6 from 1 to 2 feet bgs, and 2-3-3 from 2 to 3 feet bgs.

Left Downstream Slope

Hand augers H-2, H-3, H-4 and H-5 were completed on the embankment’s left downstream slope, from the mid-slope section to near the toe of slope.

- Between 4 and 8 inches of topsoil were observed over clayey sand and sandy lean clay extending to a depth of 10 feet bgs.

- A layer of silty sand with clay (SM) was observed from 4 – 5 feet bgs in HA-3.

- Sandy lean clay was observed for the entire depth below topsoil at HA-5.

- A sample collected from 3.5 feet bgs in HA-2 was classified as clayey sand with 44.1% passing the #200 sieve and 30% moisture content.

- A sample collected from 5.5 feet bgs in HA-4 was classified as clayey sand with 42.8% passing the #200 sieve and 25.1% moisture content. The plasticity index of this sample was 10.

- Standing water was not encountered in any of the boreholes.

- DCP test values ranged from 2 – 12 in HA-2.

- DCP test values ranged from 3 – 25 in HA-3.

- DCP test values ranged from 4 – 26 in HA-4.

- DCP test values ranged from 4 – 9 in HA-5.

Left Upstream Slope

Hand auger H-5 was completed on the midpoint of embankment’s left upstream slope.

- Six (6) inches of topsoil were observed over sandy lean and clayey sand extending to a depth of 10 feet bgs.

- A sample collected from 5.5 feet bgs was classified as clayey sand with 47.2% passing the #200 sieve and 22.4% moisture content.

- Standing water was not encountered in the borehole.

- DCP test values ranged from 3 – 8.

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2.2.3 Initial Assessment of Boring

Program Results

From a review of the boring program data, the following preliminary assessments have been developed.

Assessment of this data will be continued and refined through detailed analysis in future evaluation and design tasks under this project study.

The structural quality of soils base material and embankment fill material are compressible and subject to consolidation due to the presence of clay, which could be a causal factor in the observed settlement of the two concrete pavement slabs. While these types of soils are likely consistent with soils in place on other portions of the embankment, it is possible that higher clay content, particularly in the shallow depths, combined with relatively low densities at this particular location of the embankment may result in inadequate bearing capacities, thus allowing differential settlement of these slabs to occur.

A review of spot elevations on the parkway surface in the portion of embankment where settlement has occurred indicates that the roadway surface is pitched to the north, toward the settled slabs, causing a higher volume of runoff over these slabs than if the roadway were crowned. Higher runoff volumes over this lane, particularly if ponding occurs, could cause water to seep through openings in failed joint seals and increase the moisture content, and possibly saturate, base soils and shallow embankment soils.

Higher water content in clayey soils will increase pore pressures and reduce structural properties required to provide adequate base support of the overlying slabs. Inadequate base support would increase loading to longitudinal dowels between adjacent slabs, and over time structurally deteriorate the connection until adequate support is no longer provided and differential settlement occurs.

Dynamic loading by passing vehicles, particularly commuter buses, could also be a factor in the observed differential settlement. Once initial settlement occurs, dynamic loading of the rigid structure at the western portion of the slab would have progressively increased as the settlement depth increased until adequate bearing support was provided by the compacted base materials (and the bituminous asphalt patch was placed to avoid dynamic loading). Dynamic loading could also result in “pumping” of finer grain particles upward from shallow depths into the granular base material immediately below the slab when moisture content of this clayey soil approaches saturation. Migration of fines into the granular base material could have the effect of degrading its structural properties to the point where inadequate support is provided.

Although not assessed in this study, it is also possible that a high point in the parkway’s elevation profile immediately west of the settled slab could be responsible for “launch” of passenger vehicles or commuter buses, where the increased force of “landing” after this launch occurs at the point where settlement is observed. While this is a possible causal or contributing factor, a visual assessment of the parkway’s westbound lane immediately east of the settled slabs did not identify tire or vehicle fluid markings on the roadway surface that are typically observed where such vertical variations in roadway profiles occur.

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Saturation of the earthen shoulder and adjacent upper slope may also be a contributing factor.

Increased moisture content of shallow and intermediate soils adjacent to the parkway (due to runoff volumes) may result in gradual movement of the soil mass down the slope or otherwise away from the edge of the parkway slabs.

Movement of shoulder material could reduce lateral support of base aggregate and shallow soils below the slabs, enabling greater deformation or migration of the material below the slab(e.g., slide or slip), resulting in inadequate vertical or lateral support of base soils immediately below the slabs. A close evaluation of the shoulder noted that it is approximately 2 inches lower than the top of the concrete pavement slab immediately west of the settled/patched slab, and level with the top of the slab where settlement has occurred, reflecting a lowered shoulder elevation on this section of the embankment.

If such settlement/movement of the shoulder is significant enough, it would be expected to result in some amount of misalignment of the timber guardrail in this section of the embankment as well, due to lowering of the posts (and soil mass) down and away from the edge of the parkway. Reviews of the guardrail’s profile and alignment appear to show this occurring, as depicted in the annotated photographs provided below as Figure 5. The upper photograph shows a profile of the guardrail, with the top of the guardrail’s beam apparently lower than adjacent sections in proximity to the settled slab, resulting from a lowering of the posts in this section. The lower photograph shows the guardrail’s alignment with an apparent deflection/rotation of the guardrail’s horizontal beam toward the roadway, resulting from the buried portions of the timber posts moving away from the edge of the parkway.

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An initial assessment of the geotechnical program’s results finds general agreement with conditions used in the 1995 stability analysis.

- While the findings of this program confirmed that soil types are relatively homogenous (i.e., no zonation of different soil types by design of the embankment), it appears that the cohesion values used in the 1995 report are low (0.5 – 1.0 psi, 72 – 144 psf). Based on the observed soil condition, it is likely that a more appropriate maximum cohesion value would be on the order of 500 psf.

- Foundation soils are clayey and not highly liquefiable; this was noted as an open issue in the 1995 report due to absence of any in-situ data of foundation soil conditions.

Figure 5 – Possible Misalignment of Timber Guardrail Due to Slope/Shoulder Movement

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3 Initial Analysis of Potential Dam Failure Modes This section provides an initial, limited evaluation of three potential failure modes identified in RM-40 that are applicable to conditions/deficiencies being addressed under this project’s current scope of assessment.

Subsurface Erosion or Piping Due to Seepage Flows at an Embankment Dam Overtopping of an Embankment Dam Earthquake-related Failure of an Embankment Dam Due to Slope Instability Earthquake-related Failure of an Embankment Dam Due to Seepage Erosion Failure.

It is expected that a complete assessment of these and the remaining three failure modes discussed in RM-40 listed below will be addressed with other applicable failure modes as part of a Screening Level Risk Assessment for the dam, as discussed in Section 4.1.2 below.

3.1 Subsurface Erosion or Piping Due

to Seepage Flows at an Embankment Dam

Active seepage has been noted on the embankment’s right downstream slope, adjacent to the dam’s spillway discharge channel. While limited monitoring has been performed to date, the observations made have consisted primarily of a wet ground area, stressed vegetation, and possible soil movement due to saturation-induced creep. No evidence of piping of soil particles was observed during the site visits. A hand auger was conducted adjacent to the seepage area, which identified clayey sand, sandy lean clay and clayey sand down to depths of 6-feet below the ground surface. Exit gradients (and the potential for movement of soil particles from seepage)are perceived to be relatively low due to the relatively low permeability of the embankment soils. As a result it is believed that the soil is relatively insusceptible to internal erosion due to gradation.

Deeper borings through the embankment crest also identified clayey sand and sandy lean clay extending below the embankment fill into foundation soils, transitioning to silty sand at depths of 35-feet below the embankment crest. While a full seepage analysis has not yet been performed to assess the potential for foundation seepage below the embankment, it is noted that the relatively low permeability of these soils will inhibit seepage with high exit gradients at or near the embankment’s downstream slope.

Seepage into the embankment crest from drainage running onto the embankment from the adjacent valleys may be a contributing factor in increasing shallow seepage and surficial saturation on both upstream and downstream slopes. While this mode of action is primarily responsible for surface slides and instability observed, contributions of surface infiltration may increase seepage gradients, particularly during periods with high impoundment elevations (i.e., during or immediately after significant rainfall or flooding events), resulting in temporally-limited conditions of positive seepage gradients on the downstream embankment slopes. Such conditions would be expected to increase the magnitude of previously identified seepage on the right downstream embankment slope in particular.

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While these potential failure modes are present to the limited degrees noted, the current evaluation and repair program is anticipated to address potential seepage conditions through a geotechnical evaluation, with recommended remedial measures (e.g., trench drains) to be implemented to further reduce any potential of subsurface erosion or piping. These measures will provide the appropriate filtering media and controlled outlets for seepage flows.

3.2 Overtopping of an Embankment

Dam

Two flood studies prepared for Jones Mill Pond Dam have been included in an Evaluation Report for Jones Mill Pond Dam prepared by BOR Technical Service Center in 1995. These studies evaluated the 100-year flood and Probable Maximum Flood (PMF) at the site and determined that the spillway as designed could pass the 100-year flood (569 ft3/s) and 42% of the PMF (15,488 ft3/s) with no freeboard.

This report noted a maximum peak inflow capacity of 6,505 ft3/s, which based on the current stage of assessment is recommended for future verification/assessment. The maximum spillway capacity was reported to be approximately 2,000 ft3/s in a 2005 Dam Inventory Report completed by BOR and

WASO.

The Virginia Dam Safety Office Impounding Structure Regulations state that for a significant hazard dam the spillway design flood (SDF) is the ½ PMF and for low hazard dams the SDF is the 100-year flood. In order for the dam to by hydrologically adequate, the spillway should be able to pass 500-year flood or the ½ PMF flow (7,744 ft3/s according to the Evaluation Report). Typical methods for determining the 500-year flood rely on gaged data (typically available via USGS Streamstats) or on FEMA flood insurance studies. Because Jones Mill Pond Dam is located in a tidal region, FEMA completes its flood insurance studies based on tidal elevations, and does not calculate frequency recurrence storms received by each waterbody. USGS Streamstats does not include data for the State of Virginia and is still developing its database. Additionally, the raw data used to calculate the flood frequencies in the 1995 report is not available.

The USGS National-Flood Frequency Program publishes equations that can be used in each state to estimate flood magnitude and frequency. USGS Fact Sheet 023-01 entitled “Methods for Estimating Flood Magnitude and Frequency in Rural Areas in Virginia, 2000” offers the guidance in Table 2 below for coastal plain areas. Note that the average standard error of prediction is relatively high and estimating frequency floods based on regression equations and not actual gage data is not an accurate way of determining flood magnitude and frequency. This method does, however, provide a baseline value for comparison and analysis.

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Table 2 USGS Flood-peak Discharge Regression Equations for Rural Virginia

Obtained from U.S. Geological Survey Fact Sheet 023-01 “The National Flood-Frequency Program—Methods for Estimating Flood Magnitude and Frequency in Rural Areas in Virginia, 2000” February, 2001. http://pubs.usgs.gov/fs/fs-023-01/

Using the drainage area listed in Table 2 above, the main channel slope for the site was estimated using the value calculated for the 100-year storm from the 1995 Evaluation Report and the regression equation listed in Table 2 for the 100-year storm (Q100). The estimate for main channel slope was found to be

27.35 feet/mile. Using this value and the regression equation listed in Table 2 for the 500-year storm (Q500), the 500-year storm was estimated to be 964 ft3/s. Assuming that this value is within the average standard error of prediction (82.7%), the 500-year storm is likely to be less than 2,000 ft3/s, which is the assumed spillway capacity.

In order to determine the headwater elevation and freeboard available during the estimated 500-year storm, the spillway configuration was evaluated. The FEMA Flood Insurance Study for York County, VA indicates that the water elevation in the York River near the site during the 500-year storm is 9.3 feet NGVD (8.3 feet NAVD88). While this location is approximately 8 miles from the dam, it is the closest location with available data.