Hop Brook Pl- Rev Draft.pdf

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United States Department Of Agriculture

Natural Resources Conservation Service

SUPPLEMENTAL

WATERSHED PLAN No. 6 &

ENVIRONMENTAL ASSESSMENT

For Rehabilitation of the Hop Brook Floodwater Retarding Dam

SuAsCo Watershed Worcester County, Massachusetts

Prepared By:

U.S. Department of Agriculture

Natural Resources Conservation Service

DRAFT

June 2009 i

DRAFT

Supplemental Watershed Plan No. 6 & Environmental Assessment for Rehabilitation of Hop Brook Floodwater Retarding Dam SuAsCo Watershed

Worcester County, Massachusetts

Prepared By:

U.S. Department of Agriculture

Natural Resources Conservation Service

In Cooperation With:

Massachusetts Department of Conservation and Recreation

AUTHORITY

The original watershed work plan was prepared, and works of improvement have been installed, under the authority of the Watershed Protection and Flood Prevention Act of 1954 (Public Law 83-566) as amended. The rehabilitation of the Hop Brook Floodwater Retarding Dam is authorized under Public Law 83-566 (as amended), and as further amended by Section 313 of Public Law 106-472.

ABSTRACT

The Hop Brook Floodwater Retarding Dam no longer provides the original protection planned for the watershed due to a greater than planned increase in development of the upstream drainage area. For current and future build-out development conditions, the dam does not meet current Massachusetts or Natural Resources Conservation Service design criteria for a high hazard dam. The local project sponsors have chosen to rehabilitate the dam to address the identified safety deficiencies. The purposes of the proposed rehabilitation of the Hop Brook Dam are to maintain the present level of flood control benefits and comply with current performance and safety standards. Rehabilitation of the site will require the following modifications to the structure: extending the current auxiliary spillway width from 340 feet to 610 feet and armoring the auxiliary spillway to provide scour protection. Project installation cost is estimated to be $1,955,100, of which $1,406,400 will be paid from Small Watershed Rehabilitation funds and $548,700 from local funds.

COMMENTS AND INQUIRIES

For further information, contact Carl Gustafson, State Conservation Engineer, USDA/NRCS, 451 West Street, Amherst, MA 01002-2953, 413-253-4362.

Submit comments by [NRCS insert date 45 days after submittal to IA review] to: Christine Clarke, State Conservationist, USDA/NRCS, 451 West Street, Amherst, MA 01002-2953 or christine.clarke@ma.usda.gov.

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, age, disability, and where applicable, sex, marital status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or part of an individual’s income is derived from any public assistance program. (Not all prohibited bases apply to all programs.)

Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center at (202) 720-2600 (voice and TDD). To file a complaint of discrimination, write to USDA, Director, Office of Civil Rights, 1400 Independence Avenue, S.W., Washington, D.C. 20250-9410, or call (800) 795-3272 (voice) or (202) 720-6382 (TDD). USDA is an equal opportunity provider and employer.

mailto:christine.clarke@ma.usda.gov� ii

Supplemental Watershed Plan No. 6 & Environmental Assessment for Rehabilitation of Hop Brook Floodwater Retarding Dam

Worcester County, Massachusetts

Contents:

SUMMARY OF WATERSHED PLAN

PURPOSE AND NEED FOR ACTION

INTRODUCTION

PURPOSE AND NEED FOR SUPPLEMENT

WATERSHED PROBLEMS AND OPPORTUNITIES

SCOPE OF THE ENVIRONMENTAL ASSESSMENT

AFFECTED ENVIRONMENT

EXISTING CONDITIONS

STATUS OF OPERATION AND MAINTENANCE

SEDIMENTATION

BREACH ANALYSIS AND HAZARD CLASSIFICATION

POTENTIAL MODES OF DAM FAILURE

CONSEQUENCES OF DAM FAILURE

ALTERNATIVES

FORMULATION PROCESS

ALTERNATIVES CONSIDERED BUT ELIMINATED FROM DETAILED STUDY

DESCRIPTION OF ALTERNATIVE PLANS

COMPARISON OF ALTERNATIVES

ENVIRONMENTAL CONSEQUENCES

DAM SAFETY

HUMAN HEALTH AND SAFETY

FLOOD DAMAGES

WATER QUALITY

WETLANDS

FISH AND WILDLIFE HABITAT

THREATENED AND ENDANGERED SPECIES

AIR QUALITY

RECREATION

CUMULATIVE IMPACTS

CONTROVERSY

RISK AND UNCERTAINTY

CONSULTATION AND PUBLIC PARTICIPATION

PROJECT SPONSORS

PLANNING TEAM

PUBLIC PARTICIPATION

AGENCY CONSULTATION

PROVISIONS OF THE PREFERRED ALTERNATIVE

PREFERRED ALTERNATIVE

RATIONALE FOR ALTERNATIVE PREFERENCE

iii

PERMITS, COMPLIANCE, AND REQUIREMENTS PRIOR TO CONSTRUCTION

COST, INSTALLATION, AND FINANCING

REFERENCES

LIST OF PREPARERS

INDEX

LIST OF TABLES

Table A – Identified Concerns Table B – Existing Structural Data—Hop Brook Floodwater Retarding Dam Table C – Important Farmland Soils Table D – Highly Erodible Land Table E – Water Quality and Stream Health, Hop Brook, June 14, 2008 Table F – Fish Species Observed at the Hop Brook Site Table G – Wetlands Table H – Land Use in the Hop Brook Dam Drainage Area Table I – Land Use in the Hop Brook Dam Floodplain Table J - Summary of Socioeconomic Characteristics Table K – Hydrologic and Hydraulic Analyses Summary Table L – Summary and Comparison of Candidate Plans Table M – Comparison of Structural Data iv

APPENDICES

Appendix A. Economic and Structural Tables Table 1 – Estimated Installation Cost ......................................................................................... A-1 Table 2 – Estimated Cost Distribution, Structural and Nonstructural Measures ........................ A-2 Table 3 – Structural Data – Dams with Planned Storage Capacity ............................................ A-3 Table 4 – Estimated Average Annual NED Costs ...................................................................... A-5 Table 5 – Estimated Average Annual Flood Damage Reduction Benefits ................................. A-5 Table 6 – Comparison of NED Benefits and Costs .................................................................... A-6

Appendix B. Letters and Oral Comments Received on Draft Supplemental Watershed Plan and

Environmental Assessment

Appendix C. Support Maps Figure 1. Vicinity Map Figure 2. Project Schematic Figure 3. Wood Turtle Habitat Figure 4. DEP Estimated Wetlands Figure 5. Field-Delineated Wetlands Figure 6. Floodplains Figure 7. Current Conditions Land Use Figure 8. Ultimate Build-out Conditions Land Use Figure 9. Breach Inundation Map (1) Figure 10. Breach Inundation Map (2) Figure 11. Breach Inundation Map (3)

Appendix D. Investigation and Analysis Report

Appendix E. Consultation and Public Scoping Process

Supplemental Watershed Plan No. 6 & Environmental Assessment For Rehabilitation of the Hop Brook Floodwater Retarding Dam

Massachusetts

SUMMARY OF WATERSHED PLAN

Project Name: Rehabilitation of Hop Brook Floodwater Retarding Dam1

Description of Recommended Plan: The weir width of the auxiliary spillway on the Hop Brook Dam would be increased from 340 feet to 610 feet by extending the spillway north onto adjacent Dike A. The auxiliary spillway crest would remain at its existing elevation. The increased capacity of the auxiliary spillway would prevent the dam from being overtopped by the freeboard storm under current land use and watershed build-out conditions. The auxiliary spillway would be armored with articulated concrete blocks to prevent spillway erosion. The principal spillway, the main dam crest and embankment, and Dikes B and C would not be affected by the project. The evaluated life of the rehabilitation structure is 54 years.

Resource Information:

Size of SuAsCo watershed: 241,000 acres (377 square miles) Drainage area of Hop Brook Dam: 3,145 acres (4.91 square miles) Land use in Hop Brook Dam drainage area:

, SuAsCo watershed County: Worcester State: Massachusetts

Sponsors: Massachusetts Department of Conservation and Recreation (DCR) Worcester County Conservation District Middlesex Conservation District Massachusetts Division of Fisheries & Wildlife (DFW)

Acres % of drainage area Agricultural 153 5 Forest 1,148 36 Developed, residential 1,538 49 Developed, commercial/industrial 57 2 Other (wetlands, open land, water, etc.) 249 8

Land Ownership:

Hop Brook Dam drainage area: Private 90 % State-Local 10 % Federal 0 % Hop Brook Dam floodplain: Private 78 % State-Local 18 % Federal 4 %

1 Hop Brook Floodwater Retarding Dam is identified in the original SuAsCo Watershed Plan (NRCS 1958). It is designated as dam A-3-c in the original work plan, as MA303 in the NRCS list of PL-566 dams, as 3-14-215-24 by the DCR Office of Dam Safety, and as MA00998 in the National Inventory of Dams database.

Number of farms (Worcester County): 1,547 Average farm size (Worcester County): 69 acres Prime and important farmland:

Drainage area (acres) Floodplain (acres) Prime farmland 813 284 Farmland of statewide importance 858 469 Farmland of unique importance 130 824

Project Beneficiary Profile: The primary beneficiaries of the project are residential, industrial, and commercial property owners in the floodplains of Hop Brook and the Assabet River; the towns of Northborough, Westborough, Berlin, Hudson, Stow, and Maynard; the City of Marlborough; and the Commonwealth of Massachusetts.

Characteristic Northborough Worcester Co. Massachusetts United States Per capita income $32,889 $22,983 $25,952 $21,587 Median annual household income

$79,781 $47,874 $50,502 $41,994

Median house value $228,300 $149,784 $192,978 $120,467 Median age 37 36 36 35 Population 14,013 750,963 6,349,097 281,421,906 Population age 65 & over 9.8 % 13 % 13 % 12 % Unemployment rate 1.4 % 2.8 % 3.0 % 3.7 % Poverty level 1.7 % 6.8 % 6.7 % 9.2 % Minority population 7 % 10 % 15 % 25 %

Note: 2000 Census data.

Wetlands: Estimated wetlands within the dry impoundment area, as interpreted and classified by the Massachusetts Department of Environmental Protection (DEP):

Deep marsh 10.0 acres Open water 0.7 acres Shallow marsh, meadow, or fen 0.2 acres Shrub swamp 4.1 acres Wooded swamp deciduous 52.9 acres Wooded swamp mixed trees 7.3 acres Total 75.2 acres

A shrub swamp wetland of 2.6 acres lies just downstream of the dam. There would be no permanent impacts to wetlands. Temporary wetland impacts (less than one acre) may occur at the toe of the slope of the dam as a result of construction access to embed the proposed armoring system. All temporary wetland impact areas would be restored following construction. All other construction staging and access would occur entirely within existing cleared or previously disturbed upland areas. All disturbed areas would be revegetated and restored after construction is complete.

Floodplains: Land uses within the 3,324-acre floodplain downstream of the dam:

Acres % of floodplain area Agricultural 243 7 Forest 1,058 32 Developed, residential 312 9 Developed, commercial/industrial 297 9 Other (wetlands, open land, water, etc.) 1,414 43

Highly Erodible Land:

Hop Brook Dam drainage area: 310 acres Hop Brook Dam floodplain: 136 acres

Threatened and Endangered Species: No federally listed species are known to occur in the area. Habitat for the wood turtle (Glyptemys insculpta), a Massachusetts listed Species of Special Concern, occurs in the floodplain along Hop Brook, and the species has been documented within the Hop Brook Dam area as recently as 2007 according to the Massachusetts Natural Heritage and Endangered Species Program (NHESP) database. Wood turtles use a variety of riparian habitats but would be most susceptible to impacts when hibernating in the bottom of streams during the winter. Proposed activities would not occur within the stream.

Cultural Resources: No historic properties that are listed on or eligible for listing on the National Register of Historic Places are present in the project’s Area of Potential Effect.

Construction will occur within the area of previous disturbance for the dam.

Problem Identification: Hop Brook Dam does not meet current dam design and safety criteria.

Hydrologic and hydraulic modeling of the freeboard storm predicts that the dam would be overtopped by 1.4 and 1.6 feet for current land use and build-out conditions, respectively.

Overtopping of the dam could lead to embankment erosion and dam failure. The models also predict that maximum permissible velocities for the auxiliary spillway would be exceeded, and erosion of the spillway slope could then occur. Dam failure from one or both of these causes would result in flood damages to approximately 380 residences, 89 industrial or commercial properties, 114 roads, 2 schools, 1 fire department, and 1 dam, plus utilities in the floodplain.

Dam failure would also potentially cause the loss of life of residents, workers, or motorists.

Alternative Plans Considered:

Alternative 1 – Future Without Project (No Action Alternative) The dam owner, DCR, has stated that it will rehabilitate the dam to meet current federal dam safety standards if federal funding assistance is not provided. DCR may choose to use rehabilitation methods other than those identified in this plan or develop its own plan to bring the dam into compliance with federal standards.

Alternative 2 – Rehabilitation (NED Alternative) The dam would be rehabilitated by widening the auxiliary spillway from 340 feet to 610 feet and armoring the spillway. Federal funding assistance would be provided to the project sponsors by the Natural Resources Conservation Service (NRCS).

Project Purpose: Flood prevention. Rehabilitation of the Hop Brook Dam is necessary to meet current state and federal safety and performance standards.

Principal Project Measure: Rehabilitation of the dam involves two primary actions:

• Widening the auxiliary spillway from 340 feet to 610 feet.

• Armoring the spillway with articulated concrete blocks.

Project Cost:

PL 83-566 funds Other funds Total Structural measures Flood prevention

$1,406,400

$548,700

$1,955,100

Project Benefits: Economic benefits of the project are derived from ensuring the continued flood prevention purpose of Hop Brook Floodwater Retarding Dam by meeting current performance and safety standards. Benefits are based on continuing flood protection to the downstream area, which has an annual benefit of $215,000. Rehabilitation would also minimize the risk of loss of life to residents and motorists traveling on downstream roadways within the breach flood area. Net average annual equivalent benefits between the Future with Federal Project (Rehabilitation Alternative) and the Future without Federal Project (No Action Alternative) = $0.

Environmental Values Changed or Lost:

Resource Impact Air quality Short-term impact from construction equipment emissions Floodplains No effect; no new structures in floodplain; existing floodplain hydrology maintained Wetlands No long-term impact (0 acres affected); potential temporary impacts to wetlands adjacent to construction area (less than 1 acre)—wetlands to be avoided if possible and restored with native vegetation if affected by construction

Fisheries No effect; existing fisheries maintained Wildlife habitat No long-term effect (0 acres affected); temporary disruption near construction area (less than 1 acre)—disturbed areas would be replanted with native vegetation

Threatened and endangered species

No effect

Land use No effect Cultural resources No effect Prime farmland No effect

Major Conclusions: Rehabilitation of Hop Brook Floodwater Retarding Dam is necessary to minimize the risk of loss of life and property damage within the potential breach area and to allow the continuance of flood prevention benefits.

Areas of Controversy: There are no known areas of controversy.

Issues to be Resolved: None

Permits: The site-specific need for permits and mitigation, if required, would be determined during final design. The owner (DCR) will be responsible for obtaining the necessary local, state, and federal permits, including (1) National Pollutant Discharge Elimination System (NPDES) general permit for construction, (2) U.S. Army Corps of Engineers (USACE) permit under Section 404 of the Clean Water Act of 1972, (3) Chapter 253 Permit to Construct or Alter a Dam, (4) Chapter 91 Waterways License, (5) Order of Conditions through the Massachusetts Wetlands Protection Act, (6) Section 401 Water Quality Certification, and (7) Massachusetts Endangered Species Act approval through Massachusetts Natural Heritage and Endangered Species Program.

PURPOSE AND NEED FOR ACTION

INTRODUCTION

The Hop Brook Floodwater Retarding Dam (referred to hereafter as the Hop Brook Dam or the dam) is one of ten floodwater retarding dams built between 1962 and 1987 in the watershed of the Sudbury, Assabet, and Concord Rivers (known as the SuAsCo watershed). Nine of those dams, including the Hop Brook Dam, were authorized to provide flood protection benefits in a 48-square-mile subwatershed by NRCS’s 1958 Watershed Work Plan for Watershed Protection and Flood Prevention, SuAsCo Watershed, Middlesex and Worcester Counties, Massachusetts and five supplemental plans. The Hop Brook Dam was constructed in 1964 in the Town of Northborough, Worcester County, Massachusetts (Figure 1, Appendix C). The dam impounds Hop Brook, a tributary to the Assabet River, during rain events, but then slowly releases the water and has no permanent pool.

PURPOSE AND NEED FOR SUPPLEMENT

The Hop Brook Dam was built under the Watershed Protection and Flood Prevention Act of 1954 (Public Law (PL) 83-566) for the purpose of flood prevention. The dam was constructed in 1964 in a rural setting. Since 1964, urban development upstream of the dam has increased the quantity of stormwater runoff, and the 2005 Hop Brook Dam Assessment Report (USDA-NRCS 2005) determined:

For current and build-out land use conditions the existing dam is overtopped by 1.4 and

1.6 feet respectively during the routing of the freeboard storm. The maximum permissible velocities are also exceeded in the emergency spillway.

As a result, DCR applied to NRCS in 2005 for funding assistance for rehabilitation of the dam to comply with current standards and ensure continued flood damage protection downstream of the dam.

The purpose of the proposed dam rehabilitation project is to continue to prevent flood damages by complying with current performance and safety standards. Failure of the dam would cause serious damage to homes and commercial facilities downstream of the dam and potentially result in a loss of life. Rehabilitation of the dam is needed to protect downstream properties, public utilities, highways, and a railroad and to reduce the risk of loss of life. Rehabilitation of the dam would extend the service life by 54 years and ensure the continued safe service of the dam throughout its original 100-year evaluation period.

This Supplemental Watershed Plan/Environmental Assessment was prepared to evaluate the rehabilitation of the Hop Brook Dam. The dam was built in accordance with the 1958 SuAsCo Watershed Plan. An amendment to PL 83-566, the Watershed Rehabilitation Amendments of 2000 (PL 106-472), Section 313, authorizes funding and technical assistance to upgrade dams under the U.S. Department of Agriculture (USDA) Watershed Program. The rehabilitation upgrade of the Hop Brook Dam is authorized under this amendment. This supplemental plan documents the planning process by which the NRCS provided technical assistance to the local sponsors, technical advisors, and the public in addressing resource issues and concerns within the Hop Brook watershed. DCR cooperated in the preparation of the plan by leading the public meeting, reviewing technical studies (hydrology and hydraulic modeling, preliminary engineering), and reviewing the draft plan-EA.

WATERSHED PROBLEMS AND OPPORTUNITIES

Modeling results indicate that the auxiliary spillway does not meet all necessary design criteria for current land use and ultimate watershed build-out conditions. During a freeboard storm, pool elevation would overtop the dam by 1.4 feet under current conditions and 1.6 feet under build-out conditions, potentially leading to failure of the dam. Flow through the auxiliary spillway would exceed NRCS maximum permissible velocities, which would erode the dam slope and potentially lead to failure of the dam.

The Hop Brook Dam provides approximately $215,000 in average annual flood damage reduction benefits for the Hop Brook watershed. The beneficiaries of the structure are the Commonwealth of Massachusetts and the localities of Northborough, Westborough, Berlin, Stow, Hudson, Maynard, and Marlborough.

Primary concerns are the safety of the dam and the potential problems that failure of the dam would cause. Associated downstream hazards include residential, commercial, and industrial developments, recreational facilities, secondary highways, and local roads, as well as utilities along those roads. The Emergency Action Plan (EAP) for the Hop Brook Dam estimates that an uncontrolled breach of the dam would cause flood damages to approximately 380 residences, 89 industrial or commercial properties, 114 roads, 2 schools, and 1 fire department, plus utilities in the floodplain (GZA GeoEnvironmental 2008). The Washington Street Dam in the Town of Hudson would be overtopped and fail because it does not have the hydraulic capacity to pass the flood wave. Catastrophic failure of the Hop Brook Dam would also potentially cause the loss of life of residents, workers, or motorists.

Opportunities that would be realized through the implementation of this watershed rehabilitation plan are:

• Compliance with current dam safety criteria,

• Protection of human health and safety,

• Protection of infrastructure and transportation system,

• Maintenance of flood control benefits, and

• Prevention of increased flooding in the floodplain.

SCOPE OF THE ENVIRONMENTAL ASSESSMENT

A scoping process was used to define project needs, determine important issues, and formulate alternatives. Scoping included a public meeting; written requests for input from state, local, and federal agencies; and coordination meetings with appropriate agencies. A steering committee of

NRCS, DCR, and technical experts was also formed to assist in the formulation and evaluation of alternatives.

Stakeholder agencies that were contacted concerning the proposed project are:

Worcester County Conservation District Massachusetts DCR Massachusetts Office of Dam Safety Massachusetts DFW Massachusetts Department of Fish & Game, Riverways Program Massachusetts DEP Town of Northborough (Selectmen, Conservation Commission, Planning Board, Engineering

Department) Town of Northborough Trails Committee Organization of the Assabet River Massachusetts Executive Office of Energy and Environmental Affairs Massachusetts Executive Office of Energy and Environmental Affairs, Environmental Policy

Act Office U.S. Environmental Protection Agency (EPA) Region 1, Regulatory Section USACE, Regulatory Division

Table A presents the results of the scoping process.

Table A – Identified Concerns

Economic, social, environmental, and cultural concerns

Degree of Concern

Degree of Significance to Decision Making Remarks

Dam safety High High Primary concern of sponsors and

NRCS

Human health and safety High High Primary concern of sponsors and

NRCS

Flood damages High High Primary concern of sponsors and

NRCS

Wetlands Moderate Moderate Analysis of effects required by Clean Water Act and Executive Order 11990;

potential for minor, temporary impact from construction; no permanent impact.

Threatened & endangered species

Moderate Moderate Analysis of effects required by Endangered Species Act; no federally protected species present. State-listed species (wood turtle) habitat occurs in project vicinity. Activities are not proposed for streams where nesting or hibernation occurs.

Table A – Identified Concerns

Economic, social, environmental, and cultural concerns

Degree of Concern

Degree of Significance to Decision Making Remarks

Water quality Moderate Low Evaluated for all NRCS projects;

minimal, temporary impact.

Fish habitat Moderate Low Massachusetts Dept. of Fish and Game requested consideration of providing fish passage; project purpose does not include fish and wildlife habitat.

Wildlife habitat Moderate Low Evaluated for all NRCS projects;

minimal, temporary impact.

Prime farm lands Moderate Low Evaluated for all NRCS projects; none affected by project.

Highly erodible cropland Moderate Low Evaluated for all NRCS projects; none affected by project.

Cultural resources Moderate Low Analysis of effects required by National Historic Preservation Act; no historic sites present in Area of Potential Effect.

Air quality Low Low Minimal, temporary impact Water quantity Low Low No impact Aesthetics Low Low Minimal, temporary impact Sedimentation and erosion Low Low Minimal, temporary impact Recreation Low Low Minimal, temporary impact

AFFECTED ENVIRONMENT

The area potentially affected by rehabilitation of the Hop Brook Dam is the dam structure itself, the area adjacent to the dam that could be affected by construction, and the flood protection area downstream of the dam. The following discussions of existing conditions focus on these areas, plus the general project vicinity—the Town of Northborough—where appropriate.

EXISTING CONDITIONS

Original Project

The Hop Brook Dam was one of eight floodwater-retarding structures proposed in the 1958 SuAsCo Watershed Plan under the authority of PL 83-566. The dam was constructed in 1964 with federal assistance provided by the U.S. Department of Agriculture, Soil Conservation Service (now the NRCS). Subsequently five supplements to the original plan were prepared and approved between 1964 and 1996. All of the supplements occurred after construction of the Hop Brook Dam and were not concerned with this facility. Through these supplements, two of the original dams were deleted from the plan and three others were added, and as a result nine floodwater retarding structures were planned and constructed between 1962 and 1974 for watershed protection and flood prevention. The Middlesex Conservation District and the Northeastern Worcester County Conservation District were the original local sponsoring organizations. The three conservation districts in Worcester County have combined into one district, known as the Worcester County Conservation District. Through the supplemental planning process and reorganization of state agencies, by 1996 the local sponsoring organizations also included DFW and the Massachusetts Department of Environmental Management (DEM). Further state reorganizations since 1996 have resulted in renaming DEM as DCR.

Description of Existing Dam

The dam was originally designed and constructed as a federal Class C dam, a hazard classification given to dams whose failure “may cause loss of life or serious damage to homes, industrial and commercial buildings, important public utilities, main highways, or railroads.”

The floodwater retarding structure is comprised of six major elements: the earthen embankment or main dam, the drop inlet principal spillway, the auxiliary spillway, and three dikes referred to as Dike A, Dike B, and Dike C. Figure 2 in Appendix C presents a schematic drawing of the Hop Brook Dam.

The dam is located on the southeastern side of the impoundment area. The dam embankment has a total structural height of approximately 23 feet, a hydraulic height2

The principal spillway for the dam is located near the north end of the dam embankment. The structure consists of a reinforced concrete riser that leads to a 36-inch-diameter reinforced concrete outlet pipe. Normal flow of the brook enters the riser by means of a 30-inch-diameter of approximately 19 feet, and an overall length of approximately 410 feet. Under normal operating conditions, there is no impoundment upstream of the dam. During impoundment conditions with the water level at the crest of the auxiliary spillway, the storage capacity of the dam is estimated to be 1,340 acre-feet, which classifies the dam as a “Large” structure in the DCR Office of Dam Safety classification system (302 CMR 10.00).

The upstream slope of the dam is generally uniform with a slope of 3H:1V. The slope extends from the south abutment to the south side of the auxiliary spillway and is primarily grass covered. The top of the dam is approximately 12 feet wide and grass covered with an elevation of 313.0 feet. The downstream slope also extends from the south abutment to the south edge of the auxiliary spillway and is primarily grass covered with an average slope of approximately 3H:1V. The downstream toe of the slope contains a rock toe drain that consists primarily of an approximately 12-foot-wide by 3-foot-high triangular section of embankment of coarse gravel and rock. A 27-inch reinforced concrete sewer main passes through the south side of the dam.

Two relief wells are provided along the length of the connection ditch along the downstream right toe of the dam to convey water from the rock toe drain to the principal spillway stilling basin.

2 Hydraulic height is defined as the difference between the elevation of the maximum controllable water surface elevation (auxiliary spillway crest) and the elevation of the lowest point in the original streambed.

opening that functions as the lower stage inlet set to elevation 292.0 feet. Upstream of the orifice, a vertical steel slatted rack bolted to the low stage concrete headwall extends upstream of the concrete riser to prevent obstructions from entering the structure. Access to the interior of the intake structure is through a 24-inch-diameter manhole cover on top of the structure. The tops of the side walls of the riser are set to elevation 302.0 feet to act as overflow weirs during high stage impoundment levels, with the openings protected by galvanized steel pipe trash racks and a reinforced concrete anti-vortex cap. Three anti-seep collars are provided along the length of the outlet pipe to limit seepage along the pipe. The downstream end of the pipe is supported by a reinforced concrete cradle and bent constructed with a two stage filter upstream. The discharge at the downstream toe of the dam flows into a 36-foot-long rock riprapped trapezoidal stilling basin and then flows for a short distance through a 35-foot-wide man-made channel before joining the natural stream channel.

The auxiliary spillway is located north of the dam embankment and extends to the west end of Dike A. The auxiliary spillway is a 340-foot-long grass lined channel with a crest elevation approximately 4 feet below the top of the dam (design elevation 309.0 feet). The crest of the spillway is 20 feet wide with the upstream slope of 3H:1V and the downstream slope of 4.5H:1V. Discharge through the auxiliary spillway would spread through the wetland area at the toe of the spillway prior to entering the downstream outlet channel.

The three dikes are located north of the auxiliary spillway. Dike A is approximately 740 feet long running in an easterly direction. Dike B is located approximately 200 feet north of the east side of Dike A and runs for approximately 290 feet in a northerly direction. Approximately 600 feet north of the north end of Dike B, Dike C begins and extends north for approximately 925 feet. The downstream slopes of the dikes are protected with rock toe drains to collect seepage.

Seepage waters collected by the toe drain of Dikes B and C flow through separate culverts beneath Route 20, through a connecting ditch on the east side of the road, and through a culvert beneath Tomblin Hill Road before joining flows from the principal spillway at Smith Pond.

Each of the three dikes is constructed of a geometry similar to that of the dam embankment, with 3H:1V grassed slopes and a 12-foot-wide crest with a design elevation of 313.0 feet.

In 2005 and 2008, the NRCS sponsored inspections of the Hop Brook Dam. The inspections determined that the dam is in good to satisfactory condition. The DCR is responsible for maintenance of the dam. As of July 2008, deficiencies identified in the 2005 inspection, including wear of vegetative cover and deterioration of the primary spillway trash rack, had been repaired. There was no indication of significant repairs or changes to the dam since the date of construction (H&S Environmental 2009).

Existing Structural Data

Table B provides a summary of the existing structural data for the Hop Brook flood control structure.

Table B – Existing Structural Data—Hop Brook Floodwater Retarding Dam Year completed 1964 Drainage area 3,145 acres (4.91 square miles) Stream Hop Brook Purpose Flood prevention Dam type Earthen embankment Dam height 23 feet Dam crest length 410 feet Storage:

Total, maximum pool 1,928 acre-feet Total, auxiliary spillway crest 1,340 acre-feet Sediment 22 acre-feet

Flood 1,318 acre-feet Principal spillway:

Type Reinforced concrete Lower stage inlet height 0 feet Lower stage inlet size 30 inches Upper stage inlet height 10 feet Outlet conduit size 36 inches

Auxiliary spillway:

Type Grass-lined channel Width 340 feet

Principal spillway crest elevation 302.0 feet Auxiliary spillway crest elevation 309.0 feet Top of dam (minimum crest) elevation 313.0 feet

Physical Features and Environmental Factors

Project Location: The Assabet River flows north for 30 miles to its confluence with the Sudbury River in Concord, Massachusetts, where the two rivers form the Concord River, which flows north for 15.5 miles to its confluence with the Merrimack River in Lowell, Massachusetts.

The SuAsCo watershed encompasses a large network of tributaries that drain approximately 377 square miles in Middlesex and Worcester Counties. The watershed contains 25 tributary sub-watersheds, one of which is Hop Brook watershed. The drainage area for Hop Brook is approximately 3,145 acres (4.91 square miles) and extends through moderately developed areas within the Town of Northborough west of the dam and areas of residential development within the Town of Shrewsbury.

Climate: The average annual precipitation for Worcester County is 49.2 inches, and the average seasonal snowfall is 59.7 inches. In winter, the average temperature is 26.2 ºF, and the average daily minimum is 18.4 ºF. In summer, the average temperature is 67.7 ºF, and the average daily maximum temperature is 76.9 ºF. The average (50 percent) freeze-free period of 172 days extends from April 27 through October 16 (NRCS 2008a).

Geology and Soils: The project area is generally located at the boundary of the Nasoba Formation (OZn) of metamorphic rock and the Marlboro Formation (Ozm) of Mafic Rocks. The soils are generally described as Paxton-Woodbridge-Ridgebury banded with Merrimac- Hinckley-Windsor. Paxton-Woodbridge-Ridgebury soils consist of very deep, poorly drained (Ridgebury) to well drained (Paxton) soils on glacial till uplands that formed in firm and very firm glacial till derived from schist, gneiss, and granite. Merriman-Hinckley-Windsor soils consist of very deep, excessively drained soils on outwash plains that formed in glacial outwash derived mostly from granite, gneiss, and schists. These soils are generally centered around the stream channels extending through the area.

According to the NRCS soil survey for Worcester County, several major soil types are located within the area surrounding Hop Brook (Taylor and Hotz 1985). Richfield, Merrimac, Agawam, and Canton fine sandy loams comprise approximately 54 percent of the soils in the area around Hop Brook. Poorly drained Freetown muck accounts for 24.6 percent of the soil type around the Hop Brook Dam. Gravel pits and rocky outcrop complex account for an additional 13.7 percent of the major soil types.

The original design geology as interpreted from the boring logs provided on the as-built drawings indicated a variety of soil materials along the alignment of the dam and dikes. These materials varied from poorly graded sands to well graded silty sand and gravel. Although variation was observed across the site, all soils appeared to represent a granular type material with variable amounts of gravels and silts as is typical of the glacial history of the area (H&S Environmental 2009).

Topography: The SuAsCo watershed lies within an area of previous glaciation, and many glacial features are present. In addition, the watershed is characterized by the prevalence of swamps, ponds, and lakes. The drainage pattern is dendritic with many tributary streams.

Within the SuAsCo watershed, the Assabet River has a steeper gradient than the lower Sudbury and upper Concord Rivers and as a result has a more rapid runoff of floodwaters (NRCS 1958).

Prime Farmland: Prime farmland is protected by the Farmland Protection Policy Act in order to “minimize the extent to which Federal programs contribute to the unnecessary and irreversible conversion of farmland to non-agricultural uses” (NRCS 2008b). Soils that are designated as prime farmland and are present in the Hop Brook Dam drainage area are the Agawam and Merrimac series fine sandy loams (NRCS 2008c). Table C presents the acreages of soils in the Hop Brook Dam drainage area and the downstream floodplain that are designated as prime farmland, farmland of statewide importance, or farmland of unique importance.

Table C – Important Farmland Soils Soil Designation Drainage Area (acres) Floodplain (acres) Prime farmland 813 284 Farmland of statewide importance 858 469 Farmland of unique importance 130 824

Source: Massachusetts Geographic Information System (MassGIS 2008b)

Highly Erodible Land: As summarized in Table D, less than 10 percent of the Hop Brook Dam drainage area and less than 5 percent of the downstream floodplain are highly erodible lands.

Table D – Highly Erodible Land

Land Use Drainage Area Floodplain

Acres Percent Acres Percent Highly erodible land 310 10 136 4.1 Potentially highly erodible land 865 28 426 12.8 Not highly erodible land 1,970 63 2,762 83.1

Source: MassGIS (2008b)

Water Quality: The Organization for the Assabet River (OAR) conducts monthly water quality monitoring of Hop Brook near Otis Street in Northborough. Data from 2008 are presented in Table E (OAR 2008b). OAR rated stream health in Hop Brook as “good” to “excellent” for 12 of the 16 weeks sampled in June to September 2006 (OAR 2007). In its water quality assessment of the SuAsCo watershed, the Massachusetts Division of Watershed Management (DWM 2005) noted “some indications of water quality degradation in Hop Brook,” mainly occasional low dissolved oxygen concentrations.

Table E – Water Quality and Stream Health, Hop Brook, June 14, 2008 Parameter Result Parameter Result Total nitrogen 0.79 pH 6.77 Total phosphorus 0.087 mg/L Water temperature 22.16 ºC Total suspended solids 7 mg/L Streamflow 3.26 cfs Dissolved oxygen 4.91 mg/L Habitat availability (0-20) 18 Note: mg/L = milligrams/liter; cfs = cubic feet per second; ºC = degrees Celsius (centigrade) Source: OAR (2008b)

Hop Brook discharges into the Assabet River. DWM (2005) summarized water quality in the Assabet River:

Historically, wastewater discharges and water withdrawals for public supply have deleteriously affected the Assabet River. A nutrient TMDL for the Assabet River was completed in 2004... Implementation of the TMDL requires removal of total phosphorus to 0.1 mg/L in the effluent of the major municipal wastewater treatment plants and evaluation of the feasibility of sediment remediation to reduce phosphorus flux from the sediments.

Hop Brook Dam is not expected to have a significant effect on water quality because it has no permanent impoundment.

Fish and Wildlife Resources: As a mostly developed area, wildlife resources expected to be associated with the area surrounding the Hop Brook Dam would be species tolerant of human activities such as small mammals (gray squirrel [Sciurus carolinensis], raccoon [Procyon lotor], striped skunk [Mephitis mephitis], opossum [Didelphis virginiana], and small rodents) and resident and migrant birds (great blue heron [Ardea herodias], mallard [Anas platyrhynchos], and Canada goose [Branta canadensis]).

Fish surveys conducted by DFW at two stations in Hop Brook in 2001 identified nine fish species, as listed in Table F. Banded sunfish, bluegill, and yellow bullhead have been introduced into the Assabet River watershed and are now found in Hop Brook. The other species are native to the area. DWM (2005) determined that the aquatic life use in Hop Brook is supported based primarily on the benthic macroinvertebrate community analysis and excellent habitat quality conditions. The benthos was classified as “slightly impacted…possibly as the result of the upstream impoundment [Smith Pond, which is downstream of Hop Brook Dam] and adjacent land uses.” Hop Brook Dam is not expected to have a significant effect on aquatic life because it has no permanent impoundment.

Table F – Fish Species Observed at the Hop Brook Site Common Name Scientific Name # Observed American eel Anguilla rostrata 2 Banded sunfish Enneacanthus obesus 1 Blacknose dace Rhinichthys atratulus 50 Bluegill Lepomis macrochirus 1 Brook trout Salvelinus fontinalis 7 Chain pickerel Esox niger 3 Fallfish Semotilus corporalis 7 White sucker Catostomus commersoni 9 Yellow bullhead Ameiurus natalis 2

Total 82 Source: DWM (2005)

Threatened and Endangered Species: In response to a consultation request from NRCS, the U.S. Fish and Wildlife Service (FWS) stated that there are no listed or proposed threatened or endangered species or critical habitat in the project area (FWS 2008). A letter received from the Massachusetts NHESP in response to an information request stated that the wood turtle (Glyptemys insculpta) has been found in the vicinity of the project site as recently as 2007 (DFW 2008). Estimated habitat for the wood turtle occurs both upstream and downstream of the dam (Figure 3, Appendix C).

In Massachusetts, the wood turtle is listed as a Species of Special Concern and is protected by the Massachusetts Endangered Species Act. According to NHESP data, wood turtle habitat includes slower moving, mid-sized streams with sandy bottoms and heavily vegetated banks.

They overwinter in the stream bottom and muddy banks and nest in sand or gravel substrate near the edge of the stream. The remainder of the year they can utilize a variety of areas including mixed or deciduous forests, fields, riparian wetlands and wet meadows (DFW 2007).

Wood turtles were not observed during site surveys. Vegetated areas immediately adjacent to the dam along Hop Brook appear to be regularly cut to prohibit the overgrowth of vegetation limiting the heavily vegetated banks preferred by the wood turtle. Possible wood turtle habitat was observed farther upstream and downstream of the dam consistent with the habitat delineated by MassGIS NHESP data and outside of the potential construction area. Winter hibernation for wood turtles occurs in streams and no activity related to the rehabilitation project is planned to occur in the stream below the dam.

Wetlands: A map of freshwater wetlands, as interpreted and classified according to cover type by the Massachusetts Department of the Environment using aerial photographs, was obtained from MassGIS (Figure 4, Appendix C). Wetland types within the floodplain upstream of the dam are listed in Table G. A shrub swamp wetland of 2.6 acres lies just downstream of the dam.

Table G – Wetlands Wetland Classification Area (acres) Deep marsh 10.0 Open water 0.7 Shallow marsh, meadow, or fen 0.2 Shrub swamp 4.1 Wooded swamp deciduous 52.9 Wooded swamp mixed trees 7.3 Total 75.2

Source: MassGIS (2008b)

Wetlands on both sides of the dam where project construction could be located were field-delineated (Figure 5, Appendix C). State-regulated wetland resources identified at the site, as defined in the Massachusetts Wetlands Protection Act Regulations 310 CMR 10.00, include Bordering Vegetated Wetlands (BVWs), Banks, Land Under Water Bodies (LUWB), and Riverfront Area, as described below.

A BVW is located along the western and southeastern portions of the Hop Brook Dam, including Dikes A, B, and C. This BVW meets the definition of a Freshwater Wetland according to the Massachusetts regulations and, therefore, a 100-foot Buffer Zone is applied. The boundary of the BVW is situated either at the toe of slope associated with the fill area for the Hop Brook Dam or is located further downslope away from the dam itself.

The delineated portion of the BVW adjacent to the dam includes forested wetlands dominated by red maple (Acer rubrum) and white pine (Pinus strobus) and a small emergent wetland along Route 20 dominated by broadleaf cattail (Typha latifolia). Understory vegetation in this wetland consists of sweet pepperbush (Clethra alnifolia), winter berry (Ilex verticillata), arrowwood (Viburnum dentatum), reed canary grass (Phalaris arundinacea), sensitive fern (Onoclea sensibilis), cinnamon fern (Osmunda cinnamomea), and sphagnum moss (Sphagnum sp.). Soils are generally comprised of 6 to 20 inches of black (10YR 2/1) muck underlain by approximately 6 inches of black (10YR 2/1) mucky-sandy loam. The 100-foot Buffer Zone associated with the BVW at the site includes forested uplands and existing cleared or previously disturbed land associated with Hop Brook Dam.

Bank wetland resources immediately adjacent to the dam are limited to the banks of Hop Brook.

The majority of the Banks on-site are vegetated and comprised of mineral soil material. Woody Bank vegetation includes red maple trees, and sweet pepperbush, winter berry, arrowwood shrubs. Emergent vegetation includes sensitive fern, cinnamon fern, and sphagnum moss.

LUWB immediately adjacent to the dam is limited to land under Hop Brook. This LUWB is generally comprised of mineral soil material.

Riverfront Area is defined as the area of land between a river’s mean annual high water line and a parallel line measured 200 feet horizontally from this high water line. Hop Brook is defined as a river as it is a perennial body of water that empties into another river. The boundary of the Riverfront Area associated with Hop Brook extends landward 200 feet from the mean annual high water line. Riverfront Area located within the potential project construction areas consists of existing cleared and previously disturbed land associated with Hop Brook Dam.

Floodplain: The floodplain in Northborough downstream of the Hop Brook Dam is shown in Figure 6, Appendix C. Outside of Northborough, Hop Brook Dam becomes a smaller influence on the floodplain of the Assabet River, which is controlled by multiple other dams in the watershed. The floodplain would not be affected by the rehabilitation of the dam.

Air Quality: Northborough falls within the Boston-Lawrence-Worcester area as defined by EPA, which is a nonattainment area for 8-hour ozone. The area is in attainment for all other criteria pollutants (EPA 2009).

Recreation: The dam and the impoundment area when not flooded are used informally for hiking and biking.

Hazardous Waste: Prior to the acquisition of the land for development of the Hop Brook Dam, portions of the site (approximately 10.5 acres) were used for automobile salvage operations. In addition, to automobile salvage, illegal dumping resulted in the identification of two “automobile burn areas,” a “wire dump area,” “a bottle dump area,” a “municipal burn area,” and an “electrical component” dump area when an initial site investigation was completed in 2001 (Rizzo Associates 2005). Soil and sediment analysis in the disposal areas identified elevated concentrations of metals (lead, antimony, and zinc), polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and petroleum hydrocarbons that were primarily surficial.

Groundwater sample analysis revealed lead concentrations above the Massachusetts Contingency Plan GW-1 standard. The wire dump area was remediated in 2001; remediation of the remainder of the site was scheduled in 2006 (Rizzo Associates 2005) but has been delayed due to lack of funding. Although these areas fall within the pool area of the dam, rehabilitation of the dam would not affect any of the areas.

Cultural and Historic Resources: The Area of Potential Effect (APE) for the project is the access road into the site and the project construction area. All of the APE was previously disturbed for construction of the dam and Dike A. Other than the dam itself, there are no structures within the APE. No historic properties that are listed on or eligible for listing on the National Register of Historic Places are present in the project’s APE (National Park Service 2008). [Note: confirmed verbally; waiting for letter responses to be included in final plan.]

Land Use: In the 1958 watershed plan, the SuAsCo watershed is described as 10 percent developed and 90 percent, cropland, grassland, forest, and open water. In the 50 years since, the area has developed as a residential area for Boston and Worcester commuters (Town of Northborough 2008). Current land use in the Hop Brook Dam drainage area (based on 1999 data in MassGIS) is summarized in Table H; almost 50 percent of the area is residential, mostly low to medium density. Land in the drainage area is predominantly privately owned (90 percent), with the rest being state- or local government-owned.

Table H also summarizes land use under ultimate build-out, as projected from zoning (Bhatti Group 2005). Residential and commercial/industrial development is projected to increase by about 25 percent in the area and will result in a similar loss of forested land cover and agricultural land. Current and build-out land use maps of the Hop Brook Dam drainage area are presented in Figures 7 and 8 in Appendix C

Table H – Land Use in the Hop Brook Dam Drainage Area

Land Use Current Ultimate Build-out

Acres Percent Acres Percent Residential 1,538 49 2,134 68 Forest 1,148 36 511 16 Agricultural 153 5 63 2 Commercial, industrial 57 2 257 8 Other (wetlands, open land, water, etc.) 249 8 180 6

Source: Bhatti Group (2005)

Land use in the Hop Brook Dam floodplain is summarized in Table I. Commercial and industrial areas are located south of the dam along…