5 - 2004 Dam safety inspection report.pdf

DAM SAFETY INSPECTION REPORT

HANCOCK CREEK DAM

DAM ID 88

HANCOCK CREEK, MENOMINEE RIVER BASIN

DICKINSON COUNTY

SE 1/4, Section 25, T41 N, R28W

OWNER/OPERATOR: Department of Natural Resources Wildlife Division

1420 West US-2

Crystal Falls, Michigan 49920-9626 906-875-6622

HAZARD POTENTIAL

CLASSIFICATION: Low

INSPECTION DATE: October 28,2004

REPORT DATE: Decemberl, 2004

INSPECTED AND PREPARED BY: <2*ad 77fltJ 44\ barjies T. Pawloski, P.E.

Mithigan Registration No. 34034 Dam Safety Program Land and Water Management Division Department of Environmental Quality 2100 M-32 West

Gaylord, Michigan 49735 989-705-3443

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INTRODUCTION

The purpose of this inspection report is to evaluate the structural condition and hydraulic capacity of this dam as required by Part 315, Dam Safety, of the Natural Resources and Environmental Protection Act, 1994 PA 451, as amended, and the Memorandum of Understanding between the Department of Natural Resources (DNR) and the Department of Environmental Quality (DEQ) entitled "Inspection and Maintenance of DNR Owned Dams." The report is limited to a discussion of observations based on a visual investigation and review of any previous inspection reports, plans, and data that are available. This report should not be considered an in-depth engineering investigation.

All references to "right" and "left" in this report are based on the observer facing downstream.

CONCLUSIONS AND RECOMMENDATIONS

The Hancock Creek Dam is in good condition. There were no deficiencies observed during the inspection that would lead to failure of the dam. The dam has adequate spillway capacity under existing conditions.

The following recommended actions should be completed within the specified time frames:

1. Remove trees and brush from both upstream and downstream slopes of embankment during the year 2005 construction season.

2. Remove logs from spillway entrance and spillway riser as soon as possible.

3. By January 31 2005, prepare a written Operation and Maintenance (O&M) Plan that outlines operations and maintenance activities and provide a copy of the plan to the Dam Safety Program staff.

The current low hazard rating is appropriate.

PROJECT INFORMATION

The Hancock Creek Dam was constructed in 1951 to control a wildlife flooding originally created by a beaver dam. Reduced copies of construction plans are available in the Dam Safety Program files.

The dam consists of a 338-foot long earth embankment with a height of 12 feet, as measured from the top of the embankment to the invert of the outlet channel. The principal spillway is a concrete drop box spillway with a single 7-foot wide stoplog

L section. The spillway has a total weir length of 21 feet. The concrete inlet structure is founded on eight gauge steel sheet pilings. The inlet structure is connected to a 40-foot long, 60-inch diameter corrugated metal pipe (CMP) outlet pipe. A plank walkway with a pipe handrail provides access to the stoplogs.

The embankment consists of unspecified earth fill with an upstream slope of 3 horizontal to 1 vertical, downstream slope of 2.5 horizontal to 1 vertical, and a top width of 10 feet.

The embankment foundation consists of rock overlain by sand and gravel up to 5 feet thick, and covered with topsoil or muck varying up to 2 feet thick. Drawing notes indicate that unsuitable foundation material was to be excavated prior to placing embankment fill.

The height of the dam as defined by Part 315 is the difference in elevation from the design flood elevation to the invert of the outlet stream. That height for the Hancock Creek Dam is approximately 11 feet. The head, or difference in upstream and downstream water elevations, was approximately 6 feet on the date of the inspection.

Normal freeboard is approximately 4 feet. The impoundment created by the dam has a surface area of approximately 70 acres under normal conditions.

Previous inspection reports were completed in 1978,1979,1988, 1991,1995, and 1999.

SITE INVESTIGATION

The following discussion of the physical condition of the dam and appurtenances is based on observations and photographs obtained on the date of the inspection.

The Hancock Creek Dam is in good condition. The embankment is grass covered and in good condition, as shown in Photographs 1 and 2. No slumps, slides, cracks, or settlement were observed during the inspection. Small trees and brush also cover both the upstream and downstream slopes. Trees and brush should be removed in the year 2005 construction season.

The spillway is shown in Photographs 3 through 6. The concrete of the inlet structure is cracked and displaced just to the left of the outlet pipe. The crack does not appear to be worse than noted in previous inspections. Another crack in the headwall is located just to the right of the outlet pipe. The headwall is also cracked directly above the crown of the pipe. None of these cracks appear to be cause for concern at this time but should continue to be monitored in subsequent dam safety inspections.

Several logs are obstructing the riser, as shown in Photograph 3. These logs should be removed as soon as possible to allow free unrestricted flow through the spillway. The outlet pipe is in good condition. No leaking or deflected joints were observed.

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STRUCTURAL STABILITY

Based on the conditions observed during the field inspection, the dam appears to be stable. No further structural stability analysis is warranted at this time.

HYDRAULICS AND HYDROLOGY

The contributing drainage area of the Hancock Creek at the dam is 6.4 square miles.

The design discharge for this dam is the 1 percent chance (100-year) flood discharge, which is estimated to be 360 cubic feet per second (cfs). The existing spillway capacity is estimated to be 200 cfs, with 1.0 foot of freeboard.

The design flood event can be attenuated in the impoundment, with a maximum discharge of approximately 200 cfs and approximately 0.9 feet of freeboard. Hydraulic calculations are on file with the Hydrologic Studies Unit and the Dam Safety Program.

OPERATION AND MAINTENANCE

Operation of the dam is the responsibility of the owner or their designee. A written O&M Plan should be prepared, and a copy should be provided to the Dam Safety Program staff by January 31, 2005. The O&M Plan should include provisions for normal maintenance, such as mowing embankment grass and debris removal from the spillway.

EMERGENCY ACTION PLAN

No Emergency Action Plan (EAP) exists for this dam. Since the dam has a low hazard potential rating, an EAP is not required.

APPENDICES

A location map and photographs are included in the appendix.

DICKINSON COUNTY

DAM ID 88

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Photograph 1 -Embankment viewed from left abutment.

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Photograph 2-Embankment viewed from right abutment.

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Photograph 3-Spillway inlet. Arrow shows log lodged on spillway.

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Photograph 4-Spillway outlet.

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Photograph 5-Historical crack in spillway headwall, left side.

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Photograph 6-Historical cracks in spillway headwall, right side.