Reservoir Consultant's Report
by Mohammad Kheirallah, February 2000 SECTION 2: VISUAL INSPECTION
On October 11, 1999, a WSE engineer visited Arlington Reservoir to observe the dam and related structures. The purpose of the visit was to evaluate the surficial conditions of the earthen dam, concrete outlet works, and emergency spillway; to check for seepage; and to measure the water depth of the reservoir. We inspected the earthen dam for surface cracks, upstream and downstream condition of the slopes, seepage, debris, undesirable growth, and animal burrows. The concrete outlet works were also inspected dimensionally and for concrete damage and overall visual stability. The emergency spillway’s training wall and masonry slab were inspected for undesirable growth, missing stones, and visual stability of the walls. A boat was used to measure the depth of the reservoir at several locations. In general, we judge the dam to be in fair/poor condition.
At the time of the inspection, the crest gate spillway was lowered to a horizontal position at elevation 153 feet. This lowered the reservoir elevation to 153 feet, approximately 9.5 feet below the top of the spillway’s concrete training walls (at about elevation 162.5 feet).
We judge the upstream face of the dam to be in poor condition. The upstream face of the dam is covered with a thick growth of brush and large trees that have grown through the existing riprap layer. The upstream slope is estimated between 1.5H:1V to 2H:1V. The riprap is barely visible at most locations due to leaves and other organic materials. At the toe of this slope the riprap layer is being unraveled towards the crest of the dam by a combination of tree roots and wave action resulting in a 1- to 1.5-foot vertical drop to a gently sloping gravelly-sand beach (see Figure 2).
We judge the crest of the dam to be in fair condition. Presently the crest is used as a walkway for town residents, and is subject to considerable foot traffic, which has resulted in eroding the dam top. Regardless of the foot traffic, the northern end of the dam shows no signs of cracks, depressions, or animal burrows, and is generally free of brush and tree growth. The southern end of the dam crest is covered with wood chips, piles of logs, and other debris that prevented us from observing the crest of the dam. The southern end of the dam appears to have widened in order to facilitate the storage of organic matter and logs (see Figure 3).
We judge the downstream face of the dam to be in poor condition. The downstream face of the dam is covered in a thick growth of brush and large trees. The slope undulates but is estimated between 1.5H:1V and 3H:1V. The toe of this slope is undercut by 1 to 1.5 feet horizontally by either erosion from Mill Brook or from seepage forces. At the time of the inspection the water in the reservoir was too low to accurately determine the cause of erosion. There were no signs of cracks on this slope. The southern section of the slope had numerous animal burrows. At the northern end, we observed localized areas of shallow sloughing, possibly resulting from freeze-thaw action or seepage force. The downstream slope is bordered by Mill Brook and wetland areas.
We judge the emergency spillway to be in poor condition. The emergency spillway is approximately 15 feet wide and has a thick stand of brush and small trees growing up through the masonry stone work. The stone work of the training walls and the spillway appears to be in good shape but the process of removing the brush and small trees may dislodge the stones. The easternmost 5 to 10 feet of the southern training wall of the emergency spillway is undercut and must be rebuilt.
We judge the concrete outlet structure to be in fair shape and to require only minor repairs. Three spalls at the top of the training walls were observed and should be repaired. Two cracks in the concrete were observed at the western end of the flared portions of the training walls. The crack on the southern wall is up to 1/8-inch wide and runs the entire height of the wall. The crack on the northern wall is in the upper two feet of the wall and is less than 1/16-inch wide. Both cracks should be repaired.
The eastern ends of the vertical steel plates that extend from the side of the crest gate are heavily corroded. At the northern wall a gap has opened between the steel and concrete structure. The vertical plates themselves are thinly rusted and are missing paint. The gate was covered in water and was not available for inspection.
The downstream outlet pipe was three-quarters full of water and the exposed surface appeared to be in good condition.
The downstream end of the low-level outlet pipe was covered in water but the last section of pipe appears to have failed and the first exposed joint is parted.
Several locations in the reservoir were checked for depth. At the time of the inspection the crest gate was horizontal and had approximately 2 to 4 inches of water flowing over it. The southern half of the pond was generally 3 to 5 feet deep. The northern half of the pond was less than 6 inches deep. Throughout the reservoir a 1- to 1.5-foot layer of very soft muck existed at the mudline. Please refer to Figure 4 for depth locations and measurements.