TO: Town Manager, Philip J. Farrington
FROM: Richard A. Bento, DPW Director
SUBJECT: Reservoir Dam Project Update
DATE: February 26, 2001
The following information is forwarded as an update on progress made in recent weeks regarding the Reservoir Dam project:
Subsurface Exploration – The consultant has completed the fieldwork on the subsurface exploration program. The work was done in accordance with the Order of Conditions issued by the Arlington Conservation Commission in response to the Notice of Intent prepared by the Town.
A total of five (5) test pits and nine (9) borings were executed between January 23 and February 2. The test pits were excavated in the wide area to the left (southeast) of the gated service spillway. Two (2) borings were also done in this area. Additional borings were located as follows: Two (2) on the northwest side of the spillway, two (2) on the main embankment, two (2) in the swimming area dike, and one (1) in the town beach. Observation wells were installed in all borings except the one in the beach.
Based on the information from the test pits and the borings, it appears that the area around the spillway may be natural soil rather than constructed embankment. The presence of fine soil layering and boulders indicates that the wide area near the spillway may have been a natural knoll of some sort. This is consistent with some information in historic documents obtained from the Arlington library and a plan found in the DPW files showing two separate embankments on either side of the spillway.
The embankments and underlying soil appear to be composed primarily of silty sand and gravel. No evidence of a clay (puddle) core or a corewall was found in either the dam or swimming area dike. It should be noted that a core wall might be easily missed by a small diameter boring. Rock (presumably bedrock) was encountered in the area of the embankment at approximately 27 feet below the crest and 37 feet below the top of the swimming area dike.
The test pits on the eastern end of the so-called wide area indicate that at least some portion of the wide area is composed of unsuitable fill. It appears that debris, including brick, metal, and ash, was dumped off the downstream slope of the embankment along a length of several hundred feet. This led to the appearance of a wider top of dam in the area.
In situ permeability tests were performed by GZA using the observation wells. Soil samples taken from the borings were submitted to GZA’s geotechnical lab last week for sieve analyses.
Topographic Survey – The surveyors, H.R. Feldman, Inc, have substantially completed the topographic survey, with the exception of locating the borings. An electronic copy of the first progress print of the topographic plan was delivered to the Town last week.
Underwater Inspections – The inspection sub-consultant, Araco Co., has visited the site and requested more information before finalizing plans. Araco needs access to several manholes downstream of the dam. They suggested that the spillway culvert be sandbagged and pumped to allow for video inspection. This will require assistance from the Arlington DPW. Mark Shea of DPW will coordinate the pipe inspections with the replacement of the upstream valve on the low-level outlet once the weather improves.
Hydrologic Study - MADEM Dam Safety is in the process of reviewing the Town’s petition for using the 500-year storm as the Spillway Design Flood (SDF). Based on discussions with Scott Ryan of the MADEM, an error was found in our original computations of total drainage area to the reservoir. An approximate 0.5 mi2 area draining Little Pond (i.e. Reed Brook) was added to the total contributing drainage area, representing an approximate 22% increase in area. Runoff and dam break (IDF) analyses under this revised drainage area condition were re-run. While total volume and peak rates of runoff increased somewhat, the results and conclusions in the original hydrology report remain unchanged (i.e. no overtopping under the 500 year storm for the ungated spillway scenario, and the incremental dam break analysis indicates that the 500 year storm is proper as the SDF for this dam). A revised hydrology report will be ready this week for submission to DEM.
Environmental Permitting – As discussed in the previous project meetings, the proposal to permanently lower the reservoir may require a significant environmental permitting effort. Notices of intent to Arlington and Lexington Conservation Commissions and subsequent hearings will determine initial permitting requirements.
Generally, a MEPA threshold applies to changes in the impoundment capacity of lakes and reservoirs.
MEPA is currently in the process of trying to formulate a policy regarding reservoir drawdown as it relates to dam breaching. A formal consultation with the MEPA office will be made for an official determination on the ENF issue. There are several issues that will be raised during consideration of the environmental consequences of lowering the reservoir.
Further comprehensive study in these areas is warranted and will be investigated even without MEPA involvement.
Landscape Design Alternatives - We have recently met with Weston and Sampson and the landscape architect sub-consultant to discuss strategies for developing a landscape treatment for the area adjacent to the dam formerly used as a tree residual materials storage area and beach parking area.
Several alternatives are being developed at our request to present to town officials and interested groups. These alternatives are not ready at this time due to delay in development of the topo plan.
Schedule – I have attached a modified schedule that considers recent delays due to weather conditions. A summary follows:
Funding – Currently, $225,000 is being carried within the FY2002 capital budget request. If approved, these funds would be allocated, in part, to pay for engineering services including permitting requirements.
In addition, $350,000 is programmed in FY2004 from the Sewer Enterprise Fund capital. This falls within the annual $1,300,000 appropriation from the Water/Sewer-Stormwater Enterprise Funds. If funding allotments continue, this would be used for projected repairs and improvements to the reservoir dam, beach berm and proposed landscaping improvements.
CC Chairman, Board of Selectmen
Chairman, Parks and Recreation Commission
Weston and Sampson