07/30/2003 ZONING BOARD OF APPEALS Decisions

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Meetinghouse Hill Plan Review Page three A small area in the southwesterly comer of the site appears to be a low point where water could collect after a significant rainfall. There is also a natural drainage swale southerly of the property line that discharges storm water runoff onto the site before it finds its way toward Meetinghouse Road. This latter feature is a point of accumulation for runoff that originates on the southerly side of the access road that leads to the new water tank. A pipe culvert intercepts storm water from an old road cut and discharges it onto the north side of the access road where it collects with runoff from the surrounding ground. This runoff eventually finds its way toward the low point at the southerly side of Lot 1. The drainage calculations were reviewed with several items noted as being critical to the design for storm water control. 1. Soil types in the project area, 2. Depth of high ground water, 3. Amount of pervious and impervious cover, 4. Times of concentration allotted to the overland flow of surface runoff. Soil Types Soils are noted as Carver deep sands and Enfield loamy sand. The Carver soils are highly permeable and are located mostly within that portion of the site that has been previously disturbed and excavated to form an abandoned sand pit. The Enfield soils are much less permeable and occur on the higher slopes and contribute to runoff mostly in the northeasterly comer of the site. Ponding Area The entire ponding area of the existing site has been assumed to be impervious (flooded or frozen) for pre-development conditions. This may be a very conservative approach in determining the actual need for flood storage but serves the purpose of establishing the capability of the ponding area to contain the 100-year runoff without exiting to the discharge point near Meetinghouse Road. The importance of this is reflected in the design of the subdivision drainage system. The essential part of the design is to insure that peak runoff from the site after development is no greater than runoff calculated for pre-construction conditions. Most runoff appears to be contained within the site due to the volume of the available ponding area. Therefore post construction must also be able to contain runoff to the same extent so as not to adversely increase off site runoff conditions. Design Methods Used The proposed methods of controlling water quality and quantity are shown on Sheets 3, 4 and 5 of the plans. Water quality control is achieved through the use of deep sump catch basins, a water quality sediment trap located in the drainage line east of the house on Lot 4, and a forebay basin that is the receiving point for most storm water generated from the proposed roadway. The sediment trap is intended to receive low flows and to contain finer sediments that would otherwise reach the forebay. A by- pass line allows larger flows to go directly to the forebay. The system is not adversely affected by the by-pass line. Water quantity is controlled by the use of storage in the forebay, a detention area and a controlled outlet and leaching pit that is located at the westerly edge of the detention area. No elevation is given for the bottom of this leaching pit. It is assumed to be above the high ground water elevation of 93.6 but should be confirmed by the design engineer. Runoff that reaches the site from off-site sources is unaffected by the proposed design. The design appears to take this added source into consideration. The point of