03/28/2001 Env Bd Packet - Laserfiche WebLink

Home Search

Mr. William A. T. McCully, P.E. March 10, 2001 Page 2 numbers from Chart 1, post - development conditions will result in less runoff than the existing conditions. Model results are attached. Water quality basins were added to the development plans to treat runoff from the site. These basins are designed to be dry in between storm events and contain the runoff from a 2.5 -inch rainfall. Locations of these basins are shown in Figure 4. These basins also act to control volume runoff from roadways as required. A District Infiltration Worksheet is attached. The interior wetland on site has an open channel outlet with an invert elevation of 889.2. The existing conditions 100 -year flood level is modeled to be 890.1. The proposed outlet of this wetland is a 15 -foot wide broad crested weir at elevation 890.2 with a 4 -foot wide notch 1- foot deep cut in the middle to elevation 889.2. The effective outlet elevation will match the existing outlet elevation (889.2). The top of the weir will act as the emergency overflow. The post- development 100 year flood level is modeled to be 890.2. Basement elevations of the homes around the wetland will have to be set at or above 892.2 according to the above regulations. A 36 -inch equivalent diameter concrete arch pipe with an invert elevation of 888.7 is proposed under the roadway downstream of the weir. The capacity of this pipe with a headwater at 890.2 exceeds that of the upstream weir and will not control the level within the wetland under normal conditions. We checked extreme flood conditions by modeling the effect of blocking more than 80 percent of the culvert during a 10 -inch rainfall. The results show that the water elevation within the wetland did not exceed the minimum basement elevations recommended above. We noted a small (7200 sq.ft.) portion of floodplain fill at the northwestern part of the site. We recommend compensating for this fill by an equivalent area of excavation on the southwestern part of the site. Please review the information we have provided. If you have questions or need further information please contact me (952) 881 -3050. Thank you.