05/08/1995 Council Packet - Laserfiche WebLink

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Ms. Mary Kay Wyland April 20, 1995 Page 3 Page 1, Paragraph 7 After evaluating the numerous storm -water catch basins and the proposed ponding of this project, they are substandard, in keeping water run-off contained on-site. Most of the holding ponds have their inlet openings barely 3 feet fronz the lowest level of each pond — which does not allow much water capacity in these ponds before they ALL drain into the adjacent OFF-SITE WETLANDS OF OTHER LANDOWNERS. Because of this arrangement, this project does not have the drainage capabilities to handle the potential "100 -hear flood "", much less normal water problenis. The volume of water below a pond's outlet is not intended to retain all of the runoff from a development during a 100 -year storm event The volume of water below a pond's outlet is intended to perform storm water treatment by allowing suspended solids and adsorbed pollutants to settle out. Rate control is provided by the volume above the outlet. Water will flow into the pond during a storm event. The outlet from the pond is designed to allow water to flow out of the pond, but not necessarily at the same rate runoff is coming in. The additional volume above the pond's outlet is used for storage and the water is discharged at a slower rate to avoid flooding downstream. The ponds have been designed to contain sufficient storage to contain excess runoff within the ponds during a 100-vear storm event. The outlet structures have been designed to allow this stored water to drain from the pond at a rate no higher than runoff from the site in an undeveloped condition. Page 2, Paragraph 1 On the north site of Holly Drive West, the 3.18 -acre wetland identified as Basin A is saturated at a depth of 18 inches. Basin A adjoins County Ditch #25, which appears on the DNR Protected Waters Inventory List, and has the potential to be contaminated from the residential areas' pollution, consisting of lawn chemicals, insecticides, motor oils dripped onto driveways/roads, salts/chemicals for snow/ice removal, and other everyday toxins used. Pond #1 is a small holding pond, with an incoming culvert from the northern portion of Road J. Should the outgoing culvert into Basin A, be unable to adequately maintain the appropriate water level in Pond #1 --- it only has 5 feet, before it potentially overflows into the adjoining 40 -acre parcel to the north. Consistent with the LWMP, the P8 Urban Catchment Model was used to determine the treatment efficiency of the ponds on the proposed development site. Although small, the proposed pond will remove over 60 percent of the total phosphorus and over 87 percent of the total suspended solids in the runoff directed to the pond in accordance with the Lino Lakes Local Water Management Plan and the National Urban Runoff Program (NURP). The P8 model shows that under normal conditions, the maximum elevation of water in Pond 1 will be approximately 888.4. The P8 model was also run using the 100 -year, 24-hour storm (6 inches of rain in 24 hours). The model was run five times in sequence and resulted in a maximum elevation in Pond 1 of 889.6. It is unlikely that five 100 -year storms would occur in sequence, but if it did occur, Pond 1 would still not be expected to overflow. Should the pond overflow because of a plugged outlet, the pond will overflow to the west into Wetland Basin A, not into the adjacent property to the north.