05-13-1996 Council Packet - Laserfiche WebLink

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DNR HETRO REGION TEL :Gi:-772-7-?77 Hpr 12.'?G No.013 P.03 Ml. JflMM Mabaaih Afrill2.1996 4 Por your infonnation, I diicussod • couple other posiible ctormwiter management techniques with at one resident of the project area. These are techniques which may not be feasible for this site, but which the city may want to keep in mind for f\iture projects. a.When twmce arc near the tops of bluffs or steep slopes, such as arc oAen seen along creeks or ravines, runoiT from gutters or from the structure itself can become concentrated and cause oroaion problems. However, in oixkr to minimize these problems, the gutters on the homes can be directed to the fronts of the structures (i.e., away from the steep slopes) and run over a vegetated area or through a longer, flatter area before eotenng the area with the steep slopes. With wnalier rains, most of the water should have an opportunitt’ to infllvatc before rcac^g the steeper, erosive areas. I know, for example, that Riley>Purgatoiy«Bluff Creek Watershed District has succcssfrilly required this type of runoff rodiroction for developments along stoq) crock volleys. b.Current development standards usually require ponding to detain the excess runoff produced with a development (over pre*projoct cooditioos), with the intent to koqi the rate of ninoff and the flooding elevations no higher than before the development More recently, ponding for purposes of improv'ing the water quality of (he water that leaves the site has been a roquironiont. While the importance of the current design of ponds for water quality treatment and flooding oootrol should not be minimized, it should be recognized that even if this ponding is provided that the total volume of runoff from the more impervious sites is increased. Although the flows may not be increased for the peak flood period, the base flows or more “nonnal” flows are increased. This leads to changes in the character of streams or ravines, and can cause serious eroaioQ and sodiraontation problems as well as loss of vegetation and habitat for fish or wildlife. Thercfbre, where it is feasible, we would encourage ponding that is intended to bold the excess runoff on>sitc and not overflow to existing flowage systems (except for in oxtromo events, such as the 100«year storm). Please contact me at 772-7910 (772-7914 direct/voice mail) should you have eny questions regarding theee comments. Sincerely. Ceil Strauss Area Hydrologist c:Forest Lake File (27-139P) Lake Minnetonka - Jennings Bay (27-133-16)