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Planning Commission Regular Meeting Minutes October 18, 1994 Page 7 1 issues for Knox are 1. Conditional use permit, 2. Lot split, and, 3. Variance request. The other 2 items included are a comprehensive sign package and to review the drainage proposal for CUB 3 and Knox. 4 At 8:20 P.M., Gondorchin opened the meeting to questions from the public. 5 Charles Finn, 2709 Silver Lane, inquired as to where the run-off from the roof was going to 6 drain. He expressed doubt in the previous stated figure of 80%of the run-off going into 7 detention ponds. He asked why the existing trunk system is so deep. 8 Beduhn explained that the existing roof run-off had never drained into ponds. This has always 9 been picked up in the storm sewer system. He explained that the figure of 80%was in regards to 10 the parking area. He has reviewed this with the Rice Creek Watershed District and they feel it is 11 best to capture the run-off coming from the parking lots. In order to drain this area of St. 12 Anthony, the pipes are deep to match the entrance of the existing storm system. 13 Finn asked why there was no pond at the other end of the Plaza. He felt there would be a huge 14 volume of water in that area. He questioned if the green space in that area could facilitate a 15 pond. Beduhn reported that in this area a pond was not physically feasible since there was no way to 17 construct effective side slopes but there were other things that could be done to treat the water in 18 that,area such as using a skimmer-, directing storm water into ground, etc. He stated that the 19 green space was not sufficient to house an efficient pond and could not meet design standards. 20 He has presented other options for the area to the City and also to Cavanaugh for consideration. 21 22 Finn expressed that he did not think the area of ponds was sufficient to hold the amount of run- 23 off in that area. 24 Beduhn explained that the ponds were designed to meet the Rice Creek Watershed guidelines, as 25 well as MPCA guidelines, etc. During a storm event,there may be some run-off but the pond 26 designs had been based on a 1, 10 and 100 year storm event. He explained that these ponds were 27 designed to achieve significant removal of particles from the water. Sand, grit and clay have a 28 specific gravity 3 times greater than water so they stay behind in the pond to be cleaned out at a 29 later date. The concept for these detention ponds is that the water sits in the pond and is treated 30 between storm events. When the next storm events occurs, the treated water is pushed out into 31 the sewer-system and the ponds fill again. The water that is then left is treated until the next 32 storm event. Hopefully, there is enough detention time before the next storm event for sediment . 33 to separate.