Laserfiche WebLink
Page 3 of 5 <br /> number of conditions are met, including retention of runoff from a 10-year storm event. <br /> 20. We sized the basin to hold a 10-year event. Statistically, over a long period of time, a 10- <br /> year event is the average interval between a particular size storm and an equal or larger event. <br /> Probably a better way to look at it on a yearly basis is that a 10-year event has a 10% chance <br /> of being equaled or exceeded in any particular year (a 100-year event, a 1% chance per year; <br /> a 50-year event, a 2% chance; and a 5-year event, a 20% chance). <br /> 21. This resulted in the infiltration basin being basically the length (just under 700 feet) and <br /> width (about 55 feet) shown on the sketch Monday night. The depth will be 5 feet. The <br /> grassed slopes will be 4 horizontal to 1 vertical, which are mowable and walkable. <br /> 22. There is an existing fence between the edge of street parking area and the School's fields. <br /> The proposed improvements have this fence being put back on the School side of the <br /> infiltration basin (the east side of the basin). There is no fence planned alongside the edge of <br /> road on the west side of the infiltration basin. <br /> 23. There was a comment Monday night that we did not show a building south of the <br /> maintenance building. We did pick this other building up on our survey - it just isn't shown <br /> on the aerial mapping we used for the study-level sketch handed out Monday night. It will be <br /> shown on the final design drawings and would not adversely impact the infiltration basin <br /> volume needed. <br /> 24. In order to allow infiltration (as discussed above), the outlet is proposed to be about 2 <br /> feet up from the bottom of the infiltration basin. On Monday night, I believe I said I thought this <br /> was 3 to 4 feet. Please note that we are still only at the preliminary design stage, but in looking <br /> ahead to final design, we believe it will be pretty close to this dimension / depth. <br /> 25. When I mentioned this outlet dimension, there were concerns about standing water. From <br /> discussions after the meeting, I understand that there was a past standing water problem at <br /> the elementary school in the spring due to a plugged culvert and thawing and then freezing <br /> temperatures. I can see (now) why the Board was so concerned about the proposed infiltration <br /> basin. <br /> 26. With any public improvement, we cannot guarantee maintenance free operation or a <br /> perfectly safe situation at all times. Culverts can get plugged - usually at the worst times - they <br /> need periodic maintenance. Manhole lids and grates can be removed - kids have fallen into <br /> manholes and catch basins. There can be standing water in ditches for various lengths of time <br /> depending on frozen ground conditions and size of storms - while it takes various amounts of <br /> time, water does tend to infiltrate eventually. <br /> 27. On the flip side, we cannot just throw our hands up in the air and say there is nothing we <br /> can do. We need to be prudent and minimize dangerous situations while still having a <br /> functioning infiltration basin. <br /> 28. After reviewing this, is a fence on the School side of the infiltration basin and an outlet pipe <br /> 2-feet above the bottom of the infiltration basin still seen as a dangerous situation by the <br /> School? <br /> 29. Outlet pipe-wise, we can set it down at the bottom of the infiltration basin. However, in <br /> order to utilize the infiltration capacity of the basin, we need to constrict the outlet pipe so there <br /> 11/21/2008 <br />