Agenda - Council - 12/12/1995 - Laserfiche WebLink

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I I I I I I I I I I I I I I I I I ! I DEPARTMENT: i Natural Resources !Division of Waters STATE OF MINNESOTA Office Memorandum DATE: iNovember 21, 1995 TO: [Tom Hovey, Area Hydrologist '-Region 6 FROM: Jim Solstad iSenior Hydrologist PHONE: SUBJECT: 297-3851 ~UNWOOD DRIVE MITIGATION (2-114P) As requested, I ex',tmined possible mitigation options for protected basin 2-114. This mitigation is in regard to a p~,,rmit application (94-6209) for a bridge and fill wetland crossing. I didn't do everything you ask,ed, but hopefully the following data and recommendations meet your needs. I quickly eliminate,It consideration of any type of ditch block at the surveyed Section No. 1. The houses along KryOton Street are extremely low. Any action at Section 1 could subject these structures to additional flood risk. I therefore concentrated my review and analysis at sections 2 and 3. The contributing w~atershed area to Section 2 is 325 acres and the area of the wetland above this point is approxima!ely 75 acres. The approximate watershed to wetland area ratio is 4.3:1. This watershed is locatq~d in the Anoka sandplain. The Anoka County soil survey indicates that the soils in the CD 43 ~vatershed are highly permeable generally having a hydrologic classification of "A". The low watershed to wetland area ratio coupled with the highly permeable soils suggests that there should ~not be a significant bounce following a heavy rainfall/runoff event. This conclusion is supported by the fact that the OHW for the southern portion of the basin is only 0.8 feet above its rUnout. I developed a ver~ simple watershed model using the Corps of Engineers' HEC-1 computer program for the CD 43 watershed above Section 2. I assumed the worst-case scenario was the 10-day runoff' eveflt (curve number = 100) and ran the model for the 10- and 100-year storms (4.8 and 7.3 inchei, respectively). Rounding the numbers off, I found that the bounce for the 10- and 100~year ,events was 1.0 and 1.5 feet, respectively. These results seem reasonable irrespective of whit options we take for the ditch blocks. Again, these numbers are pretty rough, but hopefully they give a ball park estimate of the high levels which may occur. I've reconsidered m~y recommendations since our telephone conversation. Let me know if the following seems re~tsonable.