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COFFIN & GRONBERG, INC. SURVEYING, ENGINEERING AND LAND PLANNING 482•A T MARACK AVENUE LONG LAKE, MINN 56756 473•4 1+1 January 9, 1985 To: City of Orono Re: Hardcover Calculation.. & ;tormwater Runoff I have worked on numerous projects and permit applications requiring approval of the Minnehah, Creek Watershed District(MCWD) engineers over the past 10 years and I fully agree with their concerns regarding increases in stormwater runoff into Lake Minnetonka. I also commend the City of Orono for their concerns and regulations covering this runoff, Bowe 'er, I feel that certain improverr,�nts or structures have different runoff coefficients which should be taken into account, much as the MCWD engineers do in considering applications made to them. There are many publications listing coefficients of runoff(C factors), and I have enclosed a copy of the table used by the Minnesota Highway Dep- artment in their drainage manual. The usual factors I use which are accept- able to the MCWD engineers are 1.00 or 100% runoff for rooftops, 0.90 or 90b runoff from blacktop surfaces, 0.50 or 50% for gravel surfaces, and 0.30-0.40 or 30-40A from existing ground surfaces depending on slope and vegetation. Thus, I believe that gravel walks and decks should not be lumped together with rooftops in considering hardcover calculations. In the case of the William Krutzig property of Tract A, Registered Land Survey No. 565, the following proposal shows measures taken to reduce the impact of runoff from a deck so that all runoff is directed to the soil be- low. A half -inch high wood strip will be placed around the outside cf the deck to restrict water from running off the ends and keep it either on the deck surface or running down through the spaces between the deck boards. Underneath the deck a one -foot layer of sand would be placed on level ground in order to disperse the water over the entire soil area beneath the sand in much the same manner that a sand layer is used in a (round type drainfield system. With a small amount of water staying on the deck or being absorbed by the deck boards, the net runoff should be less than or equal to that from the original ground. Although not proposed in this case, it appears that. temporary storage areas or basins could be used to reduce runoff into the lake. The MCWD re- quires the use of temporary storage basins for projects in order to reduce the runoff rate after construction to the rate prijr to development. It seems that this method could also be used for individual residences since the de- sired goal of less runoff to the lake would be met.