01-28-1985 Council Packet - Laserfiche WebLink

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COFFIN A GRONBFRG, INC. `.4URVE YING, ENGINEERING AND LAND PLANNING 402•A TAMARACK AVENUE LONG LAKE, MINN. 55356 January 9, 1985 To: City of Orono Re: Hardcover Calculations & Sto rmwater Runoff I have worked on numerous projects and permit applications requiring approval of the Minnehaha 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. 1 also commend the City of Orono for their concerns and regulations covering this runoff. However, I feel that certain improvements 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 1 use which are accept- abie to the MCWD engineers are 1.00 or 100% runoff for rooftops, 0.90 or 90% runoff from blacktop surfaces, 0.50 or 50% for gravel surfaces, and 0.30-0.40 or 30-40% from existing ground surfaces depending on slope and veqetation. 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 Survev 14o. 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 of 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 gro and in order to disperse the water over the entire soil area beneath the sand in much the same manner that a sand layer is uskd in a mound 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 prior 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.