Agenda - 11-16-1993 - VIII-B (2)

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7 Attachments #3-#8 provide impervious surface calculations for one and two-acre lots to illustrate the difficulties a property owner may experience in complying with impervious surface ratios of 12% and 6% respectively. The results of these calculations are summarized in the table below. Impervious Surface One-Acre Lots Two-Acre Lots Coverage W112% Limit W/ 6% Limit W/One-Story House of 2,200 SqFt Centered on Lot 11.77% 7.00% W/Two-Story House (1,100 SqFt Ground Floor) Centered on Lot 9.26% 5.81% W/One Story House of 2,200 SqFt Placed at 40 10,58% 5.66% Foot Building Setback Line As is evident in the table, the principal limitation involves two-acre lots where homes are set back from the street. The length of the driveway, combined with lot frontage and building area, results in situations where the impervious surface standard of 6% cannot be met. Through construction of a two-story home or setting the house at the building setback line, compliance is possible. These simple illustrations point to the fact that impervious surface ratios can be met, particularly if addressed during the subdivision design phase. Lots which are limited may be dealt with through transfers of "surplus" impervious surface. Furthermore, the proposed watershed standards incorporate a provision contained in the State's model ordinance which provides for the Board of Adjustment to grant variances of up to 10% to property owners experiencing difficulty in meeting the impervious surface standards. RECOMMENDATION: Based on the above analysis, the Planning Staff is altering its original recommendation. For new subdivisions, developers and property owners must comply with the 6% impervious surface ratio for two-acre lots and 12%for one-acre lots. For example, a proposed subdivision located in the Critical Area of a WS-II watershed must have minimum lot sizes of two acres, and the development as a whole may not exceed an impervious surface ratio of 6%. For existing lots of record, the current sliding scale would apply throughout the county. 3. What does the concept of "travel time" mean in terms of watershed protection? "Travel time" refers to the time it takes pollutants to reach the reservoir once they have entered streams which drain into the reservoir. Pollutants such as phosphorus and nitrogen are washed into streams during rainstorms from fields, parking lots, etc. Travel time is a consideration in watershed protection, since the stream itself may have a "cleansing" effect through deposition and/or aeration prior to the time the pollutants enter the reservoir. How much of the pollutant load; e.g., 100%. 75%, 50%, etc., is delivered to the reservoir is referred to as the "delivery ratio".