Agenda - 02-28-1990 - Laserfiche WebLink

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Orange Water and Sewer Authority 400 ones F 1owAsAI .irR0ad Carrboro. NC 27510 (919) 968-4421 MEMORANDUM TO: Everett Billingsley FROM: Ed Holland DATE: February 9, 1990 SUBJECT: University Lake Watershed, Impervious Surface Standards Proposed by Orange County, February 5, 1990 These comments and data represent a more systematic review of the water quality implications of Orange County' s proposed impervious limits than was available when Marvin Collins called on me during the February 5 Commissioners' meeting. Orange County Findings Marvin analyzed a large number of hypothetical development configurations for different 57_ 2-acre parcels. Results showed that only 60 percent of- the design configurations for 5- acre lots would comply with a 4% impervious limit, and only .30 percent of the 2-acre patterns would comply with a 6% limit. The average, imperviousness of the 5-acre configurations was 4. 03%; the 2-acre average was 7%. County staff have recommended a sliding scale of impervious limits for different lot sizes. The maximum would be 6% for 5- acre lots, and 12% for 2-acre lots, rather than the 4% and 6% recommended by Camp Dresser i McKee and by the October 1989 13- Point Agreement. Additionally, the County proposes stricter site design standards that will require roads, homesites, and septic tank fields to be located outside of stream buffers. Effects on University Lake Water Quality In its present form, the CDM watershed model cannot directly predict the water quality effects of the County' s proposal, because the model contains built-in assumptions of imperviousness and lot size that differ from the County' s. The model does provide an indirect estimate. Table 1 lists total impervious coverage and chlorophyll concentrations predicted by the CDM model for a variety of land An Equal Opportunity Employer