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limitation of six percent ( 6%) shall apply to the entire <br /> tract to be developed. For an existing lot of record to <br /> be developed in accordance with the provisions of <br /> Article 6 .25 .2, the full range of impervious surface <br /> ratios specified in Table 6 .25 . 4 is applicable. <br /> Where a property owner can clearly demonstrate the <br /> inability to use his/her property because of the <br /> application of the impervious surface limitations <br /> specified in Table 6 . 25 . 4, a modification of the <br /> impervious surface ratios may be requested in accordance <br /> with the provisions of Article 8 (Special Uses ) of this <br /> ordinance. <br /> 6 .25 .5 Stream Buffers Required <br /> The effective use of buffer zones along streams can <br /> increase the overall factor of safety associated with a <br /> watershed protection plan. By preventing development <br /> from encroaching on streams, buffer zones can help <br /> minimize stream channel erosion downstream of <br /> development. If properly situated and kept in <br /> undisturbed natural vegetation, buffer zones will <br /> receive sheetflow runoff; i. e. , runoff which flows <br /> slowly in a spread out pattern rather than being <br /> concentrated in a swale, from adjoining development and <br /> achieve reductions in pollution discharges . <br /> Within the University Lake Watershed (ULW) District, an <br /> area of land or buffer zone along perennial and <br /> intermittent streams shall be required to remain in its <br /> natural state, unless the area is subject to serious <br /> erosion, .in which case an erosion resistant vegetative <br /> cover shall be established and maintained. Perennial <br /> streams are those streams shown in solid blue on the <br /> USGS quadrangle maps for Orange County. Intermittent <br /> streams are those streams shown by broken blue lines on <br /> the same maps . <br /> 6 . 25 .5 . 1 Calculation of Buffer Zone Width <br /> Width; i. e. , distance from the stream bank to the edge <br /> of the buffer, is a critical factor in determining the <br /> effectiveness of a buffer zone. An equally important <br /> determinant in pollutant removal in a buffer zone is <br /> travel time; i. e. , the greater the time allowed for <br /> runoff to travel through a buffer, the greater the <br /> opportunities for pollutant removal through <br /> sedimentation, filtration, absorption, and seepage into <br /> the soil profile. <br /> Monitoring studies of highway runoff have shown that <br /> pollutant removal in grass swales is at a maximum after <br /> a 15-minute travel time. Based on this standard, Table <br />