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• <br />What Are They the <br />surface <br />ground A subsurface drain is a perforated pipe or con an ' area by lowering he water t ble. eA high w ter <br />designed depth and grade. It is used to drain <br />table can saturate soils and prevent the growth of certain types of vegetation. Saturated soils on <br />slopes will sometimes "slip" down the hill. Installing subsurface drains can help prevent these . <br />problems. <br />\ Random Pattern <br />/ If Jutiet <br />Wet,Areas <br />i' Herringbone Pattern <br />J <br />Typical Subsurface Drain Patterns <br />Water Table 3e'ore drainage <br />dater Table .After Drainage <br />/------ Interceptor Drair. <br />/ Seepage Area <br />Ground Water Flow- <br />/ermeabl <br />imp . . , z , , Layer <br />Effect of Subsurface Drains on Water Table <br />FIGURE 4.19 SUBSURFACE DRAINS <br />(Modified from commonwealth of Virginia, 19801 <br />I@) <br />4-74 <br />Chapter 4-Site-Specific Industrial Storm Water BMPs