07/25/2007 Env Bd Packet - Laserfiche WebLink

Home Search

Over time, an infiltration practice will need to be regularly maintained to restore its original infiltration capacity. Restoration may include actions such as removal of the top layer of material, removal and replacement of a gravel filter or deep tilling. The purpose of the pretreatment requirement is to protect the infiltration system from clogging before scheduled maintenance and to protect groundwater from contamination. Pretreatment would include any Best Management Practice that removes settleable or particulate material or other contaminating pollutants. In addition, the proposed rule: (a) requires a 48 -hour drawdown time; (b) mandates at least three feet of separation from the seasonal high water table; and (c) requires that design provide for maintenance access (Paragraph 3(c)(i)). D. Peak Flow Management Under the present rule, a property owner must ensure that under the proposed condition, the peak rate of stormwater runoff at the parcel boundary will not exceed the "pre - project" rate for the one -, two- and 100 -year, 24 -hour storm events (Paragraph 3(b)). The rule requires that an applicant use hydrologic modeling to determine the difference in peak runoff rate from the pre - project to the proposed condition, and take measures to maintain the rate at the pre - project condition. The RCWD proposes to amend the rule to require post - project peak flow to be held to the "pre- settlement," rather than "pre- project," peak rate (Paragraph 3(b)(iii)). The goal of stormwater management is to mimic natural conditions -- and the natural dynamic equilibrium -- as much as possible to cause the least impact to downstream resources. Requiring proposals to meet the "pre- settlement" condition will provide a higher level of protection that more closely approaches the conditions that sustained the District's resources in the past. This modeling assumption will apply to those parts of a site with hard cover. If the site already has hard cover in the pre - development state, that hard cover may be reflected in modeling the pre - development condition. "Pre- project" curve numbers (numbers reflecting the proportion of precipitation that moves as surface runoff) typically are those for pasture, open space, and woods -grass uses (Table 1). 9