07/25/2007 Env Bd Packet - Laserfiche WebLink

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• • "Stormwater and Wetlands Planning and Evaluation Guidelines for Addressing Potential Impacts of Urban Stormwater and Snowmelt Runoff on Wetlands" (1997) (Paragraph 3(b)(iv)). This management schematic classifies wetlands into four categories of vulnerability (susceptibility) and applies bounce and inundation period limits accordingly. E. Flood Protection for Structures The present rule requires that the low floor of all new construction maintain a separation of two feet above the 100- year -event water elevation of a natural or artificial waterbody and of one foot above the emergency overflow of the waterbody (Paragraph 3(j)). Within a landlocked basin, low floor elevations must be at least a foot above the basin outlet. The proposed rule would restate and somewhat revise these freeboard requirements (Paragraph 3(c)(v)). As proposed, the requirement for separation above the 100 -year high water elevation and above the emergency overflow must be met for both low floor and low entry elevations. Four categories of water feature are specified, each with its own requirements for low -floor and low -entry separation from water elevations: regional flood elevations, local basins and wetlands, infiltration basins and mottled soils. In addition, the rule would allow more flexibility for building in landlocked basins. A structure would be permitted below the basin outlot if the property owner demonstrates that the low -floor elevation is at least two feet above the water level of both the 100 -year, 10 -day runoff event and back -to -back 100 -year, 24 -hour rainfall events. Finally, the RCWD currently applies its freeboard requirements to the design of stormwater treatment basins constructed adjacent to existing structures, although the language of the rules is not fully clear on this. The proposed rule would clarify that the freeboard rule applies in such situations. F. Modeling Protocol The proposed rule also would clarify and refine the protocol for modeling stormwater flows as a part of the permitting process. The proposed changes include the following: 11