20090430 Ver 1_Staff Comments_20091220

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09- Ht Johnston County Airport Extended Runway Safety Area
AML comments
December 20, 2009
Grassed swales for pollutant removal should be designed based on the 10-year
storm intensity, not the t 0-year, 24-hour storm intensity. The velocity during the
10-year storm should be 1 foot per second.
2. Per the DWQ BMP Manual, please design the swales such that they have
trapezoidal cross section with a bottom width of two to six feet.
3. If decide use a different n value for the grassed swales, please provide a technical
reference to justify the value you select. In addition, please ensure that the O&M
agreements reflect the 6" height for the grass.
4. Please provide a full-sized plan sheet that depicts the drainage areas on the
project. On this sheet, please show which drainage areas are being directed to a
grassed swale, a filter strip, or both devices in series. On this sheet, please
distinguish between new and existing areas of impervious surface.
5. As a companion to the plan sheet requested in Item (3) above, please provide a
summary of overall pollutant loadings and removal (TSS and TN) from the
project. It might be helpful to organize the summary by drainage area.
6. Please provide signed O&M agreements for the grassed swales and the filter
strips.
7. Please provide a Level Spreader Supplement Form with all required items for the
vegetated filter strips to make sure they are sized correctly in terms of slopes,
sizing, etc.