Assessing the Fraction of the Temporary Storage Volume Occupied by Emergent Vegetation in Constructed Stormwater Wetlands

摘要

Vegetation in a constructed stormwater wetland (CSW) is essential for nutrient removal and thermal pollution reduction. However, a portion of the storage volume designed to retain the water quality event is occupied by vegetation, which may result inthe CSW being under-sized from a hydrologic perspective. Twelve stormwater wetlands and one hybrid wet pond in North Carolina were sampled to document their percent vegetative occupancy by volume. Vegetation was harvested from randomly selected 0.1 m2 areas at the normal pool elevation, and displacement tests were used to calculate the volume added by vegetation at elevations of 30 cm and 45 cm. Combining these data with observed vegetative cover, a water quality volume factor of safety accounting for vegetative occupancy was calculated for use in CSW design. Of the thirteen practices sampled, the highest factor of safety calculated for temporary pool elevations of 30 cm and 45 cm are 1.019 and 1.018, respectively. Therefore, a conservative recommendedfactor of safety is 1.025. Species-specific factors of safety (FOS) were also calculated; at the 30 cm temporary pool elevation, the Iris (Iris spp.) FOS was highest at 1.020 and the mixed vegetation FOS was lowest at 1.005. When designing a wetland, amore specific FOS may be calculated using a weighted average of the FOS for each species in the planting plan. The outcomes of this research allow designers to account for the volume taken up by emergent wetland vegetation. However, because the occupancyof wetland vegetation (< 2%) is within normal construction tolerances, some jurisdictions and designers may opt not to include this as a design factor.