Assessing the Importance of Temporary Storage Volume Occupied by Emergent Vegetation in Constructed Storm Water Wetlands

摘要

Vegetation in a constructed storm water 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 in the CSW being undersized from a hydrologic perspective. The fraction of the volume associated with vegetation mass was unknown and designers have been unsure as to (1) how they should account for volume occupied by vegetation and (2) whether this was a significant design issue. Twelve storm water wetlands and one hybrid wet pond in North Carolina were sampled to assess their percent vegetative occupancy by volume. Vegetation was harvested from randomly selected 0.1 m~2 grids at the normal pool elevation, and displacement tests were used to calculate the volume added by vegetation at elevations of 30 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 and 45 cm are 1.019 and 1.018, respectively. Therefore, a conservative recommended factor 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, a more 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, the uncertainty in other factors such as discrepancies between designed and as-built bathymetry probably have a greater impact on the design volume loss when compared with vegetation occupancy (＜2%).