Stormwater control measures (SCMs) are an effective type of green infrastructure used individually or in a network within developed areas to reduce runoff and replicate pre-development conditions by restoring the hydrologic cycle. The volume and peak flow of storm runoff entering vegetated SCMs are controlled by infiltration and evapotranspiration. Several SCM design characteristics affect these two processes. Geotechnical components such as soil type, outlet structure, and underdrain structure are important examples of these characteristics. Thus, investigating these geotechnical design characteristics (GDC) may lead to a comprehensive understanding of the infiltration, volume and peak flow performance of the SCM. This general goal has been investigated considering two types of SCMs and a watershed in Pennsylvania under short-term precipitation conditions (single storm event). The first type was two rain gardens located at the Jenkintown Creek headwaters and the other one a multi-stage basin with three cells located at the Pennypack Creek headwaters. In terms of the watershed investigation, the headwater of the Pennypcak Creek with three unique regions was chosen for this study.
In the first place, the present role of the GDC of the rain garden and basin has been analyzed by modeling the two rain gardens using a variably saturated flow model, HYDRUS 2D, and the basin using the Stormwater Management Model, SWMM. The native soil type and underdrain properties were included in the GDC investigation of the rain garden, while the outlet structure properties and soil compaction were included in the GDC investigation of the basin. After calibrating the modeled underdrain outflow to the observed underdrain outflow of the rain gardens, the percolation results show that percolation and outflow were sensitive to the properties of the native soils beneath the underdrain system. The presence of native soils with high hydraulic conductivity increased percolation and decreased outflow and vice versa. In terms of the basin calibration, the SWMM model was calibrated considering the basin outlet gate setup of fully opened and fully closed. It was also essential to address the basin compaction condition during calibration. The author found that the basin compaction changed the infiltration properties of the basin soil from the sandy loam soil (preconstruction) to sandy clay loam. After basin calibration, it was found that the fully closed gate setup induced a high outflow reduction comparing to the inflow.
The calibrated rain garden models were used to examine several scenarios of different GDC including presence/absence, location, and design details of an underdrain and the type and thickness of the native soils with lowest hydraulic conductivity or highest moisture-holding capacity. Neither changing the underdrain design details nor changing native soil thickness was efficient in enhancing the rain gardens' performance. Only changing the type of native soils with low hydraulic conductivity to soils with high hydraulic conductivity was effective in increasing surface infiltration, deep percolation, and reducing underdrain outflow. However, changing the underdrain stone to less permeable stone, removing the underdrain or lowering its location, changing the native soils to soils with low hydraulic conductivity or high plasticity, and covering the underdrain with thick native soils with high moisture holding capacity can produce a further decline in the rain gardens’ performances. Both the outlet pipe diameter and soil type were investigated using the calibrated basin model under both gate setup of fully opened and fully closed. The reduction of the outlet pipe diameter was the most effective GDC in the outflow volume and flow peak reduction under fully opened gate setup while the soil type was the most effective GDC in the outflow volume and flow peak reduction under fully closed gate setup.
The impact of the SCMs’ distribution, quantity, and GDC on watershed performance was studied using the calibrated SWMM model of the basin and semi-calibrated SWMM model of the Pennypack headwaters. ArcGIS was used to identify new drainage areas in the headwater and assign their potential basin locations. After importing the new drainage areas and their basins in the SWMM model of the headwater, eight SWMM model patterns of the headwater with different basins’ distribution and quantity were investigated. One of these eight patterns with a high impact on volume and flow peak reduction in the headwater streams was designated as the effective pattern and assigned for the GDC investigation. The results show that both volume and flow peak reductions were proportional to the number of installed basins in the headwater. Distributing the basin within the downstream regions of the headwater has a higher impact on the headwater performance than that in the upstream region. In addition to the headwater hydrological performance, the health of the streams of the headwater regions was impacted as well. In terms of GDC, only stream volume and basins infiltration were impacted by changing the outlet properties and soil type. At the current compacted soil condition of the basin, closing the basins’ outlet gate allowed more water volume to be infiltrated, which reduced the stream volume. On the other hand, the outlet gate setup of fully opened was mostly effective in stream volume reduction at small outlet pipe diameter because the outlet functioned as slow release at this combined condition of GDC. Finally, removing the compaction condition of the basins' soil by restoring the soil properties before the construction significantly increased the infiltration and stream volume reduction.
|Commitee:||Sample-Lord, Kristin, Manahiloh, Kalehiwot, Komlos, John|
|Department:||College of Engineering|
|School Location:||United States -- Pennsylvania|
|Source:||DAI-B 82/7(E), Dissertation Abstracts International|
|Subjects:||Civil engineering, Engineering|
|Keywords:||Basins, Geotechnical design Characteristics, HYDRUS, Rain gardens, Stormwater control measures, SWMM|
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