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Title:Distributed surface-subsurface modelling approach for evaluating the impact of rain gardens on steep terrain
Author(s):Rababaa, Firas Moustafa Ibrahim
Advisor(s):Schmidt, Arthur R
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Stormwater BMPs
Rain gardens
Steep terrain
Distributed modelling
Abstract:As the proportion of people living in urban low-lying areas increase dramatically over time, a large number of high and steep slopes have been modified in order to increase the available land cover. This expansion usually includes vegetation removal and construction of multiple engineered impervious surfaces that can result in increased severity of flooding events at the downstream valley regions. Sloped areas at the headwaters promote rapid downward movement of water towards the low-land regions, and infiltration capacity in mountain sides is relatively small due to the near-surface bedrock layer. Therefore, stormwater BMPs-LID can be considered as one of the effective approaches to reduce urban runoff quantity and quality from steep terrain areas. Stormwater BMPs provide sustainable, cost effective, and ecologically-friendly approach to maintain natural pre-development hydrological functions and prevent the delivery of contaminants to the receiving waters. Structural BMPs, in particular rain gardens, were selected in this work. Moreover, an integrated surface-subsurface modeling tool, MIKE SHE, was utilized for simulating the performance of distributed rain garden cells in reducing urban runoff from steep residential catchments. A series of hypothetical cases of rain gardens with distinctive characteristics, including different sizes and implementation densities, are simulated and results are reviewed and compared to the catchment response with no stormwater BMPs. Three typical storm events were selected for model simulation and evaluation of rain gardens on steep terrain, including a 60-min storm with a 25-year return period, a typical small storm exactly followed by the design storm, moderate storm after a long period of dry weather. Analysis of variance (ANOVA) was conducted as part of statistical procedures in order to validate the effect of rain garden density and storm intensity on flow reduction. The statistical analysis helps to answer whether rain gardens on steep terrain systematically reduce runoff volumes or is this random noise due to random selection of hypothetical scenarios. Simulation results suggest that implementing distributed rain gardens on steep terrain would reduce peak flow rates and runoff volumes of stormwater up to 60% and 50%, respectively. However, implementation density and rain garden allocation plays a major role in mitigating urban runoff quantities. The best-case scenario includes the construction of all potential rain gardens, as well as implementing rain gardens with areas of at least 8% of the impervious area within a catchment, to accomplish a peak reduction of around 65%. The response of soil saturation to rain garden allocation within the steep catchment varied considerably in some implementation scenarios. Subsurface flows and artificial retaining walls significantly affected the performance of distributed rain gardens in reducing urban runoff. It is important to consider the impact of unsaturated zone flow when designing and planning rain gardens on sloped drainage basins for sufficient stormwater runoff control. Furthermore, rain gardens should be better allocated near upstream locations to result in better potential performance of rain gardens on steep terrain. Statistical results indicated that rain garden density has a statistically significant effect on runoff reduction over steep terrain. However, implementing rain garden systems on hillslopes requires extra attention in the allocation of each cell in order to achieve adequate stormwater runoff control at a low implementation cost.
Issue Date:2021-04-29
Rights Information:Copyright 2021 Firas Rababaa
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05

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