Browsing by Author "Towsif Khan, Sami"

Now showing 1 - 8 of 8
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    Assessing the Efficacy of Stream Restoration and SCM Retrofitting for Channel Stability in Urbanized Catchments
    Towsif Khan, Sami; Thompson, Theresa M.; Sample, David J. (2024-05-29)
    The hydrological benefits of catchment-scale implementation of stormwater control measures (SCMs) in mitigating the adverse effects of urbanization are well established. Nevertheless, recent studies indicate that Maryland's stormwater regulations, mandating the combined use of distributed and end-of-pipe SCMs, fall short in maintaining channel stability, despite their effectiveness in reducing runoff from impervious surfaces. The study objective was to evaluate the incremental impact of SCM retrofits and stream restoration on channel stability in a small, urbanized catchment (0.9 sq. km) in Montgomery County, Maryland, USA. This study employed a refined, well-calibrated, coupled hierarchical modeling approach, integrating a watershed-scale Storm Water Management Model (SWMM) with the Hydrologic Engineering Centers River Analysis System (HEC-RAS). A comprehensive methodology was developed using the calibrated SWMM and HEC-RAS models. The modeling results revealed that only retrofitting SCMs with multi-stage outlet structures designed to maintain the pre-development mobility of bed particles may not effectively reduce channel degradation. Conversely, stream restoration practices, including the removal of legacy sediments from the floodplain, significantly mitigated channel instability. Notably, the combination of SCM retrofitting, aimed at matching the sediment transport capacity of the predevelopment state, and stream restoration practices did not yield better results compared to stream restoration alone. This finding suggests that for streams impacted by legacy sediments, floodplain restoration alone might suffice to achieve channel stability, eliminating the need to retrofit SCMs designed under existing regulations.
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    Cost-Effective Methods for Reducing Sediment Loads in the Lick Run Watershed
    Thompson, Theresa M.; Sample, David J.; Stephenson, Stephen Kurt; Towsif Khan, Sami; Macdonald, Kiara (2024-05-15)
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    Do Maryland's Stormwater Management Regulations Protect Channel Stability?
    Thompson, Theresa M.; Sample, David J.; Al-Samdi, Mohammad; Towsif Khan, Sami; Shahed Behrouz, Mina; Miller, Andrew; Butcher, Jon (2024-06-20)
    Webinar for the Maryland Stream Restoration Association. 84 participants
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    Effectiveness of environmental site design in protecting stream channel stability
    Thompson, Theresa M.; Sample, David J.; Al-Smadi, Mohammad; Towsif Khan, Sami; Shahed Behrouz, Mina; Miller, Andrew (2023-05-08)
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    Effectiveness of stormwater control measures in protecting stream channel stability
    Towsif Khan, Sami; Wynn-Thompson, Theresa; Sample, David; Al-Smadi, Mohammad; Shahed Behrouz, Mina; Miller, Andrew J. (2024-04-23)
    While research on the hydrologic impact of different types of stormwater control measures (SCMs) is extensive, little research exists linking urbanization, widespread implementation of SCMs and channel stability in headwater streams. This study evaluated whether the unified stormwater sizing criteria (USSC) regulations in the state of Maryland, USA, which require the use of both end-of-pipe and distributed, smallscale SCMs, protect channel stability. To achieve this goal, a coupled hierarchical modelling approach utilizing the Storm Water Management Model (SWMM) and the Hydrologic Engineering Center River Analysis System 6.3 (HEC-RAS) was developed to predict changes in streamflow and sediment transport dynamics in a first-order gravel-bed, riffle-pool channel. Storm event discretization revealed that 88% of observed storm events during the 16 years (2004–2020) had durations less than 18 h and that the greatest peak flows resulted from storm events with durations less than 24 h. HEC-RAS simulation results also showed that both channel degradation and aggradation, as high as 1.2 m, will likely occur due to regulations which require the use of 24 h duration design storms with a target stormwater detention time rather than bed material sediment transport limits. Overall, this study provides valuable insights into the complex interactions between SCM practises, flow regimes and sediment transport dynamics in heavily urbanized watersheds. It is recommended that SCMs be designed using a continuous simulation model with at least 10 years of continuous rainfall data. Furthermore, to protect channel stability, the SCM design goal should focus on maintaining pre-development sediment transport regimes across a range of flows.
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    Effectiveness of stormwater management practices in protecting stream channel stability
    Thompson, Theresa M.; Sample, David J.; Al-Smadi, Mohammad; Towsif Khan, Sami; Shahed Behrouz, Mina; Miller, Andrew (2024-06-11)
    Presentation made as part of the Stream Restoration Webinar Series: Finding Common Ground. Webinar had 284 participants.
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    Efficacy of environmental site design in protecting channel stability under changing climate
    Towsif Khan, Sami; Thompson, Theresa M.; Alsmadi, Mohammad; Sample, David J. (American Ecological Engineering Society, 2023-06-06)
    Research on the impacts of climate change (CC) on water resources has received much attention during the past decade. However, little research has been done on how future climate will likely impact sediment transport and channel stability of first-order streams, particularly in urban environments which utilize Nature-based Solutions (NbS) for stormwater management. This study aimed to assess whether the current stormwater regulations in the state of Maryland, USA, which require the use of environmental site design (ESD), are protective of channel stability when CC is considered. ESD relies on the combination of the concepts of NbS for enhanced infiltration and evapotranspiration with the utilization of storage-based gray infrastructure. To achieve this goal, a coupled hierarchical modeling approach was developed and applied to examine projected changes in bedload transport and channel geometry for a first-order riffle-pool, gravel-bed channel draining an urban watershed equipped with the extensive implementation of ESD. The modeling approach was based on discharge from a watershed-scale hydrologic model driven by a range of spatiotemporally downscaled CC scenarios. Changes in sedimentary responses of the modeled reach were estimated using the Hydrologic Engineering Center River Analysis System 6.3 (HEC-RAS). Ensemble simulation results showed that even with the extensive implementation of ESD, the studied reach is expected to degrade over many decades developing alternate regions of aggradation and degradation due to the changes in watershed hydrology caused by urbanization under both current and future climate conditions. Mobilization of larger particles during high-magnitude storm events and their subsequent deposition upstream of narrower sections of the reach leads to the formation of several steep riffles. Results from this study show that the current stormwater regulations in the State of Maryland are not protective of channel stability and that changes in climate will likely accelerate channel degradation.
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    Impacts of Stormwater Management Practices and Climate Change on Flow Regime and Channel Stability
    Towsif Khan, Sami (Virginia Tech, 2024-06-03)
    Urbanization increases runoff during storm events due to a reduction in vegetation and an increase in impervious surfaces, which limits the land's capacity to absorb and slow down water. This increase in runoff contributes to channel erosion. While extensive research exists on the hydrologic benefits of various types of stormwater control measures (SCMs), the relationship between urbanization, widespread SCM implementation, and channel stability in headwater streams remains less explored. Additionally, the impact of climate change (CC) on SCMs, with its growing focus due to improved global and regional CC models and data, is a critical area of study. However, most existing studies rely on simplified design storm analyses and unit-area runoff models, and there is a lack of comprehensive research evaluating the long-term, continuous hydrologic response of SCMs under future CC scenarios. This study presents an in-depth evaluation of the effectiveness of SCMs in maintaining channel stability in urbanized headwater streams, with a particular focus on the challenges posed by urbanization and CC. Conducted in a small catchment in Montgomery County, Maryland, USA, the study employs a sequential hierarchical modeling approach integrating the Storm Water Management Model (SWMM) with the Hydrologic Engineering Center's River Analysis System (HEC-RAS). First, the impact of a stormwater management system design following Maryland's Unified Stormwater Sizing Criteria (USSC) on channel stability was investigated. Simulation over 16 years (2004-2020) demonstrated that the majority of storm events were short in duration, with the greatest peak flows resulting from storm events with durations less than 24 hours. However, results indicated that despite the use of multiple SCMs, channel changes, including both degradation and aggradation up to 1.2 m, are likely over a period of 16 years. Study results indicate SCMs should be designed using continuous simulation models to simulate pre- and post-development sediment transport. Secondly, the impact of SCMs and CC on flow regime and channel stability was examined, challenging the previous simplified analyses. The findings highlight that future CC scenarios, characterized by decreased total rainfall but increased intensity, will likely shift watershed hydrology towards a flashier regime, exacerbating channel erosion. To address these shortcomings, a multicriteria design approach for SCMs is required, considering local sediment transport capacity and the complexities of urban catchments under changing climatic conditions. Lastly, evaluation of the impact of proposed stormwater regulations on channel stability using a novel three-step methodology revealed that SCM design goals focused on maintaining pre-development sediment transport or excess shear stress could reduce channel disturbance. Overall, this study illustrates the need for more nuanced and holistic approaches to stormwater management to ensure channel stability, especially in the face of the challenges posed by climatic changes.