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1. Changes in event‐based streamflow magnitude and timing after suburban development with infiltration‐based stormwater management

   https://doi.org/10.1002/hyp.13593  

   Hopkins, Kristina G. ; Bhaskar, Aditi S. ; Woznicki, Sean A. ; Fanelli, Rosemary M. ( November 2019 , Hydrological Processes)

   Green stormwater infrastructure implementation in urban watersheds has outpaced our understanding of practice effectiveness on streamflow response to precipitation events. Long‐term monitoring of experimental suburban watersheds in Clarksburg, Maryland, USA, provided an opportunity to examine changes in event‐based streamflow metrics in two treatment watersheds that transitioned from agriculture to suburban development with a high density of infiltration‐focused stormwater control measures (SCMs). Urban Treatment 1 has predominantly single family detached housing with 33% impervious cover and 126 SCMs. Urban Treatment 2 has a mix of single family detached and attached housing with 44% impervious cover and 219 SCMs. Differences in streamflow‐event magnitude and timing were assessed using a before‐after‐control‐reference‐impact design to compare urban treatment watersheds with a forested control and an urban control with detention‐focused SCMs. Streamflow and precipitation events were identified from 14 years of sub‐daily monitoring data with an automated approach to characterize peak streamflow, runoff yield, runoff ratio, streamflow duration, time to peak, rise rate, and precipitation depth for each event. Results indicated that streamflow magnitude and timing were altered by urbanization in the urban treatment watersheds, even with SCMs treating 100% of the impervious area. The largest hydrologic changes were observed in streamflow magnitude metrics, with greater hydrologic change in Urban Treatment 2 compared with Urban Treatment 1. Although streamflow changes were observed in both urban treatment watersheds, SCMs were able to mitigate peak flows and runoff volumes compared with the urban control. The urban control had similar impervious cover to Urban Treatment 2, but Urban Treatment 2 had more than twice the precipitation depth needed to initiate a flow response and lower median peak flow and runoff yield for events less than 20 mm. Differences in impervious cover between the Urban Treatment watersheds appeared to be a large driver of differences in streamflow response, rather than SCM density. Overall, use of infiltration‐focused SCMs implemented at a watershed‐scale did provide enhanced attenuation of peak flow and runoff volumes compared to centralized‐detention SCMs.

    

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2. Modeling Runoff and Nitrogen Loads From a Watershed at Different Levels of Impervious Surface Coverage and Connectivity to Storm Water Control Measures

   https://doi.org/10.1029/2018WR023006  

   Bell, Colin D. ; Tague, Christina L. ; McMillan, Sara K. ( April 2019 , Water Resources Research)

   Urban development of watersheds increases runoff and nitrogen loads by adding urban impervious surfaces and increasing the hydrologic connectivity of these surfaces to streams. Storm water control measures (SCMs) are designed to disrupt this connectivity by retaining water in biologically active depressions where nitrogen retention, transformation, and removal occur. This work applies a mechanistic, spatially distributed, hydroecological model (RHESSys) to a suburban watershed in Charlotte, NC, with 15% total imperviousness (TI) and 33% watershed area mitigated by SCMs. We developed emergent relationships between watershed‐scale predictors (TI and connectivity to SCMs) and water and nitrogen response variables (storm water runoff ratios and nitrogen load by species). Results showed that annual runoff ratios were insensitive to increases in connectivity to SCMs (varying by ~1% of rainfall) because SCMs did not substantially increase evaporation but that runoff ratios increased by an average 0.2% per 1% increase in TI due to decreases in transpiration in the watershed. Generally, nitrate loads increased with TI but decreased as more surfaces were mitigated by SCMs. However, these nitrate reductions corresponded to increased export of dissolved organic nitrogen and ammonium. Together, these results indicate that SCMs act as both removers and transformers of nitrogen at the watershed scale. SCMs showed a net assimilation of nitrogen in warm months and net release in cool months, which offset the timing of nitrogen export relative to inputs. This work highlights that using a hydroecological, process‐based model reveals both the emergent relationships between watershed condition and response and the processes controlling those relationships.

    

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3. Developing a Modeling Framework to Simulate Compound Flooding: When Storm Surge Interacts With Riverine Flow

   https://doi.org/10.3389/fclim.2020.609610  

   Loveland, Mark ; Kiaghadi, Amin ; Dawson, Clint N. ; Rifai, Hanadi S. ; Misra, Shubhra ; Mosser, Helena ; Parola, Alessandro ( February 2021 , Frontiers in Climate)

   null (Ed.)

   Compound flooding is a physical phenomenon that has become more destructive in recent years. Moreover, compound flooding is a broad term that envelops many different physical processes that can range from preconditioned, to multivariate, to temporally compounding, or spatially compounding. This research aims to analyze a specific case of compound flooding related to tropical cyclones where the compounding effect is on coastal flooding due to a combination of storm surge and river discharge. In recent years, such compound flood events have increased in frequency and magnitude, due to a number of factors such as sea-level rise from warming oceans. Therefore, the ability to model such events is of increasing urgency. At present, there is no holistic, integrated modeling system capable of simulating or forecasting compound flooding on a large regional or global scale, leading to the need to couple various existing models. More specifically, two more challenges in such a modeling effort are determining the primary model and accounting for the effect of adjacent watersheds that discharge to the same receiving water body in amplifying the impact of compound flooding from riverine discharge with storm surge when the scale of the model includes an entire coastal line. In this study, we investigated the possibility of using the Advanced Circulation (ADCIRC) model as the primary model to simulate the compounding effects of fluvial flooding and storm surge via loose one-way coupling with gage data through internal time-dependent flux boundary conditions. The performance of the ADCIRC model was compared with the Hydrologic Engineering Center- River Analysis System (HEC-RAS) model both at the watershed and global scales. Furthermore, the importance of including riverine discharges and the interactions among adjacent watersheds were quantified. Results showed that the ADCIRC model could reliably be used to model compound flooding on both a watershed scale and a regional scale. Moreover, accounting for the interaction of river discharge from multiple watersheds is critical in accurately predicting flood patterns when high amounts of riverine flow occur in conjunction with storm surge. Particularly, with storms such as Hurricane Harvey (2017), where river flows were near record levels, inundation patterns and water surface elevations were highly dependent on the incorporation of the discharge input from multiple watersheds. Such an effect caused extra and longer inundations in some areas during Hurricane Harvey. Comparisons with real gauge data show that adding internal flow boundary conditions into ADCIRC to account for river discharge from multiple watersheds significantly improves accuracy in predictions of water surface elevations during coastal flooding events. 

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4. Disentangling the impacts of human and environmental change on catchment response during Hurricane Harvey

   https://doi.org/10.1088/1748-9326/ab5234  

   Sebastian, Antonia ; Gori, Avantika ; Blessing, Russell B. ; van der Wiel, Karin ; Bass, Benjamin ( November 2019 , Environmental Research Letters)

   Flooding is a function of hydrologic, climatologic, and land use characteristics. However, the relative contribution of these factors to flood risk over the long-term is uncertain. In response to this knowledge gap, this study quantifies how urbanization and climatological trends influenced flooding in the greater Houston region during Hurricane Harvey. The region—characterized by extreme precipitation events, low topographic relief, and clay-dominated soils—is naturally flood prone, but it is also one of the fastest growing urban areas in the United States. This rapid growth has contributed to increased runoff volumes and rates in areas where anthropogenic climate changes has also been shown to be contributing to extreme precipitation. To disentangle the relative contributions of urban development and climatic changes on flooding during Hurricane Harvey, we simulate catchment response using a spatially-distributed hydrologic model under 1900 and 2017 conditions. This approach provides insight into how timing, volume, and peak discharge in response to Harvey-like events have evolved over more than a century. Results suggest that over the past century, urban development and climate change have had a large impact on peak discharge at stream gauges in the Houston region, where development alone has increased peak discharges by 54% (±28%) and climate change has increased peak discharge by about 20% (±3%). When combined, urban development and climate change nearly doubled peak discharge (84% ±35%) in the Houston area during Harvey compared to a similar event in 1900, suggesting that land use change has magnified the effects of climate change on catchment response. The findings support a precautionary approach to flood risk management that explicitly considers how current land use decisions may impact future conditions under varying climate trends, particularly in low-lying coastal cities.

    

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5. Stormwater control impacts on runoff volume and peak flow: A meta‐analysis of watershed modelling studies

   https://doi.org/10.1002/hyp.13784  

   Bell, Colin D. ; Wolfand, Jordyn M. ; Panos, Chelsea L. ; Bhaskar, Aditi S. ; Gilliom, Ryan L. ; Hogue, Terri S. ; Hopkins, Kristina G. ; Jefferson, Anne J. ( May 2020 , Hydrological Processes)

   Decades of research has concluded that the percent of impervious surface cover in a watershed is strongly linked to negative impacts on urban stream health. Recently, there has been a push by municipalities to offset these effects by installing structural stormwater control measures (SCMs), which are landscape features designed to retain and reduce runoff to mitigate the effects of urbanisation on event hydrology. The goal of this study is to build generalisable relationships between the level of SCM implementation in urban watersheds and resulting changes to hydrology. A literature review of 185 peer‐reviewed studies of watershed‐scale SCM implementation across the globe was used to identify 52 modelling studies suitable for a meta‐analysis to build statistical relationships between SCM implementation and hydrologic change. Hydrologic change is quantified as the percent reduction in storm event runoff volume and peak flow between a watershed with SCMs relative to a (near) identical control watershed without SCMs. Results show that for each additional 1% of SCM‐mitigated impervious area in a watershed, there is an additional 0.43% reduction in runoff and a 0.60% reduction in peak flow. Values of SCM implementation required to produce a change in water quantity metrics were identified at varying levels of probability. For example, there is a 90% probability (high confidence) of at least a 1% reduction in peak flow with mitigation of 33% of impervious surfaces. However, as the reduction target increases or mitigated impervious surface decreases, the probability of reaching the reduction target also decreases. These relationships can be used by managers to plan SCM implementation at the watershed scale.

    

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