Extreme flooding and excess nutrient pollution have been detrimental to river health under increased environmental stress from human activities (e.g., agriculture, urbanization). Riverine flooding can be detrimental to human life and infrastructure yet provides important habitat and ecosystem services. Traditional flood control approaches (e.g., levees, dams) negatively impact habitat and ecosystem services, and cause flooding elsewhere along the river. Prior studies have shown that stream restoration can enhance flood attenuation, and increased exchange of water between the channel and floodplain can improve water quality. However, the effects of floodplain restoration during small and sub annual recurrence interval storms have not been thoroughly studied, nor have cumulative impacts of floodplain restoration on water quality at watershed scales. We used HEC-RAS to perform 1D unsteady simulations on a 2nd-order generic stream from the Chesapeake Bay Watershed to study flood attenuation under small and sub-annual recurrence interval storms (i.e., 2-year, 1-year, 0.5-year, and monthly). In HEC-RAS we varied percent of channel restored, location of restoration, bank height of restoration, floodplain width, and floodplain Manning's n. Overall, stream restoration reduced peak flow (up to 37%) and decreased time to peak (up to 93%). We found the timing of tributary inflows could obscure the attenuation achieved, and even reverse the trends with certain parameters in the sensitivity analysis. The greatest exchange with the floodplains (greater volume and exchange under more recurrence interval storms) was observed from Stage 0 restoration, which reduces bank height more than other approaches. We also conducted a quantitative literature synthesis of nitrate removal rates from stream restoration projects. We focused on how removal rates varied with properties relevant at watershed scales, such as effects of stream order. The resulting database will aid in determining which stream restoration parameters better reduce nutrient loads and in simulating the effects of stream restoration on water quality at watershed scales. Floodplain restoration practices, and particularly Stage 0 approaches, enhance flood attenuation which can help to counteract urban hydrologic effects. / Master of Science / Extreme flooding and excess nutrient pollution have been detrimental to river health under increased environmental stress from human activities (e.g., agriculture, urbanization). Riverine flooding can be detrimental to human life and infrastructure yet provides important habitat and ecosystem services. Traditional flood control approaches (e.g., levees, dams) negatively impact habitat and ecosystem services, and cause flooding elsewhere along the river. Prior studies have shown that stream restoration can enhance flood attenuation and aid in removal of excess nutrients. Previous studies have shown that stream restoration helps to transport nutrients to highly reactive soils and increases time for reactions. However, the effects of floodplain restoration during small and sub annual recurrence interval storms have not been thoroughly studied, nor have cumulative impacts of floodplain restoration on water quality at watershed scales. To fill these knowledge gaps, increased understanding of stream restoration design parameters and watershed level characteristics (e.g., tributary inflows, nutrient loads, etc.) is necessary. We used HEC-RAS to study flood attenuation via stream restoration under small and sub-annual recurrence interval storms on a generic stream from the Chesapeake Bay Watershed. In HEC-RAS we varied percent of channel restored, location of restoration, bank height of restoration, floodplain width, and floodplain Manning's n (surface roughness). Overall, stream restoration did reduce peak flow and decrease time to peak, which means that restoration can diminish negative flooding effects. The greatest exchange with the floodplains was observed under Stage 0 restoration, which reduces bank height more than other approaches. We also conducted a quantitative literature synthesis to collect nitrate removal rates from stream restoration projects. We focused on how removal rates varied with properties relevant at watershed scales, such as effects of stream order. The resulting database will aid in determining which stream restoration parameters better reduce nutrient loads and in simulating the effects of stream restoration on water quality at watershed scales. These efforts will help to inform practitioners how to construct stream restoration projects that are more efficient for flood control and nutrient reduction. Floodplain restoration practices, particularly Stage 0 approaches, enhance flood attenuation and exchange which can help to counteract urban hydrologic effects.

Human land use practices such as urbanization and agriculture contribute excess nutrients (nitrogen and phosphorus) and runoff volumes to rivers that degrade aquatic ecosystems and cause a loss of river functions such as nutrient processing and flood attenuation. Floodplain restoration increases floodplain exchange and is commonly implemented to improve water quality and reduce flood impacts at watershed scales. However, the effect of multiple restoration projects at the watershed scale is not well studied. We addressed this knowledge gap by two studies. The first study evaluated the impact of cumulative and spatially varying Stage-0 and bankfull floodplain restoration on nitrate removal in a generic 4th-order Virginia Piedmont watershed for small and sub-annual storm sizes (i.e. 2-year, 1-year, half-year, and monthly recurrence intervals). We used HEC-RAS hydraulics results from a prior study together with a nitrate removal model coded in R. Results indicated that watershed nitrate removal varied depending on the location of restoration in the watershed and where removal was evaluated. The greatest reductions in nitrate loads were observed in the same part of the river network where restoration occurred, with diminished impacts downstream. Removal also increased with increasing stream order/river size. However, removal was generally of small magnitude, with up to 1% or 19% of the watershed load removed for median or 90th-percentile removal rates, respectively. We estimated removal for our restoration scenarios under the Chesapeake Bay Program Protocols and found the removal rate to also be a critical factor in determining the efficiency of restoration project. Other controlling factors for nitrate removal were the amount of restoration and storm size. The second study entailed modeling cumulative restoration in a case study watershed to assess the impacts on nutrient removal and flood attenuation. We built a 1D HEC-RAS model of the 4th-order Gwynns Falls watershed near Baltimore MD using georeferenced HEC-RAS model geometries from the Maryland Department of the Environment and simulated unsteady stormflow hydraulics due to cumulative Stage-0 floodplain restoration for small and sub-annual storms. Restoration actually increased peak flow on the main channel (up to 0.9%) due to slowing of the flood wave on the main channel which was then better synchronized with tributary inflows. Restoration increased nitrate removal but at low levels (up to 0.12% or 2.6% removal for a median and 90th-percentile removal rate respectively) due to the small footprint of restoration in the watershed (up to 21.4% of the main channel was restored). These small and sometimes adverse outcomes occurred in response to what would be expensive restoration. Therefore, we argue for large-scale solutions to address watershed-scale water quality and flooding issues yet acknowledge re-evaluation of restoration goals against other societal priorities may be necessary. Overall, our results highlight the potential value and limitations of floodplain restoration in reducing flooding and nitrate exports at the channel network scale and provide practical insight for application of floodplain modeling at the watershed scale. / Master of Science / Human land use practices such as building cities and farms adds nutrients (nitrogen and phosphorus) and increase storm flows in rivers downstream. While nutrients and flows are needed for humans and wildlife, too much of either can harm aquatic organisms and endanger people and property. Floodplain restoration is a common river engineering technique that increases exchange between the river channel and low-lying areas next to rivers known as floodplains. Floodplains are natural features, but people have reduced river flows between channels and floodplains in many ways. For example, by allowing sediments to build up in floodplains or building levees that separate channels from adjacent floodplains. Increasing floodplain exchange by floodplain restoration is commonly implemented to improve water quality and reduce the impact of flooding in watersheds, which are large areas that drain to a single river. However, while the goals of restoration are often at watershed scales, the effect of multiple restoration projects at that watershed scale is not well studied. We addressed this knowledge gap by two studies. The first study evaluated the impact of multiple restoration projects and project locations in a generic (average/typical) watershed on nitrate removal. We used a nitrate removal model and the results from a prior study that modeled the stormflow behavior resulting from floodplain restoration. Results indicated that watershed nitrate removal varied depended on the location of restoration in the watershed and where removal was evaluated. The most nitrate was removed where restoration occurred, with less removal downstream in the watershed. Removal also increased with increasing river size. However, removal was generally small with up to 1% or 19% of the watershed load removed for a smaller and larger nitrate removal rate, respectively. Other factors that changed the amount of nitrate removed were the amount of restoration, nitrate removal rate in the floodplains, and storm size. The second study entailed modeling cumulative restoration in a case study watershed to assess the impacts on nitrate removal and reducing flooding. We modeled stormflow for multiple hypothetical restoration projects in the Gwynns Falls watershed and found that restoration can actually increase peak flow when placed in certain locations. Restoration increased removal but at low levels (up to 0.12% or 2.6% for a smaller and larger removal rate) due to the small amount of restoration simulated. These small and sometimes adverse outcomes occurred in response to what would be expensive restoration projects to construct. Therefore, we argue for large-scale solutions to address watershed-scale water quality and flooding issues yet acknowledge that re-evaluation of restoration goals against other societal priorities may be necessary. Overall, our results highlight the potential value and limitations of using floodplain restoration to reduce flooding and nutrient exports and provide practical insight for using our modeling techniques in managing watershed flows and pollution.

Severe flooding and excess nutrient pollution, exacerbated by heightened anthropogenic pressures (e.g., climate change, urbanization, land use change, unsustainable agricultural practices), have been detrimental to riverine systems and their estuaries. The degradation of riverine systems can negatively impact human and environmental health, as well as local, regional, and even global economies. Floods provide beneficial ecosystem services (e.g., processing pollutants, transferring nutrients and sediment, supporting biodiversity), but they can also damage infrastructure and result in the loss of human life. Meanwhile, eutrophication can cause anoxic dead zones, harming aquatic ecosystems and public health. To address the issues facing riverine systems, focus has shifted to watershed-scale management plans. However, it can prove challenging to quantify the cumulative impacts of multiple stream restoration projects within a single watershed on flooding and nutrient removal. Previous studies have quantified the effects of stream restoration on flood attenuation. However, our first study fills a substantial knowledge gap by evaluating the impacts of different floodplain restoration practices, varied by location and length, on flood attenuation and floodplain inundation dynamics at the watershed scale during more frequent storm recurrence intervals (i.e., 2-year, 1-year, 0.5-year, and monthly). We created a 1D HEC-RAS model to simulate the effects of Stage 0 restoration within a 4th-order generic watershed based on the Chesapeake Bay watershed. By varying the percent river length restored and location, we found that Stage 0 restoration, especially in 2nd-order rivers, can be particularly effective at enhancing flood attenuation and floodplain inundation locally and farther downstream. We addressed the water quality component by using a random forest machine learning approach coupled with artificial neural networks to find trends and predict nitrate removal rates associated with spatial, temporal, hydrologic, and restoration features. Our results showed that hydrologic conditions were the most important variable for predicting actual nitrate removal rates. Overall, both studies demonstrate the importance of hydrologic connectivity for flood attenuation, channel-floodplain exchange, and nutrient processing. / Maryland Department of Natural Resources; National Fish and Wildlife Foundation through the U.S. Environmental Protection Agency’s Chesapeake Bay Program Office; Chesapeake Bay Trust / Master of Science / Severe flooding and nutrient pollution from sources such as urban and agricultural runoff have been detrimental to the health of rivers. The degradation of rivers can negatively impact human and environmental health, as well as local, regional, and even global economies. Floods can be both helpful, by providing water quality benefits and supporting wildlife, and harmful, causing damage and even loss of life. Excess nutrients, such as nitrogen, can create underwater zones void of life, with serious consequences for aquatic life and public health. To address the flooding and water quality issues facing rivers, focus has shifted to landscapelevel river network management plans. However, it can prove challenging to understand the impacts of multiple stream restoration projects within a larger river network on flooding and nutrient removal. We address the flooding component by using a model to simulate the effects of different floodplain restoration techniques on a medium-sized watershed that is generally based on streams that flow into the Chesapeake Bay. Our model simulated small, relatively frequent storm events that, on average, occur every two years to once a month. By varying restoration length and location, we found that restoration practices with lower streambanks can be particularly effective at slowing down floods, reducing their overall severity by allowing more water to access the floodplains. This was especially true when restoration occurred in smaller streams, and the effects were seen both locally and farther downstream. We address the water quality component by using a different model to find patterns and predict nutrient removal rates associated with different landscape, seasonal, storm event, and restoration features. Our results showed that the most important variable for predicting nutrient removal rates was whether a stream was experiencing normal flow or stormflow conditions. Overall, both studies demonstrate the importance of restoring rivers in a manner that encourages water to flow from the channel into the floodplains during smaller storm events, because this will reduce the severity of downstream flooding while simultaneously improving water quality.