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Characterizing Multi-Decadal Trends in Streamflow and Design Floods in the Southeastern United States By Sarah Brannum Senior Honors Thesis Department of Geological Sciences University of North Carolina at Chapel Hill April 29, 2021 Approved: ____________________________ Dr. Antonia Sebastian, Thesis Advisor Characterizing Multi-decadal Trends in Streamflow and Design Floods in the Southeastern United States Sarah Brannum Abstract Design floods serve an important role in environmental planning and management; however, flood frequency analyses often assume that historical records are stationary despite anthropogenic changes across the watershed. Such changes are especially prevalent in the southeastern United States where rapid population growth coupled with climate change is dramatically altering catchment response. To understand how design floods have changed over time, this study investigates historical records of daily river discharge at over 5,800 USGS gauges in the Southeastern USA. When looking at trends in daily discharges, we find that 40% experienced a significant (p<0.05) increase in daily discharge, 50% experienced significant decrease, and 10% experienced no significant change in daily discharge. We observe that, in general, gages exhibiting increasing daily discharge are spatially concentrated east of the Appalachian Mountains and gages exhibiting decreasing daily discharge are spatially concentrated to the west. When looking at trends in extreme discharges using both the annual maxima and a peak-over-threshold with discharges above the 90th and 99th percentiles, we find that less than 20% of gages had a significant trend in the extreme discharges. We then conducted a flood frequency analysis at gages with the most significant (positive and negative) trends in daily discharge to estimate the 25- 50- and 100-year design flows using the Gumbel distribution based. We compare the modeled results using the annual maxima over the entire period of record and for every 30-year segment of the record. For all return periods, we find that about 75% of gages experienced a significant trend, with 50% of gages experiencing an increasing trend in the magnitude of the design flood. Future work will look to attribute our observed trends to changes in the watershed including urbanization and climate change. Understanding non-stationary trends in river discharge, especially in coastal zone, is critical for local government planning and adaptation in light of changing coastal hazards. 1 1. Introduction Floods are the natural disaster that occur at the highest frequency and can cause damage and loss of life almost anywhere around the world (UNISDR 2017). The Southeastern United States (U.S.) is particularly vulnerable to flooding due to risk of both tropical and wave cyclones, as well as orographic lifting from the Appalachian Mountains which can cause extreme rain events on the eastern side of the mountains (Lecce 2000). In fact, recent tropical cyclone events, including Hurricanes Matthew (2016), Irma (2017), and Florence (2018), as well as major precipitation events, have demonstrated the vulnerability of Southeastern U.S. communities to flooding and the catastrophic damage that can ensue (Smith 2020). Because floods can be so costly, there is a critical need to accurately quantify flood risk and plan for future events based on that risk. Scientists and engineers use models to understand and quantify flood hazards, which can provide local governments with information needed to best protect their communities. These models are often based on historical events and their frequencies in order to gauge how likely these events will occur in the future. However, if flood hazards are changing over time, not accounting for this change could lead to false assumptions about the frequency of major flood events and undermine preparedness, leaving communities vulnerable to flood damage (Beighley and Moglen, 2002). Thus, quantifying and effectively communicating flood hazards and how they are changing is vital for community resilience and has the potential to save both money and lives. It is important to observe and quantify trends in river discharge to understand and predict future flood events and recognize the characteristics that impact the damage a future flood event may have. One useful approach is to evaluate whether the magnitude of the design flood has changed over time (Blum et al. 2020). A design flood is a flood of a certain magnitude that has a specific probability of occurring (FEMA). The magnitude of the design flood can tell city planners how close to have development to a river, insurance rates, or how at risk a building is to flooding given an event of a particular return period (FEMA). The 100-year-flood is the primary design flood used by government agencies because it is severe enough to cause damage, but statistically not likely to occur in a given year (FEMA). Government flood standards use the 100-year-flood in order to avoid frequent flood exposure but avoid setting a superfluous standard that limits any development along rivers (FEMA). If the magnitude of the 100-year-flood or other design floods is changing, then flood risk for certain locations could be underestimated (Blum et al. 2020). Recent studies have begun to analyze how the magnitude of the design flood has changed (Griffin et al 2019; Slater et al 2021). For example, Griffin et al (2019) studied how the magnitude of the 30- and 50-year design flood has changed at stream gages in the UK using a moving window approach. They found that a sizeable fraction of gages showed trends in the magnitude of a specific design flood. Although this paper was specific to the UK, many of the characteristics that affect streamflow persist around the world, making it plausible that other areas could have trends in the magnitude of the design flood. Slater et al. (2021) performed a similar analysis on discharges at the global scale. Globally, for the 20- and 50-year return periods, they found that the magnitude of the design flood increased in temperate zones but decreased in arid, tropical, polar, and cold zones. However, for the 100-year return periods, they found decreased magnitudes in arid and temperate zones and an increase in tropical areas. They note that differences could also be attributed to site record length. For the Southeastern U.S. specifically, Slater et al. 2021 found no overarching spatial trend in how the magnitude of the 20- 2 , 50-, or 100- year design floods have changed. However, since this study was performed for the global scale, the finer details are often neglected in favor of a discussion the larger trends observed. In addition, several prior studies have investigated trends across stream gages in the continental U.S. in daily discharge (Sagarika et al 2014; Rice et al 2015; Rice et al 2016; Boggs et al 2011), annual maxima (Berg et al 2018; Villarini et al 2016; Aryal et al 2018), and extreme events above a specific threshold or discharge percentile (Slater et al 2016; Neri et al 2020; Lins et al 1999; Debbage et al 2018; Aryal et al 2018). Studies which focus on median, low-percentile events, or mean discharge have found statistically significant increases in discharge (Patterson et al 2013; Naz et al 2018; Sagarika et al 2014) whereas studies which analyze extreme events, or high discharge scenarios, reach conclusions which are more nuanced, with some studies finding increases in the magnitudes of extreme discharge events (Naz et al 2018; Groisman et al 2001) and others finding decreases or no change in the magnitudes of extreme discharge events (Lins et al 1999; Rice et al 2015; Blum et al 2020). In general, previous studies have attributed changes in discharge (whether positive or negative) to anthropogenic factors such as urbanization and climate change. Discharge can change due to a variety of factors affecting the hydrologic cycle or hydraulic response at any given river station. It is well understood that changes in urbanization, precipitation, land use/land cover, climate, and antecedent rainfall conditions all contribute to changes in streamflow (Neri et al 2020). For example, urbanization can lead to increases in impervious surfaces which results in increased in runoff volume and reduced time to peak (Rose and Peters, 2011; Mongollón et al 2016; Beighley et al 2002). Similarly, global warming associated with anthropogenic climate change has led to increases in precipitation intensity in certain places which can drive higher streamflows during storm events (Sagarika et al 2014). However, interactions between urbanization and observed increases in precipitation are highly non-linear, leading to even more complex streamflow responses (Sebastian et al. 2019). While it is difficult to attribute trends in streamflow to one of these variables, and it is equally difficult to predict how changes in any one of these variables will impact streamflow. It is vitally important to understand where these changes are occurring and the magnitude of the change. In this study, we collect daily discharge records at over 5,000 U.S. Geological Survey (USGS) gages at locations in the Southeastern U.S. and use them to quantify both historical trends in river discharges and the magnitude of the design flood. We used this data in order to see if there are trends in daily discharges, extreme discharges, and in the magnitude of the design flood. Additionally, we hope to see if there are any spatial trends in gages that have trends in discharges. In performing these analyses, we hope that this study will contribute to the conversation around flood risk and management in the Southeastern U.S. 2. Background Flood risk is a function of hazard, vulnerability, and exposure (UNISDR 2017). The hazard is the depth or velocity of water caused by either extreme rainfall, dam failure, coastal storm surge, or other events (UNSIDR 2017). Exposure refers to the presence of development and people that are susceptible to flood damages, while vulnerability refers to the susceptibility of infrastructure and people to flooding (Kron 2005).

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