Characterizing Trends in Design Floods along Major Rivers in the Southeastern USA

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 along the U.S. Atlantic and Gulf Coastlines 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. At gage locations with records exceeding 30 years, 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. We then conducted a flood frequency analysis at gages with the most significant (positive and negative) trends in daily discharge to estimate the 5-, 10-, 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. Preliminary analysis suggests that gages with decreasing trends in daily discharge show a decreasing trend in the design flood, whereas gages with increasing overall discharge show no significant change in the design flood over time. This suggests that despite increases in daily discharge, there is no significant change in extreme flows over time. Future work will explore whether these same observations hold when applying alternative extreme value distributions (e.g., generalized pareto) to analyze trends in flood hazards. Understanding non-stationary trends in river discharge, especially in the coastal zone, is critical for local government planning and adaptation in light of changing coastal hazards.