Recent Changes in Stream Flashiness and Flooding, and Effects of Flood Management in North Carolina and Virginia

Abstract

The southeastern United States has undergone anthropogenic changes in landscape structure, with the potential to increase (e.g., urbanization) and decrease (e.g., reservoir construction) stream flashiness and flooding. Assessment of the outcome of such change can provide insight into the efficacy of current strategies and policies to manage water resources. We (1) examined trends in precipitation, floods, and stream flashiness and (2) assessed the relative influence of land cover and flow‐regulating features (e.g., best management practices and artificial water bodies) on stream flashiness from 1991 to 2013. We found mean annual precipitation decreased, which coincided with decreasing trends in floods. In contrast, stream flashiness, overall, showed an increasing trend during the period of study. However, upon closer examination, 20 watersheds showed stable stream flashiness, whereas 5 increased and 6 decreased in flashiness. Urban watersheds were among those that increased or decreased in flashiness. Watersheds that increased in stream flashiness gained more urban cover, lost more forested cover and had fewer best management practices installed than urban watersheds that decreased in stream flashiness. We found best management practices are more effective than artificial water bodies in regulating flashy floods. Flashiness index is a valuable and straightforward metric to characterize changes in streamflow and help to assess the efficacy of management interventions.

The complexities of urban flood response: Flood frequency analyses for the Charlotte metropolitan region

“…The population density reached 746 km −2 in 2015, with an increase of 12.4% from 2010 to 2015 [ United States Census Bureau , ]. Extensive urban development has contributed to changing flood risks in the Charlotte Metropolitan region [ Smith et al ., ; Villarini et al ., ; Bell et al ., ; Mogollón et al ., ; Smith and Smith , ]. A series of major floods during the 1990s [ Smith et al ., ; Turner‐Gillespie et al ., ; Villarini et al ., ; Wright et al ., ] resulted in changes to flood mitigation strategies for the region.…”

“…The nature and history of storm water management practices have large impacts on urban flood response [ Brander et al ., ; Carter and Jackson , ; Gilroy and McCuen , ; Loperfido et al ., ; Bhaskar et al ., ; Jarden et al ., ; Mogollón et al ., ]. Stormwater detention structures are typically designed to directly and locally impact flashiness of flood response.…”

Urbanization in Arid Central Arizona Watersheds Results in Decreased Stream Flashiness

“…Since we had limited information on SCM installation year to estimate the percent SCM area for each year, we used a simpler linear regression model where each input metric was based on the mean of the 14-year period. To enable comparison between our results and those from another geographic region (Mogollon et al, 2016), we also examined the relationship between R-B Flashiness and development, rather than imperviousness.…”

How Effective Are Retention Ponds at Mitigating the Effects of Urban Runoff?

“…Creek seen today could be a result of watershed urbanization. A larger magnitude of shear stress and stream power, driven by the increased volume of runoff being discharged from the retention pond, could have resulted in a greater amount of sediment removal from this portion of the channel (Hammer, 1972;Arnold et al, 1982;Booth, 1990;Booth and Jackson, 1997;Booth et al, 2002;Walsh et al, 2005;Nelson et al, 2006;Hancock et al, 2010;Tillinghast et al, 2011;Mogollón et al, 2016;Bevan et al, 2018). This display of channel down-cutting is a commonly observed phenomenon in channels with urbanized watersheds (Hammer, 1972;Booth, 1990;Booth and Jackson, 1997;Walsh et al, 2005;Nelson et al, 2006;Tillinghast et al, 2011;Bevan et al, 2018).…”

“…For example, in their evaluation of BMPs in King County, Washington, Booth et al (2002) state, "there is no evidence that [BMPs] can mitigate any but the most egregious consequences of urbanization." Even with the construction of a retention pond, urbanized watersheds exhibit larger volumes of runoff moving through their systems and have flashier hydrographs in comparison to natural, non-urbanized watersheds (Walsh et al, 2005;Nelson et al, 2006;Hester and Bauman, 2013;Mogollón et al, 2016). As displayed in Figure 1, urbanized hydrographs (both with and without a retention pond) display higher peak discharges and shorter centroid lag times compared to the hydrograph of a stream draining a forested watershed (Hammer, 1972;Arnold et al, 1982;Leopold, 1986;Booth and Jackson, 1997;Fennessey et al, 2001;Booth et al, 2002;Emerson et al, 2005;Walsh et al, 2005;Nelson et al, 2006;Hancock et al, 2010;Tillinghast et al, 2011;Hester and Bauman, 2013).…”