Storms, channel changes, and a sediment budget for an urban-suburban stream, Difficult Run, Virginia, USA

Abstract

Determining erosion and deposition rates in urban-suburban settings and how these processes are affected by large storms is important to understanding geomorphic processes in these landscapes. Sediment yields in the suburban and urban Upper Difficult Run are among the highest ever recorded in the Chesapeake Bay watershed, ranging from 161 to 376 Mg/km 2 /y. Erosion and deposition of streambanks, channel bed, and bars and deposition of floodplains were monitored between 1 March 2010 and 18 January 2013 in Upper Difficult Run, Virginia, USA. We documented the effects of two large storms, Tropical Storm Lee (September 2011), a 100-year event, and Super Storm Sandy (October 2012) a 5-year event, on channel erosion and deposition. Variability in erosion and deposition rates for all geomorphic features, temporally and spatially, are important conclusions of this study. Tropical Storm Lee was an erosive event, where erosion occurred on 82% of all streambanks and where 88% of streambanks that were aggrading before Tropical Storm Lee became erosional. Statistical analysis indicated that drainage area explains linear changes (cm/y) in eroding streambanks and that channel top width explains cross-sectional area changes (cm 2 /y) in eroding streambanks and floodplain deposition (mm/y). A quasi-sediment budget constructed for the study period using the streambanks, channel bed, channel bars, and floodplain measurements underestimated the measured suspended-sediment load by 61% (2130 Mg/y). Underestimation of the sediment load may be caused by measurement errors and to contributions from upland sediment sources, which were not measured but estimated at 36% of the gross input of sediment. Eroding streambanks contributed 42% of the gross input of sediment and accounted for 70% of the measured suspended-sediment load. Similar to other urban watersheds, the large percentage of impervious area in Difficult Run and direct runoff of precipitation leads to increased streamflow and streambank erosion. This study emphasizes the importance of streambanks in urban-suburban sediment budgets but also suggests that other sediment sources, such as upland sources, which were not measured in this study, can be an important source of sediment.

Freeze–thaw processes and intense rainfall: the one-two punch for high sediment and nutrient loads from mid-Atlantic watersheds

“…Since fine sediments (silt and clay) are particularly susceptible to effects of freeze thaw (Couper 2003;Wynn and Mostaghimi 2006), fine-grained millpond legacy sediments along the streambanks, as in case of our study sites, would especially be vulnerable to this erosion. Large contributions of streambank sediments to suspended sediment exports have previously been reported for Piedmont watersheds (Gellis et al , 2017Voli et al 2013). Gellis et al (2017) reported that eroding streambanks accounted for as much as 70% of the suspended sediment fluvial exports from tropical storms Lee and Sandy.…”

“…Freeze-thaw cycles cause bank sediments to lose their cohesive strength with subsequent detachment and slumping/collapse at the base of the streambank (Merritts et al 2013;Wolman 1959;Wynn 2006). Freeze-thaw erosion (Couper 2003;Lawler 1993a, b;Merritts et al 2011Merritts et al , 2013Wolman 1959) along with fluvial erosion with large storms (Gellis et al 2017) and mass wasting of stream banks (Fox et al 2016) have been hypothesized as some of the important mechanisms for erosion of stream-bank sediments. Wynn and Mostaghimi (2006) have also shown that the number of freeze-thaw cycles has a cumulative destabilizing effect on streambank sediments.…”

“…Sediment and particulate nutrient inputs to water bodies are typically at their highest during storm events Gellis et al 2017;Jeong et al 2012;Jung et al 2012). The largest and most intense storms, particularly those associated with hurricanes and tropical depressions, have been found to contribute a substantial proportion of the annual sediment and nutrient export from watersheds Inamdar 2013, 2014;Inamdar et al 2015;Gellis et al 2017).…”

“…The largest and most intense storms, particularly those associated with hurricanes and tropical depressions, have been found to contribute a substantial proportion of the annual sediment and nutrient export from watersheds Inamdar 2013, 2014;Inamdar et al 2015;Gellis et al 2017). Climate data from the past 50 years for the northeast USA and projections for the future indicate an intensification of the top 1% of the storm events (Melillo 2014).…”

“…Fluvial erosion with large storm flows (Gellis et al 2017); freeze-thaw activity (Couper, 2003;Lawler 1993a, b;Merritts et al 2011Merritts et al , 2013Wolman 1959;Wynn and Mostaghimi 2006) during winter and/or desiccation and cracking (Lyons et al 2015) in summer; and mass wasting (Fox et al 2016) have been hypothesized as some of the important mechanisms for erosion of stream-bank sediments. Freezethaw cycles cause bank sediments to lose their cohesive strength with subsequent detachment and slumping/collapse at the base of the streambank (Merritts et al 2013;Wolman 1959;Wynn 2006).…”