Characteristic length scales and time-averaged transport velocities of suspended sediment in the mid-Atlantic Region, USA

Abstract

Watershed Best Management Practices (BMPs) are often designed to reduce loading from particle-borne contaminants, but the temporal lag between BMP implementation and improvement in receiving water quality is difficult to assess because particles are only moved downstream episodically, resting for long periods in storage between transport events. A theory is developed that describes the downstream movement of suspended sediment particles accounting for the time particles spend in storage given sediment budget data (by grain size fraction) and information on particle transit times through storage reservoirs. The theory is used to define a suspended sediment transport length scale that describes how far particles are carried during transport events, and to estimate a downstream particle velocity that includes time spent in storage. At 5 upland watersheds of the mid-Atlantic region, transport length scales for silt-clay range from 4 to 60 km, while those for sand range from 0.4 to 113 km. Mean sediment velocities for silt-clay range from 0.0072 km/yr to 0.12 km/yr, while those for sand range from 0.0008 km/yr to 0.20 km/yr, 4-6 orders of magnitude slower than the velocity of water in the channel. These results suggest lag times of 100-1000 years between BMP implementation and effectiveness in receiving waters such as the Chesapeake Bay (where BMPs are located upstream of the characteristic transport length scale). Many particles likely travel much faster than these average values, so further research is needed to determine the complete distribution of suspended sediment velocities in real watersheds.

Model predictions of long-lived storage of organic carbon in river deposits

“…In this way, x tran represents the length of channel required to exchange the entire sediment flux with river deposits via lateral migration. This definition of x tran is comparable to previous studies (Malmon et al, 2003;Lauer and Parker, 2008b;Pizzuto et al, 2014) and was calculated with the equation…”

“…Following previous approaches (e.g, Malmon et al, 2003;Lauer and Parker, 2008b;Pizzuto et al, 2014), we defined a characteristic length scale over which eroded sediment particles are transported before being re-deposited. While particles are transported variable distances (depending for example on particle size and current velocity), we made the simplifying assumption that the spread of the distribution of transport lengths is small relative to the mean transport length.…”

“…This simplified metric captures the expected effects of sediment supply (Constantine et al, 2014) and channel length (Malmon et al, 2003;Lauer and Parker, 2008b;Pizzuto et al, 2014) on the duration of sediment storage in river deposits and is useful for comparing systems where more direct measurements of sediment ages are unavailable. We note that field estimates of sediment fluxes are often imperfect (e.g., Kirchner et al, 2001).…”

Integrated Environmental Modelling: human decisions, human challenges

“…As another example, integrated environmental watershed models of water, sediment and nutrient inputs to the Chesapeake Bay have, generally, inadequately represented groundwater processes. The models have not accounted for the decadal timescales of groundwater processes (Sanford & Pope 2013), or for the decadal to millennial timescales of sediment processes (Pizzuto et al 2014). As a result, the IEM watershed models for the Chesapeake Bay watershed have, for the most part, ignored the response times required for best management practices and other efforts to limit sediment and nutrient transport to the Bay.…”

Sediment contributions from floodplains and legacy sediments to Piedmont streams of Baltimore County, Maryland

“…1), may be an ongoing dominant source of sediment in watersheds caused by increased runoff and reduced upland sediment supply subsequent to historical peaks of agricultural land use (Colosimo and Wilcock, 2007). Despite evidence that floodplains (Church and Slaymaker, 1989;Allmendinger et al, 2007;Voli et al, 2013) and legacy sediments (Walter et al, 2007;Hupp et al, 2013) are large components of the total sediment load delivered from streams, mobilized sediment may not exit Piedmont watersheds for 6 to 10 millennia (Pizzuto, 2001;Jackson et al, 2005;Pizzuto et al, 2014).…”