Geology topics

Allen Gellis

Publications and source records attributed to Allen Gellis.

3 recordsLinked to original sources

USGS2014

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

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.

Maryland, Pennsylvania, Virginia↗

USGS2012

Developing a new stream metric for comparing stream function using a bank-floodplain sediment budget: a case study of three Piedmont streams

A bank and floodplain sediment budget was created for three Piedmont streams tributary to the Chesapeake Bay. The watersheds of each stream varied in land use from urban (Difficult Run) to urbanizing (Little Conestoga Creek) to agricultural (Linganore Creek). The purpose of the study was to determine the relation between geomorphic parameters and sediment dynamics and to develop a floodplain trapping metric for comparing streams with variable characteristics. Net site sediment budgets were best explained by gradient at Difficult Run, floodplain width at Little Conestoga Creek, and the relation of channel cross-sectional area to floodplain width at Linganore Creek. A correlation for all streams indicated that net site sediment budget was best explained by relative floodplain width (ratio of channel width to floodplain width). A new geomorphic metric, the floodplain trapping factor, was used to compare sediment budgets between streams with differing suspended sediment yields. Site sediment budgets were normalized by floodplain area and divided by the stream's sediment yield to provide a unitless measure of floodplain sediment trapping. A floodplain trapping factor represents the amount of upland sediment that a particular floodplain site can trap (e.g. a factor of 5 would indicate that a particular floodplain site traps the equivalent of 5 times that area in upland erosional source area). Using this factor we determined that Linganore Creek had the highest gross and net (floodplain deposition minus bank erosion) floodplain trapping factor (107 and 46, respectively) that Difficult Run the lowest gross floodplain trapping factor (29) and Little Conestoga Creek had the lowest net floodplain trapping factor (–14, indicating that study sites were net contributors to the suspended sediment load). The trapping factor is a robust metric for comparing three streams of varied watershed and geomorphic character, it promises to be a useful tool for future stream assessments.

Piedmont;Difficult Run;Little Conestoga Creek;Ling↗

USGS2011

Anthropocene streams and base-level controls from historic dams in the unglaciated mid-Atlantic region, USA

Recently, widespread valley-bottom damming for water power was identified as a primary control on valley sedimentation in the mid-Atlantic US during the late seventeenth to early twentieth century. The timing of damming coincided with that of accelerated upland erosion during post-European settlement land-use change. In this paper, we examine the impact of local drops in base level on incision into historic reservoir sediment as thousands of ageing dams breach. Analysis of lidar and field data indicates that historic milldam building led to local base-level rises of 2–5 m (typical milldam height) and reduced valley slopes by half. Subsequent base-level fall with dam breaching led to an approximate doubling in slope, a significant base-level forcing. Case studies in forested, rural as well as agricultural and urban areas demonstrate that a breached dam can lead to stream incision, bank erosion and increased loads of suspended sediment, even with no change in land use. After dam breaching, key predictors of stream bank erosion include number of years since dam breach, proximity to a dam and dam height. One implication of this work is that conceptual models linking channel condition and sediment yield exclusively with modern upland land use are incomplete for valleys impacted by milldams. With no equivalent in the Holocene or late Pleistocene sedimentary record, modern incised stream-channel forms in the mid-Atlantic region represent a transient response to both base-level forcing and major changes in land use beginning centuries ago. Similar channel forms might also exist in other locales where historic milling was prevalent.

Delaware, Maryland, Newe Jersey, New York, Pennsyl↗