Michael A. O'Neal

Increased mid-twentieth century riverbank erosion rates related to the demise of mill dams, South River, Virginia

A recent hypothesis suggests that fluvial processes in areas of the eastern United States are strongly influenced by the demise of colonial mill dams, rather than reflecting a quasi-equilibrium adjustment to the current hydrologic and sediment regime. We evaluated the control of colonial mill dams on twentieth and twenty-first century bank erosion rates on the South River, Virginia, through studies of historical aerial photographs, historical documents, hydrologic and climatic records, and hydraulic modeling. Historical sources and aerial photographs document eight colonial mill dams along the study reach in the early twentieth century; all but one of these dams disappeared in the 1950s, and the last was breached by 1976. From initially low values between 1937 and 1957, mean bank erosion rates increased by more than a factor of 2 after 1957, remaining high through 2005. Accelerated bank erosion rates cannot be explained by changes in storm intensity, the frequency of freeze-thaw cycles, or by changes in the density of riparian trees. Hydraulic modeling suggests that mill dams reduced velocities of the 5 yr flood through ~80% of our study reach. By considering the timing of mill dam loss, the spatial extent of backwater influence, and the locations of our study sites, we find that the loss of mill dams explains the observed trends in bank erosion rates at 9 (and possibly 10) of our 14 monitoring sites. These results support the hypothesis that the demise of mill dams has been an important influence on fluvial processes in the region.

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

[1] 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.

Impact of topography on groundwater salinization due to ocean surge inundation

Sea-level rise and increases in the frequency and intensity of ocean surges caused by climate change are likely to exacerbate adverse effects on low-lying coastal areas. The landward flow of water during ocean surges introduces salt to surficial coastal aquifers and threatens groundwater resources. Coastal topographic features (e.g., ponds, dunes, barrier islands, and channels) likely have a strong impact on overwash and salinization processes, but are generally highly simplified in modeling studies. To understand topographic impacts on groundwater salinization, we modeled a theoretical overwash event and variable-density groundwater flow and salt transport in 3-D using the fully coupled surface and subsurface numerical simulator, HydroGeoSphere. The model simulates the coastal aquifer as an integrated system considering overland flow, coupled surface and subsurface exchange, variably saturated flow, and variable-density groundwater flow. To represent various coastal landscape types, we simulated both synthetic fields and real-world coastal topography from Delaware, USA. The groundwater salinization assessment suggested that the topographic connectivity promoting overland flow controls the volume of aquifer that is salinized. In contrast, the amount of water that can be stored in surface depressions determines the amount of seawater that infiltrates the subsurface and the time for seawater to flush from the aquifer. Our study suggests that topography has a significant impact on groundwater salinization due to ocean surge overwash, with important implications for coastal land management and groundwater vulnerability assessment.

Estimating Land Cover-Induced Increases in Daytime Summer Temperatures Near Mt. Adams, Washington

A wealth of 20th-century instrumental data indicates that daytime near-surface temperatures differ greatly between clear-cut and mature forest stands, decreasing approximately linearly with stand height and density. Although these temperatures are easily observed in individual stands, extrapolating the net temperature effect to an actively logged terrain is difficult due to the heterogeneous mixture of different forest stand heights and densities. Previous work has related the percentage of tree shade observed in remotely sensed imagery to forest stand structure; we further developed this proxy to derive an estimate of the near-surface daytime air temperature during the summer months. This temperature proxy can be integrated across a region using both actual shade from the modern landscape and an idealized data set representing old-growth forest. In our 10 × 20 km study area on Mt. Adams, Washington, the temperature difference between these two data sets implies that as much as 0.6°C of recent near-surface air temperature rise may be attributed to land cover change. Our results demonstrate that differences in regional temperatures caused by industrial-scale logging are large enough to play a significant role in other physical system responses, such as the retreat of alpine glaciers near logged forests. Because industrial-scale logging is globally pervasive and operates in the same timescale and direction as global warming trends, forcings due to land cover change may have been heretofore overlooked as a cause of the retreat of some glacier systems.

Identifying lichenometrically datable, glacierized terrains: a case study in the cascade range of western North America

Climatic interpretations of recent glacier fluctuations rely on ice-extent chronologies developed from lichenometric ages of Holocene landforms. However, lichenometry requires time- and resource-consuming field surveys, which limit our understanding of glacier chronologies, especially in remote locations. This study presents a rapid, coarse, a priori approach to predicting new field sites where lichenometry can be applied. Geologic, geographic, climatic, and landcover data were used in spatial and supervised classification analyses to identify areas in the Cascade Range of Washington and northern Oregon with similar environmental conditions to those where lichenometric dating techniques had previously been applied. These results focus the attention of researchers to only 1100 km2, or 3%, of the broader Cascade Range study area. Though this study concentrates on the utility of lichenometry for dating recent glacier activity in the Cascade Range, the screening method presented is easily translatable to a variety of geomorphic and environmental applications.

Subpixel roughness effects in spectral thermal infrared emissivity images

Emissivity spectra recovered from spectral radiance images may have lowered spectral contrast due to irradiance from nearby surface elements (“cavity effect”). For analyses based only on photointerpretation or Reststrahlen band identification, it is not always necessary to account for cavity effects, but for full spectral analyses, including spectral unmixing, it may be desirable. We present a method that is under development for compensating thermal infrared images for cavity radiation based on optical estimates of surface roughness and model inversion for percent cavity contribution. The approach is adaptable for different spectral resolutions, including hyperspectral. It will be tested on tripod-mounted Telops HyperCam hyperspectral thermal-infrared images of natural targets, LiDAR DEMs of similar targets, and optical estimates of shadowing, related to roughness.