Article

Using Tripod Mounted Lidar to Determine Riverbank Erosion Rates Along the South River, Virginia.

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Abstract

To determine the rate of supply of contaminated sediments introduced into the river from erosion, a program of bank surveys is underway along South River using ground LIDAR combined with more traditional methods of historical aerial photo analysis. During a pilot study, we surveyed two sites in January 2006. Each site consisted of a reach several hundred meters in length. Both banks of the river at each site were surveyed simultaneously. Each survey was completed in about 4 hours. Setting up equipment and around 10 targets at each site consumed most of the survey time. Three GPS (Global Positioning System) units accurate to within 0.01 m were used to georeference the LIDAR data. Over 2x106 data points were obtained at each site. Root- mean-square survey errors were all less than 0.007 m. The data provide an exceptional database for quantifying bank morphology. The survey instrument was setup so that points are spaced at scales of 0.01 m at distances of 100 m. At this scale very detailed morphological features are imaged, including overhangs, small protrusions, individual roots, and details of the riparian vegetation (including trunks and branch canopy structure). These data can be used to determine bank morphology and roughness caused by vegetation and bank irregularities. Repeat surveys will determine detailed spatial patterns of erosion along each reach. Ground LIDAR technology, therefore, has the potential to provide new insights to better understand bank morphology and related processes of erosion and deposition.

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Article
The highly stochastic nature of riverbank erosion has driven the need for spatially explicit empirical models. Detailed bank profile surveys along a meander bend of the Brandywine Creek in Pennsylvania, USA, before and after 28 high flow events over a 2·5 year period are used to develop an empirical model of cohesive bank profile erosion. Two hundred and thirty-six bank erosion observations are classified as hydraulic erosion or subaerial erosion. Threshold conditions required to initiate bank erosion cannot be defined based on field measurements. Using the near-bank velocity and the number of freeze–thaw cycles as predictors, regression equations are derived for hydraulic erosion that specify the length, thickness, and location on the bank face of eroded blocks. An empirical discriminant function defines the critical geometry of overhang failures, and the volumes removed by overhang failures are computed using another regression equation. All the regression equations are significant, but have low correlation coefficients, suggesting that cohesive bank erosion has a strong stochastic component. Individual events typically remove small masses of soil (average volume 0·084 m3/m) a few centimeters thick (median = 0·057 m) and a few decimeters in length (median = 0·50 m) from the lower third of the bank. Hydraulic erosion is responsible for 87% of all erosion. When applied to three survey sites not used in its development, the profile model predicts the total volume of erosion with errors of 23%, 5% and 1%. Twenty-four percent of computed erosion volumes for single events are within 50% of observed volumes at these three sites. Extending the approach to decadal timescales and to entire bends will require three-dimensional observations of bank failure, and spatially and temporally explicit methods to account for the influence of individual large trees on bank failures and near-bank hydraulic processes. Copyright © 2009 John Wiley & Sons, Ltd.
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