COMPARING METHODS OF QUANTIFYING RAPID TOPOGRAPHIC CHANGES

Land disturbance at construction sites exposes bare soil to erosive agents, so erosion rates in construction areas are 2-40,000 times greater than pre-construction and agricultural rates. Eroded sediments and associated nutrients impact water quality and alter stream ecosystems by blanketing streambeds, contributing to algal blooms, limiting photosynthesis due to increased turbidity, and abrading aquatic organisms. Much of the erosion at construction sites occurs in the temporary diversion ditches designed to direct runoff into catchment basins while construction is in progress. Current methods to quantify sediment loss from temporary diversion ditches involve measuring the turbidity, total suspended solids (TSS), and total sediment of the water flowing out of the ditches. Physical changes in the ditches are described using incremental measurements of ditch cross-sectional profile, which are used to calculate the area eroded. These methods are time-consuming, tedious, and limited in describing the spatial extent of sediment loss. Using Light Detection and Ranging (LiDAR) and point clouds derived from Unmanned Aerial Vehicle (UAV) and ground-based imagery, erosion in artificial ditches as well as ditches at an active construction site was quantified by calculating changes in ditch volumes. Runoff flow paths at the construction site were computed using ArcGIS to determine the effectiveness of ditch placement. Photogrammetric and remote sensing methods are valuable when measuring rapid sediment loss, as they enable spatial representation of topographic changes and identification of areas at high risk of erosion. UAV and ground-based imagery are especially promising as low-cost and spatially accurate erosion quantification methods. Remote sensing and photogrammetry may be useful both when measuring small-scale topographic changes and when determining the effectiveness and best placement of sediment control practices.