The Alaska Energy Authority (AEA) is preparing a license application that will be submitted to the Federal Energy Regulatory Commission (FERC) for the Susitna-Watana Hydroelectric Project. AEA needed a baseline set of data to provide context for predicting the likely extent and nature of potential changes to flow, sediment supply, and sediment transport capacity that will occur due to the project. Tetra Tech collected data to help provide an understanding of existing baseline geomorphic conditions, how geomorphic conditions and aquatic habitat change over a range of stream flows, and the stability of geomorphic conditions over recent decades. 

Tetra Tech, as part of the URS Corporation team, collected and processed high-resolution digital orthoimagery and airborne LiDAR for 557 square miles along the Susitna River corridor to support AEA’s fluvial geomorphology modeling study. Aerial photography, acquired with a DMC digital camera, included more than 3,000 images used to generate 1-foot-resolution orthoimagery.

Tetra Tech first evaluated a previously collected LiDAR dataset—the Matanuska-Susitna Borough LiDAR—that was acquired in 2011. After Tetra Tech and AEA’s evaluation of quality and accuracy, AEA requested that a higher density dataset be collected. The 2013 LiDAR was collected using an Optech ALTM Gemini Airborne Laser Terrain Mapper System; the 2014 LiDAR was collected using a Riegal VQ 480i at a density of >8 points per square meter (pts/sq m). Tetra Tech acquired LiDAR at low-flow conditions to maximize LiDAR point returns on the bare ground along the river. Low-flow levels were determined using data from three U.S. Geological Survey gage stations along the river.

Tetra Tech classified the LiDAR data and verified that the vertical and horizontal accuracies from collected ground checkpoints met ASPRS specifications. Our method maximized the utility of the imagery to inform the compiler’s placement of breaklines while relying on the LiDAR elevations for the vertices of the breaklines that were drawn. The resulting contours had the smooth appearance of traditional photogrammetric work but took full advantage of the vertical accuracy of the LiDAR. Deliverables included classified LiDAR point clouds, digital elevation models, 1-foot contours, and mosaicked orthoimagery at 30cm ground sample distance pixel resolution.

Tetra Tech also provided GIS services for geodatabase compilation related to the hydrographic studies. Major project components included providing digitization and geoprocessing to create new layers, creating and providing QA/QC for Federal Geographic Data Committee metadata, integrating new layers into existing geodatabases, and completing other GIS geodatabase management tasks in accordance with Alaska Department of Natural Resources standards.

Tetra Tech exceeded the client’s targeted resolution specifications of 8 points per square meter. This task was a huge success due to the steep cliffs and rock outcroppings of the river area, which presented an additional challenge for LiDAR data collection and processing.