Bathymetric and hydraulic survey of the Matanuska River near Circle View Estates, Alaska

Abstract

An acoustic Doppler current profiler interfaced with a differentially corrected global positioning system was used to map bathymetry and multi-dimensional velocities on the Matanuska River near Circle View Estates, Alaska. Data were collected along four spur dikes and a bend in the river during a period of active bank erosion. These data were collected as part of a larger investigation into channel processes being conducted to aid land managers with development of a long-term management plan for land near the river. The banks and streambed are composed of readily erodible material and the braided channels frequently scour and migrate. Lateral channel migration has resulted in the periodic loss of properties and structures along the river for decades. For most of the survey, discharge of the Matanuska River was less than the 25th percentile of long-term streamflow. Despite this relatively low flow, measured water velocities were as high as 15 feet per second. The survey required a unique deployment of the acoustic Doppler current profiler in a tethered boat that was towed by a small inflatable raft. Data were collected along cross sections and longitudinal profiles. The bathymetric and velocity data document river conditions before the installation of an additional spur dike in 2006 and during a period of bank erosion. Data were collected along 1,700 feet of river in front of the spur dikes and along 1,500 feet of an eroding bank. Data collected at the nose of spur dikes 2, 3, and 4 were selected to quantify the flow hydraulics at the locations subject to the highest velocities. The measured velocities and flow depths were greatest at the nose of the downstream-most spur dike. The maximum point velocity at the spur dike nose was 13.3 feet per second and the maximum depth-averaged velocity was 11.6 feet per second. The maximum measured depth was 12.0 feet at the nose of spur dike 4 and velocities greater than 10 feet per second were measured to a depth of 10 feet. Data collected along an eroding bank provided details of the spatial distribution and variability in magnitude of velocities and flow depths while erosion was taking place. Erosion was concentrated in an area just downstream of the apex of a river bend. Measured velocities and flow depths were greater in the apex of the bend than in the area of maximum bank erosion. The maximum measured velocity was 12.9 feet per second at the apex and 11.2 feet per second in front of the eroding bank. The maximum measured depth was 10.2 feet at the apex and 5.2 feet in front of the eroding bank.