Boat Wake Erosion of Sand Bars in Hells Canyon of the Snake River, Idaho and Oregon

Studies were conducted at four sites in the Hells Canyon reach of the Snake River to assess the significance of jet boat wakes on erosion and loss of sandbars. The studies included collection of detailed wave, velocity and suspended sediment data at each of the sites, and analysis of the data to determine if the waves caused net offshore transport of sediment. The data were collected at a discharge of about 246 cms (8,700 cfs) with an array of wave gages, a 3-D Acoustic Doppler Velocimeter (ADV) and two Optical Backscatter (OBS) sensors connected to a synchronized data collection system during both controlled (211) and uncontrolled (53) upstream and downstream passages of jet boats of known size and displacement. The location, speed and track of the boats were determined with boat-mounted GPS and/or a land-based radar gun and laser range-finder. Repeat surveys of the beach profiles and substrate were also made to document changes during the data collection period. The wave velocity data were resolved and used to estimate the bed shear stress, incipient motion conditions and sand transport rates perpendicular and parallel to the shore during each boat passage. The data showed that the sand along the margins of the bars was generally not mobile in the absence of the boat waves, but substantial entrainment occurred in the presence of the boat waves. The analysis also showed net movement of sand away from the shore at two of the sites, relatively small net transport in the downstream direction at one site, and no net long-shore transport at one site. Sand transport parallel and perpendicular to the shore at flows up to 1,130 cms (40,000 cfs) was also estimated using hydraulic output from MIKE-21C, assuming that orbital velocities from boat wakes would be similar to those measured at the lower discharge. The results indicate little potential for bed-load transport at any of the sites in the absence of waves. Under the influence of boat waves, however, there is net offshore transport at all of the sites. The sediment transport analyses demonstrated that waves generated by jet boats are an important factor in mobilizing sediment that would not otherwise be mobile in the near-shore region. Because the wave-entrained sediment is easily transported by the river when it reaches higher velocity zones adjacent to the bars, it was concluded that boat-generated waves cause net loss of sediment from the bars.