FLUVIAL GEOMORPHOLOGIC IMPACTS OF GABIONS ON THE ZEALAND RIVER, NEW HAMPSHIRE

For the past 50 years, river restoration projects have been conducted throughout the United States. Typically, man-made static features are applied to the banks of a river to prevent erosion and channel avulsions as well as improve stability. The first two restoration projects that extensively used gabions, a particular design of instream structure, were constructed on the North River, VA and the Zealand River, NH. The following study looks specifically at gabions structures that the U.S. Forest Service (USFS) implemented along the Zealand River between 1957-1965 after a series of major floods. Gabions are wire-mesh baskets filled with stones that either line the banks and are called walls or cut perpendicular to the stream flow and are referred to as sills. The Zealand River site was never resurveyed after the 1960’s, thus the impacts the gabions had on channel widths and bed elevations were unknown. This study, conducted in 2014, consisted of longitudinal profiles and cross-sectional surveys, providing evidence of the geomorphologic changes that took place over the past 50 years at the Zealand River site. Multiple cross-channel pebble counts were preformed as well as photo replications and general assessments of the gabion structures today. The data collected in this study was compared to the archived profiles and photographs from the 1960’s. Research suggests, in higher slope reaches of the river, the gabion walls narrowed the channel, thus increasing the water velocity and enhancing the failure of the sills. The destruction of the sills led to 1-2 m of vertical erosion that undercut the gabion walls and created banks that were susceptible to collapse. The downstream reaches experienced greater than 1 m of deposition, which have created avulsions. By comparing the evidence collected from the 2014 survey and the archived profiles from the 1960’s as well as a field reconnaissance, it can be concluded that although gabions were intended to stabilize the river, they have resulted in bank instability, vertical channel incision and massive downstream deposition.