THE ROLE OF CHANNEL ARMORING WITH GABIONS ON CHANNEL ADJUSTMENTS ON THE ZEALAND RIVER, NEW HAMPSHIRE

Extensive armoring projects were undertaken in the past to limit channel adjustments and prevent channel erosion, sometimes with disastrous results. From 1960-1964, 1,724 m of gabion walls, 405 m of gabion sills, 53 m of spur dike deflectors and 125 m of riprap were installed on a 4.5 km length of Zealand River in White Mountain National Forest, NH in response to flood damage from an October 24, 1959 high-flow event. Gabions are wire-mesh baskets filled with stones. Gabions and riprap walls were designed to prevent bank erosion and prevent avulsions, and sills were used to control incision. At least two T-shaped mid-channel deflectors were also installed to create fishing holes. A longitudinal profile was surveyed in 1960 along 5.6 km of the channel prior to structure placement. Limited structure evaluations were performed in 1963, 1973 and 1988. In 2014, a study was conducted to determine how many gabions remained after more than 50 years and what impact those gabions had on geomorphic stability of the channel. The 1960 longitudinal profile was resurveyed along a 4.5 km study reach, cross-sectional surveys and associated pebble counts were completed at 18 locations, and historic photographs were replicated to determine the impact of the gabions on channel change after over 50 years. The current condition of gabion structures was also assessed. In the upper, higher-slope, study reach riprap walls narrowed the channel, increasing water depths and shear stress. In the middle, moderate-slope portion of the study reach gabion walls further confined flows and increase shear stress values and enhancing the failure of gabion sills. The destruction of sills led to 1-2 m of vertical erosion that undercut nearby gabion walls, which often fell into the channel further narrowing flows. The downstream reach experienced greater than 1 m of deposition and associated avulsions that were often initiated along banks armored with gabion walls. The results indicate that the 1960s attempts to limit channel adjustments probably exacerbated channel change over the last 50 years by disconnecting the channel from its floodplain and increasing shear stress values in the main channel in steeper reaches, which increased sediment supply to the lower reach.