Session C3 - Channel Damage, Repair, and Recovery after Extreme Floods

Extreme floods cause extensive damage to both natural systems and developed communities. A series of major floods in 2012 capped by Hurricane Irene resulted in severe upland erosion, landslides, channel incision and knick points, and mass bank failures along high energy streams, creating high sediment loads. Many low gradient and downstream rivers had severe bed aggradation, channel widening, avulsions, and floodplain deposition. Numerous road culverts and bridges were damaged, blocked, or destroyed. Aquatic habitat impacts include turbidity, substrate siltation, loss of pools, riffles and side channels, wide shallow flow, increased solar exposure, fish passage blocks, and loss of bank vegetation. Even though the preferred option is normally to allow natural processes to heal flood damaged channels, there are unavoidable situations where physical, social, or ecological conditions warrant active intervention.The first response phase after the flood saw rapid deployment of contractors and public works staff to repair roads, bridges, culverts and channels to protect infrastructure and public safety plus provide emergency services. Several regulatory programs temporarily revised, modified, or waived permits to enable rapid responses. River experts were assigned to assess streams, prioritize repairs, co-ordinate contractors and direct construction work based on rapid geomorphic assessments and field designs. Evaluations of historic and potential river processes were used to review channel sensitivity and ecological risk factors with the goal of adjusting critical reaches toward equilibrium conditions and to discourage further damage by excessive channelization, dredging, straightening and armoring. The initial verbal policies and instructions evolved into written emergency channel repair guidelines for use by technical staff, public works crews, contractors, and landowners. The guidelines provide basic recommendations to classify projects and set clear goals, and select methodologies for proposed channel types and dimensions, slope, alignment, and floodplain connectivity based upon use of analog reference sites and empirical regional hydraulic data, plus use of analytical techniques for sediment continuity by professional staff. The final step is to monitor both the repaired and unrepaired river reaches to observe and document their recovery to whether other action is necessary.
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