RIVER CHANNEL ADJUSTMENTS DURING AND AFTER HURRICANE IRENE

Hurricane Irene struck the east coast causing massive flooding, river and floodplain adjustments, and subsequent channel modifications by emergency response teams. The flooding led to major alterations of upland rivers, floodplains, valley bottoms, and infrastructure. Landscape adjustments in steep terrain included irreversible processes such as landslides, bedrock erosion, mass bank failures, and channel incision that generated huge quantities of bedload sediment from previously stable threshold streams; plus downstream channel aggradation, avulsions, and floodplain accretion in the lower gradient deposition zones. The erosion of fine grain material from former glacial lake sediments that are increasingly exposed in the valley bottoms and incised channels creates a special concern to water supply utilities due to potential turbidity levels.

The emergency response included public and private efforts to clear and reestablish river channels, a process enabled by the temporary suspension of some regulatory programs. Extensive channel modifications were made, generally without use of detailed hydrologic or geomorphic studies, or with engineering plans.

River managers were able to influence some of the emergency responses and are continuing to take corrective action. Recommended guidelines include knowledge of the rivers watershed and historic context, hydrology, use of sustainable channel form, slope, and dimensions, reconnecting floodplains to rivers, removing dredged spoil piles, providing space for lateral channel adjustments, replanting barren areas, and improving habitat. Replacement culverts and bridges need to be properly sized for passage of aquatic organisms, future watershed conditions and climate change, and not just match previous structure types and sizes. One must avoid the urge to keep filling floodplains after each event, they are needed to convey runoff and store both water and sediment. Where possible, existing and new levees should be relocated farther from the rivers edge to provide more room for channel migration. Most of all, we need to educate municipal officials and the public about river and floodplain processes and good management practices that balance human needs and the natural system, and reconsider the role of flood insurance programs that encourage reconstruction on floodplains.