TO MAKE AND BREAK A FLOOD WAVE: THE ROLE OF TWO FAILED BEAVER DAMS IN TROPICAL STORM IRENE

Dam failures generate flood waves that dramatically modify river systems. Even a small dam can generate a powerful—though brief—flood wave on failure. The locally high rainfall of Tropical Storm Irene generated sufficient runoff in the White Mountains of New Hampshire to breach small dams and generate these flood waves.

This talk examines a 1.1m high beaver dam that breached during the storm. Using empirical dam break equations, the most conservative estimate of runoff generated by the breach is greater than the peak runoff rate of the storm, more than doubling the peak flow in the reach downstream of the failed dam during the storm. This estimate is corroborated by hydraulic estimates of flow rate using shear stress analysis of streambed sediments and resulting estimates of water level. Other notable geomorphic impacts include a high density of large logs deposited downstream of the breach, bank erosion in the same reach, and creation of a new scour hole in the channel bed.

In contrast with high rates of flow, the total volume of the flood wave is low. In this case, the flood wave was absorbed in a downstream meadow that had been created by a beaver dam that failed prior to the storm. The remnant dam connected the upstream channel with the riparian meadow, generating off-channel storage. Downstream of the beaver meadow, evidence for flood wave absorption includes an immediate decrease in logs, a sediment distribution equivalent to the reach upstream of the breached dam, and moss and algae-covered rocks on the streambed surface.

Given that beaver dams are built and fail over decades, we infer that these types of flood waves were a potent channel-shaping force prior to European colonization of the northeastern U.S. We discuss implications for river restoration when including beaver dams in the baseline condition. We also discuss implications for protection of modern infrastructure given the rapid rate of beaver recolonization.