Bank Softening (aka Using Explosives) to Initiate Channel Widening

PCI has been working with The Nature Conservancy since 2013 to plan, design and implement restoration projects to recover populations of salmon in the lower Ten Mile River watershed. The overarching premise for this series of projects is that off-channel rearing habitat and winter high-flow refugia are often severely limited in alluvial valleys where the channels have become simplified and disconnected from their floodplains, and recovery of salmon populations is dependent upon restoring this habitat and the processes that form and maintain it. Off-channel ponds, side channels, flooded wetlands, low-elevation floodplains, and complex in-channel habitat associated with large wood are the habitat features that provide the low-velocity environment that coho juveniles need to survive and thrive.

A video created by TOPO Collective for PCI follows the first phase of the mainstem Ten Mile project’s formal design and construction. This phase, funded by a private foundation, included several approaches to initiate channel widening and complex habitat development within a narrow, straight channel reach with simplified bed and banks; the banks are roughly 15 feet high, vertical, and locked into place by mature alder trees. One approach: constructing large engineered log jams to pinch the channel and divert flow into the banks. The flow would then scour the banks and cause trees to topple.

“The other approach we took was almost a sarcastic suggestion,” says PCI Principal Geomorphologist Lauren Hammack in the video. “It was, ‘Well, maybe we should just blow the trees up.’” So we did (in a controlled, safe and thoughtful fashion).

The video follows this novel technique, called bank softening, through conversations with regulators around safety, timing and potential impacts to any wildlife in the area, coordination with California Department of Fish and Wildlife blasting specialists, and blasting day itself. Bank softening loosens the soil in the bank while removing the mature trees and their root wads, making the banks more erosive. High flows can then provide more scour. While it might sound extreme, the technique has the potential to provide substantial habitat improvement. Use of explosives kickstarts the beneficial geomorphic processes of bank retreat and channel widening and rapidly recruits large wood into the channel for coho juveniles. Alders are abundant nearby and can regenerate rapidly after disturbance, so felling selected trees will not harm the riparian forest.

“California Department of Fish and Wildlife staff were very excited to try [the approach],” Hammack says in the video.  Our agency partners (CDFW, State Water Board, NOAA Fisheries, NOAA Restoration Center, and the Corps) were all supportive of experimenting with using explosives to soften the channel banks. “Where we’re at right now is that novel and out-of-the-box approaches – they’re willing to consider them and try them.”

Watch the video below.

PCI | Ten Mile Blasting from TOPO Collective on Vimeo.