I've also proposed a chain of reinforced rain gardens downstream, where the floods occur. These would have to be armoured in some way to prevent erosion from floodwaters--and one method could be Deltalok bags. Here's my original proposal.
The following is Lisa Coleman's initial response to the proposal.
Lisa: We have used a very similar product to the Deltalok system along Starkweather Creek (see photo below), with, in my opinion, unacceptable results.
David: How were the results unacceptable?
Lisa: Unacceptable in that we installed and planted under the original contract, then had to re-plant plugs and reseed (for an additional $20,000). Vegetation is still not established- now nearly 3 years after original installation.
At Starkweather... the plantings were watered both times post-installation by the installers. Both times wiped out by flooding and sustained high water levels....
Difficult to get vegetation established, and without vegetation, the product eventually fails. As you mention, this is a particular concern in this shady environment.
David: Difficult, yes. But not impossible? What if more intensive methods are used, rather than love it and leave it? Why don't we use this project as a trial for developing better methods to establish vegetation? From my own experience with woodland gardens, I know that keeping the area wet during dry spells is the key to growing woodland plants.
Lisa: I wouldn’t endorse a solution in which the structural integrity of the solution relies on establishment of vegetation when it’s clear that vegetation could be difficult to establish here. My goal is to fix the erosion, not to do field trials for vegetation establishment.
Also with the storm water velocities that we are talking about here, I don’t think this is a good application of this product.
David: Stormwater velocities will vary, according to location and topography. Certainly, back from the channel, water velocity will not be an issue. And at the E end of the greenway, the existing pipe can handle the big floods, allowing a resurrected channel to the North with Deltalok reinforcement to the side to handle lower-velocity flows .
Lisa: The only place where there would be room to install the product in the project as planned would be right along the bank of the channel. This is where I am concerned about velocities.
A potential application of the Deltaloc product in my opinion would be for homeowners to use it for terracing of their backyards, further back from the channel. In that case the product would not have to stand up to the velocities in the channel, and the homeowner would be responsible for establishment (or not) of vegetation. But our project addresses the channel itself (and remember we are trying to disturb as little as possible that we don’t absolutely need to disturb) – what homeowners wish to do with their yards further up the slope is their business.
Remember that once we are done with the project, we are not going to have the ability to access the area to make repairs, so we need something that’s going to work as installed, and continue to work, no matter how well vegetation does or doesn’t establish.
David: For access, why can't we have a central channel, with the bottom reinforced with interlocking blocks. The bottom undulates, creating pools. A bobcat can use the channel as a track, delivering any needed repair materials. Any rain garden at the far east end will have easy access.
Lisa: I wouldn’t recommend reliance on volunteers for an aspect of a design that is critical to the design function of the project. Based on our experience with volunteers, while they are well-intentioned, sometimes when push comes to shove the work doesn’t get done.
David: This particular location is ideal for volunteers. It's the backyard of residents. And if volunteers don't step up, can the city let a contract for maintenance of vegetation while it gets established? Eventually, tree roots will infiltrate the Deltalok bags, and make them pretty solid.
Lisa: For two items above…we generally try to design our projects to require as little maintenance as is feasible, as this makes the best fiscal sense and is mindful of our limited resources and many projects and areas to maintain. We do have a few vegetation maintenance contracts.
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Location for one rain garden, to be located in the former bed of the now-buried stream. You are looking upstream (east) from the new stormwater pipe outfall (right). The garden would extend from the middleground back past the wood pile.
Starkweather Creek. Soil-filled bags are visible in layers on either side.