14 Dec Geyserville Bridge Project sees some High Water
Now remember, the intent of the project was to “move the thalweg stream-ward”. The big issue of secondary importance was trying/implementing the construction method of “Self-Launching” rock. The emergency nature of the project and the environmental sensitivities were the primary design “drivers”. RSP (Rock Slope protection) – Rip rap in Caltrans jargon, would not be allowed by the Resource Agencies. Other design constraints were water quality and both instream and gravel bar habitat protection. In other words, we would not excavate the river bottom and we would not excavate the gravel bars.
So, this was a perfect opportunity to utilize the self-launching (graded, poorly sorted) rock as described in NCHRP Report 544, and place clean rock on the existing river substrate. This is how the Longitudinal Stone Toe Protection (LSTP doing double duty as the construction access road) was built. And then the redirective Rock Vanes, attached to the LSTP, were installed to re-direct and manage the THAWEG. So, if the thalweg and high energy stay away from the bank and out to the middle of the channel, then the project is a success. See for yourself how it is working.
David Derrick, upon seeing these photos commented ” I like that thalweg”. I like it too, it is over 60′ streamward of the bank and last winter the thalweg was at the toe of the bank. Also notice in the photos the slow velocity and low energy of the flood waters over the terrace.
Thalweg management, adding roughness (the willows, live siltation, live poles, and a flood bench), and a flood bench can sometimes replace RSP for an Environmentally-Sensitive approach to streambank protection. please note that the vertical bank will be addressed in subsequent projects.
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