Today was really exciting at Lucas Creek. We built probably the first rock vane in New Zealand, maybe the southern hemisphere! Those of you who have accessed the NCHRP Report 544 or the ESenSS manuals (www.esenss.com) know that “REDIRECTIVE” techniques (Rock Vanes, Bendway Weirs, etc) are often cost-effective and environmentally-sensitive alternatives to ” RESISTIVE” techniques such as rip rap, gabions, automobiles, refrigerators etc.
Looking downstream at the outer bend in Lucas Creek just before it flows under a bridge (on right). Gavin is shown here preparing the keyway for a redirective Rock Vane. Rock Vanes point upstream at a 30-degree angle and dip from a high water elevation down to the stream bed.
The research commissioned by Transportation Research Board (NCHRP 544 / ESenss) compiled documentation that Vanes, for instance, produce a scour pool at the tip and slow water adjacent to the bank. This substrate complexity provides much more habitat, as several fish counts and bio-assays demonstrated.
Rock Vanes are best built of self-launching stone (poorly sorted, well graded) but I like to place some big anchor stones also.
What is really cool about vanes is they redirect the high flow velocities away from the bank. Two rock vanes can move the thalweg away from the bank 20% of the stream width. Therefore, with no impinging flows against an outer bend, the bank can be protected with more vegetative type measures. One vane will convert actively eroding banks into depositional areas both upstream and downstream of the vane!!!
Next we placed the self-adjusting, self-launching stone. The average highwater here is expected to be as high as the excavator bucket! Even so, I predict that the vegetated bank behind the bucket will remain stable and that deposition will occur on the bank-side of the vane. Any bets?Next the Compost Blanket was used to stabilize the bare soil and provide erosion control. The Caltrans EC Studies at San Diego State University showed that Compost Blanket provides 96% reduction in erosion!!
Also, we got much of this weeks work area all “buttoned up” with Compost Blankets (2-3″ (50mm) thick) and some compost berm. Compost is not new here in NZ but this project gaining lots of attention so seeing the use of Compost Blankets (these are the “Eco-Blanket type), Compost Socks (“Living Walls”), Compost Silt Socks, and, my favorite alternative to silt fence, the Compost Berm should help with more widespread acceptance.
When compost blankets are placed over properly scarified soils, as we did here, one can expect greatly increased infiltration rates and soil water holding capacity. Also, the compost can absorb 10-times its weight in water, acting as a sponge to slowly wet even clayey soils! Also, also, compost provides soil microbes that are essential for sustainable plant establishment. Also, also, also, the compost organisms slowly convert the carbon into nutrients that the plants can use. FYI, properly made and aged compost does not remove available nitrogen, e.g, the carbon:nitrogen ratio is very low. Check out the Compost Berm !! This can have SO MANY applications as a silt fence substitute. How many of you have had to put in a silt fence down by the stream edge?? Is it not impossible to properly key in – often resulting in unnecessary soil/rock disturbance? Well, just put in a compost berm instead, it will catch rocks and clods and certainly capture silt or sediment from sheet-type flows. Grasses and vegetation will become established in the berm and – YOU NEVER HAVE TO RETRIEVE IT. No excess disturbance after project is stable, no silt fence going to landfill/dump. NO, properly aged compost doesn’t discharge N to waterways (EPA). No, properly aged compost doesn’t float or wash! What is not to like?Representatives from the North Shore City Council, the owners of Lucas Creek and the Park Reserve, are frequent visitors to the project. There is a lot of interest, curiosity and support for this project. Downstream view of Pool, Newbury Rock Riffle (actually a “viffle”), NZ Modified Lunker (LWD), Compost berm, Rock Vane, and Compost Blanket.
I’m reminding everybody here about the Caltrans sponsored research done in California and the new SSPs recently developed for Compost Blankets, Compost Socks, and Compost Incorporate ( see https://www.dot.ca.gov/hq/LandArch/ec/index.htm)