I recently added a second gabion down-stream about 40-feet from the original gabion built about 10-years ago (see earlier posts in this blog).
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Q) Why was a second gabion needed?
A) Because the dry-wash downstream of the first gabion became essentially a 'channel' that directed flash-flood waters from summer monsoon storms rapidly off the property and over time deepened the dry wash channel.
Q) How will this second gabion help?
A) During a flash-flood, the flowing water contains sand, rocks, clay and organic soils that settle once the water slows down. Over time, the channel upstream of the gabion fills-in with sediment and levels off the land. This action can be seen in earlier posts in this blog.
Q) Other benefits of the second gabion?
A) Just like the first gabion, this second downstream gabion will slow the flood flow to allow water to infiltrate the ground. Also, the gabion acts to spread water out the sides from the channel to saturating the ground around the neighboring Mesquite trees to supply needed water to the Mesquite bosque.
The second gabion was built using rock stripped-off the 'apron' from the first gabion (see earlier posts). The apron attached to the downstream side of the gabion, helps prevent the gabion from essentially being undermined by the flood waters and 'rolling' down the wash. At this point the apron was not needed because the first gabion was no longer in danger of being swept-away by a flash flood, since sediment has already build-itself-up to the height of the first gabion.
The gabion in the main channel is simply rocks of various sizes encased in a steel mesh. Both sides of the channel were dug into about a foot to hold the wire-mesh-cage in place. The rocks on the sides, should also be encased in wire-mesh but time didn't allow that level of detail. This risks these large loose rock being swept away during a flood, but I'm going to go with my gut feeling that the first gabion will still act to slow and disperse the water sufficiently to maintain the integrity of the second gabion.
The photo above was taken downstream looking upstream. Notice that there is no apron behind this second gabion. The reason in part was the time-allowance I gave for the project. Again, my feeling is that it will hold, but never underestimate the power of water to change the landscape - We'll see.