Surface Water Control (continued)
The number one enemy of simple drains is sediment. If the drain clogs, the water you are trying to get rid of either continues eroding its way down the tread, or just sits there in a puddle. Your job is to keep that water off, Off, OFF the tread!
The best drains are "self cleaning." But in the real world most drains collect debris and sediment; this must be removed before the drain stops working. Since a long time may pass between maintenance visits, the drain needs to handle annual high volume runoff without failing.
Most problem drains are waterbars. If the water is slowed by hitting the waterbar, sediment builds up. This can be compounded by inadequate outsloping or an outlet that is too narrow. The extra time it takes to rebuild the offending bar into a functional drain will pay off almost immediately (Figure 24).
Figure 24--The key to waterbar maintenance is to ensure that
sediment will not clog the drain before the next scheduled
maintenance. Embed the rocks or logs a little deeper,
cover them with soil, and you have a reinforced grade dip.
The best cure for a waterbar that forces the water to turn too abruptly is to rebuild the structure into a reinforced grade dip. The reset bar and curved water channel keep the sediment-laden water moving through the outlet. If this is not an option, the next best move is to reset the waterbar at a steeper angle. Usually a longer bar will be needed.
If a lot of tread has eroded below the bar, reset the bar so it is flush with the existing tread height. Regrade the water channel and outlet drain. On gentle trails, tamp the excavated mineral soil sediment into the tread on the downhill side of the bar. Scatter any organic debris well off the trail.
At grades steeper than 7 to 10 percent, or in highly erodible soils, borrow material placed below the bar will usually erode away quickly. This is particularly true on waterbars with high faces. Downhill traffic, especially packstock, will step in the same place every time and dislodge any new material you place there. If a significant step-off exists below the bar, reseat the bar flush with the existing tread level and deepen the drain above the bar.
Dig drains and leadoff ditches wide enough to prevent clogging by debris, and graded so water does not slow before it is off the trail. Ditches that allow water to return to the tread below the drain need to be reconstructed so this doesn't happen.
You may need to install additional water control structures if erosion is evident. Figure out where the water is coming from and where it is likely to go. Think about soil type, slope gradient, distance of flow, and volume of water before you start moving dirt.
Eroded trails do not always become major problems. Many eventually stabilize if the trail surface is rocky, and use, water, and slopes are moderate. The key question is whether the loss of tread will materially affect the designed challenge and risk levels of the trail. If not, the erosion isn't significant. It is exceedingly rare for an eroding trail to have a significant effect on aquatic or riparian habitat or stream function.
|A lot of learning takes place when you slosh over a wet trail in a downpour and watch what the water is doing and how your drains and structures are holding up.|
Adequate puddle drains are important. Puddles may produce several kinds of tread damage. Traffic going around puddles may widen the tread (and eventually the puddle). Standing water usually weakens the tread and fillslopes. It can cause a bog to develop if the soils are right. Traffic on the soft lower edge of a puddle can lead to step-throughs and cause tread creep.
When a crew takes a swipe at the berm with a shovel or kicks a hole through it--that's useless puddle control. These small openings are rapidly plugged by floating debris or the mud-mooshing effect of passing traffic. The puddle lives on.
Effective puddle prevention requires constructing a wide drain. The ultimate drain is when the entire section of tread is outsloped. If terrain prevents such outsloping, the next best solution is to cut a puddle drain at least 600 mm (24 in) wide extending across the entire width of the tread. Dig the drain deep enough to ensure that the water can escape the tread. Feather the edges of the drain into the tread so travelers don't trip over them. Plant guide structures along the lower edge of the tread at either side of the drain to keep traffic in the center. In a really long puddle, construct several drains at what appear to be the deepest spots.