CONTROLLING EROSION

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The erosive or cutting capacity of the water is reduced about four times. The quantity of silt that can be carried is reduced about thirty-two times. The size of particle that can be transported by pushing or rolling is reduced about sixty-four times.

As you can see, by slowing down the flow of water, you reduce the amount of damage it can do, and you very spectacularly reduce the amount of silt it can carry. If there is lots of silt suspended in the water, once you slow the water down, most of the silt will be dropped — thus building up the bottom of the gully again.

The principles of check dam architecture. There are many possible designs and materials for building check dams, but whichever one you choose must adhere to certain architectural principles of check dam construction.

HEAD-TO-TOE ALIGNMENT The most effective way of building check dams is to build them in a series where the base of the upper dam is on a level with the top of the lower dam. This will eventually stabilize the whole gully bottom and will create a series of steps or terraces.

SMALLNESS "The bigger they are, the harder they fall" applies particularly to check dams. For most gullies, the check dams should be no more than about two feet high. Anything much higher than two feet will necessitate anchors, deadmen, and other retaining-wall features. Several small dams are far more effective and easier to build than one or two big dams.

DIGGING IT IN The dam must be dug into the walls of the gully, not just laid genteelly up against them. Unless the dams are dug far enough in, water will sweep around them.

NOTCHING A notch is a place where the water can flow over the dam. This is essential. Without one, the silt builds up behind the dam, the water flows on top of the silt, and instead of being led through the notch, it may start eating away at one of the slopes. Eventually, it may make a new channel around the dam. I've seen many erosion-control dams standing proudly and nobly on dry land while gullies flowed merrily around them.

APRON Once the silt builds up behind the dam, the water flows through the notch like a waterfall. You'll need an apron to catch it befores it digs out a pool and undermines the dam. The easiest apron is a bed of stones where the water can simply knock itself out and flow tamely to the next check dam.

Building a check dam. There are several possibilities for building very good check dams: a rock dam, a wire dam, a stake dam, a pole dam, and a plank or slab dam. Which one you choose to build will probably depend more upon the materials you can scrounge up than upon anything else. I built mostly pole check dams because we had plenty of poles. Whichever one you decide on, remember to follow the general principles already laid out, and you will make out very well.

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