CONTROL Stream Erosion

Article Tools

Image Gallery

Print

E-mail

Comments

RSS

Placing riprap, however, is not just a matter of flinging rocks. First of all, the stones must be the correct size and shape for the dimensions and flow rate of the stream. If the rocks are too small, they may be moved be a flood. If they are too large, the bank may erode behind them. As for shape, flat-edged rocks are easier to place and will deflect water better than rounded ones. A shovel may be handy for securely placing the first layer of rock. From then on, the process is not unlike building a stone wall. (A layer of gravel under the riprap provides added protection for the bank.)

Riprap can redirect flow, but it doesn't appreciably slow the water. Furthermore, while well-placed riprap is scarcely noticeable on some sites, on others it can look as out of place as a concrete wall.

The preferable, but more complicated, alternative is to build some sort of in-stream structure. The most adaptable of these is known as a wing deflector. (Fig. 2 shows how a wing deflector might be sited in the situation we are discussing, and Fig. 3 shows how it's constructed.) The space behind the leading edge should always be filled. Otherwise, floodwaters passing over the top will create a new erosion site.

SOLVING SILTATION

Now, let's look at the silted downstream site. We've already helped it a little by dealing with the upstream problem. At least there will be that much less soil entering the stream which could be deposited there. But we want to get rid of most of the present silt layer, restoring the riffle area (which is the most important type of feeding and spawning habitat for many kinds of fish), and deepening the pool.

The only real in-stream problem here is caused by the willows. These trees, with their tenacious root systems, are great bank stabilizers, but they aren't so desirable when they get out into the stream. Here two willows, which face each other on opposite sides of a narrow spot, have formed a partial dam with their roots and branches, and that blockage is worsened by floating debris which collects there. We don't want to eradicate the willows, but we can prune them periodically and try to control their progeny.

To accelerate the process of desiltation, we can make further use of riprap or wing deflectors to concentrate the current and flush the silt downstream. In this instance, we may be able to save labor and cost by substituting large rocks, strategically placed, for the deflectors. If this is done, the rocks should be placed toward the middle of the stream so as not to push the current into the bank. The ideal is fast current in the middle and slow water along the banks.

RSS | Comments | E-mail | Print