The Permaculture Earthworks Handbook: How to Design and Build Swales, Dams, Ponds, and other Water Harvesting Systems by Douglas Barnes

Author:Douglas Barnes [Barnes, Douglas]
Language: eng
Format: azw3
Publisher: New Society Publishers
Published: 2017-09-18T04:00:00+00:00

FIGURE 6.6. Jessours, or check dams, are designed to capture both eroding soil and water. Trees can then be planted in the deposited soil. Agriculture is often carried out in wadis via jessours. They are traditionally constructed with earthen embankments, though gabions can be used.

A weir is a check dam built into a stream bed. In most jurisdictions, the use of weirs in streams would require special permitting. In developing nations, such permitting is usually not needed. While this frees up possibilities, it is a privilege that must not be abused. Downstream communities relying on river flow are very often opposed to the installation of check dams above them. However, a properly designed weir will prolong the flow of water, not cut it off from those downstream. The aim of a weir is to slow water, not stop it. This is particularly valuable in drylands, where large rainfalls in a small number of rain events lead to sudden torrents of water inundating streams and rivers. There is a brief spike in flow, which drops off rapidly, or stops altogether. Prolonging the availability of water provides more opportunities to support communities and wildlife both locally and downstream.

As with erosion gullies, a cutaway trench that extends into the bank is needed to anchor the dam in place. Whether constructed of gabions, concrete, or log pilings driven into the stream bed, the center portion of the dam must form a lower spillway section to allow water to pass over the top. There must also be a splash apron on the downstream base of the dam. A gap or window in the wall or a pipe can be located near the window to ensure flow in all water level conditions and to allow fish to pass through when the water is lower. Concrete weirs are often built by creating a concrete trough along the river bed that extends up above the banks of the river. Large concrete blocks can then be lifted into the trough to form the check dam.

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