307
edits
Changes
沙坝
,no edit summary
[[Image:Sand dam.JPG|thumb|right|200px|肯尼亚基图伊的沙坝建造。图片来源：M. Hoogmoed.]]
在降雨量极不稳定的地区，通常砂质河床上会有季节性水流临时流经。这些地区只在雨后才有短时大量的径流，而大量的沙会在这一时段被带到下游。在流动过程中，一些沙会被阻挡在上游突出的岩石处，于是，天然水中的这些沙子便形成砂石含水层。沙坝技术由此应运而生，重现这种集水机制，扩大现有的砂石含水层规模。
[[Image:CompletedSandDam.JPG|thumb|right|200px|竣工的沙坝。索马里兰。 Eric Fewster, BushProof / Caritas]]
* Timing is important: dams should be built during the dry season, but don’t build dams too close to the rains in order to avoid the trench filling up with water or the dam being washed away.时间安排很重要：建坝需在旱季进行，且勿太临近雨季，以避免沟渠被水填满或坝体被冲走。
* The method depends on the type of dam and the type of ground. The construction of sand dams in cascades improves total storage and efficiency and minimizes seepage losses. Dams made of concrete, stone-masonry (cheapest and easiest) and brickwork require skilled labour for construction, but are stronger and have a longer lifespan. 建坝方式由水坝类型和地质类型来决定。建造沙坝建坝方式由水坝类型和地质类型来决定。在梯级地形上建造沙坝能提升总储水量和效率，并将渗流损失降至最低。
* Usually sand dams are built onto a rock layer, but where there is no rock and only clay, it can still work but as long as the foundation is keyed into the clay layer and where the wall does not protrude more than 0.5m above original sand level, otherwise there is a risk the structure overturns during a flood event.
* Key into banks or construct wing walls to avoid erosion around edges of the sand dam. Where wing walls are built, a good technique is to start with the wing walls and work inwards to the centre, since community enthusiasm lags by the time wing walls are constructed (if not built first), yet they are essential to proper functioning. Length of the wing wall varies according to bank characteristic: loose riverbanks, 7 metres; hard soils, 5 metres; hard & impermeable soils or rocks, wing wall is not needed. Planting napier grass along upstream riverbanks controls erosion and fixes the course of the river in a flood.