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[[Image:Sand dam.JPG|thumb|right|200px|Sand dam construction in Kitui, Kenya. Photo: M肯尼亚基图伊的沙坝建造。图片来源：M. Hoogmoed.]]
Sand dams are a simple, low cost and low maintenance, replicable rainwater harvesting technology. They provide a clean, local water supply for domestic and farming use and are suited to semi-arid areas of the world.沙坝是一种建造简便、成本低、维护工作量少、易复制再造的雨水收集技术。沙坝为家庭及农场提供清洁的当地用水，适用于全球的半干旱地区。
Regions with highly erratic rainfall often have rivers with temporary seasonal sandy bed streams. They only experience heavy water run-off for short periods of time after rain. During such periods of high flows, large quantities of sand are transported downstream. Some sand can get trapped on the upstream sides of rock outcroppings along the stream. The sand traps form natural water storing aquifers. Sand dam technology provides a means to reproduce this water collecting mechanism and to increase the size of existing aquifers. 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，而大量的沙會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。
Water in natural collection sites is usually clean for drinking but limited and quickly gets depleted. Sand dams are an artificial improvement of sand beds where more water can be recharged and stored for water use. A dam of concrete, clay soil or stone masonry is constructed across the channel at specific sites to trap and hold back the sand during flooding; this creates an additional sub surface water bank to be harvested during the dry season. 天然集水的区域通常汇集了清洁的饮用水，但数量有限，很快便会耗尽。而沙坝对砂质河床进行了人工改进，能补给、储存更多水，以供备用。用混凝土、黏土或石材在河道的一些特定处建造水坝，为的是能在洪水期间留住沙子；这样便创造了额外的地下水储备，在旱季得以利用。
With an accurate and appropriate location choice the total amount of water available in the sand dams can be over 6000m³. 如果选址准确适宜，沙坝中可用的总水量至少有6000立方米。
Sand dam projects here have not only improved water availability, but benefited the community socially and economically. Locals are trained to participate in construction of the dam, maintenance, financial management, and also in natural resource management. 沙坝项目不仅提高了水的可供性，也有利于社区的社交和经济。当地人会获得培训，参与建设和维护沙坝，并管理财务和自然资源。
===Suitable conditions 适宜条件===[[Image:Sand dam flood.JPG|thumb|right|200px|Sand dam during flooding. Photo: 洪水期间的沙坝。图片来源：[http://www.rainfoundation.org/ RAIN Foundation.]]]
Sand dams can be constructed and used for household, community and even municipal uses. There are examples of sand dams which are connected via infiltration galleries, off-take wells and pump stations into municipal piped water systems.沙坝可因家庭、社区甚至市政用途建造使用。有些沙坝通过渗渠、排水井和泵站与市政水管系统连接。
Expert input, however, is required to determine the best site for the dam and to make sure the project is adapted to the physical conditions as well as social circumstances of the local community. Having a lead artisan per catchment area to decide on siting seems to help success rates. Physically, the site must be suitable for construction of a dam, provide safe drinking water, and be able to use sand sedimentation to increase water availability. Firstly it must be decided if the river is appropriate, secondly the best section of riverbed must be chosen, and then the optimum location for extraction. 然而，选定建坝的最佳位置需要专业知识的投入，确保项目能适应当地社区的实地条件和社会环境。在每一个集水区设立一位工匠领队来进行选址或可提高成功率。从实际出发，选址必须适宜建造沙坝，能提供安全的饮用水，也能利用沙子的沉积增加水的供给。首先，必须判定河流是否适宜；第二，必须选择河床的最佳区段；随后，确定最佳选址。
Generally, the dam requires a sandy riverbed in a hilly area. Preferably the riverbed consists of coarse sands laid on impermeable bedrock. The river should be seasonal but must have a base flow. Whether this is the case can be determined by the type of vegetation growing around the river. The river shouldn’t be too wide (max. 25m) and should have two high riverbanks. 通常而言，沙坝需要建在丘陵地带中的砂质河床上。河床最好有不透水的基岩，上面铺有粗砂。河流需是季节性的，但必须有基本流量。要判断河流是否符合上述条件，可以观察生长在河边的植被种类。河流不能太宽（最宽25米），两边需要有较高的河堤。
Ensure the sand dam is not built in an area where water will bypass the structure. Riverbanks should be equal in height and tall enough (height of dam 请确保把沙坝建在河水会流经坝体的区域，两边河堤要等高，高度要足够（坝高+ height of flood 洪水水位+10%), and the dam should not be constructed near the bend in a river.），且不要把沙坝建在河流弯道附近。
Site where there are no possibilities for the water to leak away:以下选址能防止漏水：* On impervious bedrock or clay rather than rock with fractures. A good indicator is whether there is some preexisting subsurface flow in the dry season or not and if there are large stones & boulders seen in the riverbed. Extra care should be taken when siting as seepage can occur under the dam in such cases. 要有非渗透性的基岩或黏土，而非有裂缝的石头。一个实用的判断指征是，在旱季是否有先前就存在的地下水流，并且，在河床上是否有较大的岩石和卵石。选址时需要特别留意，因为在这些情况下，沙坝下方可能会出现渗漏。 * On the base layer rather than on the intermediate clay lens within sand. 选址在基层上，而非在沙中的黏土透镜体上。* Between defined banks with no old riverbeds on either side which could allow sub-surface water around dam edge.在确定的河堤间，若河堤任一边有旧河床，则需确保旧河床不会使沙坝边缘有地下水存在。
Site in areas where gradient is suitable to get sand rather than silt要选择斜度适宜留住沙子而不是淤泥的地方建坝。水流速度至少要达到每秒0. A flow of at least 0.45 m/s river flow means less silt deposition, and such areas will be where there is a suitable gradient – too flat and there will be too many small particles and silt. Flatter gradients also mean wider riverbeds, and for sand dams it should really be limited to 25 metres width. An optimum gradient is said to occur between 045米才会留下较少淤泥，因此，这样的区域就需要斜坡角度适宜——若太平便会有太多小颗粒和淤泥。坡度越平，河床便越宽，而建沙坝必须要25米以下的河宽。据说最佳的斜度是在0.125% and 4至4% but can be higher than this but then the sand volume stored is less. An easier field test might be to do a sand analysis to find size distribution, or a porosity & specific yield test from which one can extrapolate the likely sand type. Medium sand will have the best balance between porosity and specific yield, and is therefore the type that is needed.之间；虽然也可以高于4%的值，但会造成沙储备量减少。若想要确定粒径分布，沙子分析是一种较简易的实地检测方法，或者可以通过孔隙率和单位出水量测试来推断沙子可能的类型。中粒砂可以最好地平衡孔隙率和单位出水量，因此是最需要的类型。
Site where river is narrower and where there is a natural barrier to groundwater flow. This results in cheaper construction while maximizing sand already present. Such barriers can be found by seeing where water will remain in scoop holes after rains, or through probing, augering & trial pits, or other techniques such as drilling with air compressor.如果河流较窄，并有阻挡地下水的天然屏障，这样的选址因沙量已达最多，能降低建坝成本。要找到这样的屏障，可以通过观察雨后积水在勺状孔中的位置，或通过探针、螺旋钻和浅探井，或空气压缩机钻孔等方法。
Avoid siting where halite (white & pink rocks) is present in riverbanks upstream. These may make the water saline.避免在上游河堤有盐岩（呈白色和粉红色）的区域选址，否则会使水中含盐。
Have a sequence of dams in the same river to avoid everyone using a single source with possible ecological damage as a result. However, having dams too close together means their areas of influence overlap. This enables water levels to rise in general, but the total quantity of water available decreases. Quantity is more important regionally, therefore minimum distances might be employed between dams (350m either side of dam was zone of influence in Kenya with a 700m minimum, but this might vary according to site).在同一条河上建坝要有秩序，以避免所有人过度使用同一水源，造成可能的生态破坏。而坝间距太近则会使各自影响的区域重叠，会造成水位普遍升高，而可供使用的总水量降低。对每一区域而言，水量更为重要，因此最好设立最小坝间距（以往，肯尼亚的最小坝间距为700米，坝每边350米是影响区域，但需视不同地点而定）。
These conditions and also others related to, for example; climate, presence of boulders, or gradient of the riverbed must be analyzed in detail to determine suitability of the site. Also, though it can be artificially improved, the water quality must be good enough for drinking (not too saline, contaminated, etc.) 建坝适宜条件也与其它因素相关，如：气候，有无卵石，同时也必须具体分析河床倾斜度来判定选址是否合适。并且，虽然实地条件可通过人为来改善，但水质必须达到可以饮用的级别（不能含盐太多，不能受污染，等）。
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- Year-round source of good quality water and groundwater recharge 全年都有质量优良的水源和地下水补给 <br>- Environmentally friendly 环保 <br>- Erosion control 控制侵蚀 <br>- Good for coping with drought, especially when accompanying riverbank storage replenishes sand dam in dry seasons 有利于应对干旱，尤其是在旱季，随着河堤储水向沙坝补充水量的时候 <br>- Silt management within river basins 控制江河流域中的淤泥 <br>- Added soil moisture infiltration 增加土壤水分入渗 <br>- Regeneration of vegetation along riverbanks 河岸植被再生 <br> - Sand dam effects on downstream river discharge are generally small (< 10沙坝通常对河水下游的流量影响较小（少于10% of runoff). 的径流） <br>
- Better water quality than surface water dams: protects the water against evaporation, animal contamination and insect & bacteria breeding. <br>
- Sand deposits can be collected and sold for extra income. <br>
