307
edits
Changes
沙坝
,no edit summary
* The height of wall built before each flood event should not exceed accumulation rate of coarse to medium sand during that flood event, otherwise ponding & silt deposition will occur, which can lower specific yield and higher capillarity, then lead to limited extraction rates in wells upstream and more water lost to evaporation. Dams at 1.3m depth showed that where finer material content (0.063 mm or less) is increased, specific yield is known to decrease remarkably. Accumulation rate and therefore height varies according to location and should be adjusted at each site after the first flood event demonstrates the rate of accumulation. Height per stage will probably be between 0.3 metre and 1 metre per stage according to experience from past projects. Some silt deposition will always occur as velocities decrease toward the end of the flood event; the idea is to limit its quantity in final sand volume.在每次洪水来临前建造的坝墙高度不能超过当季洪水里粗砂至中粒砂堆积速率，否则会出现积水成池和泥沙沉积，这将降低单位出水量且增加毛细管作用，而导致位于上游的井取水率有限且更多水分蒸发流失。观察1.3米深的水坝显示，当更精细的物质成分（小于等于0.063毫米）增高，单位出水量会显著降低。堆积速率及相应的坝墙高度根据具体位置而定，且需要在第一次洪水来临后得出堆积速率，从而对每一位置的坝墙高度进行调整。根据以往工程的经验来看，每一阶段的坝墙高度差不多在0.3米至1米之间。随着洪水消退，由于速率下降，总是会出现一些泥沙沉积；这一点用来限制最终沙量中的泥沙沉积。
* 在混凝土中的大石块底部建造防护板（消力池），防止下游的侵蚀问题。防护板尺寸需要设计，但不一定要和下游有裸露石块的区域大小相同。
* In upstream parts of a catchment it is recommended that sand dams are always built in stages, since the availability of coarse material is generally limited and base flow is small or absent (base flow = sub-surface flow which aids surface flow so that silt/clay can still be flushed away once rainfall stops, rather than sinking directly into the sand bed). It has also been suggested that to build in stages over several years is also more beneficial for functioning of dam committees. Build them within a spillway, which was part of the first stage & wing wall construction.在流域的上游，由于粗粒物质通常有限，且建议沙坝始终分阶段建造，基流很小或没有基流（基流在流域的上游，由于粗粒物质通常有限，且基流很小或没有基流（基流=有助于地表径流的地下水流，只要降雨停止，淤泥/黏土仍可被冲走，而不会直接沉入沙床）。也建议在几年中分阶段建设，这也更有益于沙坝委员会的运作。将它们建在溢洪道中，这是第一阶段和翼墙建造的一部分。黏土仍可被冲走，而不会直接沉入沙床），因而建议沙坝始终分阶段建造。也建议在几年中分阶段建设，这也更有益于沙坝委员会的运作。将它们建在溢洪道中，这是第一阶段的一部分及翼墙建造的一部分。
====取水====
