In drainage of agricultural lands, a filter or gravel envelope is used to prevent the entrance of fine sand and silt into the drains.

在农田排水中常常采用砾石或过滤层来防止粉细砂和泥沙进入排水沟。

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In drainage of agricultural lands, a filter or gravel envelope is used to prevent the entrance of fine sand and silt into the drains.

在农田排水中常常采用砾石或过滤层来防止粉细砂和泥沙进入排水沟。

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