Berms can be used to direct flow of water through permanent ponds, ephemeral pools and wetlands, or on slopes with vegetated filter strips. They may be utilised to divert water through the wetland, creating a longer flow path. This increases the efficiency of the system by increasing the hydraulic residence time. The top of the banks may be below or above water level. Flow diversion banks are usually submerged at nominal operating level.
- If possible the soil should only be moved inside the site. Selection of the bottom elevation of the wetland, together with proper positioning on the site with respect to its topography, generally allow balancing of cut and fill, avoiding import/export costs and greater environment impact.
- Compaction, the immediate increase in soil density effected by the displacement of air, should (not to be confused with consolidation, which is a slow increase in density due to the gradual
rearrangement of soil particles over time. Compaction affects the future behaviour of any earth structure. Poor compaction results in low strength, high permeability, susceptibility of tunnelling in dispersible clay, risk of erosion and risk of slip failure. Motorised rollers are usually used to compact soil. The movement of ordinary machinery during construction may provide sufficient compaction, however this technique should be used with caution. Internal clay plugs may be required to minimise berm seepage if permeable materials are used for berm construction. External seepage collection channels may be necessary if soils are unconsolidated.
Freeboards should be adequate to prevent over-topping during storm events and allow overflow of less frequent storm events through controlled and protected emergency overflow points. Berm freeboards should also consider berm soil consolidation and subsidence, and also that the wetland can gradually fill with vegetation and with sediments which increase flow resistance and decrease freeboard during wetland life.
- For vehicle access a berm needs to be ≥ 3m wide,
- For foot access a berm needs to be ≥ 1m wide,
- Berms ≥ 5m in width are less likely to be fully penetrated by muskrats or nutrias. Furthermore, water containment berms are subject to local dam safety regulations
- Bendoricchio, G., Dal Cin, L., & Persson, J. (2000). Guidelines for free water surface wetland design. EcoSys Bd, 8, 51–91. Retrieved from http://www.pixelrauschen.de/wet/design.pdf