Berms
Berms can be used to direct flow of water through permanent ponds, ephemeral pools and constructed 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.
Design considerations[edit]
- 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.[1]
Height[edit]
- 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.
- Controlled overflow points that are rip-rapped dips in the surface or concrete weirs should be incorporated into the top of the berm of each wetland cell so that if the wetland is overtopped the flow exiting the wetland does not cut channels into the top of the berm that could compromise the integrity of the berm.
Top width[edit]
- 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
Slope[edit]
Berm slope is dictated by geotechnical considerations and slope-stability analysis.
- Maximum berm slopes typically used are 2:1 (horizontal: vertical).
- However, it is better to keep slopes at 3.5:1 or lower angle to minimize sloughing of the slopes into the wetlands while maximizing wetted area in the wetlands.
- A side slope of 5:1 or greater provides easier egress in case someone falls into the pond or wetland contained within.
- Slopes up to 10:1 or 20:1 are used when a shallow littoral shelf is desired to create vegetation and habitat diversity. The side slopes may be rip-rapped with stone to reduce the potential for erosion or rodent burrowing.
- ↑ 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
- ↑ CH2MHill. (2014). Wetland Design Guidelines City of Saskatoon. Retrieved from https://www.saskatoon.ca/sites/default/files/documents/transportation-utilities/construction-design/new-neighbourhood-design/wetlands_design_guidelines.pdf