| What it is | A flexible elastomeric membrane anchored to a concrete foundation and raised by compressed air. | A flexible elastomeric membrane anchored to a concrete foundation and raised by water pumped into the internal chamber. | Both systems regulate upstream water level by changing the dam crest height. |
| Typical control method | Air compressor, pressure controls, valves, sensors and automatic venting equipment. | Water pump, filling and draining pipes, valves, level sensors and control equipment. | Automatic control should include high-level, low-level and emergency deflation functions. |
| Common crest-height range | Approximately 0.5–5 m, depending on hydraulic design and membrane configuration. | Approximately 0.5–5 m, depending on hydraulic design and membrane configuration. | The final height must be determined from flood levels, upstream operating level and downstream conditions. |
| Typical span arrangement | Single or multiple bays across the river, with intermediate piers where required. | Single or multiple bays across the river, with intermediate piers where required. | Longer crossings may be divided into bays to improve structural stability and operational control. |
| Main membrane material | Reinforced rubber or elastomeric membrane with embedded textile reinforcement. | Reinforced rubber or elastomeric membrane with embedded textile reinforcement. | Material selection should account for ultraviolet exposure, abrasion, temperature, chemicals and biological activity. |
| Response speed | Generally fast; automatic deflation can rapidly reduce the crest during rising floods. | Generally slower than air inflation because the internal water volume must be pumped or discharged. | Required response time depends on river rise rate, catchment size and flood-warning capability. |
| Level-control accuracy | High when pressure sensors and upstream water-level sensors are correctly calibrated. | High when pump control, water-level sensors and filling-volume monitoring are correctly calibrated. | A practical control system should maintain the target level while preventing excessive upstream backwater. |
| Energy requirement during holding | Usually low after reaching the target pressure, provided leakage is limited. | Usually low after filling, provided the chamber and valves maintain the required water volume. | Actual energy use depends on leakage, operating cycles, pump or compressor efficiency and control settings. |
| Flood-management function | Can be automatically deflated to provide a wider flow passage and reduce upstream water levels. | Can be drained to lower the crest and increase flood conveyance. | Flood capacity must be verified using design floods, sediment conditions and debris blockage scenarios. |
| Sediment and debris considerations | Requires protection from sharp debris, abrasion and concentrated flow near the membrane and anchor points. | Requires the same membrane protection, plus reliable filtration or clean-water arrangements for the filling system. | Trash racks, flushing provisions and routine inspection may be necessary in sediment-rich rivers. |
| Fish passage and ecological operation | Can be partially or fully deflated to support seasonal flow requirements and ecological releases. | Can also be lowered for ecological flows, although the discharge response may be slower. | Fish passage, minimum flows and water-quality requirements should be addressed during permitting and design. |
| Maintenance focus | Membrane inspection, pressure testing, compressor maintenance, valve checks and sensor calibration. | Membrane inspection, pump maintenance, pipe and valve checks, leak testing and sensor calibration. | Inspection frequency should increase after floods, debris impact, unusual pressure loss or visible membrane damage. |
| Best-fit application | Sites needing rapid flood response, frequent level adjustment or compact control equipment. | Sites with dependable water supply, moderate response requirements and preference for water-based inflation. | The best option depends on hydrology, river width, flood risk, foundation conditions, ecology and maintenance access. |
| Primary limitations | Potential air leakage, compressor dependence and vulnerability to puncture or abrasion. | Potential water leakage, greater filling and draining time, and dependence on pumps and clean filling water. | Neither type should be selected without hydraulic, geotechnical, structural and environmental assessment. |