Membranes, used as Baffles or Curtain Walls within water retaining structures, need to be positioned and/or protected to ensure that excessive hydrostic and hydrodynamic pressures or impacts from cascading or turbulent water are not exerted upon the material.
Whilst the membrane materials and their fixings used have adequate tensile strength and tear resistance, they are not specifically designed to structurally and dynamically accommodate high differential hydrostic pressure across any wall, or to resist sustained and indeterminate effects of cyclic or even one off impacts from adjacent cascading or turbulent water.
Tank layouts, inlets, outlets, and baffle locations, as well as the hydraulics through the complete tank system, need to be designed by the main civil and process works designer to ensure that the long term and cyclic process, operational, hydrostatic, and hydrodynamic loads provided onto the membrane systems are always within the membrane’s and membrane fixings’ specified design parameters for all operational scenarios, including possible but unintended operational scenarios, and for the intended life of the membrane system.
With this in mind:
• Baffle walls should always have openings at the base fixings level sufficiently continuous and large to ensure that pressure differences across the membrane wall are always no more than the hydraulic head loss generated by flow along and around that baffle – including for all tank water levels from empty to extreme emergency top water level, and also allowing for all possible surge flows into, through, and out of the tank.
• Baffle openings at the wall top should be similarly sized to limit any similar excessive pressure difference across the membrane during any surcharging of the tank, including any dynamic effects of short circuit overflows over the top of any membrane from one flow channel to the next.
• The process/civil designer should ensure that inlets, outlets, and overflows to and from the tank should be located, designed, and their flows managed within the hydraulic design upstream and downstream of the tank, such that local surcharging from cascade flows, gulping, vortexing, or any other excessive hydrodynamic loading onto the membrane baffle or curtain is avoided for all operational conditions. Where necessary structural shielding or flow management (diffuser) baffles are to be provided by the civils designer/contractor to so protect the membranes.
• The Operation and Maintenance Manuals for the Works, as provided by the process/civil designer and contractor, should include the express requirement for the tank to be drained down, say every 12 months, for a complete inspection of all membranes and their fixings. This is specifically required to identify and rectify any areas of local damage and/or overloading or failure of the membrane or fixings, to modify where required any process/civil works as deemed necessary to avoid any repetition of any such damage or failure, and to avoid or at least minimise the extent and consequences of any progressive failure or deterioration arising from a localised problem.
• The responsibility and liability of the membrane supplier and the membrane fabricator and installer should be strictly limited, as noted above in more detail, to the design, materials supply, manufacture, and installation of the membrane system and fixings to a layout and details as provided by the process/civil designer, for a tank system designed by the process and civil designers, and for a tank system constructed by the civil contractor.
Where Baffle Curtains are included within a geomembrane lined lagoon, with perhaps a floating cover installed, care should be taken to ensure that Hydrodynamic forces acting upon the curtain are not transmitted through to the liner. Where possible, the baffle curtain should not be fixed to the liner via a welded or mechanical joint, but should be anchored by ballast. A suitable slip liner (say 2mm thick HDPE sat on a geotextile) can be positioned along the Baffle Curtain centre line. Pre-cast concrete anchor blocks can then be positioned along the centre line and connected to the curtains lower fixing arrangement.
Given that most Baffle Curtain installations are in contact with potable water, it is important to remember that the accessories (fixings, hem poles, anchor blocks etc) will need to comply with the relevant authority standards for materials in contact with potable water even though they have a relatively small surface area to volume ratio.
