Inflatable storage

Hidden deep within the tropical rainforests of the Atherton Tablelands in north-eastern Australia lies Koombooloomba dam and it’s 7MW hydro station. Further downstream, below the spectacular Tully Falls, is the larger 72MW Kareeya hydro station. Both stations are boosted by the additional water storage capacity provided by an inflatable rubber dam installed on the fixed crest of the dam.

The 1.8m high inflatable dam installed on Koombooloomba dam is one of numerous inflatable dams supplied by the Australian-based Trelleborg Queensland Rubber. Stanwell Corporation, the owner of the Koombooloomba and Kareeya hydro stations, previously operated one of the original ‘Fabridams’ to provide additional water storage. This rubber dam was nearing the extent of its design life and Stanwell selected Trelleborg to supply the replacement membrane and associated items.

Water-filled

Over the past five to 10 years, the vast majority of inflatable dams installed worldwide have been designed to be inflated with air rather than water. One of the primary reasons is the height-to-circumference ratio for an air-filled dam is much higher (compared with a water-filled dam) resulting in significantly less membrane area to achieve the same storage capacity. However the existing membrane on Koombooloomba dam was designed for water inflation. As such, there were some economical advantages to maintain this same control system that had operated successfully for over 20 years.

Trelleborg has previous experience with both air-filled and water-filled inflatable dams and were focussed on supplying a dam to meet Stanwell’s specific requirements. The client decided to remain with the water-filled dam design as this ensured the existing control system could be fully utilised. Another operational advantage of water-filled dams is their inherent stability under a large range of operating conditions (compared with air-filled dams). Koombooloomba dam is designed with a steep ogee crest so stability of the dam membrane was one of the key design criteria.

The steep ogee crest presented a number of other challenges on this project. Stanwell Corporation has traditionally experienced problems fully deflating the dam. As a result, Trelleborg modified the existing operating system to include the option of a vacuum deflation system to suck any water remaining in the bag after normal deflation. Also, the downstream side of the dam was equipped with a series of straps to ensure the bag would not roll forward over the upstream anchor line during the inflation process.

The new dam membrane was commissioned under full overflow flood conditions earlier this year.

General details

Trelleborg Queensland Rubber’s Flexidam is basically a large fabric-reinforced rubber membrane inflated with either air or water. The membrane is secured to the dam’s concrete foundations to form a barrier for the water. Inflatable dams are designed to be easy to install, maintain and operate, and are resistant to the effects of impact, corrosion and abrasion.

Since their inception in the 1950’s thousands of inflatable dams have been installed all over the world. Other dam control mechanisms such as steel gates do not provide the flexibility and reliability of inflatable dams. During heavy rain periods, inflatable dams can be designed to automatically deflate and allow quick release of floodwaters and debris.

Applications for rubber dams include: raising the crest height of existing dams or weirs to increase storage capacity and provide a system of flood control; tidal barriers to protect rivers from salt water contamination; recreational lakes; low control weirs for flood mitigation and waste water treatment works; and replacing existing steel gate systems.

Benefits

Inflatable dams provide a number of advantages compared with other available dam control options, such as steel gates and flashboards.

• Capital Cost: Inflatable dams can be more cost effective compared with steel gates. Inflatable dams can be installed as a long span (in excess of 100 metres) without intermediate piers.

• Installation Cost: Inflatable dams are quick and easy to install and can be attached to existing dam crests at marginal extra cost. Minimal skills and personnel are required to embed the anchorage system and pipework into the foundations and to attach the rubber membrane. Quick installation is an advantage when considering the possibility of fluctuating water levels interrupting completion.

• Operating Cost: Inflatable dams are economical to operate. The only significant running costs are power requirements for the air blower or water pump during bag inflation.

• Maintenance Cost: Inflatable dams are not subject to corrosion compared with steel shutters that require periodic shutdowns for grinding and painting (with the possible installation of coffer dams).

• Sediment Buildup: Sediment and debris buildup is periodically flushed away from the upstream side of the deflated dam during flood periods. The deflated dam provides a low, smooth profile with few or no intermediate piers to catch logs and debris, and any sediment buildup on the membrane is automatically removed during reinflation.

• Impact Resistance: Inflatable dams are specifically designed to provide superior resistance to rocks, logs and other debris. Inflatable dams absorb shock and vibration with their inherent flexibility and energy-absorbing properties, and transmit less impact through to the foundations.

• Abrasion Resistance: The dam membrane is manufactured using abrasion-resistant rubber compounds that are less susceptible to scouring from sediment compared with concrete and steel.

• Corrosion Resistance: The dam membrane is resistant to the effects of weather and oxidation. The only metal parts underwater are the anchor plates and bolts and these items can be supplied in stainless steel for salt water and other corrosive environments.

• Water Tightness: Inflatable dams provide superior water tightness compared with steel gates and flashboards.

• Flexibility: Inflatable dams operate effectively either inflated with a range of different head heights, or deflated during flood conditions. They can be easily installed on existing foundations and are suitable for all climates.

• Reliability: The deflation and inflation mechanisms are reliable due to their simplicity of operation.

• Flow Disturbance: The deflated bag lies flat on the concrete foundations resulting in minimal disturbance to the natural river flow and reduced danger of bank erosion.

Membrane construction

The construction of the rubber membrane is the key to the inflatable dam’s continued operation under a variety of adverse conditions. Using advanced technology compounds, the membrane is manufactured with layers of weather-and-wear resistant rubber and high strength reinforced fabric. Rubber dams with less superior rubber and fabric compounds have been in operation for over 30 years.

The membrane’s fabric reinforcement provides exceptional tensile strength, impact and puncture resistance. Abrasion and puncture resistance can be further improved by compositing other materials, such as stainless steel wire mesh or carbon fibre, into the membrane (e.g. rubber dam locations with a high risk of vandalism). The membrane is usually designed with a nominal safety factor of not less than 8.

Membranes can be supplied with a range of thicknesses and reinforcement layers according to the design height of the dam and tension on the downstream face. Seams are usually designed circumferentially around the membrane to secure against downstream tension. The deflated membrane is designed to lie flat on the foundations providing minimal flow resistance.

Anchorage system

Inflatable dams are secured to the concrete foundations using a single or dual row of clamping plates. These plates are attached to anchor bolts that are either cast into new foundations or embedded into existing foundations.

Design features of the anchorage system include: easy to install on new or existing crests; recess in concrete foundations to protect from large floating debris; cast iron anchorage designed with cathodic protection; Stainless steel or carbon fibre fixings can be installed if there is a risk of corrosion.

A dual anchorage system is preferred to single anchorage for some applications.

• Tidal barrages with pressure acting on the upstream and downstream sides of the membrane

• Large inflatable dams requiring high clamping forces

• To enhance the bag’s stability and reduce vibration under high overflow conditions

• To reduce stress concentration on the membrane pipework attachments

• To minimise variations in the dam height under fluctuating head heights

Control system

Inflatable dams can be designed to operate unsupervised under all adverse conditions. The membrane is inflated with either an air blower/compressor or water pump. Deflation is achieved by electrical and/or mechanical operation of the exhaust valve. Drain pads or spacer pipes can be installed inside the dam membrane to assist with complete deflation and reduce the risk of increased membrane wear.

The control system depends on the customer’s specific requirements and choice of either an air-filled or water-filled inflatable dam. There are various control systems available ranging from simple manual control of inflation and deflation to advanced control of the upstream water level within a tight range. In general, inflatable dams are designed with both an electrical and mechanical failsafe system for automatic deflation.

Installation

Inflatable dams can be easily installed on either new foundations with anchor bolts and pipework cast into the concrete or an existing dam crest using grout emplacement. If the rubber dam is to be installed on an existing crest, a recess for the anchor bolts is formed, the holes for the anchor bolts are drilled into the recessed foundation and the anchor bolts are fixed to the foundation using epoxy resin.

The dams can be supplied to site rolled on a mandrel to assist with easy installation. Trelleborg Queensland Rubber usually include provision for an Installation Manual, lifting equipment to aid with installation of the membrane, and experienced personnel to be on-site during the unrolling, clamping and commissioning of the dam.

Installation steps for securing the membrane to the foundations are simple: (1) Unroll the rubber membrane onto the concrete foundations; (2) Drill anchor bolt holes; (3) Install spacer pipes and secure the pipe attachments; (4) Secure the anchor plates; (5) Retorque the anchor bolts after at least 24 hours; (6) Complete commissioning tests.

No specialised skills or equipment are required to install the dam. Contractors duties include installing the pipework into the foundations, securing the membrane, constructing a control area and installing control equipment and instrumentation.

Hydraulic model studies

Hydraulic model studies can be conducted on inflatable dams operating either inflated or deflated for a range of upstream water levels, including: single versus dual anchorage; dam shape characteristics; bag tension at varying upstream and downstream head heights; dam vibration during overflow; river flow conditions during and after deflation.