Rubber dams are weir systems in which an elongated rubber body is firmly connected to the foundation of the weir system. The rubber body can be filled with water or air, depending on the design. The height and thus the overflow of the weir can be determined by filling and draining the weir. Water-filled rubber dams by Hydroconstruct are usually filled with river water, controlled by the hydrostatic pressure of the water in the regulating shaft.
Theoretically, independent regulation by the pressure of the overflowing water is also possible; the rubber dam lowers independently by reducing the overpressure via an overflow threshold. This is a feature that is otherwise only possible with electronic control.
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In Hydroconstruct’s water-filled version, the regulating shaft system comprises a shaft structure with individual chambers equipped with corresponding hydraulic regulating and control devices such as flaps and gates. The regulating chamber is connected to the interior of the rubber dam via a pipe system incorporated in the weir foundation structure. Control is via a headwater probe. The weatherproof control cabinet with the electrical controls and regulating equipment is located directly at the weir or very close by.
In the standard Hydroconstruct design, all steel components (clamping system and pipes in the regulating shafts) are hot-dip galvanized, the pipes set in concrete are made of PVC plastic, and the anchor bolts are made of stainless steel.
The water-filled rubber dam system is particularly notable for delivering unrestricted regulation of the dam level. When the outflow increases, the dam height lowers continuously. When the inflow decreases again, the height is continuously raised again by the filling pump.
In winter operation, when the rubber dam is stationary and not overflowing, the fill-water can be protected against ice formation by pumping through a small amount river water with a temperature greater than 1°C.
The air-filled dam has a regulation apparatus consisting of a piping system leading from the inside of the rubber dam via a shaft system into the control room. The bottom of the shaft is below the level of the weir sill in order to be able to drain off the condensed water that has formed.
The operating room above houses the regulating and control equipment with fan. Control is via a headwater probe. The control cabinet with the electrical control and regulating equipment is also located in the operating room.
In the standard Hydro Construct design, all steel components (anchoring and clamping system) are hot-dip galvanised, the piping system and anchor bolts are made of stainless steel.
The main advantage of the air-filled system is faster recovery after a flood event.
Under normal operating conditions, the rubber dam is inflated, and can be used to regulate the water level with continuous lowering up to 15%, after which it is usually completely deflated, but can also be operated further if the irregularity of the weir discharge does not play a role.
This results in a V-shaped buckling of the membrane.
For air-filled weirs, which harbour condensation within the rubber membrane, make sure the valves are located in a frost-free area.
Water-filled rubber dams are also suitable for winter operation. Many years of experience, especially in Austria, show that outside temperatures with double-digit degrees below zero do not cause any problems.
Precautions taken when there is no water flow over the top of the dam (when a danger of frost can occur) such as pumping through, are usually sufficient to keep the inside of the membrane free of ice. In principle, however, ice formation inside the membrane is not a major problem when the weir height stands at 0.5 m.
Over the past 15 to 20 years, hydraulic structures using FRP composites have been designed, manufactured and installed in several countries, including the United States, the Netherlands, Spain, China, the United Kingdom and France. Many of the original waterway navigational structures around the world have far exceeded their original design life – some to the point of failure. Corrosion-resistant, low-maintenance composite hydraulic structures are poised to take their place in locks, dams, levees, pump stations and other systems that involve valves, gates, walls, piling, mooring facilities and more.
As in any composite application, careful examination of design considerations for hydraulic structures are paramount. Because the market encompasses a bevy of structures and accompanying components, it’s impossible to cover specific design principles within this column. For instance, the design process for a vertical lift gate varies from that of a stoplog on a dam. However, there are several overarching considerations, five of which are presented here: design requirements, material selection, structural concepts, deflection requirements and cost.
Before examining those considerations, it’s important to note work that has already been done to create best practices and design standards for hydraulic structures. In , the U.S. Army Corps of Engineers led publication of a guide specification entitled “UFGS 35 20 15 FRP Composites for Low-Head Water Control Structures” that covers stoplogs, vertical lift gates, weir dams and more.
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Another effort is currently underway by PIANC, the World Association for Waterborne Transport Infrastructure. In order to identify “best practices” of composites in the hydraulic structures market and develop applicable guidance documents, PIANC established an international working group – WG-191, Composites for Hydraulic Structures – in . The WG-191 has developed a draft report entitled “Composites for Hydraulic Structures,” which was submitted to PIANC’s Inland Navigation Commission in February for review, editing and publication.
The report summarizes state-of-the-art FRP composites for hydraulic structures, including material characterization, engineering (science) evaluations, design, construction, repair and cost analyses. Emphasis is placed on successful applications of composites in waterfront, marine and navigational structures, such as lock doors, gates, in-situ and under water repair, and flood protection systems.
In particular, the report highlights several applications of FRP composites for hydraulic structural systems that enhance durability, reduce downtime to replace or rehabilitate traditional structures, reduce embodied energy and exhibit other advantages. (Two of the applications are depicted in this column.) Ideally, the contents of this report will help designers and promoters of hydraulic structures by providing guidance to develop and operate safe, economical, durable structural systems for waterways utilizing FRP composites.
In the meantime, here is an overview of the general design considerations for composite hydraulic structures:
Identify Design Requirements
The requirements of a new FRP structure are usually based on the requirements of the existing structure made of conventional materials, including the geometric boundary conditions and various loads on the structure. Geometric boundary conditions focus on the dimension of the structure, fatigue cycles, sustained stresses and environmental conditions, such as the range of operating temperature. The loads considered may include complex wave loads, dead load, torsion and longitudinal loads, ice loads, wind loads, water flows and potential barge collisions.
Consider Material Selection
The properties of composite materials and their manufacturing process usually drive the selection of FRP composite materials. The lamina properties of FRPs, including the properties of core materials (if involved), are calculated, and the material safety factors and design values are determined. Manufacturing methods suitable for the proposed structure should be selected to achieve high fiber content and to enhance the strength and stiffness of the material. Designers also can take advantage of the fact that composite materials are anisotropic, so they can be designed and manufactured with properties varying in different directions to suit the application needs, dependent on load paths.
During the material selection process, designers should consider numerous factors, including corrosion, weathering and the material safety factor. With regard to safety factor, uncertainties due to a combination of effects corresponding to various knock-downs (under temperature, moisture, sustained stress/creep and fatigue) should be taken into account. In addition, uncertainties regarding targeted material design properties should be considered. It is extremely important to note that proper selection of the manufacturing process, as well as the constituent materials (resins, fibers, additives) for a tailor-made composite structure, are essential to meet minimum design and performance requirements.
Define the Structural Concepts
The next step is to define the structural concepts of the proposed FRP structure. Usually, a minimum of three important aspects should be considered: 1) use of a limited number of connections, 2) optimal design under water or barge impact loads and leakage flows and 3) uniform load transfer over the entire structure. The design includes guiding principles for the structural concepts, description and calculation of different options, such as sandwich structure versus box beam. Then, the most suitable option of optimal design is selected to meet the strength and deflection requirements.
Take Deflection Requirements into Account
In some cases, the maximum deflection requirement influences the design of a composite hydraulic structure since the stiffness of the GFRP composite is low. To meet the stiffness requirement, CFRP can be used at a different cost scenario, recognizing that the potential for galvanic corrosion exists when in direct contact with steel.
Perform a Cost Comparison
The cost comparison should be made among different design options. A life cycle cost analysis should be used when designing with composites as an alternative to conventional materials because FRP offers longer service life and lower maintenance costs, but at a higher initial cost in some scenarios and at a competitive price in other cases, such as wicket gates.
These five considerations only begin to address the budding hydraulic structures market. Educating owners and designers about composites – including specification developments and training fabricators of composite systems – are real challenges facing the market. Even so, the future is promising, provided that high-quality workmanship and installation procedures are ensured. Based on limited in-service data, it is safe to conclude that GFRP systems for hydraulic structures are resulting in longer service life, lower initial costs (in some cases) and reduced life-cycle costs with enhanced structural performance.
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