Tilt-Dam springs to the challenge17 March 2009
Spring Dam and Tilt-Dam provide a simple but effective civil engineering contribution to meeting the challenge of flood defence
In 2003, after yet another flood disaster in the UK, civil engineer John Forrest decided to look into harnessing gravity to help provide an effective flood defence. The result was a trench with a counterbalanced tilting lid where gravity did all the work in mobilisation and keeping the flood at bay. From being a path or roadway in the dormant state, the lid would be rotated manually through 90 degrees to form a wall. Tilt-Dam was born. This permanent in-situ demountable solution can be considered where fixed flood defences are deemed inappropriate, or may also be used to add to the height of a fixed defence.
The following year, Forrest presented his concept to a peer group at the Institution of Civil Engineers (ICE) and was awarded development funding by the Research and Enabling Fund at ICE. This allowed development work to proceed, resulting in an established design and a full scale dry prototype demonstration unit being fabricated at the Building Research Establishment (BRE) at Watford. In March 2005, Tilt-Dam was launched on the market with a presentation at ICE headquarters in London. This was followed by further presentations to organisations involved in flood defence (such as the Environment Agency, consulting engineers and contractors). Much of this was based around a full size wet prototype (two units each 2.8m wide for a 1.2m flood depth), installed at Lea Marston in the Midlands. This was used to provide inspection and demonstration for interested parties and included flood conditions with dynamic loading.
In 2008, as a result of ongoing innovation and system development, Spring Dam has now arrived on the scene. The same principles are retained, but with added significant economic benefits including minimum ground penetration (ie no trench), better load carrying capacity, lighter lids plus easier access for inspection, maintenance and cleaning after a flood. At licensed contractor, Littlehampton Welding, a full scale Spring Dam has been designed, fabricated and subjected to wet testing. This unit is 3m long and 1.5m high, which is the related flood depth, and is available for inspection and demonstration.
The fundamental design principles for these demountable systems, where permanent walls (or bunds) are inappropriate or access is required through them, are:
• Reliability and longevity – permanently available in-situ over a 50-year life.
• Safety – provides operational and public safety in both active and dormant states.
• Simplicity – design uses gravitational forces for manual operation and water sealing.
• Robustness – heavy duty conventional structural materials designed for static and dynamic loading conditions.
• Rapid mobilisation – vital for demountable flood defence coupled with short warning times. Two people can mobilise at one metre per minute.
• Ease of inspection and maintenance – has only two moving parts, bearings and locks, and uses long life seals.
The Tilt-Dam system consists of two main elements, a trench and a lid. The trench, in reinforced concrete, is set below ground level, designed for the forces arising from the lid (and loading applied to it in both dormant and mobilised conditions) and the surrounding ground. On the floor of the trench a steel angle carries the continuous horizontal seal. Drainage from both wet and dry sides of the trench is catered for, with non-return valves on the dry side. Railings can be provided for personal protection in the area of the trench on the wet side.
The lid, which may be concrete, steel, hybrid or an alternative material, is designed to take loading conditions associated with the dormant state as a path or roadway and the mobilised state under water pressure. The most important feature here is the level of the bearings (just above one third of the height of the dam) which ensures that the water pressure keeps the dam closed in flood mode, and also determines the weight of the counterbalance. The lateral edges of the lids have channels to house the vertical seals between adjacent units. Unit length is determined principally by the weight of the lids arising from the flood depth to be catered for. Each lid has two locks, securing it to the trench below, operated by a special key, the only item of the system not in-situ.
Props are available for assembly between the top of the vertical lid after mobilisation and the dry side trench wall. These props allow the horizontal seal to be compressed before the flood water continues this process and they also provide additional stability to the system. The props and vertical seals are stored on the underside of the lids. Should leakage occur, or the dam become overtopped, then pumping from the dry side trench can be undertaken using standby equipment.
For Spring Dam the lid is set in a frame, using either steel or aluminium, supported on a concrete slab at ground level. There is now no trench required, since the lid is hinged at one end as opposed at the one third point for Tilt-Dam. The gravitational power for mobilisation is now delivered through energy stored in torsion springs, on the hinge line, which are energised by the weight of the lid when it is lowered from the vertical. Hence, as for Tilt-Dam, the process is a quick and simple manual exercise. Once again the system is self contained in-situ. Here, however, the loading from the water pressure in flood mode is resisted purely by the props. The seal configuration and operation is similar to Tilt-Dam with the horizontal seal in a recess in the concrete ground slab.
In the ground below either system, piled (or equivalent) support may be required if the allowable ground bearing pressure is inadequate. In addition, but complementary to the design of this surface based flood defence, a cut off may be required in porous ground.
The lower flood depth limit is around 0.5m and in theory there is no maximum, but in practice there is a limit brought about by adhering to the principles. Beyond a 2m flood depth, for both Spring Dam and Tilt-Dam, the principle of simplicity will begin to be compromised with regard to the simple manual assembly of props and vertical seals. Two metres, however, covers a very high proportion of flood cases likely to be considered. In addition, due to the one third/two thirds design criteria in relation to the water pressure diagram for Tilt-Dam, for every one unit of flood depth upwards the trench must go an extra half a unit downwards. Hence at a 2m flood depth the ground excavation is over 1m deep (allowing for the trench base). Flood depths over 2m can nevertheless be catered for.
Automation has been considered but not generally recommended for various reasons. First of all there is reliability, where human intervention is seen to be more appropriate and indeed probably more reliable, considering the potentially large intervals between mobilisations. Secondly, there is added cost and complexity, followed by potential injury or damage caused when a piece of infrastructure suddenly rotates through 90 degrees even if automatic warnings were to be included.
The systems can be used as front line protection against fluvial or surface water flooding and as a secondary system in coastal locations. They can be either a single opening for access through a fixed wall/bund or a sequence of units forming any length of defence along say a river bank or around a property or an industrial installation. Tilt-Dam can only be installed in straight lines and hence change of direction is achieved using a fixed element such as a pillar or section of wall. Spring Dam can cater for curved alignments (either concave or convex) such as a river bank, since the bearings are now at one end of the lids and adjacent units can be rotated without clashing.
Both systems are considered to have many benefits, including: no permanent wall/bund to obscure views or sight lines; allows access which is then easily closed off at flood warning; no offsite storage requirements and costs; no transportation risks or costs; no risks of damage or loss of items in handling or assembly; protected below ground in the dormant state; quick and easy mobilisation; environmental acceptance including upper surface integrated with surroundings.
For further information contact Jim Barrack, Director, Tilt-Dam Ltd, [email protected]