Old theory behind new flood defence system

Proof of concept for a new automatic form of flood defence has been successfully demonstrated by a Herefordshire company, Repel Limited. Unlike so many increasingly ‘high-tech’ solutions employed to address everyday problems, the concept relies on an idea as old as science itself - that of Archimedes.

The ‘Higginson Barrier’ is an automatic flood defence that employs flood water itself to raise and lower a protective mechanism around an entire property, be that domestic, agricultural or commercial.

Design

The patent pending design is likened by its designer to the concept of a boat moored in a harbour. At low tide, the boat rests on the sand; but as the tide rises, the buoyancy of the boat causes it to rise with the tide until it is floating. For Repel, the differences between boat and ‘Higginson Barrier’ are that the sides of the boat are ‘fixed’ to the seabed and the boat has no bottom.

The mechanism comprises a concertina arrangement of uniform width glass reinforced polymer hinged panels fronted by a waterproof material membrane. The bottom of the membrane and the lower edge of the bottom panel are both attached to the base of a concrete trough, while the top of the membrane and upper edge of the top panel are attached to a float that runs around the length of the barrier. The top of the concrete trough (and consequently the top of the float) lie flush with a previously graded ground level.

Under non-flood conditions, the mechanism sits in its folded state within the trough, the top of which lies at ground level. The rigid float, which is also made of glass reinforced polymer, and concertina panel mechanism are said to offer sufficient strength to allow vehicles to drive over the barrier and afford access to the property.

On commencement of a flood, the mechanism is activated by the ingress of water into the front section of the trough through drainage slots. The ingress of water and buoyancy afforded by the float is sufficient to raise both panels and waterproof membrane as the level of the flood water rises, and to return the barrier to its folded state as the flood water recedes. There is no reliance on any power source other than the rise and fall of the flood water level to operate the barrier.

The panels of the barrier act as a support for the waterproof membrane when the barrier is activated. The panels are fabricated in increasing thickness from top to bottom of the barrier to counter the greater lateral force acting on the barrier with increasing depth of flood. The panels themselves are supported by a series of poles which may either be fixed on installation or which are activated and rise to support the barrier as the flood water rises in order to maintain the aesthetics of the property in question.

Free-standing

A particular feature of the design is that it lies remote from the property it protects; it neither relies on the strength of a property’s walls to support the barrier, nor is it joined to the property’s foundations.

Since a fluid exerts a normal force on any boundary it is in contact with, other whole-property defence systems that rely upon a building’s walls to provide the structural support for a barrier or that are attached to a property’s foundations are constrained as to the maximum height of flood they can defend.

A flood of 1.5 metres for example exerts a pressure of 14,715 N/m2, a force greater than that needed to crack un-reinforced concrete of the type used for the fabrication of most domestic floors. The resultant lateral forces acting on a property defended in this way may be sufficient to cause the failure of a conventional brick wall (depending upon its condition), the buckling of walls from the action of vertical forces acting on the foundations, or structural damage to flooring.

By providing an area of ‘breathable’ ground between barrier and property, the design overcomes two of the constraints that limit the maximum height of flood that existing barriers are able to defend.

By separating the ‘Higginson Barrier’ from the property, the action of the lateral forces acting on the walls of the property are removed, while creating an area of ‘breathable’ ground in the resultant gap between property and barrier allows the pressure beneath the property to be reduced. By grading this area to slope downward from the property towards the barrier’s inner base allows water seepage through the ground to be collected and expelled using a low cost bilge pump arrangement powered from the mains with battery override.