Tidal Gates

Due to their location at the top of the tide, tidal gates have the potential to impact upon most if not all fish and eels moving into a watercourse, if appropriate mitigation is not put in place.

One solution to improve the ‘passability’ of tidal gates or flaps is the Tidal Gate Damper (TGD), which delays closing of the tidal flap until after level equalisation.

This solution to fish passage at tidal gates (which has been granted a full legal patent) was developed by Fishtek and other partners (including the Environment Agency) as a novel way of allowing elvers and fish to pass tidal gates. They consist of a compressible material that increases the resistance of the flap and hence the pressure required for them to close. As a result, the gate is kept open over the critical period of level equalisation, allowing fish to pass upstream through the gate.

TGDs

Monitoring work carried out has shown that TGDs are an extremely effective means of improving passage at tidal gates. Research carried out at a pair of tidal gates located side by side (one with dampers fitted and the other without dampers fitted) found that the presence of the dampers held the gate open for 25 minutes while the other gate was closed and this resulted in a ten-fold increase in the number of elvers moving upstream through the tidal gate.

Retarders

An alternative option for improving fish and elver passage at tidal gates is a spring retarder. Retarders are a spring or tension device that holds the tidal gate open above level equalisation, allowing more saline ingress and elver and fish migration. The size of retarder for each site must be calculated and will depend on the weight and size of door and the degree of saline ingress deemed acceptable.

The retarder is relatively easy to fit to installations that have a steel winch cable and pulley wheel. Larger gates generally have a steel cable/winch installed for maintenance. As such, retarders are a low cost, adjustable option to improve eel and fish passage at tidal gates. They are relatively easy to fit and require minimal maintenance.

Experience in design and fitting

Effective, low maintenance, low cost solution

We have designed and fitted numerous spring retarders and TGDs to tidal gates around the UK, including on the Severn estuary and at various sites along the south coast. The TGD in particular is an effective, low-maintenance, low-cost solution for improving fish and eel passage at tidal gates.

Monitoring of the effectiveness of the TGD has been carried out at a site with a pair of tidal gates, where TGDs had been fitted to one gate, with no improvements made to the other. The monitoring found that the presence of the damper caused the gate to close much more slowly, allowing a window of approximately 25 minutes at the crucial point of equalisation when the damper was kept open, while the gate with no damper on was closed. The result was a 90% increase in the number of elvers moving through the gate with a damper on, relative to the gate with no damper on.

The TGD is therefore clearly an effective means of increasing elver passage at tidal gates and it is easy to retrofit to a gate, with installation typically taking one or two days.