Flap Gate Performance in Hydraulic Models

Flap gates are common at waterway confluences where flow in a particular direction (usually upstream flow) is to be prevented. As such structures are relatively common in waterways that are subject to flood-related constraints, and since hydraulic modelling is often used in such situations to aid with decision making, it is often important that the performance of flap gates is adequately represented in such models. Many models allow the user to permit flow only in one direction, and it is this representation of what in reality might be a flap gate that requires clear understanding.

The degree of opening of a flap gate is dependent on the head difference across the structure. For a small head difference the opening is severely constrained, and the flap gate opens wider with increasing head difference. Full open flow is seldom achieved with older flap gates, since these are generally heavy and do not lift fully even under high velocity conditions. More modern counter-weighted flap gates open more under small head differences, and are arguably more efficient.

There have been studies undertaken to evaluate the head loss across flap gates for a range of different flow rates, and it is this type of accuracy in performance that often needs to be incorporated into hydraulic models. Simply permitting flow in only one direction implies a fully open structure regardless of head difference across it, where in reality the degree of opening (and hence head loss) at such a structure is dependent on head difference across it. This is further complicated when water level either side of the structure is below obvert level, and full flow does not occur.

This paper examines methods for adequately representing flap-gated outlets in hydraulic models.