Analysis of flows and water levels near tidal flap gate

A method is proposed for assessing the impact of rising sea levels on areas upstream of tidal flap gates. These devices, designed to allow drainage of surrounding land during low water and flood defence during high water, are commonly used in areas where tidal high water is higher than the level of the land behind the sea wall. Studies were undertaken at Pagham Harbour, West Sussex, UK, where a number of tidal flap gates are used to control the flow of water from channels carrying drainage waters away from the nearby town of Chichester. On one of these channels, water level data were collected at 15 minute intervals at three sites near the flap gate and analysed in order to obtain values over each tidal cycle of maximum and minimum water levels downstream of the gate, and the time of closure of the gate. Water level was also monitored at two locations upstream of the gate. By obtaining estimates of mean fresh water discharge for each tidal cycle, and combining these with peak tidal water level, a functional relationship (in the form of a three dimensional plot) was obtained for the parameter Tc, the length of time in hours for which the flap gate is closed during each tidal cycle. These values of tc were also used in conjunction with mean tidal fresh water discharge to produce another three-dimensional plot showing the effect on peak tidal water level upstream of the flap gate, thus providing an illustration of the sensitivity of local flooding of varying tc under differing tidal conditions. Finally an estimate was made of the likely impact of a 300mm rise in mean sea water level on tc for the same set of fresh water flow data, using a set of 'look-up' tables based on the three-dimensional plots obtained. These indicated that the frequency of local flooding was likely to increase significantly for fresh water flows greater than 0.8m3/s. Although the increase in local flooding illustrated here is not thought to be a cause for immediate concern, particularly in view of the recent construction of a further tidal flap gate adjacent to the existing one, nevertheless the methodology employed is applicable to similar systems.