The overtopping of seawalls A comparison between prototype and physical model data

Herbert, D.M. (1996) The overtopping of seawalls A comparison between prototype and physical model data. Technical Report. HR Wallingford.

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Abstract

Significant sections of the United Kingdom coastline are protected from flooding by sea walls. These sea walls, which are commonly fronted by sand or shingle beaches, have a wide range of cross-sections ranging from vertical faces to relatively shallow sloping structures with gradients approaching 1:5. Whatever the sea wall cross-section, the selection of the crest elevation is of primary importance in determining the overtopping discharge performance of the structure and hence the susceptibility of the hinterland to flooding. Traditionally the overtopping performance of simple sea wall cross-sections has been determined from empirical equations whilst complicated cross-sections have been assessed using site specific physical models. The empirical equations employed to estimate overtopping have generally been derived from physical model data obtained during wave flume tests at scales ranging from 1:15 -1:30. A concern with using empirical equations derived from wave flume tests is that the physical model does not reproduce all the physical effects present at prototype sea walls. The most obvious deficiency of physical models is the omission of onshore winds which generally accompany storm events. This comission has two major influences. Firstly the onshore wind raises the still water level at the structure (called wind set-up) and secondly often causes water thrown into the air to be blown over the sea wall. This latter influence may be particularly important for vertical or near vertical walls and slopes topped with a recurve where water reflected from the structure is commonly thrown up into the air. A research project was therefore undertaken to measure overtopping at prototype sites. The aim of the study was to compare prototype discharges with those obtained using physical model techniques. Two sites were subsequently selected on the North Wales coast to complete the fieldwork exercise. The first was a vertical wall whilst the second was a 1:4 simply sloping sea wall. This report discusses the section of the sites, the measurements made and how whey compare with existing prediction methods. The study forms part of a continuing programme of research into the behaviour of sea walls being carried out at HR Wallingford with support from the Ministry of Agriculture, Fisheries and Food under Commission FD0201, Marine Flood Protection, Sea Defence Structures.

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