ICE - Institution of Civil Engineers - Virtual Library

In the Humber estuary approximately 235 km of flood defences protect a tidal floodplain of about 90 000 ha, containing the homes of more than 300 000 people and assets valued at more than £7 billion including the conurbations of Hull, Grimsby, Cleethorpes, Immingham and Goole and extensive areas of highly productive farmland. With a projected expenditure of £500 million over 50 years, there was a need to propose a prioritised programme of works for the 50-year period and to develop proposals in sufficient detail to get approvals for the first five-year programme of works. This paper describes the phased approach that was adopted and the methodology used to establish the extent and timing of works needed. Following prioritisation of the works and preparation of a preliminary programme of works, more detailed assessment of the works identified in the first 15 years was undertaken, including environmental assessments and economic analysis. The paper concludes with a brief description of the works proposed in the first five years of the strategy period.

In low-lying areas of the coast and particularly around estuaries, the option to realign some of the seawalls is becoming increasingly attractive. Although the wholesale removal of existing walls is possible in some cases, frequently this is not possible for a variety of reasons and it is necessary to consider simply breaching the existing wall over a limited length. In order to undertake this in a controlled manner the size of the breach needs to be determined in advance. Current guidance on breach design was derived from a regression analysis of data from sites on the east coast that have breached during storms. This provides a simple relationship between the tidal prism of the site and the breach width. The basis of this approach is re-examined and shown to have limited explanatory power. An alternative method is presented, which uses the basic flow conditions, a limiting threshold of erosion and a description of the site hypsometry (variation in area with level) to estimate the breach profile. The proposed method is tested using the east coast data for seawalls that have breached and shown to provide better estimates. As the basis of the method is quite general, it can be used in widely varying situations, allowing rough estimates to be made with the minimum of data, or more refined estimates when more site-specific information is available.

The way in which the plan area of a drainage basin varies with elevation, is known as the hypsometry and is a useful way of capturing key elements of the geomorphological form. Variations in this form can alter the way the tide propagates and change the ebb and flood properties of the tide. The hypsometry can be particularly relevant where a sea wall is to be breached to create new mudflat and saltmarsh within a managed re-alignment site. A number of parametric relationships are examined and used to fit data obtained from bathymetry and topography for a range of estuaries, creeks and breached sea wall sites. These results are used as the basis for a method to characterise the hypsometry of inlets and breached sea wall sites. This is particularly useful for the design of breaches in managed re-alignment sites because the hypsometry has a major influence on the rate of infilling and hence the size of breach required to avoid scour problems. It also provides an indication of the likely habitat composition of the site ranging from mudflat to saltmarsh to supra-tidal.

The depth of erosion of cohesionless sediments underneath horizontal circular cylinders that are oscillated periodically in still water was of primary interest in this study. In this investigation anthracite was used for the sediments. First the formation of vortex ripples was studied by oscillating a box containing anthracite on its own in still water without the cylinder. The geometrical characteristics of the ripples obtained compared well with those of other investigations. In the experimental range studied, the low density of sediments had no explicit influence on the features of the vortex ripples. The box filled with anthracite and the cylinder were oscillated together to perform periodic motion and the depth of erosion beneath the cylinder was measured. For the diameters of cylinder tested, the height of the vortex ripples formed on both sides of the cylinder was comparable with the depth of scour underneath the cylinder. The experimental results are categorised as clear water scour and live bed scour. For both cases, the relative scour depth was weakly dependent on the Shields parameter for Keulegan–Carpenter numbers less than about 30. For Keulegan–Carpenter numbers greater than about 30, it increased with the increasing value of the Shields parameter.