Practical implications of tidal flat shape

Arising from high precision empirical measurements of whole shore shape involving determination of incremental areas in narrow height bands undertaken during the 1980s, now confirmed by physical and mathematical models and theoretical studies, it has been established that accretion-dominated mudshores tend to be high and convex in cross-sectional shape, whereas their erosion-dominated counterparts tend to be low and concave. High and convex mudshores are preferable in coastal defence, port engineering and conservation terms to low and concave ones. In this paper the practical implications of these contrasted shapes are drawn out. The natural shapes of mudshores contrast with what might be anticipated from the well-established Bruun Rule for sandy shores. Accordingly, a new mudshore conceptual model, the Mehby Rule, is proposed which embraces all aspects of mudshore behaviour and will stand alongside the Bruun Rule. A new mudshore management tool, a top-down model, called S-factor analysis, is proposed and developed to a semi-quantitative state by way of a demonstration of its possibilities. It can be applied to quantifying the stability of mudshores. Mudshores with an S close to zero are stable in shape and approaching one type of equilibrium condition. As S departs progressively from zero in the negative direction shores become more accretion-dominated and, similarly, as they depart from zero in the positive direction they become more erosion dominated. A new mudshore management technology, tidal flat regeneration, is also proposed. The Mehby Rule shows that, to be stable and self-sustaining, whole shore profile manipulation is required, whilst S-factor analysis indicates these will have an S close to zero. By simultaneously placing a minimal wave attenuator and feeding with dense muddy dredge material, a low and concave mudshore can be converted into a more desirable high and convex one. The technology is proved from physical model tests and by analogy with the natural response of low and concave mudshores to a sudden change in the degree of wave protection. Thus, the new method seeks deliberately to copy accidentally induced shape change. The next stage will be to undertake full-scale field trials.