Numerical simulation of vertical marsh growth and adjustment to accelerated sea‐level rise, North Norfolk, U.K.

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Abstract
In parts of North America and Europe, present and future sedimentary deficits translate into major areal losses of coastal salt marsh. Physically based simulations of medium‐ to long‐term adjustment to accelerated sea‐level rise are few, partly due to the difficulty in extrapolating imperfectly understood sedimentation parameters. This paper outlines the implementation and application of a simple one‐dimensional mass balance model designed to simulate the vertical adjustment of predominantly minerogenic marsh surfaces to various combinations of sediment supply, tidal levels and regional subsidence. Two aspects of marsh growth are examined, with reference to sites on the macro‐tidal north Norfolk coast, U.K.: (i) historical marsh growth under a scenario of effective (long‐term) eustatic stability but slow regional subsidence; and (ii) marsh response to various non‐linear eustatic rise scenarios for the next century. In contrast to more organogenic North American marshes, sedimentation rates in Norfolk are strongly time‐dependent. Where the overall sediment budget is so closely linked to marsh age and relative elevation, some form of numerical simulation offers a preferred means of predicting the impact of accelerated sea‐level rise. Simulations performed here show that only the most dramatic eustatic scenarios result in ecological ‘drowning’ and reversion to tidal flat within the conventional 2100 prediction interval. Currently favoured scenarios give rise to accretionary deficits which are clearly sustainable in the short‐term, albeit at the expense of increased inundation frequency and consequent changes in the distribution of marsh flora and fauna.