Salt-marsh growth and stratification: A numerical model with special reference to the Severn Estuary, southwest Britain

Abstract

Numerical modelling is one way to improve understanding of the growth and response to external factors of high tidal mudflats and marshes. Theoretically, mudflat-marsh growth is determined by the rates of minerogenic and organogenic sedimentation, the rate of change and tendency of relative sea-level, and the rate of “long-range” sediment compaction. A numerical simulation model for mudflat-marsh growth using these ideas is described and implemented for the Severn Estuary on the basis of empirical knowledge of its tidal and fine-sediment regimes and of the present-day order of magnitude of the deposition rate of fine sediment in its upper intertidal zone. The predicted curve of vertical growth relative to the tidal frame at first rises very steeply but thereafter flattens rapidly. Its precise form depends on the balance between the terms of the governing equation. When relative sea-level is rising, a marsh may grow, after a certain maturation time, to a stable elevation in the moving tidal frame that is substantially less than the elevation of the extreme tide, a situation that could inhibit peat formation. When the tendency of relative sea-level is downward, or when the rate of organogenic sedimentation is sufficiently large, a marsh after a certain emergence time may rise above the tidal frame, with the consequence that peat growth is favoured. The exploratory model is supported by a published growth curve for tidal marshes in eastern England, and by the observed internal stratification and relative elevations of the salt marshes developed along the Severn Estuary.