Lateral extent • Elevation change • Changes in creek and marsh • Changes in floristic composition • Changes in vegetation vigour

Changes in creek and marsh surface morphology

Changes in tidal creek morphology (exemplified by channel density, channel volume, plan form and bifurcation ratio) naturally take place in a saltmarsh as it evolves. Based on an analysis of thirteen British marshes, Steel and Pye (1997) developed a model (see below) which proposes that tidal creek systems in natural marshes undergo a series of stages which are related to the marsh elevation and frequency/duration and volume of tidal flooding.

Channels initially develop on the precursor mudflat or sandflat surface but become more numerous and better organised as the marsh begins to rise in the tidal frame. The drainage density increases to a maximum before starting to decline again, as some sections of creek are abandoned. The critical point at which expansion changes to contraction is related to the marsh surface elevation and tidal frame. As a general rule, the maximum drainage density is found when the marsh surface is covered by approximately 280 tides per year.

Variations to this natural pattern of development can be brought about if, for example, a new marsh begins to develop (see figure below) to seaward, effectively lengthening the main creeks, or if some creeks are 'beheaded' by embanking and reclamation. Large-scale reclamation within an estuary may lead to a significant reduction in the estuarine tidal prism, or the tidal prism relating to a specific section of flanking marsh. In some circumstances, the reduction in tidal volume entering and leaving the marsh will result in reduced flow velocities and net sediment accumulation within the creek system. The major channels will become narrower and shallower, while some finger-tip tributaries may be abandoned and develop into linear salt pans or vegetated depressions. In other circumstances, an excess of tidal energy may be maintained within the creek system, resulting in local scouring, creek enlargement and the development of mud basins. Which of these two scenarios occurs is dependent on the general morphology, tidal energy conditions and sediment transport regime within the estuary in question.

Schematic model showing stages in the development of tidal creek networks on tidal flats and saltmarshes (after Steel and Pye, 1997)

Many estuarine marshes in Essex and North Kent have suffered from expansion of the creek network following reclamation and/or renewed flooding of formerly reclaimed land. Coalescence of the creeks in some areas has resulted in the formation of extensive mud basins and many such marshes are characterised by a relatively low ratio of vegetated to bare mud surface (e.g. Canvey Island, Essex see Figure below). The pattern of erosion within such marshes is often heavily influenced by natural and human-induced differences in the sediment properties (e.g. Crooks and Pye, 2000). Predictions of the likely results of re-introducing tidal waters to a reclaimed area can often be made using data from airborne LiDAR and ground surveys (e.g. Blott and Pye, 2004).

Aerial photograph of part of Canvey Island, Essex, showing severely eroded area of marsh formed after the breaching of the surround sea wall in 1881 (from Toft et al., 1995)