The Long-term sedimentary regime of the outer Medway Estuary

Estuary evolution over the last 300 years is evaluated, based on pronouncements by some that the recent erosional phase was a temporary response to human intervention and is now shifting back in favour of accretion. In contrast, a painting from 1698, 1700ʹs surveys and accurate surveys from 1800 and 1865 confirm that the erosional phase commenced naturally around 1700. From the early 1840s the saltmarshes were heavily exploited for their clay by the brick and cement industries. One or more storm surges in 1875, 1877, 1881 and 1897 breached embankments of many “inned” (reclaimed) marsh pastures. In deference to this exploitative industry these were never reclaimed. Natural loss due to sea level rise-induced erosion was thus exacerbated by saltmarsh digging. Fine sediment budget measurements show saltmarsh cliff, creek and tidal flat surface losses greatly exceed gains on vegetated salt-marsh surfaces. Stoke Marshes is losing about 160,000 tonnes/yr and loss for the entire intertidal zone is thought to lie close to 1 M tonnes/yr. The exceptionally low-lying tidal flats are highly vulnerable to sea level rise. Relative sea level will rise due to 3 overlapping factors and is suggested to be accelerating. 50 cm increments of relative rise would induce losses to 28, 44, 72 and 83% respectively. In respect of the major tributary and estuary channels, in a short period coupled embankment breaching together with mud digging greatly increased the tidal volume passing Sheerness. Tidal velocities rose significantly. North Sea Pilot extracts, plus tidal cycle measurements at major creek entrances, reveal a strong ebb-dominance, which is further confirmed by a fluorescent tracer test on labelled muddy dredge material. Over a 132 year period the estuary bed has self-deepened and a further increase in tidal volume has arisen. With accelerating sea level rise in a starved ebb-dominated system there is no known mechanism which could shift the Medway back in the direction of accretion.