Severn Estuary

The hypertidal Severn is suited to the kind of shipboard investigation of cohesive sediment behaviour envisaged by the Instititute of Oceanographic Sciences on grounds of exhibiting high and variable turbidities in which fine sediment is cycled through the estuary in a regular and predictable manner. The latter arises because it is tidally dominated, as opposed to the more common situation where natural systems are episodically (storm) dominated.

In respect of undertaking research, tides can be predicted and anticipated whereas storms are random and unpredictable. This facilitates planning shipboard experiments, which have a long lead time. Starting from 1969 new and dedicated instruments were developed to measure suspended sediment concentration rapidly and continuously in what became known as “dynamic mode”, ie. from a vessel hove-to as opposed to anchored (see Home Page). Parallel developments in tidal current measurement supported such work (see CV).

The sequence of 24 large shipboard research investigations undertaken by IOS in the 1970’s spun-off into twin academic and commercial pathways.

In the academic sphere Dr. Kirby and colleagues discovered and explained the now widely perceived cohesive sediment phenomena:

  • Lutoclines
  • Suspended Sediment Fronts
  • Slug Flow

These represent segregations by various mechanisms of the suspended sediment population in the vertical, lateral and longtitudinal planes with respect to flows.Academic scientists around the world have taken up and advanced these new and important discoveries (see CV).

In the commercial sphere the instruments developed for this research have been adopted widely around the world by other academics, as well as being applied commercially to understand other estuaries and coastal regions, including in the port and shipping and industry.

Similarly this academic understanding renders the Severn the best understood turbidity zone (turbidity maximum) of any estuary in the world. Consequently the academic knowledge feeds directly into small and large commercial engineering projects in the estuary. Examples:

  • Severn Tidal Power Options
  • Deep Sea Container Terminal, Avonmouth
  • Hinkley “C” Nuclear Power station
  • Shoreline Management Plan
  • Second Severn Crossing
  • Habitat creation by set-back at Stert Flats and Kingston Seymour

It is true to say that the suspended sediment population is so extremely high and predictably variable that it dominates any engineering consequences of works themselves, as well as all aspects of estuary ecosystems from a regional scale down to the micro-structure of a floc.


Plan view of average bed suspended particulate material concentrations for neap flood tides between Watchet and The Shoots in mg l-1 . Dots are standard stations for which data is available. Concentrations routinely reach 10,000-15,000mg l-1 on the turbid English side. The strong lateral concentration gradient indicates a suspended sediment front.

Core sample localities and distribution of erosional, stable and depositional zones in the subtidal mud patch in Bridgewater Bay. Since making the map, the entire muddy intertidal zone is also recognised to be eroding.