COHBED

Realistic Sedimentary Bedform Prediction: Incorporating Physical and Biological Cohesion

The
Challenge

The United Kingdom is a coastal nation with the majority of the population living within a few miles of an estuary or the sea.

The nature of the coastline depends on the local conditions of geology and water flow. Rocky coastlines are found where the energy of the sea is high, while mud and sand are found where the energy is lower and these sediments can be deposited. These low energy muddy and sandy (depositional) habitats, are very important for the ecology and economy of the UK. They provide food for many species of birds and fish, but also protect the coastline from the erosive forces of the sea. In addition, they act as a "filter", where pollutants from the rivers are captured and eventually degraded.

The natural behaviour and stability of these muddy and sandy habitats is of increasing concern as sea levels rise.

Because of the importance of these systems, their natural behaviour and stability is of increasing concern as sea levels rise and storm events increase in frequency with climate change.

The movement of sediment around the coast of Britain has vast economic and ecological consequences, but surprisingly we have very little scientific information that helps us to predict how natural mudflats and beaches will respond to the changing forces of the tides, wind and waves. Information allowing us to predict the shape, size and movement of bedforms is essential for environmental management, hydraulic engineering, benthic habitat biology, computer modelling of particle transport, sedimentary geology, and many other scientific disciplines.

The
Approach

Lead researchers

Project partners

COHBED utilised the latest developments in measurement technologies to produce information about the growth, movement and stability of bedforms consisting of natural mixtures of sands and muds, a natural condition that is very common but has rarely been studied before.

In a new departure, this work included a multidisciplinary team to combine the physics, mathematics, sedimentology, and biology of these systems, in recognition that the organisms inhabiting natural systems also change the erosional characteristics and bedform behaviour. Hence, COHBED included laboratory experiments and field surveys. A series of experiments in laboratory flow channels investigated key factors that control the behaviour and properties of bedforms, such as:

System energy: effects of flow velocity, bed friction and flow depth
Bed properties: particle size, proportion of mud and sand, and biological effects
Time: the speed of bedform growth and rate of change as flow energy changes
Particle erosion: changes in the bedforms as smaller particles are eroded away

The Impact

This project revealed that the physical cohesion imparted by cohesive clay within mixed sand-mud substrates has a profound influence on the dimensions and dynamics of sandy bedforms such as ripples, dunes and bars. It also identified the significant role that biological extracellular polymeric substances (EPS) has in generating cohesion and influencing bed form dynamics. Results showed that bedform height, length and steepness decrease linearly with clay and biological content; existing equations that predict bedform dimensions significantly over-predicted those dimensions for all but the smallest clay contents examined. The profound effect substrate clay content has on bedform dimensions has a number of important implications for interpretation in a range of modern and ancient environments, and offers a step-change in our understanding of bedform formation and dynamics in these environments. In particular, project outcomes will help: 1. improve engineering predictions of bedform dimensions, which will help guarantee engineering success when extracting sediment or building structures in and on the sea bed (e.g. pipelines, wind turbines, tidal barrages, oil platforms); 2. provide a vehicle for facilitating more reliable palaeo-environmental reconstructions; and 3. improve confidence in reconstructing and predicting the porosity and permeability of sedimentary rocks in cores and outcrops. These latter outcomes bring significant benefits to the hydrocarbon industry.

The Challenge

The Approach

The Impact

Other completed research projects

The
Challenge

The
Approach