Methodologies

The methods used in sampling exposed and submerged sediment are very specific and transferring a method from the environment in which it was devised into a different one presented problems. Additionally replicating the conditions of flow experienced

in situ was vital.

4.5.2.1 Measuring sediment stability

The use of a CSM to measure sediment stability in exposed and submerged sediment allowed the stability of submerged sediment to be tested in a new way, bringing many of the advantages the CSM has on exposed sediment into the submerged environment.

timing and measurements. Additionally, being able to use the CSM in the field and the flume allowed sediment to be subjected to normal sub-critical erosion conditions prior to being tested. Of practical consideration was the addition of the plate to the erosion chamber (section 3.2.3.2), when deploying the chamber directly onto a submerged sediment surface it is very difficult to view the base of the chamber in relation to the sediment surface. Once on the surface the weight of the chamber was rarely supported by the sediment, meaning it was often pushed too deeply into the sediment, potentially reducing the distance from nozzle tip to sediment surface. The increased surface area of the plate allowed the chamber to be supported by the sediment surface and therefore be lowered accurately onto the surface without it accidentally being pushed too far into the sediment. The use of the CSM to measure stability removed the necessity to subject a sample to increasing flow speeds prior to erosion. The CSM was restricted to working in very shallow depths and the use of bunged cores to bring undisturbed sediment to the surface was sufficient for the needs

of this experiment but would prove inefficient and also depth limiting (≈5m snorkelling and ≈30m SCUBA, personal observation) for extensive sampling. This

may need to be assessed if more depth was required or the conditions were not suitable for snorkelling or SCUBA. The use of a Jenkins corer was attempted but the highly consolidated nature of intertidal sediment prevented it collecting an undisturbed core. The use of the CSM in the EPS experiment and river transect was highly advantageous. Deploying the erosion chamber in the flume (EPS experiments), directly on to the sediment surface (river samples) and then within sediment cores (estuary sample) allowed direct comparisons of results from three different submerged environments.

4.5.2.2 Sampling exposed and submerged sediment

Two methods of sediment sampling were used, the course core and contact core, both devised for use on exposed sediment and adapted for submerged sampling. Both methods sampled the upper 2mm as a homogenous layer, however with potential for delicate and micro scale changes in sediment properties (Sutherland et al., 1998) both

were deemed insufficient for highly accurate measurements of dry bulk density and water concentration. Of the two the contact core required access to the exposed

course corer was used in the same fashion on exposed and submerged sediment with the inner core placed onto the sediment surface, again this is probably too destructive to measure very small changes in dry bulk density and water concentration but if it could be modified to sample the overlaying water it may suffice. However, this would not be possible for exposed sediment. Despite this if the nature of the experiment required less accurate measurements such as the organic and colloidal carbohydrate concentration readings taken in the two supplementary experiments then either method of sampling cores is probably sufficient.

4.5.2.3 Replicating submersion in the laboratory

The potential for gradients in dry bulk density and water concentration to occur within the upper millimetres of the sediment (Sutherland et al. 1998) makes replicating

submersion conditions in the laboratory very important. While simulation of the conditions and hydrodynamics may be possible, great attention needs to be paid to the state of the cores. The surface layer of sediment needs to remain consistent from the estuary to the test chamber. Maintaining this state is highly important as the erosion threshold of the sediment increases dramatically once this sediment layer becomes more consolidated. This must be checked with comparisons of stability between laboratory and the field. In the one experiment in the laboratory where stability was equal to that in the field (initial submersion of Golf Course sediment) the results of submersion matched those found in the estuary. Of equal importance appears to be the need to replicate the hydrodynamic conditions of initial and continued submersion. When stationary water was used instead of flowing water in the laboratory or in situ the results differed from those submerged in flowing water. In

both the laboratory and field, submersion in stationary water for 2-3 minutes did not replicate the conditions that caused an increase in stability found with flowing water. Equally, prolonged submersion in stationary water caused a continuing decline in sediment stability which was not found in flowing water where stability remained constant after submersion. It appears that without the mechanisms and energy of flowing water the fluff layer is not removed in the initial submersion, causing the stability to remain equal to the exposed sediment. With prolonged submersion in stationary water stability continually decreases, possibly as the sediment becomes more unconsolidated, in flowing water these unconsolidated sediment particles might

be removed but in stationary water they remain on the sediment surface lowering the overall critical erosion threshold.

4.5.3 Supplementary experiments

The effect of submergence on the stabilising influence of EPS

The addition of EPS dramatically increased the stability of exposed sediment, and as expected this stability dropped as the sediments were submerged. To what extent this was caused by the EPS dissolving from the sediment and into the water is unclear as surprisingly measurements of organic and colloidal carbohydrate concentration both remained unchanged throughout the experiment. However, the large increase in initial stability, coupled with the drop while submerged, indicates that this is an area of study that is worth further investigation.

Submerged sediment stability along a river transect

A large change in sediment stability was found between sediments in different parts of the River Eden. Although water concentration and dry bulk density both changed in relation to the stability it is considered unlikely that these are the only variables that will affect sediment properties along such a transect. Grain size remained constant throughout the sample sites (data not shown) while salinity significantly increased between each site towards the sea (data not shown). The implications for this are highly significant with the movement of sediment and related chemicals along a catchment area of great importance to coastal management (Gerbersdorf et al., 2007).

Therefore, hopefully the work achieved in measuring underwater sediment properties and stability can be applied.

4.6 Conclusions

 Intertidal sediment stability changes dramatically under submersion with moving water. However, this change is not found when sediment is submerged in stationary water.

 Sediment properties are not affected universally by submersion with a large degree of heterogeneity occurring in the variation of properties.

sediments to overall system properties are important although not clarified greatly by this work.

 If there is a fluff layer on exposed sediment then the current methods of detecting it are inadequate and this will require more research.

 If the fluff layer is being removed with the incoming tide then this needs to be quantified as over a tidal flat it will account for a large volume of sediment.

 Methods of sediment sampling devised on exposed sediment are not adequate for sampling submerged sediment.

 Laboratory replication of sediment submersion is possible, however, stationary water is not a suitable substitute to replicate natural flowing conditions.

 The CSM is an ideal machine for measuring stability on exposed and submerged sediment.

 The development of a CSM machine that can be used in situ on deeply

submerged sediments, removing the dependence on collection of sediment cores, would be highly advantageous.