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In intertidal systems the initial submersion is by a small tidally driven “wave” of flowing water. From the data presented, this “wave” can cause an increase in sediment stability, with a faster incoming tide, with more energy, more likely to result in an increase in stability. In laboratory-based experiments, submersion in stationary water and the slowest flow speed did not change stability in Golf Course sediments, while faster flows increased stability. Papermill sediments were not affected by submersion. In situ results show an increase in stability in Golf Course sediments

with submersion and while not significant the pattern was repeated in Papermill sediments. Again submersion in stationary water did not cause a change in stability. After longer periods of submersion, sediment stability from the laboratory and in situ

experiments appeared to contradict each other, with in situ sediments increasing in

stability with submersion, continuing the trend from the initial submersion, but laboratory-based sediments remaining stable or reducing in stability.

The presence of a layer of unconsolidated sediment upon the intertidal exposed sediment surface, similar to the fluff layer found on submerged sediment (Sutherland

et al., 1998; Wang et al., 2003; Kuhrts et al., 2006; Schaaff et al., 2006), may be the

explanation for the change in stability with submersion and also the apparently contradictory results between laboratory and in situ experiments. Such a layer is

fluff layer has a significantly lower critical erosion threshold than the deeper sediments and can be easily eroded, exposing the more stable sediment beneath. If such a layer were present on exposed sediment then it may possibly be eroded quickly during the initial act of submersion, exposing the deeper more consolidated sediment. This could explain the increase in stability with submersion found in situ as the lower

critical erosion threshold on exposed sediment may be attributed to the erosion of this fluff layer. The increased stability once submerged may be because the fluff layer has been removed by the tide and it is the deeper, more consolidated, sediment that is being eroded. The difference in results from exposed and submerged sediments would therefore not be a result of a change in the properties of the sediment, but rather a change in the sediment that is being tested.

The presence of a fluff like layer upon the surface of exposed intertidal sediments could be generated by similar mechanisms to that found in submerged sediment. Deposition of sediment could occur with the receding tide, and bioturbation of sediment will be most evident on the sediment surface resulting in a layer of unconsolidated sediment and organic debris (Orvain et al., 2003). Such a fluff layer

may consist of well spaced unconsolidated sediment particles with large inter-particle spaces filled with water or air which may allow it to be eroded at low energy levels. The contradictory results from the laboratory and in situ can be used to support the

theory of a fluff layer. The critical erosion thresholds of the exposed sediments from the laboratory and in situ differ hugely, with sediments from both sites having critical

erosion thresholds of about 104Nm-2 and 25Nm-2 respectively. The higher stability of the exposed laboratory sediment is possibly because the fluff layer has become consolidated onto the underlying sediment during the time from collection of cores to testing. Due to the experimental design this time gap could have been up to 10 hours for some cores, potentially allowing compaction of the sediment particles or loss of water from the sediment surface through evaporation or drainage into the deeper sediment. This is supported by the similarity in stability of the exposed laboratory cores and the submerged in situ sediment from which the fluff layer has been

That such a layer has not been evident in previous studies on intertidal sediments is possibly due to the methods used in detecting erosion. The CSM is capable of very high resolution at low erosion thresholds which makes it ideal for detecting the erosion of the unstable fluff layer. The CSM is also a small and relatively delicate machine to use upon the sediment surface, whereas the use of larger flumes or transportation of sediment cores may result in a disruption to the fluff layer, either allowing it to become more consolidated or inadvertently removing it before it can be tested. When coring submerged sediment it is important to maintain the overlaying water column to prevent disruption to the sediment surface before testing (Hauton & Paterson, 2003; Schaaff et al., 2006; Spears et al., 2007) and it is possible that

maintaining the environment in which exposed sediment is sampled is equally vital. After submersion for the longer time periods that the laboratory experiments allowed, stability of the sediment from the two sites differed, with Papermill sediments remaining unchanged and Golf Course sediments dropping in stability after an hour then remaining constant. However, the validity of these results has to be questioned if a fluff layer has become consolidated into the sediment then the properties of the sediment surface are probably different to that of sediment from which the unconsolidated layer has been removed.

4.5.1.2 Sediment properties

Tolhurst et al., (2005) argued that a given volume of intertidal sediment is made up of

six components; non-cohesive mineral grains, cohesive mineral grains, water, gas, biota and other matter. Ignoring the last two components (which is probably incorrect but useful for simplicity) a sediment core can be divided into solid (sediment particles), liquid (water) and gas (air), the contribution of each component to the overall volume of the core can be given as a concentration. When comparing exposed and submerged sediment it is possible that the meanings of these values will change. The dry bulk density (sediment concentration) will also give some indication of the volume of the inter-particle spaces. In exposed sediments this space will be filled with water and air, of which only water can be measured (expressed as water concentration) and is therefore a measure of how much of the inter-particle space is filled with water. However, once submerged it is probable that air is removed from

volume of the inter-particle space. Therefore, while dry bulk density measures the density of the sediment particles in both exposed and submerged sediment, the interpretation of the water concentration measurement is potentially different.

Over all, sediment water concentration tended to increase with submersion although this was not found on longer in situ submersion. However, dry bulk density did not

consistently change with water concentration, implying that water was replacing air in the inter-particle spaces without causing a change in the density of the sediment particles.

There was no consistent change in organic and colloidal carbohydrate concentrations with submersion. This is not surprising for organic concentration since it is a measure of all organic matter within the sediment, most of which would be expected to remain within the sediment. However, it was expected that the concentration of colloidal carbohydrates would drop with submersion due to their solubility. This did not occur possibly because the colloidal carbohydrates are bound within a mixture of non- soluble components and as such are prevented from dissolving into the water column. However, this does contradict the findings of Blanchard et al., (2000) where

carbohydrate levels dropped with increased water levels

If changes in sediment stability are in part due to an unconsolidated fluff layer on exposed sediment it would be expected that dry bulk density would increase and water concentration decrease with its removal. As with measurements of stability this would not be a change in the sediment properties but rather a change in the actual sediment that is being tested. Organic and colloidal carbohydrate levels may also drop if they are present in high proportions within the fluff layer. That this was not found may be a result of the sampling methodology being inadequate to detect the removal of the fluff layer. Sutherland et al. (1998) used X-ray tomography in

submerged sediment to quantify the bulk density of the fluff layer. They found a low bulk density on the sediment surface which increased with depth until a constant value was obtained in the consolidated sediment. At most the low bulk density extended to a depth of 1.5mm. Both the contact core and course core methods used in

this destroyed the fine scale gradient in water concentration and dry bulk density that a fluff layer would display.

4.5.1.3 Sediment properties relating to sediment stability

The changes in stability that occurred between exposed and submerged sediments within each experiment were not found to relate to a change in sediment properties. On a larger scale, decreases in both dry bulk density and colloidal carbohydrates, and increase in organic concentration all correlated with increasing stability. The changes in dry bulk density and colloidal carbohydrates would appear to contradict expectations, as usually increases in the density of the particles and colloidal carbohydrates (as a proxy for EPS) cause increases in the stability of sediment surfaces.

4.5.1.4 Extrapolating from exposed to submerged sediments

There are many changes in sediment stability and related properties between exposed and submerged sediments, and these appear to vary depending upon the nature of the submersion. As erosion will occur when submerged this has important implications for using measurements taken on exposed sediments as an indication of the erosional properties of the sediment. However, this work did not present many definitive answers to these problems, indeed, as with work on exposed sediments it is quite possible that the influences on sediment stability once submerged are as site specific as they are on exposed sediments (Defew et al., 2003; Friend et al., 2003a).