Geomorphology and Sedimentology of Estuaries (Developments in Sedimentology)

References

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The bedrock geology of St Ives Bay and the Hayle catchment is largely composed of Devonian metasediments mudstones, siltstones, slates and sandstones. Permian granitic intrusions bound the bay to the west at St Ives , and border the Hayle catchment to the south and southeast. The bay is considered to be a closed sediment cell, suggesting that over the mesoscale , the sediment budget is balanced by morphodynamic adjustment across the estuary, inlet, beaches and nearshore [ 16 ].

Photographs showing surface conditions in the four main sedimentary environments surveyed. This means that the sites sampled for this study were those allowed by the conservation authority in the District with the limit of 15cm core permitted to be excavated at the sites. Cores acquired at low tide, were sealed, tagged and returned to the laboratory intact. Short 15cm sediment cores obtained from 80 sample sites using a 65 mm diameter tube were sliced at 1cm intervals and the grain size analysis was undertaken on these subsamples, using a Malvern MasterSizer particle analyser, which uses a laser diffraction principle, detecting sizes across the range of 0.

Grain size analysis has been widely used to statistically examine spatial changes in sediment size properties. Recent applications include the studies by Jitheshkumar et al. Principal Component Analysis PCA was used to reduce the grain size distributions across all samples into a smaller number of key variables.

Sediment Characterisation in an Estuary-Beach System

Hierarchical cluster analysis using Euclidean distance and average linkage was applied to the grain size distribution to organise samples into groups comprising similar sedimentological characteristics, specifically for each of the system. These calculations were undertaken in Matlab. This section presents the results and discussion of the analyses carried out on the sediment. Grain size distributions of sediment sampled from sites A to D are summarised and presented in Figure 3. Surface sediment characteristics in the St Ives Bay - Hayle estuary system.

Grain size distributions are shown for all samples obtained within the depth range cm at locations in the 4 main sedimentary environments examined: The mean distribution at site C is very similar to that at site B, but site C lacks consistency between samples which range between medium coarse and very coarse sand.

Site A in the inner estuary is distinct in the significance presence of finer material, either as a distinct clay population, a silty population or a silty tail to a dominant sandy population. Clay and silt are not present in the surface samples obtained at sites B-D. The presence of the fine-medium sand population in all of the sites however suggests that sediment exchange is active between sites.

Exploratory sediment analysis — grain-size statistics — A Mean vs.

Sorting B Mean vs. Skewness C Median vs. Sorting, and, D Median vs. These sites do show some discrete differences. Site B is moderately sorted whilst C and D are moderately well sorted. The combination of grain-size statistical parameters such as the median, sorting and skewness, obtained from the analyses of grain-size distribution in this study have been used in understanding the pattern of estuary - beach sediment exchange. The overlap of marine derived and fluvial-derived sediment populations is evidenced in identifiable areas where there is exchange between the estuary and the beach coast , for example the samples from the inlet Figure 3C.

This means that to a large extent, there is at least some exchanges between the inner estuary and the beach, evidenced by the sediment at, and close, to the surface. This has probably reflected the characteristics of the primary sediment source for this region, which could be marine, largely derived from glacigenic shelf sand which contribute a fine - medium sand population to a wide range of coastal sedimentary environments in northwest Europe [ 26 ].

The large-scale tidal current forced bedforms noticed in the inlet and inner- sub-estuarine environments of the study sites Figure 2A and 2C , which demonstrates that tidal forcing is clearly significant in the region especially in transferring sediments within and between the sedimentary systems, therefore enabling the delivery of core fine-medium sand to most parts of the estuaries Figure 4.

However, it should be noted that the sand circulation in the system does not appear to be in equilibrium as the greater amounts of sand entering the estuarine environments are transported to the beach, thereby producing a landward extension of the sand inlet into the estuaries Figure 2C. Tidal currents alone are not responsible for sediment transport - other energies are also important in maintaining the estuary - beach sediment exchanges.

Prominent of these forcings is the wave energy [ 27 ]. The combination of tidal and wave energies is of great influence and is enhancing the flood tide erosion in the processes, and this tend to increase the supply of sand to the estuarine environments. Within the estuarine environments, however, there is a possibility of a prolonged downstream bedloads transport of sediments as a result of low tide thereby causing the remixing of sand populations with other sediment populations presumably sourced locally or representing locally specific processes Figure 4A.

In the system here, river flow is active. The beds and the banks of the River Hayle and its tributaries entering the estuaries do carry wide range of materials including but not limited to considerable amount of mud and organic matter. The significant departure of the inner estuarine environment from the results presented in Figures from other environments e. Sand transport processes are the predominant sediment movement and dynamics between estuary and beach in this study site.

Mud and gravel are present, primarily at the margins of the inner Hayle estuary. Mud is certainly important to the development of inner estuary tidal flats and saltmarshes, whereas gravel is likely an inherited relic sediment along estuarine margins. But the beach and estuary sediment system is dominated by sand. Based on the spatial patterns in particle size of surface sediments, two dominant populations can be used to determine the nature of sediment linkage between the estuary and beach — marine and fluvial. Marine sand, which is predominantly of medium-coarser sand population Figure 3B-3D , has limited exchange with the estuaries Figure 3A.

Within this sub-environment, there is possibility of sand being transferred from the inner shelf to the beaches, either by the prevailing south westerly west to east longshore currents induced by wave activity or by tidal currents that counter-balance the littoral drift in the sub-tidal zone. This applies to the beaches of St Ives Bay. The lack of the coarse sediment population from the Hayle estuarine environment Figures 3A and 4A implies transport processes connecting the beach with the estuary are either unable to transport this grade of material, or that the transport linkage does not exist.

In most cases however, the beach St Ives Bay sediment populations do comprise a significant fine-medium sand population that is also present within the estuaries. Inlet sediments, but also those associated with the flood tidal delta, comprise this fine-medium population Figure 3C. This is considered here as evidence of a sediment linkage between the estuary and beach. The spatial analyses of the composition reveal that the medium-fine sand population is found just landward of the inlet in the Hayle estuarine valleys, marking the boundary between the beach and the estuary.

The predominant sand deposits here are likely derived from the adjoining beaches.

A silty population is present in the inner estuary sample site, particularly the area in sheltered inner embayment, close to estuary margins or close to the estuary head Figure 3A. Despite the compartmentalisation of sediments in Hayle system, it is clear that the sediment populations indicative of one depositional environment or rouse are found at other sedimentary locations, i. This shows that mixing is important throughout this system. Interaction between estuarine and coastal processes leads to the mixing of the sediment populations. Summary of sediment statistics for sedimentary environments at the Hayle Estuary.

Data are divided based on Site A - D, stated in figures 1 and 2 and depth using cm, cm and cm stratigraphic units. Comparison of grain size statistics grouped by site sedimentary environment and sample depth cm, cm and cm from the intertidal sediment surface reveals little systematic variation in grain size parameters with depth. However, differences between sub-environments are evident, with the estuarine samples Site A being finer, less well sorted, and more strongly negatively skewed. Results p-value of one-way analysis of variance of selected sediment statistics, considering groupings based on sample site and depth, using the Kruskal Wallis non-parametric method.

This is primarily driven by the properties of sediments from site A which is significantly different from all other sites for all metrics except skewness for site B and sorting for site D. There is no significant difference between the median grain sizes at sites B and C, and no significant difference between skewness at sites C and D. The results show the consistency in sediment characteristics with depth suggesting that the depositional environments are well mixed to at least 15cm depth.

Duplicate citations

The analysis shows that variability in sediment size characteristics within the St Ives Beach - Hayle intertidal system is the product of sedimentary environment, not sample depth. Multivariate analyses were undertaken to explore patterns in the full particle size distribution.

Principal component scores in relation to the grain size distribution for the St Ives Beach - Hayle system see text for explanation. PC1 is strongly correlated negatively with sorting, whereas PC2 is very strongly correlated positively with median grain size. Combined plots of PCA and cluster analysis of the grain size distribution A , comparing the relative sub-environment B and relative stratigraphic depth C. Samples from sites B and D west and east extent of the open coast are strongly separated on PC2: Only samples from site A estuary show any significant distribution along PC1, reflecting a mix of well to poorly sorted sediments at this site, in addition to the presence of very fine material.

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Birding Australia 2 All Regions Reference Collections Management Reference: Out of Print Details. About this book First book to be devoted entirely to the geomorphology and sedimentology of estuaries. Contents Geomorphology and sedimentology of estuaries - an introduction, G. Perillo; definitions and geomorphologic classifications of estuaries, G. Perillo; sedimentary systems of coastal-plain estuaries, H. Bokuniewicz; geomorphology and sedimentology of rias, P.

Guilcher; sedimentology and geomorphology of fjords, J. Shaw; tide-dominated estuaries and tidal rivers, J.

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