Bedform migration in a mixed sand and cohesive clay intertidal environment and implications for bed material transport predictions

Abstract

a b s t r a c t a r t i c l e i n f oMany coastal and estuarine environments are dominated by mixtures of non-cohesive sand and cohesive mud. The migration rate of bedforms, such as ripples and dunes, in these environments is important in determining bed material transport rates to inform and assess numerical models of sediment transport and geomorphology. However, these models tend to ignore parameters describing the physical and biological cohesion (resulting from clay and extracellular polymeric substances, EPS) in natural mixed sediment, largely because of a scarcity of relevant laboratory and field data. To address this gap in knowledge, data were collected on intertidal flats over a spring-neap cycle to determine the bed material transport rates of bedforms in biologically-active mixed sand-mud. Bed cohesive composition changed from below 2 vol% up to 5.4 vol% cohesive clay, as the tide progressed from spring towards neap. The amount of EPS in the bed sediment was found to vary linearly with the clay content. Using multiple linear regression, the transport rate was found to depend on the Shields stress parameter and the bed cohesive clay content. The transport rates decreased with increasing cohesive clay and EPS content, when these contents were below 2.8 vol% and 0.05 wt%, respectively. Above these limits, bedform migration and bed material transport was not detectable by the instruments in the study area. These limits are consistent with recently conducted sand-clay and sand-EPS laboratory experiments on bedform development. This work has important implications for the circumstances under which existing sand-only bedform migration transport formulae may be applied in a mixed sand-clay environment, particularly as 2.8 vol% cohesive clay is well within the commonly adopted definition of "clean sand".

Integrating field and laboratory approaches for ripple development in mixed sand–clay–EPS

“…However, the age of the sandiest ripples at the field site might extend further back in time, because it is unknown how long these bedforms had been at equilibrium. Yet, it is unlikely that these bedforms formed more than three or four principal lunar semi‐diurnal tidal cycles before the ripple wavelength data were collected because strong waves on 24 May 2013 generated wave ripples, combined flow ripples, and upper‐stage plane beds (Lichtman et al ., ). The model predictions for S L and L SL in Fig.…”

“…A) have proven valuable as a natural laboratory for studying bedform dynamics in mixed sand–mud, owing to the large variety in sand–mud ratio, ranging from pure sand to sandy mud. During the field campaign in May 2013, the intertidal flats also contained significant amounts of EPS, ranging from 0·02% to 0·21% by weight (Lichtman et al ., ). The present paper uses textural and morphological data collected from a 1400 m 2 area on 26 May 2013, where conditions had been essentially wave‐free in the previous two tidal inundations.…”

“…Because of time constraints during low slack water, it was not possible to collect EPS samples that exactly corresponded to these clay samples, since the former needed to be frozen in liquid nitrogen in the field (Hope, ). It was thus necessary to rely on the relationship between clay content and EPS content by weight, e , determined nearby (Lichtman et al ., ): ( R 2 = 0·41, P < 0·05, n = 20), yielding EPS contents ranging from 0·04 to 0·17% by weight. Adjacent to the area where the bedforms were measured, near‐bed flow velocities were measured continuously during water cover using two frame‐mounted acoustic Doppler velocimeters (ADV) located at 0·25 m and 0·40 m above the sediment surface.…”

“…Following Lichtman et al . (), the cohesive clay fraction, c , can therefore be expressed in terms of the mud content, m , as: thus yielding cohesive clay fractions ranging from 1·4 to 13·4% by volume. Because of time constraints during low slack water, it was not possible to collect EPS samples that exactly corresponded to these clay samples, since the former needed to be frozen in liquid nitrogen in the field (Hope, ).…”

“…. The difference in cohesive clay content in the samples from the subsurface and the ripples agrees with previous studies that associated the development of bedforms with the winnowing of fine particles (Baas et al ., ; Schindler et al ., ; Parsons et al ., ; Lichtman et al ., ). The winnowing depth is limited by the height of the bedforms, which explains why the clay content in the subsurface is up to three times higher than the clay content in the ripples on the surface (Table ).…”