Groundwater dominated chalk streams regularly exhibit higher quantities of accumulated fine sediment (inorganic and organic particles <2 mm) within their gravel beds compared with other UK systems due to their natural flow conditions, notably low bed mobilising flows. This characteristic, in combination with their fine sediment-sensitive species, creates a high propensity for lethal/sub-lethal ecological impacts. Current approaches to management targets and targeted interventions have failed in chalk streams due to a lack of scientific knowledge underpinning them. Although research has quantified fine sediment infiltration and accumulation in chalk stream gravel beds, little is understood regarding remobilisation that leads to the 'cleanout' of fine sediment. To address this gap, flume experiments were carried out to investigate the remobilisation depths of fine sediment (especially cohesive sediment <62.5 mu m) from the ecologically sensitive surface layer (0-10 cm) of a typical chalk stream gravel bed, across a range of flow conditions. Bed shear stresses in the flume experiments ranged from 0.6 to 8.1 Pa; increases in bed shear stress corresponded to increases in fine sediment cleanout depth. Fine sediment remaining after experiment runs indicated two processes of remobilisation important in keeping the surface layer of gravel beds clean of excessive fine sediment: flushing from the bed framework and hydraulic winnowing within the bed framework. The data were used to evaluate the validity of established models for predicting fine sediment remobilisation from gravel beds. Comparisons between observed and predicted cleanout depths demonstrated that established models tend to overpredict cleanout depths. Existing models appear unsuitable for use in chalk streams due to assumptions within these models and their failure to represent the natural characteristics of chalk stream gravel beds. The novel data generated by this study can be applied to direct revised fine sediment targets, management and restoration activities.

The Mar Menor is the second largest coastal lagoon in the Mediterranean Sea, with a surface area of about 136 km2. It is restricted from the open sea by a sandy barrier system (La Manga) interrupted by three tidal inlets. As a result of high evaporation, it is hypersaline (42-47 ppt) in parts. This study examines the factors leading to the rise in sea surface temperature in the Mar Menor through an analysis of long-term sea surface temperature using HadSST1.1 data together with shorter-term Moderate-Resolution Imaging Radiometer and Optimum Interpolation Sea Surface Temperature data. A thermal box model has been constructed for the lagoon in an attempt to balance major heat sources and sinks. Additionally, a thermal probe was deployed in 0.3 m of water to evaluate the benthic flux of heat of the shelly fine sand that covers the lagoon seabed. The results show that the vertical thermal gradient in the seabed inverts between the day and night. Prior to circa 1977, there was no clear trend in SST, and variations were strongly associated with the Atlantic Mutidecadal Oscillation and the North Atlantic Oscillation. Post circa 1980, the maximum summertime sea surface temperature showed a steady increase of 0.34 degrees C/decade. The cross-correlation of SST in the Mar Menor with external drivers showed that it is dominated by the sea surface temperature of the Western Mediterranean, followed by local air temperature, with a minor contribution from the Indian Ocean Dipole. No other significant correlations were evident, suggesting that local temperature was dominated by local drivers. In addition, a Spearman rank order evaluation and principal component analysis showed that the general trends of the Mar Menor SST were also influenced by the Atlantic Multidecadal Oscillation, CO2, and GDP.

The temporal clustering of storms can present successive natural hazards for coastal areas in the form of extreme sea levels, storm surges and waves. Studies have investigated the prevalence of the temporal clustering of such hazards but are hindered by the rarity of the phenomena combined with short records and a lack of data availability around the coastline. This has made it difficult to determine if the levels of clustering reported were typical for the location or were being masked by natural variability or climate change over different timescales. In this study, we assess a near 500-year model simulation of extreme sea levels and storm surges forced with pre-industrial meteorological conditions to quantify the levels of temporal clustering seen from natural variability around Great Britain. We then utilise a 50-year rolling window to see how clustering statistics can change through time when dealing with time periods that are representative of the average length of a record in the United Kingdom National Tide Gauge Network. When using near 500-year timeseries, we highlight that many clustering statistics return values close to their statistical expectancies. However, when analysing discrete 50-year windows, results can vary dramatically. The percentage of years with an extreme sea level or surge exceedance at a given location at the 1 in 1-, 5-, and 10-year return level, can vary by up to ~ 33%, ~ 24%, and ~ 18%, the mean number of days between consecutive sea level or surge exceedances can vary by ~ 231, ~14,780, and ~ 17,793 days, and the extremal index can vary by ~ 0.37, ~ 0.64, and ~ 0.79, respectively. Although these results represent the best estimate of the levels of clustering to be expected under natural variability, a comparison of the longest records in the tide gauge network and their nearest model grid nodes shows a tendency for the model to underestimate the clustering statistics that are calculated from the measured data (apart from the extremal index). As such, these can be considered to represent the minimum levels of temporal clustering around Great Britain, as the potential underestimation of clustering, combined with climatic change and sea level rise, means that the temporal clustering of sea levels and storm surges are likely to be far greater over the next 500 years.

Understanding temporal variations in nearshore sea states is crucial, as they affect shoreline evolution and coastal hazard potential. Local sea state conditions are influenced by large-scale climate modes, yet the underlying mechanisms remain not fully understood. Previous studies have mainly established the climate–sea state links through correlation analyses or other statistical methods. This study investigates whether weather typing, a statistical downscaling method, can provide a physically interpretable link between climate modes and local wave and storm surge variability. The analysis was conducted at Hartlepool, UK, where 36 weather types were previously developed to assess the exposure to coastal hazards for a local nuclear power station. Six climate indices were examined, and we found that the North Atlantic Oscillation (NAO) and the Scandinavian pattern (SCAND) have significant correlations with local wave and storm surge variables. The analysis reveals that, in response to the phases of NAO or SCAND, storm surge distributions exhibit changes in the mean and standard deviation, peak wave period distributions shift between bimodal and near-unimodal shapes, and wind waves and swell show different dominant directions. Using weather types, these response patterns can be traced back to synoptic circulation conditions characterized by different prevailing winds, spatial patterns of storm activity, and local atmospheric pressure. NAO and SCAND modify the occurrence probabilities of these synoptic conditions, thereby providing a probabilistic link between large-scale climate modes and local sea states. This research demonstrates the potential of weather types to offer new perspectives on the impact of climate modes on local sea states.

There has been limited exploration of the nearshore zone of mixed sediment beaches despite being widespread globally. Our work aims to build a more complete picture of coastal change by looking at seabed evolution from the very upper reaches of the swash zone, down to the edge of the nearshore zone, at a variety of timescales. Bathymetry surveys, completed using both traditional and automated surface vessels collected single and multibeam sonar data over a 19-year period, were complimented by a shorter period of weekly radar sea surface roughness images which are indicative of nearshore bed morphology. Additionally, grab samples were collected from across the nearshore to show the bed sediments composition variation over a year, providing valuable insight on bed response to varying hydrodynamic conditions. Process based analysis gave long and cross shore drift rates to help understand the observed changes. We found that the nearshore zone experiences significantly larger volumetric bed changes in comparison to the upper beach but were limited to approximately +/-0.3 m, which is equivalent to the upper error limits of the surveyed data. The depth of closure, a term used to mark the offshore point of no change over a defined time scale, varied across the mixed sediment bay at both seasonal (ranging between –4.7 and –8.4 metres Ordnance Datum(mOD)) and decadal (ranging between –7.3 and –8.2 mOD) timescales, yet was consistently shallower than all predictive equations of this depth. Moreover, our results indicate a loss of volume in the nearshore zone over time which is coupled with a simultaneous steepening of the upper beach for two thirds of the frontage. The observed steepening poses questions for the current ‘hold the line’ management strategy, which is achieved through active beach management works, and accommodation space. Overall, our observations highlight the substantial sediment transport occurring within the predominantly sandy nearshore zone and demonstrate evidence of impacts to the upper shingle beach. Consequently, to ensure the sustainability of management practices in the upper beach, a comprehensive understanding of the nearshore dynamics becomes imperative. The presented findings emphasize the necessity of integrating nearshore considerations into coastal management strategies, providing a more holistic and effective approach to making sustainable management decisions.

Transverse finger bars have largely been associated with sandy coasts. Here we show that these features persist within a wider mixed sediment environment, adjacent to a shingle cuspate foreland, which has not been previously reported. Details of the bars' characteristics were gleaned from analysis of bathymetry data, whilst weekly migration rates were inferred from remote sensing of the sea surface roughness as a proxy of undulating bedforms, using X-band radar reflectance data. The bars were on average 380 m long, had wavelengths of similar to 160 m, amplitudes of approximately 0.2 to 0.6 m and were orientated 30 degrees to shore normal. They were found in water depths between -3.3 and -5.8 m Ordnance Datum. The bars migrated by approximately 150 m over the first 'winter' observation period (15/11/2020-02/04/2021) and 70 m in the following winter period (Sept 2021-Feb 2022) but showed virtually no signs of movement during the intervening summer months. Analysis of hydrodynamic conditions suggested the bar mobility was related to the dominant longshore currents resulting from high angle, south westerly waves. Low amplitude rhythmic bedforms were also found in the upper beach, migrating at a similar rate to the nearshore bars, which are thought to be driven by high-angle wave instability.

Evaluating risks from external hazards is crucial for the safety of nuclear power stations throughout their lifecycle. In coastal areas, a key threat arises from the risks of coastal flooding and erosion via a combination of simultaneous processes (e.g., tides, waves, and storm surges) acting on varying spatial and temporal scales. Therefore, an accurate characterisation of local sea state conditions is essential for risk assessment and mitigation. In this paper, we use a weather typing method to downscale local wave climate and storm surge conditions at the Hartlepool nuclear power station. Model validation suggests that the use of 36 weather types can effectively downscale multivariate wave variables (wave height, period, and direction) and storm surge with overall good performance, though the accuracy is limited for wave direction and extreme wave height. Comprehensive sensitivity tests are conducted to investigate key factors influencing the downscaling process, including predictor variable, spatial and temporal definitions, predictor resolution, the number of weather types, and the weighting parameter in semi-supervised classification. For example, we find that the model with sea level pressure and sea level pressure gradient as the predictor has better overall performance in downscaling multivariate predictands than the model using either one individually. These results can facilitate the development of weather typing models to enable efficient and reliable estimations of local predictands in wider applications. This approach links atmospheric conditions to potential coastal threats, which offers a valuable tool for proactive hazard preparedness and risk management in nuclear power and other critical infrastructure sectors.

Coastal flooding, driven by extreme sea levels, is a significant threat to the coastline of the United Kingdom. The primary contribution to extreme sea levels is the combination of tide and surge and understanding how these components interact is critical to assessing extreme sea levels at the coast. Here, we analyse the interactions of skew surge and tidal high water, non-tidal residual and tidal phase, and non-tidal residual and tidal level using the entire observational tide gauge network of the UK, a near 500-year model, and a model run of 2013/14 with an artificially adjusted forcing to examine how storm arrival time impacts these interactions. We show that the levels of tide-surge interaction at most sites are relatively insensitive to the magnitude of the extreme value threshold and the declustering window size. Measured data show greater levels of interaction than modelled data and although there is little interaction between skew surge and tidal high water, there are sizeable tide-surge interactions between the non-tidal residual and the astronomical tide, the largest being for tidal phase. Around the UK, extreme non-tidal residuals generally occur favourably between 1 and 5 h before tidal high water and at tidal levels that are at, or below, the average tidal level. When storm arrival time is artificially shifted, the overall change in interaction around the UK is relatively small, with skew surge and non-tidal residual maxima occurring at similar respective tidal high waters, tidal phases, and tidal levels, although variation is seen on smaller spatial scales. Our findings advance the understanding of non-linear tide-surge interactions around the UK, which is essential for the accurate estimation of extreme sea level probabilities and thus the defence of the coastline against coastal flooding.

Mixed sediment beaches are globally commonplace, yet little is understood of the extent and behaviour of their nearshore zones, potentially underestimating total cross-shore change. This paper is the first study to investigate the lateral and vertical extent of the active zone of the gravel-rich mixed beach in Pevensey Bay, a study site on the South East UK coastline. Morphodynamic change in the nearshore zone was studied at a range of timescales (days, months, years) suggesting that the width of the active nearshore zone correlated with the magnitude of the peak morphological change, whilst the depth of closure was influenced by bed slope, grain size and local variation in wave conditions. A conceptual model detailing the physical parameters responsible for local variations in the depth of closure was used to help understand differences between the observed and predicted depths of closure. Finally, ongoing chronic loss of sediment from below Mean Sea Level (MSL) was examined, which was shown to be independent of the depth of closure, but closely linked to the wider geomorphic setting of the bay.

Saltmarsh restoration such as managed realignment (MR) projects often include excavation of simplified tidal creek networks to improve drainage and marsh functioning, but their design is based on limited evidence. This paper compares the morphological evolution of creek networks in current MR projects in the UK with creek networks in natural saltmarshes, in order to provide improved guidance. The evolution of creek networks was monitored for 2-20 years post-breach at 10 MR sites across the UK by semi-automatically extracting 12 morphological creek parameters from lidar. The rates of creek evolution in MR sites are linked to the initial tidal, morphological and sedimentological conditions using principal component analysis, then compared with power law relationships of morphological equilibrium defined from 13 mature natural saltmarshes. MR creeks evolved into larger, more complex, better distributed systems, with a total creek length and volume statistically similar to their natural counterparts. However, the creek volume remains poorly distributed, with a mean distance between creeks ranging from 33 to 101 m versus 5-15 m for natural mature saltmarshes. MR creeks are also clustered around the breach area, leaving the marsh interior poorly drained. MR creek network morphologies remain strongly influenced by the initial creek template, as evidenced by unnaturally straight creeks inherited from former drainage ditches. A combination of external conditions (i.e., tidal range, sediment concentration in the wider estuary) and local conditions (i.e., site elevation, topographical heterogeneity, soil compaction) controls how easily creeks can form within MR sites. This in turn determines the amount of engineering effort required to help achieve reference site conditions. The end goal of creek design is to create MR sites that closely resemble reference site conditions, however the final design is also likely to be affected by a range of practical factors (e.g. engineering/cost) unique to each site and project.

This paper presents the results of an autonomous seafloor imaging survey to map the distribution of seagrass around 10 eco-moorings installed to protect seagrass meadows in the Studland Bay Marine Conservation Zone (MCZ). The survey was carried out using the University of Southampton’s Smarty200 Autonomous Underwater Vehicle (AUV) in July 2022. Approximately 10,000 stereo image pairs were gathered from an altitude of 1 m along transects totaling 2.95 km. Images were classified according to the density of seagrass using a location- regularised semi-supervised deep-learning method developed at the University of Southampton. Three hundred expert-labelled images were used to train the classifier and the accuracy of the results were evaluated on a separate set of two hundred expert-labelled images. The results show the detailed distribution of habitats at the site and qualitative comparisons with high- resolution satellite imagery are made.

Few studies focus on the changing morphology of the nearshore zone of mixed sediment beaches, despite the fact that these beaches are found across the world. In the UK, these beaches make up ~25% of the coastline, and are often utilised as a first line of defence against coastal flooding. In Pevensey Bay, East Sussex, active beach management (sediment recycling and recharge) maintains the mixed gravel barrier beach to protect around 10,000 properties, culturally significant landmarks and internationally important wildlife sites. During the past 25 years, this management approach has successfully maintained the volume of the upper shingle part of the beach. However, the sandy foreshore area is experiencing a continuing loss of 8000 m3 of sediment per annum.This study seeks to understand the drivers behind the sustained loss of volume. Examination of multibeam bathymetry data revealed the presence of transverse finger bars with a wavelength of approximately 80 – 120 m, orientated at 45 degrees from the shoreline in the subtidal zone extending between the -3.0 to -6.0 mOD contours. Sediment grab samples taken perpendicular to the crests and troughs, revealed the surface sediments to be comprised of very well sorted fine sand, with D50 ranging between 150 – 169mm. Strong tidal currents flowing over these bed features modulate the sea surface roughness which can be detected in the X-band radar reflectance imagery. Using weekly averages of X-band radar reflectance imagery we show that the bars were a permanent feature over the 18-month period of observation and provide an indicative migration rate of approximately one wavelength a year to the east, which was validated against monthly bathymetric data. This novel approach of studying mobile sea bed features revealed a steady migration rate during the winter months, and virtually no movement during the summer period, suggests that the movement of the bars is driven by relatively higher energy south westerly waves. It is thought that the movement of these bars may be linked to erosive and accretive pulses which move easterly across the bay on the upper beach face. Understanding the process dynamics and broader role within the bay-wide sediment budget of these features is essential in comprehending the loss of sediment from the bay and will contribute to the future sustainable management of the site, where the management strategy for the next 100 years is currently under review.

This paper reports a new experimental method applying medical X-ray computed tomography (CT) to estimate the bed load in sand transport. A set of current-generated sand ripple experiments were conducted in a small hydraulic flume inserted in the CT scanner. The methodology is based on the measurements of height, velocity, and density of bed forms to estimate bed load. A temporal series of bed topography is first extracted from the CT scan images. The velocity is estimated by tracking the displacement of bed forms from two successive bed topographies. The sand bed density (rho(sb)) is estimated from the CT scan measurements using a calibration technique. The method measuring rho(sb) to calculate bed load is validated comparing measurements made with sand traps. The advantages and limitations of the CT method applied to bed-load transport are discussed. (C) 2022 American Society of Civil Engineers.

Sea surface temperature (SST) trends along the south coast of England (northern English Channel) were examined based on data from systematic buoy measurements deployed by the National Network of Regional Coastal Monitoring Programmes of England (NNRCMP) since 2003. These data were supplemented with: (1) long-term, coastal SST measurements by the Centre for Environment, Fisheries and Aquaculture Science (CEFAS); (2) global data sets compiled by the Hadley Centre since 1900, and (3) satellite-derived observations from Moderate Resolution Imaging Spectroradiometer (MODIS) (Aqua) since 2002. These data sets were used to evaluate de-seasoned nearshore trends in SST along the south coast of England and examine links to regional ocean-atmosphere teleconnections. The analyses of long-term, CEFAS data support the proposal that prior to the mid-1980s there were no de-seasoned trends in SST and conditions from year to year were relatively stable. Subsequently, interannual fluctuations appear to have increased, associated with a period of warming between 1985 and 2003 (0.28 degrees C/decade). Post 2003, interannual fluctuations in SST monitored by the NNRCMP buoys continued, and the warming trend appears to be greater (0.42 degrees C/decade). This trend in SST is greatest in the nearshore and decreases with distance offshore. The warming in SST also varied greatly from month to month. The greatest warming took place from December to March, whilst the least heating (and sometimes cooling) occurred between September and November. Analysis of Hadley (HadSST1.1) and MODIS data sets substantiated these trends. The greatest warming (post 2003) was found west of Portland Bill (up to 0.76 degrees C/decade) and decreased towards the Strait of Dover. Despite this west-to-east trend, all 12 NNRCMP stations between Penzance and Folkestone showed remarkably similar results, suggesting regional and global sources of heat rather than local sources. This is corroborated through wavelet coherence analysis linking SST anomalies to regional/global ocean-atmosphere teleconnection indices at seasonal scales.

Abstract Saltmarshes are important natural ecosystems along many temperate (and other) coastlines. They stabilize sediments and act as biofilters for a range of industrial pollutants and, potentially, microplastics. Accumulation of microplastics along estuarine coastlines may be enhanced by the presence of saltmarsh species, as they offer better particle trapping efficiency than adjacent intertidal mudflats under prevailing flood and ebb tidal currents. However, the trapping efficiency of entire saltmarsh systems under varying flow conditions has not been widely assessed. While the effects of saltmarsh systems on water flow, and on sediment transport and trapping, have been relatively well studied, little is known about the contributions of saltmarsh halophytes, resident organisms and the associated saltmarsh sediments to the trapping of microplastics. To address this, a series of flume experiments were undertaken to examine transport and accumulation of Bakelite particles (~ 500 µm) and PVC nurdles (~ 5 mm) as model plastics in sub-sampled saltmarsh and intertidal mudflat monoliths. The results showed that saltmarsh systems influenced the hydrodynamics within and above the canopy, enhancing turbulence and shear stresses. With increasing flow velocities (≤ 0.51 m s−1), negligible quantities (2 $$\times$$ × 10−4 mg L−1) of sediments and Bakelite particles were eroded and resuspended. The algal biogenic roughness from the mudflat, and the vegetative roughness from the Spartina plants on the saltmarsh, inhibited the transportation of the microplastics within the tested systems. Resident burrowing crabs (Carcinus maenas) promoted the burial, release and transport of microplastics. The results of this study provide evidence of the contributory roles of saltmarsh systems in the sequestration of microplastics and sediment stabilization. Estuarine saltmarsh systems can act as sinks for microplastics with enhanced burial from burrowing crabs under favourable flow conditions.

Biologically-mediated muds and sand-mud sediment mixtures are prevalent in lowland rivers, coastal, marine, and estuarine environments. These systems are highly sensitive to ongoing sea-level rise and environmental change. Effective management of these environments and adaptation to future changes, including mitigation to flood risk, requires accurate prediction of how flow and bed morphology changes over time, which has recently been shown to strongly depend upon substrate composition and the mud-to-sand ratios. Mud is cohesive and helps stick granular sediment together, potentially reducing sediment transport rates and bedform growth, which impacts hydraulic resistance and thus the fluid flow. We examined the co-evolution of bedform growth (morphodynamics) and hydraulic resistance (hydrodynamics) in muddy, shallow coastal environments subject to the simultaneous action of waves and currents (combined-flow) through controlled physical experiments in the Total Environment Simulator at the University of Hull. We conducted experiments with combined flow (regular waves plus a steady current in 0.4 m water depth) over 1.5 m wide channels constructed within the experiment basin (11 m long). The channels were each filled with a homogeneous sediment mixture of kaolin clay (D50 = 8 microns) and medium sand (D50 = 390 microns) in mud-to-sand ratios ranging between 0% (clean sand, baseline) and 16% by mass, to a substrate depth of 0.10 m. We ran the experiments to equilibrium conditions whereby steady-state bedform dimensions were approached with respect to the flow conditions. As such, longer experimental run-times were required for beds with higher mud-to-sand ratios. We quantified bedform formation and evolution, and flow velocities with a suite of acoustic sensors. With the 3D flow velocity data, we quantified turbulent fluctuations to assess the flow dynamics and estimate shear characteristics of the flow. We used these data to quantify hydraulic resistance. Our results show that there is a mud-content threshold of approximately 8-11% (depends on hydrodynamic conditions) below which clean sand ripples form once the finer sediment is winnowed out, leading to similar ripple heights as those measured for clean sand conditions at equilibrium. This in turns results in comparable hydraulic resistance (friction) to the low mud or sand-only substrates. However, increasing clay content suppresses bedform dimensions (shorter and smaller ripples), and thus reduces hydraulic resistance. Above the mud-content threshold, ripples are inhibited and sand transport rates are insignificant, resulting in minimal form drag and subdued skin friction. Our results suggest that hydraulic resistance predictors for muddy-, shallow-coastal environments need to account for the presence of mud and its modulating effects in sediment transport and friction, which ultimately affects flow properties and associated flood risks.

The temporal clustering of storms presents consecutive storm surge and wave hazards that can lead to amplified flood and erosional damages; thus, clustering is important for coastal stakeholders to consider. We analyse the prevalence of storm clustering around the UK coastline by examining the temporal and spatial characteristics of storm surge, wave height, and high still sea level exceedances at the 1 in 1- and 5-year return levels. First, at the interannual timescale, we show that there are periods of high/low exceedance counts on national and regional scales. Elevated annual counts of exceedances with smaller magnitudes can occur without a respective signal of higher-magnitude exceedances. Secondly, at the intra-annual timescale, we show that high proportions of exceedances are clustering over short timescales. Storm surge, wave height and still sea level exceedances occurring < 50 days after the prior exceedance at a given site account for between ~ 35–44% and ~ 15–22% of all exceedances at the 1 in 1- and 5-year return levels, respectively. Still sea levels have the highest proportion of exceedances clustered in quick succession, with ~ 25% of 1 in 1-year exceedances occurring < 2 days after the previous at the same site. Spatially, for UK storm surges and still sea levels, the North Sea has the lowest proportion of clustering, whereas the North Atlantic and Bristol Channel have the highest. For English wave records, the highest proportions of clustering are found in the North Sea for exceedances of a lower magnitude and the English Channel for exceedances of a higher magnitude. These findings illuminate the prevalence of the clustering of coastal hazards around the UK—helping coastal stakeholders evaluate the threat of surges, waves, and sea levels clustering over short periods.

Coasts, estuaries, and lowland river environments are some of the most sensitive systems to climate-induced environmental change. In order to manage these systems, and adapt to future changes, we need to be able to predict how they might change. However, most available models have formulations based on assumptions that these systems are composed of only non-cohesive sands, even if mud is the most common sediment on Earth. Therefore, we need to find ways to incorporate the effect of sticky mud in predictors of flow resistance, sediment transport rates, and bedform geometries associated with form drag. In this paper, we show results from seven experiments conducted with different mud contents (0% 12.6%) under the combined action of waves and currents. Experiments were conducted over a time period that was greater than the time needed for the bedform ripples to be at equilibrium with the imposed flow conditions. Our results reveal a mud content threshold above which friction coefficients are 4-6 times smaller than for clean sand and lower mud content counterparts. Below this threshold, flow resistance decreases with increasing mud content even though ripple migration rates are similar. Near the threshold, ripple migration rates increase as shorter ripples form and travel faster; the associated bedload transport rates also increase near the threshold and decrease beyond it. The threshold mud content, likely to vary with substrate composition and hydrodynamic conditions, is key to better estimating the behavior of different systems and needs further studies to properly incorporate it into predictors.

A benthic annular flume, Sea Carousel, was deployed at both sand-dominated (Baynes Sound) and mud-dominated (Carrie Bay) stations in British Columbia, Canada, to examine the character of near-bed flow over these contrasting bottom types and its control on particle size of resuspended sediment. An assessment has also been made of the turbidity-induced drag reduction due to suspension of bottom sediments. The median sizes of suspended material from the sandy sites have been compared with the well-known Rouse theory, whereas the aggregates resuspended from muddy stations were scaled with the energy dissipation rate (epsilon) determined from high-frequency three-dimensional flow measures in the flume. There was no evidence in the turbulence spectra in the Sea Carousel of energy inputs in the paddle and lid rotational frequencies, and a f(-5/3) slope for f>2 Hz in turbulent transitional flows was evident. The bed roughness length of sandy sites was Reynolds-number dependent but was asymptotic to a constant value of 2 mm at high flows. This equated to a dimensionless drag coefficient at 1 m above bed of a constant, 3 x 10(-3) (also at high Reynolds numbers), which agrees well with values reported in the literature. The median size of suspended sand (from the sandy sites) and equivalent still water settling rate (w(s)) scaled with the friction velocity (u*) in the form w(s)/u* = D*/8. The median size of resuspended aggregates (d(f)) scaled inversely with dissipation (epsilon) in the form d(f) = 5 x 10(-6)epsilon(-0.)(24)m, which is close to the relationship found in the literature.