High Suspended Sediment Concentration Research Articles

Wave-induced Maintenance of Suspended Sediment Concentration during Slack in a Tidal Channel on a Sheltered Macro-tidal Flat, Gangwha Island, Korea

A field campaign was conducted to better understand the influence of wave action, in terms of turbulence and bed shear stress, on sediment resuspension and transport processes on a protected tidal flat. An H-frame was deployed in a tidal channel south of Gangwha Island for 6 tidal cycles during November 2006 with instrumentation including an Acoustic Doppler Velocimeter, an Acoustic Backscatter System, and an Optical Backscatter Sensor. During calm conditions, the current-induced shear was dominant and responsible for suspending sediments during the accelerating phases of flood and ebb. During the high-tide slack, both bed shear stress and suspended sediment concentration were reduced. The sediment flux was directed landward due to the scour-lag effect over a tidal cycle. On the other hand, when waves were stronger, the wave-induced turbulence appeared to keep sediments in suspension even during the high-tide slack, while the current-induced shear remained dominant during the accelerating phases of flood and ebb. The sediment flux under strong waves was directed offshore due to the sustained high suspended sediment concentration during the high-tide slack. Although strong waves can induce offshore sediment flux, infrequent events with strong waves are unlikely to alter the long-term accretion of the protected southern Gangwha tidal flats.

Dynamics of Sediment Resuspension in the Inner Harbor Under Different Forcing Conditions: A Case Study of Ulsan, Korea

Ha, H.K.; Seo, J.Y.; Jung, Y.H.; Ha. H.J.; Kim, S.B.; Kang, J.W.; Kim, Y.H., and Ryu, J., 2018. Dynamics of sediment resuspension in the inner harbor under different forcing conditions: A case study of Ulsan, Korea. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 451–455. Coconut Creek (Florida), ISSN 0749-0208.Bottom mooring data of currents and backscatter strength derived from a 1200-kHz acoustic Doppler current profiler were collected in the inner Onsan Harbor, Ulsan during the summer season (July 5 to August 5, 2016). The mean volume backscattering strength (MVBS), which is a proxy of suspended sediment concentration (SSC), continued to exhibit distinct signatures of sediment erosion (or resuspension) under different forcing conditions such as tides, winds, and precipitation. Results exhibit the different patterns of sediment resuspension between spring and neap tides. During spring tide, the extensive tidal forcing provided a favorable condition to resuspend sediments trapped near the bed. The MVBS with high SSC was consistently recorded. During neap tide, on the other hand, the MVBS with high SSC was intermittently recorded, which was related to the strong wind events. While the flow was being intensified by winds, the short-lasting (2–4 hours) sharp spikes of MVBS were observed. The stratification developed by increased precipitation might cause to block the upward transport of sediments near the bed, even though the strong winds (>13.6 m s−1) lasted over 4 days. These outcomes could give insight into the complexity of hydrodynamics and sediment transport occurring in the harbor where various natural and artificial forcings are combined.

The Variations of Sediment Transport Patterns in the Radial Sand Ridges along the Jiangsu Coast, China over the Last 30 Years

Tao, J.; Xu, F.; Yao, P.; Zhou, Z., and Zhang, C., 2018. The variations of sediment transport patterns in the radial sand ridges along the Jiangsu coast, China over the last 30 years. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 216–220. Coconut Creek (Florida), ISSN 0749-0208.The research objective is to clarify the primary factors lead to the variations of the suspended sediment transport in the radial sand ridges (RSRs) along the Jiangsu Coast over the last 30 years. The suspended sediment concentration (SSC) fields measured in several field surveys from 1980 to 2010 were analyzed. The field data indicate that the SSCs in 1980 are higher than those 2010. A two-dimensional numerical model was developed to simulate the inter-annual variations of the suspended sediment transport. Generally, the magnitude of the SSC is highly relevant with the current speed, which peaks around the Old Yellow River Delta (OYRD) and in the central RSRs. However, the northern part of the RSRs indicates higher SSC than the southern part, which is attributed to their different bed sediment composition. The net suspended sediment flux was calculated. In the north of the RSRs, the sediments are transported in the south direction, which implies a sediment input from the OYRD to the RSRs. In the south of the RSRs, a net sediment loss was observed, which is consistent with the decreasing of the sediment flux from the Yangtze River. The simulated suspended sediment transport in 1980 and 2010 mutually show similar spatial patterns, while SSCs in 1980 are overall larger than those in 2010. It is concluded that the decrease of the SSCs from 1980 to 2010 is attributed to the reduction of the OYRD and the large-scale reclamation along the Jiangsu coast, while the sediment fluxes from the Yangtze River is less important.

Contrasting suspended sediment export in two small agricultural catchments: Cross‐influence of hydrological behaviour and landscape degradation or stream bank management

AbstractThe study aimed to identify hydrological and landscape factors that influence sediment transport in 2 agricultural catchments in north‐western France—Moulinet and Kervidy‐Naizin—with the same climate, size, and bedrock but with contrasting suspended sediment (SS) fluxes. Discharge and SS concentrations were continuously monitored at the catchment outlets for 9 hydrological years. Data were examined at annual, seasonal, and individual storm‐flow event scales. Storm events were classified into 3 types—single small, successive, and major events—whose effect on sediment transport was studied. At the annual scale, SS yields were higher in Moulinet (16–63 × 103 kg km−2) than in Kervidy‐Naizin (3–22 × 103 kg km−2) despite similar water fluxes. However, Kervidy‐Naizin had higher potential for hillslope erosion because of more frequent occurrence of saturation excess runoff, more frequent bare soil in cultivated area, and fewer hedgerows. Major storm events were also stronger in this catchment. Although high SS concentrations and fluxes occurred only during a few major events in Kervidy‐Naizin, SS export was always substantial throughout the year in Moulinet. We hypothesised that bank degradation due to cattle trampling generated the higher SS export in the Moulinet catchment. This hypothesis was confirmed by the large decrease in SS fluxes following construction of watering troughs in the middle of the study period. In the Kervidy‐Naizin catchment, natural woody vegetation or grass buffer strips along the stream effectively protect the stream from bank erosion or a possible influx of hillslope erosion particles, except during some major events in winter.

Numerical study of sediment dynamics during hurricane Gustav

In this study, the coupled ocean-atmosphere-wave-and-sediment transport (COAWST) modeling system was employed to explore sediment dynamics in the northern Gulf of Mexico during hurricane Gustav in 2008. The performance of the model was evaluated quantitatively and qualitatively against in-situ and remote sensing measurements, respectively. After Gustav's landfall in coastal Louisiana, the maximum significant wave heights reached more than 8 m offshore and they decreased quickly as it moved toward the inner shelf, where the vertical stratification was largely destroyed. Alongshore currents were dominant westward on the eastern sector of the hurricane track, and offshoreward currents prevailed on the western sector. High suspended sediment concentrations (>1000 mg/l) were confined to the inner shelf at surface layers and the simulated high concentrations at the bottom layer extended to the 200 m isobaths. The stratification was restored one week after landfall, although not fully. The asymmetric hurricane winds induced stronger hydrodynamics in the eastern sector, which led to severe erosion. The calculated suspended sediment flux (SSF) was convergent to the hurricane center and the maximum SSF was simulated near the south and southeast of the Mississippi river delta. The averaged post-hurricane deposition over the Louisiana shelf was 4.0 cm, which was 3.2–26 times higher than the annual accumulation rate under normal weather conditions.

Reprint of Mechanisms of maintaining high suspended sediment concentration over tide-dominated offshore shoals in the southern Yellow Sea

An understanding of the dynamics and behaviors of suspended sediments is vital in analysis of morphological, environmental, and ecological processes occurring in coastal marine environments. To study the mechanisms of maintaining high suspended sediment concentrations (SSCs) on a tide-dominated offshore shoal, we measured water depths, current velocities, SSCs, wave parameters and bottom sediment compositions in the southern Yellow Sea. These data were then used to calculate bottom shear stresses generated by currents (τc), waves (τw), and wave–current interactions (τcw). SSCs time series exhibited strong quarter-diurnal peaks during spring tides, in contrast to the semidiurnal signal during neap tides. A Fourier analysis showed that suspended sediment variations within tidal cycles was mainly controlled by resuspension in most stations. There existed relatively stable background SSCs (maintaining high SSCs among tidal cycles) values at all four stations during both windy (wind speed > 9.0 m/s) and normal weather conditions (wind speed < 3.0 m/s). The background SSCs had strong relationship with spring/neap-averaged τcw, indicating background SSCs were mainly controlled by mean bottom shear stress, with a minimum value of 0.21 N/m2. On account of the strong tidal currents, background SSCs of spring tides were greater than that of neap tides. In addition, on the base of wavelet, statistics analyses and turbulence dissipation parameter, background SSCs during slack tide in the study area may be maintained by intermittent turbulence events induced by a combined tidal current and wave action.

Use of the Cone Penetration Testing (CPT) method to interpret late Quaternary tide-dominated successions: A case study from the eastern China coastal plain

We evaluate the applicability of cone penetration testing (CPT), calibrated using adjacent cores, as a tool for the sedimentological and stratigraphic examination of late Quaternary tide-dominated successions in the eastern China coastal plain. The results indicate that the sedimentary facies and sequence-stratigraphic surfaces can be readily distinguished using CPT profiles in the Qiantang River incised-valley system because of their distinctive mechanical behavior. The lithologic character of the various facies, which is controlled mainly by sediment supply, dynamic processes and post-depositional diagenesis, is the key factor affecting how well the CPT technique works. Within this particular macrotidal environment, which is dominated by non-cohesive sand and silt in the tidal channels, the accumulation of fluid mud is rare. Consequently, the tidal-channel deposits exhibit the geotechnical properties of coarse-grained sediments, and can be easily distinguished from the mud-dominated facies. However, in the nearby Changjiang delta system which is characterized by very high suspended-sediment concentrations and an abundance of fine-grained cohesive sediments, the presence of channel-bottom fluid muds makes it difficult to recognize channel deposits, because of the lack of a sharp lithologic contrast at their base. Consequently, the CPT method might not be as universally effective in tide-dominated systems as it appears to be in wave-dominated settings. Care is needed in the interpretation of the results from tide-dominated successions because of the widespread presence of fluid muds, the heterolithic nature of tidal deposits, the rheological similarity between adjacent facies, and the averaging of geotechnical properties between the alternating finer and coarser layers.

Technical note: False low turbidity readings from optical probes during high suspended-sediment concentrations

Abstract. Turbidity, a measure of water clarity, is monitored for a variety of purposes including (1) to help determine whether water is safe to drink, (2) to establish background conditions of lakes and rivers and detect pollution caused by construction projects and stormwater discharge, (3) to study sediment transport in rivers and erosion in catchments, (4) to manage siltation of water reservoirs, and (5) to establish connections with aquatic biological properties, such as primary production and predator–prey interactions. Turbidity is typically measured with an optical probe that detects light scattered from particles in the water. Probes have defined upper limits of the range of turbidity that they can measure. The general assumption is that when turbidity exceeds this upper limit, the values of turbidity will be constant, i.e., the probe is “pegged”; however, this assumption is not necessarily valid. In rivers with limited variation in the physical properties of the suspended sediment, at lower suspended-sediment concentrations, an increase in suspended-sediment concentration will cause a linear increase in turbidity. When the suspended-sediment concentration in these rivers is high, turbidity levels can exceed the upper measurement limit of an optical probe and record a constant “pegged” value. However, at extremely high suspended-sediment concentrations, optical turbidity probes do not necessarily stay “pegged” at a constant value. Data from the Colorado River in Grand Canyon, Arizona, USA, and a laboratory experiment both demonstrate that when turbidity exceeds instrument-pegged conditions, increasing suspended-sediment concentration (and thus increasing turbidity) may cause optical probes to record decreasing “false” turbidity values that appear to be within the valid measurement range of the probe. Therefore, under high-turbidity conditions, other surrogate measurements of turbidity (e.g., acoustic-attenuation measurements or suspended-sediment samples) are necessary to correct these low false turbidity measurements and accurately measure turbidity.

Sediment–Water Surface Area Along Rivers: Water Column Versus Benthic

Aquatic ecosystems have two distinct zones: the water column and benthic zone. Although the benthic zone has received considerable attention, recent studies have found the water column capable of accounting for a majority of whole ecosystem processes in rivers. The relative role of these zones inevitably varies across a size continuum of rivers, from headwaters to large transcontinental systems. A fundamental question in aquatic science is where along this size continuum do ecosystem processes potentially shift from occurring largely in the benthic zone to largely in the water column? Sediment structures the physical template of the benthic and water column zones of rivers and the contact area between water and sediment mediates ecological, geochemical, and physical processes. High concentrations of suspended sediments are hypothesized to cause a shift from benthic to water column dominance in rivers. We developed an analytical model for the contact area between surface water and all sediment particles in benthic and water column volumes. The model was implemented with empirical data along the main stem of major US rivers. The ratio of water column to benthic sediment contact area scaled as a power function of watershed area. There was more sediment–water contact area in the water column than the benthic zone in rivers equal to or greater than 5th to 9th order depending on the river basin. This suggests material processing could be occurring largely in the water column in rivers greater than 5th order. However, dams and variation in discharge caused rivers to oscillate between water column and benthic dominance over time and space.

The hyperturbid state of the water column in estuaries and rivers: the importance of hindered settling

Over the last few decades, some estuaries have undergone a transition to a hyperturbid state, characterised by suspended sediment concentrations of several grammes per litre averaged over the water column. To improve our understanding of this transition and of naturally hyperturbid estuaries, we systematically identify the processes allowing for high suspended sediment concentrations using a water column (1DV) model. Under a range of realistic forcing conditions, the state of the water column can be characterised by one of two equilibrium states. The first is an erosion-limited state, in which there still is sediment available for erosion at the bed. We find that this state only occurs with relatively low concentrations. The second is a supply-limited state, in which all erodable sediment is in suspension. The concentration in this state depends entirely on the amount of sediment in the system and can potentially be very high. We identify the conditions under which the state of the water column can jump from a low to a high concentration and identify hysteresis in the transition between the two states. The mechanism responsible for this hysteresis is hindered settling. It thus follows that hyperturbidity is only possible in a supply-limited state. From this observation we derive a necessary condition for an estuarine system to make the transition from low turbidity to hyperturbidity in a 1DV context. This is an important step towards understanding why some estuaries are hyperturbid and assessing the risk that particular estuaries may become hyperturbid in the future.

Effect of suspended clay on growth rates of the cyanobacterium Cylindrospermopsis raciborskii

Recent studies have shown that sediment resuspension may lead to the collapse of C. raciborskii dominance, which suggests that clay might have a negative effect on the growth of C. raciborskii. To test the hypothesis that suspended clay creates an unfavorable environment for growth of C. raciborskii, we exposed four different strains of this species to various concentrations of the clays kaolinite and bentonite, and monitored the biomass of each strain over the course of 1-week microcosm experiments. Contrary to our hypothesis, C. raciborskii was able to grow in suspensions of both clays. While kaolinite clay caused higher turbidity than bentonite, the growth rates of all four C. raciborskii strains were higher in kaolinite than in bentonite suspensions. C. raciborskii could still grow in clay concentrations that cause turbidity far above the levels found in natural lakes. Our study suggests that the reported collapse of C. raciborskii blooms with high concentrations of suspended sediments in tropical shallow lakes is probably not caused by the effects of suspended clay on light attenuation, but rather is a consequence of cell sinking or, possibly a response to disturbance events responsible for sediment suspension.

Measurement and control of high suspended sediment concentration during the Kurobe River sediment flushing with submersible type SMDP

In the Kurobe River, coordinated sediment flushing and sediment sluicing of Dashidaira and Unazuki dams have been executed since 2001. From the view point of the comprehensive sediment management in the sediment routing system, monitoring of quantity and quality of sediment transport during these events in rivers and reservoirs is very important. This paper shows the results of continuous measurement of high suspended sediment concentration during sediment flushing and sluicing operation of Unazuki dam in 2015 and 2016. These data have been obtained by submergible type of Suspended Sediment Concentration Measuring System with Differential Pressure Transmitter (hereinafter we call ‘submergible type SMDP’), which is designed to be placed underwater. Measured data have good correlation with manual sampling and effectively utilized to control gate operation by real-time basis during the most critical timing producing extreme high concentration just before free flow starts for sediment flushing.

Quantification and Analysis of Suspended Sediments Concentration Using Mobile and Static Acoustic Doppler Current Profiler Instruments

The application of Acoustic Doppler Current Profiler (ADCP) can be used not only for measuring ocean currents, but also for quantifying suspended sediment concentrations (SSC) from acoustic backscatter strength based on sonar principle. Suspended sediment has long been recognized as the largest sources of sea contaminant and must be considered as one of the important parameters in water quality of seawater. This research was to determine SSC from measured acoustic backscattered intensity of static and mobile ADCP. In this study, vertically mounted 400 kHz and 750 kHz static ADCP were deployed in Lembeh Strait, North Sulawesi. A mobile ADCP 307.2 kHz was also mounted on the boat and moved to the predefined cross-section, accordingly. The linear regression analysis of echo intensity measured by ADCP and by direct measurement methods showed that ADCP is a reliable method to measure SSC with correlation coefficient (r) 0.92. Higher SSC was observed in low water compared to that in high water and near port area compared to those in observed areas. All of this analysis showed that the combination of static and mobile ADCP methods produces reasonably good spatial and temporal data of SSC.

Near bed suspended sediment flux by single turbulent events

The role of small scale single turbulent events in the vertical mixing of near bed suspended sediments was explored in a shallow shelf sea environment. High frequency velocity and suspended sediment concentration (SSC; calibrated from the backscatter intensity) were collected using an Acoustic Doppler Velocimeter (ADV). Using quadrant analysis, the despiked velocity time series was divided into turbulent events and small background fluctuations. Reynolds stress and Turbulent Kinetic Energy (TKE) calculated from all velocity samples, were compared to the same turbulent statistics calculated only from velocity samples classified as turbulent events (Reevents and TKEevents). The comparison showed that Reevents and TKEevents was increased 3 and 1.6 times, respectively, when small background fluctuations were removed and that the correlation with SSC for TKE could be improved through removal of the latter. The correlation between instantaneous vertical turbulent flux (w′) and SSC fluctuations (SSC′) exhibits a tidal pattern with the maximum correlation at peak ebb and flood currents, when strong turbulent events appear. Individual turbulent events were characterized by type, strength, duration and length. Cumulative vertical turbulent sediment fluxes and average SSC associated with individual turbulent events were calculated. Over the tidal cycle, ejections and sweeps were the most dominant events, transporting 50% and 36% of the cumulative vertical turbulent event sediment flux, respectively. Although the contribution of outward interactions to the vertical turbulent event sediment flux was low (11%), single outward interaction events were capable of inducing similar SSC′ as sweep events. The results suggest that on time scales of tens of minutes to hours, TKE may be appropriate to quantify turbulence in sediment transport studies, but that event characteristics, particular the upward turbulent flux need to be accounted for when considering sediment transport on process time scales.

The effect of source suspended sediment concentration on the sediment dynamics of a macrotidal creek and salt marsh

Seasonal variability in the sediment dynamics of a Bay of Fundy tidal creek and salt marsh system was analyzed to better understand the ecomorphodynamics of a high suspended sediment concentration intertidal habitat. Data were collected over 62 tides for velocity, suspended sediment concentration, deposition, and grain size at four stations from the creek thalweg to the marsh surface. Five topographic surveys were also conducted throughout the 14-month study. Deposition rates per tide varied spatially from 56.4gm-2 at the creek thalweg to 15.3gm-2 at the marsh surface. Seasonal variations in deposition in the creek and marsh surface were from 38.0gm-2 to 97.7gm-2 and from 12.2gm-2 to 19.6gm-2 respectively. Deposition and erosion were greatest in late fall and winter. This seasonal change, led by higher suspended sediment concentrations, was observed in the creek and at the marsh bank but notably absent from the marsh edge and marsh surface. Sediments were predominantly deposited in floc form (76–83%). Because of high floc content, higher suspended sediment concentrations led to more rapid loss of sediment from suspension. With increasing sediment concentration, deposition increased in the tidal creek and at the marsh bank but not at the marsh edge or marsh surface. This suggests that in highly flocculated environments the water column clears fast enough that very little sediment remains in suspension when the water reaches the marsh and that the sediment concentration during marsh inundation is independent of the initial concentration in the creek.

Seasonal and intra-seasonal variations of suspended-sediment distribution in the Yellow Sea

Seasonal and intra-seasonal variability of hydrodynamics and suspended sediment concentration (SSC) in the Yellow Sea were investigated using combined ship measurements and physical fields extracted from the global Hybrid Coordinate Ocean Model (HyCOM). Model-data comparison indicates that the HyCOM solutions fairly agree with the observed temperature fields, both showing strong horizontal and vertical seasonality. High SSC was observed in waters near the Chengshantou Cape and above the old Yellow River subaqueous delta in November 2012, and was coincident with enhanced velocity shear. Thermal fronts were identified in waters less than 60m deep with their prominent appearances in November 2012, inhibiting the offshore sediment transport. The previously reported depo-center of distal Yellow River mud-patch off the Chengshantou Cape was confined between the positions of the strong velocity shear in a shoreward direction and the thermal fronts in an offshore direction. We concluded that the nearshore mixing front and the offshore thermal front play critical roles in both capture of the fine-grained suspended sediment and formation of the Omega-shaped mud patch.

Hyperpycnal flows control the persistence and flushing of hypoxic high conductivity bottom water in a High Arctic lake

In the deepest portions of many lakes, zones of high-conductivity bottom water (HCBW) depleted in dissolved oxygen (DO) are present. HCBW and DO are important for determining benthic diversity and abundance, nutrients, and contaminant cycling and understanding the long-term evolution of lakes. We investigate the persistence and removal of HCBW and DO replenishment in a High Arctic lake using physical properties and flow velocity data along with hydrometric and suspended sediment inflow data over a 4 year monitoring period (2007–2010). HCBW was removed in 2007 and 2008 but largely remained in 2009 and 2010. Catchment disturbances in 2007 increased suspended sediment concentrations (SSC) in the inflowing river in 2007 and 2008. In the later two years of monitoring (2009 and 2010), fluvial sediment availability relaxed to pre-disturbance levels. High SSC in 2007 and 2008 caused by landscape disturbances formed sustained river-generated hyperpycnal flows during the snowmelt period that are linked to HCBW removal. In 2009 and 2010, inflowing river water was periodically denser than lake water; however, HCBW was not removed in these years. Hyperpycnal flows were likely either of insufficient strength or duration, deposited on the delta front, or followed paths that led away from the deepest portion of the lake. Results suggest that hyperpycnal flow frequency will increase and HCBW persistence will decrease with projected climate change due to an increase in fluvial SSC inflow. Water density changes resulting from increased electrical conductivity and water temperature are not likely to have a similarly large effect on hyperpycnal flow frequency and HCBW.

Simulating high ebb currents in the North Passage of the Yangtze estuary using a vertical 1-D model

A strong maximum ebb current (>3 m/s) in the upper water column was observed at Station CS3 in the middle of the North Passage of the Yangtze River Estuary during the wet season, which was higher than either its upstream or downstream counterparts. To better understand the mechanisms and factors causing the strong ebb current, a vertical one-dimensional (1-D) model was used to conduct a diagnostic study. The model used time series of observed tidal amplitudes, vertical salinity, and suspended sediment concentration (SSC) profiles to compute the density and turbulence. Two tunable parameters, the tidal amplitude attenuation coefficient (i.e., the phase lag) and the background surface pressure gradient that represents the net pressure gradient induced by the freshwater discharge and baroclinic effect, were used to determine the best match with the observed high velocity amplitudes in addition to the bottom roughness height. Three hypotheses of possible causes are tested: (1) the large freshwater discharge, (2) the bottom stratification effects (which were caused by a possible high near-bed suspended sediment gradient), and (3) the unique location of the CS3 station that was influenced by local geometry. The findings show that neither of the first two factors has much influence on the pronounced ebb velocities. Instead, the energy loss caused by the change of channel geometry and a maximum convex bathymetry in the North Passage of the Yangtze River Estuary are the main reasons behind the extremely high observed ebb current velocity profiles. The high near-bottom SSC and gradient located within 0.5 m above the bed only slightly alters the velocity profiles. This 1-D model is convenient for testing a different hypothesis and for coupling with other selected variables to account for the floc size distributions in future studies.

The importance of a small ephemeral tributary for fine sediment dynamics in a main‐stem river

AbstractStudies of ephemeral streams have focused mainly in arid and semi‐arid regions. Such streams also occur widely in temperate regions, but much less is known about their influence on fluvial processes in main‐stem rivers here. In this paper, we present evidence of the importance of a small ephemeral temperate stream for main‐stem fine sediment dynamics. The paper focuses on a restoration project (River Ehen, North West England) which involved the reconnection of a headwater tributary to the main‐stem river. We present data on suspended sediment transport 2 years prior to and 2 years following the reconnection. Despite the small size and non‐perennial flow of the tributary, its reconnection resulted in an increase of 65% in the main‐stem sediment yield. During both the pre‐reconnection and post‐reconnection periods, a higher proportion of the annual yield was conveyed during short events with relatively high suspended sediment concentrations. Following the reconnection, the magnitude and frequency of such events increased, primarily due to sediment being delivered from the tributary at times when main‐stem flows were not elevated. Overall, the main‐stem remains supply limited and so is highly dependent on sediment delivered from the tributary. The study helps stress that even non‐perennial tributaries yielding only a small increase in catchment size (+1.2% in this case) can have a major influence on main‐stem fluvial dynamics. Their role as sediment sources may be especially important where, as in the case of the Ehen, the main‐stem is regulated and the system is otherwise starved of sediment.

Effects of combined dredging-related stressors on sponges: a laboratory approach using realistic scenarios

Dredging can cause increased suspended sediment concentrations (SSCs), light attenuation and sedimentation in marine communities. In order to determine the combined effects of dredging-related pressures on adult sponges, three species spanning different nutritional modes and morphologies were exposed to 5 treatment levels representing realistic dredging scenarios. Most sponges survived under low to moderate turbidity scenarios (SSCs of ≤ 33 mg L−1, and a daily light integral of ≥0.5 mol photons m−2 d−1) for up to 28 d. However, under the highest turbidity scenario (76 mg L−1, 0.1 mol photons m−2 d−1) there was 20% and 90% mortality of the phototrophic sponges Cliona orientalis and Carteriospongia foliascens respectively, and tissue regression in the heterotrophic Ianthella basta. All three sponge species exhibited mechanisms to effectively tolerate dredging-related pressures in the short term (e.g. oscula closure, mucus production and tissue regression), although reduced lipids and deterioration of sponge health suggest that longer term exposure to similar conditions is likely to result in higher mortality. These results suggest that the combination of high SSCs and low light availability can accelerate mortality, increasing the probability of biological effects, although there is considerable interspecies variability in how adult sponges respond to dredging pressures.