Hydro-morphodynamic numerical modeling indicates risk zones for riverbed clogging

A theoretical model is developed to describe the process of fine sediment infiltration into immobile coarse sediment deposits. The governing equations are derived from mass conservation and the assumption that the amount of fine sediment deposition per unit vertical travel distance into the deposit is either constant or increases with increasing fine sediment fraction. Model results demonstrate that fine sediment accumulation decreases rapidly with depth into coarse substrate initially void of fine sediment, which is consistent with experimental observations that significant fine sediment infiltration occurs to only a shallow depth. Comparisons of the theory with flume data indicate that the model adequately reproduced the weighted-averaged fine sediment fraction values from experiments. An early model developed by Sakthivadivel and Einstein for fine sediment infiltration is in part based on the generally incorrect assumption that intragravel flow remains constant following fine sediment infiltration. Applying a correction to the Sakthivadivel and Einstein model based on alternate hypothesis that introgravel flow is driven by a constant head gives similar results as the proposed model.

Abstract. Empirical measurements on fine sediment dynamics and fine sediment infiltration and accumulation have been conducted worldwide, but it is difficult to compare the results because the applied methods differ widely. We compared common methods to capture temporal and spatial dynamics of suspended sediment (SS), fine sediment infiltration and accumulation and tested them for their suitability in a small, canalized river of the Swiss Plateau. Measurement suitability was assessed by data comparison, relation to hydrological data and in the context of previously published data. SS concentration and load were assessed by optical backscatter (OBS) sensors and SS samplers. The former exhibit a better temporal resolution, but were associated with calibration problems. Due to the relatively low cost and easy mounting of SS samplers, they can provide a higher spatial distribution in the river's cross section. The latter resulted in a better correlation between sediment infiltration and SS load assessed by SS samplers than SS concentrations measured with OBS sensors. Sediment infiltration baskets and bedload traps capture the temporal and spatial distribution of fine sediment infiltration. Data obtained by both methods were positively correlated with water level and SS. In contrast, accumulation baskets do not assess the temporal behaviour of fine sediment, but the net accumulation over a certain time period. Less fine sediment accumulated in upwelling zones and within areas of higher mean water level due to scouring of fine sediments. Even though SS and sediment infiltration assessed with the bedload traps increased from up- to downstream, less fine sediment accumulated downstream. This is probably also attributable to more scouring downstream.

The impact of fine sediment accumulation on the survival of incubating salmon progeny: Implications for sediment management

Fine sediment deposition and infiltration into the bed of lotic ecosystems, such as sedimentation, siltation and colmation, has been widely recognised as one of the most important causes of degradation within lotic ecosystems. The impact of increased fine sediment loading as a result of agricultural practices, urban development and channel management activities for flood defence purposes, have been widely acknowledged but poorly quantified. This chapter quantifies the influence of increasing sediment input that is sediment loading on the benthic invertebrate community inhabiting an artificial channel with an impervious concrete bed. This approach provided highly controlled conditions but also reflected channel and habitat characteristics typical of many highly modified and managed urban streams. Significant advances have been made recently in the development of biomonitoring tools which quantify fine sediment impacts on instream communities and which facilitate identification of vulnerable locations within river channels.

The long‐term performance of measures to restore in‐stream habitat in gravel bed rivers is uncertain in the presence of impoundments, land use pressures, and fine sediment inputs. The goal of this study was to evaluate the longer‐term performance of five bottom ramps, designed to facilitate fish passage, and constructed similarly to artificial riffles to provide compensatory gravel riverbed habitat for benthic invertebrates and lithophilic, coarse‐substrate‐preferring fish in a channelized lowland river. Bottom ramp age did not significantly influence habitat conditions indicated by a lack of correlations with the percentage of fine sediment less than 2 mm, the organic content of the substrate, and the years since construction. A significant decrease in the relative abundances of coarse‐substrate‐preferring benthic invertebrates corresponding to project construction age was found, but there were no significant differences in the density of rheophilic, fast‐flow‐preferring or lithophilic fish species among sites. This study presents substantial evidence that similarly constructed bottom ramps in comparable environmental settings provided sufficient habitat for sensitive benthic invertebrates and fish to be present for over 13 years. However, a sudden decrease in habitat suitability cannot be excluded in the long‐term if there is a fine sediment deposition threshold, which results in ramps becoming full. Nevertheless, bottom ramps are recommended as effective measures to enhance longitudinal connectivity, fish passage, and gravel habitat provision in channelized lowland rivers.

Elevated fine sediment accumulation in a river system's gravel bed is known to cause detrimental ecological impacts. Current sediment targets and approaches to mitigation have failed due to the oversimplification of geomorphological processes controlling fine sediment accumulation and the lack of relevant scientific knowledge underpinning them. This is particularly apparent in chalk streams (groundwater‐dominated systems) which regularly exhibit high rates of sediment accumulation despite low suspended sediment yields. A necessary first step is to better characterise their sedimentology; thus, the novelty of this study was to determine the sedimentological characteristics of chalk stream gravel beds, specifically the quantity and distribution of fine sediment with depth. We collated published and unpublished freeze‐core data, encompassing 90 sites across 11 UK chalk streams. Results showed average quantities of fine sediment (<2 mm) in chalk stream gravel beds were 25% by weight, with >75% of beds exceeding thresholds for ecological degradation. Quantities of fine sediment increased with increasing depth into the bed, with an average increase between surface and subsurface layers of 54%, and 89% of the gravel bed over‐saturated with fine sediment. Regional differences were attributed to differences in stream power and local sediment sources, including surficial geology and catchment land use. Additionally, a major contrast was identified between experimental conditions in flume studies used to establish models describing interactions/mechanisms of fine sediment infiltration into immobile gravel beds and the natural conditions observed in chalk streams. As such, the use of such models as a basis to explore sediment management scenarios is unlikely to predict the outcome of such management techniques correctly in a real‐world situation.

Introduction to the special issue ‘Tracer Applications in Sediment Research’

Globally, salmon populations are under pressure and in those where numbers are severely depleted, density independent factors such as the accumulation of fine sediment and subsequent mortality of incubating embryos are critical factors in their conservation. Although progress has been made in identifying the processes by which fine sediment can lead to embryo mortality, this has focussed on the physical blockage of gravel pores. This paper presents new knowledge on the role of sediment‐associated organic matter in controlling the supply of dissolved oxygen to incubating salmon embryos within gravel river beds. We report a new approach to the measurement of sediment oxygen demand (SOD) from interstitial sediments accumulated within salmon redds and demonstrate this across a range of salmon river types. The effects of varying SOD on dissolved oxygen supply to incubating salmon embryos are quantified for the first time, using the physically based Sediment Intrusion and Dissolved Oxygen (SIDO‐UK) model. The results reveal the importance of the mass of accumulating sediment and sediment consumption rates (sediment oxygen consumption [SOC]), which constitute the overall SOD over time for a particular sediment sample. Higher SOC result in reductions in dissolved oxygen that are deleterious to salmonids; however, these are subordinate to the effects of sediment mass. Larger quantities of sediment accumulating within the redd not only create a higher SOD but also physically block the pores within the gravels, leading to a more drastic decline in oxygen supply through the combined effects of reduced seepage velocity and dissolved oxygen concentration. We seek to generalize the results by exploring the influence of catchment characteristics on field measures of SOD and SOC. This work makes a significant and novel contribution to improving our fundamental understanding of hyporheic processes within salmonid spawning gravels whilst providing resource managers with evidence of the deleterious effects of sediment‐associated organic matter on salmon recruitment.

Stream bed and mid‐column velocities, depth, redox, and mussel densities were measured in 76 quadrats across five transects in Margaritifera margaritifera habitat supporting high densities of adult mussels and evidence of recent juvenile recruitment.At relatively low flow (Q85), the near‐bed velocities over the mussel habitat remained high, with optimal habitat having stream velocities of 0.27–0.31 m s−1.The average near‐bed velocity for mussel densities greater than 20 m−2 was 0.3 m s−1, with a range from 0.18 m s−1 for mussel densities lower than 10 m−2, to 0.3 m s−1 for mussel densities greater than 50 m−2.The average velocity at 60% depth for mussel densities greater than 20 m−2 was 0.37 m s−1, with a range from 0.21 m s−1 for mussel densities lower than 10 m−2, to 0.39 m s−1 for mussel densities greater than 50 m−2.Percentage redox losses from open water to water at 5 cm substrate depth were an average of 18% for mussel densities of 1–10 m−2 to 13% at mussel densities >100 m−2.The conclusion from this study is that relatively high velocities are present at relatively low flows in sustainable Margaritifera habitat, suggesting that this species is adapted to a combination of stable substrate conditions that are kept clean through high water velocities with low fine sediment infiltration, rather than habitat that is subject to regular fine sediment infiltration followed by substrate cleansing. Copyright © 2014 John Wiley & Sons, Ltd.

Microplastic (MP) pollution is an important challenge for human life which has consequently affected the natural system of other organisms. Mismanagement and also careless handling of plastics in daily life has led to an accelerating contamination of air, water and soil compartments with MP. Under estuarine conditions, interactions with suspended particulate matter (SPM) like fine sediment in the water column play an important role on the fate of MP. Further studies to better understand the corresponding transport and accumulation mechanisms are required. This paper aims at providing a new modeling approach improving the MP settling velocity formulation based on higher suspended fine sediment concentrations, as i.e. existent in estuarine turbidity zones (ETZ). The capability of the suggested approach is examined through the modeling of released MP transport in water and their interactions with fine sediment (cohesive sediment/fluid mud). The model results suggest higher concentrations of MP in ETZ, both in the water column as well as the bed sediment, which is also supported by measurements. The key process in the modeling approach is the integration of small MP particles into estuarine fine sediment aggregates. This is realized by means of a threshold sediment concentration, above which the effective MP settling velocity increasingly approaches that of the sediment aggregates. The model results are in good agreement with measured MP mass concentrations. Moreover, the model results also show that lighter small MP particles can easier escape the ETZ towards the open sea.

Riparian zones are especially dynamic due to occasional river floods that provide powerful physical disturbances. Northern Hemisphere riparian woodlands are often dominated by cottonwoods, and studies in semi-arid ecoregions have indicated associations between flood events and cottonwood colonization. Here, we investigated the association between a major flood and cottonwood recruitment along a mountain river in a humid, forested ecoregion. We investigated riparian processes and black cottonwoods (Populus trichocarpa) along the free-flowing Elk River in the Rocky Mountains of British Columbia between 1993 and 2005, a period with a 1-in-100 year flood in June 1995. We analyzed hydrology, floodplain conditions, and aerial photographs and monitored vegetation establishment and survival. We excavated cottonwood saplings to determine origin and considered spatial patterns of cottonwood colonization. Results revealed that the flood induced channel migration and abrupt avulsion that created extensive barren nursery sites. This enabled prolific cottonwood establishment, but few seedlings from the flood year survived. Survival increased in subsequent years, but mortality remained extensive due to sediment burial, scour by high flows and ice, and drought. The patterns of successful colonization revealed seedling safe sites, survivable locations with appropriate elevation relative to water (0.6 to 2.8 m above base stage) and geomorphic context (<5 cm scour and <40 cm deposition). By 2002, cottonwood saplings covered islands and bars created by the flood, and excavation revealed seedling origin, not from clonal fragments. Clonal root suckers were stimulated by flood scarification, expanding previously established groves. The study confirmed that a major flood enabled extensive cottonwood colonization, but the underlying processes were somewhat different than along adjacent rivers in semiarid ecoregions. Along the Elk River, sediment erosion and deposition were predominant, producing patches of even-aged cottonwoods, in contrast to the arcuate banding along rivers in drier regions with finer sediments and progressive channel migration. Thus, water was less limiting in the humid ecoregion and geomorphic disturbance was the more vital component of the flood event.

The prediction of velocity and attentuation in acoustic waves which propagate through marine sediments is of key importance in many surveillance and exploration techniques. A mathematical model of the sediment must reflect the influence of such parameters as porosity, overburden stress, particle size, and lithification. The general theory of Biot has been used to construct a model which has been successful in estimating propagation characteristics in a wide variety of sediments. Two kinds of viscous damping are included in the model. One deals with the overall motion of the interconnected fluid field with respect to the skeletal frame. The other accounts for local fluid motion near the grain contacts which produces a viscoelastic response similar to the “squeeze film” phenomenon in lubrication theory. Recent experimental data for sandstone, sand, silt, and other sediments show that the various forms of viscous damping produce a frequency dependent Q in virtually all water-saturated sediments. New data are presented for sand and silt in the frequency range of 2–1500 Hz which demonstrates the effects of both kinds of viscous damping. [Work supported by ONR.]

Dynamics of the turbidity maximum zone in a macrotidal estuary (the Gironde, France): Observations from field and MODIS satellite data

The geophone spacing and offset distances for MASW field investigations are commonly chosen based on common rules of thumb and experience. This method of survey design usually can not guarantee the adequacy of the final results in terms of penetration depth and resolution. Further, in complex geologies existence of higher modes of surface waves will make the interpretation of collected data difficult. One possible reason for these shortcomings is that the commonly used rules of thumb are empirical, do not have sufficient accuracy, and are not costumed for various geological conditions. To overcome the existing uncertainties, the authors previously proposed the multi geometry approach in which field data are collected using multiple geophone spacings along the same line, with active shots at multiple offset locations. Then the obtained dispersion images are overlaid to obtain one combined image with a better quality than the individual dispersion images. Field and numerical data are presented to show the efficiency of the multi geometry approach. Field data are complemented by borehole information. Comparisons are made between the curves obtained from different set ups. Field data shows that dispersion curves usually show different trends than the trends predicted by empirical rules of thumb. Specifically, the penetration depth to array length ratio depends on site conditions, the effect of array length on the quality and frequency bandwidth of the dispersion curve is not well understood and relations between offset distances and dispersion curve quality is not well known. Further, in complex geologies the observation of different modes depends on the array geometry. Synthetic data from numerical models are used to generate dispersion curves for various layering systems. Relations between penetration depths and array length are investigated. It is confirmed that due to the dependency of the field data quality to the site condition a multi geometry approach is required to reduce the risk of misinterpretations.

In gravel-bed rivers, bedload transport exhibits considerable variability in\ntime and space. Recently, stochastic bedload transport theories have been\ndeveloped to address the mechanisms and effects of bedload transport\nfluctuations. Stochastic models involve parameters such as particle\ndiffusivity, entrainment and deposition rates. The lack of hard information on\nhow these parameters vary with flow conditions is a clear impediment to their\napplication to real-world scenarios. In this paper, we determined the closure\nequations for the above parameters from laboratory experiments. We focused on\nshallow supercritical flow on a sloping mobile bed in straight channels, a\nsetting that was representative of flow conditions in mountain rivers.\nExperiments were run at low sediment transport rates under steady nonuniform\nflow conditions (i.e., the water discharge was kept constant, but bedforms\ndeveloped and migrated upstream, making flow nonuniform). Using image\nprocessing, we reconstructed particle paths to deduce the particle velocity and\nits probability distribution, particle diffusivity, and rates of deposition and\nentrainment. We found that on average, particle acceleration, velocity and\ndeposition rate were responsive to local flow conditions, whereas entrainment\nrate depended strongly on local bed activity. Particle diffusivity varied\nlinearly with the depth-averaged flow velocity. The empirical probability\ndistribution of particle velocity was well approximated by a Gaussian\ndistribution when all particle positions were considered together. In contrast,\nthe particles located in close vicinity to the bed had exponentially\ndistributed velocities. Our experimental results provide closure equations for\nstochastic or deterministic bedload transport models.\n