Critical Erosion Threshold Research Articles

Relating spatial and temporal variability in sediment chlorophylla and carbohydrate distribution with erodibility of a tidal flat

Trends in the spatial distribution of chlorophylla (chla) and colloidal and total carbohydrates on the Molenplaat tidal flat in the Westerschelde estuary, Netherlands, reflected spatial differences in physical properties of the sediment. Results from a Spearman Rank Order Correlation indicated that many of the physical and biological measures covaried. Multiple regression analyses describing the relationship between colloidal carbohydrates and sediment properties resulted in several highly significant equations, although in all cases chla was able to predict colloidal carbohydrate content. Relationships between sediment surface chla and colloidal carbohydrate, and sediment erodibility (i.e., critical erosion threshold, Ucrit, and mass of sediment eroded at a velocity of 30 cm s−1) determined in annular flume experiments were examined. Overall sediment erodibility was lowest (i.e., high thresholds, low mass eroded) for the siltiest sediments in June 1996 when chla and colloidal carbohydrates were high (56.9 μg gDW−1 and 320.6 μg gluc.equ. gDW−1, respectively), and greatest (i.e., low thresholds, high mass eroded) at the sandier sediments in September 1996, when chla and colloidal carbohydrates were low (1.0 μg gDW−1 and 5.7 μg gluc.equ. gDW−1, respectively). When sediments were grouped according to relative silt content, the most significant relationships were found in muddy sand with a finegrained fraction (<63 μm) of 25–50%. Thresholds of erosion increased, while mass of sediment eroded decreased, with increasing chla and colloidal carbohydrate. A similar trend was observed for the sand-muddy sand (63 μm 10–25%). In the sand (63 μm 0–10%), there were no relationships for Ucrit, whereas mass eroded appeared to increase with increasing chla and colloidal carbohydrate. The increased carbohydrate may stick sand grains together, altering the nature of erosion from rolling grains to clumps of resuspension.

Impact of biotic and abiotic processes on sediment dynamics and the consequences to the structure and functioning of the intertidal zone

This paper reviews field and laboratory studies using flumes to quantify the erodability of undisturbed intertidal sediments as a function of changes in (1) the natural benthic community structure and sediment properties, and (2) the abundance of key intertidal species. Sediment erodability, which varies spatially and temporally, is dependent on the interactions between physical processes, sediment properties and biological processes, particularly the balance between two functional groups of biota, the stabilisers and the destabilisers. Bio-stabilisers can influence the hydrodynamics and provide some physical protection to the bed (e.g. mussel beds, macroalgae, salt marsh macrophytes), or can enhance cohesiveness and alter the critical erosion threshold (e.g. microphytobenthos). In contrast, bio-destabilisers (e.g. bioturbators such as Macoma balthica, Hydrobia ulvae) increase surface roughness, reduce the critical erosion threshold and enhance the erosion rate. Field studies in the Humber (England) and Westerschelde (Netherlands) have shown that interannual changes in sediment erodability were a result of a shift from a stabilised sediment dominated by microphytobenthos to a destabilised sediment dominated by M. balthica. Interannual changes in key biota, their influence on sediment erosion, and the consequences for intertidal ecology and morphology, appear to be driven in part by climatic factors (primarily a shift from mild to cold winters). Quantification and understanding of these benthic processes has been used to parameterise mathematical models of intertidal sediment dynamics, and this has provided insight into the relative importance of biological and physical factors in determining sediment erosion/accretion in the intertidal zone.

Structure, Stability, and Transformation of Contaminated Lacustrine Surface Fine-Grained Laminae

An in situ annular flume (Sea Carousel) was deployed in Hamilton Harbour, Lake Ontario, to assess the structure, stability, and transformation of lacustrine surface fine-grained laminae (SFGL). Such laminae typify depositional lacustrine environments and occur at the sediment-water interface. The critical erosion thresholds, erosion rates, and internal friction coefficients were determined for natural undisturbed and physically disturbed sites in order to assess changes in bed stability. Sediment cores, taken at each site, were analyzed for bulk density using a CT scanner in order to provide an intercomparison of our results. Flocs and aggregates, pumped from the flume during erosion experiments, were analyzed for morphological characteristics using conventional optical microscopy and transmission electron microscopy to evaluate the mechanism and development of the SFGL and its structure. A general three-layer model is developed which depicts the organic flocs of the SFGL (Layer 1) compressing within a collapse zone (Layer 2) to form a consolidated bed (Layer 3). The SFGL was observed to be porous, of low density, and of high water content which possessed yield resistance due, in part, to binding by extracellular polymeric substances secreted by a colonized microbial population. Transmission electron microscopy observations showed an active biological community in the sediment promoting biostabilization. Time series of erosion thresholds and friction coefficients of the disturbed bed showed that the SFGL reconsolidated rapidly and increased in shear strength. The artificial disturbance of the sediment resulted in incorporation of the SFGL and the collapse zone with the consolidated bed through mixing. This stimulated the process of consolidation chiefly through removal of gas and breakdown of the organic fibril network. Reconsolidation began with the formation of a collapse zone at the surface of the disturbed sediment. High friction coefficients and an increasing density with depth in the new surface layer indicated that rapid consolidation occurred after disturbance.

Variations in sediment properties, Skeffling mudflat, Humber Estuary, UK

The generic importance of biogenic mediation of sediment erosion and transport is a matter of debate and a multidisciplinary approach is required to investigate biologically mediated mechanisms of sediment stability. Biogenic influence on sediment behaviour can be inferred from a variety of correlative parameters that act as proxies for biological effects. These include pigment content, organic content and biomass. These biological “indicators” are routinely measured by biologists on a number of differing scales and depth resolutions. Few attempts have been made to examine the importance of an appropriate “match” between the erosion process, the measured physical response and the scale/resolution of the measured biological parameter. This scale dependency was examined along an extensive shore normal transect on the Skeffling mudflat (Humber Estuary, UK). Measurements of physical sediment properties, macrobenthos and selected biogeochemical properties (extracellular polymeric substances) were made. Biogeochemical properties were measured on a “traditional” cm scale and at a depth resolution of 5 mm but also on a microspatial scale, at a 0.2 mm depth resolution. Sediment stability was measured using a cohesive strength meter (CSM). Correlation analysis was used to determine the interactions between variables. A complementary investigation of the sediment micro-fabric (low-temperature scanning electron microscopy) was also conducted. Results demonstrate that the depth resolution of biogeochemical measurements is an influential factor in the interpretation of the biogenic stabilisation of intertidal cohesive sediments. Sediment stability varied with time and with bed feature. Stability increased with time except where influenced by other factors such as rain which markedly reduce surface stability. Critical erosion threshold increased towards the shore whilst suspension index (erosion rate) decreased, and crests were generally more stable than troughs. The study emphasises the temporal and spatial variability of mudflat stability and the importance of biological processes on the erosional behaviour of cohesive sediments.

Tephrochronology, Environmental Change and the Norse Settlement of Iceland

The first human impacts on the Icelandic environment came with the Norse colonisation or Landnám of the ninth century AD. The colonisation represents a fundamental environmental change that is both rapid and profound. In this paper we assess geomorphological dimensions of the initial settlement period using a tephrochronology that includes the Landnám Tephra, erupted ca. 870 AD, two tenth century AD tephras KR 920 and E 935, and 11 other well dated tephra layers. We report a new 14C age of 1676 ±12 14C yr BP (cal AD 345 (400) 419) for the tephra SILK-YN which forms a key prehistoric marker horizon that constrains rates of environmental change in the centuries before Norse Settlement. Aeolian sediment accumulation rates show five geomorphological responses to settlement that differ in the rate and trajectory of change. These distinct anthropogenic signals are the result of spatially variable sensitivity to grazing and deforestation, and reflect the extent of local soil erosion. This critical erosion threshold is variable in space and time.

Tephrochronology, Environmental Change and the Norse Settlement of Iceland

The first human impacts on the Icelandic environment came with the Norse colonisation or Landnám of the ninth century AD. The colonisation represents a fundamental environmental change that is both rapid and profound. In this paper we assess geomorphological dimensions of the initial settlement period using a tephrochronology that includes the Landnám Tephra, erupted ca. 870 AD, two tenth century AD tephras KR 920 and E 935, and 11 other well dated tephra layers. We report a new 14C age of 1676 ±12 14C yr BP (cal AD 345 (400) 419) for the tephra SILK-YN which forms a key prehistoric marker horizon that constrains rates of environmental change in the centuries before Norse Settlement. Aeolian sediment accumulation rates show five geomorphological responses to settlement that differ in the rate and trajectory of change. These distinct anthropogenic signals are the result of spatially variable sensitivity to grazing and deforestation, and reflect the extent of local soil erosion. This critical erosion threshold is variable in space and time.

Determination of the Critical Erosion Threshold of Cohesive Sediments on Intertidal Mudflats Along the Dutch Wadden Sea Coast

The bed shear strength on an intertidal mudflat bordering salt marshes along the Dutch Wadden Sea coast was determined by means of an in situ erosion flume (ISEF). Characteristic erosion patterns (type I- and type II-erosion) were observed for most measurements. The critical erosion threshold varied between 0·11–0·18 (Pa) for all measurements and erosion rates varied between 5×10−5and 3×10−3(kgm−2s−1). No clear relationship was found between distinct parameters like bed density, moisture content or biological activity and the critical erosion threshold.A more pronounced effect of the erodability of the mudflat was found when the erosion rate was taken into account. Basically two clusters were distinguished. High mean peak erosion rates of the substratum were found at locations which were characterized by a low mud content of the substratum (below 20% mud by weight). In these situations bed load transport is expected to be the most important process which determined erosion of the bed and erosion of predominantly the sand fraction was found. At higher rates of the mud content (over 20% mud by weight) mean peak erosion rates decreased substantially (an order 6 to 10 times lower) and both erosion of the mud and sand fraction was found.

Seed Dispersal in a Marine Macrophyte: Implications for Colonization and Restoration

Seagrasses rely on both vegetative (rhizome elongation) and sexual (seeds) propagation for maintenance of existing beds and colonization of new areas. Yet mechanisms of seed dispersal and survival of seeds in new areas remain poorly described. We conducted seed dispersal experiments in the field and laboratory to better describe seed dispersal characteristics in one species, Zostera marina L. (eelgrass), the dominant seagrass species in the temperate zone of the United States, Japan, and Europe. Seeds were broadcast by hand into unvegetated 5 m diameter plots at three locations over 3 yr (1989—1991) in the York River, Virginia (Chesapeake Bay). These sites had been previously vegetated but were devoid of any vegetation prior to (since 1972) and during the course of the experiments. Resultant seedling distributions closely matched broadcast patterns, with 80% of all seedlings found within the 5 m diameter plots, despite the fact that geophysical processes would appear sufficient to transport seeds greater distances. Wind records for the 2—mo period between seed broadcasting and germination revealed time—averaged wind speeds in excess of 40 km/h on > 12 d in each of the 3 yr and galeforce winds (17 km/h) in 2 of 3 yr. A three—dimensional hydrographic computer simulation model of the York River provided instantaneous current velocity estimates from which maximum bottom shear velocities (u*) in the study area were approximated (flood tide: 1.26 cm/s, ebb tide: 1.20 cm/s). These estimates exceeded the critical erosion threshold (U*crit = 0.7 cm/s) for Z. marina seeds determined from laboratory flume experiments. We postulate that small—scale topographic features on the bottom (burrows, pits, mounds, ripples) shield the seeds from the flow. Our results suggest that seeds settle rapidly, dispersing only up to a few metres under the influence of currents and become rapidly incorporated into the sediment. The limited dispersal capabilities of seeds underscore the need to address restoration goals and questions of seagrass ecology in the context of landscape—scale distributional patterns and metapopulation analyses.

Comparative structure, primary production and biogenic stabilization of cohesive and non-cohesive marine sediments inhabited by microphytobenthos

The microstructure, rate of primary production and erodibility of noncohesive and cohesive marine sediments colonized by autotrophic microbial assemblages were investigated. Microbial development on non-cohesive sediments (Island of Texel, The Netherlands) formed millimetre-thick stratified mats. A surface sheath of extracellular polymeric substances (EPS) was visible on these mats; diatoms dominated the surface 500 μm with an underlying layer of cyanobacteria, mainly Microcoleus chthonoplastes. In comparison, biofilms on cohesive sediment (Bristol Channel, U.K.) were relatively thin (100 μm) and unstratified. The microspatial distribution of the algal biomass was confirmed by measurements of oxygen evolution and rates of primary production using oxygen micro-electrodes. While gross primary production was greater in the non-cohesive sediments, peak rates of photosynthetic activity and algal biomass were similar for the two sites. The erodibility of colonized areas was measured and the mechanisms of biogenic stabilization were examined by low-temperature scanning electron microscopy (LTSEM). Cohesive sediment stabilization was purely by the secretion of EPS while both EPS binding and network formation by cyanobacteria were visualized within the non-cohesive sediments. Areas where mat and biofilm systems were visible were more resistant to sediment erosion than adjacent areas without biofilm development. Direct comparison between biogenic effects at the two sites must be treated with caution because of the different nature of the sediments but the index of biogenic stabilization for both sites is reported. The stability of the non-cohesive sediments was greatly increased by the presence of a microbial mat. The hydration state of the EPS matrix was considered to have a potential effect on the critical erosion threshold of the sediments.

Soil erosion and conservation on land cultivated and drained for afforestation

AbstractErosion of soil from pre‐afforestation plough furrows has been measured on four soil types in Scotland for 12 to 18 month periods between 1987 and 1990. Rainfall‐run‐off was also measured at one site. Run‐off is directly proportional to furrow length and rainfall intensity, and for a wide range of intensities (typically &gt; 6 mm hr−1) small amount of soil is flushed out of the furrows. However, for furrow spacings of 3.8 m, a critical downslope run‐off increment associated with significant soil loss is of the order of 25 cm3 s−1 m−1, which is in accord with a storm of five years return period and a maximum intensity of 25 mm hr−1. The total run‐off volume for any hydrograph is commensurate with the total rainfall in the rainstorm — typically 40–80% by the hydrograph peak and approaching 100% by the end of the hydrograph; i.e. long term storage is negligible. A positive relationship was recorded between furrow length, slope angle and sediment yield, with deposition predominating in furrows less than 30 m in length on slopes less than a few degrees. Soil loss is proportional to the excess streampower expended by the run‐off with an exponent in the range 1–1.5. For the soils examined, significant differences in soil loss when comparing sites for low power expenditure become undifferentiated at high power expenditures. For the rainfall regimes monitored, maximum soil losses were in the region of 40 kg per meter run‐length of furrow, when soil peds were ripped from the bed. Laboratory data concerning the critical erosion threshold power and shear stress to erode soil peds are in general accord with the threshold furrow run‐lengths defined using the field data for a five year storm and the soil losses observed.

Evidence for the seasonal deposition of detritus to the deep-sea floor and its subsequent resuspension

A free-fall benthic time-lapse camera and current meter system has been used to examine changes in the appearance of the sea bed in the Porcupine Seabight 50°N, 13°W — northeast Atlantic). Changes in the benthic environment due to rapid deposition of phytodetritus occur down to 4000 m. Photograhs taken every 8 h from 1 May to mid-August showed dramatic changes in the appearance of the sea bed between mid-June and mid-July. The sinking rate of the detritus was calculated from the time of the spring bloom until its arrival on the sea bed. In both 1982 and 1983, year which differed greatly in the timing of the spring bloom, the sinking rates were probably between 100 and 150 m d −1. Individual aggregates up to 12 mm diameter arrived between frames at all depths and up to 50 mm at 4000 m. Their subsequent disintegration was monitored over the few days following arrival on the sea bed. Some such aggregates were collected from the sea bed and a sinking rate experiment was carried out on them. Once on the sea bed, the detrital carpet moves over the sediment surface due to bottom currents; when currents exceed about 7 cm s −1 (at 1 m altitude), the material is resuspended. Increases in the quantity in suspension occur at the same time as decreases in the quantity visible on the sea bed. The tidal nature of the current gives a strong tidal component to the variation in suspended particles near the sea bed. The significance of a detrital critical erosion threshold to the structure of benthic and benthopelagic communities is discussed.