Critical Erosion Velocity Research Articles

Temporal changes in intertidal sediment erodability: influence of biological and climatic factors

Annular flumes were used to measure the impact of current velocity on material fluxes associated with undisturbed sediment and its natural benthic community. Sediment erodability was quantified in terms of critical erosion velocities ( U crit), mean erosion rate and the mass of sediment eroded in relation to increasing current velocities. Variations in the erosion potential of intertidal sediments were measured in the Humber estuary (UK) and were related to changes in the balance between biological and physical processes of sediment ‘stabilisation’ and ‘destabilisation’. Flume experiments quantified the impact of the clam, Macoma balthica, when added to upper shore sediments (above HW neap tide level) at various stages during the spring-neap tidal cycle. The marked reduction in sediment erodability on the upper shore was due to a combination of prolonged air exposure/dehydration during the neap tidal cycle and the absence or low density of the bioturbator, M. balthica Temporal-spatial changes in sediment erosion potential ( U crit ranging from 14 to 40 cm s −1 and ·>100-fold differences in erosion rate) largely reflected the temporal shift in the balance between key biota acting as sediment ‘stabilisers’ (benthic algal films) and ‘destabilisers’ (density of bioturbating clams, M. balthica). There was no evidence of a consistent seasonal cycle but long-term changes in sediment erodability correlated with the density of M. balthica. These temporal changes in sediment erodability and biota in the Humber were also apparent over a wider geographical scale, which implies that parallel changes in benthic community structure and sediment erosion may be climate driven, probably via long-term biological cycles in ‘stabilisers’ and ‘destabilisers’. This study provides evidence of an important coupling between long-term temporal changes in the sediment erodability of mudflats and sediment accretion on the upper salt marsh. Between March 1996 and June 1996, sediments had a high erosion threshold, which was associated with the presence of well developed benthic algal films and low densities of bioturbating clams. This resulted in significant sediment accretion on the intertidal mudflat, but little accretion on the upper salt marsh ( Spartina anglica). In contrast, from July 1996 to July 1997 there was a marked increase in the sediment erosion potential due to the higher densities of M. balthica, and this was associated with significant erosion of the intertidal mudflat. As a consequence of enhanced erosion of the mudflat and re-suspension of sediment, there was greater sediment accretion on the upper salt marsh, thus providing benefits to the natural coastal defence. Conversely, reduce sediment erodability on the mudflat, due to bio-stabilisation, resulted in a lower accretion rate on the upper salt marsh.

Influence of biota on spatial and temporal variation in sediment erodability and material flux on a tidal flat (Westerschelde, The Netherlands)

Annular flumes were used to quantify benthic-pelagic exchanges in relation to changes in current velocity and sediment biota. Various physical, chemical and biological parameters were determined for 4 sites on the Molenplaat tidal flat in the Westerschelde (The Netherlands) during 2 field campaigns in May-June and August-September 1996. These included: the mass of sediment eroded, maximum and mean erosion rates, critical erosion velocity (U crit ), suspension feeding/biodeposition rate, oxygen and ammonium fluxes, density of macrofauna species, chlorophyll a (chl a), colloidal carbohydrate, and physical properties of the sediment. The study showed marked and statistically significant spatial and temporal differences in the potential for sediment erosion. Sites in the centre of the tidal flat were less erodable than those on the edge. All sites on the tidal flat had a significantly (p < 0.001) lower erosion potential in June (i.e. higher U crit and lower erosion rates) compared to September. The presence of a well-developed microphytobenthos community in June resulted in a statistically significant relationship between U crit and chl a/colloidal carbohydrate (r = +0.85, p = 0.01). There was a significant relationship (r = +0.88, p = 0.005) between sediment erodability (mass of sediment eroded and erosion rate) and the density of the clam Macoma balthica, a major bioturbator of the surface sediments. Analysis of the data also demonstrated significant relationships between clearance rate and Cerastoderma edule biomass (r = +0.91, p < 0.001), and between sediment chl a/colloidal carbohydrate and C. edule biomass (r = +0.92; p < 0.001), reflecting the influence of suspension feeders on both the water column and the sediment, through their biodeposits. The significant increase in sediment erodability between June and September reflected the shift in the overall balance between the major 'biostabilisers' (microphytobenthos) and 'bio-destabilisers' (bioturbating bivalves). This interpretation of field-derived correlations is consistent with previously established relationships and mechanistic understanding derived from controlled flume experiments.

Sedimentological characteristics of small rivers with loessic headwaters in the Chaco, South America

Some sedimentological characteristics of #uvial environments located in the northeastern region of the Chaco}Pampa plain were analysed in order to determine the possible in#uence of the loessic headwaters of local basins. Concentrations of suspended sediments, as well as grain size of the river beds, were compared. Unexpectedly, it was found that the suspended load has an inverse relationship with the hydrometric levels, with the exception of highly saline streams (conductivity values between 4300 and 11,000 l Sc m~1). The mean concentration of suspended sediments was 63.1 and 206.3 mg l~1 in #ood and low water periods, respectively. The grain size of bed sediments was relatively homogeneous along the whole hydrological cycle. Sandy}silty bottoms predominate in channels, followed by silty}sandy and clayey}silty bottom types. Sediment transport mainly occurs in accelerating #ow suspension, overcoming the critical erosion velocity. During #ood periods, some parameters exceeded by 70% the values recorded during the low water phase. ( 2000 Elsevier Science Ltd and INQUA. All rights reserved.

Use of in situ flume to quantify particle flux (biodeposition rates and sediment erosion) for an intertidal mudflat in relation to changes in current velocity and benthic macrofauna

Abstract A portable in situ annular flume was deployed on eight occasions at five stations along a shore normal transect from immediately below the saltmarsh to the mid-tide level of the Skeffling mudflat in the Humber estuary. Both large scale (km) and small scale (m) spatial differences (i.e. between ridges, gullies and pools on ridges) were examined. Biodeposition rates were measured at low current velocities (5 cm s −1 ), while sediment resuspension and erosion rates were determined in response to step-wise increases in current velocities between 10 and 50 cm s −1 (equivalent to a range in shear stress from 0.02 to 1.6 Pa). Maximum biodeposition rates (6.6 g m −2 h −1 ) were more than an order of magnitude higher than the mean sedimentation rate in the absence of sediment and biota. There was a significant correlation between biodeposition rate and the density of the suspension feeder Cerastoderma edule ( r = +0.90; P &lt; 0.05). The intertidal sites could be separated into three distinct groups on the basis of sediment stability/erodibility. Upper shore sites had the highest stability with critical erosion velocities ( u crit ) of 30 cm s −1 , whereas the pools and gullies were the most easily eroded with u crit of 15 cm s −1 . Site differences in sediment erodibility were not significantly correlated with measured physical properties of the sediment (bulk density, moisture content, % POM, % sand). However, there was a significant correlation between sediment erodibility and the density of Cerastoderma ( r = +0.89; P &lt; 0.01) and an index of the bioturbation activity of Macoma balthica ( r = +0.96; P &lt; 0.001).

Mussels and mussel beds ( Mytilus edulis ) as stabilizers of sedimentary environments in the intertidal zone

Abstract Coastal zone protection of the intertidal zone is of great importance in view of the threats of sea level rise around the globe. Environmentally-friendly methods involving biological control may offer a solution. This paper studies mussels ( Mytilus edulis ) as stabilizers of the intertidal zone because they form beds that armour some intertidal sediments, hence reducing erosion. In field experiments we seeded high energy lower intertidal and low energy upper intertidal environments with isolated mussels and preformed clumps of mussels. More animals were lost from the higher energy site. Those that remained were transported landward for significant distances. Clumping protected animal loss at the low energy but not at the high energy site. Gravel placed under mussels improved their stability because mussels attached threads to them. Laboratory experiments show that mussels form clumps by attaching to other mussels, and when available to gravel which has to be at the sediment surface. Flume studies show single mussels decrease the critical erosion velocity of sediment near them. Similar effects are seen in the field at the edge of small mussel beds.

A Benthic Annular Flume forIn SituMeasurement of Suspension Feeding/Biodeposition Rates and Erosion Potential of Intertidal Cohesive Sediments

A portable benthic annular flume for thein situmeasurement of suspension feeding (biodeposition rates) and erosion potential of undisturbed intertidal cohesive sediments with benthic biota is described. The experimental procedures used during deployment are also outlined. Results showing the changes in biodeposition rates, sediment resuspension, erosion rates and critical erosion velocities associated with large-scale (km) and small-scale (m) spatial differences are presented. Comparison between laboratory andin situmeasurements demonstrated marked differences in sediment erodability if the physical and biological properties of the sediment are not maintained.

Transport of recently settled soft-shell clams ( Mya arenaria L.) in laboratory flume flow

A laboratory flume was used to examine the retention of juvenile soft-shell clams (2 wk postsettlement) in sediment when exposed to flow. Clams resisted erosion until the initiation of sediment transport, after which they were rapidly advected from sections of test substrate. Comparisons between living and killed clams indicated that burrowing behavior was instrumental for maintaining position in flow ≤ the critical erosion velocity for sediment movement. The ability of low density, shallow-burrowing juvenile bivalves to avoid transport as bedload or resuspended particles is probably minimal during erosional periods, despite that fact that the position of larger, deeper living individuals may be unaffected. In energetic regimes, infaunal recruitment patterns can be dominated by hydrodynamic forces affecting juveniles.

Transport of Juvenile Gem Clams (Gemma gemma) in a Headland Wake

Accumulation of bivalve recruits in the bottom convergence at the center of coastal eddies has been suggested as a possible mechanism resulting in locally abundant adult populations. We investigated transport of juvenile gem clams (Gemma gemma) in a headland wake to determine whether they accumulated, and where. Velocity measurements during three flood tides showed that a wake consistently formed, but that flow speeds were too slow to transport juvenile clams to the eddy center. Instead, the clams were deposited just inside the wake perimeter, where shear velocities decreased to levels below critical erosion velocities of the clams. This result demonstrated that accumulation in a coastal flow separation can occur even in the absence of a well-defined eddy or a strong bottom convergence. Juvenile gem clams were carried, probably as bedload, to regions in the wake dominated by sediments with similar grain sizes, rather than similar fall velocities, suggesting that bedload transport was particularly dependent on particle diameter in this flow regime. Adult gem clam populations tended to be locally abundant in regions receiving transported juveniles, but clam transport on any specific flood tide was not sufficient to fully predict the adult distributions.

Formation of sand banks due to tidal vortices around straits

Sand banks around straits are used as a commercial fishing ground. In order to clarify the mechanism of sand bank formation, the Lagrangian method was used to measure currents and turbidity around the banks in the Neko Seto Sea in the Seto Inland Sea of Japan. A neutrally buoyant float released in the Neko Seto Strait at the maximum tidal flow stage was engulfed in a pair of tidal vortices and moved around one of the sand banks. The vertical distribution of turbidity, which was measured by the vessel moving with the neutral float, showed an extremely high turbidity in the bottom layer of this bank area. According to the analysis of these observational data, the process of sand bank formation around straits is as follows. The tidal vortex transports water mass with suspended materials (including sand) which are whirled up at the bottom by the tidal jet. In the decaying stage of the vortex, the materials in the bottom layer are gathered in the central part of the vortex by the secondary convergent flow in the vortex. Among these materials, a large-size sand particle with a high critical erosion velocity accumulates at the bottom and forms banks. The distribution of bottom sediment and the thickness of alluvium support this result.

Bioturbation, microbial activity and sediment properties in an estuarine ecosystem

SynopsisMacrobenthic bioturbation and microbial activity can have major ecological effects on present day marine sediments some of which are also seen in the fossil record. The coastal and estuarine sediments in the Clyde Estuary area are described. They contain very high densities of benthic macroorganisms and microorganisms. These organisms have dramatic effects on the physical and chemical properties of sediments, and field and laboratory studies have shown that many geotechnical and geochemical properties of sediments are dictated by them. The effects can be on a microscale of a few millimetres horizontally or vertically (micro-spatial variation), or can have large scale consequences over hundreds of metres (macro-spatial variation).Sediment stability (shear strength and critical erosion velocity), permeability and particle sedimentation are all altered by macro-invertebrate bioturbation or microbial activity, and many chemical properties such as Eh and pH may be influenced in a similar manner.Biological activity may therefore play a major role in modifying sediment geotechnics and early sediment diagenesis. It also has direct relevance to the siting of man-made structures on the sea-bed and to sediment stability and transport in estuaries.In situseeding experiments may lead to biological control and manipulation of these most important field phenomena.

Bed forms and stratification types of modern gravel meander lobes, Nueces River, Texas

ABSTRACTAll major streams draining the southwestern flank of the Edwards Plateau in south‐central Texas transport large volumes of gravel and sandy muddy gravel and are developing meander lobe sequences consisting predominantly of coarse gravel. The largest of these streams, the Nueces River, has a sinuosity index of 1.3 and an average stream surface slope of 1.8 m/km in the study area. Stream discharge is variable and has ranged from no flow to more than 17,000 m3/s.Mean clast b‐axis length for the ten largest clasts at thirteen sample sites ranged from 2.5 to 10.8 cm. Velocities of 2.7‐4.4 m/s 1 m above the stream bed are required to transport these clasts. Stream velocities of these magnitudes occur about once in 8 years when discharge of the Nueces River exceeds 3300 m3/s. Mean grain size of Nueces River alluvium ranges from 1.2 to 3.4 cm. At a flow depth of 1 m, sediment of this size has a critical erosion velocity of 1.8‐3 m/s. Velocities of this magnitude occur about once in two years when discharge exceeds 340 m3/s. Under these conditions flow is subcritical, with critical shear stresses on depositional surfaces ranging from 6.4 to 12.7 kg/m2.Gravel clasts are imbricated and channel bed forms are predominantly transverse gravel bars with slip faces ranging up to 2 m high and wavelengths in excess of 100 m. Stratification includes graded planar crossbeds and horizontal beds. Lower lateral accretion face sediments are also predominantly transverse bars; upper lateral accretion face deposits occur as longitudinal gravel ridges deposited in the lee of vegetation and, less commonly, as chute bars. Near the upper limit of meander lobes where vegetation is heavy, mud and muddy sand occur as overbank deposits; in these deposits sedimentary structures other than desiccation cracks are rare.Sedimentary sequences in gravel meander lobe systems deposited by low sinuosity streams are graded or non‐graded horizontal beds and planar cross‐beds overlain by mud and muddy sand interbedded with horizontally bedded gravels. Sequences may be several metres thick, but probably do not exceed 8‐10 m in thickness. These deposits in turn are overlain by overbank deposits of mud and muddy sand. Similar sedimentary sequences occur in the extensive Quaternary terraces that parallel the Nueces River.

New Jersey Continental Shelf Near Bottom Current Meter Records and Recent Sediment Activity

ABSTRACT Modern sedimentary processes associated with the ridge and depression topography of the New Jersey continental shelf were investigated with the aid of near bottom current meter measurements and also estimates of near bottom wave orbital velocities derived from classical wave theory and available surface wave observations. Four current meters were set 1.5 to 2.0 m above sea bottom in 30, 59, 74, and 143 m depths for nine to eleven days during the late springs of 1970 and 1971. The root mean square speeds of the meter records were 13.7, 11.8, 12.9, and 19.5 cm/sec. in order of increasing depth, with a maximum recorded 2.5 minute average speed of nearly 40 cm/sec. and a net southwesterly transport. Both current and wave data indicate reworking of the surface sand cover on the shelf. In d termining the relative importance of wave vs. current activity the true value of the critical erosion velocity is of primary importance. The present day physical reworking of the surface sediments is indicated independently by bottom photographs of ripples and by sedimentary structures in box cores, with reworking being generally limited to the upper meter of sediment. No mechanism for the present formation of the larger scale ridge and depression topography is indicated by the currents measured or the wave considerations.