Median Sediment Grain Size Research Articles

Hydrogeomorphic Recovery and Temporal Changes in Rainfall Thresholds for Debris Flows Following Wildfire

AbstractWildfire‐induced changes to soil and vegetation promote runoff‐generated debris flows in steep watersheds. Postfire debris flows are most commonly observed in steep watersheds during the first wet season following a wildfire, but it is unclear how long the elevated threat of debris flow persists and why debris‐flow potential changes in recovering burned areas. This work quantifies how rainfall intensity‐duration (ID) thresholds for debris‐flow initiation change with time since burning and provides a mechanistic explanation for these changes. We constrained a hydrologic model using field and remotely sensed measurements of soil‐infiltration capacity, vegetation cover, runoff, and debris‐flow activity. We applied this model to estimate rainfall ID thresholds for debris‐flow initiation within three burned areas in the southwestern United States over a postfire recovery period of three to four years. Modeling suggests ID thresholds are lowest immediately following the fire (below a one‐year recurrence interval [RI] storm) and increase with time, such that a 10‐ to 25‐year RI storm would be required to generate a debris flow after three years of recovery. Modeled changes in rainfall ID thresholds result from increases in soil infiltration capacity, canopy interception, hydraulic roughness, and median grain size of sediment entrained in an incipient debris flow. The relative importance of each of these factors varied among our three sites. Results improve our ability to assess temporal changes in postfire debris‐flow potential, highlight how site‐specific factors may alter the persistence of postfire debris‐flow hazards, and provide additional constraints on the timescale of recovery following wildfire.

Technical Feasibility of Using Frac-Packed Wells for Producing Natural Gas from Offshore Gas Hydrate Reservoirs

This paper presents a feasibility analysis of using frac-packing completion techniques to produce natural gas from offshore gas hydrate reservoirs. A case study was carried out for the gas hydrate accumulations in the northern South China Sea. The feasibility analysis covers the requirements of proppant size, fluid injection rate, fracturing pressure, and well productivity. For the median grain size of sediments in the studied formation from 2.60 to 28.96 μm with an average value of 8.49 μm, the required range of proppant size is between 333 mesh to 748 mesh (0.001 inch ~ 0.003 inch). Since the proppants in this size range are not commercially available, it would be economical to use screened natural sands as proppants in frac-packing operations. The minimum flow rate of fracturing fluid required to carry the 0.003 inch proppant/sand into the fracture tip at 510 ft is 3.64 bpm, which is much lower than the practical values ranging from 20 bpm to 100 bpm. Therefore proppant/sand transport during frac-packing is not a concern. To create a horizontal fracture of 510 ft radius with a fracturing fluid injection rate of 72 bpm, the maximum bottom hole injection pressure is predicted to be 2,378 psi, which is only 334 psi above the reservoir pressure and can be handled by most pumps used in frac-packing operations. Well productivity forecast with a simplified mathematical model shows that a commercial gas production rate of 16 MMscf/day is achievable with the fracture radius of 510 ft. However, the model requires further validation.

Regulation effectiveness of a window-check dam on debris flows

Window-check dams are widely used to regulate sediment transport in debris-flow hazard mitigation. However, the effectiveness of window-check dams on modulating debris flows is still unclear. In this study, dimensional analyses combined with physical experiments were conducted to evaluate the sediment-trapping and particle-regulating functions of a window-check dam. Results showed that both of the attenuation ratio of unit weight and (the difference between upstream and downstream divided by upstream) the retention ratio of mass (upstream divided by downstream) decreased with an increase in openings area, drain holes width, and bottom slope. However, the increase in inflow unit weight of debris-flow led to larger retention ratio of mass. Besides, an optimal attention ratio corresponding to dimensionless unit weight of 1.8 was found. Also, the particle sizes sorted by the window-check dam were found to be associated with the median grain size of sediment. Furthermore, two improved expressions were obtained to assess the effectiveness of window-check dams on trapping sediments. The comparison between these expressions and laboratory data is in reasonable agreement, serving a useful reference for the optimal design of check dams.

The Incipient Motion Features of Sediment from Yangtze Estuary: Annular Flume Experiments

In this study, annular flume experiments are carried out, using the sediment samples collected from Yangtze estuary. The incipient velocity and the incipient shear stress of three different groups of median grain size of sediment are given. The turbulent kinetic energy method (TKE) is used to determine the bed shear stress (τ), by evaluating variations in the suspended sediment concentration (SSC) within the water column. The suspended sediment concentration increases with the increase of the bed shear stress. When the sediment concentration reaches a certain concentration, the rate of change in τ obviously slows down. As the concentration increasing again, the bed shear stress grows rapidly with different growth rate attributed to different grain size. The results of the experiments indicate that SSC and grain size have strong influence on τ.

The paleoenvironmental significance of spatial distributions of grain size in groundwater‐recharged lakes: A case study in the hinterland of the Badain Jaran Desert, northwest China

AbstractFor lakes in desert hinterlands that are not recharged by river runoff, sediment input solely comes from wind transport. While the processes of sediment transport and deposition in these lakes differ significantly from those with river discharge, the spatial distribution of sediment grain size in these groundwater‐recharged lakes remains largely unknown. Moreover, whether the grain size distribution in these lake sediments can be used as a proxy in the study of past climatic change and environmental evolution studies is unclear. In this study, five lakes with a range of surface areas that had no runoff recharge were selected from the hinterland of the Badain Jaran Desert of north‐western China, and a total of 108 samples of lake surface sediments were collected to examine the spatial distribution of grain size. Moreover, an end‐member‐modeling algorithm was used to calculate end members from all grain size measurements. Our results showed that both the median and mean grain sizes in the lake sediments decreased from the nearshore to the offshore, deep‐water zone. However, the lowest median and mean grain sizes were not found in the center of the lakes, in contrast to lakes recharged by surface runoff. The median grain size of sediment in the lake center was negatively correlated with lake level, and thus could help reveal lake evolution at low resolutions. Moreover, EM1 and EM2 were interpreted as wind transported sediment, and sediment perturbed by lake waves after wind transport, respectively. The modal grain size of EM1 varied slightly between lakes, while changes in the modal grain size of EM2 were related to lake area. Given the positive relationship found between EM2 content and lake level, changes in the EM2 content (%) can serve as a rough indicator of lake level fluctuations at low temporal resolutions. Copyright © 2017 John Wiley & Sons, Ltd.

Trend and dynamic cause of sediment particle size on the adjacent continental shelf of the Yangtze Estuary

Based on the measured data in recent 20 years, the variation trends of the median grain size of the surface sediment, the sand-silt boundary and the mud area on the adjacent continental shelf of the Yangtze Estuary were analyzed in depth, and the effects of natural mechanism and human activities were discussed. The results show that: (1) In recent years (2006-2013), the median grain size of sediment and the distribution pattern of grouped sediments in the adjacent continental shelf area to the Yangtze Estuary have presented no obvious change compared with those before 2006; (2) The median diameter of the surface sediment in the continental shelf area displayed a coarsening trend with the decrease of sediment discharge from the basin and the drop in suspended sediment concentration in the shore area; (3) In 2004-2007, the sand-silt boundary in the north part (31°30′N) of the continental shelf area presented no significant changes, while that in the south part (31°30′S) moved inwards; In 2008-2013, both the sand-silt boundaries in the north and south parts of the continental shelf area moved inwards, mainly due to the fact that in the dry season, a relatively enhanced hydrodynamic force of the tides was generated in the Yangtze River, as well as a decreased suspended sediment concentration and a flow along the banks in North Jiangsu; (4) The mud area where the maximum deposition rate is found in the Yangtze Estuary, tends to shrink due to the drop in sediment discharge from the basin, and the decrease in suspended sediment concentration in the shore area and erosion in the delta. Moreover, it tended to shift to the south at the same time because the implement of the training works on the deep-water channel of the North Passage changed the split ratio between the North and South Passages with an increase in the power of the discharged runoff in the South Passage.

Community structure changes of macrobenthos in the South Yellow Sea

The ecological environment in the Yellow Sea has changed greatly from the 1950s to 1990s and this has had significant impact on marine organisms. In this study, data on soft-sediment macrobenthos occurring in depths from 25 m to 81 m in the South Yellow Sea were used to compare changes in community structure. The agglomerative classification (CLUSTER) and multidimensional scaling (MDS) methods were applied. Five communities were recognized by cluster analysis: 1. The Yellow Sea Cold Water Mass community dominated by cold water species, which changed slightly in species composition since the 1950s; 2. The mixed community with the coexistence of cold water species and warm water species, as had been reported previously; 3. The polychaete-dominated eurythermal community in which the composition changed considerably as some dominant species disappeared or decreased; 4. The Changjiang (Yangtze) River Estuarine community, with some typical estuarine species; 5. The community affected by the Yellow Sea Warm Current. The greatest change occurred in the coastal area, which indicated that the change may be caused by human activities. Macrobenthos in the central region remained almost unchanged, particularly the cold water species shielded by the Yellow Sea Cold Water Mass. The depth, temperature and median grain size of sediments were important factors affecting the distributions of macrobenthos in the South Yellow Sea.

Seagrass colonization: Knock-on effects on zoobenthic community, populations and individual health

This study provided evidence that Zostera noltii presence affects macrofauna community structure independently from median sediment grain-size and that the notion of ecosystem health is rather subjective: in the present case, we recorded “good health” in terms of seagrass development, “no impact” in terms of macrobenthic biotic indices and “negative effect” for a given key-population. The occurrence and development of a Z. noltii seagrass bed was surveyed at Banc d’Arguin, Arcachon Bay (France), to estimate the modification of the macrozoobenthic community and of the dynamics of a key-population for the local ecosystem, – the cockle Cerastoderma edule. Even though median grain-size of the sediment decreased only at the very end of the survey, i.e. when seagrass totally invaded the area, most of the macrofauna community characteristics (such as abundance and biomass) increased as soon as Z. noltii patches appeared. The structure of the macrofauna community also immediately diverged between sand and seagrass habitats, without however modifying the tested biotic indices (BENTIX, BOPA, AMBI). The health of the cockle population (growth, abundance, recruitment) was impacted by seagrass development. Related parasite communities slowly diverged between habitats, with more parasites in the cockles from seagrass areas. However, the number of parasites per cockle was always insufficient to alter cockle fitness.

An investigation of the relationship between sediment particles size and the development of green algal mats (Ulva prolifera) on the intertidal flats of Muan, Korea

The relationship between median grain size of sediments and the abundance in the wild of green algal mats (Ulva prolifera) on the intertidal flats of Muan, Korea, were investigated. The impact of substratum particle size on the growth and survival of germlings was examined in the laboratory. In the wild, the average annual density of algal mats was 7,950 ind m−2. The algal mats mainly occurred in sands and exhibited patchy distribution. Statistical analysis indicates significant spatial analysis differences and a significant relationship between density and the ratio of sands to silts, suggesting that the distribution and density of this species were related to particle size. In laboratory experiments, the survival rate of U. prolifera germlings was the lowest value (22%) on sediments with a median grain size of 63–125 μm. Laboratory experiments have generally shown a positive relationship between attachment or survival of the alga and substratum particles size. Our laboratory results indicate a clear link between germling settlement/survival and substratum particle size. These results explain the spatial differences in abundance observed in the field in relation to the distribution and ratio of sands to silt on the Muan flats.

Spatial distribution and assemblage structure of macrobenthic invertebrates in a brackish lake in relation to environmental variables

The distribution of organisms relative to their habitat is of central importance to ecology. The nature of this distribution provides an initial insight into the types of ecological processes that regulate populations and assemblages. Estuaries provide diverse habitats in terms of spatial heterogeneity in physical and chemical conditions. The purpose of the present study is to determine the assemblage structure and the spatial distribution of macrobenthic invertebrates in relation to environmental variables in a brackish lake, Lake Hinuma, in Japan. Thirty-seven sites in the lake were selected to cover the entire area of the lake along gradients for water depth, salinity and median grain size of sediment. A total of 23 species and five higher taxa groups were found. In terms of total individuals, Corbicula japonica, Prionospio japonica, Notomustus sp. and Einfeldia sp. were the most abundant species. The results of cluster analysis and canonical correspondence analysis using 14 dominant species revealed that median grain size and salinity have significant effects on the spatial distribution of 11 macrobenthic species. That is, the two species C. japonica and P. japonica were found at sites with larger grain size and higher salinity, the six species Gradidierella japonica, Assiminea lutea japonica, Tylorrhynchus heterochaetus, Cyathura sp., Melita sp. and Potamilla sp. were found at sites with smaller grain size and higher salinity, the one species Einfeldia sp. was found at sites with larger grain size and lower salinity, and the two species Polypedilum sp. and Notomustus sp. were found at sites with smaller grain size and lower salinity. The results demonstrated significant differences in macrobenthic assemblage structures along environmental gradients in a brackish lake.

Relationships between meiofaunal population densities and physico-chemical properties of unpolluted sandy beaches

Water and sediment samples from ten unpolluted sandy beaches have been analysed for various chemical parameters and meiofauna. The results have indicated that, within the range of grain sizes found, (196-337 μm), short term fluctuations in the natural levels of the various chemical constituents do not significantly influence the densities of meiofauna populations. These are, however, governed largely by the median grain size of the sediments. The relationships between median grain size and the population densities of nematodes and harpacticoid copepods have been established. The optimum grain size for the populations studied was 284 μm, and as the sediment size deviated from this, the population density decreased. Thus knowledge of the median grain size of sediments should indicate the approximate meiofaunal density which can be expected in the absence of any other external influence. Deviations from this can then be attributed to perturbation.