In parts of the developing world, the expansion of industrial sand mining activities has led to serious environmental concerns. However, current understanding of the effects of this activity on an ...inland water ecosystem remains limited. Herein, we choose the “most affected” lake in China (Dongting Lake), to assess short-term (1year) effects of sand dredging on key environmental parameters and on the structure of the macroinvertebrate assemblage. Within the dredged area we observed increases in water depth (on average 2.17m), turbidity and changes in sediment composition (e.g., increase in % medium sand, and a decrease in % clay). In addition, dredging was associated with a 50 % reduction in taxa richness, Simpson and Shannon-Wiener indices, and a 72 and 99 % reduction in abundance and biomass, respectively. Indirect effects were also observed in the zone surrounding the extraction sites (ca. 500m), most likely as a result of the dredging processes (e.g., sediment screening and overspill) and water flow. No such effects were observed at a nearby reference site. The direct removal of sediment and indirect alteration of physical conditions (e.g., water depth, turbidity and sediment composition) appear to be the most likely cause of variations in the benthic community. Implications of our findings for the planning, management and monitoring of sand dredging in inland waters are discussed.
The environmental concerns of the development of the sand dredging industry in Dongting Lake, the middle-lower Yangtze River, China (a, the sand market in East Dongting Lake; b, the dredgers in South Dongting Lake). Display omitted
•Effects of sand dredging on inland freshwater ecosystems have rarely been studied.•Short-term sand dredging activity resulted in changes to water depth, transparency, sediment particle size, and lakebed fauna.•Taxa richness, abundance and biomass were enhanced in area adjacent to dredging result of changes in physical parameters.•Alterations to the benthic community were caused by direct removal of sediment and indirect changes to physical conditions.
Sediment is one of the key components of free water surface constructed wetland (FWS), and sediment dredging has become common practice in the long-term management of FWS. However, insufficient ...research has been carried out on the effects of sediment dredging in FWS, and limited information is available regarding appropriate dredging depth and methods in FWS sediment dredging. This study investigated the physicochemical characteristics of sediment in a five-year operational FWS. The effects of different dredging depths (0 cm, 1 cm, 10 cm, and 25 cm) and different dredging methods (mechanical excavation dredging and hydraulic purged flushing dredging) were compared based on the improvement of overlying water quality via indoor static simulating experiments. The results showed that the 0–10 cm depth of sediment was heavily contaminated by organic matter and nitrogen, and slightly contaminated by phosphorus. A boundary layer (0–1 cm) was formed with obvious different physicochemical characteristics which could suppress nutrient release into the overlying water. The 25 cm dredging depth was the appropriate dredging depth according to engineering practice and control performance of water quality. The pollutant diffusion fluxes of 25 cm dredged depth were 31.8%, 58.3%, 30.7%, 41.5% and 20.6% lower than those of undredged sediment for total phosphorus, soluble reactive phosphorus, total nitrogen, ammonia nitrogen, and permanganate index, respectively. Mechanical excavation dredging had a better control of water quality than hydraulic purged flushing dredging due to the low sediment water content and low residual sediment on the dredged sediment surface.
•The sediment characteristic in a full-scale FWS treating drinking water was studied.•A boundary layer was formed in the FWS sediment after five-year operation.•Dredging at 25 cm depth could significantly decreased the pollutants concentrations.•Mechanical excavation dredging was an acceptable choice for FWS sediment dredging.
The Tulang Bawang river is one of the largest rivers in Lampung Province, Indonesia, transporting humans and goods. Changes in upstream land use, climate, and sedimentation are silting the riverbed ...and disrupting transportation. To this end, investors and government agencies have submitted various proposals to carry out sand mining downstream to assist the local government in revitalizing transportation. However, some government and community assets are likely to be affected in the upstream part that is planned to be dredged. Therefore, this study aimed to conduct a modeling scenario of riverbed dredging in the lower reaches of the Tulang Bawang River from the estuary to 11.8 km upstream. It also aimed to review the impact on the environment, especially the impact of flooding and sedimentation by 17.8 km upstream, using the HEC-RAS software. The scenarios of upstream and downstream boundary conditions were used to determine the significance of the impact. The results showed that dredging would cause water level elevation to drop upstream and sediment deposition along the river section dredged. However, the decrease in river water level was insignificant for the upstream assets and beneficial for reducing flood inundation. The result of sedimentation analysis shows that river dredging leads to morphological changes and may have an environmental impact. Therefore, effective environmental management for dredging needs to be applied to minimize the environmental impact.
► Intensity, duration and frequency of sediment disturbance determine dredging impacts on corals. ► Sediment stress effects have been investigated in ∼10% of known reef-building coral species. ► ...Tolerance limits of coral reefs to suspended sediment vary widely from <10 to >100mgL−1. ► Tolerance limits of corals to sedimentation vary widely from <10 to >400mgcm−2day−1. ► Sensitivity to sediment stress is significantly related to coral growth form, but not calyx size.
A review of published literature on the sensitivity of corals to turbidity and sedimentation is presented, with an emphasis on the effects of dredging. The risks and severity of impact from dredging (and other sediment disturbances) on corals are primarily related to the intensity, duration and frequency of exposure to increased turbidity and sedimentation. The sensitivity of a coral reef to dredging impacts and its ability to recover depend on the antecedent ecological conditions of the reef, its resilience and the ambient conditions normally experienced. Effects of sediment stress have so far been investigated in 89 coral species (∼10% of all known reef-building corals). Results of these investigations have provided a generic understanding of tolerance levels, response mechanisms, adaptations and threshold levels of corals to the effects of natural and anthropogenic sediment disturbances. Coral polyps undergo stress from high suspended-sediment concentrations and the subsequent effects on light attenuation which affect their algal symbionts. Minimum light requirements of corals range from <1% to as much as 60% of surface irradiance. Reported tolerance limits of coral reef systems for chronic suspended-sediment concentrations range from <10mgL−1 in pristine offshore reef areas to >100mgL−1 in marginal nearshore reefs. Some individual coral species can tolerate short-term exposure (days) to suspended-sediment concentrations as high as 1000mgL−1 while others show mortality after exposure (weeks) to concentrations as low as 30mgL−1. The duration that corals can survive high turbidities ranges from several days (sensitive species) to at least 5–6weeks (tolerant species). Increased sedimentation can cause smothering and burial of coral polyps, shading, tissue necrosis and population explosions of bacteria in coral mucus. Fine sediments tend to have greater effects on corals than coarse sediments. Turbidity and sedimentation also reduce the recruitment, survival and settlement of coral larvae. Maximum sedimentation rates that can be tolerated by different corals range from <10mgcm−2d−1 to >400mgcm−2d−1. The durations that corals can survive high sedimentation rates range from <24h for sensitive species to a few weeks (>4weeks of high sedimentation or >14days complete burial) for very tolerant species. Hypotheses to explain substantial differences in sensitivity between different coral species include the growth form of coral colonies and the size of the coral polyp or calyx. The validity of these hypotheses was tested on the basis of 77 published studies on the effects of turbidity and sedimentation on 89 coral species. The results of this analysis reveal a significant relationship of coral sensitivity to turbidity and sedimentation with growth form, but not with calyx size. Some of the variation in sensitivities reported in the literature may have been caused by differences in the type and particle size of sediments applied in experiments. The ability of many corals (in varying degrees) to actively reject sediment through polyp inflation, mucus production, ciliary and tentacular action (at considerable energetic cost), as well as intraspecific morphological variation and the mobility of free-living mushroom corals, further contribute to the observed differences. Given the wide range of sensitivity levels among coral species and in baseline water quality conditions among reefs, meaningful criteria to limit the extent and turbidity of dredging plumes and their effects on corals will always require site-specific evaluations, taking into account the species assemblage present at the site and the natural variability of local background turbidity and sedimentation.
•Sediment peak regulation is adopted to discharge more sediment downstream.•Sediment deposition has been greatly reduced by management methods over 30 years.•Fluctuating backwater area of the TGR has ...been scoured.
The Three Gorges Dam (TGD), on the upper valley of the Yangtze River, China, is the largest water conservancy and power generation project in the world. Problematic sedimentation in the Three Gorges Reservoir (TGR) is related to the reservoir lifetime, hydropower generation, riverbed erosion, and sustainable development of ecosystems. In this study, measures to holistically and systematically address sedimentation in the TGR are -discussed and demonstrated, including upstream sediment trapping, regulatory optimisation with respect to sediment peaks and reservoir tail sedimentation reduction, and sediment dredging. Owing to upstream reservoir sediment trapping, the incoming sediment load of the TGR has decreased markedly. Results showed that the annual incoming sediment was only 164 million tons, nearly 70% lesser than the preliminary design value. By intercepting the sediment in the upstream reservoirs and conducting sediment peak regulation, the volume and rates of sedimentation in the TGR decreased significantly. Meanwhile, through sediment peak regulation during the flood season, increased volumes of sediment could be discharged downstream, thereby relieving sediment starvation downstream of the TGD. Overall, through adoption of the related sedimentation management strategies, sediment deposition decreased considerably, to approximately 33% of the preliminary design value, thereby successfully addressing the sediment problems in the TGR. Useful sedimentation management strategies learned from the TGR experience can be applied elsewhere to ensure the sustainable use and management of reservoirs.
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