Abstract: Nature-based coastal protection is increasingly recognised as a potentially sustainable and cost-effective solution to reduce coastal flood risk. It uses coastal ecosystems such as mangrove forests to create resilient designs for coastal flood protection. However, to use mangroves effectively as a nature-based measure for flood risk reduction, we must understand the biophysical processes that govern risk reduction capacity through mangrove ecosystem size and structure. In this perspective, we evaluate the current state of knowledge on local physical drivers and ecological processes that determine mangrove functioning as part of a nature-based flood defence. We show that the forest properties that comprise coastal flood protection are well-known, but models cannot yet pinpoint how spatial heterogeneity of the forest structure affects the capacity for wave or surge attenuation. Overall, there is relatively good understanding of the ecological processes that drive forest structure and size, but there is a lack of knowledge on how daily bed-level dynamics link to long-term biogeomorphic forest dynamics, and on the role of combined stressors influencing forest retreat. Integrating simulation models of forest structure under changing physical (e.g. due to sea-level change) and ecological drivers with hydrodynamic attenuation models will allow for better projections of long-term natural coastal protection.
Abstract: Understanding the sensitivity of tidal flats to environmental changes is challenging. Currently, most studies rely on process-based models to systematically explain the morphodynamic evolution of tidal flats. In this study, we proposed an alternative empirical approach to explore tidal flat dynamics using statistical indices based on long-term time series of daily surface elevation development. Surface elevation dynamic (SED) indices focus on the magnitude and period of surface elevation changes, while morphodynamic signature (MDS) indices relate sediment dynamics to environmental drivers. The statistical analyses were applied to an intervention site in the Netherlands to determine the effect of recently constructed groynes on the tidal flat. Using these analyses, we were able to (1) detect a reduction in the daily SED and (2) determine that the changes in the daily SED were predominantly caused by the reduction in wave impact between the groynes rather than the reduction in tidal currents. Overall, the presented results showed that the combination of novel statistical indices provides new insights into the trajectories of tidal flats, ecosystem functioning, and sensitivity to physical drivers (wind and tides). Finally, we suggested how the SED and MDS indices may help to explore the future trajectories and climate resilience of intertidal habitats.
Abstract: An increasing number of marine aquaculture facilities have been placed in shallow bays and open sea, which might significantly affect hydrodynamic and solute transport processes in marine aquaculture waters. In this study, a coupled hydrodynamic and solute transport model was developed with high-resolution schemes in marine aquaculture waters based on depth-averaged shallow water equations. A new expression of drag force was incorporated into the momentum equations to express the resistance of suspended culture cages. The coupled model was used to simulate the effect of suspended structures on tidal currents and the movement of a contaminant cloud in the marine aquaculture of the North Yellow Sea, China. The simulation results showed a low-velocity area appearing inside the aquaculture cage area, with a maximum reduction rate of velocity close to 45% under high-density culture. The results also showed that tidal currents were sensitive to the density of suspended cages, the length of cages, and the drag coefficients of cages. The transport processes of pollutants inside aquaculture facilities were inhibited away from the vicinity of the culture cage area because of the diminished tidal currents. Therefore, the suspended cages significantly affected the transport processes of pollutants in the coastal aquaculture waters. Furthermore, the reduced horizontal velocity significantly decreased the food supply for the aquaculture areas from the surrounding sea.
Abstract: Extreme climate events threaten human health, economic development, and ecosystems. Many studies have been conducted on extreme precipitation and temperature changes in the Yarlung Zangbo River Basin (YZRB). However, little attention has been paid to compound climate extremes. In this study, the variations of wet/warm compound extreme events in summer and dry/cold compound extreme events in winter over the past 42 years in the YZRB were investigated using eight extreme climate indices that were estimated using monthly temperature and precipitation observations. The results showed that the numbers of frost days and ice days tended to decrease on the spatiotemporal scale, while the maximum values of daily maximum temperature and daily minimum temperature exhibited increasing trends. The frequency of wet/warm compound extreme events was significantly higher from 1998 to 2018 than from 1977 to 1997. Dry/cold compound extreme events became less frequent from 1998 to 2018 than from 1977 to 1997. The rate of increase of wet/warm compound extreme events was about ten times the absolute rate of decrease of dry/cold compound extreme events. With regard to the spatial pattern, the frequency of wet/warm compound extreme events increased significantly in almost all parts of the YZRB, while that of dry/cold compound extreme events decreased across the basin. This study helps to improve our understanding of the changes in compound precipitation and temperature extremes in the YZRB from a multivariable perspective.
Abstract: A comprehensive assessment of representative satellite-retrieved (Integrated Multi-satellite Retrievals for Global Precipitation Measurement (IMERG) and Tropical Rainfall Measuring Mission Multi-satellite Precipitation Analysis (TMPA)), reanalysis-based (fifth generation of atmospheric reanalysis by the European Centre for Medium Range Weather Forecasts (ERA5)), and gauge-estimated (Climate Prediction Center (CPC)) precipitation products was conducted using the data from 807 meteorological stations across Chinese mainland from 2001 to 2017. Error statistical metrics, precipitation distribution functions, and extreme precipitation indices were used to evaluate the quality of the four precipitation products in terms of multi-timescale accuracy and extreme precipitation estimation. When the timescale increased from daily to seasonal scales, the accuracy of the four precipitation products first increased and then decreased, and all products performed best on the monthly timescale. Their accuracy ranking in descending order was CPC, IMERG, TMPA, and ERA5 on the daily timescale and IMERG, CPC, TMPA, and ERA5 on the monthly and seasonal timescales. IMERG was generally superior to its predecessor TMPA on the three timescales. ERA5 exhibited large statistical errors. CPC provided stable estimated values. For extreme precipitation estimation, the quality of IMERG was relatively consistent with that of TMPA in terms of precipitation distribution and extreme metrics, and IMERG exhibited a significant advantage in estimating moderate and heavy precipitation. In contrast, ERA5 and CPC exhibited poor performance with large systematic underestimation biases. The findings of this study provide insight into the performance of the latest IMERG product compared with the widely used TMPA, ERA5, and CPC datasets, and points to possible directions for improvement of multi-source precipitation data fusion algorithms in order to better serve hydrological applications.
Abstract: It is essential to minimize soil quality degradation in sloping agricultural fields through stabilization and improvement of soil hydraulic properties using sustainable soil management. This study aimed to analyze the impact of different tillage practices, including conventional tillage (CT), minimum tillage (MT), and zero tillage (ZT), on soil hydraulic conductivity in a sloping agricultural field under maize—wheat rotation. The results showed that the highest runoff volume (257.40 m3), runoff coefficient (42.84%), and soil loss (11.3 t) were observed when the CT treatment was applied. In contrast, the lowest runoff volume (67.95 m3), runoff coefficient (11.35%), and soil loss (1.05 t) were observed when the ZT treatment was adopted. The soil organic carbon and aggregate mean weight diameter were found to be significantly greater (with mean values of 0.79% and 1.19 mm, respectively) with the ZT treatment than with the CT treatment. With the tilled treatments (CT and MT), substantial changes in the saturated soil hydraulic conductivity (ks), near-saturated soil hydraulic conductivity (k), and water-conducting porosity (ε) were observed between two crop seasons. These three soil parameters were significantly higher in the period after maize harvesting than in the wheat growing period. In contrast, no significant difference in these soil parameters was found when the untilled treatment (ZT) was carried out. With regard to the slope positions, ks, k, and ε showed different behaviors under different treatments. The toe slope position showed significantly lower ks and ε values than the summit and middle slope positions. Of the evaluated tillage practices, ZT was found to be the most promising means to improve the soil hydro-physical properties and effectively reduce surface runoff and soil erosion.
Abstract: Different hydraulic retention times (HRTs) were tested in a mixed anoxic/oxic (A/O) system at 5 C and 10 C to investigate the effects of HRT and carrier on nitrogen removal in wastewater at low temperatures. The results showed that the addition of the fillers improved the treatment effect of each index in the system. With an optimal HRT of 7.5 h at 5 C, the removal efficiencies of NH4+-N and total nitrogen (TN) reached 91.2% and 75.6%, respectively. With an HRT of 6 h at 10 C, the removal efficiencies of NH4+-N and TN were 96.7% and 82.9%, respectively. The results of high-throughput sequencing showed that the addition of the suspended carriers in the aerobic zone could improve the treatment efficiency of nitrogen at low temperatures. The microbial analysis indicated that the addition of the suspended carriers enhanced the enrichment of nitrogen removal bacteria. Nitrospira, Nitrotoga, and Nitrosomonas were found to be the bacteria responsible for nitrification, and their relative concentrations on the biofilm at 5 C and 10 C accounted for 98.11%, 92.79%, and 69.98% of all biological samples, respectively.
Abstract: Treating waste with a waste material using freely available solar energy is the most effective way towards sustainable future. In this study, a novel photocatalyst, partly derived from waste material from the coal industry, was developed. Fly ash hybridized with ZnO (FA—Zn) was synthesized as a potential photocatalyst for dye discoloration. The synthesized photocatalyst was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and ultraviolet—visible/near infra-red spectroscopy. The photocatalytic activity was examined with the discoloration of methylene blue used as synthetic dye wastewater. All the experiments were performed in direct sunlight. The photocatalytic performance of FA—Zn was found to be better than that of ZnO and the conventionally popular TiO2. The Langmuire—Hinshelwood model rate constant values of ZnO, TiO2, and FA—Zn were found to be 0.016 min-1, 0.017 min-1, and 0.020 min-1, respectively. There were two reasons for this: (1) FA—Zn was able to utilize both ultraviolet and visible parts of the solar spectrum, and (2) its Brunauere—Emmette—Teller surface area and porosity were significantly enhanced. This led to increased photon absorption and dye adsorption, thus exhibiting an energy-efficient performance. Therefore, FA—Zn, partly derived from waste, can serve as a suitable material for environmental remediation and practical solar energy applications.
Abstract: Due to the difference in density between the discharge effluent and coastal water, partially treated wastewater is often discharged into the marine environment as a buoyant jet via submarine outfalls with multiport diffusers. The dilution characteristics of effluent discharge (dual buoyant jets) in a wavy cross-flow environment were studied in a laboratory. The planar laser-induced fluorescence technique was used to obtain the concentration data of the jets. The effects of different environmental variables on the diffusion and dilution characteristics of the jets were examined through physical experiments, dimensional analysis, and empirical formulations. It was found that the dilution process of the dual jets could be divided into two components: the original jet component and the effluent cloud component. The jet-to-current velocity ratio was the main parameter affecting the concentration levels of the effluent cloud. The merging of the two jets increased the jet concentration in the flow field. When the jets traveled further downstream, the axial dilution increased gradually and then increased significantly along the axis. Under the effects of strong waves, the concentration contours branched into two peaks, and the mean dilution became more significant than under the effects of weak waves. Therefore, the dilution of the effluent discharge was expected to be significant under strong wave effects because the hydrodynamic force increased. A dilution equation was derived to improve our understanding of the dilution process of buoyant jets in a wavy cross-flow environment. This equation was used to determine the influences of the jet-to-current velocity ratio, wave-to-current velocity ratio, and Strouhal number on the minimum jet dilution. It revealed that the wave and buoyancy effects in effluent discharges were significant.
Abstract: Local scour around bridge piers and abutments is one of the most significant causes of bridge failure. Despite a plethora of studies on scour around individual bridge piers or abutments, few studies have focused on the joint impact of a pier and an abutment in proximity to one another on scour. This study conducted laboratory experiments and flow analyses to examine the interaction of piers and abutments and their effect on clear-water scour. The experiments were conducted in a rectangular laboratory flume. They included 18 main tests (with a combination of different types of piers and abutments) and five control tests (with individual piers or abutments). Three pier types (a rectangular pier with a rounded edge, a group of three cylindrical piers, and a single cylindrical pier) and two abutment types (a wing—wall abutment and a semicircular abutment) were used. An acoustic Doppler velocimeter was used to measure the three-dimensional flow velocity for analyses of streamline, velocity magnitude, vertical velocity, and bed shear stress. The results showed that the velocity near the pier and abutment increased by up to 80%. The maximum scour depth around the abutment increased by up to 19%. In contrast, the maximum scour depth around the pier increased significantly by up to l71%. The presence of the pier in the vicinity of the abutment led to an increase in the scour hole volume by up to 87% relative to the case with a solitary abutment. Empirical equations were also derived to accurately estimate the maximum scour depth at the pier adjacent to the abutment.
Abstract: This study aimed to investigate whether saline silt and sandy loam coastal soils could be reclaimed by micro-sprinkler irrigation. The experiments were run using moderately salt-tolerant tall fescue grass. Micro-sprinkler irrigation in three stages was used to regulate soil matric potential at a 20-cm soil depth. Continued regulation of soil water and salt through micro-sprinkler irrigation consistently resulted in an increasingly large low-salinity region. The application of the three stages of soil water—salt regulation resulted in an absence of salt accumulation throughout the soil profile and the conversion of highly saline soils into moderately saline soils. There were increases in the plant height, leaf width, leaf length, and tiller numbers of tall fescue throughout the leaching process. The results showed that micro-sprinkler irrigation in three soil water and salt regulation stages can be used to successfully cultivate tall festuca in highly saline coastal soil. This approach achieved better effects in sandy loam soil than in silt soil. Tall fescue showed greater survival rates in sandy loam soil due to the rapid reclamation process, whereas plant growth was higher in silt soil because of effective water conservation. In sandy loam, soil moisture should be maintained during soil reclamation, and in silt soil, soil root-zone environments optimal for the emergence of plants should be quickly established. Micro-sprinkler irrigation can be successfully applied to the cultivation of tall fescue in coastal heavy saline soils under a three-stage soil water—salt regulation regime.
