Gravel–sand mulching is a traditional no–tillage technique used for more than 300 years on the Loess Plateau. Applying gravel–sand mulch (mixture of gravel and sand) has an impact on the soil ...environment, soil microorganisms, and crop yield. However, we know surprisingly little of how respiration dynamics at the soil surface is affected by gravel–sand mulching. To study the response of soil respiration (Rs) dynamics to different in gravel–sand mulch thickness and its driving factors, we recorded Rs under a gradient of gravel–sand mulch thickness consisting of six layers (0 cm CK, 3 cm, 5 cm, 7 cm, 9 cm, 11 cm), in situ, during the growing season by using the LI–6400–09 soil respiration measurement system on the Loess Plateau in northwest China. The trends of all six treatments in diurnal and monthly variation of soil respiration followed a single–peaked curve; the diurnal dynamics trends were consistent with soil temperature changes. With gravel–sand mulch applied, the daily peak of soil respiration decreased, and the peak in Rs under the gravel–sand mulch treatments lagged that of bare land by 2 h. Monthly changes in soil respiration were consistent with soil temperature and precipitation changes, with peaks concentrated in July and August. Compared with bare land (CK), the monthly average soil respiration is lowest at 0.67 µmol∙m–2∙s–1 under a 11–cm thick mulch layer, and reduced by 26.63–61.03% by all mulch applications. Gravel–sand mulch reduced the temperature sensitivity of soil respiration, while soil temperature and soil water content were the main factors determining soil respiration. The thickness of gravel–sand has a significant, negative exponential function relationship with the changes in monthly soil respiration. Further, compared with bare land, the presence of gravel–sand mulch reduced the positive impact of precipitation on Rs. Soil temperature (T), gravel–sand mulch thickness (M), and soil pH were all significant predictors of soil respiration (Rs) (multiple regression model: Rs = 8.561 + 0.077 T – 0.056 M – 0.989 pH), together accounting for 73% of its variation. Hence, by considering all these three factors simultaneously we can better explain the changes to soil respiration in response to gravel–sand mulch. Overall, our results show that as the thickness of the gravel–sand layer increases, soil respiration decreases.
Display omitted
•Gravel–sand mulch reduces CO2 emissions: the thicker the mulch thickness, the lower the emissions.•Gravel–sand mulch controls soil respiration by changing both soil temperature and soil moisture.•Soil temperature, gravel–sand mulch thickness, and pH can serve as good environmental predictors of soil respiration.•Gravel–sand mulch reduces carbon emissions and helps to achieve carbon neutrality in agriculture.
•A theoretical calculation method for rockfall impact force and penetration depth is explicitly raised.•The range of strengthening coefficient of the gravel soil is identified.•The strengthening ...coefficient is used to predict rockfall impact force and penetration depth.•The determined range values of rockfall impact force and penetration depth compare well with the literature data.•The effects of strain rate and pressure dependencies on the estimation of rockfall impact force and penetration depth are verified.
Gravel soil backfilled on the top of a rock shed serves as a cushion and protects structures in mountainous districts that are endangered by rockfalls. Strengthening effect under rockfall impact is one of the factors that can influence the properties of the backfilled gravel soil; therefore, this effect should be considered in studies on the mechanical behaviour of the structure protected by the gravel soil. This paper introduces a bilinear enhanced constitutive model considering the strengthening coefficient and proposes a theoretical calculation method for rockfall impact force and penetration depth based on the Hertz contact theory. Back analysis and mathematical statistical analysis of the results of rockfall experiments produced a range of strengthening coefficients for the gravel soil. Based on the obtained values of the strengthening coefficient and the proposed calculating method, ranges of values of impact force and penetration depth are predicted. Based on the comparisons with previous experimental results and the results calculated using other previous methods, the calculated upper and lower limits of impact force in this study prove to be in good agreement with the previous experimental results; however, the calculated medium values are lower than the test medium values. Furthermore, the predicted medium values of the penetration depth were, on average, 20% larger than the experimental values. This study shows that the effects of the strain rate and pressure dependencies produce a certain influence on the estimated results of rockfall impact force and penetration depth. It also shows that the effects of pressure dependencies are comparatively more distinct than those of the strain rate.
Evaluation of flow resistance is necessary for the computation of conveyance capacity in open channels. The significance of the friction factor in channels with bedload conditions is paramount. The ...response of flow resistance in gravel-bed channels in bedload transport conditions is distinct from that of a fixed bed. The paper studies the different empirical approaches in the literature to determine the friction factor under bedload transport conditions and proposes an expression by genetic programming for the same. Various hydraulic and geometric parameters affect flow resistance in the bedload transport condition. The present study includes bed slope, relative submergence depth, aspect ratio, Reynolds number, and Froude number as influencing factors for such flow conditions. A wide range of experimental datasets is employed to determine the effect of these influencing parameters and develop a customised single expression for the friction factor. The experimental data set has also been moderated for sidewall corrections. The predictability of the proposed model is compared to various empirical equations from the literature. Unlike the existing models, the proposed model provides a more extensive expression for effectively predicting the friction factor for a wide range of datasets. The conveyance capacity of a river is validated from the estimated value of friction factor, as compared to other standard models. The developed Multi-Gene Genetic Programming (MGGP) model reasonably predicts discharge in the rivers, signifying that the model can competently be applied to field study within the specified range of parameters.
The paper deals with mathematical and physical modelling of flow of partially-stratified settling slurry in inclined pipes. Experimental results are presented of flows of aqueous slurries of four ...fractions of sand and gravel in two laboratory loops of different pipe diameters. They show a considerable variation in pressure drop and distribution of solids with the pipe slope. The degree of flow stratification and frictional pressure drop decrease with the increasing angle of inclination in the ascending pipe while the opposite applies in the descending pipe. A predictive mathematical model is presented which is based on a description of a layered structure of partially stratified slurry flow and takes flow inclination into account. The experimentally observed effects of pipe inclination are well reproduced by model predictions. Predicted pressure drops and solids distributions are in a very reasonable agreement with the experimental results and indicate suitability of the model for engineering practise.
Display omitted
•New model predicts pressure drops and solids concentration in inclined pipe.•Model predicts solids distribution and bed velocity both sensitive to flow incline.•Experiments in inclined pipes of two diameters verify model predictions.•Worster-Denny correlation is not suitable for stratified settling slurry flows.
•Sand blocks can be cemented by seawater based MICP.•The urease activity in the seawater based UPB was measured.•Building materials can be prepared by the seawater based MICP.•One/two-phase seawater ...based MICP was compared.•Biocementation sand blocks of mechanism was analyzed and discussed.
The Sporosarcina pasteurii strains can grow and multiply in seawater medium and have certain urease activity. The two-phase and one-phase cementation methods can be used for the construction of ocean islands and reefs far away from the mainland in seawater seawater based MICP (Microbial-Induced Carbonate Precipitation). The compressive strength, carbonates content and porosity of sand blocks cemented by seawater based MICP with two-phase and one-phase methods are measured and compared. The results show that the strength and carbonates content of sand blocks with two-phase biocementation are greater than that of one-phase. The SEM images show that the principle of seawater based MICP cementing loose sand blocks is that carbonates particles grow between the sand particles, and them act as a bridge to bind the loose sand particles into a whole. The seawater based UPB of different pH can induce calcium carbonate deposition at different time. The seawater based UPB of different pH can be selected according to the actual project to reinforce the island reef sand foundation and prepare building materials under one-phase biocementation.
Flow measuring locations and its influence on the dynamics of macro-turbulent flow structures has received substantial interest of research in gravel bed hydrodynamics. Present study aims to ...elucidate the near-bed turbulent flow fields and bursting events over the stoss side, crest and leeside cavities in a gravel bed stream applying double averaging (DA) methodology. Time averaged flow velocities are measured at thirty six vertical planes with an acoustic Doppler velocimeter. The DA velocity distributions for the lee side cavities resemble S-shape profile expressed by polynomial series below the crest level otherwise, velocity profile over the stoss side and crest follow the logarithmic law of wall. The DA Reynolds shear stress distributions reveal damping at the crest level of lee side cavities and compensated by enhanced DA form induced shear stress (FISS). The experimental results are further explored in order to delineate different turbulent bursting events over the stoss side, crest and lee sides. The quadrant analysis of DA Reynolds shear stress production confirmed that within the interfacial sub-layer the sweep is the dominant event towards the RSS production.
The wadis in Algeria organize the beds as gravel by transporting fine sediments form through hydrosedimentary dynamic of intermittent flood events along the time. In this paper, a new flow resistance ...equation is proposed for gravel bed bedload regime. Due to the field measurements lack in wadis, the study is based on 2,147 flume data with friction factor measurements. The development of convenient equation is achieved through theoretical analysis by taking into account wadi dominant parameters based on dimensional analysis. Hence, deduced relationships are considered between friction coefficient
f
, the flow characteristics, bed grains size, geometric parameter and bed slope. The proposed equation gives satisfactory results; the estimated mean normalized error is less than 17%.
The current study performed the laboratory flume to examine the velocity distribution and the velocity moments of turbulent flow over a sand-gravel mixture bed. The 3D instantaneous velocity data of ...water is collected by using acoustic doppler velocimeter at the test section which will provide an important data related to the flow turbulence. The parameters of turbulence measured that the lower value of longitudinal velocity (velocity along flow direction) is observed close to the bed surface and increases with the flow depth. The lower magnitude of von Karman constant is achieved than the universal value. The damping of Reynolds shear stress observed close to the boundary wall indicating lower exchange of flow energy towards the boundary and vice versa. The sand-gravel bed influenced the turbulence intensities with higher magnitude in the streamwise than those in vertical direction. The velocity moments which show the structure function of fluctuating components of velocity, has been analysed. The present study also analysed the flow anisotropy, and the flatness factor in the flow.
PDF
