Pit Mine Research Articles

Flow and morphological characteristics in mining pits of a river through numerical and experimental modeling

Flow and morphological characteristics in mining pits of a river through numerical and experimental modeling

Research on temporal and spatial evolution of land use and landscape pattern in Anshan City based on GEE

Frequent mining activities can bring about problems such as soil erosion and environmental pollution, which are detrimental to the efficient use of land and the sustainable development of cities. Existing studies have paid little attention to mining areas and lack comparative analysis of landscape changes in multiple mining pits. In this paper, the main urban area of Anshan City, where the mining areas are concentrated, was used as the research area, and the Landsat TM/OLI surface reflectance (SR) data of the Google Earth Engine (GEE) platform and the random forest algorithm were used to map the land use in 2008, 2014, and 2020. On this basis, land use dynamics and landscape pattern indices were used to analyze the changes in land use and landscape patterns in the Anshan City area. In addition, a moving window method was combined to further analyze and compare the landscape changes between different pits. The results show that:1. From 2008 to 2020, the construction land in Anshan urban area continued to decline, the forest land continued to expand, and the construction land was shifted to the forest land and cultivated land. Mining land increased before 2014 and remained almost unchanged after 2014, which is in line with the actual situation. 2. During the study period, the landscape fragmentation degree and landscape heterogeneity in the urban area of Anshan kept increasing. The high value areas of landscape fragmentation were the urban-rural combination areas and the mining areas. Among them, the reclamation of Dagushan and Donganshan is better, while the reclamation of Anqian, Yanqianshan and Xiaolingzi mines needs to be strengthened. 3. The random forest algorithm based on GEE shows a high degree of accuracy for land use classification. The overall classification accuracy in 3 years exceeds 90% and the kappa coefficient exceeds 0.85. The study results can be used as an essential reference for optimizing the urban ecological environment and provide technical backing for the urbanization construction and rational use of land in Anshan City.

Impact of mining-induced deforestation on soil surface temperature and carbon stocks: A case study using remote sensing in the Amazon rainforest

The Amazon rainforest is one of the most important ecosystems on the planet; however, the environmental pressure created by anthropic activities require monitoring of this critical biome. In this study, we assessed the evolution of deforestation in an open mine pit and its impact on surface environment (i.e., temperature and carbon stocks) using remote sensing techniques. The study was carried out on an area of 11,283.3 ha in the municipality of Marabá, Pará State, Brazil, where the “Salobo” copper mine is located. A temporal analysis was conducted, using Landsat satellite images (2005–2020). Subsequently, the Land Surface Temperature (LST) and the Normalized Difference Vegetation Index (NDVI) of the deforested area were determined. Mining-induced deforestation has expanded from 0.9 ha in 2005 to 2214 ha in 2020 with an increase in surface temperature of 10 °C in the period. The temperature difference between the pit and the adjacent forest ranged from 30 to 40 °C over the 15 years, while the temperature at the forest edges rose by 4 °C. The correlation coefficient between exposed soil temperatures and mining deforestation was 0.66. CO2 emissions, increasing from 0.005 Tg CO2 in 2005 to 1.82 Tg CO2 in 2020 due to mining deforestation. The findings demonstrate the significant environmental impact of deforestation, which may also have an influence on local climate. The results can provide scientific support for public policies aimed at mitigating the issue.

The Hegemony Of The Coal Mining Corporation, The Destruction Of The Kutai Rice Barn Center And The Damage To The Living Space Of Transmigrants

Kerta Buana Village, became the location for transmigrants from Bali, Lombok (NTB) and a few local transmigrants in 1980. This village later grew to become part of the main rice granary area of ​​Kutai Kartanegara Regency, as well as being known as the Bali Village of East Kalimantan. Kerta Buana is also known as the Pancasila Village, because of the harmony of the villagers with the background of 2 different dominant religions, Balinese transmigrants are Hindu and Lombok Muslim transmigrants. However, at present Kerta Buana Village has suffered severe damage to life, has changed its face into a village surrounded by coal mines, surrounded by 14 mining pits, agricultural productivity is low and a class of farmers without rice fields has emerged. This article analyzes how a transmigrant village that was once known as part of the district's food barn has now turned into a village surrounded by mining. Data was collected by in-depth observation, in-depth interviews with 16 key informants from each transmigrant group and repeated focus group interviews. Antonio Gramsci's concept of hegemony is used to explain how company power builds historical blocks, hegemony villagers with extractive economic ideology so that villagers agree to massive coal exploration around the village. The results show that companies use corporate and government institutions, community leaders, security forces and even local communities to lead transmigrants to obey the

The Hegemony Of The Coal Mining Corporation, The Destruction Of The Kutai Rice Barn Center And The Damage To The Living Space Of Transmigrants

Kerta Buana Village, became the location for transmigrants from Bali, Lombok (NTB) and a few local transmigrants in 1980. This village later grew to become part of the main rice granary area of ​​Kutai Kartanegara Regency, as well as being known as the Bali Village of East Kalimantan. Kerta Buana is also known as the Pancasila Village, because of the harmony of the villagers with the background of 2 different dominant religions, Balinese transmigrants are Hindu and Lombok Muslim transmigrants. However, at present Kerta Buana Village has suffered severe damage to life, has changed its face into a village surrounded by coal mines, surrounded by 14 mining pits, agricultural productivity is low and a class of farmers without rice fields has emerged. This article analyzes how a transmigrant village that was once known as part of the district's food barn has now turned into a village surrounded by mining. Data was collected by in-depth observation, in-depth interviews with 16 key informants from each transmigrant group and repeated focus group interviews. Antonio Gramsci's concept of hegemony is used to explain how company power builds historical blocks, hegemony villagers with extractive economic ideology so that villagers agree to massive coal exploration around the village. The results show that companies use corporate and government institutions, community leaders, security forces and even local communities to lead transmigrants to obey the

Geochemical Investigation of Cassiterite at Bisichi Kara II and Kuru-Jentar, Plateau State, Nigeria Using X-Ray Fluorescence Analysis (XRF)

Background: Artisanal Mining of Cassiterite at Bisichi (Kara II) and Kuru-Jentar is a very tedious activity that is carried out by artisanal miners. It involves the use of primitive tools like digger, spade, shovel and to mention afew. Most of the artisanal mining activity in the study area is done with no prior knowledge about the geochemical constituent of the mineral deposit in the area but in order to solve the above stated problem, a geochemical analysis was conducted on the obtained samples of cassiterite from the study area and a geologic map of the mining pit was digitized using Ilwis 3.1 academic and Surfer 12. Result: A random sampling method was used to obtain ten (10) samples of cassiterite from the various visited pit at the study area with each weighing 10g. A laboratory analysis was also conducted using X-ray Fluorescence (XRF) analysis which shows that samples from Pit 1 to Pit 5 in the mine site at Kuru-Jentar has 44.45% - 44.9% of Tin (Sn) and 7.86% - 9.00% of Iron (II) oxide (Fe2O3) while samples from Pit 5 to Pit 10 in the mine site at Bisichi (Kara II) has 28.2% - 32.2 % of Tin (Sn) and 15.57% - 16.67% of Iron II oxide (Fe2 O3 ). Conclusively, an understanding of the geochemical constituent of a mineral deposit within a given study area would help increase the knowledge of miners and also attract interested investors.

Analysis of SLAM-Based Lidar Data Quality Metrics for Geotechnical Underground Monitoring

Adverse ground behavior events, such as convergence and ground falls, pose critical risks to underground mine safety and productivity. Today, monitoring of such failures is primarily conducted using legacy techniques with low spatial and temporal resolution while exposing workers to hazardous environments. This study assesses the potential of novel simultaneous localization and mapping (SLAM)-based light detection and ranging (Lidar) data quality for rapid, digital, and eventually autonomous mine-wide underground geotechnical monitoring. We derive a comprehensive suite of quality metrics based on tests in two underground mines for two state-of-the-art mobile laser scanning (MLS) systems. Our results provide evidence that SLAM-based MLS provides data of the quality required to detect geotechnically relevant changes while being significantly more efficient for large mine layouts when compared to traditional static systems. Additionally, we show that SLAM-specific processing can achieve an order of magnitude better relative accuracy relevant for change detection than quality metrics derived from traditionally deployed tests would suggest while reducing SLAM drift error by up to 90%. In collaboration with an operating block cave mine, we confirm these capabilities in field tests on a mine-wide scale and, for the first time, demonstrate methods of rockfall detection using MLS data. While more work is required to investigate optimal collection, processing, and utilization of MLS data, we demonstrate its potential to become an effective and widely applicable data source for rapid, accurate, and comprehensive geotechnical inspections.

Prediction of Water-Blocking Capability of Water-Seepage-Resistance Strata Based on AHP-Fuzzy Comprehensive Evaluation Method—A Case Study

Coal mining under the aquifer in Northwest China has brought a series of ecological problems, such as the decline of groundwater levels and the death of surface vegetation. The study of the impermeability of rock strata between coal seams and the overlying aquifers is of great significance to solve these problems and realize water-preserving coal mining (WPCM). Based on mining-induced overburden damage and permeability deterioration, the concept of the “three seepage zones” of overburden is proposed, namely the pipe flow zone, water seepage zone and nominal water-seepage-resistance zone (NWSRZ). Meanwhile, the concept of water-seepage-resistance strata (WSRS) is put forward from the aspects of initial permeability, structural strength, swelling and the stratigraphic structure of the overlying strata. AHP-fuzzy comprehensive evaluation (AHPF) is employed to construct a model to evaluate the water-blocking capacity (WBC) of the WSRS. The model includes three secondary factors and nine tertiary indicators, and the weights and membership functions of the indicators are determined. Subsequently, the model is generalized and applied to the Yu-Shen mining area. The membership degrees are spatially visualized by means of thematic maps. The comprehensive evaluation values Φ of the WBCWSRS of 400 boreholes in the mining area under backfill mining, narrow strip mining, slice mining and longwall cave mining are calculated. Then, the Kriging method is employed to plot the zoning maps of Φ under four different mining methods. In view of different grades of WBCWSRS, three corresponding countermeasures, i.e., mining methods optimization, curtain grouting and underground reservoir construction, are put forward. The fluid–solid coupling embedded in FALC3D software is employed to establish a numerical calculation model to simulate the water table fluctuation of the underground aquifer under the four mining methods, and the reliability of the model is verified indirectly. In this paper, mathematical theory is combined with WPCM to develop an evaluation model of WBCWSRS, which provides a reference for the coordinated development of coal extraction and water resource preservation in arid and semi-arid mining areas.

Prevention of water and mud inrush in longwall top coal cave mining under shallow buried thin bedrock

Prevention of water and mud inrush in longwall top coal cave mining under shallow buried thin bedrock

Recent expansion of artisanal gold mining along the Bandama River (Côte d’Ivoire)

• Artisanal and small-scale gold mining is a serious threat to the environment. • In Ivory Côte d’Ivoire, this activity has developed very quickly recently. • The lack of geographical information is an obstacle to the regulation. • The monitoring of artisanal mining from space can help governements. Recent development of small-scale gold mining activities in Côte d’Ivoire is a serious threat to the environment because of deforestation, soil scrapping, pit mining, over-use of water resources and pollution of surface and ground waters by mercury, cyanides, and acids. The challenge for the governance of this activity is to resolve the tension between the desired development of a small-scale mining activity, which may benefit the local and national economy, and the impacts of this activity on the environment. The regulation of the exploration and exploitation of mining sites and the promotion of best practices was part of the National program for the rationalization of gold-panning in Côte d’Ivoire. The capacity of the government to monitor the expansion of numerous mining sites disseminated all over the country is one of the key aspects for successful implementation of these policies. This study explores the potential value of computer-assisted mapping of artisanal mining sites based on Sentinel-2 imagery. The detection method, using artificial intelligence and training data sets generated during field campaigns, was inspired from a previous experience in Senegal. It was applied to a region of about 600 km 2 in Central Côte d’Ivoire. Annual maps of areas affected by the mining activities were produced for the period 2018 – 2021. The areas affected by artisanal mining activities expanded from 3.39 km 2 to 8.80 km 2 in December 2021, corresponding to an average growth rate of 0.24 km 2 /month. The temporal and spatial resolution of the Sentinel satellite imagery proved to be useful to map and quantify the expansion rate of artisanal mining sites in Côte d’Ivoire. Recommendations are made for the integration of these tools into plans for the development of small-scale mining activities in Côte d’Ivoire that would be more respectful of the environment and societies.

Numerical modelling of rock mass bulking and geometric dilation using a bonded block modelling approach to assist in support design for deep mining pillars

As underground mining operations move deeper and the stresses encountered become more adverse, mine pillars are experiencing increased stress-induced damage and rock mass bulking. This has led to significant challenges in designing effective support systems, leading to costly mine development, production delays and support maintenance and rehabilitation requirements. Efforts to assess rock mass bulking have relied almost exclusively on the use of continuum modelling techniques, employing plasticity theory to predict the depth of brittle failure and associated post-yield deformations. However, rock mass bulking in high stress environments is largely driven by brittle fracturing resulting in geometric dilation, which involves shearing and buckling of spalled rock pieces as they deform into the excavation. Continuum techniques are inherently limited in their ability to model this behavior. This paper presents the results of research investigating the application of discontinuum-based numerical modelling techniques for assessing rock mass spalling and bulking in highly stressed pillars. The 3-D distinct-element program 3DEC is used to develop a bonded block modelling approach, referred to here as 3DEC-BBM. The method developed allows for the explicit modelling of brittle fracturing, spalling and geometric bulking, and the corresponding support responses, in reaction to the simulation of different mine stress paths and operation scenarios. Several different support strategies are investigated to test their effectiveness as a means to improve displacement-based support design in highly stressed pillars. These are based on the simulation of an extraction-level drawpoint pillar for a block cave mining operation that is exposed to the abutment stresses from an undercut passing over it. Based on the results, three key findings are presented and discussed: (1) Adding a small support pressure has a significant impact on the performance of the pillar. However, the benefits gained by further increasing this pressure diminish and have a less dramatic impact on spalling damage and bulking with increasing effort. (2) It is crucial to install appropriate support before the undercut passes over the extraction level pillars. The depth of damage and amount of bulking can increase significantly during pillar unloading following passing of the undercut. Extra confinement applied to the failed pillar skin allows it to carry more load and thus provide more confinement to the pillar core, helping to limit further damage and bulking. (3) The addition of support pressure through shotcrete and rock bolting performs similarly to “perfectly” installed steel sets . Thus, a heavier support system will not necessarily result in better pillar performance.

Uneven Trajectories and Decentralisation: Lessons From Historical Planning Processes in Saint-Étienne

Once the industrial capital of France, Saint-Étienne has faced the closure of mining pits, steel plants, and textile firms in close succession, leading to population loss and social and economic challenges, and making the city an outlier in France as a large-sized shrinking city. There has generally been a lack of temporal approaches to urban shrinking processes and calls to incorporate historical institutionalism in planning research. This research will use path dependence—a conceptual framework where a critical event causes a process that is marked by reproductive logic—as a central explanatory tool to assess historical planning processes in Saint-Étienne. This article identifies a critical event—the publication of the first spatial plan for the Saint-Étienne region—and then considers temporal self-reinforcing processes, reviewing subsequent local spatial planning strategies through a culturalist theory frame. It shows that spatial strategies have not adapted over time to the reality of shrinkage; local beliefs in growth displayed path-dependent features and resulted in decentralisation and deepening socio-economic inequalities both within the metropolitan area of Saint-Étienne and with its larger neighbour, Lyon. More broadly, for metropolitan areas to be able to adapt to future changes and be resilient, it will be crucial for urban planning policy and research to consider the extent to which planning strategies can self-reinforce and to find ways to adapt these strategies in the face of global urban transformations.

Groundwater-level recovery following closure of open-pit mines

Open-pit mining has increased substantially over the past two decades. Many currently operating open-pit mines are facing the end of mine-life over the next few decades and, increasingly, focus is shifting towards mine-closure planning that provides evidence on available closure options under the given geological, hydro(geo)logical and climatic conditions. This study uses synthetic groundwater modelling to build basic process understanding of closure options and how these will determine the formation of pit lakes. This governs the long-term pit lake water quality and how postmining landscapes may be utilised. Simulations show that the recovery time of postmining groundwater levels increases with decreasing aquifer transmissivity. Final postmining water tables are predominantly controlled by the implemented mine closure options and climatic conditions. The most important decision is, thereby, whether to backfill the pit to above the water table or allow a pit lake to develop. Under moderately transmissive aquifer settings, backfilling of pits leads to rapidly rising groundwater levels within the first decade after mining, with water-table recoveries of above 70%. If mine voids remain unfilled, evaporation from the pit lake surface becomes a governing factor in determining whether the unfilled mine pit becomes a terminal sink for groundwater. Lake levels may remain subdued by several 10s of metres in arid to semiarid climates. If surplus surface water can be diverted into open pits, rapid filling can accelerate groundwater recovery of open pits in regions of low permeability. This is a less successful management option in transmissive aquifers.

Experimental study of sediment mining pit locomotion and morphological evolution

Experimental study of sediment mining pit locomotion and morphological evolution

Influence of Long-Term Mine Activity on Hydraulic Relations between Separate Hydrogeological Units—New Aspects of Regional Water Circulation Assessment

In the course of the documentation works aimed at updating exploitable resources of the groundwater intake, an unexpected change in hydrodynamic field arrangement was observed in a fragment of the area. In relation to the observations made so far, lasting several decades, the groundwater table switched its slope, indicating the flow in the opposite direction than originally. Initially, in the range of the surveyed hydrogeological structure, no object was identified whose draining character could cause such an effect. No significant groundwater extraction or drainage points were documented within a radius of several kilometers. It was only the extended scope of spatial analysis that made it possible to identify the probable cause of a local change in the water table position as a mining pit located in the neighboring hydrogeological structure. The two adjacent hydrogeological structures treated separately so far revealed an experimentally confirmed hydraulic connection. An in-depth analysis of the problem showed the necessity of modifying the existing model of the structure in order to map the observed interactions. An additional simulation module of General-Head Boundary type was applied. The quantity of lateral groundwater exchange between two hydrogeological structures was estimated using mathematical modeling at 3.4 thousand m3/d (reconstructed current state, after model calibration). Further work should aim at treating reservoirs as hydraulically connected after exceeding limiting parameters.

A drone- and field-based investigation of the land degradation and soil erosion at an opencast coal mine dump after 5 years’ evolution of natural processes

Opencast coal mining has a large impact on the land surface, both at the mining pits themselves and at waste sites. After artificial management is stopped, a reclaimed opencast coal mine dump is affected by wind and water erosion from natural processes, resulting in land degradation and even safety incidents. In this paper, the soil erosion and land degradation after 5 years of such natural processes, at the Xilinhot opencast coal mine dump in Inner Mongolia, were investigated. A multi-source data acquisition method was applied: the vegetation fraction coverage (VFC) was extracted from GF-1 satellite imagery, high-precision terrain characteristics and the location and degree of soil erosion were obtained using a drone, and the physical properties of the topsoil were obtained by field sampling. On this basis, the degree and spatial distribution of erosion cracks were identified, and the causes of soil erosion and land degradation were analyzed using the geographical detector. The results show that (1) multi-source data acquisition method can provide effective basic data for the quantitative evaluation of the ecological environment at dumps, and (2) slope aspect and VFC are the main factors affecting the degree of degradation and soil erosion. Based on above analysis, several countermeasures are proposed to mitigate land degradation: (1) The windward slope be designed to imitate the natural landform. (2) Reasonable engineering measures should be applied at the slope to restrain soil erosion. (3) The Pioneer plants should be widely planted on the platform at the early stage of reclamation.

Stress modelling using cellular automata for block caving applications

In underground mining, rock mass stress is commonly modeled as a continuum. However, in block cave mining discrete modelling should be used to properly represent the stress over the extraction level in the broken column where there are high rock columns of large rock fragments. Unfortunately, we lack methods that use discrete modelling of stress in the broken column at block caving scale. In this work, we propose a vertical stress model of granular material to simulate static and dynamic flow conditions. The model is developed within a gravity flow simulator based on cellular automata to simulate the scale of the problem and flow conditions. The vertical stress model proposed is calibrated through four experimental models for the static condition. Then, based on the results from experimental testing, the dynamic condition is calibrated and compared with different flow scenarios. The results show that the proposed model can correctly simulate the vertical stresses in static conditions as well as dynamic conditions under the different flow setups tested. This vertical stress model with its flow simulator based on cellular automata has the potential to be applied at block caving scale once calibration parameters are defined.

Analyses of abandoned coal mine pit water quality compared to the water of regular, conventional ources in respect of its utilitarian perspectives: a special reference to selected abandoned open cast mine pits of Satgram area, Raniganj coalfield, West Bengal

The present study was conducted to investigate the status of water quality and to determine the WaterQuality Index (WQI) of three abandoned open cast coal mine pits named HarabangaO.C.P., Damalia O.C.P.and Ratibati O.C.P. of Satgram area, Raniganj coalfield, West Bengal, India for three consecutive seasonalphases. These are pre-monsoon, monsoon and post-monsoon from 2019 to 2020. This paper deals with thestudy of the most commonly used water quality parameters as per IS 10500: 1991 Drinking Water-Specification and developed Water Quality Index (WQI) from the mean value of the season-wise Analysisof the said selected O.C.P.s water samples. The WQI of Harabanga O.C.P., Damalia O.C.P. and RatibatiO.C.P. are 90.3243, 82.8558 and 92.9864 respectively. The results indicate that all the O.C.P.s show verypoor water quality. Compared to the water of regular, conventional sources in Raniganj coalfield withrespect to its utilitarian perspectives. Simultaneously during the investigatory phase voting technique wasalso applied to determine or understand the ground reality related to the said abandoned O.C.P.s. An idealquestionnaire was prepared on the basis of associated issues and problems like pollution, water quality,fishing activity, agricultural activity and domestic activity among selected respondents, like Local people,Mining workers, Local administrators, Environmentalists, Policymakers and Researchers. From the surveyit is clearly revealed that local people are utilizing the O.C.P.s water only for agricultural activity andfishing activity etc. not for drinking and domestic purposes. Therefore the implementation of suitable watertreatment and sustainable management plans are needed from the competent authorities for utilitarianperspectives of the O.C.P.s.

Implementation of a QAQC program for in-situ grade control by pulsed fast and thermal neutron activation methods

Following years of development and testing, in-situ chemical assay by Pulsed Fast and Thermal Neutron Activation (PFTNA) has been implemented in mining grade control at BHP Western Australian Iron Ore as a world first. Demonstrating the technical capability and aptness of a new methodology, however, is not sufficient to ensure the sustained quality of reported assay data. The success of moving from testing stage to implementation in active mining grade control, is chiefly dependent on the robustness of ongoing quality control and quality management. This paper shows the steps undertaken to achieve end to end monitoring of data acquired by Blasthole Assay Tools (BHAT) using PFTNA methods. The main challenge forin situ chemical assay by the BHAT is to design a quality assurance/quality control program (QA/QC) without a physical sample being collected, and in consequence, without the conventional separation into the focus areas sample collection, sample preparation and laboratory analysis. In this context, the BHAT combines all in one in-strument, and different ways to monitor data integrity, repeatability and accuracy need to be established as outlined below. After the validity of a BHAT calibration has been verified and a tool is in operation, data is monitored on a daily basis to check that relevant operational parameters inside the tool are working within defined acceptance limits. Measurement error in the field is monitored with repeat logs in Blastholes, and inter-instrument error by replicate logs of different BHAT units in the same Blastholes. Accuracy and instrument drift over longer periods are monitored by repeated logs in Reverse Circulation (RC) drill holes. Operational parameters, such as neutron output and spectral resolution of the instrument detector are monitored by scheduled logs in dedicated testing facilities. Also, duplicate manual sampling in Blastholes isused to compare grade populations obtained by different sampling methods in mining pits to aid grade reconciliation from mining to production. By routine application of these QA/QC steps, in conjunction with close communication of results to mining teams, the new BHAT technology has been successfully embedded in day to day mining operations.

Fractal dimensions of hydraulic parameters in sand mined alluvial channel

Abstract Long-term extraction of sand from a river causes large mining pit on the river bed, and these pits change their fluvial processes. In this paper, an experimental study is executed to interpret the turbulence flow structure in a mining pit. This research mainly focuses on the fractal dimension of velocity fluctuations and Reynolds shear stress (RSS) of flow in the longitudinal and vertical plane for a mining pit region. The results show that the magnitude of fractal dimension for velocity fluctuation and RSS is smaller in the mining pit than the upstream and downstream of the pit. The large values of fractal dimensions at the pit downstream show an increase in flow roughness. It is found that the Taylor microscale decreases towards the channel bed, and this is because of increasing turbulence dissipation near the bed region. We have observed a significantly lower value of Taylor microscale in the mining pit. The higher value of the Taylor microscale is found far downstream of the pit. Nondimensional turbulence mixing length shows a decrease in the traversing length of an eddy at the far downstream of the pit.