Role of riverbed sand mining on planform and cross-sectional morphology of Mayurakshi River, India

Introduction

Sand mining changes the river morphology by inducing an imbalance between discharge and sediment load since these two hydraulic parameters are directly correlated with channel slope and sediment competence (Lane, 1955). It causes disturbances in geomorphic processes that result in changing channel form and function (Yuill et al., 2016). Sand mining reduces the available sediment load if extraction is greater than the annual volume of deposition and greatly affects velocity as channel slope changes due to extraction pits formation (Koehnken and Rintoul, 2018). These are all local effects depending upon the volume of extraction but it create channel morphological instability which can be reflected in a wider spectrum of channel morphological adjustments (Rentier and Cammeraat, 2022). Apart from channel morphological changes sand mining often results in serious environmental issues too (Park, 2024), i.e. decreasing flood frequency (Park et al., 2020), decreasing water level due to channel incision (NG, W. X., and Park, E., 2021), changing freshwater biodiversity and spread of vector-borne disease from the sand mining pits (Iversen et al., 2024). The burrow pits affect the net transportation of sediment load and the rate of infilling actually determines the lifespan of the pit (van Rijn, 1986). However, if the pits do not get replenished fully in an annual cycle of infilling and excavation, the enlargement process often changes downstream and upstream channel morphology (Kondolf, 1997a, Kondolf, 1997b). Currently sand is one of the most consumed products on the earth with increasing demand (UNEP, 2014) due to rapid urbanisation mostly in developing countries (Schandl et al., 2016). To meet this high demand, globally the volume of sand extraction exceeds the volume of fossil fuels and natural gas extraction (Yuen et al., 2024). River sands from the east-flowing rivers to the lower deltaic surface of the Ganga are often in great demand in the eastern part of India due to their high construction grade and increasing urbanisation (Prabhakar et al., 2019; Daham et al., 2024). Thus, under political influence and lack of administrative restrictions, the extent and intensity of sand mining in these rivers have increased manifold, creating a hydrologically disturbed fluvial environment. The direct and indirect impact of such a scenario is very little known in these rivers. Thus, a systematic contribution to understanding it and trying a future prediction necessitates a scientific venture.

The swarms of scientific inquiry into the potential impact of sand mining on fluvial systems date back >50 years to now (Costea, 2010). Scientists from all over the world have tried to understand the fluvial process-form linkage to the extraction of aggregate materials from riverbeds and floodplains. Warner et al. (1977) documented disturbance in erosion-accretion phases in fluvial systems due to sediment extraction, while Erskine et al. (1985), a decade later identified the issue of channel bed degradation due to gravel extraction. Erskine (1990) stated multiple process-form discontinuities because of excavation pit development. Sediment starvation is caused by increased transport capacity issues and fresh and increased entrainment from the pit bed and it results in upstream propagation of erosion. Similarly, bed material supply deficiency to transport capacity led to downstream propagation of erosion (Galay, 1983; Guchhait et al., 2016). The formation of sediment detention basins limits the downstream supply of coarse sediments (Brenna et al., 2021) and it affects bed coarsening and bed composition which affects habitat conditions in return (Kondolf, 1994; Sarkar et al., 2021). Bed material characteristics are dependent on grain size’s interaction with channel hydraulics during transportation (Church, 2006). Sediment transportation during peak flow conditions is controlled by surface boundary shear stress and it usually changes in the excavation pits (Kondolf, 1997a, Kondolf, 1997b; Islam and Guchhait, 2020). The lack of shear stress in the excavation pit leads to active downcutting and eventually triggers channel morphometric changes (Parker et al., 1982). The increased cross-section and changed channel gradient at the sediment extraction site lead to upstream and downstream incisions (Rinaldi et al., 2005) and also trigger the erosion potentiality of the tributaries due to the lowering of local base level (Surian and Rinaldi, 2003). Channel incision influences the lateral stability of the channels thus, increasing channel width and channel shifting due to changes in erosion-aggradation dynamics (Rinaldi, 2003). Often, sediment extraction affects the flow velocity and creates vorticity during the peak flow period which exaggerates localized scouring (Kim, 2005; Brestolani et al., 2015; Ghosh and Islam, 2024). It may result in the leaking of aquifer reserves (Kondolf, 1997a, Kondolf, 1997b). The cessation of mining and quick attainment of quasi-equilibrium of a river defies the actual degree of how sediment extraction can drive morphological changes (Brenna et al., 2021). These impacts are usually scale-dependent as channel geometric changes (width and bank erosion) can be prompt and be caused by moderate annual flood (Islam et al., 2021) while alteration of bed morphology and stretch-specific incision can be felt later (Kondolf et al., 2002).

Rivers flowing through the basin fill alluvial tracts of the Bengal basin are highly impounded (both longitudinal and across channel structures) and experience a high degree of human intervention. Featuring the global view of sand consumption for infrastructure development (Martín-Vide et al., 2010), in a country like India, the excess demand for sand is still being met from the river beds (Ghosh et al., 2016) because it is considered of better utility due to hydrological grading and abrasion (Padmalal and Maya, 2014a). In 2020, the estimated sand consumption in India was 2.5 billion tons (Kelkar et al., 2020) with a per capita consumption of 200 kg (Aghor et al., 2015). This high demand leads to illegal sand mining from river beds and even the official lease holders intensify excavations beyond their permitted limits (Koehnken and Rintoul, 2018). The effect of sediment mining on channel morphology including planform and cross-sectional morphology has previously been conducted on the rivers draining the western flank of the Bengal basin, like the Damodar River (Ghosh et al., 2016), Kangshabati River (Bhattacharya et al., 2019), Dwarakeswar River (Ghosh, 2024) and Dwarka River (Mandal and Pal, 2022). Similarly, on the northern edge, which is connected to the Himalayan frontal plains, studies were carried out in the Balason River (Tamang, 2013). Islam and Deb Barman (2020), Islam et al. (2021) and Nag et al. (2023) have integrated the effect of sand mining in the channel morphology of the Mayurakshi River marginally. It lacks any dedicated study focusing on the direct impact of sand mining in this river. Sand extraction from the Mayurakshi River bed has been linked with channel morphological changes at the local level and is also augmented as one of the factors affecting bank failure (Islam et al., 2021). The factual understandings of the direct role of sand mining are only certain fragments of a bigger story that the planners and administration are concerned with. An in-depth analysis of the direct role of sand excavation from the river bed of the Mayurakshi River is important from the river rehabilitation perspective since the catchment contains fertile agricultural land and thousands of inhabitants while a considerable amount has already been invested in constructing major engineering projects. A robust prediction method is also necessary to provide ideal forecasting of the extent of channel morphological degradation for future decision-making. The intensity of channel excavation and burrow pits are capable of changing the channel morphology which can create multi-faceted problems.

The central objective of this research is to portray sand mining-induced channel morphological changes. The specific research questions to address the central objective are – (1) to what extent sand mining of the Mayurkshi River has escalated from 1970 to 2020? (2) how do mining activities impact the channel meandering, braiding and cross-sectional morphological changes? (3) what would be the mining-induced planform morphological transformation in the short (2030) and long run (2050)? (4) what are the factors and probable pathways for destabilizing fluvial systems affected by sand mining? This research leverages a varied range of datasets, containing topographical sheets and satellite images for a time span of 50 (1970–2020) years and annexes topographic measurements for further quantifications. Thus, the present work would be a novel attempt to unlock the complex system behaviour in response to sand mining that would be helpful for academic research and regional planning.

Section snippets

Study area

The Mayurakshi River located in eastern India (Fig. 1A) originates from Trikut Hill, a Precambrian remnant of the Chotonagpur Plateau in Jharkhand, and converges with the Bhagirathi River at Kalyanpur, covering an estimated flow path of about 250 km with a basin area of 9596 km² (Islam and Deb Barman, 2020; Islam et al., 2021). The upper stretch receives heavy monsoon rainfall (e.g., 1285 mm at Massanjore), with 60–70 % falling from July to September (Bhattacharyya, 2013). Kandi station

Temporal and spatial dynamics of sand mining

Sand mining exhibits a specific temporal and spatial trend (Fig. 3 A-D). There are four phases dotted with varying magnitudes of sand mining (Fig. 3A). Temporal variation (1970–2020) in sand mining areas across different segments is striking with a rapid increase in sand mining in 2020 (Fig. 3B) often through illegal measures (Fig. 3C). The interquartile range (IQR) of 1970 is relatively small with the median close to zero, indicating minimal sand mining activity. The presence of an outlier

Sand mining-controlled geomorphic system destabilization

The work uncovers a geomorphic discontinuity state in the Anthropocene River, where the system’s planform and processes are altered as it transitions from a steady-state to a dis-equilibrium condition. Here, the human-made threshold to a fluvial system in the local spatial scale (i.e. channel segment) may be seen in the form of sand mining pits and intentional deformation of the riverbed (through roads, berms and heavy vehicle paths). The sand mining industry’s estimated growth rate has also

Conclusions

The study executed over the Mayurakshi River, India based on the planform characteristics and cross-sectional morphology indicates a nuanced control of sand mining over channel meandering, brading and cross-sectional morphological dynamics. Planform analysis was done based on the topographical maps, corona images and Landsat images from 1970 to 2020 while cross-sectional behaviour was detected using 2018 field survey data from 25 cross sections of the river. The study found that sand mining was

CRediT authorship contribution statement

Aznarul Islam: Writing – original draft, Supervision, Software, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Balai Chandra Das: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Sandipan Ghosh: Writing – original draft, Investigation, Formal analysis. Abdul Mannan Saheb: Writing – original draft, Software, Methodology, Investigation, Formal analysis, Data curation. Suman Deb Barman:

Funding

The study acknowledges the financial assistance of the Indian Council of Social Science Research (Grant no. F.No. 02/295/2016–17/RP), Government of India given to the first author (AI) for carrying out the major research project from which some cross sections are used in the present work.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We truly acknowledge the kind support and cooperation received from the Department of Geography, Aliah University, Kolkata, India. Besides, we also extend our sincere thanks to those persons involved in the collection of the data from the intensive field investigation. …

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