and mining is considered to be the most revenue generating business as it does need much investment. It is also the most important commodity, as sand is required for construction activities but reckless mining has caused catastrophic for environment and hadoften led to social conflicts.
Following directives by the National Green Tribunal (NGT) to regulate mining, the Goa government in June 2019 hired the services of the National Institute of Oceanography (NIO), Dona Paula, Goa, for monitoring riverine and riparian biodiversity on a sampling mode, carrying out assessment studies for sand replenishment/accretion and erosion studies, along with assessment of biodiversity for rivers of Chapora, Terekhol, Mandovi and Zuari.
The NIO has so far submitted two Environment Impact Assessment/Environmental Management Plan (EIOA/EMA) Sand Modelling Study for the Chapora River and Mandovi Estuary, which would enable the State government decide on granting permits to contractors for sand extraction and regulate the mining activities on a sustainable basis.
In its second report on Mandovi Estuary, the NIO has detailed out the current status of the river and has recommended various measures considering the ongoing activities including sand mining, site sensitivities, national guidelines, international best practices and concerns raised by local community as well as directions by the NGT.
The NIO study states that considering the results of the baseline, Mandovi estuary has been influenced by cumulative anthropogenic activities and natural riverine and costal processes. These effects have changed the geo‐morphology and bathymetry at many stretches of the river/estuary. Over the years sand extraction has caused many pits and deep gorges in the lease areas as well as outside the lease areas.
The lower stretches of the estuary are influenced by higher salinity regime thus the adjacent banks have been protected by traditional embankments. These embankments allow the locals to carry out agriculture predominantly paddy cultivation. These traditional embankments consistently face the problem of saline water intrusion and require continuous repair and maintenance. There are several reasons for the damage of embankments viz high tidal surges, subsidence of land, removal of sand/sediment from the river bed, non‐maintenance of embankments and flooding due to rainfall and upstream water flow.
The salinity intrusion has also changed the land use, converting many paddy fields and riparian zones and mudflats into unproductive areas or mangrove areas.
Based on the baseline data and the prevailing site conditions (cumulative impacts herein) sand extraction at unregulated rates or in the sensitive area will have negative impacts on the environment. Accordingly, certain areas in the stretches of the river have been demarcated where sand extraction from the river bed requires restrictions.
Based on the side-scan sonar imaging, single beam echo-sounder bathymetry and high resolution seismic (HRS) data morphological features of river bed have been interpreted across a stretch of 38 km of Mandovi River. The side-scan sonar image shows various geomorphological features (external structures) like ripple marks (associated with sand), rocky outcrops, river banks, erosion features, mud zones and shadow zones of the road and railway bridges across the Mandovi River. Apart from this, active sand mining zones in different regions of Mandovi River have also been identified. The single-beam echo sounding data is used for estimating the depth of the river bed and the high‐resolution seismic data is utilised for estimating the sediment thickness.
The microscopic grain size analysis shows a decrease in grain size while moving from Khandepar to the river mouth. The bulk sediment grain size in most of the samples is dominated by sand fraction. Most of the stations contain a considerable amount of quartzite, iron oxide, and fewer lithic fragments.
The comparative analysis has revealed that Khandepar, Cotombi, Volvoi, Navelim and Betqi are the locations where mid‐channel/point bars erosion and deposition of non‐cohesive sediment in the nearby downstream region is prominent; whereas Navelim‐Betqui, Amona, Piligao and Narve are the regions where the morphological changes are not prominent, except sand mining signatures and mining patterns. Integrated analysis of SSS and HRS data at respective locations revealed that a local redistribution of the sediment exists in most of the region.
The study suggests that the erosion and depositional processes are simultaneously working at very local scale in the river. In the Mandovi estuary the bed level changes for the post‐mining scenario are mostly around ‐0.15 m to 0.25 m during both simulated monsoon (August‐November 2020) and non‐monsoon period (November, 2020‐May, 2021) of Mandovi estuary. The changes in bed morphology in the post‐mining scenario at different sections of the river are highly dynamic and patchy in nature.
Considering the stretches which have low sensitivities, the section can be considered for sand extraction while following the national statutory and legal guidelines. While recommending the lease areas the authority should conduct a reconnaissance site survey and a consultative discussion with the stakeholders and accordingly work out areas that do not have conflict of interest.
The report states that sand extraction has to be conducted by traditional (manual) method only and no mechanised boats or machinery should be operated. The activity should be restricted to the available areas only and the depth of extraction should not exceed three metres from the existing baseline profiles.
Once the lease areas are issued, the delineated “Environmental Conservation and Management Plan” needs implementation. The plan also describes the team/committee which should be capable to address, execute and monitor all the aspects of the plan.
Some strategies suggested by the management plan for replenishment
River bed mining recommendations
Permit mining volume based on measured annual replenishment in the first year following adoption of the management plan, a volume equal to the estimated annual replenishment could be extracted from the reach of channel. Replenishment (up to the elevation of the selected channel configuration) would need to occur before subsequent extraction could take place.
Establish an absolute elevation
The absolute elevation below which no mining could occur or “redline” would be surveyed on a site‐specific basis in order to avoid impacts to structures such as bridges and to avoid vegetation impacts associated with down‐cutting due to excessive removal of sediment. An extraction site can be determined after setting the deposition level at 1 m above natural channel thalweg elevation.
Extraction of sand/gravel from the bar
Retaining the upstream one to two thirds of the bar and riparian vegetation while excavating from the downstream one to two third of the bar is accepted as a method to promote channel stability and protect the narrow width of the low flow channel necessary for aquatic life. Sand and gravel would be re‐deposited in the excavated downstream one to two thirds of the bar (or downstream of the widest point of the bar) where an Eddy would form during sediment transporting flows.
Limit river bed extraction to bar skimming
If mining is limited to the downstream end of the bar with a riparian buffer on both the channel and hill slope (or floodplain) side, bar skimming would minimise impacts. Other methods such as excavation of trenches or pools in the low flow channel lower the local base level, and maximises upstream (head cutting and incision) and downstream (widening and braiding) impacts.
Concentrate activities to minimise disturbance
River bed extraction activities should be concentrated or localised to a few bars rather than spread out over many bars. This localisation of extraction will minimise the area of disturbance of upstream and downstream effects. Skimming decreases habitat and species diversity – these effects should not be expanded over a large portion of the area.
Review effects of sand & gravel extraction
The cumulative impact of all mining proposals should be reviewed on an annual basis to determine if cumulative riverine effects or effects to the estuary are likely.
Maintain flood capacity
Flood capacity in the river should be maintained in areas where there are significant flood hazards to existing structures or infrastructure
Establish a long-term monitoring programme
Monitoring of changes in bed elevation and channel morphology, and aquatic and riparian habitat upstream and downstream of the extraction would identify any impacts of sand and gravel extraction to biological resources. Long‐term data collected over a period of decades as sand and gravel extraction will provide data to be used in determining trends.
Minimise activities that release fine sediment to the river
No washing, crushing, screening, stockpiling, or plant operations should occur at or below the stream’s average high‐water elevation, or the dominant discharge. These and similar activities have the potential to release fine sediments into the stream, providing habitat conditions harmful to local fish.
Retain vegetation buffer against river bank
Riparian vegetation performs several functions essential to the proper maintenance of geomorphic and biological processes in rivers. It shields river banks and bars from erosion. Additionally, riparian vegetation, including roots and downed trees, serves as cover for fish, provides food source, works as a filter against sediment inputs, and aids in nutrient cycling.
An annual status and trends report
This report should review permitted extraction quantities in light of results of the monitoring programme or as improved estimates of replenishment become available. The report should document changes in bed elevation, channel morphology, and aquatic and riparian habitat.
Seasonal ban in sand mining
The River bed mining should only be allowed during the dry season. No River bed mining should be permitted during rainy season.
Prevention and management of illegal mining
Prevention and mitigation to illegal mining can be achieved through close monitoring, cancellation of mining licence of violators, imposing heavy penalties including long‐term jail for mining on sensitive areas, and confiscation of mining machinery and vehicles. Department of Mines and Geology, law and order authorities need to restrict some areas for example, riverbanks, near schools, clinics, or residential areas.
Management of infrastructure
Temporary access roads or Katcha roads shall be formed between the banks of the river and the mining area with locally available bio‐degradable materials such as sugarcane waste, hay, etc. The preliminary works such as the construction of temporary sheds, bio‐toilets, drilling of bore wells, waste management facilities need to be developed before mining operations. CCTV cameras need to be installed at the entry and exit points.