How does sand mining affect rivers?: Guest post by Melissa Nemhara
Rivers are key conveyer belts for the delivery of sediment to the oceans. The continued extraction of sand and gravel through mining causes the river flow to be sediment starved and more prone to eroding the channel bed and banks. As sediment moves downstream, it becomes increasingly rounded and well sorted (due to flow changes and erosion by abrasion and attrition). As a result, in stream sand is desirable as it requires less processing for use in industry. However, the excavation and extraction processes lead to the disruption of channel morphology; changing the steady state of the river. Generally, after excavation sediment passing the sandpit in the flow is deposited and trapped there. This then leads to sediment-starved water passing downstream which then erodes the channel bed.
Source: https://www.wou.edu/las/physci/taylor/g407/kondolf_97.pdf
Excavation of pits in the channel also creates a steeper gradient in this area. This over-steepened nick point has higher stream power and erodes upstream. Sand mining changes the equilibrium profile of the stream bed and this (especially when coupled with dam building) leads to downstream channel erosion, levee erosion and coarsening of bottom sediment. An example of an area where this is occurring is the Yangtze’s subaqueous delta. Many rivers worldwide have experienced reduction in sediment fluxes of up to 90%. This sediment starvation increases the effects of subsidence and increases erosion at the delta. Sand mining forms a sand pit in the area cutting the river bed and these changes in morphology are thought to take several hundred years to recover from.
References:
Kondolf, G. Environmental Management (1997) 21: 533. Available at https://www.wou.edu/las/physci/taylor/g407/kondolf_97.pdf
Yang, H., Yang, S., Meng, Y., Xu, K., Luo, X., Wu, C. and Shi, B. (2018). Recent coarsening of sediments on the southern Yangtze subaqueous delta front: A response to river damming. Continental Shelf Research, 155, pp.45-51.
About the author:
Melissa Nemhara is a Geology undergraduate at Imperial College, London. She attended the De La Beche society talk by Kiran Pereira and was really interested in the topic as she had never heard about it previously. She subsequently read journals and papers on the sand mining crisis. She currently works as a summer volunteer at SandStories.