Last night, I read a strangely moving 125-page white paper titled Freshwater Gravel Mining and Dredging Issues. It was authored by G. Mathias Kondolf, Matt Smeltzer and Lisa Kimball of UC Berkeley for the Washington Department of Fish and Wildlife, Washington Department of Ecology, and Washington Department of Transportation in 2001. Despite “Gravel” in the name, the paper is an encyclopedic review of the scientific literature that surrounds both sand and gravel mining in all of their various forms (river, flood plain, wet, dry, bar scalping, in-stream sand traps, etc.).
It’s a virtual primer on how aggregate mining affects rivers, infrastructure, the water table, people and the environment. The study argues that many of the impacts of sand and gravel mining are never reflected in the cost of the products because government, in effect, subsidizes them. The authors also argue that if the full costs of mining were reflected in the price of aggregate, that we might be producing it in ways that were safer.
About the Author(s) and Focus
The lead author, Kondolf, is Professor of Landscape Architecture and Environmental Planning at Berkeley. He specializes in hydrology, environmental geology, environmental impact assessment, and riparian zone management with an emphasis on stream channel processes as they relate to natural resource management. Kondolf’s research is widely cited in scientific literature concerning sand and gravel mining.
While a large part of this white paper discusses aggregate mining’s impacts on salmon, it also addresses issues related directly to humans.
Sources for Aggregate and Their Cost
“Sand and gravel deposited by fluvial processes are used as construction aggregate for roads and highways (base material and asphalt), pipelines (bedding), septic systems (drain rock in leach fields), and concrete (aggregate mix) for highways and buildings.
Page 21 discusses two primary sources of construction aggregate. In many areas, aggregate is derived primarily from alluvial deposits, either from pits in river floodplains and terraces, or by in-channel (instream) mining, removing sand and gravel directly from river beds with heavy equipment.” The primary type of mining done in the Houston area is floodplain mining. However, the industry is beginning to push mining in rivers as a way to reduce excess sedimentation. (Ironically, many governments see floodplain mining as the answer to the dangers of river mining.)
Pages 22 and 23 discuss another novel source: reservoir deltas (much like the West Fork of the San Jacinto between US59 and FM1960). “Extraction of reservoir deposits serves to restore some (albeit a small fraction) of the reservoir capacity lost to sedimentation.” However, in the late 1990’s and early 2000’s, the cost of building new reservoirs in California was approximately $3,000/acre foot, while the cost of mechanically removing sediment from old reservoirs was $20,000/acre foot, almost 7X more.
“The economic value of avoiding further reservoir capacity loss could be a significant factor making removal more economically attractive in the future, especially if the environmental costs of instream and floodplain mining become better recognized and reflected in the prices of those aggregates.”
Kondolf, et al. also discuss other potential sources of aggregate such as recycled concrete. “Recycling concrete rubble not only avoids environmental impacts of new aggregate production, but avoids impacts of disposing the rubble as well.” Further, they found that the quality of recycled concrete could meet half of current aggregate uses.
Dangers Associated with Floodplain Mining
“As in-channel mining is increasingly discouraged or prohibited, mining of floodplain pits is encouraged as a less damaging alternative,” say the authors. However, there is no shortage of dangers associated with floodplain mining. The authors catalog those.
Where mines intersect the water table, dangers include:
- Lowering of alluvial water table
- Loss of wells
- Loss of riparian vegetation
- Prevention of seedlings from establishing
- Die-off of trees
- Reduced summer base flow in rivers
- Increased water temps in river during summers due to shallower water
- Fish kills due to river lowering
- Increase in evaporative losses.
During excavation, if floodplain pits are kept dry by pumping, they:
- Lower local water tables
- Potentially dewater nearby tributary channels
- Desiccate riparian vegetation and floodplain wetlands.
Floodplain pits are often accompanied by channelization to maximize the floodplain area accessible for mining and to prevent the channel from eroding into pits. Miners may straighten channels and stabilize banks with rip rap. Even when successful in keeping pits “isolated,” the principal biological effects of floodplain and terrace-pit mining include:
- Conversion of riparian forest to open pond habitat
- Reduced habitat complexity in the channel
- Loss of dynamic channel migration processes due to levees and bank protection
- Lack of natural channel banks
- Loss of riparian vegetation along hardened banks
- Changes in the hyporheic zone dynamics potentially affecting stream water temperature and water quality
- Increased potential for contamination of the alluvial aquifer due to the operation of equipment
- Spills and the direct route to groundwater through the pit
- Loss of floodplain wetlands
- Dewatering of tributaries due to lowered water tables.
Pit Capture Inevitable
Often old pits are used to settle fines. Once filled, the pits act as fine sediment plugs in the floodplain. “Subsequent channel migration can erode these, releasing concentrated fine sediments into the channel,” say the authors.
After off-channel pits “inevitably” (authors’ wording) become captured by the channel, other impacts often result:
- Bed and bank erosion upstream and downstream
- Potential loss of infrastructure, such as roads and bridges, through “head cutting”
- Bank erosion
- Property destruction
- Excessive downstream sedimentation
The authors claim capture is inevitable for floodplain pits, though not necessarily for terrace pits, which are usually higher in elevation and farther from the channel. Pit capture is most rapid when:
- The pit lies inside of a meander
- The upstream end of the pit is much lower than the adjacent channel
- The river floods, creating a pressure difference inside and outside of pits that causes dikes to collapse.
Captured pits become lakes within the river, transforming lotic (moving water) environments into lentic (still water) environments, thereby inducing changes in the ecology of the reach.
In the Naugatuck River, Connecticut, captured pits have become lakes with seasonally stagnant water and low oxygen levels. Authorities there expect the pits to persist for hundreds of years.
“Moreover, channel incision and instability induced upstream of captured gravel pits could trigger other pit captures, resulting in widespread and long-term cumulative effects,” say the authors.
Floodplain pits, when abandoned without remediation, “can be viewed as substantial liabilities for future generations, either to maintain their separation from the current channel, or if already breached, to suffer consequences of resultant channel incision … or to pay the price of re-isolating the breached pits.” They also pose safety hazards because of their steep sides.
On page 95, Kondolf et al. cite the costs of several public projects which became necessary after miners had abandoned pits. “The actual costs of isolating gravel pits will depend, of course, upon the surface area extent, excavation depth, and geometry of the pit and channel, as well as the availability and cost of suitable fill material. Experience to date in the Central Valley of California suggests that the costs … have been around $3-4 million per pit, although all these projects use dredger tailings available nearby” (and the costs are in 2001 dollars).
Decommissioning Costs Should Be Paid Upfront
“…pit isolation is a costly exercise, and given the likelihood of pit capture, these costs of “decommissioning” should probably be taken into consideration when permits for the gravel pits are initially awarded. It would be an interesting exercise to estimate the value of gravel extracted from these pits during their period of commercial operations compared to the current costs of reclamation.”
Alternative Sourcing Dependent on Full-Cost Accounting
Future regulation of aggregate mining should emphasize incentives to use alternative sources, such as … reservoir deltas, quarries and recycled concrete rubble. There is currently little incentive to use alternate sources. They generally require higher transport or production costs than aggregate taken from channels and floodplains.
Because the full costs of extracting aggregate from rivers and flood plains are not incorporated in the price paid for the product, it will be difficult to encourage use of alternatives. In effect, extraction of river/floodplain aggregate is subsidized.
Another study by M.D. Harvey and T.W. Smith found that the cost of mining-induced infrastructure damage was equivalent too $3/ton in a California river.* That’s equivalent to about $4.50/ton in today’s dollars.
Neither does the price of aggregate reflect the cost of dredging, which was necessitated here in part by the choice to locate mines in floodways. Dredging costs for Phase 1 of the West Fork already exceed $70 million. Phase 2 could easily cost another $100 million – all borne by taxpayers.
If such costs were incorporated into the price of river and flood plain aggregate, alternatives might look much more attractive.
*Gravel Mining Impacts on San Benito River, California. In: Proceedings of 1998 International Water Resources Engineering Conference, Hydraulics Division, ASCE, Memphis, TN, August 1998.
Posted by Bob Rehak on August 7, 2018
435 days since Hurricane Harvey