Fluvial Gravels as Sources of Construction Aggregate

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Fluvial Gravels as Sources of Construction Aggregate

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Fluvial Gravels as Sources of Construction Aggregate

Fluvial Gravels as Sources of Construction Aggregate

Fluvial Gravels as Sources of Construction Aggregate

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. 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.

Fluvial and Glacial Outwash Deposits

Sand and gravel that have been subject to prolonged transport in water (such as active channel deposits) are particularly desirable sources of aggregate because weak materials are eliminated by abrasion and attrition, leaving durable, rounded, well sorted gravels (Dunne et al. 1981, Barksdale 1991). Sand and gravel are commercially mined from the active channel (instream mining) and from floodplain and terrace pits (Figure 11). Instream gravels thus require less processing than many other sources, are easily worked by heavy equipment, and suitable channel deposits are commonly located near the markets for the product or on transportation routes, reducing transportation costs (which are the largest costs in the industry). Moreover, instream gravels are commonly of sufficiently high quality to be classified as “PCC-grade” aggregate, suitable for use in production of Portland Cement concrete (Barksdale 1991).

River channels and floodplains are important sources of aggregate in many settings by virtue of the durability of river-worked gravels and their sorting by fluvial processes. The relative importance of alluvial aggregates is a function of the quality, location, and processing requirements of alluvial aggregates, and the availability of alternative sources in a given region.

aggregate depicted in relation to river channel morphology and alluvial water table

Of the 120 million tones (132 million short tons) of construction aggregate produced annually in California (Carillo et al. 1990, Tepordei 1992) virtually all is derived from alluvial deposits. Annual aggregate production from alluvial deposits in California exceeds estimated annual average production of sand and gravel by erosion in the entire state by an order of magnitude (Kondolf 1995). In Washington State, however, riverine sources account for less than 17 percent of the state’s production (Collins 1995) thanks to the availability of extensive glacial outwash deposits convenient to many markets, especially in the Puget Sound region (Leighton 1919, Lingley and Manson 1992). Kroft (1972) and Dunne et al. (1981) mapped the distribution of glacial deltas along the Snoqualmie, Cedar, and Green Rivers. Dave Knoblach (WDNR) is presently mapping potential gravel sources, including glacial outwash deposits, at 1:100,000 scale (D. Norman, WDNR, personal communication 2000). Maintaining these supplies into the future will depend, in part, on protecting outwash deposits from being rendered inaccessible by urban development.

Other sources can supply suitable aggregates for most purposes, although more processing may be required.

Other Potential Aggregate Sources

Reservoir Deltas
Sand and gravel are mined commercially from some debris basins in the Los Angeles Basin and from Rollins Reservoir on the Bear River in California. In Taiwan, most reservoir sediments are fine-grained (owing to the caliber of the source rocks), but where coarser sediments are deposited, they are virtually all mined for construction aggregate (J.S. Hwang, Taiwan Provincial Water Conservancy Bureau, Taichung City, personal communication 1996). In Israel, the 2.2- km-long (1.4 mi) Shikma Reservoir is mined in its upper 600 m (1970 ft) to produce sand and gravel for construction aggregate, and in its lower 1 km (0.6 mi) to produce clay for use in cement, bricks, clay seals for sewage treatment ponds, and pottery (Laronne 1995, Taig 1996). The zone of mixed sediments in the mid-section of the reservoir is left unexcavated and vegetated so it permits only fine-grained washload to pass downstream into the lower reservoir, thereby insuring continued deposition of sand and gravel in the upstream portion of the reservoir, silt and clay in the downstream portion (Figure 12). The extraction itself restores some of the reservoir capacity lost to sedimentation. Similarly, on Nahal Besor, Israel, the off-channel Lower Rehovot Reservoir was deliberately created (to provide needed reservoir storage) by gravel mining. Water is diverted into the reservoir through a spillway at high flows, as controlled by a weir across the channel (Cohen 1996).

Extraction of reservoir deposits serves to restore some (albeit a small fraction) of the reservoir capacity lost to sedimentation. Replacing lost capacity through new reservoir construction is expensive, especially since the most favorable reservoir sites have already been developed. The cost of new reservoir construction (estimated from projects proposed or under construction on the Carmel and Santa Ana rivers in California) is approximately US$ 2.50 m-3 ($3000/acre- foot), and the cost of mechanical removal of sediment can exceed US$200 m-3 ($20,000/acre- foot), based on costs in Sierra Nevada hydroelectric diversion dams (Kondolf 1995). 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. In the US, construction of reservoirs was often justified partially by anticipated recreational benefits, and thus reservoir margins are commonly designated as recreation areas, posing a potential conflict with an industrial use such as gravel mining. Many reservoir deltas are relatively inaccessible or distant from markets, such that transportation costs make their exploitation uneconomical under present conditions. Wetlands may form in reservoir delta deposits, posing potential conflicts with regulations protecting wetlands. The likely opposition of nearby residents to gravel-truck traffic would be another obstacle to development of these resources.

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