Landrum, Keith., C. Monohan, D. Brown. Quantifying Process Based Erosion Rates in an Abandoned Mining Pit: Malakoff Diggins. California State University, Chico. Summer 2014.
Abstract
Malakoff Diggins State Park, California contains one of the largest historic (1849-1910 gold rush) hydraulic gold mines in California. Chronic erosion within the 200’deep, mile long pit resulted in a State water quality 303(d) listing of Humbug Creek for sediment, mercury, copper and zinc. In this study, quantification of annual erosion and deposition was performed using a combination of GIS analysis, and direct field measurements to create an estimate of annual sediment discharge.
Measurements of material lost due to cliff retreat were made over a 60 year period using ArcGIS 10 by comparing aerial photographs from 1952 and 2012. During the 60 year time span, 100,600 m2 ± 25 of pit rim surface area was lost to mass wasting and gully erosion. Estimates of sediment volume associated with changes in pit rim area was 10,900 ± 100 m3/ yr. Direct field measurements of elevation change from surface wash were made with seven, 1-meter erosion bridges in representative areas of the pit. The average annual erosion rate measured from erosion bridges in 2012/2013 water year was -15.4 ± 8.41 mm/yr. The annual volume contributed by surface wash erosion was estimated as 9,100 ± 50.5 m3/yr. Field measurements of the annual deposition rate were made using sequential measurements of trail markers and t-posts placed in 2005 on trails and in transects on the pit floor. Deposition was measured as 47.5 +/- 4.5mm/yr, and the estimate of annual volume of deposition throughout the pit was 12,700 ± 889 m3/yr.
Annual sediment yield discharged from the mine was estimated using a simplified sediment budget, where cliff retreat erosion (10,900 ± 99.5 m3/ yr) + surface wash erosion (9,100 ± 50.5 m3/ yr) – deposition (12,700 ± 889 m3/ yr) = discharge (7,300 ± 1,039 m3/yr). It is estimated that 7,300 m3/yr of sediment is discharged from the pit as suspended and bed load.
Student Bio
Keith works at a Consulting firm in Chico California.
