SEDIMENTARY GEOLOGY OF URBAN ENVIRONMENTS: AN EXAMPLE OF MEASURING SEDIMENT PRODUCTION AND COMPOSITION IN TEMPE, AZ
We selected internally-drained neighborhood retention basins to isolate two watersheds and calculate yearly-scale urban sediment production rates in Tempe, AZ. Using aerial photography and the city’s GIS data; we mapped the landcover of each watershed and estimated the areas of impermeable surfaces, greenspace, and desert landscaping (unvegetated areas with dirt and gravel cover). We estimated the total sediment accumulated over one and two years for the two different watersheds and analyzed sediment composition. Urban sediments contained materials from desert landscaping (crushed granite and round Salt River gravels), construction, urban trash, and organic materials (dried grass, leaves and seeds). Pebbles derived from desert landscaping were 82% of the sediment by mass. We calculated a range of erosion rates using total, pervious, and desert landscaping areas. Annual-scale sedimentation rates for these neighborhood watersheds suggest erosion rates on the order of 1 m/My; a rate similar to estimates for bare bedrock surfaces in arid environments.
The simple methods used in this study begin to explore uses for urban sedimentary geology. This type of sedimentary geology approach provides urban erosion rates and sediment composition data that are pertinent to geomorphologists, anthropologists, city planners, and storm water managers. We plan to expand these studies to evaluate a diverse range of urban landcovers over longer sedimentary records to establish a relationship between erosion and human landscape modification.