SEDIMENTARY GEOLOGY OF URBAN ENVIRONMENTS: AN EXAMPLE OF MEASURING SEDIMENT PRODUCTION AND COMPOSITION IN TEMPE, AZ

Urban sedimentary geology reveals information about human interactions with the landscape and helps predict associated changes in hydrology and geomorphology. In urbanized areas storm water runoff and storage is modified by transportation structures, buildings, and constructed landscapes. Urban runoff is routed through storm water engineering structures into retention basins where eroded sediments accumulate in trash collectors and drywells. Because pre-existing channels and riparian ecosystems depend on surrounding watersheds an understanding of storm water drainage and management is required to determine the extent of urban watersheds’ that are contributing to the sedimentary basins and develop a method for measuring sediments in the urban setting.

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.