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About I-RITE

Simulating Nature's Version of the Landfill

Nick Martin
Department of Geological and Environmental Sciences
Stanford University
March 2002

How can you see below the ground surface? The easiest way to see beneath the surface is to dig or drill a hole. Seeing below the earth's surface is important because subsurface dirt and rock control the occurrence and movement of groundwater, water that resides below ground. An understanding of the pattern of dirt, rock, and other material beneath the surface allows the construction of productive wells and permits the identification of sources of groundwater pollution. Although knowledge of the arrangement of subsurface material is critical and creating a hole is straightforward, digging and drilling are destructive and expensive. My research involves computer simulation of the layering of valleys with sediment. The computerized reconstruction of the contents of a valley provides a non-destructive method to determine the organization of material beneath the earth's surface.

The simulation of the filling of a valley requires an understanding of the history of the valley and a representation of the natural mechanisms to transport and arrange material. One way to visualize the valley-filling process is to think about the operation of a landfill. A landfill is constructed by digging a large hole in the ground while a valley is a depression in the earth's surface created by natural forces. The large hole, whether valley or landfill, furnishes storage space. Trash, broken appliances, and other unwanted material pack a landfill. The material stored in a valley is sediment, small fragments of rock that have been detached from the surrounding hills.

Just as trucks and bulldozers transport unwanted material to a landfill, flowing water and the force of gravity move sediment from adjacent hills to the valley floor. Gravity rolls or slides all sizes of sediment downhill to the edge of the valley. Flowing water carries sediment into the valley from the neighboring highlands and transports sediment across the valley floor.

Once trash reaches a landfill, the unwanted material is sorted for proper storage. Similarly, the sediment stored in a valley is sorted by size and density. A stream rushing down a hillside will slow as the stream crosses the flat valley floor. As the stream slows, only the smaller and lighter rock fragments can be transported. Smaller and lighter sediments will come to rest in the center of the valley. Larger and heavier sediments are dropped at the margins.

Although a landfill may be filled with trash in a few years, a valley fills with sediment over thousands to millions of years. Over such long time periods, climate and landscape changes influence sediment movement and sorting. In the course of a thousand years, the streams flowing into a valley may increase or decrease in size as the amount of rainfall varies with climate. The land surface shape, or topography, of the valley and neighboring hills may also vary during the valley-filling process. The erosion of the adjacent hills lowers the height and decreases the slope of the hillsides. Alternatively, a series of earthquakes can elevate the hills relative to the valley increasing the elevation and the slope of the hillsides. Larger streams, or streams which run down steeper slopes, can move larger and heavier sediments. Streams traversing shallower slopes, or streams which are smaller in size, transport only smaller and lighter rock fragments across the valley.

As a valley fills over time, climate and topographic variation augment the natural sorting process of flowing water to create patterns of material of different size and density beneath the valley surface. My research focuses on the development and application of a computer program that simulates the movement and sorting of sediment under conditions of varying climate and topography. Information on climate and landscape change for a particular valley can be obtained from scientific studies concerning climate history, landscape evolution, and geologic history. The application of the program to the climate and topographic history of a specific valley can reproduce the pattern of sediment below the valley floor.

The simulated valley fill provides a three-dimensional description of the material, deposited sediment, that governs groundwater resources. Certain types of sediments can store large quantities of water and provide excellent locations for water wells. Other deposits quickly transmit water from one location to another and accommodate the movement of contaminants below the ground surface. The simulated sediment pattern can be employed to analyze the amount of groundwater stored in the valley and to calculate the optimal location for new wells. Additionally, the recreated sediment arrangement can constrain the location of pollution sources and facilitate the prediction of areas of future contamination.