Abstract

Facilities for storing hazardous waste on or slightly below the Earth’s surface must be located or designed to avoid disruption of the containment system and dispersal of the wastes by the maximum credible flood in their vicinity. Because many forms of hazardous waste must be isolated for hundreds or thousands of years, the historical record is not adequate to estimate the magnitude of the ‘maximum’ flood requiring consideration. Paleohydrologic techniques, based commonly on data derived from the geometry and sedimentology of alluvial gravels, allow the extension of the historical record into the Holocene and late Pleistocene. This method, unlike statistical manipulations of the historical hydrologic record, includes the manifested effects of the range of past climates on floods in the specific drainage basin. Properly interpreted, including estimation of the span of the paleohydrologic record compared to the desired containment period and the physical relation between processes leading to past extreme floods and future ones, paleohydrology is a valuable asset for aiding the assessment or design of a near-surface storage site for hazardous waste.

Paleohydrologic methods are based on the mechanics of flow and sediment transport in alluvial streams. The governing equations relate the flow width, depth, slope, and discharge, and the maximum size of sediment transported. Paleohydrologic studies involve determining or estimating as many of these variables as possible in the field, and calculating the unknowns. Different techniques are subject to varying degrees of uncertainty, but generally the most reliable estimates can be made for paleoflow surface elevation or depth, followed by width, and then slope with least reliability. Multiple, independent approaches are desirable where adequate data are available; confidence in a paleoflow estimate is increased if different approaches give similar results. Combined methods produce the least uncertainty, but data are seldom adequate to allow their use.