MATRIX STORAGE OF FLOODS IN KARST CAVE CONDUITS

Over forty years ago (1963) Cooper and Rorabaugh (CR) of the USGS developed a simple but important one-dimensional, linear model of streambank storage of a passing flood wave in surface stream or channel. Given the flood wave history, the model explained water level changes in the aquifer bounding the channel, estimated flux rates between channel and aquifer, and modeled the time history of the volume of bank storage. From the latter you can determine the portion of the aquifer impacted by the unique chemical fingerprint of channel water, including contamination. The model did not account for feedbacks, such as attenuation of the flood wave by bank storage. We borrow the concepts in this model to study matrix storage of a passing flood wave in a phreatic (fully saturated) cave conduit using a two-dimensional, cross-sectional, transient finite element method. For a conduit below a water table or caprock, we study the fluid exchange between the conduit and matrix, flood volumetric storage in the matrix, and storage of flood water with a unique fingerprint disturbing matrix water quality. Note that volumetric storage of the flood wave involves a pressure or head pulse moving into (and back out of) the matrix, while storage of chemically tagged flood water involves displacement and dispersion. In both cases water is transferred to the matrix much faster than it returns, and is stored for a much longer period than the duration of the flood itself. The volume of stored chemically tagged flood water is greater than the volume of hydraulically stored volume, and the geometric pattern of storage is significantly different. These influences are characterized by dimensionless numbers accounting for matrix, conduit and flood properties, and aquifer geometry.