AN ANALYSIS OF HYDRAULIC GEOMETRY IN A KARST CONDUIT, PARKS RANCH CAVE, EDDY COUNTY, NEW MEXICO

Numerous studies have quantified the hydraulic geometry of natural stream channels in terms of such parameters as velocity, width, depth, and roughness. These factors have been shown to vary with discharge as simple power functions. It is proposed that the direct application of these studies to flow through karst conduits is possible, though limited. As the hydrology of a karst conduit transitions from open channel to conduit flow, the relationships between these hydraulic factors will change. Additionally, the impact of nearby passages on flow at a given cross section can be demonstrated through the use of these parameters.

A conduit was selected which had only minimal inputs through a distance of approximately 200 meters. Five stations were chosen for analysis. Three of these were in relatively uniform, elliptical passage. Two were located where flow split between upper and lower passages. A random stage height of 10 cm. was selected for velocity and discharge profiling in the conduit. Scallop measurements taken between the stage levels were used to determine the average velocity and discharge at the station under varying flow conditions.

The discharge values obtained were compared to width, depth, and velocity at each of the stages. In the elliptical stations, the calculated discharge followed the trends of surface models up to approximately 65% of its maximum. At that point, a change in the rate at which velocity increased reflected the closing of the top of the channel, rather than the continued widening seen in surface channels. Flow parameters at the final two stations reflected the impact of nearby passages on the discharge. The transition from open channel to conduit flow at all stations was seen as the stage heights reached 100% of capacity. The Froude number and Mannings n were calculated at each stage for both cross sections. While a sub-critical flow seemed to dominate, changes in Mannings n were observed which appeared to be proportional to changes in velocity and the hydraulic radius.