Air Content Measurements in
Natural Hydraulic Jumps

Gregory B. Pasternack and Brett L. Valle
Department of Land, Air, and Water Resources, UC Davis

Background

Hydraulic jumps are turbulent mixtures of air and water that occur extensively in bedrock rivers. Only two ingredients are needed for a jump to form - a flow obstruction and appropriate water discharge. Natural boulders, woody debris, alluvial bedorms, and abrupt channel geometry changes are examples of frequently occurring natural flow obstructions in channels. In addition, engineered structures such as weirs, check dams, and spillways can induce hydraulic jumps. Along with a flow obstruction, a hydraulic jump needs a specific water flow. If discharge is too low, water will flow around the obstruction. If discharge is too high, excessive water depth will drown the jump.

Because hydraulic jumps are features of rivers throughout California, it is necessary to consider their structure and function when attempting river restoration. Existing restoration practice primarily focuses on channel shape as identified through stream classification and secondarily on substrate conditions. Scientists are beginning to recognize the important role of in-stream features such as large woody debris, but natural jumps have not been assessed.

Past research on hydraulic jumps has investigated idealized conditions in laboratory flumes, on spillways, and below weirs but has never measured conditions in the natural setting. The primary research motivation has been maximizing energy dissipation on man-made structures and minimizing maintenance cost. Despite the prevalence of hydraulics, no studies have assessed their structure and dynamics, let alone their role in fluvial geomorphology.

Objectives

Test Time Domain Reflectometry (TDR) under controlled laboratory conditions as a potential instrument for measuring air content in hydraulic jumps.

Conduct preliminary field measurements to measure air content in natural hydraulic jumps on the American River in California.

The long term goal of this research is to investigate hydraulic jump fluid mechanics in the natural setting and its relevance to fluvial geomorphology and river restoration.

Please send comments about this WWW site to: gpast@ucdavis.edu
Copyright © 1999 Gregory B. Pasternack and Brett L. Vallé. All Rights Reserved.