Field Site

A preliminary field campaign encompassed an 8.4 km bedrock-controlled section of the South Fork American River that contains a very large number of hydraulic jumps. The overall slope of the section is 0.0058 (31 ft/mi). The discharge is largely regulated by Chili Bar Dam, which lies shortly upstream of the research sites. Water releases from Chili Bar Dam can vary by up to a factor of 20 over a single day depending on time of the year. During the summer low flow period hourly discharge changes rapidly, though flow at a point is typically constant for a two hour period at the peak value. During winter and spring high flows, discharge can be nearly constant for days on end. Two small tributaries to the main channel only contribute flow during high intensity storms when no monitoring will take place.

South Fork American River Field Map

Field Methods

Measurements of the air content in each of four hydraulic jumps along the South Fork American River were made using one of two approaches. For the three small jumps studied - Upper Chili Bar, Upper Meatgrinder, and Little Maya - the TDR probe was manually held in the hydraulic jump for up to 8 minutes at a time. Larger and less accessible jumps such as First Threat required the probe to be strapped to a floatation device that keeps the wave guide pointed down and then hurled into the jump. The probe would then 'surf' the jump for several minutes. The orientation of the wave guides was recorded each run so that any accidental emergence could be accounted for.

Field Results

Upper Meatgrinder

Meatgrinder, South Fork American River, California

A transverse bedrock protrusion forms a hydraulic jump in Upper Meatgrinder. This jump had a mean air content of 18% and a range of 0 - 72%.

Upper Meatgrinder Air Content

First Threat

Gregory Pasternack, First Threat, South Fork American River, California

A non-transverse flat bedrock slab forms a hydraulic jump at First Threat. This jump had the highest mean air content measured thus far at 58%. Also, the relative variation about the mean was the lowest of all jumps.

First Threat Air Content

Upper Chili Bar

A transverse bedrock protrusion forms a hydraulic jump at Upper Chili Bar. This jump had a mean air content of 26% with a standard deviation of 23%. Sharp peaks in air content occurred when the probe was placed in the foam layer at the top of the jump.

Upper Chili Bar Air Content

Little Maya

A non-transverse flat bedrock slab forms a jump at Little Maya. This hydraulic had a mean air content of 23% and a range of 4 - 70%.

Little Maya Air Content

Comparison of Hydraulic Jumps on South Fork American River

Jump Meatgrinder First Threat Maya Chili Bar
Mean Air Content (%) 18.06 58.28 23.24 25.59
Standard Deviation 16.83 13.73 8.75 22.71
Coefficient of Variation 0.93 0.24 0.38 0.89
Minimum Air Content (%) 0.26 8.50 3.70 0.10
Maximum Air Content (%) 71.58 89.40 69.50 98.40

Overall, the minimum and maximum observed air contents were 0.1 and 98%, both at Upper Chili Bar. The highest mean air content (58%) was observed at First Threat, which is the biggest of the jumps. The lowest content (18%) was recorded at Upper Meatgrinder.

Cumulative Distribution Functions for Hydraulic Jump Air Content

Cumulative Distribution Functions for Air Content

The air content cumulative distribution function shown above indicates some similarities but also significant differences between jumps. The air contents of both Little Maya and First Threat are normally distributed, as indicated by their near linear fit. However, the former does not exceed 13% air for 90% of the time, while the latter has up to 38% air at the same nonexceedance level. In contrast to the distributions for these two jumps, those for Upper Chili Bar and Upper Meatgrinder are both strongly skewed toward much less air.

Background - Objectives - Air Content Measurements - Conclusions - Lab Methods - Single Phase Test Results - Air-Water Mixture Calibrations - Turbulent Air-Water Mixture Time Series
Field Methods - Field Site - Field Results


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