surface processes research group

 
 

M.S. (WRE) student Jon Sanfilippo and I are instrumenting a gravel-bed stream, Porter Creek, in the North Fork Siuslaw River basin in the Oregon Coast Range in order to measure sediment transport as a function of hydraulic variables. Instrumentation was funded in part by a RAPID grant from the National Science Foundation’s Geomorphology and Land-Use Dynamcs Program. We are working in cooperation with fish biologists, Jack Sleeper, Justin Gerding, and Paul Burns, at the Siuslaw National Forest. They have allowed us to design two log jams, which were built by a helicopter pilot. Other cooperators and collaborators include Todd Jarvis of OSU’s Institute for Water and Watersheds; and Lindsay Olinde and Joel Johnson of the University of Texas at Austin.


There are at least two fundamental problems with testing gravel transport models with data from natural streams. The first is the problem of measuring transport rates. Gravel transport typically varies greatly in time and space, so measurements that involve placing a relatively small sampler on the bed for a relatively short time have inherently large uncertainties. Moreover,  gravel transport occurs predominantly during relatively high flows that are too fast and too deep to enter safely. The second fundamental problem is that of measuring the bed shear stress that leads to a given transport rate. In a wide, rectangular, plane-bed channel, we would measure water surface gradient, or even just bed gradient, and flow depth and assume that “skin friction,” or the shear stress applied to the grains on the channel bed, was proportional to the product of flow depth and gradient. Natural streams, however, typically have large roughness elements, such as undulations in the bed and banks and large woody debris, which is often bunched up to form wood jams that span all or a large fraction of the channel width. These large roughness elements have correspondingly large form drag that, in turn, is a large fraction of the total boundary shear stress.


To measure sediment transport, we embedded passive integrated transponder (PIT) tags in about 800 pieces of gravel and spread these tagged rocks over the channel bed. We installed 8 transceiver antennas on the channel bed to detect the passage of the tagged rocks, and each detection event is recorded by a data logger. Provided that we know, from measurements, the concentration of grains of a given size fraction on the bed and the concentration of tagged rocks in the same size fraction, we can calculate the transport rate of that size fraction from the rate at which the tagged rocks pass a given antenna.


To measure boundary shear stress, we installed piezometers in the channel banks to record flow stage upstream and downstream of each antenna. In conjunction with surveyed channel cross sections and discharge measurements at known times, we can calculate time series of total boundary shear stress. We are hoping to develop a simple measure of the degree of interaction between the flow and in-stream wood, the variation of that interaction with flow stage, and hence, a relationship to quantify the partitioning of boundary shear stress into form drag and skin friction.


The piezometers began logging water levels in mid-December 2012, and the antennas began logging in early January 2013. Unfortunately, winter and spring 2013 lacked storms of any consequence, so the antennas did not detect any gravel transport before the equipment was shut down in early September 2013 in preparation for log placement in October. Shortly after shutdown, a significant storm occurred. Tagged rock locations were determined by sweeping the stream with a mobile antenna in November 2013 and January 2014, and this mobile tracking relocated about 85% of the tagged rocks.


We’ve also been working on methods for constructing high-resolution topography from digital photographs with Agisoft PhotoScan software. See OSU’s MediaSpace video and the Institute for Water and Watershed’s H2OSU feature.

BEDLOAD SEDIMENT TRANSPORT IN A GRAVEL-BED MOUNTAIN STREAM

In March 2014, we installed a pico-hydropower generator to supply power to the equipment. Previously, the antenna array, which draws about 0.5 Amp of current, has been powered by deep-cycle 12-volt batteries, and the batteries have been recharged every two weeks with an on-site gasoline-powered generator. In February 2014, the batteries suddenly deteriorated, apparently as a result of repeated draw-down and recharging, to the extent that they can no longer power the antenna array. The site, at the bottom of a deep valley in a seasonal rain forest, is not a good candidate for solar power, but small, steep, perennial streams provide enough flow and head to turn a turbine attached to a permanent-magnet alternator and thus generate sufficient power to keep the antenna array going. Finicky electronics require isolated power, so we’re installing a second generator on the same penstock. See the video below and stay tuned!