Big Cr nr Roosevelt Beach

The gage at Big Creek at Roosevelt Beach, Oregon was a gage on a small river basin draining into the Pacific Ocean in the Oregon Coast Range. While all of the major river basins flowing directly into the Pacific Ocean in the Coast Range had one or more gages, there are very few in small basins, which react differently during storm events.

This gage was off Highway 101 on the Oregon coast north of Florence. It's a near two hour drive just to the small dirt road of the highway from the Eugene office. It meant for a long day of driving. And servicing the gage was always a challenge. Once you turn off the highway you drive about an half an hour plus up the creek valley which is heavily forested and damp on good days and soaking on bad days. It was so wet that ferns were so abundant that many people earned money collecting them for florist shop in Florence.

It was a gage that no matter the weather when you left the highway you always wore your rain suit, coat and pants, when you walked to and serviced the gage and made your measurement. It was just a day you accepted as part of the work and job. And besides you got to spend a day in a beautiful place, ok, minus the occasional dumped cars and trucks people would haul in and leave for the State of Oregon to remove.

But the neat thing about this gage after you had finished was that you drove out to the highway. And you were right there on the ocean, just a parking lot away. So, after being thoroughly wet, you got the chance to park the truck, get your thermos out, and sit on the beach for awhile, before you drive the two hours home. That's hard to argue is a bad thing or a bad day, even if the gage work and measurement weren't all the fun that day.

It was here that Jack Mrowka tested an idea. The USGS method to measure the velocity of water in a profile is to measure the speed and direction for about 40 seconds, only less during high flows or floods. The idea is that the speed variation during the 40 seconds will average out to a value within an acceptable accuracy. But this method, developed in the 1950's using measurements on larger rivers, has never retested.

Jack had the idea that the speed in a river cross-section has harmonics, meaning tthat the speed varies over a cyclical pattern where the length for a full cycle varies from about 15-60 seconds, usually 30-45 seconds. He had been testing this using A velocity meter and chart recorder, but he had never tested on a small stream and against the standard USGS metering equipment and method.

He was aware that the variation not only changes in the cross-section, it changes as you move up and down the river, meaning it's a three dimensional characteristic, and it changes with time, so it's a four dimensional characteristic of any river. So it introduces the problem it may be so complex it can't really be determined, and meaning individual station-based method would require consistent updates to ensure it representative.

This may be why the USGS established the 40 second minimum. With all the data they originally collected which determines the optimum minimum time was 40 seconds, it would be impractical to redo the research over a wider number of streams and rivers under varying conditions over time. It's a situation where you can't really disprove it because you could never have enough data to establish the method is inaccurate.

But Jack was taking his shot. He had this spirit that he liked to pursue a lot of ideas. He would collect some data to get an idea off the ground and into some degree of insight, and like a lot of academics, he never collected enough to prove or disprove anything, but just enough to make you pause that it's a realistic possibility and even a probablity. But how much? Well, as all academics write, more research was needed.

So, being friends, and getting permission from my boss, he tagged along one day to service this gage. He got his equipment ready and made some initial tests while I serviced the gage. When I was ready to measure, meaning tagline set, equipment in hand and ready to start, Jack followed me in each section with his equipment. I did a standard measurement and he did his, usually recording the speed for 2-3 minutes to ensure he had several complete cycles in the speed variation.

When we were done I let him copy my notes to his so he could make the comparison later, but he had some initial observations. He discovered that during the measurement I was making the harmonic cycle of the velocity was consistently in the cross-section to be about 30 seconds. This meant I was likely over measuring in some sections and under measuring in other sections.

The questions is, with that thought in mind, did it average out during my measurement, or did I bias the total with too many over or under velocities? I don't know, except that there are many factors effecting the accuracy of a measurement, from the cross-section conditions, equipment, choice of measuring sections, and so on, it seems likely the harmonics of the velocity is lost in the overall error.

It was an interesting day, something few streamgagers get to do, working directly with a professor as an equal, to understand the reality of rivers in the field. Most streamgagers don't really care for this but then a good number like to understand the theory behind what they do. And you couldn't have had a better companion for this than Jack.

And Jack knew where on the drive home there was an ice cream shop that made their own 18% butterfat ice cream.