I've just returned from a site visit to a coastal bridge in New Jersey, making observations and collecting data necessary for tidal modeling efforts. With a budget too tight for extended tide data collection using our usual suite of bubbler pots and acoustic doppler velocimeters, I was faced with the prospect of using the age-old technique of tossing oranges in the water to assess streamflow. I don't mind going old-school to get the job done, but with an absence of NOAA tide data in the vicinity, I really didn't want to calibrate my highly advanced models with floating fruit, no matter how sustainable and earth-friendly the approach may be. I needed an equally advanced but inexpensive and quick method to gage stream velocity and angles of attack. What I came up with was basically a reusable, electronic orange - a sort of digital streamflow tracer. Here's how it worked: Borrowing an outrigger real from the survey locker and mounting it onto a Home Depot purchased aluminum rod, I created a rugged casting reel. Also borrowed from the survey guys was a water-resistant Garmin Rhino GPS/walk-talk handheld unit, which I placed in a waterproof low-profile plastic pouch with external anchor point, purchased from West Marine. Visiting the bridge at a full moon spring tide cycle, I repeatedly cast the unit into the oncoming tidal stream and allowed it to float freely under the bridge and around the piers before retrieving. The Garmin unit was set to provide track data at 1 second intervals, including lat/long position, distance traveled per leg, velocity, heading, elevation and time/date. Using mapping software on my field laptop, I was able to periodically download and check the data visually as it was acquired. Post-processing of the data shows very few data fall-outs as the unit passed beneath the bridge, with very clean-looking lines of track data obtained after a little trimming of the toss and retrieve periods. Samples of the streamflow tracers are attached. Surface velocities will later be converted to depth-averaged velocities using open-channel flow principles, and used in model calibration at periods of peak flood and ebb tides. Angles of attack will be used in pier scour predictions. I will post more later on the results of this effort. Some additional notes if you try this technique: Don't bother with ziplock bags - they leak no matter how many times you double and triple bag and tape them up! Get a real ocean casting rod and reel with 20 lb filament test if you can. My homemade unit did not cast well - I ended up letting out a couple of bridge widths worth of slack and tossing the unit by hand. Squeeze as much air as you can out of the pouch while still allowing floatation - you don't want the wind catching it at all during the drift. The low-profile West Marine pouch shown lay flat on the water and worked very well. Lastly, have a witty retort memorized and ready for use when the local fisherman ask "what the heck are you fishing for??"
Happy casting!
