Introduction

Artificial streams allow replication of flowing water conditions in the lab, where conditions for making detailed observations can be optimized, and where replication is simplified. PTA is a simple, yet powerful method for determining flow characteristics in the vicinity of benthic organisms without physical intrusions which will affect flow. The stream described here is inexpensive (<$75 US) and delivers useful flow regimes for a variety of research or instructional purposes. The PTA system described uses simple, consumer grade electronics and optics; a workable system can be assembled for under $1,000 US.


Methods - Stream Construction

An artificial stream was created by nesting a 40x20x25 cm aquarium inside a 75x30x30 cm aquarium, forming channels of variable width between the aquaria. Water was driven by one or two 1000 L/hr water pumps. White plastic grids (1.5 cm2 openings) were placed as collimators. Two 2-liter plastic containers were placed upstream of the study channel to divert water and straighten flowlines as water entered the study section.

A small, separate section of the grid material used for the collimators was covered on both sides with 1mm2 mesh aluminum screen. The pieces of screen, separated by the 1cm thick plastic grid, were connected to a 10 amp automotive battery charger. With current applied, the screen produced small bubbles ( 1mm-2).


Methods - PTA System

The videomacroscopic PTA system used a Sony Video 8 Pro CCD V-220 camcorder with an 8x macro-zoom lens fitted with a +7 diopter close-up lens set. Video was recorded on 1.27 cm (1/2") VHS videotape using a Sony SLV-676UC videocassette recorder. With the camcorder lens 5.5 cm from the stream, magnification on the 69 cm (27") diagonal Sony K27TS30 monitor was 10x. The electronic shutter on the camera was set to 1/500 sec.

Consumer video in the United States is based on the assembly of an image from two interlaced images; each of these images is refreshed 60 times per second. Consumer VCR's typically have a frame advance feature which provides still images spaced 1/30 sec apart. Current velocity was inferred from the distance traveled by particles (bubbles produced via electrolysis) between two such images.


Methods - Flow Measurement

To facilitate measurements, grids were produced by copying 1mm2 graph paper onto plastic transparencies. One of these transparencies was placed on the outer wall of the stream; the other was placed on the monitor where it was held in place by static electricity. The grid on the stream, while out of focus, was visible on the monitor. This grid provided coordinates for objects in the stream and a reference for determining magnification. To measure flow, the stream was run for a 15-minute period, with the VCR recording continuously. Later, the tape was advanced to the beginning of the fifth minute. Only particles (usually bubbles) which were clearly in focus (midstream) and which fell within the coordinates under study were used.


Methods - PTA Analysis

Using the time counter on the VCR, the first ten particles to be seen were tracked between two successive frames; both x (downstream) and y (vertical) displacement were measured. If a particle moved out of the plane of focus (z-displacement) it was not used and the next particle to be seen was used in its stead. At this point, the tape was advanced to the beginning of the next minute, and the next ten particles were tracked. 100 particles were measured for each location of each stream configuration. The data were entered into a spreadsheet (Quattro Pro for Windows) and analyzed using various descriptive statistics.

Stream characteristics were measured for a 5 cm wide channel and a channel varying in width from 5 cm (upstream) to 2.5 cm. Nominal water depth in all cases was 10 cm. Flow was analyzed from 3-4 and 4-5 cm above the channel bottom.