Visualizing Hyporheic Flow Through Bedforms Using Dye Experiments and Simulation

The overall goal of this procedure is to demonstrate hypo EIC flow experimentally using modeling software that creates a simulation which agrees strongly with the physical experiment. This method can help demonstrate key concepts in the field of hydrology by showing how the flow of water through sediments under a stream is influenced by sediment, topography, and surface water properties. While this method can be used to investigate hyper IC flow, it can also be used in educational laboratory is to demonstrate hyper IC flow to students of all levels.

The main advantage of this technique is that it couples physical laboratory experiments with interactive computer software that simulates the same phenomenon. Visual demonstration shows the spatial similarities and discrepancies between the physical experiments and the simulations, which encourages the development of a deeper understanding of hyper principles. Begin with installing the required software, which is net logo and two scripts to run in net.

Logo, mouse drop and interface. Next, following instructions in the text protocol, set up the laboratory flume so that all parameters fall within the mouse drop simulation parameter range constraints. Run the flume for 12 to 24 hours to create a bed form with the desired characteristics.

Adjust the flume slope and water depth to achieve a uniform flow over the bed form. The goal is for the sediment grains in bed forms to not appear to be moving, although a little movement may be unavoidable. First, make the flow uniform while the pump is running.

Select two points at the bottom of the flume and record the distance to the water surface for each line. Then adjust to the slope of the flume or the water depth until these vertical distance measurements are the same. Second, stop the pump and wait for the water to stop moving.

Then at the same locations as before, measure the distances from the bottom of the flume to the water surface and measure the distance between these vertical measurements. Calculate the channel slope as the difference between these measurements divided by the sloped horizontal distance between them. Now, restart the pump and select a test section.

Choose a location near the middle or downstream end of the flume where dunes have formed a regular pattern. This section must encompass at least one full bed form. In the test section.

Make a few measurements using a transparent ruler. First, determine the average sediment depth by taking measurements at a dune trough and at a dune crest. The difference between these measurements is the bed formm height.

Next, find the average water depth, which is the average distance from the water surface to the sand bed surface. Then measure and record the average bed form wavelength by measuring the distance between successive dune crests. Next, record the channel flow rate from a flow meter in the recirculation loop and calculate the average flow velocity.

Now, open the mouse drop simulation and check that all these measurements are within the ranges specified in the user interface. If a measured parameter falls outside of the constraint range, adjust the parameter range by right clicking on the slider, selecting, edit, and adjusting the minimum and maximum values. First, set up a camera on a tripod pointed orthogonally to the flume wall.

The picture should be centered on a single bed form in the test section. If reflections are a problem, fix the position of the camera and adjust the lighting, including a ruler in the picture can help with scaling. Next, using a syringe and needle, make two or three small dye injections near the flume wall.

These injections should form two centimeter patches of colored pour water that should be placed at a variety of vertical and horizontal locations. Record the start time of the dye injections and take an initial picture. It can be educational to use transparency paper to trace the initial D fronts and the boundaries around the dye.

Thus, it is easier to observe their movements in the lab, but this method has its trade-offs. Using the camera, capture the D fronts positions at the appropriate time intervals. For time-lapse photography, use 32nd intervals for smooth results.

For a simulation. First run mouse drop and compare the results with the observed dye transport. In mouse drop, adjust the physical system parameters to match the flumes experimental conditions.

Be sure to pay careful attention to the units when entering these parameters. Next, adjust the sliders to indicate at what times the simulation tracking color will change. Set these color changes to match the observed times.

If the time parameters are all set to zero, the simulation will display a single color throughout. After all the parameters are set, click the setup button. The bed formm should appear in the simulation view.

Next, click the mouse drop button to indicate the starting locations of the virtual tracers. Multiple locations in the bed may be clicked. Hold the mouse down to release more virtual tracer.

Once all of the virtual tracers have been placed, you can click the advance to next time button to run the simulation. Until time one, do not re-click the setup button or the tracers will have to be placed again. You can also click the go stop button to run the simulation.

The tracers will keep moving until all virtual tracers have left the system unless you hit the go stop button. Again, this can be used to pause the simulation, so comparisons can be made between the simulated and measured dye distributions. Once the simulation starts running, the velocity is calculated for the location of each tracer.

Based on the simulation parameters, the tracer moves to a new location using that velocity, then the procedure is repeated until the tracer leaves the system. Next, run the interface simulation by clicking setup followed by go stop. This will run the simulation with the default settings.

The interface simulation introduces the virtual tracers on the streambed surface in a flux weighted manner based on the calculated subsurface velocities by default particles leave paths showing where they have been. Turn the show paths button to off to eliminate these paths. Turning the red drop switch to on disables the cumulative residence time distribution plot and releases a new particle each time.

One exits the system. After observing the simulation with the default parameters, click go stop to stop the simulation. Then change one or more parameters, restart the simulation with the new parameters by clicking setup, followed by go stop here, we adjusted the bed form height, ran the simulation and then repeated the process, adjusting the bed depth to compare the simulation to the experimental results.

The initial photograph was used to determine the placement of the simulated D tracer at time zero. Then the simulation was run for 34.2 minutes and compared with a photograph taken at that time. Overall, the model did an excellent job.

Each D blob moves in the same general directions as the model and deforms similarly to the simulated D blobs. However, careful inspection shows some discrepancies. For instance, the D blob on the right forms more of a bean shape than the simulation.

This is probably due to the observable dip in the bed form topography immediately above this blob, which was created during its injection into the sediment. Another common discrepancy is timing, which was also not perfect. This is likely due to slight errors in the sediment property measurements.

Common discrepancies are formed from a combination of measurement errors and second order physical effects due to irregular bed formm geometry variability and sediment packing, and so forth. Once mastered, this technique can be done in 24 hours. While attempting this procedure, it's important to let the bed form stabilize, be patient, and pay attention to units while making and entering measurements.

Following this procedure, other experiments can be performed in order to answer additional questions about the influence of topography, hydraulic conductivity, and surface water properties on hyper flow. After watching this video, you should have a good understanding of how to visualize hyper flow experimentally and how to use our computer simulations.