Measuring River and Floodplains Flows Using PIV

Section of the Iowa river next to the IIHR main building overlaid with estimates of free surface velocities.

Animation MPEG (size 1.4MB)   GIF (size 3.2MB)   AVI (size 2.5MB)

Investigators

A.A. Bradley, A. Kruger, M.V. Muste and E. Meselhe

Introduction

Floods produce dramatic changes to river channels, floodplains, habitat and vegetation. Deposition of sediments on the floodplain and the loss of vegetation affects riverine habitat populations. Furthermore, suspended material and changes to the river bed produce both short and long-term effects on river habitat.

Flow measurements can provide critical information for study of physical and biological processes in rivers during floods. In some locations continuous stage measurements are available for estimating discharges. In other instances discharges must be estimated by indirect methods. In recent years, new measurement techniques such as acoustic Doppler velocimetry (ADV) have played a role in river studies. In particular, ADV has been used to measure two- and three-dimensional flow fields in natural waterways.

ADV equipment is relatively expensive to purchase as well as to operate, but it has proven to extremely valuable. However, when one is able to observe a river during a flood, measurement flows is difficult and dangerous. Therefore few direct measurements of the two-dimensional river and floodplain flows are available.

Research Objectives

Researchers at the IIHR are developing a method for measuring free surface flows in rivers and floodplains.The method is based on particle image velocimetry (PIV), a well-known method in laboratory-scale fluids experiments.

The free surface velocities (in image space) in the figure above were obtained as follows. First the flow was recorded using a video camera at 30 frames per second. Every 30th frame in the sequence was digitized using a computer-controllable VCR and framegrabber.

Then a search was performed for the most likely movement of groups of pixels between frames. With the displacement as well as the time difference between frames known, a estimated velocity was computed.

Research Issues

There are many important issues that need to be addressed in order to make accurate estimates. One of these are image registration, where the image coordinates are related to the physical coordinates. This and other issues are the focus of ongoing research at IIHR.

Field Testing

The Iowa river flows through the University of Iowa and there are several bridges and buildings that provide access to and a good view of the river that is required for developing the method. A USGS streamgage is located 10 m south of the IIHR main building, providing continuous river stage and discharge estimates.

Two views of a stretch of Iowa River on the campus of the University of Iowa.

More Information

For more information regarding this project, please contact Dr. A.A. Bradley abradley@iihr.uiowa.edu