Flood discharge of rivers

The current discharge measurement methods in rivers are intrusive and often manual. During flood season (Fig.1) the discharge measurement is difficult to assess and of limited accuracy. Further, access to a station can be difficult, sediment transport may be considerable and velocities prevent the use of conventional instrumentation. A novel method to predict flood discharge in rivers was developed for small changes of the bed geometry during a flood. Based on a proposal of Prof. W. Kinzelbach (IHW), two research projects were conducted, one as a numerical approach and the second as an experimental approach to be described in the following.

Experiments and discharge computation

The new method proceeds in two steps:

• In a first step the water surface is recorded with three cameras to determine its geometrical position and the free surface velocity field.
• In a second step a finite element program is employed to compute the discharge with an iterative procedure.

The method was developed in a laboratory channel. The free surface was recorded with three calibrated cameras connected to a computer with the same frequency, such that three pictures are recorded at the same time step t₁. In order to visualize the free surface, the flow was fed with floating tracer particles. A photogrammetric procedure developed at the institute of Geodesy and Photogrammetry allowed to find the coordinates of the tracer particles and the corresponding time-averaged free surface. The position of a specific particle during several time steps was determined with a tracking-algorithm. This information allowed calculation of the corresponding velocity field (Fig.2).

The free surface and its velocity field were then used to compute the discharge. The bed geometry is measured conventionally prior or after the flood. A computational volume is defined between the measured free surface and the river bed geometry. An iteration procedure based on the commercial finite element program CFX allows finding the discharge. For a selected discharge the velocity field (Fig.3) at the free surface is computed and the difference 'delta' between computed and measured velocity fields is calculated. The iteration is continued until the difference 'delta' reaches a minimum. This discharge deviated less than 5% from observation.

Main goals

• Developing a 3D-PTV method to measure the water surface and the velocity field.
• Presenting a method to compute the discharge based on the commercial program CFX.