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Jesper Nielsen

Modelling of Small Rivers and Creeks

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Hi there.


Modelling small rivers and creeks in TUFLOW-FV can be a challenging undertaking for several reasons. Representing the bathymetry and flow fields of the channel with cells sufficiently large enough to allow for a sensible timestep and thus acceptable run-times is certainly one of the biggest challenges. I will touch on this here...


Say you have a natural channel approximately 60m wide and with a maximum depth of 5m - a moderately sized creek or small river connected to the ocean with upstream freshwater inflows.

You have used rectangular cells which are typically 15m wide and 30m long.

You have inspected your cell center elevations off a detailed and highly reliable DEM or other bathymetric data set.

You think that this model will represent reality.

You run your model in 3D and compare to some salinity measurements.

You discover that your model has far less stratification than the data.


What is going on?

Each element has a discrete elevation. Lets consider, for example, the chain of elements defining one of the banks of your channel connecting the downstream boundary to the upstream boundary. The chain of cells consists of cells which have elevations inspected from the bathymetry data at 30m intervals. Thus the model represents the relatively smoothly undulating bathymetry with a series of discrete steps. Abrupt changes in cell elevations force abrupt changes in velocity. Where the velocities are abruptly increased the turbulent mixing is consequently abruptly increased. This phenomenon is probably contributing to the lack of stratification in your model.


How to fix it?

The matlab utility "fv_cellcentres_channel_smooth.m"

It is an almost fully automated way to smooth the bathymetry along the channel - it has worked a treat for me.

I have not uploaded it yet but if you would like to give it a whirl just send through a msg and I'll send it through.


Plenty more to follow on this topic...





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