[RM]

Thanks Scott.

I guess I was expecting the concentration graphs of the mass injection with specified duration to be more “squashed” as the injection moved downstream, ie with lower concentrations but increased distance between the rising and falling limbs such as in the sketch below.

Obviously the mass injected into the system would be retained as it is a conservative pollutant.

[RM]

Thanks Scott,

So is the HECRAS water quality module unsuitable for observing decreases in concentrations in receiving waters from a constant pollutant discharge?
Or does the distance between the observation points just need to be great enough to observe a greater decrease in concentration?
The diffusion will also greatly depend on the hydraulic flow regimes in the channel?

Thanks again.

[RM]

Hi, thanks for the prompt reply.

The cell size = 1m, the total channel length =1500m.

The time step interval is calculated by the software by ensuring the Courant and Peclet constraints are satisfied. Hand calculations using equations 19-34 and 19-35 from the user’s manual estimate the timestep to be 0.09s. This is the timestep value that satisfies the Courant condition. The peclet timestep and the user assigned maximum allowable timestep were greater values (0.12s and 10s respectively). (The model code selects the smaller of these three values.)

The dispersion coefficients used are computed by the software. A quick hand calculation using equations 19-1 and 19-2 from the user’s manual estimates the dispersion coefficient to be 2.5m2/s.
The channel geometry consists of identical trapezoidal cross-sections with a constant slope of 0.0015, so the velocities at each computation cell face are also identical. Therefore, the dispersion coefficients are constant throughout the channel also.

The dx, dt and dispersion values for the two runs are identical, the only difference is duration of mass injection.
Why is it that the instantaneous mass injection simulation produces results that illustrate dispersion but the mass injection with a given duration (say 1hr) seems to illustrate no dispersion but mainly advection?

Many thanks

[RM]

I managed to fix my error.
It was simply due to entering time in the incorrect format in the run window.
I was using the HH:MM format instead of just HHMM.

Thanks guys.

[RM]

Using HEC-RAS version 5.0.6

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