D.9 Mass balance - Interventions

This section repeats the analysis of Section D.8, but includes the action of two interventions using Demonstration_009.tcc.

D.9.1 Concept

The concept is the same as that described in Section D.8.1, but includes consideration of the mass removed by interventions so that the pollutant mass generated, less that removed by settling and interventions, should equal that exiting the TUFLOW CATCH domain. This was undertaken by applying bespoke post processing tools (which are the same as those used above) to the predictions of Demonstration_009.tcc. The intention was to show that the cumulative released mass and the cumulative mass leaving the overall domain are different by an amount equal to that removed by the interventions. This demonstrates mass balance.

D.9.2 Configuration

The TUFLOW CATCH simulation Demonstration_009.tcc was configured to be the same as Demonstration_008.tcc but with the following modifications:

• Pollutant Export configuration
  • Two intervention devices
    • Wetland
    • Riparian revegetation strip
  • Two removal methods
    • Lookup table (PFAS)
    • Constant (Tailings)
• This simulation cannot be run in demo mode as it too long due to the time taken in writing the highly temporally resolved outputs. It must therefore be run with a licence

D.9.3 Method

Mass balance was computed separately for tailings and PFAS so as to demonstrate performance over different pollutant generation methods. A slightly different numerical recipe was applied to each, as described above in Section D.8.3. The only difference in this case was the inclusion of mass removal due to the two interventions.

D.9.4 Results

In both Shear1 and Washoff1 cases, the key points of comparison are between:

• The final cumulative released, removed and exiting masses. This is a single number to number comparison, with (cumulative released - cumulative removed) required to be equal to cumulative exiting mass. In the Shear1 case (i.e. Tailings), a timeseries comparison between released and exiting mass is also possible
• The Shear1 method also allows for a timeseries comparison of water borne mass of pollutants, computed indirectly from mass balance and directly from post processing of concentration and volume cell outputs

The MATLAB scripts used to perform all post processing are provided in the Modelling\TUFLOWCATCH\matlab folder in the Demonstration Model download, and are self explanatory. Some functions from the TUFLOW FV MATLAB toolbox are required to execute these scripts.

D.9.4.1 Tailings

For Tailings simulation using the Shear1 export method:

• Exported tonnage: 477.98 (via the shear1 method)
• Removed tonnage: 105.49 (via interventions)
• Received tonnage: 372.49 (entering the downstream polygon)

Timeseries of these are presented in Figure D.11. Note that the shape and end points of the export and receiving curves are different (as expected). Specifically:

• The shift in time between exported and received masses reflects TUFLOW CATCH’s simulation of on-ground hydraulics and associated pollutant transport: time is taken (and explicitly simulated) between upstream pollutant export and subsequent delivery downstream
• The exported curve is non-monotonic and this reflects dynamic settling of previously released pollutant occurring in the catchment simulation
• The endpoints differ by exactly the amount removed due to interventions, as expected: the removed mass does not exit the model through the downstream polygon. The final masses between (exported - removed) and (received) are the same.

Figure D.11: Tailings mass conservation, with action of interventions

An interesting feature of the above analysis is that TUFLOW CATCH predicts a greater tonnage of Tailings is exported (not received downstream) from the overall catchment when interventions are put in place (477.98 tonnes with intervention compared to 451.92 tonnes without interventions). This is because the Tailing mass that is being removed is not able to settle and be resuspended downstream. Rather, fresh material - that would otherwise not have been released - is being liberated from regions downstream of interventions instead of settled material being (effectively) recycled.

As above, a complementary measure of mass conservation performance is to compare the timeseries evolution of the total Tailings mass \(M\) in the water column at every timestep \(i\) computed reported concentrations (see Equation (D.1)) and mass flux analysis (see Equation (D.2)). These two independent calculations of water column mass are presented as timeseries in Figure D.12. As expected, the timeseries differ throughout and at their end point in this case where interventions are included, confirming that:

• Mass is progressively removed
• The final difference in the two masses is the same as the mass removed by the intervention measures

The difference between these two mass timeseries provides the corresponding cumulative timeseries of mass removal, which has also been included in the figure. As expected, the final value of the cumulative removed mass equals the difference between the two mass computation methods, most obviously at simulation end.

Figure D.12: Tailings mass comparison, with action of interventions. The divergence is expected and correct

D.9.4.2 PFAS

For PFAS simulation using the Washoff1 export method, with interventions:

• Generated tonnage: 17.37 (total, via the washoff1 method)
• Removed tonnage: 0.96 (total removed by interventions)
• Received tonnage: 3.49 (entering the downstream polygon)
• Remaining dry store tonnage: 12.88 (remaining in the catchment as not washed off after all rainfall has ceased)
• Remaining water phase tonnage: 0.04 (remaining in the catchment in trapped water after all rainfall has ceased)

The sum of received and remaining tonnage (3.49 + 12.88 + 0.04 tonnes) equals that generated less removed by interventions (17.37 - 0.96 tonnes), as expected.