3.3 Interventions
TUFLOW CATCH supports the simulation of spatially distributed catchment based measures to remove pollutants from surface waters. These measures are referred to as interventions, and might include devices such as constructed wetlands, bioretention systems, grassed swales, riparian revegetation strips or similar. TUFLOW CATCH does so by allowing users to specify:
- The location and orientation of any number of intervention devices via a user digitised polyline for each
- The pollutant removal properties of each intervention device, with these properties adjustable within a device on a pollutant by pollutant basis
TUFLOW CATCH explicitly predicts surface hydraulics (i.e. water location, depth, velocity and associated pollutant concentrations) to the spatial resolution of a grid cell. This offers a powerful potential to users when simulating interventions: it means that intervention devices can be digitised in the actual locations they are intended to be (or already exist) and to therefore interact only with the water volume that TUFLOW CATCH predicts to pass through those locations. This obviates the need to somehow parameterise this volume by relying on proportioning or other assumptions - TUFLOW CATCH explicitly avoids this for the user as a matter of course, and does not resort to top down or lumped assumptions.
TUFLOW CATCH performs removal calculations based on mass fluxes. TUFLOW CATCH does not modify concentrations directly. This is because mass flux based approaches are meaningful in terms of environmental process simulation and understanding - concentrations are very much less so. This is discussed in detail in one of TUFLOW’s global webinars. The pollutant mass removal methods offered by TUFLOW CATCH’s intervention devices are (per pollutant):
- Equation: An equation based reduction in pollutant mass flux passing through a device
- Table: A two dimensional array of proportional pollutant mass removals that vary based on (dynamically simulated) incoming pollutant concentration and flow rate
For a model demonstration of the above mass removal options, please refer to TUFLOW CATCH Tutorial Model 2.
Different mass removal methods across the above options can be used within a single TUFLOW CATCH intervention device for different pollutants. Commands are provided in detail in Section 4.5.3.5, however the fundamental premise is that interventions are specified via device blocks (akin to material based pollutant export), as follows.
Multiple methods can be specified within a device block (one for each pollutant), and subsequent block specifications can be constructed to progressively overwrite previous commands. For example, the above applies a suite of methods to ALL devices in the first block, and these are then partially overwritten (modified) for Wetland1. In a manner akin to specification of pollutant export properties, sequences of blocks affect this overwrite in the order presented. In the above example, device Wetland1 would therefore be interpreted as
Other intervention devices would be as per the ALL specification above. The relevant TUFLOW CATCH commands are described in Section 4.5.3.5.
3.3.1 Mass removal
TUFLOW CATCH offers a range of mass removal methods. Regardless of individual removal details however, all methods rely on initially computing the incoming mass flux of pollutant \(j\), \(F^{in}_j\) across cell faces intersected by the intervention polyline, at each simulation timestep. It is this mass flux (not concentration) that is subsequently adjusted by TUFLOW CATCH based on the operation of user defined mass removal methods. These methods are described following.
3.3.1.1 Equation
3.3.1.1.1 Constant
The constant mass removal method is the simplest of all available methods. At each computational timestep, this method multiplies the incoming mass flux of pollutant \(j\) by the user specified factor. This factor (in the range 0.0 - 1.0) is a mass removal factor for a given constituent, \(R_j\), where the corresponding outgoing pollutant mass flux for constituent \(j\), \(F^{out}_j\) is given by Equation (3.5).
\[\begin{equation} F^{out}_j = (1 - R_j) \times F^{in}_j \tag{3.5} \end{equation}\]
The removed flux \(F^{rem}_j\) is therefore given by Equation (3.6).
\[\begin{equation} F^{rem}_j = (R_j) \times F^{in}_j \tag{3.6} \end{equation}\]
For example, if a user sets \(R_j\) to be 0.7 for a given intervention device and constituent, then 70% of pollutant \(j\)’s mass flux is removed at each timestep, and 30% is allowed to pass.
The relevant TUFLOW CATCH commands are described in Section 4.5.3.5.1.
3.3.1.1.2 Other
Other equation based interventions may be implemented in future. Contact support@tuflow.com with requests or suggestions.
3.3.1.2 Table
The table mass removal method is an extension of the constant mass removal method in that it allows the user to specify different \(R_j\) for different (dynamically computed) environmental conditions. Specifically, this method allows the user to specify the variation of \(R_j\) as a function of both:
- Incoming flow rate (m\(^3\)/s), and
- Incoming pollutant concentration (variable units)
This approach is supported in acknowledgement that some intervention devices are able to treat higher concentrations of pollutants entering a device at lower flow rates more efficiently and effectively than the same pollutant entering at lower concentrations and higher flow rates. This variation in \(R_j\) is provided via a text based table input, with:
- A rows defining the variation in \(R_j\) with incoming concentration from left to right, for a given incoming flow rate, and
- A columns defining the variation in \(R_j\) with incoming flow rate from top to bottom, for a given incoming concentration
At each timestep, TUFLOW CATCH uses incoming concentrations and flow rates to dynamically interpolate a value of \(R_j\) from the user specified table, and applies Equation (3.5) accordingly.
Support for this mass removal method is only possible because TUFLOW CATCH solves the equations of motion and transport to predict temporally and spatially varying water velocities (flow rates) and concentrations. Such a capability is difficult to offer under, for example, the application of lumped assumptions that do not admit the explicit simulation of water as it moves through environmental systems.
The relevant TUFLOW CATCH commands are described in Section 4.5.3.5.2.