N.7 Self shading

Photosynthetically available radiation is provided to the WQ Module from TUFLOW for each computational cell at each water quality timestep. The WQ Module allows for modification of this TUFLOW field to incorporate the effect of light attenuation due to the presence of particulate and dissolved organic matter. This additional light extinction is computed dynamically at each water quality timestep, based on instantaneous organic matter concentrations in individual computational cells.

For labile organics, the associated additive specific light attenuation coefficient is computed as per Equation (N.26). \[\begin{equation} Ke_{\langle computed \rangle}^{OM} = Ke^{POM} \times \left[POC\right] + Ke^{DOM} \times \left[DOC\right] \tag{N.26} \end{equation}\] \(Ke^{POM}\) and \(Ke^{DOM}\) are the user specified (or default) specific shading coefficients for labile particulate and dissolved organic matter respectively, and \(\left[POC\right]\) and \(\left[DOC\right]\) are the ambient labile particulate and dissolved organic carbon concentrations (used as proxies for organic matter) in each cell. For each cell and at each timestep, this quantity \(Ke_{\langle computed \rangle}^{OM}\) is computed and returned to TUFLOW as a two- or three-dimensional field which TUFLOW then includes these in subsequent light distribution calculations.

For refractory organics (if computed), the associated additive specific light attenuation coefficient is computed as per Equation (N.27). \[\begin{equation} Ke_{\langle computed \rangle}^{ROM} = R_{CDOM}^{RDOM} \times CDOM + Ke^{RPOM} \times \left[RPOM\right] \tag{N.27} \end{equation}\] \(R_{CDOM}^{RDOM}\) is a user specified (or default) dimensionless multiplier applied to the CDOM computed in Equation (N.22), where the same CDOM is used in photolysis calculations. \(R_{CDOM}^{RDOM}\) is not a specific shading coefficient, but can be thought of as (light attenuation (/m) / CDOM (/m)), or the strength of CDOM in attenuating light. \(Ke^{RPOM}\) is the user specified (or default) specific shading coefficient for refractory particulate organic matter, and \(\left[RPOM\right]\) is the instantaneous refractory particulate organic matter concentration in each cell. For each cell and at each timestep, this quantity \(Ke_{\langle computed \rangle}^{ROM}\) is computed and returned to TUFLOW as a two- or three-dimensional field which TUFLOW then includes these in subsequent light distribution calculations.