Tidal networks 3. Landscape-forming discharges and studies in empirical geomorphic relationships

In this final part of our study [Fagherazzi et al., this issue; Rinaldo et al., this issue] we propose a simple model for predicting the local peak ebb and flood discharges throughout a tidal network and use this model to investigate scaling relationships between channel morphology and discharge in the Venice Lagoon. The model assumes that the peak flows are driven by spring (astronomical) tidal fluctuations (rather than precipitation-induced runoff or seiche, sea surge, or storm-induced tidal currents) and exploits the procedure presented by Fagherazzi et al. [this issue] for delineating a time-invariant drainage area to any channel cross section. The discharge is estimated using the Fagherazzi et al. model to predict water surface topography, and hence flow directions throughout the channel network and across unchanneled regions, and the assumption of flow continuity. Water surface elevation adjustment, not assumed to be instantaneous throughout the network, is defined by a suitable solution of the flow equations where significant morphological information is used and is reduced to depending on just one parameter, the Chezy resistance coefficient. For the Venice Lagoon, peak discharges are well predicted by our model. We also document well-defined power law relationships between channel width and peak discharge, watershed area, and flow, whereas curved, nonscaling relationships were found for channel cross-sectional area as a function of peak discharge. Hence our model supports the use of a power law dependency of peak discharge with drainage area in the Venice Lagoon and provides a simple means to explore aspects of morphodynamic adjustments in tidal systems.