The purpose of this project was to develop tools and knowledge that could inform stormwater management policy and investment decisions to ensure that metropolitan Adelaide’s coastal water quality is adequate to support desired environmental values, specifically the presence of seagrass meadows closer to the shore. This was achieved by repurposing/building on the Integrated Urban Water Management (IUWM) and Catchment models of the Goyder Institute’s Optimal Water Resources Mix Project and the pilot of the Adelaide Receiving Environment model (AREMp) developed by Deltares (for SA Water), as well as by exploring new lines of evidence, to assist government to:
This study has found that sufficient and suitable data exist to underpin development of computational models of the metropolitan Adelaide catchment (ICUWM model) and the adjoining coastal waters model (the AREMp) focused on suspended sediment and nitrogen. The ICUWM model and the AREMp developed in this study and the ACDC model formed by their coupling, have been demonstrated as new tools with potential to be used to inform the design of stormwater interventions aimed at achieving coastal water quality suitable for healthy seagrass. Using the models developed, this study investigated two lines of evidence to assess the impact of stormwater on coastal seagrasses. Determination of Area Specific Loads (ASLs) enabled the identification of the areas along the coast where the load limit recommended by the ACWS is exceeded based on load inputs and hydrodynamics, and where potentially seagrasses are at greater risk of loss. Use of habitat suitability maps went a step further, and took into account not only load inputs and hydrodynamics, but also resuspension and thresholds of impact for several water quality parameters including light as affected by direct and indirect shading. The ASL approach suggests that load limits are only exceeded in localized areas nearshore. Results of the habitat approach are tentative given the AREMp limitations, but also suggest low suitability nearshore, albeit as a function of wave dynamics (physical forcing) rather than light. These two lines of evidence should be viewed as complementary in the assessment of spatial impact of loads and their effect on habitat suitability. The analysis of underpinning data and operation of the models yielded new knowledge of particular relevance to both the targeting of stormwater interventions and conceptual models of the Adelaide catchments and coastal waters. While this project was not scoped to fully answer all of the questions relating to targeting stormwater interventions, based on the findings it is possible to suggest preliminary and generic guidance regarding the type and location of stormwater interventions that might best facilitate coastal water quality suitable for healthy seagrass from a suspended sediment perspective.
While this project has achieved its objectives there are several areas where greater value could be derived through more modelling, additional data collection and/or analysis. These comprise: improved understanding of seagrass distribution at times and locations of differing discharge loads, estimation of the residence time of sediment within the coastal system, increased knowledge of CDOM loads and sources, increased knowledge of fine sediment loads and sources, and upgrading of the ICUWM to deliver finer granularity in timestep and inflow node data. In addition, while the results of this project suggest that stormwater-borne sediment loads may not drive hotspots of impact in the southern parts of the coastal waters from a seagrass perspective, no assessment has been made of the impact of sediment loads on reefs. This could be achieved by augmentation of the AREMp to include habitat suitability for reefal communities.