Modelling Effects on Marine Water Quality Due to Urban Stormwater Discharge and Climate Change

In this paper the role of marine models in assessing urban stormwater discharges in relation to marine water quality is presented. A calibrated hydrodynamic model is used to quantify the dynamics of stormwater discharges along the Eastern Bays area in Auckland. The effects of future sea level rise on the dynamics of the discharges and subsequent changes in water quality in the marine receiving environment are discussed.

Traditionally, downstream static sea level conditions for urban flood models are developed based on estimates of extreme sea levels from relatively short duration tide records combined with regional estimates of sea level rise scenarios. Applying a regional estimate of future sea level rise maybe appropriate on the open coast where downscaling effects of sea level rise may be negligible. However, in settings where complex inshore topography exists (such as the many harbours and embayments in New Zealand) regional scale sea level rise scenarios may not provide realistic estimates of local scale sea levels.

Climate change modules within the MIKE by DHI software enable the sensitivity of near shore sea level conditions to be assessed under a range of global sea level rise scenarios. Similarly, estimates of changes in precipitation can be determined from a number of global circulation models. Outputs from these modules can be used to assess the effects of changes in precipitation on stormwater discharges and to quantify local scale effects of sea level rise.

The paper discusses the effects of climate change on water quality in the marine receiving environment based on predicted changes in hydrodynamics, precipitation, mixing and the dynamic nature of the interface between the marine environment and the urban stormwater system. From a planning perspective, the need for quantifying the joint probability of extreme rainfall events and sea levels are discussed.