ASSESSING THE IMPACT OF WET WEATHER OVERFLOWS USING EFFECTS-BASED ASSESSMENT

K. Chakravarthy (DHI Water & Environment), C. Jackson (Urban Utilities)

ABSTRACT

Water utilities in major cities in New Zealand and Australia maintain designed overflow structures to cope with extreme storm events. These structures allow wet weather overflows to discharge to receiving waters instead of overflowing or backing up into the properties. These overflows are carriers of a wide range of pollutants of concern which can adversely impact the ecological and recreational health of the urban receiving waters and endanger public health. Currently, there is no standard framework which considers both the impact of the overflows on the receiving environment and the management of the impact.

To address this limitation, Urban Utilities and DHI Water & Environment worked on implementing “Effects-based Assessment (EBA)” approach. EBA is a data driven decision making approach to quantify the impact of overflows and test mitigation options. EBA approach can test and compare the effectiveness of a wide range of mitigation options to protect or improve the water quality of the receiving environment. Using this approach, decision makers rely less on judgement calls by using reliable datasets. This approach assists to quantify the proportion of impact which is related to overflows.

EBA approach was implemented using an integrated hydrodynamics and water quality model of the Brisbane River Estuary to dynamically predict the fate and transport of bacteria levels. The model was used to assess the impact of wet weather overflows at a number of recreational sites. The impact was assessed using historical storm events with different ARIs (3 months, 1 year, and 5 years). The assessment determined the percentage of permissible swimmable hours available for primary (e.g.: swimming) and secondary recreation (e.g.: rowing) during the storm event (two days) and seven days after the storm event (in total nine days).

Results showed that the percentage of sites which support both primary and secondary recreation during the 9-day period for a 3-month ARI is 60%. This number decreased to 38% for a 1-year ARI and 24% for a 5-year ARI. In contrast, the percentage of sites which support secondary recreation only during the 9-day period for a 3-month ARI, 1-year ARI, and 5-year ARI are 81%, 76%, and 54% respectively. About 86% of the sites support primary recreation for at least 50% of the 9-day period for a 3-month ARI storm event. This reduced to 70% for a 1-year ARI, and 46% for a 5-year ARI. Interestingly, the percentage of sites which support secondary recreation for at least 50% of the 9-day period reduced from 100% to 96% to 91% for 3-month ARI, 1-year ARI, and 5-year ARI respectively.

Urban Utilities plans to use this EBA approach to support the transition towards more proactive management of its infrastructure, whereby different processes and systems can be operated to mitigate disturbances before they have adverse impacts on the receiving environment. This is envisaged to have large potential for economic savings and more effective protection of the environment.