Detention Time Design Criteria to Reduce the Risk of Excessive Algal Growth in Constructed Waterbodies

Urban waterbodies are highly valued by the community; however excessive algal growth can be a serious management issue. It can have adverse effects on aquatic ecosystem function, aesthetics and public amenity. Cyanobacterial species (Blue-green algae) are of particular concern because of their ability to produce harmful toxins. A range of measures are available to control excessive algal growth but they are often not possible or inappropriate for small, shallow urban waterbodies. A theoretical approach, using literature values for in situ algal growth rates is developed in this paper. Various factors that influence algal growth are considered and incorporated in to the model design, including: hydraulic detention time, in situ growth rates, temperature and starting algal cell concentrations. Based on the algal growth rate curves (for different ambient water temperatures) design detention time values are proposed. A probabilistic design approach is proposed that takes into account the inherent variability in aquatic ecosystems.