title: Effects of local on-site stormwater detention systems on regional catchment hydrology
Creators: name: Ronalds, Rodney
Publisher: name: Griffith University
Year: 2019
Subjects: subject: Stormwater detention systems
subject: Regional catchment hydrology
subject: Water quality
subject: Regression analysis
subject: Numerical modelling

description: Stormwater detention is an essential component in conventional urban drainage systems. Its function is to attenuate the increase in peak discharge of stormwater runoff that inevitably results from the urbanisation of land. Its purpose is to manage adverse impacts to downstream hydrology, including flood, erosion and degradation of water quality. Theoretically, detention should be a stormwater management technique that is adopted at carefully selected locations using comprehensive catchment wide analysis. Installing detention at inappropriate locations within the regional catchment is known to have potentially adverse impacts on the regional hydrology. The size and performance of detention can also be optimised when there is freedom to select its location in the regional catchment. In practice however, stormwater detention is commonly mandated for all new urban development projects, regardless of their location within the regional catchment. Where stormwater detention is mandated for all urban development sites, the allowable outlet flow rate should also ideally be established using catchment wide analysis. In practice however, the allowable outlet flow rate is typically specified as the maximum pre-development flow rate from the development site for design storms of equal return intervals. This research builds upon many previous studies that have highlighted this practice as a serious concern. The usage of variable rainfall distribution probabilities for the creation of hydrographs in the engineering design of infrastructure like stormwater detention is not common, largely due to the complexities of model set-up, the analyses of results required to arrive upon a deterministic design outcome, and a lack of prescriptive guidelines for practitioners. As part of this research this limitation in current practice has been shown to be the cause of a significant extent of failure to meet objectives in peak flow management of development site discharge. This research has also shown conclusively that the selection of the number of rainfall patterns used for the design of a detention system is proportional to its success in achieving peak flow reduction objectives. This thesis provides research into the development of a practical solution to these issues, whereby stormwater detention can continue to be mandated for all new development projects, with regional hydrologic impacts able to be considered without full-scale regional catchment assessment. At the core of this is the development of a numerical model for hydrologic assessment of regional catchments with urbanisation and detention installed at varying hypothetical locations throughout. Under the dominating influence of rainfall pattern variability, recurring trends are revealed that describe the impact of urbanisation and detention on the regional catchment peak discharge flow rate. The trends are shown to be respective of two key factors, being the ratio of development site area to the regional catchment area and the location of the development within the regional catchment. With these two input parameters, it is shown to be possible to estimate the mean impact of urbanisation and detention on the regional catchments peak outflow at a specific downstream location. A new system of equations is presented within this thesis that is considered to be a significant tool for better ensuring that detention is designed with due consideration given to its location and impacts on the hydrology of its regional catchment. This outcome is expected to provide a means for decision making regarding the installation or avoidance of stormwater detention in favour of infrastructure upgrades. The development of the new equations has used a regression analysis of numerical modelling results taken from a number of case study catchments located along the eastern coastline of Australia. This geographical area has been selected due to the intensity of urban development, availability of recorded rainfall data, and the prevalence of local government policies that mandate the usage of stormwater detention for urban development projects in these regions. Whilst the significance of the spatial variance of rainfall patterns is recognised, the usage of a Monte Carlo technique to account for variable rainfall probability has been adopted as a means to universalise the results. In a world of future climate change and uncertainty, the variability and unpredictability of the temporal patterns of rainfall are a major concern. This research serves to better understand these issues and provides a direction forward for the design and assessment of stormwater detention with due consideration given to its potential impact on the hydrology of its regional catchment.