Modelling the impact of Water Sensitive Urban Design technologies on the urban water cycle

Abstract

Alternative stormwater management approaches for urban developments, also called Water Sensitive Urban Design (WSUD), are increasingly being adopted with the aims of providing flood control, flow management, water quality improvements and opportunities to harvest stormwater for non-potable uses. WSUD structures (WSUDs) are typically small, decentralized systems for managing stormwater runoff near the source. These systems interact with the urban hydrological cycle, modifying the evapotranspiration, runoff and groundwater recharge fluxes. It is challenging to quantify these hydrological changes because of the cost and complexity of modelling multiple WSUD systems in larger scale urban catchments. For this reason, new modelling tools are needed. These tools must be simple enough to be computationally efficient, while still describing the observed hydrological responses of urban catchments. The models must be able to simulate both the response of single WSUDs and many coupled WSUDs in an urban catchment. This thesis aims to develop new models of two WSUD technologies: green roofs and infiltration trenches/soakaways. In particular the thesis has the following objectives: 1. To identify and develop new models of green roofs and infiltration devices relevant for urban drainage applications, and integrate them into urban hydrological models. 2. To quantify the long term hydrological performance of green roofs and infiltration devices using a statistical analysis of WSUD performance. 3. To model the interaction of infiltration based WSUDs with groundwater. 4. To assess a new combination of different WSUD techniques for improved stormwater management. 5. To model the impact of a widespread implementation of multiple soakaway systems at the catchment scale. 6. Test the models by simulating observed data describing the performance of single WSUD units, and the performance of multiple systems at a catchment scale. To address these aims, new models of green roofs and soakaways are developed and tested using observations from several urban catchments. The models are used to quantify the hydrological performance of single devices relevant for urban drainage applications. Moreover, the coupling of soakaway and detention storages is also modeled to analyze the benefits of combining different local stormwater management systems. These models are then integrated into urban drainage network models and groundwater models in order to analyze the impact of stormwater infiltration and local detention on drainage networks and groundwater flows. Results show that soakaways/infiltration trenches and green roofs significantly reduce annual stormwater runoff. Annual runoff from green roofs is 43-68% of the incoming rainfall and 0-62% for soakaways. Peak flow and volume reductions during single events are also quantified as a function of the return period. Using a part of a soakaway as detention storage significantly improves its ability to reduce single event peak runoff without significant changes to its annual performance. Peak flow and annual runoff reductions are quantified for different soakaway and detention volume combinations. These systems also avoid problems of sewer network surcharge in a small catchment during a 10 year return period event. The thesis quantifies the hydrological performance of infiltration devices interacting with groundwater. A threshold distance between infiltration devices and groundwater is estimated in order to classify whether infiltration devices are affected by groundwater or not. The threshold distance is determined as function of the soil hydraulic conductivity and the storage volume of the infiltration device. For instance, it is shown that in clay soils, infiltration trenches must be more than 11-12m above the water table if they are to be fully effective. Widespread stormwater infiltration leads to increased groundwater recharge and the risk of groundwater flooding in areas with shallow groundwater. The increased occurrence of groundwater seepage above terrain is quantified in a case study by a catchment hydrological model that is calibrated to observations. Moreover, the performance of existing stormwater infiltration systems is affected by landuse changes in other parts of their catchment. These changes were quantified for the case study by a model and observations over a 20 year period. It was shown that urbanization with widespread stormwater infiltration increased the risk of groundwater flooding. WSUDs are useful technologies for controlling urban stormwater runoff and the models presented in this thesis can help by simulating their hydrological impact. Careful engineering design is required to ensure that optimal results are achieved and to avoid unexpected outcomes such as increased groundwater flooding.