Control of urban runoff through the use of permeable pavements
In order to control stormwater runoff engineers and hydrologists have used various techniques to attempt to reduce or delay the volume of water which reaches the sewer system. Recently, international approaches have favoured the idea of "source control" or "on-site" retention. This technique stores water in areas close to the point at which precipitation lands. Permeable pavements and similar stormwater control devices have not been exploited in the United Kingdom. Their adoption has been hindered by a lack of knowledge of their hydrological performance. This research aims to produce information on the hydrological performance of a car park surface and to produce a model which can predict the hydrological response to varying rainfall inputs. The objective of this thesis is to examine the hydrological behaviour of a model car park surface under varying rainfall conditions. The study has involved the construction of full-scale permeable pavement model car park structures and a rainfall simulator for use in the laboratory. A monitoring procedure was developed in order to measure inputs and changes in drainage, storage and evaporation over short (less than 2 hours) and long (up to 3 months) time scales. A range of rainfall simulations were applied to the model car park surfaces which differed in intensity, duration and volume. Hydrological processes were monitored over an 18 month period. Results suggest that evaporation, discharge and retention in the structures were strongly influenced by the particle size of the bedding material and the surface blocks. In general an average of 55% of a 15 mm 11-1 rainfall event could be retained by an initially dry structure. Subsequent simulations suggest that approximately 30% of a 15 mm rainfall event could be stored by an initially wet structure (with a minimum time interval of 72 hours). Sediments were also applied to the car park structures in order to monitor the effects of clogging on hydrological performance and to quantify the ability of the structures to act as a primary screening site for sediments. The application of sediments to the structure showed that evaporation from the structure increased by as much as 25-30%. Laboratory simulation of clogging effects was also compared to data gathered from field sites and the results suggested that laboratory simulations provided a good approximation of the migration of sediments in the structure. A model of the hydrological performance of the structure has been developed to be used as a predictive tool. The model relates rainfall inputs to water retention and discharge output over consecutive rainfall events. It also allows evaporation and long-term retention by the structure to be estimated over differing lengths of dry periods. The model results indicate that discharge was predicted to an accuracy of 78% (based on a percentage difference between the observed and predicted values), and evaporation and retention were predicted to an accuracy of 80%.