A GIS-based methodology for selecting stormwater disconnection opportunities.

The purpose of this paper is to introduce a geographic information system (GIS)-based decision support tool that assists the user to select not only areas where (retrofit) sustainable drainage systems (SuDS) could be implemented within a large catchment (>100 ha), but also to allow discrimination between suitable SuDS techniques based on their likely feasibility and effectiveness. The tool is applied to a case study catchment within London, UK, with the aim of increasing receiving water quality by reducing combined sewer overflow (CSO) spill frequency and volume. The key benefit of the tool presented is to allow rapid assessment of the retrofit SuDS potential of large catchments. It is not intended to replace detailed site investigations, but may help to direct attention to sites that have the greatest potential for retrofit SuDS implementation. Preliminary InfoWorks CS modelling of 'global disconnections' within the case study catchment, e.g. the removal of 50% of the total impervious area, showed that CSO spill volume could be reduced by 55 to 78% during a typical year. Using the disconnection hierarchy developed by the authors, the feasibility of retrofit SuDS deployment within the case study catchment is assessed, and the implications discussed.

Towards a generic rainfall-runoff model for green roofs.

A simple conceptual model for green roof hydrological processes is shown to reproduce monitored data, both during a storm event, and over a longer continuous simulation period. The model comprises a substrate moisture storage component and a transient storage component. Storage within the substrate represents the roof's overall stormwater retention capacity (or initial losses). Following a storm event the retention capacity is restored by evapotranspiration (ET). However, standard methods for quantifying ET do not exist. Monthly ET values are identified using four different approaches: analysis of storm event antecedent dry weather period and initial losses data; calibration of the ET parameter in a continuous simulation model; use of the Thornthwaite ET formula; and direct laboratory measurement of evaporation. There appears to be potential to adapt the Thornthwaite ET formula to provide monthly ET estimates from local temperature data. The development of a standardized laboratory test for ET will enable differences resulting from substrate characteristics to be quantified.

The use of deconvolution techniques to identify the fundamental mixing characteristics of urban drainage structures.

Mixing and dispersion processes affect the timing and concentration of contaminants transported within urban drainage systems. Hence, methods of characterising the mixing effects of specific hydraulic structures are of interest to drainage network modellers. Previous research, focusing on surcharged manholes, utilised the first-order Advection-Dispersion Equation (ADE) and Aggregated Dead Zone (ADZ) models to characterise dispersion. However, although systematic variations in travel time as a function of discharge and surcharge depth have been identified, the first order ADE and ADZ models do not provide particularly good fits to observed manhole data, which means that the derived parameter values are not independent of the upstream temporal concentration profile. An alternative, more robust, approach utilises the system's Cumulative Residence Time Distribution (CRTD), and the solute transport characteristics of a surcharged manhole have been shown to be characterised by just two dimensionless CRTDs, one for pre- and the other for post-threshold surcharge depths. Although CRTDs corresponding to instantaneous upstream injections can easily be generated using Computational Fluid Dynamics (CFD) models, the identification of CRTD characteristics from non-instantaneous and noisy laboratory data sets has been hampered by practical difficulties. This paper shows how a deconvolution approach derived from systems theory may be applied to identify the CRTDs associated with urban drainage structures.

The prediction of solute transport in surcharged manholes using CFD.

Solute transport processes occur within a wide range of water engineering structures, and urban drainage engineers increasingly rely on modelling tools to represent the transport of dissolved materials. The models take as input representative travel time and dispersion characteristics for key system components, and these generally have to be identified via field or laboratory measurements. Computational Fluid Dynamics (CFD) has the potential to reveal the underlying hydraulic processes that control solute transport, and to provide a generic means of identifying relevant parameter values. This paper reports on a study that has been undertaken to evaluate the feasibility of utilising a CFD-based approach to modelling solute transport. Discrete phase modelling has been adopted, as this is computationally efficient and robust when compared with the time-dependent solution of the advection-dispersion equation. Simulation results are compared with published laboratory data characterising the dispersion effects of surcharged manholes, focusing specifically on an 800 mm diameter laboratory manhole for a flowrate of 0.002 m(3)/s and a range of surcharge depths. Preliminary indications are that the CFD results adequately replicate the measured downstream temporal concentration profiles, and that a threshold surcharge depth, corresponding to a change in hydraulic regime within the manhole, can also be identified.

The utilisation of engineered invert traps in the management of near bed solids in sewer networks.

Large existing sewers are considerable assets which wastewater utilities will require to operate for the foreseeable future to maintain health and the quality of life in cities. Despite their existence for more than a century there is surprisingly little guidance available to manage these systems to minimise problems associated with in-sewer solids. A joint study has been undertaken in the UK, to refine and utilise new knowledge gained from field data, laboratory results and Computational Fluid Dynamics (CFD) simulations to devise cost beneficial engineering tools for the application of small invert traps to localise the deposition of sediments in sewers at accessible points for collection. New guidance has been produced for trap siting and this has been linked to a risk-cost-effectiveness assessment procedure to enable system operators to approach in-sewer sediment management pro-actively rather than reactively as currently happens.

Fine sediment retention in storage chambers: an assessment of time-dependent effects.

The optimisation of the design of a storage chamber is generally based upon some measure of the chamber's sedimentation efficiency. In the UK, chambers that minimise the deposition of fine sediments are preferred. Previous laboratory and CFD-based studies to measure efficiency have focused on steady flow conditions. However, both the flow hydraulics within a storage chamber and the pollutant loading in the incoming sewage vary markedly during storm events. This paper outlines a CFD-based approach for determining "overall" chamber efficiency. The approach employs an unsteady volume-of-fluid multiphase model and stochastic particle tracking. Preliminary results from a simplified two-dimensional model are presented.