Behaviour and effectiveness of in-ground sustainable urban drainage systems in Scotland

Abstract

Infiltration trenches and filter drains are the most common types o f sustainable urban drainage systems (SUDS) in Scotland. Despite their extensive use there has been only limited examination of their performance, with the general expectation that failure through lack of maintenance and poor detailing design would necessitate reconstruction within a limited time period.

This research worked towards enhanced detailing and improved operation and maintenance of in-ground SUDS. It focused on information gained from on-site monitoring of three filter drain and three infiltration trench systems and combined the outcomes with information gathered from some 40 assessments of in-situ systems in Eastern Scotland. Performance results were produced using a newly developed scoring system, named the Schliiter Score, and results showed good performance at only 19% of systems; 19% were rated as poor and a high failure rate of 23% was discovered. Similar results were produce from a conventional environmental risk assessment identifying more than 30% of systems which require immediate mitigation measures to reduce their environmental risk. These findings give an indication of the varied performance of systems in Scotland and also show the need for knowledge enhancement in the field of in-ground SUDS. A main outcome from this research is a list of recommendations which are applicable to design engineers, developers, and authorities and contribute to achieving optimum long-term performance in terms of outflow quality and flow attenuation.

A total number of 43 sites were investigated, the majority being systems located in small-to-medium size housing developments. The average age o f the sites was 4 years, the oldest and youngest being 10 and 1 years, respectively. Almost 75% o f all systems discharge to natural watercourses, disconnecting a significant amount of impermeable area from combined sewer systems. Catchment areas varied from 392m² to 200,000m², typically consisting of road and roof surfaces. High-level by-passes are used to ensure hydraulic performance and these were found at more than 50% of all systems. Maintenance programmes were generally not in place but this study showed that regular maintenance is vital for the longevity of in-ground SUDS. A significant number o f systems require major upgrading before they may be considered satisfactory and a maintenance appraisal is provided for each system.
Hydraulic results from on-site monitoring showed good-to-satisfactory performance with flow volume reduction of 34-80% and peak flow reduction of 47-86%. The system’s design, treatment volume and soil permeability were found to be the main influence on the hydraulic performance. Simulation of flows significantly contributed to the conclusions drawn and hydraulic simulation was carried out for each o f the monitored systems. It was found that existing models did not represent in-ground filter systems adequately and an improved model was developed based on the finite-volume-method and Darcy’s law. This model, which uses the acronym FVD, was developed in collaboration with HR Wallingford Ltd and enables flow- simulation through gravel-filled SUDS. The FVD model was validated using on-site monitored data and an excellent agreement with observed data was achieved. Wallingford Software have proposed to include the FVD model in their next release o f Info works Version 6.5.

Date of Award	Jul 2005
Original language	English
Supervisor	Christopher Jefferies (Supervisor)