Impact Of Run-off Coefficients On Urban Drainage Design
Authors:   ,   Cork Institute of Technology, O'Sullivan Ross 
Published: September 2024

This study assesses the impacts that runoff coefficients have on an urban drainage design. The first part of the report reviews how drainage design is carried out. This assesses the traditional hand calculations method, where charts and graphs are used, versus the more modem approach of using a computer modelling system such as WinDes. Both approaches were reviewed, as both will be used to design drainage systems at a further stage in the project. The objective of this project is to see how runoff coefficients impact on urban drainage design. The methodology used to determine this, is to test six different test surfaces. The test surfaces selected for testing during the implementation phase are based on the most common surfaces found in Ireland, in an urban area, with the addition of green roofs. The test surfaces are concrete tile roof, slate tile roof, concrete pavement, tarmac, intensive green roof and an extensive green roof. To design these test surfaces, the building standards and details for all types of surfaces had to be reviewed. After this, a scaled down version of each test surface was designed as closely as possible to the industry standard, in order to achieve the most accurate and applicable results. Sustainable Drainage Systems (SUDS) were assessed in this report, as SUDS components, in the form of green roofs, will be tested during testing phase of the project. SUDS are a great way to reduce the amount of runoff entering a drainage system. SUDS are also excellent at delaying the time taken for runoff to reach the drainage system. This has positive implications on the underground pipe network, as it is under less pressure, and can possibly be sized smaller. In addition, positive effects will occur at the discharge point of the drainage system, which is typically a river in Ireland. The rivers will be under less pressure, as the volume of water from the drainage system will be reduced, and the time the runoff takes to enter the river will be delayed. Traditional developments in Ireland were designed to transport the runoff into the drainage system as quickly as possible. A literature review was then carried out on the different runoff coefficients. It was intended to review national studies first. However, no national studies were found on runoff coefficients for the surfaces of interest. International studies in the U.K, Europe and U.S.A were then reviewed. The green roofs proved to be the most interesting, as the analysis carried out is a lot more detailed. For the green roofs, it was found that a typical green roof can have a runoff coefficient of 0.5 i.e. 50% of the rainwater that hits the roof enters the gutter, the other 50% gets retained. The runoff coefficient values of interest when reviewing these studies were the runoff coefficient values during large rain events, as a drainage system will be designed for a 5-year return period. The average and maximum runoff coefficients for significant rain events were 0.61 and 0.71 respectively. The maximum runoff coefficient value found in the US (EPA), US (Georgia), Belgium and New York was 0.8, 0.64, 0.8 and 0.79 respectively. The average runoff coefficient values for the concrete tile, slate tile, concrete pavement and tarmac, were 0.75, 0.85, 0.9 and 0.95 respectively. The next part of this report assessed the use of WinDes, familiarity with this program was required during this stage of the project, even though, it will not be implemented until the implementation phase. WinDes is a common tool used by industry to design drainage systems. An investigation into how it works and what it does was carried out. The last chapter outlines the methodology and plan for the implementation phase. This sets out a list of objectives, and the plan of action required to achieve these objectives.