Impacts of Urbanization and Event Magnitude on Runoff Contributing Area and Runoff Coefficients 13th International Conference on Urban Drainage, 7-12 September 2014 Nora Sillanpää Postdoctoral researcher, D.Sc.(Tech.) Water and environmental engineering research group nora.sillanpaa@aalto.fi

Motivation and objectives The main objective: to identify and quantify the impacts of urban development and storm event magnitude on the runoff contributing area and volumetric runoff coefficients. What is the true magnitude of the changes that urbanization causes to runoff generation? There is lack of studies monitoring the hydrological effects of an incremental increase in catchment imperviousness, from rural to urban conditions, at the same catchment (e.g. Shuster et al., 2005; Dietz and Clausen, 2008) What is the runoff contributing area and how it changes between storms? Uncertainties in the estimates for catchment imperviousness cause substantial errors in runoff modelling (Pitt, 1987; Lee and Heaney, 2003; Park et al., 2009) The combined impact of storm event magnitude and impervious surfaces on runoff generation has not been widely studied (Said and Downing, 2010) What kind of hydrological principles can be established in order to promote the redefining the current stormwater management principles? Lee and Heaney (2003): it is necessary to determine a lower threshold of storms that initiate runoff from areas other than directly connected impervious surfaces in order to prorate the costs of stormwater systems among major and minor storms.

Study catchments Developing catchment (DEV) Year 2001: a forested catchment, total area 8.5 ha, total impervious area (TIA) 1.5 % Year 2006: a medium-density residential area, total area 13 ha, TIA 37 % 2001-2003 2004 2005-2006 Monitoring data: continuous hydrological and water quality data (2001-2006), temporal resolution 2-10 min, measured at the catchment outlet

Study catchments Low-density residential catchment (LOW) DEV HIGH Low-density residential catchment (LOW) Total area 31 ha, TIA 20 % High-density residential area (HIGH) Total area 13 ha, TIA 50 %

Data and methods Data from the warm period rainfall-runoff events (2001-2006): 126-282 events per catchment For all events: 1) total rainfall depth, PTOT (mm), 2) direct runoff depth, RDIR (mm), 3) volumetric runoff coefficient, CVOL (-) PTOT RDIR CVOL= Event data were classified into different groups: Event groups based on storm event magnitude: minor storms and major storms Two development phases for the DEV catchment: the predevelopment period (TIA 1.5%, the years 2001-2003) the post-development period (TIA 32…37%, the years 2005-2006) Statistical analysis: Simple linear regression: the effective impervious areas (EIA) and 2) initial abstractions (e.g. Chiew and McMahon, 1999) Multiple linear regression with a dummy variable z indicating the event groups

Effective impervious area, EIA Total impervious area, TIA (%) Results: effective impervious area (EIA) Effective impervious area, EIA 32…37% 1.5% Total impervious area, TIA (%) 50% 20%

Results: minor and major storms DEV catchment 2005-2006 DEV catchment 2001-2003

Results: precipitation threshold Multiple linear regression analysis of minor and major storms: Event storm depth, PTOT (mm) Direct runoff, RDIR (mm) Threshold PTOT 17…20 mm For all catchments and both development phases, statistically significant (p<0.05) regression coefficients were obtained for the dummy variable indicating different storm groups For all study catchments, a threshold rainfall depth was defined as the intersection of the regression lines for the minor and major storms Threshold precipitation for each catchment: DEV catchment (TIA 1.5%): PTOT 16.2 mm LOW catchment (TIA 20%): PTOT 19.8 mm DEV catchment (TIA 32…37%): PTOT 18.5 mm HIGH catchment (TIA 50%): PTOT 17.7 mm

Results: changes in runoff coefficients During the construction works, the greatest (relative) changes in runoff generation occurred in the volumetric runoff coefficients during minor storms. Small storms are key events when the stormwater management objective is to maintain predevelopment water balance.

Results: the need for local values The observed CVOL values do not well correspond to those derived from sewer sizing. Local values also differ from those presented in international literature. Small storm coefficients for the US (Roesner et al. 1990) The runoff coefficients in the local stormwater manuals and in the international literature are too high compared with the observed volumetric runoff coefficients (CVOL) for most events.

Conclusions: hydrological principles It is possible to identify a threshold precipitation that can be used to divide storms into different categories based on rainfall depth and runoff contributing area. Event storm depth, PTOT (mm) Direct runoff, RDIR (mm) Threshold PTOT 17…20 mm Storm depth > 17-20 mm ”Infrequent” large storm 20% of all events Impervious and pervious areas Large runoff coefficient Storm depth <17-20 mm Frequently occurring small storm 80% of all events Effective impervious area Small runoff coefficient Predevelopment water balance Water quality Flood control

Thank you! During the construction works, the greatest (relative) changes in the runoff generation occurs during the frequent small storms. Local data are needed for redefining management criteria and design objectives. It is possible to identify a threshold rainfall depth that inidcates a change in direct runoff response. This threshold can be used to classify storms based on the event magnitude, runoff contributing area, volumetric runoff coefficient, and the management objective. This study was funded by the Academy of Finland, the European Regional Development Fund, Maa- ja vesitekniikan tuki ry., Aalto University School of Engineering, Sven Hallin Research Foundation and Tekniikan edistämissäätiö.