A new approach of monitoring and physically-based modelling to investigate urban wash-off process on a road catchment near Paris
Graphical abstract
Introduction
It is predicted that by 2050, 64% of the “developing world” and 86% of the “developed world” will be urbanized (Montgomery, 2008). This trend of rapidly increasing urbanization requires better understanding of the urban wash-off phenomenon in order to develop more advanced management strategies.
Among the various substances of urban stormwater pollutants, suspended solids, heavy metals and Polycyclic Aromatic Hydrocarbons (PAHs) are widely considered as the major causes of contamination in receiving environments (Fletcher et al., 2013, Zoppou, 2001). Most of these heavy metals and PAHs are found in the particulate phase and associated with fine particles (Aryal et al., 2010, Bressy et al., 2012, Gasperi et al., 2014). Therefore, the studies of stormwater quality can focus on the urban sediment transport during stormwater events.
Numerous urban stormwater quality models exist, however, most of them are still unable to adequately reproduce urban wash-off dynamics (Dotto et al., 2012, Egodawatta et al., 2007, Elliott and Trowsdale, 2007). One of the major reasons is the lack of available and reliable local data. According to Duncan, 1995, Vaze and Chiew, 2003, accurate urban stormwater quality models require detailed spatial and temporal data of rainfall intensity, water runoff characteristics and pollutants’ features (e.g. Weight, Size, Settling velocity). Since it is impossible to collect sufficient water runoff data over different temporal and spatial points of an urban catchment, the application of Full Shallow-Water equations with extremely high-resolution topographic data is a promising approach for representing stormwater runoff processes (Grayson et al., 1992a, Grayson et al., 1992b). Another challenge of modelling urban stormwater quality is the shortage of physical descriptions of pollutant wash-off mechanisms. Until now, current urban wash-off models are generally based on exponential wash-off functions (e.g. SWMM, M-QUAL, HSPE, STORM etc.), assuming the rate of particle loss on a catchment scale is directly proportional to the availability of the pollutants on the road surface and to the water flow. With these equations, urban spatial heterogeneities are neglected, leaving models to rely on extensive calibration of empirical wash-off coefficients, a fact that limits their predictive capacities (Tsihrintzis and Hamid, 1997). Thus, greater insight into the physical processes of particulate detachment and transport will provide a more detailed understanding of the movement of pollutants in urban landscapes.
Only very few studies have been performed for the physically-based modelling of urban wash-off processes. Shaw et al. (2006) proposed a saltation-type wash-off model in which particles were repeatedly detached from the impervious surface by raindrop impacts and were transported laterally by overland flow while settling back to the surface. Massoudieh et al. (2008) presented a wash-off model in which detachment and reattachment of contaminants were considered as rate-limited processes and the detachment rate was assumed to be a function of flow velocity by a power expression. These existing models have provided a basic perception of developing new mechanistic wash-off models for urban surfaces. However, the wash-off processes in the above models were not combined with two-dimensional water-flow simulations, and the detachments were only represented by single effects of raindrops impacts (Shaw et al., 2006) or flow power influences (Massoudieh et al., 2008). These inadequate assumptions limit the reliability of such physically-based models for stormwater quality modelling in urban areas (Deletic et al., 1997, Dotto et al., 2012, Wijesiri et al., 2015).
In this study, the Hairsine-Rose (H-R) model (Hairsine and Rose, 1992a, Hairsine and Rose, 1992b) coupled with the FullSWOF (Full Shallow-Water equations for Overland Flow) modelling system (Delestre et al., 2014, Le et al., 2015) is applied. Unlike other physically based approaches, the H-R model calculates raindrop-driven detachment, flow-driven detachment and deposition processes separately, with the net outcome being the difference between these process groups. The H-R model also simulates a deposited layer that differs from the original soil in its composition and detachability, which allows us to distinctively model urban dust and road pavement.
This study is the first time that the H-R model is applied and analyzed within the context of urban stormwater wash-off, using the example of a road catchment near Paris. With this new approach, our objective is to examine urban surface wash-off dynamics for several stormwater events. This approach couples detailed monitoring surveys and physically-based modelling, which may help to advance the understanding of stormwater wash-off mechanisms. The following sections will provide details on monitoring surveys for the road catchment, model configurations, and sensitivity analysis.
Section snippets
Study site
A small urban road catchment near Paris (Le Perreux sur Marne, Val de Marne, France), including a segment of high traffic volume (more than 30,000 vehicles per day) and its adjacent sidewalk and parking zones, are selected for this study. A gutter is located between the road and the sidewalk, allowing water flow from the upper part of the catchment to the sewer inlet (Fig. 1). The total surface of the study basin is 2661 m2, where approximately 65% of the surface are roads, 30% are sidewalks,
Rainfall events selection
56 rainfall events have been identified during the study period of September 20, 2014 to April 27, 2015. Analysis of rainfall depth, mean intensity, event duration and antecedent dry days are performed for all the precipitation events in order to highlight their characteristics (Fig. 7).
According to Fig. 7(a) and (b), we can observe that most rainfall events within the study area of Eastern Paris, are considered low. In fact, more than 88% of rainfall events have a rain depth of less than 8 mm,
Perspectives
In the previous sections, we introduced a novel approach to model urban stormwater quality with the physically-based FullSWOF and Hairsine-Rose (H-R) model. This new method has good potential for future studies in the following two aspects: (1) a helpful research approach for increasing understanding of urban wash-off mechanisms; (2) a useful tool for designing innovative stormwater management technologies.
Current urban wash-off models rely on empirical, catchment-scale functions (Sartor
Conclusion
In the current study, the monitoring survey coupled with a simulation approach by the physically-based FullSWOF model and the process-based Hairsine-Rose model (H-R) is applied to a road catchment near Paris, in order to model the dynamics of urban wash-off. Centimetric resolution of topographic data, continuous measurements of rainfall intensities, water flows and turbidity measurements, as well as road dust and stormwater samplings are used in the model in order to obtain realistic input
Acknowledgement
The research work of PhD student Yi Hong is financed by ANR-Trafipollu project and Ecole des Ponts ParisTech (No. 13/66). Firstly, the authors would like to thank OPUR (Observatoire des Polluants Urbains en Île-de-France) for providing the platform for changing idears and elaborating collaborations with different researchers from various institutions. The author would also want to thank B. Béchet (IFSTTAR) and B. Soheilian (IGN) for providing valuable experimental data which help to implement
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