Differentiation of Mass and Flow Limited Rainfall-Runoff Events for Overland Flow from Small Urban Catchments

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ABSTRACT
Water quantity and quality are coupled phenomena in urban rainfall-runoff. These rainfall-runoff quantity and quality relationships have been significantly altered by the built environment and associated anthropogenic activities. Understanding the relationship between quantity and quality of urban overland flow for a given catchment facilitates development of in-situ wet weather control. One aspect of this relationship for a given catchment is the differentiation between mass and flow-limited event delivery for water quality indices. While there have been many incarnations of mass-limited behavior, such as the much-maligned mass-based first-flush, there have been fewer investigations for differentiating mass limited behavior from flow-limited behavior. The concept of a firstflush can be shown to be one limiting form of such coupled phenomena for small urban catchments, a mass-limited phenomenon. While the common assumption of mass-limited behavior is generally implicitly assumed for urban catchments (generally irrespective of size), this study illustrates that such behavior is only one limiting form of quantity-quality phenomena. This study examines such limits of transport in urban rainfall-runoff and differentiates between coupled quantity and quality behavior in order to classify rainfallrunoff events. Results from two small urban catchments are examined, a fully-paved catchment in Baton Rouge, Louisiana and a fully-paved catchment in Cincinnati, Ohio. Both sites were urban, paved, and transportation land use and the upper end of the watershed. Results indicate that the derived physical-based differentiation criteria are able to fit experimental data from these two different sites. Results are useful for mass delivery differentiation, physical understanding and for use in models such as SWMM.

INTRODUCTION
The quantity and quality relationship of rainfall-runoff in urban areas has been significantly changed by both the impervious built environment and anthropogenic activities. For example, the combination of pavement and automotive transportation in the urban environment has a significant impact on the relationship between quantity and quality. Impervious pavement functions as an efficient water quantity conveyance surface for transport of water quality constituents as compared to more pervious preconstructed conditions such as soils and vegetation. Impervious pavement cover conditions also significantly alter the relationship between rainfall and runoff. This altered relationship between rainfall and runoff results in increased peak flow, increased runoff volume, reduced infiltration, reduced evaporation and reduced depression storage. Especially if the pavement is asphaltic, there is an increase in urban temperatures and runoff temperatures.

Common water quality indices associated with urban rainfall-runoff include a wide gradation of particulate matter, ranging from colloidal (< 1 μm), suspended (~ 1 to 25 μm), settleable (~ 25 to 75 μm), sediment (~ 75 to 4750 μm), and gross solids (>4750 μm). It is becoming more common to measure this material in terms of an aggregate mass measurement such as suspended sediment concentration (SSC) (Gray et al. 2000), that recognizes the problems associated with the application and use of total suspended solids (TSS) given the complexity of rainfall-runoff. These water quality indices also include measurement such as the total dissolved solids (TDS) (that potentially overlaps the larger colloidal fraction based on operational method. Most of this particulate matter is anthropogenic; however depending on the land use and season, there can be an important biogenic fraction. Given the largely inorganic nature of most particulate matter in urban rainfall-runoff (on a gravimetric basis) and the potentially inhibitory nature of rainfall-runoff, chemical oxygen demand (COD) methods, for example, APHA 1998, are used for oxygen demand considerations. Challenges to such methods occur because of the wide gradation of particulate matter that can exert an ultimate COD and current COD methods are more amenable to the dissolved and suspended fractions of COD in rainfallrunoff; given that the settleable, sediment and gross solids are much more challenging to representatively sample and analyze, whether for COD or other water quality parameters. Other common water quality concerns include nutrients (N, P), a wide range of toxics including metals and organic compounds. These water quality parameters also
dynamically partition between dissolved and particulate phases and distribute across the wide range of particle size gradations. This distribution is a function of total surface area and total surface charge (as opposed to specific surface area) and indicates that pollutant loads are associated with the fraction of the gradations which contributes the highest total surface area and total surface charge. In addition, neutrally-buoyant pollutants, there is trash, litter and floatables; all of which contribute to this complex, dynamic and heterogeneous mixture that is transported under unsteady conditions during a wet weather event.

Accretion of constituents associated with such indices and measurements result from vehicular-pavement interaction, deposition and leaching of deposited anthropogenic materials and infrastructure, and maintenances practices (Brown 1978; Sansalone and Buchberger, 1997, USEPA 1995, Yu et al 1994). This dry deposition material is entrained in runoff and transported to sewer systems and some fraction thereof transported to receiving waters.

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