The urban environs are a complex constructed interface that significantly alters rainfall-runoff relationships, and through the generation of anthropogenic and biogenic constituents results in significant transport of chemical, thermal, microbiological and particulate matter (PM) loads. These loads are largely coupled with the altered rainfall-runoff or snow-snowmelt relationships. In North American cities with municipal separate storm sewer systems (MS4s) the load of volumetric runoff, chemicals and PM are equal to or greater than the untreated influent loads to municipal wastewater treatment plants (WWTP) yet the management of urban drainage (runoff) loads is at least a half century behind municipal wastewater management and is greater in scale and cost. Urban runoff management is very challenging; in part due to PM hetero-dispersivity, interactions between aqueous and PM phases, stochastic hydrology and highly unsteady hydrodynamics. Such challenges and associated costs with resolving such challenges have led to the relatively common historical examination of a spectrum of urban constituent control systems as black-box systems and rainfall-runoff chemistry utilizing lumped measurements; and many indices and measurement methods adopted from wastewater treatment.
Urban drainage systems that do not provide some level of hydrologic restoration are not sustainable. Additionally given the PM, gross solids and constituent load inventories that build up in urban areas, source control practices and load credits must be an integral part of any management plan that includes restoration, treatment and reuse. Urban drainage treatment, urban maintenance and green infrastructure will play an increasingly critical role in the entire urban water cycle and therefore we must develop maintenance practices such as pavement cleaning, source control and near-source control. Advances with respect to sustainability of urban water require tools such as continuous simulation models (SWMM), smart sensors and computational fluid dynamics (CFD) and a focus on fundamental UO concepts.
The role of our constructed environs and activities therein on the urban water cycle and coupled delivery of constituent loads is examined. Results indicate the need for continuous simulation models such as SWMM to quantify hydrologic restoration. State of the art tools, specifically computational fluid dynamics (CFD) are introduced to illustrate the role of CFD in examining treatment of urban water. Results also illustrate the current practice of MS4s with thousands of BMPs is not sustainable. The future will require these advanced management tools, centralized treatment and frequent maintenance practices. Sustainability will ultimately require source control of loads, green infrastructure and hydrologic restoration.