Smart Network Modeling for Effective Planning of Sustainable Urban Areas

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1 IN RESPONSE TO UNPRECEDENTED URBAN AND POPULATION GROWTH AND RELATED STRAIN ON STORMWATER COLLECTION SYSTEMS, SMART-WATERNETWORK MODELING TECHNOLOGY HELPS MANAGE AND REDUCE THE RISKS OF URBAN RUNOFF. PA U L F. BO U L O S, TAY L O R C. BAR N ET T, AN D R O BERT E. D I C K I N SON Smart Network Modeling for Effective Planning of Sustainable Urban Areas U rbanization can dramatically alter the natural water cycle, resulting in diminished water quality, increased frequency and severity of flooding, channel erosion, and destruction of aquatic habitat. Smartwater-network (SWN) modeling technology plays a crucial and growing role in resolving these challenges by providing watershed managers with a comprehensive set of analytical and decision-making tools designed to help manage and reduce the risks of urban runoff. These advances transform routine watershed runoff computation and network routing from basic planning and design into two-dimensional surface/subsurface flow modeling and optimal selection and placement of best management practices/low-impact developments (BMPs/ LIDs). Additionally, SWN technology provides critically needed support to federal, state, local, and watershed practitioners not only for optimizing stormwater management strategies but also for addressing environmental 54 quality restoration and protection needs in urban and developing areas. THE NEED FOR SUSTAINABLE URBAN WATERSHED MANAGEMENT The world s urban population has grown rapidly from 746 million in 1950 to 3.9 billion in 2014 and is expected to surpass 6 billion by 2045, according the US Department of Economic and Social Affairs (UNDESA 2014). Conversely, the world s rural population has grown slowly since 1950 and is expected to reach its peak around It is now close to 3.4 billion and is expected to decline to 3.2 billion by The report also notes that in 2014 there were 28 mega-cities with 10 million inhabitants or more worldwide, and this number is projected to increase to 41 mega-cities by 2030 (UNDESA 2014). Managing urban areas has become a critical development challenge of the 21st century, and sustainable urban watershed management is of paramount importance.

2 As urban population grows and sprawling urban development expands, more land is converted to impervious surfaces, such as driveways, sidewalks, residential rooftops, patios, public buildings, commercial structures, parking lots, and roads. These surfaces deflect rather than absorb falling rain and snowmelt, allowing runoff into streets, lawns, and other sites. The resulting runoff can carry many types of pollutants, including trash, road salt, heavy metals, bacteria, hydrocarbons, organics, fertilizers, pesticides, nutrients, oil, grease, toxic chemicals, pet droppings, construction materials, and particulates from pavement breakdown into ponds, rivers, streams, drinking water aquifers, and beaches. Such runoff is one of the major threats to water quality in the United States and is linked to chronic and acute illnesses (Gaffield et al. 2003). The US Environmental Protection Agency (USEPA) has documented widespread impairments in surface-water quality that is largely attributable to stormwater runoff, noting that these impairments constitute approximately 7,641 mi2 of estuaries, 10,451,402 acres of lakes, and 246,002 mi of rivers (USEPA 2004). In addition to contributing to the deteriorating quality of the receiving waterways and negatively affecting aquatic life, impervious surfaces also can affect the quantity of runoff, increasing the occurrence and intensity of downstream floods, thereby lowering Constructed wetland. groundwater tables and possibly decreasing recharge rates. Urban development also causes changes in the watershed hydrologic regime with higher peak runoff that can result in increased stream bank erosion, collecting and transporting stormwater runoff through a structural conveyance network to a centralized facility, such as a detention basin or wet pond, where runoff is stored, treated, and discharged downstream. Managing urban areas has become a critical development challenge of the 21st century, and sustainable urban watershed management is of paramount importance. clogged stream channels, suspended sediments, and habitat damage. The absence of open land to absorb extra urban runoff and remove excess nutrients and other chemical contaminants can permanently affect water quality and damage natural habitat. Adding to this problem, global climate change is expected to increase the frequency and magnitude of rain and large storms in some regions, which can cause more runoff, coastal flooding, and coastal erosion. Climate change may also lead to warmer water and air temperatures, spurring the growth of harmful algae that can threaten the health of humans and wildlife, as well as altering the seasonal water cycle. STORMWATER MANAGEMENT Traditional stormwater management approaches have focused on Infiltration basin. Today, advances in site planning and city design have produced a number of effective stormwater control strategies to minimize and even prevent adverse runoff impacts and provide necessary treatment closer to the origin of such impacts. These costeffective, sustainable, and environmentally friendly strategies are known collectively as BMPs/LIDs. The photographs included in this article show commonly used BMP/LIDs types. BMPs include the creation of wet and dry ponds, rain gardens, porous pavement, infiltration trenches, grass swales, and filter strips. LIDs have been characterized as sustainable stormwater practices an approach to land development or redevelopment that works with nature to manage stormwater as close to its original source as possible. LIDs retain a site s natural or predeveloped hydrologic response to precipitation by Bioretention. BOULOS ET AL. 107:12 JOURNAL AWWA DECEMBER

3 Surface sand filter. combining impervious area controls with small-scale green infrastructure. Instead of simply removing stormwater from a site without treatment and potentially creating water quantity and quality problems downstream, water is stored, reused, and treated closer to its source. Additionally, LID, also known as green infrastructure, can help communities protect the environment and human health while providing other social and economic benefits (USEPA 2014). When BMP/LID principles and practices are implemented, water can be managed in a way that reduces the impact of built areas and promotes the natural movement of water within an ecosystem or watershed. This approach complements and sometimes replaces traditional stormwater management systems, transforming runoff from a nuisance that must be discharged to a resource Cistern. 56 Nonsurface sand filter. to be preserved and protected. Roof runoff, for example, can be captured and stored in rain barrels for future plant watering. Runoff can also be directed to rain gardens for treatment and then used for landscape enhancements. Applied on a broader scale, BMP/LID strategies can maintain and restore a watershed s hydrologic and ecologic function. ADDRESSING URBAN RUNOFF CHALLENGES Recent advances in SWN modeling technology have taken on a crucial and growing role in resolving urban runoff challenges. SWN technology equips watershed managers with a comprehensive set of analytical decision-making tools designed to help manage and reduce the risks of urban runoff. Such advances have transformed routine watershed runoff computation and network routing Wet pond. Rain barrel. from basic planning and design to two-dimensional surface flow models that support optimal selection and placement of BMP/LIDs based on cost and effectiveness. SWN models are seamlessly integrated into geographic information systems to support development of watershed simulation networks, twodimensional surface-mesh generation, and spatial placement and site suitability of BMP/LID options, as well as to provide geoprocessing functions and visualization facilities for display and manipulation of data and results. These models provide critically needed support to federal, state, local, and watershed practitioners at all levels for optimizing stormwater management strategies and addressing environmental quality restoration and protection needs in urban and developing areas (USEPA 2011, 2009). SWN models Dry pond.

4 Grassed swale. can be divided into three general categories: dynamic rainfall runoff simulation models, integrated onedimensional and two-dimensional models, and urban stormwater treatment and analysis models. Dynamic rainfall-runoff simulation models. These models represent the most effective and viable means for simulating runoff quantity and quality conditions from a single storm event or from long-term, continuous storm events in urban areas. The runoff model operates on a collection of subcatchment areas that receive precipitation and generate runoff and pollutant loads. The routing model, or dynamic wave model, transports runoff through a system of pipes, channels, storage/treatment devices, pumps, and regulators, and models backwater effects, flow reversals, pressurized flow, and entrance/exit energy losses. Routing models also solve one-dimensional St. Venant equations that govern the unsteady flow of water through a drainage network of channels and pipes and use the continuity principle at the junctions. Together, the runoff and routing models track the quantity and quality of runoff generated within each subcatchment, as well as monitor the flow rate, flow depth, and quality of water in each pipe and channel during the simulation period. Through their predictive capabilities, the models can estimate the production of pollutant loads associated with stormwater runoff, including dry-weather pollutant Infiltration trench. Vegetated filter strip. buildup from different land uses, pollutant wash-off from specific land uses during storm events, direct contribution of rainfall deposition, reduction in dry-weather buildup caused by street cleaning, reduction in wash-off load resulting from treatment in BMPs, and entry of dry-weather sanitary flows and user-specified external inflows at any point in the drainage system. Additionally, the models can track routing of water quality constituents through the drainage system and the reduction in constituent concentration through treatment in storage units or by natural processes in pipes and channels. The models can also mimic the hydrologic performance of small LID controls, such as permeable pavement, rain gardens, green roofs, street planters, rain barrels, infiltration trenches, rooftop disconnection, and vegetative swales. Such capabilities allow engineers and Integrated one-dimensional and twodimensional models. These models are essentially made up of one-dimensional hydrodynamic simulation of flows in rivers, open channels, and pipe networks that are then paired with two-dimensional hydrodynamic simulation of surface, or overland, flooding in an urban environment and river floodplain. Sources of twodimensional surface flows can include overbank flooding from rivers and streams, flooding from minor systems, and distributed rainfall. The surface area is represented by a detailed two-dimensional mesh that fits around complex urban geometries. The spatial and temporal distribution of water depths and flows are determined by numerically solving the shallow-water equations. Both structured and nonstructured meshes using triangles and quadrangles make up the surface grid. By allowing the The absence of open land to absorb extra urban runoff and remove excess nutrients and other chemical contaminants can permanently affect water quality and damage natural habitat. planners to accurately represent any combination of LID controls within a study area to determine their effectiveness in managing stormwater and combined sewer overflows (Rossman 2010). simultaneous solution of all rainfall and runoff processes in complex urban areas, an integrated onedimensional and two-dimensional model can more effectively support drainage design and management and BOULOS ET AL. 107:12 JOURNAL AWWA DECEMBER

5 innovyze.com. He has coauthored more than 200 articles and nine books on water and wastewater engineering, available in the AWWA Bookstore ( Taylor C. Barnett is a water resources engineer with Innovyze. Robert E. Dickinson is a product sector leader with Innovyze. REFERENCES Green roof. better predict flood risks. The models can also be used to estimate any potential flood risks of a drainage system and develop reliable and costeffective design, management, and operational strategies (Boulos & Walker 2015). Urban stormwater treatment and analysis models. These models are used to determine the best type and placement of BMP/LID controls at strategic locations in urban watersheds and are used to address the dual objectives of pollution and flood control. The optimization module a stochastic search procedure like a genetic algorithm performs cost estimating and systematically compares cost and performance of BMP/LID options and their placement criteria for meeting desired flow and water quality targets. The decision variables consist of BMP/LID unit numbers and dimensions, a constraints set comprising runoff volumes and pollutant load reductions, and an objective function that minimizes total costs. The output consists of costeffectiveness curves that relate flow and pollutant load reductions with costs for a series of optimal solutions. These powerful features greatly assist green infrastructure planners and modelers in developing cost-effective and reliable flowand pollution-control implementation plans that are aimed at protecting source waters and meeting water quality goals (USEPA 2011, 2009). 58 Porous pavement. CONCLUSIONS A better understanding of sustainable watershed management has become increasingly more important as the world s population and urbanization increase. Unprecedented urban and population growth has started to strain many stormwater collection systems that were originally designed using traditional management methods. As a result, SWN technology modeling is becoming increasingly valuable in meeting this challenge because of its inherent ability to optimize the placement and type of BMP/ LID infrastructures in urban areas. SWN models ensure that installation and operational costs are minimized and pollutant load removal and flood mitigation are maximized. By using these increasingly sophisticated and accurate models, engineers and planners can use BMP/LIDs in ways that improve the quality of urban runoff. This in turn reduces pollutant transportation, decreases the intensity of flooding, increases groundwater recharge rates, and helps ensure a sustainable future for large urban areas. ABOUT THE AUTHORS Paul F. Boulos (to whom correspondence may be addressed) is the president, chief operating officer, and chief technical officer of Innovyze Inc., 370 Interlocken Blvd., Broomfield, CO USA; paul.boulos@ Boulos, P.F. & Walker, A.T., Fixing the Future of Wastewater Systems With Smart Network Modeling. Journal AWWA, 107:4:72. org/ /jawwa Gaffield, S.J.; Goo, R.L.; Richards, L.A.; & Jackson, R.J., Public Health Effects of Inadequately Managed Stormwater Runoff. American Journal of Public Health, 93:9:1527. Rossman, L.A., Storm Water Management Model User s Manual Version 5.0. National Risk Management Research Laboratory, Office of Research and Development, US Environmental Protection Agency. EPA600-R , Washington. nrmrl/pubs/600r05040/600r05040.pdf (accessed June 18, 2015). UNDESA (United Nations Department of Economic and Social Affairs), Population Division, World s Urbanization Prospects: The 2014 Revision, Highlights. un.org/unpd/wup/highlights/wup2014highlights.pdf (accessed June 18, 2015). USEPA (US Environmental Protection Agency), Enhancing Sustainable Communities With Green Infrastructure: A Guide to Help Communities Better Manage Stormwater While Achieving Other Environmental, Public Health, Social, and Economic Benefits. www2. epa.gov/sites/production/files/ / documents/green-infrastructure.pdf (accessed June 18, 2015). USEPA, Enhanced Framework (SUSTAIN) and Field Applications for Placement of BMPs in Urban Watersheds. EPA 600-R11-144, Washington. USEPA, SUSTAIN A Framework for Placement of Best Management Practices in Urban Watersheds to Protect Water Quality. EPA 600-R , Washington. USEPA, National Water Quality Inventory: Report to Congress. water.epa.gov/lawsregs/guidance/ cwa/305b/upload/2009_01_22_305b_2004 report_factsheet b.pdf (accessed June 18, 2015).