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| Project Springdale, Ark., five-year wastewater master plan |
| Consulting firms McGoodwin, Williams, and Yates, Fayetteville, Ark. Applied Geographic Technologies, Forth Worth, Texas Pipeline Analysis, Dallas |
| Product application Firms use InfoWorks CS software to create and calibrate a dynamic digital sewer system model for the Springdale Water and Sewer Commission. |
All of north Arkansas is experiencing significant growth. In particular, during the last few years the city of Springdale annexed a large property on its northwest side that it needs to develop, and a site to the southeast is also growing rapidly. The city needed to know how to accommodate this growth while continuing to provide water and wastewater services to its core system.
Springdale, with a population of just more than 60,000, is one of the four main cities in the northwest corner of the state. It is Arkansas’s second-most populated metropolitan area and fourth-largest city. The conurbation is near the centre of the Springfield Plateau, which is the westerly part of the Ozark Plateau.
Arkansas Health Department guidelines and regulations suggest that water districts and cities undertake a comprehensive master plan every five years. This is important for the authorities as it enables them to understand how much funding they require and to analyze their systems to know what sort of framework is required to accommodate future development.
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| Growth in Springdale and the surrounding area has necessitated expansion of water and wastewater services. |
MWY oversaw two studies—one for the water network and the other for the wastewater network. It created an expert team by working with hydraulic/hydrologic modeling and GIS consulting firm Applied Geographic Technologies (AGT) of Forth Worth, Texas. Because of its specialist input in this area, AGT ran the wastewater modeling element, using Wallingford Software’s wastewater network modeling solution, InfoWorks CS.
A third consulting firm, Pipeline Analysis of Dallas, was brought onboard to handle the flow-monitoring aspect of the sewer modeling. These results where then used by AGT to calibrate its model, as well as to identify areas of the existing sewer system that had significant infiltration and inflow, which would require rehabilitation.
Model build
AGT’s task was to develop a hydraulic model from the as-built drawings of the network, CAD atlas maps, studies, and feedback from operators. Some gaps existed, so a minimal amount of survey work had to be done to include these areas in the model. Finally, some missing data was both calculated and inferred, one of the strengths of InfoWorks CS.
The task was challenging because the drawings were of varying ages and some indicated the presence of assets that no longer existed or were not in use. The modelers began with the oldest drawings and worked through to the newest to develop an evolving picture of the system.
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| The Springdale Pollution Control Facility is a biological nutrient removal wastewater treatment plant completed in 1988 and capable of treating as much as 15.6 million gallons per day. |
AGT modeled pipes 12 inches in diameter and larger—the collector element of the system, plus specific known areas that were either critical or under-performing. Also modeled were a number of special structures such as divergent manholes, for which there was one entry pipe and two exit pipes, so flow control was critical.
In all, the finished model contains about 1,000 nodes, 326,000 linear feet of pipe, four siphons, and several significant force mains, one 4 miles long. The force mains all finish at individual lift stations and then merge into the one long main. The network also has one unusual element: a large offline equalization (detention) basin intended to hold the high flows that can accumulate rapidly during wet weather events because of the terrain.
The service areas were divided into a hierarchy of areas to support model loading: 38 flow meter basins (based on the location of the flow meters and the topography), and 7,047 subcatchments (micro-basins created using Thiessen Polygons within the edges of each flow meter basin). These areas were then used to load and calibrate the model.
For calibration, population data was added and distributed based on the city’s master plan and census data. Information from 38 flowmeters and six rain gauges was also collated over a 48-day period, as was diurnal flow curve information and other data including pipe roughness and major water users.
The flow monitors and rainfall gauges were set strategically throughout the Springdale wastewater collection system to obtain the necessary information about the wet and dry weather flows to develop the hydraulic model. The monitoring found that the dry weather wastewater contribution is approximately 12.11 million gallons per day (mgd). Currently, dry weather flows are adequately transported and treated. Wet weather rainfall-induced infiltration/inflow averages 2.5 million gallons per inch of rainfall, representing around 110.1 million gallons on an annual basis. No sanitary sewer overflows were observed during dry or wet weather flows.
AGT did a significant amount of scenario modeling. The city had strict design criteria that allowed pipes to be surcharged, but not in excess of 1 foot above the top of the pipe. By comparison, many cities allow surcharging almost to the manhole ramp.
After the baseline model was calibrated, future proposed elements of the network were then added to the network in four areas where development was planned during the next 10 to 20 years. Models were run for various planning horizons—2006, 2011, 2016, and final build-out in 2026. AGT also compared the design flows with data held by the city from a study of existing and forecast flows to the wastewater treatment plant. The data matched well.
Results
The Har-Ber Meadows lift station was built to blend in with the surrounding residential homes. Both structures add value to the surrounding properties while also providing necessary services to the community in an aesthetically pleasing manner.
The work provided a detailed dynamic hydraulic model of the Springdale sewer system. MWY used the results, as well as good engineering input from the client, to develop recommendations, which fell into two distinct sets. The first involved recommendations for improvements to the existing infrastructure and the second involved recommendations for new infrastructure to serve the growth areas.
The first set of recommendations related to upgrades, looking at existing pipe capacities and considering the output from the modeling to recommend necessary upgrades, including installing new pipes in parallel with the old ones or complete replacement, dependent on the system and the condition of the pipes.
The modeling highlighted data about the projected capacity requirements for pumping stations in the future, and identified those that needed to be upgraded. Various alternatives for increasing station capacities were evaluated, including whether additional pumps or a replacement pumping station were needed, or whether upgrading the force main beyond the pumping station would provide the necessary additional capacity. The study yielded a comprehensive plan that allowed for growth in the most economical way.
The recommendations for new facilities to serve the Springdale growth area also affected the core area because the additional flows would also mean further upgrades were required for existing facilities, since the new facilities would connect to the older core system. The suggested improvements have been phased in five-, 10- and 20-year design horizons to spread out the work and costs. It is likely that in the interim the city will also require further modeling for future five-year plans as the city’s demographics change.
The master plan project received a key accolade. The design team won an award from the American Council of Engineering Companies—Arkansas for its work in the Studies, Research, and Consulting category.
Dale E. Orth is project manager for Applied Geographic Technologies, Fort Worth, Texas. He can be contacted at dorth@overflows.com. Brad B. Hammond, P.E., is president, McGoodwin, Williams, and Yates, Fayetteville, Ark. He can be contacted at bhammond@mwyusa.com.
