Utilizing GIS for a Stormwater Drainage Utility Implementation by Susan Smith, Editor, |
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In Williams’ part of the world, that concern boils down to how we make enough money to fund infrastructure, namely, |
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a stormwater drainage utility implementation of GIS? For the purpose of this implementation of future storm water management applications,it was decided that since all impervious surfaces in her city are part of the storm drainage network, they should be incorporated into management applications. The municipal committee Williams worked with would only let her focus on driveways in her assessment of impervious surfaces. The database contained carports and patios, and all it didn’t have was driveways. Pools were not to be considered in the equation. The first step was to develop a storm drainage fee methodology, for which Williams, a one-person GIS department, did background research and investigated the geography and the performance of existing stormwater drainage systems. In response to the Clean Water Act Amendment of 1986, the EPA developed the National Pollutant Discharge Elimination System (NPDES): NPDES Regulations (NPDES) Stormwater program, whose primary objective is to improve water quality. Highland Village needed to establish fees by conducting inventories of parcels within the city. The primary challenge became data collection, and it was decided records gathered for the appraisal district would be the best way to go about it. They did not have all the data for all the square footage in the city, or records of pavement area. So they needed aerial photography, and other resources such as digital site plans and permit paperwork. For areas obscured by trees, Williams created algorithms. She then performed statistical analysis using Pearson’s correlation coefficient statistics. Williams found there was a stronger correlation between driveways and lot square footage, than between that of driveways and garages. Williams determined that the best methodology was to charge fees on proportional data collection, at a base rate of $1.20 per 1,000 square foot of impervious surface. “I was tasked with collecting planimetric data, and providing a viable algorithm of driveways obscured from view.” Data collection methods used included appraisal district records, georeferenced digital site plans, aerial photography, permit paperwork, and estimation equations. Residential data was collected from four sources: residential footprint and lot size, those parcels not in residential area collected from permit paperwork, planimetric from aerials concerning mostly the driveway size. It was discovered that 1% of the driveways did not have impervious surfaces. “I calculated the percentage of the driveway relative to lot size – the average of all the percentages in the category is the value multiplied by the lot size to estimate driveway area,” said Williams. She came up with three ways of estimating, arriving at a residential total estimate of driveway values per a standard base of $403 total drainage fee. In the commercial data collection, planimetrics, appraisal district records, and permit paperwork were used. In most cases commercial buildings were visible on aerial photos, others were gathered from permit paperwork and digital site plans. “Commercial properties presented a situation that residential did not in that more than one driveway was attributed to a lot in some cases,” explained Williams. “Equations were created to accommodate these situations. This equation was calculated so that pavement is proportionally allocated to each business.” The equations used to calculate the commercial drainage fees are as follows: Advantages to using GIS: Storm water drainage utility management includes: In summary, Williams said that she welcomed the opportunity to share the GIS as a support process with non-GIS professionals. “I could actively demonstrate the value of GIS to decision makers that helped facilitate municipal operations.” Her recommendation to those who may be tasked with doing a similar type of investigation: “Invest in building a solid GIS and investigate a wide range of GIS applications, such as a full set of planimetrics, DEM, current aerial photos, GPS devices, and GIS to integrate and analyze data. Allow time to research GIS applications.” Funding was provided by the Public Works Department, and basically the storm drainage fees fund the storm drainage system. |
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