 |
|
|
|||||||||||||||||||||||||||||||||||||||||
 
|
A master drainage study conducted for the Metropolitan Nashville Airport Authority is guiding decisions regarding development at the city’s international airport. Like many cities, Nashville, TN, has stormwater regulations that require new development to maintain pre-existing peak runoff rates. As a result, the Metropolitan Nashville Airport Authority (MNAA) sought to establish baseline hydrologic data for its Nashville International Airport. The existing practice had been to assess the current conditions to establish the potential stormwater impacts associated with each future site that is to be developed on a case-by-case basis, adding time and expense to the development process. Opened in 1937 on 340 acres, the Nashville International Airport today encompasses more than 4,500 acres. The airfield’s four runways are up to 11,000 feet in length and include three parallel runways for simultaneous takeoffs and landings and a crosswind runway. Averaging 400 daily flights, the facility serviced about 9.8 million passengers and approximately 75,000 tons of cargo in the last year. Unlike many other US airports whose growth is limited by space constraints, Nashville International Airport has undeveloped area adjacent to many parts of the airfield. To facilitate future development, the MNAA realized that it would be beneficial to assess the airport’s current hydrological conditions all at once. “We thought if we had a study of the entire property, we could use that information when we enter into projects to expand the airport,” says Traci Cooper, P.E., project engineer–design with the MNAA’s Planning, Design, and Construction Department. In addition to the hydrologic information, the MNAA wanted to have a complete picture of the airfield’s existing stormwater infrastructure. “We didn’t have a good database detailing this information,” Cooper notes. “Anytime we start the design process, one of the first things needed is a survey of the site. Surveying the drainage infrastructure all at once will make that part of the process a little easier for each project. The design of the grading and drainage can influence a lot of other factors in the development of a site, and knowing what drainage structures are on or near a site also helps us to better plan for the scope of the project before the design process really begins.” Cooper also notes another way that the study can be used as a planning tool: “Having a good record of site drainage patterns throughout the airport also enables us to effectively isolate any spill and prevent the offsite migration of spilled materials.” The recently completed drainage study included the development of a database that houses information relating to the airfield’s stormwater infrastructure. Along with giving the MNAA a more complete picture of its existing system for managing stormwater, the study has determined the airfield’s current baseline hydrologic conditions. In the few months since its completion, the study’s results already have helped significantly with the design of a parking lot at the airport, and it promises to inform and facilitate future development as well. Collecting the Data The Nashville International Airport comprises roughly 60% of the study area, while the rest includes upstream areas that drain onto airport property as well as some downstream areas that receive runoff from the airport. Where these upstream off-property areas are developed, the effects of the development were taken into account. Drainage structures were catalogued in these off-property areas, as well as the characteristics of detention ponds that influence the quantity of runoff directed onto the airport property. In other situations, stormwater runoff crosses the MNAA’s property boundary as a diffuse, overland flow and leaves again before collecting into a quantifiable concentrated flow at a discrete point downstream from the airport property. In these cases, the study’s limits were extended not only upstream to the extents of the contributing area but also downstream to the stream course of the concentrated flow that ultimately receives this runoff. The study began with an evaluation of aerial photographs and aerial survey data provided by the MNAA and local governments. Based on this initial review, the airport property was divided into 23 major basins, which were defined as the drainage areas contributing stormwater runoff that leaves the airport property as concentrated flow. Larger basins were then divided into sub-basins if they contained discrete areas draining to a culvert or other significant infrastructure components. A map depicting the airport property and the basins was provided to surveyors from Thornton & Associates of Nashville, who located and noted key features of the existing stormwater infrastructure in each basin. Data collected as part of this process included the physical size, shape, and material type for such items as drainage pipes, culverts, drainage inlets, pipe or culvert headwalls, and trench drains located throughout the drainage areas. For example, every pipe shown on the map was located and surveyed at its upstream and downstream ends. Each survey shot identified the location of either the upstream or downstream end of a culvert, a drainage inlet, a headwall, or another visible, definable element of storm drainage infrastructure. For each structure, the surveyors completed a data form to note key information and provided a digital photograph as well as a survey shot that included a point number and coordinates recording its location and elevation. In addition to cataloging the measurable data attainable at each visible inlet, outlet, and junction structure, the surveyors recorded subjective assessments of the visible infrastructure’s current overall structural and hydraulic condition. Engineers from PBS&J reviewed the photographs to ensure that the surveyors’ subjective evaluation of an infrastructure element’s condition was accurate. In some cases, pipes connect underground without a visible surface structure that can be surveyed; in other cases, deep drainage structures complicated attempts to ascertain the types, sizes, and exact depths at which one or more pipes might enter or leave a structure. In such situations, attempts were made to supplement the data from the survey with data from record drawings on file with the MNAA or solicited from the owners of adjacent developed properties. Where supplemental data were not available, elevations were interpolated or extrapolated from data on hand. Developing the Database Records in the database include all the information from the survey shots, survey worksheets, and corresponding digital photos. More than simply a static list of structures, the database links each structure to those that are directly upstream and downstream from it. Through this link, the database calculates such information as the slopes and lengths of pipes. Because field data pertaining to the pipes was collected only at the pipe endpoints, the surveyors were unable to locate and survey a unique point with which the pipe data could be associated. However, by referencing the appropriate survey data records correlating to each end of a given pipe, the database calculates the state plane coordinates representing the pipe’s midpoint for this purpose. This calculated theoretical horizontal location was assigned a vertical elevation of zero as a placeholder, and the appropriate data pertaining to the pipe, including size, shape, and material, was associated with this spatial reference point and used to complete the pipe’s data record. Benefiting From the Database Because the database enables users to quickly sort through thousands of pipes and structures based on location and physical characteristics, the MNAA’s maintenance crews will be able to use the database to identify and locate those structures most in need of maintenance. In addition to providing a photo and the precise location of a structure, the database will enable work crews to “know exactly what they have ahead of them,” Cooper says. “Such information will help crew members determine the type of equipment they’ll need to do the job.” The database is easily updated, so as new facilities are constructed at the airport, pertinent data can be added readily. Furthermore, the MNAA is in the process of implementing a geographic information system (GIS) for the entire airport. The authority wanted the data from this study to be compatible with the GIS, and so the Access database used for the current effort has been crafted to communicate easily with the Arc GIS software. Modeling Stormwater Runoff Beyond identifying likely ponding elevations, the ultimate concern was quantifying the amount of runoff that could be expected from any sub-basin and each major basin where it leaves the property for each of the design storm events. Although the software capabilities allow for the modeling of many different design storms, flow rates from the basins were reported for four specific design storms with the following return periods: two, five, 10, and 100 years. For each of the 23 basins, a report was prepared summarizing the effects of the various design storms and how the runoff is handled by the existing infrastructure. Maps depict the layout of the drainage basins and the cataloged infrastructure within the basins. Anticipated outcomes from certain storm events, such as overtopping of roadways and ponding within the runway and taxiway safety areas, were highlighted in the report. Benefiting From the Study The site where the new valet parking lot is to be constructed is located along the west side of the airport’s Discrete Access Road, a divided boulevard-style roadway that leads directly from Interstate 40 to the airport terminal. The site is bounded to the west, by Sims Branch, a major perennial stream; to the south by a major wet-weather conveyance; and to the north by a minor wet-weather conveyance. The lot’s design had to account for mandated buffer zones associated with the major streams. During the design process, it was decided that major relocation or encapsulation of the minor stream should be avoided. Because of these design considerations, the resulting available area for the parking lot was severely constricted. Moreover, local requirements pertaining to stormwater treatment called for the creation of a small pond to detain the initial flush of runoff from the lot during small, regularly occurring storm events. The pond had to detain the runoff long enough so that most of the suspended solids would settle out before the stormwater is released downstream. In addition to the buffer zones and settling pond, recent revisions to the local stormwater management regulations were enacted during the design of the lot, making compliance even more challenging.
Complying With Stricter Regulations Accommodating all of the requirements simultaneously while meeting the MNAA’s need to store more than 1,200 vehicles required extensive coordination with the airport’s parking services provider as well as an innovative design solution. The small settling pond incorporated in the site design was able to be enlarged a bit, to maximize the degree to which it would help mitigate post-development peak flows from some of the smaller design storms (two-year, five-year, and 10-year). However, the small pond could not significantly reduce the peak runoff rates of the larger design storms (25-year, 50-year, and 100-year). Because of the inflexible demand for the parking lot to store more than 1,200 cars and an inflexible project budget, neither a large aboveground pond nor supplemental underground detention was a viable option. The project’s stormwater detention needs were further exacerbated by the fact that the construction of the western portion of the proposed parking lot (closest to Sims Branch) would require an earthen-fill embankment to create a more level parking surface. This embankment would encroach into an existing regional detention area that had been built to mitigate the effects of the vast impervious areas associated with the airport terminal and terminal apron constructed in the late 1980s. Therefore, not only did the site have insufficient room to mitigate peak runoff rates from the currently proposed construction; the proposed project would eliminate some of the existing detention storage volume from a previous development as well. Identifying an Alternative Solution The contributing drainage basin for Sims Branch, upstream of the proposed development, encompasses more than 1.2 square miles. It would have taken weeks to establish a comprehensive hydrologic model of the upstream conditions that could take into account the design parameters for mitigating the runoff from the proposed parking lot. However, because PBS&J had recently completed the master drainage study for the airport, the firm had such a hydrologic model at its disposal. Along with having an existing definition of the current flow rates to be replicated as part of the proposed development, PBS&J had an adjustable computer model with which to test various design options, simulate their effects, and optimize the design. Modeling the Solution By incorporating the excavation of a relatively steeply sloped area adjacent to the east buffer zone associated with Sims Branch, the design increased the amount of storage volume for water that would back up behind the Interstate 40 culvert. By including this increased storage volume in the model, the designers could see the degree to which this added volume would reduce peak flows passing under the interstate. In addition to accounting for the lost storage area in the regional detention pond because of the encroachment of the project embankments, the model could simultaneously account for variations in the size of the settling pond on the parking lot site and the enlargement of the adjacent stormwater detention area just upstream from Interstate 40. By observing the results from individual design changes, the design team determined that increases in the onsite settling pond’s volume had a greater mitigating effect than increases in the volume of the detention area just upstream from the interstate. However, each enlargement of the onsite settling pond reduced the number of revenue-producing parking stalls. Therefore, the design team enlarged the downstream stormwater detention area to the extent that its own site constraints would allow and then increased the size of the small settling pond on the parking lot site until the resulting developed flows were sufficiently mitigated. In this way, the design effectively maximized the size of the valet parking lot without exceeding the available project budget. A comprehensive analysis of the Sims Branch watershed was essential to designing the enlargement of the floodplain area just above the culvert that carries the stream beneath Interstate 40 for use as a stormwater detention pond. Having this analysis in the form of an adjustable computer model of the watershed was essential to the design team’s efforts to track and optimize the effects of design changes to the onsite ponding and downstream ponding associated with the parking lot development. The master drainage study provided that model and saved what would have otherwise been a time-consuming element of the design process. Without the model, the only alternatives would have been to mitigate the peak runoff with an onsite pond, resulting in a much smaller parking area being available this fall, or to commission the time-consuming analysis and do without the much needed additional parking until the spring of 2008. Instead, the study allowed for an innovative and quick solution that made the most of the available site. With a tight construction schedule calling for the project to be completed before Thanksgiving, it is the hope of the project team that hundreds more holiday travelers will find readily available parking at the Nashville International Airport this peak travel season and in the future. As shown by the example of the valet parking lot, the master drainage study conducted for the MNAA is expected to provide a host of benefits. Armed with the necessary hydrologic baseline information, the MNAA will be able to assess how future development will affect runoff from the airport. Such information will play a critical role in determining whether to proceed with certain development plans and how to mitigate effects associated with such development. Dave Schilling, P.E., is the program manager for aviation services in PBS&J’s Nashville, TN, office. Darren Duckworth, P.E., is a project manager and Peter Nielson, E.I., is an engineer II. Both are also with PBS&J’s aviation program in Nashville. SW November/December 2007 |
||||||||||||||||||||||||||||||||||||||||||
 |
|
|
Home + About + Subscribe + News + Calendar + Glossary Distributed Energy | Erosion Control | Grading & Excavation Contractor | MSW Management | Onsite Water Treatment | Water Efficiency | StormCon | ForesterPress | Forester Media
|
||||||||||||||||||||||||||||||||||||||||
