With $2.5 billion in projects planned through 2035, Charlotte Douglas International Airport (CLT) is in the middle of a massive expansion and renovation programme.
Indeed, over the coming 15 years, passengers can expect larger concourses with more gates, wider roadways, a bigger terminal and a fourth runway.
All of that work means that the airport will be adding a lot of new paved (impervious) area. And, as a result, its stormwater management strategies required a new approach to accommodate the increased run-off that will follow.
HDR assisted the airport in creating an innovative “water quality credit strategy” that bundles all of the stormwater projects under one umbrella instead of its previous piecemeal approach.
In the past, CLT approached stormwater planning on the individual project level. Each project was evaluated for its stormwater needs, then an appropriately sized stormwater control effort was permitted and constructed alongside.
This project-by-project approach had the potential to constrain future development. It could also mean an added burden because of the maintenance of many more stormwater facilities.
“We’ve built a lot of independent facilities for stand-alone purposes in the last 35 years at CLT and it has become quite a challenge to manage all these detention features,” admits says CLT’s environmental manager, Jimmy Jordan. “Our impervious surface area has increased 850% since 1982.”
HDR led the drafting of a more proactive and comprehensive stormwater management strategy in 2013. In 2018, it was updated for the next five years. A key consideration in the development of that strategy was minimising the impact and number of new facilities.
Space availability for stormwater controls is not a current issue at the airport, but as development continues, space constraints could limit the placement of stormwater projects while removing valuable space for expansion.
What emerged from these concerns was an innovative treatment trading strategy. By calculating total treatment demand, all existing impervious areas at the airport were considered to be equal and available for stormwater management (water quality treatment, for instance) within a given watershed.
That means an acre of existing impervious area on the east side of the airport, for example, may be treated in lieu of treating the new acre of impervious development created by construction.
This trading, in effect, allows the airport to maximise the potential treatment at selected locations. Stormwater is now managed within four distinct watersheds that cover the airport’s more than 5,500 acres of land.
The location of newly constructed basins was centralised downstream of planned projects and excess capacity was built in to treat and control the rate of run-off. As future projects are completed, the airport can tap that capacity instead of constructing new stormwater facilities.
For example, on the south side of the airport, a new regional detention basin proposed in the 2018 management plan will handle all of the construction planned in the area through 2023 and beyond.
Building the excess capacity is an initial step. In order to take full advantage of it, the airport also needed a robust method for tracking it. To help facilitate these exchanges and track their impacts, CLT’s stormwater plan includes an ESRI geo-database that captures all the supporting data in a single electronic location.
As regulations change, priorities shift or unforeseen constraints are realised, the geo-database can quickly re-evaluate stormwater management needs. It can also be used to track the evolution of the airport master plan as projects are constructed and associated treatment options are implemented.
Jordan says: “We recognise that our airport is physically constrained by major travel corridors, and we must re-focus our strategies now to ensure we’re utilising our existing real estate footprint for necessary airport developments that are being supported by balanced comprehensive stormwater management strategies.”