Mountain States Chapter of IECA

Complicated Compliance

by Catherine A. Rafferty, PE, CPESC, CFM, CCCA
 Sr. Engineer & Project Manager-Drainage & Utilities
Denver International Airport
City & County of Denver Department of Aviation
Planning & Development Division-Construction

Prepared for the October 2011 Newsletter of the
Mountain States Chapter of the International Erosion Control Association

As we are all aware, creating and implementing an erosion and sediment control plan to maintain compliance on a construction project is difficult to begin with and it's getting more difficult every day. Now imagine trying to maintain compliance on multiple projects, some of which may disturb over 100 acres at a time over a vast development area, occur adjacent to or within waterways, occur in high security areas; and, due to competing operational requirements, do it with less than half the standard BMPs in your toolbox. Welcome to the world of construction at Denver International Airport (DIA).

So why exactly is erosion and sediment control compliance so complicated at DIA? To begin to understand the question, you need to know a little about DIA and to grasp the scope of the problem.

DIA is the 10th busiest airport in the world (2010) with over 50 million passengers served each year. DIA is also unique in the fact that it occupies the largest land mass of any airport in North America.   At 53 square miles, DIA also represents one-third of the land mass of the City and County of Denver. In addition, not all of DIA is in Denver. Approximately ten square miles are located in the City of Aurora and unincorporated Adams County.   Peruse the local regulations and codes and you'll quickly discover that these communities each have unique permitting requirements and sooner or later you can bet there will be a project that crosses jurisdictional lines and those requirements will conflict.

Located at the leading edge of the Great Plains, DIA is also situated in one of the most difficult environments for managing erosion. DIA crosses eight watersheds, some of which include perennial streams and others that include ephemeral streams. Soils are typically silty clays that are highly erodible at the surface while underlain by highly expansive clays and claystone. None of the soils at DIA could be classified as "nutrient rich" or capable of supporting anything more than sparse, shallow-rooted vegetation. Precipitation and wind can be unpredictable and vary widely across a project site. Microclimates are common across DIA and heavy rain could be occurring on one side of a project site while the opposite side of the site could experience nothing but sunshine on the same day. The truth is you could actually have runoff appear on a sunny dry day because it's raining heavily adjacent to your site. The question is do you perform a post-event inspection of your BMPs when it didn't rain on your project; or how about on a portion of your project?

Operationally, DIA is complex to say the least. It operates 365 days a year, 24 hours a day, 7 days a week. It continuously serves over 30 airlines, an average of 143,000 passengers per day (2010), and safely supports an average of 1,741 flights per day (2010). To do that, aviation safety is paramount in every aspect of DIA's operations. Every project, every task, every decision hinges on the impact it will have on operational safety. As a result, DIA Airport Operations, which is essentially an authorized agent of the FAA through DIA's Part 139 certification, always holds the trump card in determining what, when, and how construction activities occur at DIA and flight operations always have priority. After all, you can't exactly set out some cones, put up detour signs, and position flaggers for pilots to navigate a jet through. First and foremost, airplanes always have the right-of-way.

Construction may be restricted to occurring only at night to avoid delaying flights; or it might be compressed into a short window of time that infrastructure (like a runway) can be shutdown , which eliminates any chance of phasing a project to manage erosion; or it may occur out of season when revegetation would be inopportune at best. Three-dimensional imaginary boundaries exist throughout DIA that define protected airspace. It's not as if a hydroseed truck and boom can be brought out on a moment's notice to stabilize and revegetate as a project progresses. Trust me, it takes almost of a week to get something like that planned, scheduled, and approved on an active airfield.

As if the actual construction activity wasn't hard enough to perform at an operating airport, compliance with stormwater management regulations offers its own set of challenges. By their very nature, airports are very impervious. Vast areas of concrete and large rooftops are a hallmark anyone can recognize on a map. At DIA, the concrete runways, taxiways, and aprons cover approximately 2,000 acres, roads cover another 335 acres, parking and garages cover another 600 acres, and rooftops cover another 170 acres. So out of 34,000 acres, approximately 3,000 acres are impervious and that acreage will only increase as DIA continues to expand. The result is an extremely large volume of runoff to be managed. Combine that with the FAA's requirement that runways, taxiways, and aprons are clear of runoff in five minutes or less and you have an extremely large volume of runoff that's moving at a high velocity.

So how do you deal with it? Again, Airport Operations dictates what BMPs you can and cannot use. Silt fence and erosion control blankets have limited application airside. If either one is not sheltered from wind and secured above and beyond manufacturer's recommendations, the odds are that they're going to become airborne and could get sucked into a jet engine or obscure a pilot's vision. Crimped mulch is also prohibited airside because blowing straw creates "foreign object debris" or "FOD" that could be harmful to aircraft. Tackifiers and hydromulches become issues if any overspray comes in contact with the aircraft or with the concrete surfaces the aircraft may use. Within the Object Free Areas that surround runways, nothing can exceed a height of three inches. How many effective BMPs are less than three inches in height and can withstand high-volume, high-velocity flows coming off of vast quantities of concrete onto Class D soils? Not many.

To add to the difficulty of stabilization, the FAA partners with the USDA Animal and Plant Health Inspection Service (APHIS)-Wildlife Service to implement the Wildlife Hazard Mitigation Plan (WHMP) at DIA. Recall if you will the US Air flight that landed in the Hudson River a while back. It drew attention to the long-standing fact that birds (or wildlife) and planes don't mix well. Every airport in the nation implements a WHMP and DIA is regarded as having one of the leading programs in the nation. Needless to say, USDA weighs in on erosion control plans as well. Wildlife is attracted to disturbed soil, so phasing and rapid revegetation is critical. Straw BMPs (bales, wattles, mulch, etc.) are discouraged because they provide shelter and nesting material. Seed mixes are reviewed and approved to ensure seed used is not an attractant to wildlife. Positive drainage must be maintained to prevent standing water from occurring. Riprap and soil riprap are a concern since they provide shelter to small rodents, which in turn attract larger predators. On your next trip out to DIA, you'll see miles of grouted riprap. Local agencies are not thrilled with it, but it's the only way to control the potential for habitat.

So what's left in the toolbox? In summary, our SWMPs rely heavily on:

• Administrative controls
• Continuous sweeping
• Buffer zones
• Dense, large-diameter wattles
• Gabions
• Grouted riprap
• Extended vehicle tracking pads
• Compacted vegetated berms
• Inter-project swales
• Rock socks
• Select tackifiers
• Select hydromulches
• Pre-approved seed mixes
• Revetment mats
• Concealed micropools
• Heavily anchored erosion control blankets

I believe you can see now why construction compliance is challenging at DIA. As we embark on our next phase of major expansion with the South Terminal Redevelopment Project and the addition of RTD's FasTracks East Corridor project, erosion and sediment control will, I am sure, present even more challenges. We've already been working with CCD Public Works to devise a permit structure that will best serve these overlapping projects. We've been challenged by multiple CMGCs working on top of each other in a small area, by state-of-the-art construction methods, by long steep slopes, by continuous operations occurring immediately at the perimeter of the project site, and of course by the high visibility of the work and the high interest in it. We know that many challenges lie ahead, such as being most likely to set a precedent for ELG-based compliance should that come into effect, but we're ready to tackle anything that crops up. After all, the challenges and the innovation needed to address them are what make it fun to work on construction at DIA in the first place.

If you want to hear more on how DIA is addressing these challenges, come see Catherine's talk at the Winter Conference!