Category: Stormwater

3 Things to Consider When Designing for Stormwater on Airports

Stormwater that ponds or collects on airports can be considered a hazardous wildlife attractant and pose a significant risk to aircraft and the flying public. According to the Federal Aviation Administration, there were 17,228 wildlife strikes at 753 US airports in 2019. In a previous blog post by Senior Airport Engineer Wilbur Mathurin about protecting wildlife, he mentions the importance of proper drainage on an airport, calling the stormwater design “a delicate balance between aircraft safety and providing adequate infrastructure to manage stormwater runoff during a storm.”

There are of course, some challenges when it comes to finding the right balance. Stormwater design is not necessarily straightforward, and there are quite a few different goals and standards that need to be met. Below are three important airport stormwater design concepts that provide some insight into what is considered during design.

1. Elevation

For the safety of the moving aircraft, airfield pavement and airport turf areas are designed with minimal slopes, making most airports relatively flat. When the differences in elevation from inlet to outlet are minor and the slopes of any channels or pipes might be minimal it can make it difficult to move stormwater quickly off the pavement and away from the aircraft. Additionally, stormwater may need to be stored to reduce the peak flow, or treated, both of which could impact some of the elevation you need to outlet. Stormwater chambers and/or some treatment systems are located underground, so you need to make sure that it isn’t too deep that you can’t get the water out.   

Another potential limiting factor with underground storage may be possible contamination from nearby fueling operations or by the depth to groundwater and/or bedrock. Impermeable liners can be used, but this eliminates the possibility of infiltration to groundwater and requires an outlet to an appropriate discharge location or integration into a closed drainage system.

2. Above ground stormwater detention

It would be ideal to avoid any and all areas of stormwater detention, but as mentioned above, underground operations are not always feasible. If above-ground detention is used, it is important to make sure that the stormwater will drain as quickly as practicable so that it does not become an attractant for birds. When using detention in combination with a soil filter for treatment, state design standards typically require that the water drains in no less than 24 hours for proper filtration and no more than 48 hours to avoid prolonged detention.

When treating stormwater, multiple infiltration ponds may be needed to collect stormwater from multiple impermeable areas. Each soil filter or infiltration basin requires a certain amount of space to collect properly, store, and filter the required volume of stormwater runoff. In some areas of an airport such as adjacent to the runway or taxiway, a long soil filter design might be more feasible, but it can be challenging to find enough space for the proper size system near aprons and buildings.

3. Permitting

To safeguard water quality stormwater treatment is required is required for runoff from impervious areas including airport building rooftops and airfield pavement. In addition to federal standards, the permitting process is specific to each state, so permitting requirements and treatment design solutions vary by airport. Stormwater treatment design at airports can include infiltration ponds, soil filters, subsurface sand filters or underground storage chambers with filtration, open-lined channels to convey the stormwater to another location, or meadow buffers if they are available. Before permitting, it is essential to check the data you will need to submit with the design. This can include test pits, bedrock and groundwater depth, soil type, hydraulic conductivity of existing soils, etc. Information will need to be obtained relatively close to the area of the proposed stormwater treatment, so the design must be closely coordinated with the data collection process. It is ideal for permitting to happen before the design is complete to start the review process with the permitting agency.

As airport design engineers, we understand the direct correlation between aircraft safety and stormwater management. Proper stormwater drainage design considers each airport’s unique terrain, storage capacity and treatment standards.  I am available to discuss any of these elements may be impacting your airport’s stormwater management and how we can help improve your facility’s system.

Sustainable Drainage: What are the Techniques for Protecting the Watershed?

Sustainable Drainage Systems are a collection of practices used to mimic natural processes of the hydrologic water cycle, which is the path of water as it moves around the earth and includes condensation, precipitation, infiltration, runoff, and evapotranspiration.  These sustainable drainage systems can consist of natural features or man-made features made to look and act like natural features (bioretention facilities, rain gardens, vegetated rooftops).  In the United States, Sustainable Drainage Systems are more commonly referred to as Best Management Practices (BMPs) or Low-Impact Development (LID).

What are Best Management Practices (BMPs) and Low-Impact Development (LID)?

The Environmental Protection Agency (EPA) defines Low-Impact Development as systems and practices that use or mimic natural processes that result in the infiltration, evapotranspiration, or use of stormwater to protect water quality and associated aquatic habitat. Why does this matter? As EPA notes, applied on a broad scale, LID can maintain or restore a watershed’s hydrologic and ecological functions.

Implementing LID practices allows the treatment of stormwater closer to the source using natural processes. Closer to the source means treating the water as close to where it reaches the earth’s surface as possible.  For example, stormwater that sheet flows off a roadway and is collected in a swale then treated by an LID practice is treating the water closer to the source than if the stormwater were collected in a closed drainage system within the roadway and conveyed several hundred feet away to a larger detention pond.  The swale (known as a level spreader) acts as a level swale that collects the stormwater and infiltrates it slowly into the ground, similar to what the water would do if the paved roadway were not present. For more significant flows that overtop the level spreader, a best management practice can provide some treatment of common pollutants, including total phosphorus (TP), total nitrogen (TN), and total suspended solids (TSS) (gravels in the stormwater) before the stormwater reaches a waterbody. 

Best Management Practices (BMPs) are defined as methods that have been determined to be the most effective and practical means of preventing or reducing non-point source pollution to help achieve water quality goals. Non-point source pollution includes TP, TN, and TSS.  Some BMP’s commonly used include bioretention facilities, rain gardens, vegetated rooftops, and tree box filters. Methods used to treat the stormwater include infiltration, filtration, detention, retention, and disconnection.  Infiltration and filtration are similar in the way the water flows through a media which provides the treatment.  The media used in infiltration practices it the natural soils which then convey the stormwater to the groundwater below. In filtration systems, the media is a man-made media, sometimes consisting of sand, sometimes a combination of sand/soil/and compost mixture, and sometimes a manufactured filter similar to filters you find in your house or car.  Detention and retention are similar methods, they detain water. Detention practices detain water for a short time while retention practices typically have standing water at all times. Both treat the stormwater by allowing pollutants to settle out of the water over time. Disconnection methods includes conveying stormwater from an impervious surface (rooftop or pavement) to a pervious surface (grass) to allow the stormwater to naturally filter through the grass and into the soils prior to reaching surface water or groundwater sources.

Protecting our Watershed with These Methods

So how do BMPs or LID practices help with flooding or protecting the watershed?

One way these methods protect our watersheds are through the filtration.  A rain garden is designed as a small depression in the ground that consists of various native plants planted on top of a filter media.  The filter media allows the stormwater to be conveyed through it and also allows for uptake of the stormwater through the roots of the plants providing treatment of the stormwater and evapotranspiration of the stormwater (release of water to the atmosphere from soil and plant leaves).  Stormwater that filters through the media can either infiltrate into the groundwater if the soils are conducive to that, or the treated stormwater can be collected in a pipe and conveyed to nearby surface waters.  Treating the stormwater is important because untreated stormwater that reaches a waterbody (wetland, stream, pond) can affect the plant and animal life in that waterbody.  This can lead to the degradation of ecosystems across the watershed.

A construction photo of underground storage chambers for flood control at Manchester-Boston Regional Airport.

Another way we can protect the watershed using LID is through flood control. Underground storage chambers can be used as flood control in areas where there is limited above-ground storage. These best management practices are common in urban developments, including shopping centers and stadiums.  They are commonly placed beneath parking lots (such as this one pictured above, at Manchester-Boston Regional Airport) and act as a large storage facility for stormwater which can then be released at a controlled rate to nearby surface waters or infiltrated into the groundwater.

This photo shows the park where an underground storage system will be provided. The usability and aesthetics of the park will remain the same as existing conditions after construction.

Our Recent Drainage Projects

In Massachusetts, we are working on two projects that are the same roadway and therefore have similar properties, although they are in two different towns.  A large portion of these projects is within a water supply reservoir watershed.  Its location means that treating the stormwater runoff is critical so that any contaminants in the runoff do not compromise the clean water in the reservoir. Massachusetts stormwater regulations are geared toward treating the stormwater at the source as opposed to collecting large volumes of stormwater and treating it in a larger detention basin somewhere down the road.  To achieve compliance with the regulations and treat the stormwater, we have designed LID practices such as forebays, level spreaders, and grass swales at as many outlet pipe locations (outfalls) as allowable based on site constraints, including right-of-way and topography. 

Sample engineering plan and section for sustainable drainage methods.

A town in Vermont is having erosion issues at the base of a steep, dead-end, gravel road.  The erosion issues are due to the lack of stormwater conveyance practices and the lack of storage of more significant storm events.  This section of town is upstream from a large wetland, however it is not hydrologically connected to it, which means stormwater does not directly get conveyed to the wetland.  We were tasked by the regional planning commission to design two stormwater treatment practices that would convey the stormwater to the wetland area without creating additional erosion or flood control issues downstream. Both treatment practices included underground storage chambers for flood control of the larger storm events.  More formal ditch lines and a closed drainage system were designed to collect the stormwater that currently flows over the gravel roadway. 

This photo shows the location of a proposed underground storage facility for stormwater with an above-ground rain garden. In the bottom right corner of this photo, there is an existing rain garden that will be expanded.

The upstream BMP was designed to infiltrate the smaller storms.  It was requested this BMP not permanently impact the adjacent Town Green area; therefore with the underground chambers, the BMP will not be visible from above, thus not impacting the character of the Town Green.  It was requested the downstream BMP include a bioretention area (or rain garden) above the underground storage chambers.  The town requested a more natural stormwater collection process and liked the visual aspect of what a rain garden offered.  The soils beneath this BMP were not conducive to infiltration, therefore the flow out of the storage chambers was conveyed toward the downstream wetland via a pipe.

Sustainable Drainage Systems are everywhere – you have probably seen them and not even known it!  Take a look around next time you are out and about and see what you can find. For more information about sustainable drainage, reach out to me!

Celebrating St. Patrick’s Day with a Focus on Green (Infrastructure)

Stormwater Detention Pond Construction

To celebrate St. Patrick’s Day this year, we wanted to highlight some the ways we embrace green infrastructure. Sometimes our projects are naturally geared towards sustainability out of necessity (i.e. so we don’t disturb wildlife or certain plant species), while other projects may not have a need for sustainability but we see a way to incorporate it (i.e. better stormwater management).

Before we get into some of the specific green infrastructure examples, it is important to establish some background. First, green infrastructure is not just a marketing term; it is defined by the US Environmental Protection Agency (EPA) as “the range of measures that use plant or soil systems, permeable pavement or other permeable surfaces or substrates, stormwater harvest and reuse, or landscaping to store, infiltrate, or evapotranspirate stormwater and reduce flows to sewer systems or to surface waters.”

In other words, green infrastructure is using natural means to move water rather than building more infrastructure like pipes or drains. These projects are highlights of our recent green stormwater infrastructure.

Green Methodology

As part of a consultant team, we have developed a methodology for the Vermont Department of Environmental Conservation (VTDEC) to prepare cost estimates for the implementation, and operation and maintenance of stormwater projects for all land use sectors (agricultural, wetlands, river/streams, lakeshore, forest roads/trails, roads, urban, etc.), as well as developed an O&M standards manual for continued operation of these practices. The final product was recently submitted to the State of Vermont and will outline how stormwater projects are planned, implemented and maintained throughout the State.

Green Bioretention

Bioretention

One project I worked on was to conduct an Engineering Feasibility Analysis for the City of South Burlington Municipal Office complex which had been identified as a location to evaluate and implement stormwater management improvements. The analysis resulted in the implementation of Low Impact Development (LID) measures. The designed stormwater management improvements included a bioretention facility, vegetated swale, and underground storage system.

The bioretention facility works to collect and absorb runoff from rooftops, sidewalks, and paved surfaces.  Bioretention practices mimic natural hydrology by infiltrating and evaporating and transpiring stormwater runoff.

Green Gravel Wetlands

This project addressed collected stormwater flowing into Bartlett Brook, crossing beneath Route 7 and ultimately into Shelburne Bay in Vermont. The design concept that had been developed by others through the Bartlett Brook Flow Restoration Plan process included capturing collected stormwater runoff from the neighborhood and conveying this stormwater to a City-owned parcel on the west and downgradient side of the neighborhood to allow for underground infiltration. Upon further investigation, site soil conditions were found to be unfavorable for stormwater infiltration. As a result, the Hoyle, Tanner Team designed a gravel wetland at the City-owned site to provide for flow detention and phosphorus removal of the collected stormwater runoff from the contributing residential drainage area.

Green Plans

Finally, but not least in importance, is planning for green infrastructure’s future. As part of a large integrated water quality planning effort for the City of Burlington, green stormwater infrastructure (GSI) projects were included as an important means to reduce stormwater flows and phosphorus loading to the combined sewer system in Burlington.

As part of meeting requirements for different Clean Water Act programs, Burlington’s Integrated Plan is intended to identify and prioritize water quality opportunities that result in a cost-effective program to achieve water quality goals. Tasks included the development of a comprehensive clean water alternatives analysis, including identification of enhanced phosphorus optimization at wastewater treatment facilities; identification of City-wide runoff (stormwater and wet-weather) opportunities; planning concepts for high-priority runoff mitigation projects; financial capability analysis; facilitation of public outreach; and development of an Integrated Plan with an implementation schedule. Burlington’s Integrated Plan development is currently in progress with an anticipated submission to State regulators later this year.

Stormwater runoff is a major cause of water pollution in urban areas, carrying trash, bacteria, phosphorus, and other pollutants from the urban landscape into our waterways.  Implementation of green infrastructure can help mitigate stormwater runoff by providing natural areas within the urban landscape that provide habitat, flood protection, and cleaner water. At Hoyle, Tanner, we look for ways to incorporate green infrastructure into our planning and design concepts to maximize the environmental benefits they can provide.