Tag Archives: GSI

KPFF’s Stormwater Cinema

Over the past few weeks, I have presented some concepts, design guidance and project examples around the topic of low impact development (LID). This post will wrap up the topic, at least for now.

As you may know – or should by now at least strongly suspect – my employer, KPFF Consulting Engineers, has a long history of designing LID projects. KPFF’s history with LID in Portland mirrors the city’s own experience, starting with our design of the bioswales in the OMSI parking lot, Portland’s first large scale LID project. This legacy carries through to today with our work on the green street retrofit of SE Division Street, which will be completed this summer and has been billed by the city as America’s first green main street. Through our project work, KPFF has worked closely with the city to stay on the leading edge as standards have evolved, to the point of helping to develop some of the tools and guidance that are now required elements of stormwater design in Portland.

Last year, in order to celebrate this history and to take stormwater design to the streets, KPFF launched the Stormwater Cinema series. This group of short films has highlighted a few of our more unique stormwater projects along with some of the design considerations that went into those projects. Having featured one of these shorts – The Stormwater Toolbox – in a previous post, I thought this might be a good opportunity to share the other parts of the series.

Under One Umbrella

Though many had faith in the idea, no one was quite sure how successful Stormwater Cinema would ultimately be, but Under One Umbrella – the series’ first release – passed even the highest expectations. Highlighting a unique stormwater art installation, this clip went about as viral as any stormwater engineering content could ever be expected to.

A Garden to Play In

The series’ second video focuses on Tabor Commons, a project that brought together neighborhood residents, local designers and community building organizations. Engineers at KPFF have donated many hours to this effort over the years, both in the office and on the site.

Stay tuned to KPFF’s Vimeo channel so that you won’t miss the next great installment of the Stormwater Cinema series!

LID at Metro’s Gleason Boat Ramp


Last week I introduced some design ideas for Low Impact Development (LID). One of these ideas is what I am calling LID layering – a decentralized approach in which the designer incorporates multiple layers of LID elements throughout the drainage path. The land side component of Metro’s M. James Gleason Boat Ramp site is a good example of this technique. While nothing more than a big parking lot, design features throughout the project combine to create a system that treats and infiltrates stormwater runoff onsite in all but the biggest storms.

Completed in 2013, Gleason is the result of a master planning effort that reaches back to 1999. Oregon’s most used boat launch provides urban access to the Columbia River for users of all types, and conveniently hosts extensive parking for both trailered and non-trailered vehicles, restrooms and a river patrol office. Master planning and design was completed by KPFF Consulting Engineers with landscape architecture by Mayer Reed and plumbing and mechanical design by MFIA. Engineering features of the site include a debris deflection wall and both traditional and bio-engineered slope stabilization measures.


The tree canopy is the first possible point of contact between falling rain and the Gleason site. The site’s main purpose is providing parking for people using the river and admittedly landscaping was a secondary consideration in the design process, but there are trees and over the coming years they will grow to cover portions of the parking lot. The truth is, though, that most of the rain at Gleason will land on pavement.


Pavement type is Gleason’s second LID layer. While heavy loads, high levels of use and cost considerations dictated that most of the site be paved in asphalt, permeable pavers were used on the single car parking spaces, which will see much less traffic. Rain on these areas directly infiltrates, reducing the amount and delaying the concentration of runoff from paved areas. The change in material also adds a nice aesthetic. As with canopy cover, permeable pavement is a small piece of the storm drain system, but the cumulative impact of small measures early in the drainage path can be as important as larger downstream measures when developing a layered LID system.


Perhaps the most significant piece of the drainage approach at Gleason is the fact that, except in isolated areas, all of the pavement is sloped to sheet flow to stormwater facilities. This avoids concentrating flows – both at the surface and in piped systems – and led to more flexibility in designing the site’s stormwater planters. This is not to mention the fact that a flat, smooth parking lot with few or no drainage structures makes a better driving surface in both wet and dry conditions than one littered with dips, valleys, warps and catch basins.


Flush curbs were used on most parts of the Gleason site, providing both edge restraint for the pavement and walls for the adjacent stormwater planters. As opposed to collecting water in catch basins, shedding it from the surface into the planters delays flow concentration and allows the system to be much shallower. Wheels stops were added where the curb line also needed to serve as protection for parking vehicles, and slotted curbs where planters run adjacent to traffic lanes.


At this point, it is worth noting an often overlooked LID layer. As stormwater flows across paved surfaces, it picks up a large amount of the sediment that cars track onto the pavement. Much of this sediment can be quickly filtered out by directing water through a short conveyance swale or across a small filter strip before it enters a larger stormwater facility, resulting in better water quality and less maintenance. These features were incorporated into Gleason’s design where possible, but – as can be the case – space and geometry constraints precluded their use in some instances.


Runoff entering Gleason’s stormwater planters infiltrates fairly quickly, thanks to the site’s well drained, sandy soils. Because of this, it was possible to over design some planters by basing the planter depth on what the surrounding curb walls could support and not on what was required to meet the minimum standards. Designing to the “maximum extent feasible” like this is a core LID principle. In many cases it is possible to exceed minimum requirements without adding cost to the project. Where planter bottom elevations varied, steel weirs were installed to ensure that different levels filled together.


Some larger storms will exceed Gleason’s onsite stormwater management capacity. In these cases, water will overflow through raised catch basin structures and outfall into the river. Each planter also has an overland overflow route to the river that would be used if its basin were to clog. This route could also be used in extreme storms, but most of the site is below the flood plain and could be underwater in those cases.

Gleason’s storm drainage system is a good example of using LID on a pavement and vehicle centric project and counters the view of LID as a “greener” design option that only works on heavily landscaped or naturalized projects. In truth, while they work better in some cases than in others, LID concepts can and should be used on every project. These concepts don’t as much represent a new group of options, as they signify a shift in what is considered best practice in stormwater engineering design.

While there is a significant environmental benefit from this shift, there are economic and functional benefits, too. LID measures saved Metro money on the Gleason project by reducing the amount of pipe and number of drainage structures that needed to be installed. These same measures resulted in a simple grading and storm drain design that makes the site more friendly to drivers.

With simultaneous benefits to the function, economics and sustainability of civil design projects, it is clear that LID is worth the focus and advocacy it is currently being given.

Engineering Concepts in LID

Last week’s post compared how the forest ecosystem and Portland’s municipal sewer manage stormwater. The vision offered for closing the gap between these systems is one spin on what has been dubbed low impact development (LID) – a stormwater management approach that emphasizes using natural features and systems to protect water quality. LID carries a promise of improving water quality and lessening our impact on the natural environment while saving municipalities money by reducing the demand on our aging sewers. Because of this potentially big benefit, public agencies have grown increasingly interested in adopting LID methods, especially here in the Portland area.

As a part of the “Stormwater Cinema” series, KPFF has produced an excellent short film that introduces LID and the engineering behind it, the stormwater toolbox.

So, with that, how should we as engineers approach LID design? What are the key design issues that will help create a successful LID project? To oversimplify things a bit, the goal of LID design is to create a man-made environment that mimics the natural one. The LID designer can best address this goal by decentralizing and layering the design and lengthening the time it takes stormwater to pass through the system. The impact of the resulting design can best be analyzed using three unique but closely related dimensions: time of concentration, stormwater quality and stormwater disposal.

Time of Concentration

Time of concentration (TOC) is a site characteristic, much like the amount of shade or amount of vegetation are. While it can change over time, TOC is independent of intensity, duration and other storm characteristics. Measured in time, TOC is defined as the length of time between when it starts raining and when the flow through that site’s storm drain system reaches its peak.

An interesting relationship exists between TOC and the performance of a storm drain system. As TOC increases, the peak flow rate in the system decreases. This is not to say that increasing TOC will reduce the total amount of water leaving the site – though depending on the method it can – but instead that it will result in the same amount of runoff leaving the site over a longer period of time, creating a longer, lower peak. This relationship is illustrated in the Intensity-Duration-Frequency curve at the link below. Such curves are commonly used to calculate peak flow rates.


Stormwater Quality

The quality of stormwater runoff is a measure of how much pollution that water contains. Higher quality stormwater has less pollution while lower quality stormwater has more. Most municipalities require that the pollutant level in stormwater be brought below some defined level before it can be discharged into a public sewer. Many LID designs – including rain gardens, vegetated swales and flow through planters – have become widely accepted methods of addressing this need. Mechanical systems for treating water are also available and are good options when site constraints preclude the use of LID designs.

Less concentrated, slower moving flows typically can help in the quest for higher stormwater quality as they are less likely to result in erosion and more likely to allow suspended sediments to settle out. While pavement and pipes allow for efficient drainage designs, they also increase velocities and concentrate flows, leading to a need for additional downstream water quality treatment. Project designers often have to take conscious measures to prevent water from concentrating in order to keep velocities down.

Stormwater Disposal

Every drop of water that falls on a site is ultimately disposed of in one of three ways.

  • Offsite disposal – which could include connecting to an offsite sewer system or sending water directly to a water body
  • Infiltration – which could include sending water to a rain garden or infiltration gallery
  • Onsite use – which could include rainwater harvesting for irrigation and greywater systems as well as evaporation and use by plants

Before large scale human development, much more runoff infiltrated and the current thrust of design is to return to this condition. Of course, this goal can be at odds with the need to have hard, durable walking and driving surfaces, so it is important to carefully consider how these surfaces are designed and what areas can be reserved for stormwater management.

The space available for stormwater management can play a large role in determining which methods of disposal can be considered on a particular project. While connecting to an offsite sewer system is usually the least desirable outcome, it is often the only viable option, especially when stormwater management is not a priority in design decisions. Infiltration is a low cost option for disposing of stormwater, but requires space and works best when facilities can be decentralized and runoff can be directed to them without the use of catch basins and pipes. Onsite use is an attractive disposal option, but the reality for most projects is that the money saved by using onsite water will outweigh the added cost of rainwater harvesting. Further, there is usually much more water available during storms than can be captured and stored for future use, especially considering that there can be little need for irrigation during the wetter times of year.

LID Layering

As stated earlier – and to wind the points above together a bit better – a solid approach to LID design includes:

  • Lengthening the amount of time that it takes for runoff to pass through the storm system
  • Avoiding concentrating flows and increasing velocities
  • Maintaining a decentralized approach to stormwater management

Because individual LID techniques often can not address all of the stormwater needs on a site, many projects can benefit from a decentralized, layered approach to stormwater management. By layering LID elements, designers can address TOC, stormwater quality and stormwater disposal issues in a simpler form, before flows have had the chance to concentrate and accelerate. Many elements – like eco-roofs, trees, pervious pavements or check dams – can add significant value at the beginning or middle of a drainage path, leading to a less complicated design challenge at the end of that path.

Since it can be helpful to approach problems from several different angles, try imagining you are a rain drop falling on your site. What is your first contact with the project? Do you land directly on pavement, where you immediately start picking up speed and pollutants; or is there a chance that your first contact could be with the canopy of a tree? Once on the ground, are you allowed to move in a well spread sheet flow; or are you directed toward a drainage structure where you will become part of a larger, concentrated flow? As you move across the site, do you pass through any areas – either in the air, at grade or underground – that could be considered a missed opportunity for bettering the storm drain system?

Hopefully consideration of these questions and the core design issues raised in this post will help you in developing high quality, innovative designs that address the unique challenges presented by your projects.

Three Raindrops


The big leaf maple lower canopy from Forest Park’s Leif Erikson Drive

On a rainy winter evening, three raindrops fall over Portland.

Raindrop A falls into Forest Park, passing through the canopy and the undergrowth and landing softly in the soil. ‘A’ makes its way across the forest floor, travelling over terrain, around fallen trees and through beds of needles until it enters a seasonal creek. Life speeds up here and before long A arrives at the Willamette River, the Columbia River and the Pacific Ocean.

At the same time, raindrop ‘B’ lands in Portland’s downtown, seven miles upstream. B splashes on the pavement and rolls across the ground into a catch basin grate. B enters the combined sewer and mixes with other raindrops as well as sewage from nearby buildings. Continuing downhill to the Ankeny Pump Station under the west end of the Burnside Bridge, B is pumped across the Willamette River and travels through another 20 miles of sewer before arriving at the Columbia Boulevard Wastewater Treatment Plant in North Portland. The plant, one of two in Portland, will clean B through a series of physical and chemical processes and eventually discharge it to the Columbia River, about 10 miles upstream from where A entered.

Though they ended up in the same place, there are many differences between the routes A and B followed. First, the forest ecosystem avoids concentrating flows and encourages local disposal of stormwater while the urban streetscape is designed to concentrate flows and deliver runoff from a huge area to a single place. In both cases this is by design. The forest evolved over millions of years to value water as a resource. The city sewer system was designed to avert the health, safety and convenience issues that come with standing and improperly treated water.

By slowing and spreading flows, the forest protects the quality of its water. Slower moving water does not pose the same risk of erosion that fast moving, concentrated flows do. If water does become polluted with sediment or excess nutrients, the sediment will likely settle out and the nutrients will be used by native vegetation. Conversely, nearly every step of the urban process increases water pollution. Tail pipe exhaust is trapped on pavement surfaces and washed into the sewer. Stormwater in combined sewers mixes with sewage making black water, a blend that is both difficult and expensive to clean.

There is no single way to reconcile the gap between natural and urbanized systems and still maintain the level of stormwater infrastructure needed to support a city. There is, though, a growing toolbox of stormwater facility designs that has given engineers the means to begin to address the issue.

Imagine now raindrop C, falling on a new greenway trail along the Willamette. The project design team prioritized establishing a healthy canopy so C lands in a tree and may not ever reach the ground. If it does, C might land on pervious pavement and soak into the soil. Though the trail supports a high level of traffic, it is bicycle traffic, so if C leaves the pavement it will not carry pollutants with it. Once on the shoulder, C might run through a short swale, allowing sediment to be filtered away before it enters a rain garden where it will sit until it either soaks into the ground or evaporates. Much like raindrop A, each step along C’s path keeps flow dispersed, encourages local disposal and protects water quality.

C’s path illustrates just a few of the tools in the stormwater toolbox, a collection that has been evolving for several decades now and will continue to do so. As it does, it is important to remember that, like the forest, the process starts small. Creating a more natural urban environment takes time, and significant improvements are only possible with continued commitment and dedication from everyone involved.


This post is the first in a series about green stormwater infrastructure and low impact development. Check back over the coming weeks for more on design considerations and some examples from KPFF’s project experience.