Detailing Vegetated Riverbanks

South-Waterfront-Jute

Awhile back, in “The Engineer and the Fisherman”, I explored the unique challenges posed by trying to stabilize urban riverbanks using natural or vegetated measures. This post generated a lot of interesting feedback – much of it through the Urban Design Network LinkedIn group. This interest combined with the fact that the subject is so broad and complex presents an opportunity to deepen the conversation and more specifically target some of the engineering issues contained in these designs.

Approaching such a complex issue is daunting, but as with most engineering problems the discussion can be simplified by breaking the original topic into understandable pieces. This approach is the basis of my “Framework for Understanding Details”. Under this framework, we segregate constraints into one or more of three categories – programmatic, environmental and physical constraints – with the intersection of the categories representing the available design choices. This relationship is represented in the Venn diagram below.

Framework-Venn

Programmatic constraints are the limitations imposed by the project owner. Looking at the topic of naturalized riverbank design through the lens of this framework, there are two likely programmatic constraints that should concern the designer. First – and especially in urban settings – it is very likely that the project owner wants a bank that will not move in the long term. When some bank movement is acceptable, the amount is usually small and more or less only serves as an indication of the level and urgency of necessary maintenance. In these cases, the bank is usually returned to its originally designed configuration with routine maintenance.

Second, by targeting a naturalized riverbank, the project team adds a programmatic constraint that severely limits the group of available designs. Native vegetation provides the primary long term scour resistance needed to hold these banks in place, but its presence is more an indication of the underlying bank structure than it is the cause of that structure. In the same way that a building’s architecture is supported by a tailored structural design, many factors must be just right before vegetation can be sustained over a long period of time. Of course, the degree of this constraint can vary. There is a big difference between a design that uses natural elements as the primary structure and a design that has the desired outward appearance but is supported by a more conventional engineering approach below ground level.

These programmatic constraints are at least to some degree at odds with each other. Natural riverbanks are flexible, moving and changing over time – a feature that does not mix well with urban development. Project owners spend huge amounts of money developing property and they hire engineers to give them a level of assurance that their investments will be protected through storms, floods, earthquakes and whatever else time might bring. Very few options are left after constraining the design to solutions that use natural elements and create a relatively inflexible bank, and this is after only one category of constraints.

Environmental constraints are the limitations imposed by the interactions between the project and its environment. Not surprisingly, the driving interaction on riverbank projects is that with the river. From the river’s view, most projects occupy a very small amount of space – sitting high on the bank. This can mean that there are global stability problems that can’t be dealt with in the scope of a typical project. Many of these problems radiate from deep within the river’s main channel and can only be dealt with by creating a last line of defense – often soil improvements or deep foundations – at the face of the development. The sketch below illustrates one possible way that a global problem could affect the stability of a project.

Riverbank-Global-Stability

These sorts of topography considerations are one of many factors that contribute to a river’s flow characteristics, which in turn define the stresses that the river imposes on its banks. Variations in the bank line can result in an infinite number of scenarios in this regard, but longitudinal stress – the stress imposed by water moving along the bank or down the river – and perpendicular stress – the stress imposed by waves moving up the bank – are the two parameters that will combine to drive the design. These values depend on the precise conditions for each project and vary from site to site, but generalizations can often be made for specific stretches of a river. For example, bank design on large river systems that carry shipping traffic may be controlled by the waves thrown by passing barges, while design on smaller creeks or streams might be controlled by the speed of the current during times of high water.

Many of the river related habitat and scour issues that our communities face today result from designs that did not account for their impact on downstream systems. The limitations imposed by downstream interactions are very much environmental constraints and need to be quantified and addressed as such. That said, most vegetated bank designs result in slower, less channelized flows and positively affect downstream properties. This leaves global bank stability, river flow related shear stress and wave related shear stress as the key environmental constraints for typical naturalized riverbank designs.

Physical constraints – the limitations imposed by such aspects as location and constructibility – make up the third category in the framework. The most global of these constraints for this conversation involves slope. Over time, many of the riverbanks that support our cities have been steepened to create more developable land. Returning these banks to their natural state usually requires returning them to something closer to their natural slope, creating two challenges.

Just as banks were originally steepened to create more developable land, flattening these banks requires giving some of that land back to the river. This is possible in some cases, but can be problematic on sites that host buildings or other features that are slated to remain. It can also be difficult to transition the re-flattened bank to meet adjacent properties. These transitions often have to be made gradually to avoid creating scour pockets and can lead to a scenario where most of the site is transition and very little is actually a fully naturalized design.

Along with topography and slope considerations, plant establishment time is a very real limitation that must be addressed in order for a naturalized design to succeed. Most natural riverbanks rely on native vegetation for scour protection, without it these banks would wash away. But even in the best cases it takes several years for a newly planted bank to develop the root system necessary to provide this protection, meaning that an interim solution must be integrated into the design. Ideally, this interim measure is biodegradable so that the support and structure it offers will lessen as the vegetated system gets stronger.

Coir mat* – made from coconut shell fibers – can be a useful cover in these situations as it is natural, biodegradable and available in many forms. Like most erosion control blankets, coir mat can be found in both woven – think burlap – and nonwoven – think furnace filter – forms. The woven version is strong and provides good resistance to shear stress. At the same time, though, it has large openings and on its own will allow soil to wash away. Nonwoven coir mat has small openings and is very effective at preventing soil loss, but it is not very strong and would likely rip under the shear stress imposed by swift river currents. Because each type of coir matting has a strength that supports the other’s weakness, an obvious solution is to layer both woven and nonwoven matting on the bank. In this application, nonwoven matting is placed directly on the graded bank to provide soil retention, then woven matting is laid over the top to provide shear resistance.

Of course, even the best matting design will be of little help if it isn’t properly attached to the bank. The attachment system has to be simple and constructible while at the same time providing resistance to horizontal and vertical movement across the entire bank. After working through several designs, the process that I feel best meets these goals is to lay the matting on the bank, stake it in place with a close grid of long, square stakes, then weave twine around and between these stakes. In this configuration the stakes offer resistance to horizontal movement and hold the blanket in place during construction while the twine offers vertical support, preventing the mat from floating or warping. The photo at the beginning of this post shows one example of this type of installation, taken at Portland’s South Waterfront.

With all of this as background, the following is a revised version of the previously presented Venn diagram summarizing the programmatic, environmental and physical constraints that are imposed on a typical naturalized riverbank design. Each project will include many more constraints than those discussed here, but this set is meant to be a fairly standard collection for this type of project. In order to succeed, a riverbank design must address all of these constraints.

Framework-Venn-Riverbank

You can see that while there are many design options that can be categorized as naturalized or vegetated solutions, there are really very few choices that will work with all of the constraints laid out here. This is not meant to imply that designers shouldn’t try to solve this problem though. The civil engineering profession exists to develop creative solutions to problems like this one and civil engineers have to some degree sidelined themselves in recent years by avoiding the risks necessary to confront these challenges. By dissecting and analyzing the project constraints, we can better understand both the steps we need to take to address them and the implications of pushing these boundaries.

Have I overlooked any common constraints that you have encountered on these types of projects? Please feel free to post them along with any other thoughts you might have on the topic in the comments section.

*The original version of this post recommended jute mat, which is also a good product for this application, but is not as strong as coir.

A Framework for Understanding Details

Desk

Well thought out details are the essence and foundation of good engineering design. Too often engineers are tempted to reuse something that has already been developed without analyzing whether it is the right solution for the project at hand. Details must respond to the specific programmatic, environmental and physical constraints that are imposed on the project. When details miss this goal, the project falls short of what it could have been, even if no real problem is created. When they are successful though, the project has the opportunity to reach its full potential.

My vision in developing this blog is to use my perspective and experience as a civil engineer to promote and demonstrate the design processes behind the infrastructure that surrounds us. By analyzing engineering details we see a microcosm of these processes – a slice of design that can be understood independently from the larger project. Before we can approach this analysis though, we need a framework for understanding what a detail is and what it does.

Fight as we may, the truth is that every project has a list of solutions that would be great, but for different reasons don’t quite work. For example, by building roads out of permeable pavements we could significantly change how our infrastructure affects natural water quality and flow patterns, but the materials presently available just can’t support the level and type of traffic carried on even a typical city street. Maybe instead we could put our road network underground; then stormwater would never come in contact with pavement and we could use any material we like. It may not surprise you that the price alone of such a project would prevent it from being considered, particularly in an age when transportation funding can barely keep up with road maintenance costs.

It is then not hard to argue that many of the most significant design criteria for a project are in place before the design team ever begins work. For the sake of our framework, I am going to break these constraints into three categories.

Programmatic constraints are the limitations imposed by the person or group that oversees the project. This could be a public agency, a private developer or people in any number of other positions. By the time the design team has begun work, this owner has usually established much of the program for the project including what functions the site will serve, how much area will be committed to each of these functions and how much money will be made available for the project. Programmatic constraints must be observed because it is difficult to justify spending money to develop a project if that project does not meet them.

Environmental constraints are the limitations imposed by the interaction between a project and the surrounding environment – both upstream and downstream. This is not limited to the ecological environment – though that is certainly included – but extends to all outside interactions including elements like traffic. Upstream interactions – how the environment affects the project – are created independently from the project, and the designer has no control over them. The designer has more control over downstream interactions – how the project affects the environment – but this control is still limited. In either direction, the project will not fit into the bigger picture without close attention to environmental constraints.

Physical constraints are the limitations imposed by such aspects as location, topography and constructibility. These are real world issues with an active presence on the site. Physical constraints are primarily determined by the project site, the properties of the selected materials and the means and methods available to the contractor. A good idea is a wonderful thing, but that idea won’t become a reality if it doesn’t work with the physical constraints.

In order to be a good solution for a particular project, a design must simultaneously respond to all of these constraints. Showing this requirement as a Venn diagram results in the following infographic.

Framework-Venn

Projects are rarely equally constrained in all three categories. One category or another will often dominate the system. In general, imposing more constraints will lead to having fewer options, shrinking the circle and its intersection with the other categories. Conversely, imposing fewer constraints results in more options, expanding both circle and intersection. This change can be shown visually by making a few small changes to our original diagram. The main point here is that fewer constraints and more available solutions will usually lead to more flexibility in design and a better final solution.

Framework-Venn-Resize

In addition to representative circle size, the amount of overlap between categories can vary. In a perfect world you would find a long list of solutions that work for multiple categories. Occasionally one category will even be completely contained as a subset of another. On the flip side though, it is possible for projects to be constrained to the point that one category is completely separated from the others. Such an overly constrained system leaves no design options that fit the overall project and means that a change has to be made to some programmatic, environmental or physical element for the project to be viable. Again revising our original Venn diagram results in the following graphic, showing how a more closely aligned set of constraints leads to more flexibility in design.

Framework-Venn-Shift

In applying this framework to design work, it is important to note that there is a difference between given constraints and self imposed constraints. Again, by the time the design team begins work on a project, many of the constraints are already established. Seeking flexibility and creativity in our solutions and design work, we should avoid adding to these constraints when possible or at least recognize when we are doing so. For example, by limiting ourselves to designs that have a standard detail or designs that we have worked through on previous projects, we can artificially make the circles in our Venn diagram smaller across the board. This is not always a problem, but it is important for this to be a conscious decision.

Next, while it is important to identify self imposed constraints, it is equally important not to dwell on things that can’t be changed. It can be tempting to argue editing a project element to make a seemingly otherwise perfect idea fit, and doing this can be an important part of an open creative process. There is a point though where we have to realize what is possible and what isn’t and direct our energy and creative effort toward the former.

Finally, whether you agree with the way I have categorized project constraints or not, hopefully it is clear that grouping these things in some way will allow you to see the options you have in approaching design challenges. Design is a creative process and it is easy to hit a dead end and not know what to do next. By stepping back and approaching the project through the lens of a different group of constraints, you can continue to make progress while giving your mind the time and space it needs to work through the more difficult problems.

I hope this post has given you some new ideas that you can put to work in your own design efforts. I would love to hear how this framework compares to your own views on the subject and I encourage you to post your thoughts in the comments section below. Moving forward, I plan to use this framework to explore the process that goes into developing details, to analyze common designs and to continue to explore the form and function of the infrastructure around us.

Achieving Zero by David Evans and the Value of Experience

Achieving-Zero_David-F-EvansAmong a constant stream of celebrity news, it can sometimes feel like we engineers and our profession go unnoticed, unappreciated or both.

The big news here last week was that Portland Trailblazer Damian Lillard might be changing shoes next season, with the branding decision going to the company most willing to spend. Lillard had a very good rookie season last year – making the NBA All-Star team and being voted Rookie of the Year – and this gives him the opportunity to opt out of or at least renegotiate his endorsement deal with Adidas. You could guess that a lot of money is hanging in the balance.

Lillard has been an outstanding professional basketball player for a little more than one year. Before that, he had three and a half great seasons as a college player. He has done all of this while playing in arenas supported by municipal power, water and sewer utilities. Each night, fans arrive at these arenas on regional streets, highways and public transportation networks. From home, more fans watch Lillard through our country’s far reaching fiber optic and satellite services. Systems pile on top of systems to build  a whirring machine of public infrastructure, all of it designed and built by architects, engineers and contractors with decades of success, hard work and experience. It is this machine that makes Lillard’s fame even possible, yet the people who conceived of it and gave it life are forgotten in history – if they were ever known.

In his 2013 autobiography Achieving Zero, David Evans – founder of the multidisciplinary civil engineering firm David Evans and Associates – argues that this has missed the point. Consulting engineers do not aspire to be exalted for their work, but instead set out to deliver to the client all that was originally offered.

“The consulting engineer virtually promises to make the required miracles happen, to meet and exceed all expectations, and to do so on time and on budget. Upon doing so, no significant recognition of this success is expected from the client. You have only accomplished what you proposed. You have achieved a Zero. Doing less than you have proposed, well, you go down from there.”  – David Evans, Achieving Zero

Herbert Hoover - 31st President of the United States and perhaps the most famous civil engineer ever
Herbert Hoover – 31st President of the United States and perhaps the most famous civil engineer

Famous civil engineers seem to fall into one of two categories – those with dams named after them and those with companies named after them. Much has been made of the personal history and philosophies of the first group, while those of the second have gone relatively undocumented, leaving them with only their professional success and personal satisfaction. Which is plenty! That isn’t my point. My point is that not every engineer retires having founded their own successful, stable, long standing firm – in fact very few do – and it is to the detriment of the profession that we do not seek to better understand the principles and practices that set apart the most successful among us. Politicians and musicians are expected to understand these things for their fields. Why shouldn’t the same be true for engineers? Every successful engineering project starts with an understanding of the work that came before it. Why shouldn’t the same be true for every successful engineering career?

Achieving Zero is a short, enjoyable read. Dropping the specification heavy language we engineers so love, Evans writes in an approachable, conversational style. You can imagine him sitting in front of you telling his story just as easily as you can picture him sitting down to grind out a personal memoir. This is not to say that the book lacks content. In fact, content is mostly all that is left, with every passage boiled down to what the author really wants the reader to take away from his book.

David Evans (photo from deainc.com)
David Evans (photo from deainc.com)

Almost everything I know about David Evans I learned in this book. Still, I feel like this gives a good depiction of who Evans is and the qualities that made him a successful engineer, business man, citizen and leader. It would appear that Evans’ career was equal parts ability and ambition, hard work and risk taking. Evans clearly must be an exceptional engineer – this after all is what keeps the clients coming back. Still, there are many points in his firm’s story where you could imagine the thing fading away and where Evans recognized that a big risk was needed. These risks paid off with amazing frequency, which always seems like a good indication that someone is being humble and really has expertise beyond what has been admitted. It is one thing to be lucky, quite another to be lucky all the time.

The more experience I gain in my career, the more I realize how easy it is to under value it. Like a tree, a career starts with the knowledge that is in the seed and grows over a long time. Through the seasons, there are always chances to be cut short or give in to the elements. Those that make it to maturity have weathered these challenges and more, and bear visible scars from the experience. The true beauty of old growth is not its perfection, but its lack thereof – the blown out top, the fire scarring, the trunk that has bent and twisted over a lifetime of reaching for the light. These are signs of the tree’s many triumphs and it rightly wears them proudly.

David Evans is the giant fir tree that people hike miles into the forest to see – the one in the picture with you and your friends trying wrap your arms around its trunk. Now nearing the end of his career, with Achieving Zero, Evans has provided the profession with the nurse wood that is often missing in our profession. In a culture that is prone to labeling a quick, alder-like rise to the top as triumph, David Evans is an example of true professional success and deserves to be respected as such.

I highly recommend Achieving Zero to anyone who is interested in the consulting business or the engineering profession in general and give it a perfect Zero.

The Engineer and the Fisherman

The grand canyon of the Yellowstone River in Yellowstone National Park
The grand canyon of the Yellowstone River in Yellowstone National Park

Engineering and fishing have a lot in common. The engineer and the fisherman must both understand the setting they are working in. They must both recognize and anticipate the natural properties and behaviors of the water and land. They must both work not with a strict formula, but with a goal in mind, adjusting their approach as conditions change.

As a civil engineer I’ve had the opportunity to work on several riverfront projects in the Portland area, and as a fly fisherman the good fortune to explore the western US’s many rivers. Through these experiences, I have come to realize there is a large gap between how fish and I see and use our rivers. Even after fluid dynamics, hydraulics and hydrology classes, my ability to find the fish’s favorite seam between protection from the current and a steady stream of food is crude at best. I have spent hundreds of hours standing in the river, watching the bank, but my trust of nature’s methods in protecting land from water is still given only with hesitation.

The former is a fact of experience and there is probably little that can be done about it. The latter, though, hints at a basic conflict between nature and engineering – at least traditional engineering – in the area of bank stabilization. In short, nature doesn’t stabilize banks, but instead leaves banks to stabilize themselves. Nature’s system is based on balance. If that balance is maintained, the bank is “stable”. If a characteristic of the river changes and upsets that balance, the bank changes, too. This is why river banks move from year to year and season to season as river levels go up and down; it is a natural and beneficial part of the river ecosystem.

Cross section of a typical riprap slope (source: FHWA)
Cross section of a typical riprap slope (source: FHWA)

But engineers usually prefer to have more control than this system allows. Or, maybe better put, property owners prefer that the river stays where it is if they are going to invest in site development, and they hire engineers to make sure this happens. This is traditionally done by “armoring” the bank with a layer of rocks that are sized to be big enough that the river can’t move them, even during extreme flood conditions. Sizing these rocks involves analyzing the flow, forces and movements of the river in both its present and future forms.

The issue gets more complicated when you imagine multiple sites being developed along the same stretch of river over a period of time. Armored banks channelize stream flow, increasing the speed of the water flowing through the river and pushing erosion and bank stability problems downstream. Seeking balance, the river starts to move and downstream property owners are forced to either armor their banks or watch their land wash away. As more armor is added to the bank, there are more impacts to segments that are not armored and the cycle continues.

So, traditionally, engineers have had little choice but to armor up in response to upstream changes and imbalances, especially in urban environments. This is an unwelcome trend for fish and fishermen alike. Varying, vegetated bank lines provide fish traveling on the river with much needed feeding and resting ground. Channelization and armoring put pressure on resident fish like trout, making their survival more difficult, and create a sort of gauntlet for migratory fish like salmon and steelhead, decreasing their chance of ever reaching spawning ground.

Fish aren’t alone in their dependency on natural river banks either. While the conditions necessary for them to thrive are a good baseline for other species, the net casts much wider. Osprey, hawks and eagles need a plentiful supply of fish near their nesting sites. Otter, beaver and other small river mammals need lush bank habitat. Even the plants that the ecosystem turns on need other vegetation to protect them while they take root and begin growing.

Recognizing these many needs, the current thrust in river bank design is toward naturalization – using nature’s own systems to protect the bank. This bio-engineered approach has seen great success in many settings, creating extensive environmental benefits and decreasing downstream impacts. As with low impact development, bio-engineering techniques hold much promise. It is important to note, though, that they carry uncertainties and complications that more traditional methods do not.

Cowlitz River in Washington's Giford Pinchot National Forest
Cowlitz River in Washington’s Giford Pinchot National Forest

In a balanced system, all players do their part to maintain equilibrium. Conditions may change and the river may go through a period of instability, but it will eventually find a new balance point. But, after decades or more of channelization and armoring, many developed river stretches are far from balanced and are more inclined to wash out to sea with the first chance they get than to immediately establish any kind of natural order. The big picture planning in this type of environment can be as difficult to approach as the site specific engineering design.

Engineers like to use design elements that allow them to derive direct solutions to the problems posed on a site. Riprap bank protection is one example of this. The behavior of riprap slopes has been extensively studied and is well understood. For a given configuration, the engineer can determine expected damage that the slope will sustain in a particular storm, addressing site needs and defining and limiting the risk for everyone involved in the project. This is the definition of sound and responsible engineering.

Bio-engineered solutions present a challenge to this definition as there is less research to draw on and they often lead to more nuanced designs. Designs using these methods are developed empirically – instead of directly – by following available best practices and learning from failures that resulted from similar projects, and can require that the developer be willing to rework the bank in response to post-construction problems. Of course, uncertainty is usually a bad thing in this context as no one starts out with the goal of spending a large amount of money to create a failure that will inform future designs.

The challenges are even greater when a project attempts to naturalize a section of bank in a fully armored environment. Success or failure in such an instance can hang on characteristics like where the site is located with respect to movements along the river or what neighboring property owners are planning for their sites – issues that the project team doesn’t have any control over. Blindly incorporating design elements without regard to these characteristics is irresponsible engineering and can lead to catastrophic problems. Similarly, though, disregarding bio-engineered solutions off hand forgoes ever starting the naturalization process and could lead to missing big opportunities.

The effort to transition from traditional to more naturalized bank designs highlights a clash between two responsibilities that all engineers share: a responsibility to the public to create safe, well founded designs and a responsibility to the profession to innovate and improve on traditional methods. Reconciling this conflict requires being realistic about what is possible, taking risks when appropriate and maintaining a consistent effort to change the paradigm. What does this mean in the context of bank design, then? How can we as a society and professions push process forward?

South-Waterfront-Slurry
A rubble bank at Portland’s South Waterfront. The area has undergone a significant cleanup and restoration effort since this picture was taken.

As an example, consider Portland Harbor – the stretch of the Willamette River that runs through downtown Portland. An industrial area historically, the banks of many of the sites through the Harbor are protected by exposed concrete rubble from demolished buildings and roads. Planners and developers filled much of the Willamette’s flood plain with this rubble decades ago in an effort to create more developable land. As could be expected, the river has shaped the resulting bank over time into what is now a stable, but channelized and heavily armored system. Riprap placed to protect more recent development – more or less an engineered equivalent to the rubble bank – has only reinforced this channelization.

Change in this system can only be made incrementally, with slow and steady progress made over a long period of time. The City of Portland has made a great effort in recent years to support, encourage and require the use of bio-engineered solutions on redeveloped waterfront sites. Since the protection offered by these solutions depends on where the site is along the river and what redevelopment is being planned on nearby properties, however, the results of pushing change through regulation can be mixed. Still, with few other options, it seems like there are two approaches within this system that combined could – and perhaps have started to – begin the process of naturalization.

First, there are almost always a few small changes that can be made to incorporate some level of bio-engineering into a site. Mixing bio-engineered and more traditional elements can result in a sort of stepping stone to the ultimate goal of a naturalized bank. Gradually, as sites are redeveloped again and again, these small changes will accumulate and result in noticeable progress. Of course, this process will take a long time. If every site in Portland Harbor were redeveloped every 20 years – they aren’t, but just imagine – it could be sixty, eighty or a hundred years before the bank is naturalized. It took a long time for Portland Harbor to reach the state it is in and it is unrealistic to think that it will not take a long time to restore it to what it could be. That said, the process risks being abandoned if the community can’t see meaningful changes within relatively short windows of time. Something must be done to accelerate this cycle.

Fortunately, just as a large variety of sites can support a few targeted solutions, a few targeted sites should be able to support a large variety of these measures. At least in theory, big changes could be made in short amounts of time by carefully identifying properties that require a lower level of protection, including sites that are in favorable locations along the river and those that support less critical development. Because of this promise, the temptation can be to focus on biting off these big chunks one at a time, but considering that the most attractive properties might be owned by people who can’t or aren’t inclined to redevelop in this way, the reality is that the slow and steady approach will likely have to be the backbone of the movement.

As with many other design issues surrounding the concept of increased sustainability, Portland and the Northwest in general are at the forefront of the issues surrounding river bank naturalization. Though some skepticism peaked through in this post, to me this is one of the most exciting areas of current engineering design. The problems posed by bio-engineering require careful attention to the fundamentals, in-depth analysis of past projects and the kind of creative problem solving that engineers savor. Along with the cascade of environmental benefits that could result, the fundamental issues presented in these more naturalized designs represent an opportunity to push the boundaries of our profession and make a lasting improvement for future engineers to build upon.

Restored low water habitat at Portland's South Waterfront
Restored low water habitat at Portland’s South Waterfront

This post is based on my experiences with riverbank design in Portland and exploring the rivers in the western US. How do they relate to your experience and and views on the subject?

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

Gleason-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.

Gleason-Trees

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.

Gleason-Pavers

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.

Gleason-Flat-Pvmt

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.

Gleason-Curbs

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.

Gleason-Swales

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.

Gleason-Planter

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.

Gleason-Overflow

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.

Portland-IDF-Curve

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.

Engineering commentary from Portland, OR

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