Tag Archives: riverbank

South Waterfront Greenway Nearing Completion

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Relatively fresh off of working in construction management, one of my first big design assignments was the civil engineering work for the South Waterfront Greenway Central District – a combination park and active transportation corridor through Portland’s South Waterfront area. Our team spent the better part of 2007 through 2009 designing a gentler, more natural riverbank and stormwater infrastructure to support the Greenway.

Then, at the end of 2009, with the design process nearly complete, my family and I decided to move to Haiti to work on water and sanitation projects. This decision led to an incredibly unique adventure in engineering, world view and community.

After a life defining two year journey we returned home and I was fortunate enough to be re offered my old job at KPFF. It was the same position, the same desk and – surprisingly – some of the same projects. During one of my first days back in the office I was greeted by a smiling PM who warmly shook my hand saying, “It’s so great to have you back. You can help us finish South Waterfront!”

Indeed, I had assumed that the 90% complete plan set that I had left two years previous had in the those years become an actual park. I imagined bikers happily zipping along the river. I imagined families picnicking. I even imagined juvenile salmon resting on their journey up the Willamette. None of these existed. What did exist was a much different project – the result of two years of agency reviews and an economy that had forgotten the bold development of 2006.

So with that as prologue, I am excited to report that this past week I participated in the final walk through for the actual, constructed South Waterfront Greenway! The project is a stunning addition to Portland’s collection of waterfront trails and parks. The following is a highlight of some of the primary functions and features of the newly constructed park.

Pathways and Parklets

South Waterfront Greenway
Facing north up the Greenway. Note the lawn terraces on the slope and the separated pathways.

As originally envisioned, the Central District project would set the vision for a greenway trail that would run throughout the South Waterfront area. The completed corridor would include: separated bike and pedestrian paths for increased modal safety; parklets and overlooks where people could stop and enjoy the riverfront setting; and a notable focus on environmental stewardship and habitat restoration.

South Waterfront Bike Symbol
Brass bicycle (above) and pedestrian (below) symbols were cast into the trail at nodes along the corridor to direct traffic.

South Waterfront Greenway Ped Symbol

South Waterfront Greenway Trails
A view south from the top slope of the project. Note that though the geese began their journey on the asphalt bike path (above), they were able to correctly interpret the pathway symbols and soon moved to the concrete pedestrian path (below).

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Riverbanks and Low Water Habitat

As mentioned, the vision for the Greenway included an intentional focus on habitat restoration. The riverbank within the project – once stabilized by concrete rubble and demolition debris – has been cleaned up and flattened to create low water habitat for migratory fish.

South Waterfront Greenway Low Water Habitat
A view of the “cove” created to provide low water habitat. Much of my own work on the project is now underwater here. Note that the flatter banks have already begun recruiting beneficial woody debris.

South Waterfront Greenway Woody Debris

River Overlooks

The newly restored riverbank can be viewed from several overlooks. The overlooks vary in size and style and add to the sense that the Greenway is a destination and not just a transportation corridor.

South Waterfront Greenway Small Overlook
A small overlook where pedestrians can stop and admire the Willamette.
South Waterfront Greenway Large Overlook
A larger overlook near a node between the trails.

Stormwater Management

Runoff from the trails and adjacent features is treated and managed through a gravel swale that runs between the bike and pedestrian paths. Flow in these swales ultimately collects in one of several catch basins, which discharge to “level spreaders” that redistribute the flow along the riverbank.

South Waterfront Greenway Swale
A catch basin in the central water quality swale. Note the ponded water surrounding the grate. By forcing the water to pond, sediment is allowed to settle out of the runoff before it crosses to the river.
South Waterfront Greenway Level Spreader
Stormwater from the swale is piped to one of several level spreaders, where it fills the gravel trench shown and redistributes along the riverbank.

Site Furnishings and Artwork

Like the neighborhood that it supports, the Greenway has an obvious upscale character. The site is dotted with multiple, unique styles of benches and there is a riverbank stabilization themed art piece that was specifically commissioned for the site.

South Waterfront Greenway Benches
Multiple bench types provide ample opportunity to sit and enjoy a close up view of the Willamette River.
South Waterfront Greenway Concrete Bench
The use of wood decking is a common and unique theme throughout the project.
South Waterfront Greenway Artwork
Artist Buster Simpson designed this riverbank stabilization themed art piece specifically for the project. Root wads – which are often used to create bank roughness – are supported by jack type objects that were inspired by a barbed structure that has been historically used in coastal bank applications.

As you can see, the only thing missing from these pictures are the walkers, bikers and park enthusiasts who will undoubtedly discover this great new place once construction and the winter weather pass.

If you enjoyed this article, you may also be interested in the following past posts. Thanks for reading!

Detailing Vegetated Riverbanks

The Engineer and the Fisherman

LID at Metro’s Gleason Boat Ramp

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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.