20140911

Portland Civil Links 09/12/14

It was another week of great weather in Portland, as summer continues to hang on. Here are some civil engineering stories for the week that I found interesting.

  • Here at Civil PDX, I investigated a sinkhole that appeared along my daily bike route. Holes like this are not uncommon – a product of the City’s aging infrastructure. This one looks to be the result of an improperly functioning drywell. The process of discovering and writing about this problem has been an eye opening one for me and has made me rethink the possibilities of what could be happening beneath our failing streets.
  • With the school year underway, the Architecture Foundation of Oregon is gearing up for another round of the successful Architects in Schools program. This program teams AEC professionals with elementary school teachers to help kids learn about the design process. This is a great opportunity for working folks to contribute to educating the youth in our communities. I will be volunteering for my first time this year and I encourage you to consider doing the same.
  • Private development is going strong in Portland. The latest example being Gerding Edlen’s bid to buy 3/4 of a block in Old Town from the Portland Development Commission, where the developer plans to build a $37 million mixed use building. The story offers a slight twist in that Mark Edlen, the company’s CEO, is expected to joint the commission in October.
  • Speaking of a changing Portland, the Bureau of Planning and Sustainability launched a web site this week that allows you to tour the City’s Comprehensive Plan Open House from the comfort of your own couch. This seems like a great way to get more people involved in the process. The downside is that you will have to supply your own coffee and donuts.
  • Finally, the Hillsboro Hops – the Portland area’s minor league baseball team – beat the Vancouver Canadians 4-3 on Monday to claim the Northwest League Championship. This is a big accomplishment, coming off the second season after the team’s move from Yakima. Having been to games during both seasons, I have to admit that this year’s team was much improved and their record showed it. If you are thinking that this has nothing to do with civil engineering, you are right, but it is great anyway.

That’s a full lid for this week. Have a great weekend!

Enter your email address at the top of this page to have the latest content from Civil PDX, including links like these, sent directly to your inbox.

Sinkhole-Warning

And then there was a sinkhole…

At times our city streets look like a patchwork of potholes, trench cuts and repairs – all signs of a long life of hard work. Some of these also point to the unseen problems and hazards that lie beneath the pavement.

Concord & Killingsworth Sinkhole

Cycling to work is one of my favorite parts of living in Portland. Engineering work is interesting and engaging, but like many modern professions it involves a good amount of sitting at a computer, and it is nice to be able to start and end each day with some exercise. I also feel like regular riding helps me as an engineer to better understand the needs of drivers, cyclists and pedestrians, and the effects of pavement conditions and different levels of pavement maintenance.

Each evening, my commute home takes me along Concord Street, a quiet bicycle boulevard through North Portland’s Overlook neighborhood. It’s often getting dark at this point in my trip, making it more difficult to see potholes and other potential ground level hazards. Hitting one of these isn’t serious, but a cyclist has to stay aware and be prepared to recover from a random jarring bump.

For months one particular pothole seemed to surprise me every time I passed. Each day I tried to remember the cross streets and location of the pothole, and each day I seemed to hit it head on. The situation was even more frustrating because this bump wasn’t really that big and should have been easy to avoid. It was less a pothole and more a dimple really. Instead of being broken or missing, the pavement here was depressed about 2 or 3 inches within a 6 or 8 inch diameter circle.

A dimple is a strange shape to see pressed into the pavement, even in a place that sees a pretty wide range of pavement distresses. Being an engineer, I would inevitably spend the next mile or so after hitting the dimple wondering what could have left such a weird shape in the ground. Construction equipment? A meteor? A bowling ball falling off a passing moving van?

And then, one day, there was a sinkhole. This video was from about a month after it first appeared.

The hole shown here is about 8 inches in diameter through the pavement – about the size of the preceding dimple – and widens to roughly twice that below ground. The cut extends down and around the adjacent manhole for about 5 or 6 feet. The axis of the hole is on a slight angle, but there is a fair amount of loose dirt on the low side, which could mean that the walls of the collapse are below the surface that we see here.

So where did this hole come from? It is a strange thing for something as strong and reliable as pavement to suddenly be swallowed up by the earth, and the question is (understandably) asked whenever a sinkhole appears. Believe it or not, even the biggest sinkholes are usually caused by very small, almost unnoticeable problems occurring over long periods of time. The overwhelming majority of these problems involve two things: rats and leaky sewers.

Our urban sewer systems are essentially massive underground tunnel networks and they make perfect highways for rodents to move around the city. Once in the sewer, a rat would exit through a break in the pipes – of which there are many more than we would like to believe. Digging upward, he would eventually reach a point where the roadway subgrade is too hard for him to continue, would flatten his trajectory and would head for softer ground, usually ending up in a landscaped area.

Inside the sinkhole: bad picture, bad problem.
Inside the sinkhole: bad picture, bad problem.

This rat burrow is now an open conduit for surface and ground water to enter the sewer system, much like our catch basins and pipes except that this conduit isn’t planned or lined. Over time, flowing water will wash soil into the sewer, which will in turn carry it away. The pavement above the hole will grow weaker and weaker as it loses support until it is entirely relying on its own material strength to stay in place. The hole will of course continue to grow and then, one day when either the hole has grown too big or a heavy load is placed above it, there will be a sinkhole.

While ratholes are a major cause of this type of problem, I don’t think they are the cause of this specific instance of the problem. First off, the hole is bigger and more wandering than might be expected from a rat. Second, as you can see on the map below, the manhole next to our sinkhole isn’t part of some long continuous pipe network. In fact, it isn’t a manhole at all. It’s actually a drywell, disposing of stormwater runoff collected from nearby local streets.

Concord & Killingsworth Sewer Map
The nearby sewer system as recorded on portlandmaps.com.

This map comes from the engineer’s best friend portlandmaps.com – a database of a wide range of GIS information for Portland and its outlying areas. The red dot on the map is the approximate location of the sinkhole. You can see that in this location water flows from the intersection into one of four catch basins, then into a manhole and through a series of two drywells.

I don’t know much more about the system than what you see here, but I think this is enough information to suspect that if a rat wanted to move from one end of the system to the other it might just cross the street. However, even if rats aren’t to blame for this sinkhole, the description of how it may have formed is basically the same. Somehow water starts to enter a leaky sewer, bringing dirt with it. Slowly over time the leak creates a hole until, one day, there is a sinkhole.

In this case the sinkhole borders a drywell. Drywells basically act as local disposal points for stormwater, which is directed into a well and then infiltrates into the ground. Systems like this one work well and are used throughout Portland to keep stormwater from overloading our ancient combined sewer system, in turn keeping sewage from flowing into the Willamette River. These drywells and the streets that surround then, however, are quite old themselves.

The City of Portland requires that all new conveyance pipes be designed for a 100 year service life. That span is shortened to 50 years for manhole structures. It is unlikely then that this drywell has “failed.” More likely is the possibility that groundwater, in addition to the intended surface water, is entering the drywell. The walls of a drywell are perforated and water is intended to move from inside the well to outside. Reversing the flow direction usually results in a small amount of dirt washing into the well each time there was a storm, creating a void under the pavement and eventually a sinkhole.

Sinkhole-Top
Unfixed, how much will this hole grow over the coming months?

So, we have a suspected cause for the sinkhole! What does that mean, though? The hole has still been sitting in the street for nearly two months with a single traffic barricade and a paint circle the only signs that the City knows it exists. My suspicion is that this repair may be complicated and that the City is having difficulty working it into their already overburdened maintenance budgets and schedules. Still, there is now a hole in the road and it will probably keep growing, especially as we start to see more rain.

Also, thinking back to the original dimple, I am pretty sure that I have seen similar marks on the pavement. Could these be hidden sinkholes waiting to surface? Could there be a way to use pavement distress to easily and proactively identify possible sinkholes like this one before they result in a potentially dangerous collapse? Does this process already in exist?

I plan to explore all of these questions in future posts. I am keeping a close eye on this particular sinkhole since I ride past it every day, and will track any developments here on the blog. In the meantime, I wish you safe travels. Watch out for those odd dips in the pavement. They might not be anything, but sometimes who knows.

To stay up to date with future Civil PDX sinkhole content, subscribe to this blog by entering your email address at the top of this page. New blog posts will automatically be delivered to your inbox.

If you found this post interesting, you might also like the following past posts from Civil PDX.
Passive vs. Proactive Design
A Framework for Understanding Details
Three Raindrops

20140903

Portland Civil Links 09/04/14

Summer vacation and clear 90 degree days are fading away. School and rain are on their way back, but so are football and the bike commute challenge. See how the week’s themes relate to civil engineering and the AEC industry.

  • Here at Civil PDX, we toured the pavement rehabilitation and preservation work underway at the Portland International Airport. I will leave it to you to decide if I deserve part of the blame for throwing off your travel plans this summer.
  • The BTA’s annual bike commute challenge is underway! Bike commuting is a great way to stay fit while making a small but real step toward air cleaner and less congested roads. Whether you are a cycling enthusiast or have never ridden, the challenge is a great way to get into using your our streets in a whole new way.
  • NFL football starts this week and the Oregonian had an idea of where you can go if you weren’t able to get tickets to see the Seahawks take on Green Bay. This is a great way to capitalize on the impressive architecture of both CenturyLink Field and downtown Seattle.
  • Finally, school is back in session, which means that school zone traffic safety should return to the front of our minds while driving. Here are 10 things to remember about driving in school zones, courtesy of Joseph Rose. Item 4 especially bears attention: “Research shows pedestrians have a 90 percent chance of surviving car crashes at 18 mph or below. However, their chances of survival from an automobile impact drop to less than 50 percent at 28 mph or above.”

Have a great weekend – thanks for reading! Make sure to enter your email address at the top of this page to have civil engineering links like sent to your inbox each week.

IMG_0800

Roadway Construction at PDX

If you flew in or out of Portland International Airport (PDX) this summer you probably noticed that there has been quite a bit of street construction underway near the terminal building. Here’s what’s been happening.

PDX Pavement Rehabilitation

The Port of Portland,  KPFF and Kerr Contractors are nearing the end of a project to preserve and rehabilitate the pavement leading to and from PDX’s arrival and departure gates. Because of the overlap between the peak construction season and the peak travel season, this has been a high profile project for everyone involved. As an engineer at KPFF, I have had the opportunity to work on the planning, development and implementation efforts over the past year and a half.

Our team’s work began with building a 40 year life cycle cost analysis to compare the construction and maintenance costs associated with three options to extend the pavements service life: rehabilitation with an asphalt concrete inlay, reconstruction with asphalt concrete pavement and reconstruction with portland cement concrete pavement. Following selection of a preferred combination of work, we developed plans and specifications detailing the project, and we will continue to provide support until construction is completed in early October.

The following photos are from recent site visits and cover the major components of the work.

Lower Bus Lanes

PDX-LOR

Each day about 540 bus trips are made shuttling people from the PDX economy lots, to the terminal building and back again. The buses making these trips are big, double rear axle vehicles and they all follow the same route. Understandably, the asphalt pavement along this route had lived a difficult life. The buses’ weight combined with starting and stopping forces had lead to severe rutting in the pavement – so much so that it had begun shoving the adjacent curb and sidewalk.

Based on our life cycle cost analysis and existing pavement conditions, the team determined reconstruction with portland cement concrete pavement to be the best rehabilitation option in the bus lanes. The resulting roadway provides a more rigid platform for heavy bus loading, which should result in a more durable pavement structure.

Lower Passenger Lanes

PDX-LIR

Near the bus lanes is a separate roadway dedicated to carrying the thousands of passenger cars that visit the arrival gates each day. These lanes carry a high volume of traffic, but it is not heavy traffic, so pavement distress was limited to wear and minor cracking in the top surface.

Under typical conditions, asphalt concrete pavement fails from the top down. Loading creates small cracks in the top surface and these cracks gradually work their way down through the pavement. If the cracks reach the bottom of the pavement section, the pavement fails and must be replaced. However, if those responsible recognize that this cracking has begun, the corrective work can be limited to removing and replacing the top surface to the crack depth, resulting in a significant savings of both time and money.

During predesign work, the project geotechnical engineer determined that with proper top surface maintenance, the pavement section in this area will essentially never fail under passenger car loading. Such a design is called a “perpetual pavement.” As such, our life cycle cost analysis showed there to be a significant savings in using an asphalt concrete inlay here. The completed work involved removing and replacing approximately 2 inches of the existing pavement.

Upper Passenger Lanes

PDX-UIR

Traffic to and from the airport’s departure gates is carried on an elevated roadway that is split into two sections. The section furthest from the terminal building is used by taxis and other commercial traffic and exists entirely on a concrete bridge structure. This section was not included in the project and remains unchanged. The section nearest the building is used by all other passenger car departure traffic and is built partly on that same bridge and partly on the roof of the lower baggage claim area. Because it is constructed on top of an occupied space, the original construction of this roadway included a waterproofing membrane on top of the building, overlain by a layer of asphalt concrete pavement. The existing pavement and membrane were both at the end of their expected service lives and had begun to show signs of distress.

This past week, the contractor finished removing this existing pavement and waterproofing membrane and began preparing the surface to receive a new spray applied membrane. Spray applied waterproofing membranes are relatively new in the US and, in the right applications, offer superior performance in almost every respect when compared to more traditional products. After the new membrane is placed, the road will receive a layer of pavement. The completed roadway will look much the same as the original, but will have a renewed service life.

Pedestrian Signing

One of the project’s smallest but most visible changes involved removing the miniature stop signs that used to guard the airport’s raised crosswalks. These signs have long created a headache for both drivers and maintenance personnel. The new crosswalk delineation includes cross hatch striping and overhead “Stop for Pedestrians” signs.

So, it has been a busy summer at PDX. Pavement preservation work can create a temporary inconvenience from time to time, but streets are expensive and proper maintenance is a critical part of protecting our infrastructure investments. Hopefully this post has given you some insight into the work and the engineering behind this particular preservation project.

Portland Civil Links, 8/28/14

Weekly links from Portland’s AEC industry.

20140827

With development projects jumping up daily and a steady stream of new residents, the Portland metro area is always an interesting place in the AEC industry. The following links highlight civil engineering and regional news over the past week.

  • Here at Civil PDX, I explored the differences between passive and proactive design approaches and how a designer’s approach can affect the long term benefits and implications of a project.
  • BikePortland.org is wrapping up a week long series about the BTA’s five new advocacy campaigns, which include driving big functional changes on the Broadway commercial corridor and defining the future of the Safe Routes to Schools program.
  • Brian Libby of Portland Architecture wrote a great article about the past, present and future of Tilikum Crossing, Portland’s newest bridge, which is scheduled to open in September 2015.

That’s all for this week. Have a great Labor Day weekend!

Passive vs. Proactive Design

Greeley

Each weekday nearly 300,000 Portlanders take to the streets as they make their way to work, and most of them are driving. Despite the city’s mostly successful efforts to promote public transit and active transportation, most commuters still prefer the convenience of their own cars.

MODE COUNT PERCENT
DROVE ALONE 180,107 64%
CARPOOL 25,736 9%
TRANSIT 34,289 12%
BIKE 18,912 7%
WALK 21,336 8%
OTHER 3,247 1%
TOTAL 283,627

 

This isn’t a revelation. We know that Americans rely on driving to get where they need to go. This preference has shaped our communities for almost two hundred years and defines many of the habits of our everyday lives. As an example, my family and I live about one mile from the nearest grocery store and without much trouble or effort could walk there every day or every other day to buy what we need. It is much more convenient though to make the trip once a week, and while I can walk to the store in about fifteen minutes, what I can’t do is carry a week’s worth of food back with me. Isn’t it great that we have a car to help us with that!

To go further, every aspect of the typical supermarket grocery store – from the size of the carts to the size of the parking lot – is designed to support the weekly shopping trip. This is just one piece of our car centered culture. During my one mile drive to the store, I also pass three gas stations, three auto parts stores and two rental car agencies. If I’m willing to make an extra fifteen minute commitment, my options open up to several more stores that might offer better selection, lower prices or both. None of this surprises you, of course, because this is normal to us and normal things aren’t usually that surprising. There is a good chance that my routine is not that different from your own.

There is no community in the United States that isn’t in some way designed around cars. Cars affect every place and every moment of our lives. As our reliance on them has grown, so too has our need for safe streets. To this end, engineers determine street widths, speed limits and a host of other factors according to well established guidelines with the goal of preventing cars from crashing into each other as they move from place to place. While this framework for design has been intentionally evolved over time, many of its effects on our communities have not.

As traffic speed increases, so does the frequency and severity of traffic accidents. Research tells us this, and we expect it from our own driving experience – so, we each limit our speed to our own comfort range. The street’s designers are also trying to manipulate this range to what they feel is safe not only through posted speed limits, but also with more subtle traffic calming measures like lane widths, lane configurations, curve radii and building setbacks. Your driving speed, then, is controlled by two factors: it is directly controlled by how fast you are comfortable driving and indirectly controlled by how fast the designers would like you to be comfortable driving.

The sometimes ignored aspect of this conversation is that there is a reciprocal relationship between these factors. Imagine I am designing a street through an already developed part of your town. I want to understand how the road can better serve your community, so I perform a traffic study to determine how people use the existing street and how that relates to transportation in the surrounding area. From this study, I learn that while the posted speed limit is 25 mph, most people drive closer to 35 mph. This is a big difference and represents a decision point for my design. I want to create a street that is safe for the people who use it, but I also want to create a street that encourages people to use it safely. There are two general approaches that I can take from here.

Approach 1 – Passive Design

Seeing the discrepancy between posted and actual speeds, I have decided that while the street’s classification doesn’t warrant raising the speed limit, it is prudent to design the road to accommodate traffic travelling at a higher speed. Having made this decision, I use some of my available right of way to provide slightly wider lanes. I also lengthen the curves along my corridor to increase how far up the road you can see and give you more time to make decisions while you are driving. Finally, I provide moderately more protection for both drivers and pedestrians by setting street trees and sidewalks away from the edge of the road.

This is of course oversimplified, but the decision making process that I have followed is very reasonable. Each design choice was made to better protect you – the citizen, tax payer and user – based on my understanding of how you drive. The problem is that space in the right of way is limited. Each of my decisions came with a trade off, and because my approach was entirely vehicle-centric, I prioritized driver safety over other street uses at every step. I gave you slightly wider driving lanes so you have room to safely travel at your desired speed. This likely meant making a choice like building narrower bike lanes, leaving cyclists with less room to travel. I set the streetscape features and sidewalks away from the road so they would be less likely to obstruct your driving. This likely left less width for sidewalk and other pedestrian facilities.

Approach 1 has been dubbed “passive design”, a term that is not generally used in a good way. By focusing solely on vehicle needs and safety, we provide less room for cyclists and walkers. We are then encouraging or at least accommodating higher speeds in closer proximity to the street’s most vulnerable users, who are forced to use less robust facilities. At the very least this presents a serious safety concern. At the most it discourages people from using the street for anything other than driving a car. There has to be a better approach to designing streets that accommodate multiple uses, and fortunately there is.

Approach 2 – Proactive Design

Seeing the discrepancy between posted and actual speeds and deciding that the street’s classification doesn’t warrant increasing the speed limit and understanding the goals for the area, I have decided that something must be done to discourage speeding. I create lanes that will still accommodate traffic, but will also feel tighter to drivers, encouraging them to slow down and drive more precisely. Having chosen the lower posted speed for my design, I can bring streetscape features closer to the curb line, reinforcing the narrower feel. Right of way width is also less of an issue now and I can include safer, more generous bike lanes and sidewalks, making the street a more attractive corridor for active transportation.

By creating a safe environment for cyclists, pedestrians and other non-vehicular users, we build on our goal of creating streets that better serve our communities. By encouraging more people to use active transportation options and by giving them safe places to ride and walk, we create new growth opportunities and bring a different type activity to business corridors. The service for car traffic is still there, but the newly imagined street can also be so much more. From public health to business development to transportation, proactively designed complete streets are an important component of strong communities.

None of this is meant to suggest that we can avoid difficult design decisions by being proactive. If there were no difficult design decisions, there wouldn’t need to be engineers to help make those decisions. But our long term visions for how we would like our communities to grow and thrive are fragile things. Civil works projects represent great opportunities to strengthen these visions, as they focus on bringing some kind of change. These opportunities are lost, though, if we stop at passively identifying and accommodating existing uses. By proactively reviewing these uses against long term goals, we still allow ourselves to take the steps needed to maintain our infrastructure, but we also capitalize on a rare opportunity to strengthen and build our communities.

How have passive and proactive approaches affected development in your community? I would love to hear your stories and encourage you to leave them in the comments section below.

[From photo above] The design of Greeley Avenue in North Portland attempts to balance the needs of passenger car, freight, transit, cyclist and pedestrian users, all on a relatively high speed road. With the high projected growth, the City is developing proactive designs that would significantly change traffic patterns here, including a greenway trail along the Willamette River that would move cyclists and pedestrians off the road.

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.

Engineering commentary from Portland, OR

Follow

Get every new post delivered to your Inbox.