Tag: Bridges

Part 1: Why & How 2D Hydraulic Modeling is Enhancing Bridge Engineering

Image of colorful 2D hydraulic modeling image with arrows indicating which way water flows

A bridge, culvert, the road nearby or above, the banks, and the surrounding ecosystem are affected by water’s flow. It is no surprise, then, that studying hydraulics and hydrology when designing bridges is paramount for safety, road users, and engineers.

The purpose of hydrology is to study water itself with respect to the land, whereas hydraulics studies what the water is doing within a channel or pipe. So, when engineers develop 2D hydraulic models, they are looking at how water behaves in a given area, and ultimately use that information to build safer bridges. This type of modeling tells engineers not only where the water is going, but where it wants to be.

Basically, 2D modeling determines and depicts water flowing back and forth (on a horizontal plane) instead of horizontally and vertically (3D modeling). The modeling is presented as a dynamic graphic that shows the flow of the river or water body. With a bridge in the model, engineers can determine how the water will move around piers and abutments (the bridge foundation), what could happen with scour and decide how to design for it, and predict the bridge’s impact on the environment (and the environment’s impact on the bridge) for years to come.

2D hydraulic image of black water on a map with white, moving lines showing water flow direction
2D hydraulic model

First things First

When a bridge engineer designs a bridge or culvert associated with water, hydraulic modeling isn’t an afterthought; it’s one of the first things that gets done when a project starts. Structural engineers already have data about the area and usually the existing bridge. Still, they often need more specific information to understand the entire project better – for example, data points on the lowest part of the bridge and how the structure is situated in relation to the water below.

2D hydraulic modeling at the beginning of the project gathers that information and helps the engineers better plan for the project, and is welcomed by this engineer as a preferred alternative to the conventional 1D modeling. It’s not just a benefit to engineers who want to know a system’s details, though. Clients, municipalities, and everyday citizens benefit from engineers using 2D hydraulic modeling – because it helps convey to them what’s happening with the water and helps the engineers better protect the infrastructure we use every day.

When it Rains, it Pours

When you think back to the Mother’s Day floods in 2006 or any other time flooding threatened New England, you probably don’t think much about the bridges you drive unless the water pools over the road. What few people think about is what’s happening under and over the bridge; with faster rushing waters and more force, there’s the potential for three big events: scour, rising water carrying debris, and pressure on the bridge caused by flooding water.

Scour is what happens when sediment around the bridge foundation (or along the roadway) erodes and starts washing downstream, leaving the potential for the material under the bridge to become unstable. In some cases, sediment from upstream of the bridge will wash downstream and fill in these holes during the storm before anyone realizes how big of a scour hole actually developed. We use 2D hydraulic modeling to help better predict the scour that might occur during these events even though we may not see it.

Image of a flooding truss bridge in Ludlow, VT during a large storm

This erosion can also occur beside and/or below the roadways leading up to the bridge if the water flows over the roadway. The 2D model enables us to see how much of the water is going over the roadway as well as provides us with the depth and velocity of this water. We can use this information to determine if the sides of the roadway might be in danger of washing away with the water. If the embankments might erode, we can properly armor them and keep them protected.

Part of our job is looking out for this erosion – if we determine that scour might be a potential issue during a storm, we can get ahead of it. One way of doing this to help prevent it under the bridge is by putting riprap (larger stones) in front of and around the bridge foundation to help keep the natural, finer sediment of the streambed and below the foundation in place. Another way is potentially changing the foundation type: if the anticipated scour is deep, we might change from a shallow foundation to a deep foundation. Scour is just one of the dangers associated with large storms, and 2D hydraulic modeling gives engineers the insight they need to help prevent dangerous situations.

Rising water is another danger to bridges, not just because it could potentially overtop (flow over) the bridge, but because the rushing waters can carry debris (say, a fallen tree) that could hit the bridge and cause damage. When we plug in potential flooding scenarios into the 2D hydraulic models, we use the models to predict how high the water could potentially get during a storm. That way, we can plan to build a new bridge or raise an existing bridge above the water level, reducing the chance for the bridge to be damaged.

2D hydraulic image showing velocity and water flooding on top of bridge

The third big concern with large storms is the power of flooding waters pushing on the bridge. This pressure could be going into and on top of the bridge, but also could come from under the bridge (buoyant forces trying to lift the bridge up). For this scenario, 2D hydraulic modeling allows us to see where the water would want to go during a flood and determine how much of it is going under the bridge, over the bridge, and around the bridge. This allows us to evaluate what an existing bridge might experience, and to design a new bridge to eliminate these forces (locate the deck above the water) and to resist the forces that can’t be eliminated for a certain storm event.

Garbage in, Garbage Out

The more data we can put into the model and the more accurate that data is, the more confident we can be with the results. That means that for the most part, 2D hydraulic modeling provides valuable information that we had to assume or make educated guesses about 30 years ago. While 2D hydraulic modeling doesn’t solve every problem, it gets us closer to understanding the hydraulics of the bridge. In the event that a solution doesn’t quite make sense, it could be because we need more or better data to put into the 2D hydraulic model.

For example, we’re working on a project right now that is analyzing an existing bridge in a river. Using 2D hydraulic modeling, we noticed that the water isn’t flowing as expected. We looked upstream and noticed old bridge abutments causing a constriction in the water flow. This slight constriction causing the river to backwater and holding part of the flow back is a great example of how limited data can affect the hydraulic modeling, or what I like to call “garbage in, garbage out.” If we didn’t have the data that depicted this constriction, we might have missed how it influences the river and the flow at our crossing.

Another project currently underway involves an upgrade and analysis of two culverts, with a third culvert right upstream, that have all been causing water constriction. The 2D hydraulic model shows us how these constrictions are causing backwater and increased flooding. The 2D hydraulic analysis also allows us to see what happens when we change the structures in the water. With some tweaks to the 2D model, we’re able to see that if we replace the two downstream culverts with a wide open structure that spans the bankfull width, the flooding is significantly reduced in the model. Meaning that if we replace these culverts with this other bridge system we designed, the next huge rainstorm won’t cause so much issue.

Want to know more about 2D hydraulic modeling?

We’re only as good as our data and our engineers’ analysis of that data. 2D hydraulic modeling has helped us foresee challenges to certain bridge structures while advocating for others to serve an area better. It replaces 1D hydraulic modeling at a time when computers now have the bandwidth to handle the massive programs required to use.

If you have any further questions, reach out to me and stay tuned for the follow-up blog on the different programs and math needed for this undertaking!

The Flow of the River: What 2D Hydraulic Modeling Can Teach us about Movement

GIF image of 2D hydraulic modeling showing water under a bridge

Imagine trying to measure water in a beaker or in a measuring cup; it is stagnant and easy to follow the line of meniscus to see if it’s a ½ cup or 3/4. Then imagine measuring water in a river in order to build safer bridges; it tumbles over rocks, it changes speed, it experiences different water levels throughout a season.

Believe it or not, water movement is one of the most difficult phenomenon to solve. Yes, you can apply mathematics or numerical methods to solve complicated differential equations, but there are always some unknowns about turbulent flows (class 4 rapids) where general assumptions are made.

Rivers require intricate numerical models for river-type engineering problems, and I have been accepted to present on these intricate models at this years biennial National Hydraulic Engineering Conference (NHEC) in Columbus, Ohio. The Conference spans a week from 8/27 to 8/31, and I will be presenting on Friday, August 31st.

Per the NHEC website (https://www.ohio.edu/engineering/nhec/), the conference is themed “Advancing Hydraulic Engineering through Innovation and Resilient Design,” and will address the challenges that transportation agencies face to construct, maintain, sustain, and improve hydraulic structures in the physical, natural, social, and economic environments of today and tomorrow. At this conference, I will be presenting on Two-Dimensional (2D) Hydraulic Modeling with Tidal Boundary Conditions.

Modelers typically use computer software packages where you input topography, flows, roughness parameters, and hydraulic structures. The software package uses the input to solve mathematical equations. It seems simple enough, but a modeler needs to have a conceptual understanding of numerical methods and know the limitations of the software package being used.

Whenever you hear the term “3D,” you think of an object in a space that has 3-dimensions, right? Similarly, water moves within a 3-dimensional space, where there is a z-component (up, down), y-component (left, right), and x-component (back, forth). What if I were to tell you that the movement of water in the z-direction (up, down) is not considered?

What would that mean? Well, what that means is that mathematically, we are simplifying a very complicated problem:  we are restricting movement of water to flow/move in 2D, 2-directions (x and y) and that is what 2D hydraulics is all about. Similarly, a one-dimensional (1D) hydraulic model is defined when the y-direction is neglected and water is confined to moving in the x-direction.

2D hydraulic modeling is not that new and has been available in an academia setting since the 80s. But in recent years, tools to develop 2D models have been readily available to engineers. A 2D model can’t be developed for every problem that we tackle, but it allows us to accurately represent actual real world conditions, make less assumptions and judgment calls, and communicate and show visualizations of flow movement to stake holders.

 

Written by Jeff Degraff

What Droughts can Teach us about the Importance of Proper Culverts

July 2016 struck New England with an extreme drought and dry weather patterns for an entire year in most of the region. Many people are seeing the drought disappear as heavy rainfall replenishes those dry wells. Showers are taken a little less guiltily.

Yet ironically, the seacoast areas of Maine and (some) of New Hampshire are still considered abnormally dry for this time of year. The drought.gov website says that the percent of dry conditions for the Northeast is a total of less than 10 percent. In general, around 90 percent have no dry conditions at all. Despite this time of year being dryer for the coast, long-term totals actually appear normal.

So, why the pesky persistence with this abnormally dry issue?

“Much of the Northeast remains drought free with the exception of coastal Maine, which has been plagued by below-normal precipitation over the summer,” Deborah Bathke reported in the National Drought Summary for August 8, 2017.

Lack of rainfall may seem relatively insignificant in the engineering world to some. Too much rainfall can cause road erosion, mud slides, sewage overflows, and building floods (among other glorious things). Too little rain? Aside from a crispy lawn, what could go wrong?

Well, for starters, a dry season can mean that ground water levels are low. Low water levels mean that engineered structures, like culverts, don’t work like they are supposed to. Which can lead to problems for an entire ecosystem.

Culverts are a great example. Culverts allow for water passage — such as streams, creeks and brooks — to move under roads. Many aquatic species migrate during their lifetimes, so in order to do that, they need to be able to swim or wade through water freely. The National Oceanic and Atmospheric Administration (NOAA) explains that incorrectly engineered or installed dams and culverts can contribute to declining fish populations by not allowing continuous water flow and creating a physical barrier to fish passage. Throughout the watershed, there can be several examples of perched road crossing culverts (where a drop in elevation exists between the end of the culvert and the water body) and culverts that are too narrow, steep or collapsed.

As rain levels increase and droughts are ending, aquatic life has the chance to move more freely through these constricted passageways.

The importance of culverts can be partly attributed to the way the water flows.

culverts

 

The New England states have turned their attention to the importance of designing culverts that are eco-friendly for the past two decades, with regulations in place in each of the five states that require certain levels of flows, both high and low, to be maintained through culverts in order to protect migrating organisms. From an article by the US Fish & Wildlife Service of Alaska comes the challenge to make roads more fish-friendly:

“What’s under our roads should ideally mimic what’s upstream and downstream,” the article says. “This helps ensure a seamless transition for fish passing underneath. … So how wide is wide enough? To answer that, we must understand the stream’s range of flows. A stream gauge that tracks water level and documents flood events over time can help.”

When accurate stream gauge data is not available, particularly for the smaller creeks or brooks, engineers must examine the existing conditions and develop assumptions on flows, typically using hydrologic models that are standard industry practice.

In short, as you drive from place to place during your day, take time to notice the road culverts you pass over. They have an important role in keeping an ecosystem functioning at its best, even under drought conditions.

14 Steps for Preserving Steel Structures

Piermont, NH-Bradford, VT Steel Bridge

*This post has been updated in 2020.

Preventative maintenance is defined as scheduled work at regular intervals with the goal to preserve the present condition and prevent future deficiencies. On bridge structures, this work is typically performed on structures rated in ‘fair’ or better condition with significant service life remaining. Minor repairs may be necessary to maintain the integrity of the structure and prevent major rehabilitation. Structures that are not maintained are more likely to deteriorate at a faster rate and require costlier treatments sooner than maintained structures; therefore, it is more cost effective to maintain structures to avoid replacement or major rehabilitation needs.

Side image of a steel bridge with orange vehicle to inspect

New England’s weather causes extreme conditions for steel bridge trusses, such as flooding, ice and snow. Corrosive de-icing agents are used in the winter, which can accelerate deterioration of exposed bridge elements. Preventative maintenance is critical for steel truss bridges to reach their intended design service life and, therefore, attain the lowest life-cycle cost of the bridge investment. Presented are minimum recommended guidelines for preventative maintenance of steel truss bridges.

Three images of paint loss and debris on a bridge
Paint loss and debris

Here are 14 actionable maintenance tasks to preserve historic truss bridges:

  1. General: Remove brush and vegetation around structure. Annually.
  2. Bridge Deck & Sidewalks: Sweep clean sand and other debris. Power wash with water to remove salt residue. Annually.
  3. Wearing Surface: Check for excessive cracking and deterioration. Annually. 
  4. Expansion Joint: Power wash with water to remove debris, sand and salt residue. Annually.
  5. Bolted Connections: Inspect for excessive corrosion or cracking of the steel fasteners. Check for any loose or missing bolts. Annually.
  6. Welded Connections: Check for cracking in the welds. Annually.
  7. Truss Members: Power wash with water to remove sand, salt and debris, particularly along the bottom chord. Give specific attention to debris accumulation within partially enclosed locations such as truss panel point connections or tubular members. Annually.
  8. Bridge Seats: Clean around bearings by flushing with water or air blast cleaning. Annually.
  9. NBIS Inspection: Complete inspection of all components of the steel truss bridge. Every 2 years unless on Red List.
  10. Painted Steel: Scrape or wire brush clean, prime and paint isolated areas of rusted steel. Every 2 to 4 years.
  11. Steel Members: Check for rust, other deterioration or distortion around rivets and bolts, and elements that come in contact with the bridge deck which may be susceptible to corrosion from roadway moisture and de-icing agents. Every 3 to 5 years.
  12. Bearings: Remove debris that may cause the bearings to lock and become incapable of movement. Check anchor bolts for damage and determine if they are secure. Every 3 to 5 years.
  13. Exposed Concrete Surfaces: Apply silane/siloxane sealers after cleaning and drying concrete surfaces. Every 4 years.
  14. Bridge & Approach Rail: Inspect for damage, loose or missing bolts, sharp edges or protrusions. Every 5 years.

Actions to Avoid

  • Do not bolt or weld to the structural steel members.
  • Do not remove any portion of the structure.
  • CAUTION! Paint may contain lead.

Additional resources can be found through the New Hampshire Division of Historical Resources website.

Pi vs. Chocolate Cream

Pi… I did not forget the “e”, I am referring to the mathematical constant, π, for the value 3.141592…, a ratio of the circumference of a circle to its diameter. For some it was junior high and others it was high school, but almost everyone is taught the concept of Pi in geometry class in America. The staggering question asked by so many students over the years is “how do we use this in ‘real’ life?” Well we have answered that question for all of you as it relates to engineering:

When designing bridges many of the structures utilize reinforced concrete to provide the strength necessary to support its daily use by vehicles. For many of our bridge projects, the circle is most often representing the area of reinforcing steel used in the reinforced concrete beam.  We determine the total amount of the (steel) reinforcing to determine the capacity of a structural member such as a beam, deck or slab.

In associated roadway design, Pi is used in a slightly different manner, to calculate curvature. A maximum curvature (minimum radius) is used to ensure adequate sight distance at differing speed limits. This promotes safe vehicular travel by providing a level of comfort and expectation to the driver.

Another application for the mathematical constant is in airfield markings. Their purpose is simple – to safely guide pilots during aircraft take-offs and landings, and while taxiing around the airfield. To create these markings, Pi is utilized when calculating the amount of airfield paint required for runway designation markers, taxiway centerlines and edge lines.

Pi is also used extensively in the calculation of areas of gravity sewers, wastewater force mains, water main pipes, storm drains, drainage culverts and other types of utility pipes. These calculations are used to establish the area of the pipe for the purpose of determining flow velocities and flow volumes as well as other types of hydraulics calculations.

Now that we have proved your mathematics teacher correct, and that someday you may need to know the value of Pi, the obvious question remaining is “what does pi and pie have in common?” My answer is Pi is focused on circles, radius and diameters… and so does pie! If you want a great Chocolate Cream Pie recipe check this out!