Tag: 2D hydraulic modeling

Part 2: Six Programs that Contribute to a 2D Hydraulic Model & Why They Matter

Hydraulic modeling software image with arrows and blue green and orange colors showing water movement

We introduced 2D hydraulic modeling and its purpose in Part I as a way to calculate how water moves and why we use the modeling to help us engineer better bridges. With that understanding, we can further explain the different ways we get the data to make those 2D models work.

The math and programming are not simple, so using simplified terms, we will break down six common programs and types of data to show how it all comes together.

#1: Google Earth

Google Earth: Many people know Google Earth as a fun tool to see their house or maybe where they want to vacation either in an aerial or street view. Engineers use Google Earth not only to see existing conditions, but also to see the aerial history of a location, as Google has images of the land as it has changed over time. For example, we can use Google Earth to look at a river 20 years ago and see how that same location has changed over that period! Is it wider because the banks are eroding? Has it meandered, causing our bridge to need re-alignment to better fit the path it has taken? Has it taken a shortcut and cutoff any oxbows (or U-shaped bends in the river) over time?

Google Earth graphic showing how the land can change over time. This image shows oxbows cut off over time (change of 22 years shown from 1996 to 2018).

The aerial images also provide information as to the roughness (think texture) or the topography. When the river floods, will the water pass quickly over the smooth fields, or will it slow down because it has to travel through a dense forest with brush scattered over the forest floor? We apply a Manning’s roughness coefficient, n, to this area in the water modeling software to help it figure out where the water wants to move. The street view can help show what the surrounding area is like from the ground to give a better idea of what the roughness should be. Sometimes it is challenging to determine how dense a forest is from the aerial view, but the street view could show if the forest floor is clean with few branches at water level, or if there’s a lot of low branches and additional brush that will slow the water down in the event of a flood.

#2: StreamStats

StreamStats: StreamStats is a web-based geographic information system (GIS) application for water-resources planning that we use to delineate watersheds and determine flow (or how much water is passing through a section). This application uses gage data of existing streams and known flows with regression equations to determine the flood flows at a location without a gage. Now first off, you might be asking yourself “what the heck is a gage?” A streamgage uses instruments to measure and record how much water is flowing in a stream at a particular location. You can learn more about it on the US Geological Survey (USGS) website. Secondly, you might be thinking “what is a regression equation?” This is a type of equation used in statistics to determine a parameter (e.g. flow) based on the relationship between sets of data like watershed size and storage. Essentially, the equations use the information from streams with gages to extrapolate what the flow most likely is at an ungaged site. You can click on a stream point (as long as it’s inside applicable limits) and the program will then delineate the watershed and tell you what your flow is! But as is true with most software, you need to check what it gives you to make sure the delineated watershed is accurate, and the flows are reasonable.

#3: Mathcad

Mathcad: We use Mathcad to perform calculations. For example, New Hampshire requires engineers to complete hydrology calculations for bridge designs; alternative methods to determine the flood flows at the bridge are done to check that the flows determined by StreamStats are reasonable. Again, it’s not as straightforward as “StreamStats is giving me this data, let’s use it,” it needs to be checked! Mathcad allows us to efficiently check the flows from StreamStats by using other methods (i.e. equations). Mathcad is also commonly used to calculate the scour depths around the bridge foundations and size the riprap protection mentioned in Part I.

#4: LiDAR

LiDAR: We also use LiDAR data. While not a specific program, LiDAR gives us images from ground penetrating radar so that we can bring these images into a hydraulic model to merge with survey data. From this information, we can get a fairly accurate account of land to model. LiDAR will reflect things like a knoll in a plane underwater, but it won’t show the water itself; it will show why the water is moving around something we can’t see without this data.

#5: SMS

Surface-water Modeling Solution: More commonly known as SMS, this program is the graphical user interface developed by Aquaveo; this is the software that we use to put together all of the data to create a model that can showcase contours and streamlines representing water. We use the aerial image from Google Earth in the program to define the various areas of terrain roughness; use the flows from StreamStats to tell the program how much water it needs to include; and use the LiDAR to define the topography throughout the model. SMS is especially useful when we are explaining to clients the need for different types of infrastructure or to show the public what’s going on with the water, but it’s also useful for the engineers to be able to see the reason we would need different foundations for a bridge or different materials to construct with because of water flow. For example, although the bridge may span the bankfull width, is the bridge still a constriction in the overall floodplain that could cause deep scour? Or is there an area of the roadway that is still overtopping and might experience erosion?

#6: SRH-2D

Sedimentation and River Hydraulics – Two-Dimensional model: This program is known as SRH-2D and was developed at the US Bureau of Reclamation in collaboration with the Federal Highway Administration and the Water Resources Agency in Taiwan. This is the software that deals with the computational efforts that go on behind the scenes after the model is built in SMS. It’s a 2D hydraulic, sediment, temperature and vegetation model for river systems. SMS and SRH-2D usually confuse people because they think they are the same, but you use each of them for different purposes: SMS is for setup and reviewing results, and SRH-2D is for processing for the information.

There are some programs we haven’t talked about here. For simple culverts, the HY-8 Culvert Analysis Program developed by the Federal Highway Administration may be used. SRH-2D can utilize HY-8 to incorporate culverts within the 2D model; this development of SRH-2D was in cooperation with Aquaveo and the Environmental Modeling Research Laboratory at Brigham Young University. For 1D hydraulic modeling we would use the Hydrologic Engineering Center’s River Analysis System (HEC-RAS) developed by the US Army Corps of Engineers – which can be used for 2D but is not as widely used as SMS, especially in the Northeast. We could also use 3D hydraulic modeling technology, such as FLOW-3D, but that will be more prevalent in the future when computational power can better handle the programs required for it.

Computers have come a long way in being able to process the complicated software to create 2D hydraulic models. While 1D hydraulic modeling gave us more capabilities with bridge hydraulics than just calculators, we are pleased with the extra capabilities 2D hydraulic modeling has afforded us and are always looking for ways to use it better. Want to find out more about 2D hydraulic modeling?

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!