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Utilization of 2D Hydraulic Modeling Saves CDOT Millions of Dollars

Shalice Reilly, Intern for the Office of Process Improvement
Posted: October 19, 2018

Source: Brian Varrella, PE I, Hydraulics   [email protected]

2D Model

Example of a 2D model used by Region 4’s Hydraulics Department

In 2018, the use of 2D hydraulic modeling has saved the Colorado Department of Transportation(CDOT) a whopping $4.4 million and prevented $16 million worth of projects from being abandoned. These savings can be attributed to the 2D modeling advocacy efforts of Brian Varrella, a professional engineer in CDOT’s Region 4 Hydraulics Department.

The Region 4 Hydraulics department runs fifty-three different projects in-house and annually supports over $600 million worth of work. Currently, most of the hydraulic modeling done in hydraulics departments across CDOT is done with a 1D model. However, by opting for a 2D model on just four of his projects, Varella was able to save projects that were considered impossible to address, as well as accelerate the scoping and reduce material costs on his projects.

Hydraulic Modeling

A hydraulic model is a mathematical system used to predict and analyze the behavior of moving liquids within a confined space under pressure. There are three types of hydraulic models; 1D hydraulic models use only one equation for their analysis, while 2D uses two equations and 3D uses three (however, these types of model systems are still under initial development). In simplified terms, these models create virtual rivers and roads to calculate the distribution of velocity and depth of waterways near infrastructure features. This information is required to understand the long-term stability of bridges and culverts against the inherent erosive nature of flowing water.

“Hydraulic models give us a more granular view of the impact of our projects on the rivers, and the rivers on our projects,” explained Varrella.  At CDOT, these models are used by hydraulics departments to analyze the pressures of waterways on surrounding structures and to identify bottlenecks. This then allows them to identify areas of potential attack1, scour2, or breakdown of structures. It is also used to digitally test potential mediations to structural concerns, without having to actually spend time and funds on constructing anything.

When to Use 2D vs. 1D Models

A 2D model is able to analyze waterways at thousands or tens of thousands of points, while a 1D model is more of an estimation of waterway conditions between a dozen or so cross sections. If you have a fairly straight waterway over a culvert, or under a bridge, with a fairly consistent depth, then the 1D model should be used. But in situations where the waterway is not straight, and/or the depths of the waterway are changing, then it is best to use a 2D model.

“With 1D modeling, engineers would often end up with about a dozen assumptions that, in reality, wouldn’t apply to the waterway,” explained Varrella. “With more complex situations, a 2D model is going to be more cost effective to put together and is going to give us more input at the beginning of the design process than a 1D model ever could”.

How Varrella’s Use of 2D Modeling Saved CDOT $4.4 million

2D model analysis

Results of 2D analysis showing shear stresses on the overtopping section of SH 144 near Orchard, CO.

“If I am correctly doing my job, then nobody should have to worry about the stability and safety of their bridges and roadways,” said Varrella. “That’s what we as State taxpayers are all paying for; assurity and safety.”

Within the first eight hours of implementing a 2D model on a CDOT project, Varrellas knew he was onto something. “I was getting calls from people asking how I managed to complete the vetting process in a number of hours when it normally takes months,” Varrella recalled.

2D modeling saved a total of $2 million on the State Highway 144 project, compared to how much would have been spent using a 1D model. As a part of Varella’s proof of concept, he then took out the cost of teaming with experts at AECOM3 and the cost of his own time, and found that “despite some time spent deliberating on this project, the return on investment for the project was still $33 thousand an hour to the taxpayer," emphasized Varrella.

On the following project of the South Platte River and State Highway 55 in Logan County,  the team was evaluating how the river was pushing up against an abutment, putting the road at risk of washout. The original 1D model indicated that river training4 efforts would only provide a two-year protection of the structures. By creating an in-house 2D model, the team was better able to calculate deterioration and was able to assemble a team to construct a solution that would provide a fifty-year protection of the structure, at nearly half of the original cost. This 2D model was completed in four hours, making Varrella’s return on investment $55,000 per hour. “That seems like a pretty good investment to the taxpayer” Varrella joked.

With the other two projects from the study, the benefits of using 2D came in other forms than cost savings.

For a project on Highway 287 in Larimer County, using 2D modeling actually saved the project. Before creating a 2D model, the project was scoped as needing 36-inch riprap5, when the largest amount of riprap CDOT can physically get is 24 inch. While creating this 2D model resulted in many hours of work for Varrella, it also allowed him to reduce the needed rock size from 36 to 12 inch, making the project not only feasible but also much less expensive. The calculated material cost savings for that project was still about $300,000. Not to mention, hauling large riprap is not entirely environmentally friendly, so the lessening of size and volume of riprap also decreased CDOT’s environmental footprint for construction.

On the fourth project for this study, the task was to retrofit sixteen structures at six different crossings over the South Platte River. Based on inferences from a 1D model, this project originally required $2.1 million worth of riprap, and still had a number of analytical discrepancies. The use of 2D models created a material cost savings of $500 thousand because they showed that in order to repair and protect these bridges, two structures from the original scope did not need to be touched at all and that the remaining fourteen structures would need 25% less riprap than the 1D model would lead one to believe. So the biggest benefits from this project came in the form of optimizing the design for protecting sixteen different structures, and significantly reducing CDOT’s environmental impact for hauling riprap.

The Progression to 2D Modeling

For other purposes, CDOT has already begun the process of collecting LiDAR imagery of all of the roadways in the State. The Colorado Water Conservation Board (CWCB) is also under contract to collect LiDAR for waterways across the entire state. This means that the combined datasets of two state agencies will soon be available for every area in the State where a roadway passes over a body of water. This data can then be used to construct a three-dimensional model of the road and waterway, which can then be entered into a two-dimensional hydraulic analysis -- making 2D analyses more accessible to CDOT’s Hydraulics departments than ever before.

One limitation to the widespread transition to 2D modeling amongst hydraulic engineers comes from the common belief that 2D modeling is more expensive than 1D. This belief is so widespread because 2D modeling requires a large amount of computer power to process, and in the early 2000s, computer power was very expensive to obtain. But, with advancing technology, this level of computer power is now actually quite common and easy to obtain. 2D modeling programs can now be run on current standard-issue laptops.

With cheaper computer power and lots of data available statewide, 2D models are on track to become the standard of practice. Discussions with the Federal Highway Administration (FHWA) and faculty at Colorado State University (CSU) indicate that CSU’s hydraulics program almost exclusively teaches 2D modeling, which says a lot about the future of hydraulic engineering and the young professionals entering the workforce.

Resources to Learn about 2D Modeling

“We’re just scratching the surface of what we can achieve here,” expressed Varrella, “Everybody is learning at a pace that no one entity could keep up with.”

Varrella explained that his biggest limitation in leading CDOT in a transition to 2D modeling has come from a lack of awareness. Griffin was the first CDOT Hydraulic Engineer to try and normalize this technology in 2009. Despite Varrella’s work over the past three years to keep the 2D model flame burning, many CDOT Hydraulic departments are still in the early stages of understanding the potential cost and time savings associated with 2D modeling.  

In an effort to remedy this, Varrella has begun a series of ‘work sharing sessions’, where he and other CDOT engineers meet with employees of private engineering firms to collaborate on best practices, to share roadblocks they have encountered, and to showcase effective solutions discovered in the 2D hydraulic modeling process. Varrella and Griffin have hosted four of these sessions to this date and, after getting a lot of positive feedback and interest, they are now in the process of planning more.

Scott Hogan, from the Federal Highway Administration (FHWA) Resource Center, holds one-hour long seminars on the fundamental processes of 2D modeling, which he calls the ‘2D Modeling Users Forum.’ These virtual seminars are free to join6 and are broadcasted nation-wide.

Varrella and Hogan presented a ‘2D Modeling Practices’ seminar at a national conference in June of 2018. The two are now working on transforming that seminar into a one-day training7, which they would offer every six months in different locations around Colorado. The course would be comprised of two presentations, two demonstrations, and then two hands-on problem-solving exercises for the participants to complete with the assistance of Varrella and Hogan.

For individuals interested in learning more about 2D modeling, Varrella suggests listening in on the online FHWA Users Forum broadcasts, obtaining the software from Aquaveo, then utilizing the series of simple tutorials that are provided on Aquaveo’s website.

“[By working to transition to 2D modeling] I am trying to do exactly what our mission tells us to do by creating an efficient system that keeps everyone safe and moving,” said Varrella.


  1. When a river changes course and begins to deteriorate a structure.
  2. Bridge scour is the removal of sediments such as sand and gravel from around bridge abutments or piers. Scour, caused by swiftly moving water, can scoop out scour holes, compromising the integrity of a structure. In the United States, bridge scour is one of the three main causes of bridge failure (the others being collision and overloading). It has been estimated that 60% of all bridge failures result from scouring and other hydraulic-related causes (
  3. AECOM is an American multinational engineering firm that provides design, consulting, construction, and management services to a wide range of clients. AECOM has approximately 87,000 employees and is number 161 on the 2017 Fortune 500 list.
  4. River training refers to the process of mitigating a river back to a historic and functional path of water flow.
  5. Riprap can be defined as the loose stone used to form a foundation for a breakwater or other structure - used to protect structures or embankments from scour
  6. If you would like to join the 2D Modeling Users Forum, please email Scott Hogan at [email protected] and ask to be added to the listserv.
  7. If you would like to get involved with this training, reach out to Brian Varrella at [email protected] and ask to be added to the listserv.

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