2D Overland Flow or Not 2D Overland Flow - That is the Question

Introduction

While two-dimensional (2D) overland flow modeling is becoming increasingly common, many hydrology and hydraulic (H&H) modelers are either in the early stages of adoption or completely unfamiliar with the approach. Through discussions with both current and potential users interested in StormWise™ Expert license 2D modeling capabilities, the Streamline Technologies, Inc. technical support team has identified some frequently asked questions, which include:

1. What is 2D overland flow?
2. Do I need to use 2D overland flow for my projects?
3. How do I review the 2D overland flow results?

So, let’s dive in and explore these fundamental 2D overland flow questions!

What is 2D Overland Flow?

Surprisingly, the easiest way to define 2D overland flow is to start with a simple one-dimensional (1D) flow example. As the foundation for calculating rainfall excess, flow rates, and flood stages within a computational network, most modelers are familiar with the conventional 1D H&H modeling approach. In this system, the flow direction is clear, as illustrated in the channel example below. There is no doubt that this example is 1D flow confined within the channel banks in a single direction.

This channel could be easily modeled as a 1D channel link in StormWise™, with other common examples of 1D hydraulic elements in StormWise™, including pipes, weirs, bridges, and pumps.

Contrary to 1D flow, 2D overland flow can “move” in multiple directions. Extreme flooding events, in particular, offer a prime depiction of 2D overland flow.

For the rainfall-driven flooding example below, flood waters spread out over the surface, flowing over flat terrains, roadways, between houses, and through urban corridors. Consequently, this system is better represented by a 2D overland flow approach. As shown in the photo below, there are no clearly defined flow paths on the flooded surface. 

Where the example above showcases shallow 2D overland flow, the example below depicts very deep 2D overland flow.

In this case, the water level of a nearby river has overtopped the bank, flooding surrounding areas, including roadways and wetland sloughs. These flood waters also have no definitive flow path, thus benefiting from a 2D overland flow model.

Do I need to use 2D overland flow for my projects?

Now that 2D overland flow has been defined for H&H modeling, how do we know if it’s applicable to a particular project? The short answer is that most systems can be modeled with 2D overland flow. While this modeling technique is beneficial when the flow is not well defined, it is not necessary for all applications. First and foremost, the modeler should consider whether a 1D or 2D approach adequately represents the hydrology and hydraulic network for their study area to meet the overall project needs. It should be noted, however, that there are certainly cases where a 2D model accurately captures the hydrologic conditions and terrain/flow paths that are not as easily accomplished when employing a conventional 1D approach. These can include:

• Non-dendritic Systems: It is usually advantageous to model non-dendritic systems with 2D overland flow methods because basin delineation can be quite time-consuming and difficult. Non-dendritic systems often take on a more web-like network layout with multiple overflow connections between adjacent basins.
• Wetland Sloughs: Modeling flow through wetland sloughs is also much easier using 2D overland flow methods.  Given the nature of these systems, it is often difficult to model the “flow path” as 1D without incorporating multiple overflow connections between adjacent basins. These conditions are common in relatively flat areas like coastal Florida. Consequently, it is advantageous to set up these systems using a 2D overland flow model.
• Channel Overbank Flooding: Channels or rivers that are expected to reach out-of-bank conditions can benefit from 2D overland flow models. The 2D approach captures the surface flow and velocities in every outreaching direction, which can be important when out-of-bank flooding impacts urban and residential areas.

Another important consideration is the application of the model results. Elevation, depth, and flow & velocity vectors are examples of results that can be animated in StormWise™. You can also pause the animation at any time during the simulation and export a raster grid file results for further analysis or record the animation to demonstrate flooding to your clients. 

How do I set up a 2D overland flow model?

With the increasing data availability from both private and public sources, it can be easier to develop a 2D overland flow model as opposed to a conventional 1D H&H model. However, in order to optimize the computational load, it is beneficial to simplify 2D models as much as possible.

“Everything should be made as simple as possible – but no simpler.”

- Albert Einstein

This may mean developing hybrid 1D/2D models to leverage 1D model components. A general workflow for 2D overland flow model setup is as follows:

1. Collect base data (i.e., surfaces, land use, soils) 
2. Develop 1D model network components (optional) 
3. Incorporate 2D overland flow features to refine the computational network
4. Build the 2D overland flow computational network 

The 2D overland flow computational network in StormWise™ uses the same building blocks as a 1D StormWise™ model. This means both 1D and 2D work from the same system of equations and are fully integrated. Additionally, the 2D overland flow computational network consists of an irregular triangular mesh with a honeycomb constructed around each vertex, which is a highly-important concept. The irregular flexible triangular mesh resolution can be varied across the study area which reduces the complexity of the model computational network.
 
It is generally advisable to integrate 2D overland flow features selectively, providing detailed representation where necessary while minimizing unnecessary detail for analysis. The 2D overland flow computational network should capture the flow paths and ridges based on the underlying surface, as it creates a reasonable representation of the ground surface for the 2D computational network for analysis.

How do I review the results?

Once the 2D model (or 1D/2D hybrid) is created and simulations have been executed, reviewing model results for 2D overland flow is best completed visually. While you may review results for individual 2D nodes, links, and basins in tabular or chart formats (like typical 1D model results), animations offer an enhanced visual representation of the model results. In StormWise™, several animation report types can be leveraged to expedite model review. These include, but are not limited to, depth and velocity animations, as shown below.

Depth Animation:

Velocity Vector Animation:

In addition, animation results can be exported at any point during an animation. Results can be exported as CSVs, shapefiles, and raster files, which can be further analyzed outside of StormWise.

Conclusion

We are hopeful that this provides a comprehensive explanation of 2D overland flow applications! A 2D overland flow analysis opens up many possibilities compared to a standard 1D H&H approach. However, always keep in mind the available data and project goals to confirm if 2D overland flow is appropriate for the project.

Still have questions? Contact our team today for more information on utilizing 2D overland flow within StormWise™!