Deciding between steady state and extended period simulations in InfoWorks ICM

It is not uncommon for numerical hydraulic models to inform the scale and size of built infrastructure. After all, one of the clear benefits of creating a model is the ability to experiment and test out your theories about how to optimize a design before you build. Thus, the validity of our built infrastructure ultimately comes down to the accuracy of hydraulic simulations. There are two common approaches when it comes to hydraulic simulations: the first is steady state (SS) and the second is extended period simulations (EPS).

Steady state vs extended period simulations

To aid in understanding the resemblance and contrast between both simulation approaches, let’s first imagine steady state modeling as a still photograph and extended period simulation modeling to be more representative of a dynamic video, which at a basic level is comprised of a series of still photographs.

A still photograph captures a single moment in time, with everything frozen and unchanging. Every detail in the photo, from the expressions on people’s faces to the position of each object, is fixed and can be studied in-depth. Similarly, steady state modeling examines a stormwater or wastewater network at a specific point in time with all boundary conditions and network variables being constant.

On the other hand, a video captures the dynamic and interactive nature of the scene. It shows how people move, react, and interact, and how the scene evolves. In terms of hydraulic modeling, extended period simulation modeling studies a system over time, observing how the boundary conditions fluctuate and how various network elements adapt and respond to the fluctuations.

Both modeling approaches are invaluable to understand the behavior and response of our infrastructure networks, and that is true to drinking water systems, stormwater network, and wastewater networks. However, both modeling approaches also have stark differences. While a photograph gives you a snapshot of the system at a specific moment, a video provides a broader perspective of the system’s performance over an extended period.

Sensing the industry norms

In a recent webinar in partnership with WEF, Holistic wastewater collection system modeling using InfoWorks ICM, we conducted a survey of hydraulic modelers to understand how they were utilizing steady state and extended period simulations. Interestingly, 28% said they primarily used steady state, 35% said they primarily used extended period simulations, and 37% said their work was evenly split between the two approaches.

In further questioning, only 11% of respondents indicated that their application of steady state modeling was due to their perceived adequacy for their intended application. The majority highlighted limitations with steady state modeling: that steady state simulations represent a static snapshot in time, that they are not holistic in nature, and that relying solely on steady state simulations could lead to oversizing infrastructure and ultimately higher construction costs.

We wanted to dive deeper into what might be limiting the adoption of extended period simulation models. Key barriers we heard from participants were the absence of extended period simulation capabilities in some hydraulic modeling software, the number of required inputs, and the perceived complexity of extended period simulations.

The general consensus from our queries was that even though steady state simulations provided insight into the behavior of a network at a snapshot in time, the dynamic and holistic nature of extended period simulations would provide greater awareness into the fluctuation of load and boundary conditions, the variation of network attributes, and the response to changing operational modes.

Deciding between steady state and extended period simulations

Fortunately, in InfoWorks ICM, there are ways for you to tackle modeling either approaches, using either steady state or extended period simulations. In fact, the initialization process in InfoWorks ICM is basically a SS solution that warms up the network for the extended period simulation.

At its core, a key difference between SS and EPS is that EPS can take into account the attenuation of flow throughout the system, otherwise known as in-line storage. That being said, even if a model does not have a SS option as a simulation type, that doesn’t mean that it can’t effectively run a SS simulation. Any model could be turned into a SS model simply by applying a constant amount of flow over a period of time (usually fairly short), until it gets to the point that the flow entering all the nodes is equal to the flow exiting those same nodes, which eliminates any kind of storage through the system.

By removing the effects of storage, infrastructure will need to be larger in order to meet capacity requirements. This leads to oversized infrastructure and higher construction costs. Further, because all loads are applied at the same time in SS, which neglects the moving dynamic characteristics of the real system, EPS modeling is much closer to reality. Reality is dynamic and very much holistic!

Understanding holistic wastewater modeling using InfoWorks ICM

We go into the issue in the webinar, which you can watch on demand (👇). It covers everything from the initial steps of creating a network and performing calibration to the modeling analysis and output generation, through the full end-to-end workflow. Discover how InfoWorks ICM simplifies and streamlines this modeling workflow to inform capital planning and support operational decision making, deeming it the right companion as you tackle your wastewater collection system challenges.

To learn more about InfoWorks ICM and even book a custom demo for your team, you can get in touch with our team of experts by filling out this form. Additionally, Autodesk offers free on-demand training for InfoWorks ICM, which has modules that walk through setting up steady-state and extended period simulations.