Steady-State, Semi-Dynamic or Fully-Dynamic Choosing a Hydraulic Model That is Most Appropriate for Your Needs

Wastewater agencies are constantly challenged to stay on top of the latest technology and data trends. At the same time, there is an emphasis on keeping costs down. Establishing the proper balance between these two competing objectives is always a consideration when an agency decides to perform hydraulic modeling as part of a sewer master plan or other sewer planning study. The following questions often present themselves:Should we keep the same steady-state or semi-dynamic model we've been using for years, or upgrade to a fully-dynamic model?What insight will we gain by doing so?Is it worth the cost?Dynamic models include iterative hydraulic computations that accurately represent the effects of flow routing of hydrographs as they move through the collection system. Steady-state models do not hydraulically route flows through the pipe system, but instead simply compute the average flow in each pipe as the sum of all upstream flows, and apply generalized peaking factors to that sum. Semi-dynamic models represent the attenuation due to hydrograph timing and some aspects of pipe storage, but they cannot realistically represent complex hydraulic situations like backwater and flow splits.Some readers may be surprised that steady-state models are still in use today. But in parts of the country where sewer collection systems are separate, have insignificant levels of wet weather infiltration and inflow, and have other qualifying characteristics, sewer hydraulics may be simply modeled with acceptable results. In addition, steady-state models are commonly used at most agencies and engineers for the design of new sewers up to a certain size. Agencies that fit into this category need to determine if and when investment in a dynamic model is worthwhile.This paper informs readers of the issues that must be considered when selecting a hydraulic model for sewer collection system analysis. It presents the benefits and drawbacks of three model types: steady-state, semi-dynamic, and fully-dynamic. It includes illustrations that compare results and describe specific examples of analyses that can be completed with each model type.The paper concludes that the following guidelines can be used to determine which model type is best for an agency:Steady-state models can be used to conservatively size sewers that service new development and to model existing sewers in situations where wet weather response is negligible and the modeled system has uniform characteristics such as slope, diameter, and land use that produce a reliable relationship between peak flow and average base flow.Fully-dynamic models are highly accurate and can be used to address a wide array of planning, operational, and regulatory issues. Although they may be more costly to purchase, build, and maintain, they can save an agency millions of dollars by deferring or eliminating capacity projects that are developed using less accurate methods.Semi-dynamic models have many of the capabilities of both steady-state models and fully-dynamic models. They are a good option for agencies with relatively simple systems who would like to develop a dynamic model solution for planning or operational purposes while also retaining their steady-state solution for design purposes.