State departments of transportation (DOTs) face growing pressure to maintain and rehabilitate aging culvert infrastructure, especially with frequent and intense storms, and limited funding. Culverts, typically less than 5 feet in diameter, are particularly at risk. Full replacement is often costly and disruptive, especially in areas with deep fills or heavy traffic. Rehabilitation methods, such as sliplining, offer cost-effective alternatives, but they often result in downsized culverts, reducing hydraulic capacity and potentially compromising system resilience. This reduced capacity can lead to increased headwater elevations, overtopping, and downstream scour, particularly in urbanized environments. There is a need for an energy dissipator system for the outlet end of culverts and storm sewers that are more compact than currently available designs.

Outlet diffuser systems potentially offer a promising solution. By integrating improved inlet designs, flared diffuser outlets, and outlet control weirs, these systems may enhance culvert capacity. Research indicates potential capacity increases of 30–40% for culverts of the same nominal diameter. Diffuser systems also help mitigate scour and downstream channel degradation, offering a more compact and cost-effective alternative to traditional energy dissipators, which are typically large and expensive and require extensive right-of-way. 

With the rise of computational modeling like computational fluid dynamics (CFD), rigorous testing is needed to validate performance, develop reliable design tools, and establish accepted design strategies. Current energy dissipation solutions lack flexibility, are often based on decades-old research, and were designed for dam spillways rather than roadway culverts. A more compact, effective, and retrofit-capable design would not only extend the service life of existing culverts but also offer significant savings in right-of-way acquisition and construction costs. Research is needed to advance diffuser systems as a nationally recognized, credible design alternative for small culvert rehabilitation and replacement. Such systems could help preserve culvert capacity, mitigate scour, and enhance system resilience, providing state DOTs with an essential tool for cost-effective infrastructure management.

The objectives of this research are to develop a practitioner’s guide that (1) identifies compact energy dissipator systems for the outlet end of culverts and storm sewers; (2) documents and verifies the capacity improvements offered by energy dissipator systems (e.g., diffuser systems) through CFD analysis, physical modeling, and field testing; and (3) provides performance equations and design strategies to support widespread, standardized implementation.