Throughout the United States there are millions of miles of storm culverts that have exceeded their design life. Many of these structures were built by masons and craftsmen more than 100 years ago and have rusted or deteriorated over time.

It is common to see news reports about failing bridges or roads that have collapsed into sinkholes. As is often the case, these sinkholes are formed over time due to deteriorated culverts underneath the roadway. Historically, culvert maintenance had been seen as less of a priority when compared to road resurfacing or bridge repair.

This article will explore the common factors that determine culverts hydraulics, and the positive impact inlet control measures will have when implementing trenchless rehabilitation solutions.

Culvert replacement will seldom be a viable alternative in significant and congested locations where long detours or extensive disruption to vehicular traffic will occur. In areas where replacement is not an option, structural rehabilitation should be considered the primary means to restore the safety, hydraulic capacity and longevity of storm water structures.

By understanding the hydraulic conditions that influence culvert performance, designers and maintenance engineers can utilize economical techniques to satisfy various engineering requirements.

The Federal Highway Administration’s “Hydraulic Design of Highway Culverts” manual (Publication No. FHWA-HIF-12-026) is an extremely useful reference for assessing hydraulic characteristics for culverts that have a cross-sectional span of less than 20 ft. It provides a technical review of the key concepts that govern culvert hydraulics. These consist of the following:

  • Headwater Depth & Flow Rate
  • Culvert Barrel Cross Sectional Area
  • Barrel Shape & Haunch Radius
  • Culvert Inlet Size and Configuration
  • Friction / Manning’s Coefficient
  • Culvert Barrel Length
  • Barrel Slope / Gradient
  • Culvert Outlet Size & Configuration

Chapter 3 of the FHWA hydraulic design manual focuses on the types of Culvert Flow Control. As stated in the manual, “Many different flow conditions exist over time, but at a given [time] the flow is either governed by the inlet geometry (inlet control) […] or by a combination of the culvert inlet geometry, the characteristics of the barrel, and the tailwater geometry (outlet control).”

A culvert flowing in “Inlet Control” is typically characterized as having a shallow, high-velocity flow through the culvert barrel. This is referred to as a “supercritical” condition.

Conversely, a culvert flowing in “Outlet Control” is described as having relatively deep, lower velocity flow termed “subcritical.” In these conditions the culvert could also be flowing full or at capacity. As stated for subcritical flow, the control feature is at the discharge end of the culvert barrel.

The National Highway Institute, the training branch of the FHWA, has published several culvert hydraulic videos specific to the hydraulic performance of sliplined culverts. These are recommended videos to watch for designers and engineers engaging in culvert rehabilitation projects.

It is recorded that several factors influence Inlet Control. These include:

  • Headwater Depth
  • Inlet Area
  • Inlet Configuration
  • Inlet Shape
  • Barrel Slope

Because conditions at the inlet are critical for culvert’s exhibiting Inlet Control, the inlet configuration is a major factor in the hydraulic performance. This is especially true when assessing slip-line rehabilitation options for culverts with a nominal diameter of 24- in. or smaller.

It is common practice to downsize liner dimensions approximately 6 inches from the host structure. As an example, on a 24-in. culvert pipe this would require a liner of 18 in. in diameter. Downsizing from 24 in. to 18 in. in diameter results in a 44 percent reduction in allowable flow area. This can create significant issues such as over-topping or upstream flooding if the water-way area is too severely restricted.

However, as we have previously discussed, implementing changes at the culvert barrel entrance will dramatically enhance the existing hydraulic conditions. The FHWA positions that one of the best methods for increasing inlet performance is the use of beveled edges at the culvert entrance. Although any beveling technique will assist in enhancing inlet hydraulic conditions, the FHWA design charts typically recommend bevel configurations of 45 and 33.7 degrees.

Hydraulic performance of an inlet control culvert will be improved by utilizing a liner, such as InfraSteel’s Flow Bell, that creates a beveled inlet condition between the host structure and the new liner. The hydraulic performance of the culvert can then be recorded by measuring the following three variables:

Barrel Entrance Design + Friction Coefficient of the Barrel Material + Change in Exit Velocity.

The inlet factors influencing the performance of culverts in Inlet Control conditions also influence culverts exhibiting Outlet Control. The following conditions influence Outlet Control culvert performance:

  • Barrel Roughness
  • Barrel Area
  • Barrel Shape
  • Barrel Length
  • Barrel Slope
  • Tailwater Elevation

The Manning’s equation is an empirical evaluation commonly used to calculate the barrel roughness or friction loss in culvert designs. Corrugated Metal Pipe (CMP) culverts are typically assigned a Manning’s Value of .024. As the structure material moves away from rigid corrugations this value goes down. Manufactured from Smooth-Wall, Carbon Steel Plate, each InfraSteel liner has a Manning’s Value of .012. Reducing the “N” value is often used to confirm the improved flow rates when choosing liners with less surface profiles, that also provide less surface friction.

In Outlet Control culverts, the culvert barrel typically exhibits “Full Flow” or standing water at the outlet end. In these conditions, improving flow rates with slip-lining can be more difficult to achieve when compared to Inlet Control methods, especially in structures with less than 48 in. in nominal diameter.

However large culvert structures, especially those 48 in. in diameter and up, can often be improved due to the fact that any loss in the cross-sectional area is nominal compared to the overall hydraulic capacity of the pipe. For example, an existing 60-in. CMP pipe is sliplined with a 54-in. smooth wall, carbon steel liner. This results in only a 19 percent reduction in flow area, while also improving the roughness coefficient from .024 to .012.

The Outlet Control flow conditions are calculated by summarizing the various elements that contribute to the hydraulic energy balance (HL). These include the Entrance Loss (He) “case of E,” the friction loss of the barrel (Hf), and the exit loss (Ho). Pipe sizes over 48” diameter, the reduced Hf value greatly enhances hydraulic performance.

Another consideration is if the existing structure was initially overdesigned, or oversized as is often the case with Box Culverts, slip-lining allows for a straight-forward approach to rehabilitating legacy structures. InfraSteel’s Box Culvert and Formed Corner Radius (FCR) designs maximize the liner flow area by matching the shape of the liner to the existing structure.

Considering the “worst case” scenario, most modern culverts are designed as Stand-Alone Structures with design life’s that surpass 70 years. Trenchless rehabilitation techniques such as Slip-Lining with InfraSteel provide a simple solution that achieves this objective and provides a new structure inside the existing pipe. By incorporating inlet control features into the rehabilitation technique, the hydraulic capacity is maintained or even enhanced and the culvert structure is positioned to serve for decades to come.

The team at InfraSteel works hand-in-hand with design engineers in order to satisfy hydraulic and structural priorities. Innovation is driven by evaluating the most appropriate applications demanded by the project site. To help aide in determining the most appropriate rehabilitation solution, we offer a Culvert Rehabilitation decision-making matrix that allows designers to assign values to the areas of the structure deemed most important for the site, and affords the ability to impartially evaluate various rehabilitation options.

If you are interested in learning more about the InfraSteel Culvert Rehabilitation design process, Case Studies, or topics relevant to the trenchless rehabilitation industry, please visit infrasteel.com.