Culvert Design Considerations
This section presents culvert design considerations.
3-3.1.1 Flow
The first step in designing a culvert is to determine the design flows to be used. The flow from the basin contributing to the culvert can be calculated using the methods described in Chapter 2. Generally, culverts will be designed to meet criteria for two flows: the 25-year event and the 100-year event. If fish passage is a requirement at a culvert location, contact the State Hydraulics Office (see Chapter 7). Guidelines for temporary culverts are described further in Section 3-3.1.9. The PEO will be required to analyze each culvert at each of the design flows, ensuring that the appropriate criteria are met.
3-3.1.2 Additional Requirement for Culverts over 20 Feet
Once a culvert exceeds 20 feet along the centerline of the roadway, it is defined as a bridge and all hydraulic analyses on bridges are the responsibility of the State Hydraulics Office (see Chapter 1). The federal definition of a bridge is a structure, including supports, erected over a depression or obstruction, such as water, highway, or railway, and having a track or passageway for carrying traffic or other moving loads with a clear span, as measured along the centerline of the roadway, equal to or greater than 20 feet. (i.e., a 16-foot culvert on a 45-degree skew is a bridge, a 10-foot culvert on a 60-degree skew is a bridge, and three 6-foot pipes 2 feet apart is a bridge).
The two primary types of hydraulic analysis performed on bridges are backwater and scour. As noted above, all hydraulic analysis of bridges is performed by the State Hydraulics Office or a hydraulics engineer approved by the State Hydraulics Office; however, it is the responsibility of the PEO to gather field information for the analysis. Chapter 7 contains more information about backwater and scour analysis, along with the PEO list of responsibilities.
3-3.1.3 Alignment and Grade
Culverts shall be placed on the same alignment and grade as the natural channel, especially on year-round streams. This tends to maintain the natural drainage system and minimize downstream impacts.
In many instances, it may not be possible or feasible to match the existing grade and alignment. This is especially true in situations where culverts are conveying only hillside runoff or streams with intermittent flow. If following the natural drainage course results in skewed culverts, culverts with horizontal or vertical bends, or excessive and/or solid rock excavation, it may be more feasible to alter the culvert profile or change the channel alignment upstream or downstream of the culvert. This is best evaluated on a case-by-case basis, with potential environmental and stream stability impacts being balanced with construction and function ability issues.
3-3.1.4 Allowable Grade
Concrete pipe may be used on any grade up to 10 percent. Corrugated metal pipe and thermoplastic pipe may be used on up to 20 percent grades. For grades over 20 percent, consult with the RHE or the State Hydraulics Office for design assistance.
3-3.1.5 Minimum Spacing
The use of multiple culvert openings is not allowed.
3-3.1.6 Culvert Extension
Culvert extensions shall be done in-kind—using the same pipe material and size and follow the existing slope. All culvert extensions shall follow the guidelines for the culvert sizes noted in Section 3-2.2 and Chapter 1. The PEO shall follow the manufacturer’s recommendations for joining pipe. For situations not listed, contact the RHE.
• Culvert pipe connections for dissimilar materials, when approved by the RHE, must follow Standard Plan B-60.20-02 of WSDOT’s Standard Plans.
• For cast-in-place box culvert connections, contact the Bridge Design Office for rebar size and embedment.
• Precast box culvert connections must follow American Society for Testing and Materials (ASTM) C 1433, AASHTO M 259, M 273, and Standard Specification 6-02.3(28).
3-3.1.7 Minimum Culvert Diameter
The minimum diameter of a culvert under a main roadway must be 18 inches. Culvert pipe under roadway approaches must have a minimum diameter of 12 inches. If replacing an existing culvert, the new culvert shall have at least the same or larger diameter as the existing culvert even if the hydraulic analysis shows that a smaller-diameter culvert would meet hydraulic design requirements in that location.
3-3.1.8 Culvert Pipe at Walls and Foundations
Culvert pipes in the reinforcement zone of walls or the soil-bearing zone of foundations should be coordinated with the geotechnical engineer.
3-3.1.9 Temporary Diversions
Temporary diversions for a single construction season shall be sized for the 2-year storm event, unless the PEO can provide hydrologic justification for a different storm event and receive State Hydraulics Office or RHE approval. The design storm for multiple-season construction projects shall be a risk-based decision and shall be determined by the PEO and RHE.
For design-build projects, the design and flow rate are determined by the design-builder based on the requirements of project permits.
For design-bid-build projects on fish-bearing streams, the State Hydraulics Office calculates the flow rates necessary for temporary diversions and that value is part of the contract documents. A conceptual-level plan is required for permits, but no plans for the temporary diversion system should be put into the final plan set and should not be documented in the specialty report, unless otherwise approved.
Temporary diversions for fish-bearing streams shall be designed for the following storm events:
• Single season: For a temporary diversion expected to be in place for a single fish window, the design flow rate shall be, at a minimum, equal to the expected 50 percent exceedance flow rate during the window when the temporary diversion is in place with a contingency plan that shall be in place within 2 hours or less to bring the system to meet the expected 10 percent exceedance flow rate during the window when the temporary diversion is in place. The expected flow rates during the window when the temporary diversion is in place can be determined through stream gage data (if available) or through an MGSFlood seasonal flow analysis (western Washington only). The flows can also be measured in the previous fish window years to get a base flow followed by an analysis for a 2-year storm based on rainfall for that fish window. If there are no data to calculate the flows during the construction window, then the expected 2-year flow rate shall be used for the design flow (contingency not necessary in this case) unless the PEO can justify a different flow if approved by the State Hydraulics Office.
• Multiple season: A gravity bypass is required if the stream diversion is expected to remain in place over the winter; pump bypasses will not be allowed. The culvert shall be the lesser of the size required to pass the 25-year flow event or that required to meet the existing culvert capacity. The length of the stream bypass contained within a culvert shall not be longer than the existing culvert unless otherwise approved by the State Hydraulics Office. Fish passage shall not be decreased from the existing conditions as evaluated by the Fish Passage Inventory, Assessment, and Prioritization Manual.
The design flood for temporary structures over water bodies shall be determined by the State Hydraulics Office.
