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The critical depth for various culvert cross-sections may be found from charts in HEC 5. An example is given in Fig. 5.16 for a rectangular section. In this case, the ratio of the flow Q (ft3/s) to the width B (ft) is used to find the critical depth dc (ft). Of course, dc cannot exceed […]

The headwater depth for a culvert operating under outlet control may be determined with the aid of the outlet control nomographs in HDS 5. An example of these nomo­graphs for box culverts is shown in Fig. 5.15. The following procedure may be used to determine the head H from the nomographs. The length L (ft), […]

Once the design discharge and allowable headwater are determined and the culvert alignment and slope decided upon, an efficient culvert size may be found through the use of nomographs as shown in “Hydraulic Design of Highway Culverts,” Hydraulic Design Series 5, FHWA. An example of an inlet control nomograph is shown in Fig. 5.14. Since […]

The allowable headwater depth is the depth of ponded water permitted at the entrance to a culvert. Allowable headwater depths are generally classified as either physical controls or arbitrary controls. Physical headwater controls are topographic features that should be protected against periodic inundation. They include the roadway pavement and upstream pro­ductive property or structures. Additionally, […]

There are two types of flow in culverts: inlet control and outlet control. Accurate pre­diction of the condition of flow is difficult, and an assumption of the most conservative control may at times be warranted. Figures 5.12 and 5.13 depict several conditions of inlet and outlet control. For inlet control, the discharge capacity is controlled […]

The most common materials used are concrete, steel, aluminum, and plastic. The material used may affect the hydraulic capacity of the culvert, as different materials and wall configurations have different entrance loss coefficients and coefficients of roughness. The choice of the material is often controlled by structural and durability considerations. The inlet configuration generally has […]

Culverts convey surface flow from one side of the roadway to the other. Culvert design comprises three general considerations: culvert size, location, and shape. The size of the culvert is directly related to the results of the hydrologic investigation. The location of the culvert is derived from the site geometry and comprises the alignment, length, […]

As indicated previously, subgrade drainage is designed to handle surface water inflow, whereas subdrains are designed to accommodate encroaching groundwater. Surface water can enter the pavement subsection through joints, cracks, and infiltration of the pavement. Rapid drainage of the pavement structural section is necessary to minimize piping and swelling of the subgrade material, and the […]

Saturation of the structural section under the roadway (subgrade and base course) and the foundation materials is a primary cause of early roadbed failure because of decreased ability to support heavy truck loads. Saturated conditions can lead to piping of fines and frost damage or icing of the roadway surface. Designs to prevent water from […]

The open-end conduits used to convey water from one side of the roadway through the embankment to the other side are typically referred to as culverts. A network or system of conduits to carry storm water intercepted by inlets is referred to as a storm drain system. Conduits for culverts and storm drains are available […]