Lecture 21 Culvert Design & Analysis Much of the following is based on the USBR publication: Design of Small Canal Structures (1978) I. Cross-Drainage Structures Cross-drainage is required when a canal will carry water across natural drainage (runoff) channels, or across natural streams; otherwise, the canal may be damaged In some cases, cross-drainage flows are collected in a small channel paralleling the canal, with periodic cross-drainage structures over or under the canal; this is especially prevalent where there are poorly defined natural drainage channels In culvert design for carrying runoff water, usually one of the big questions is what the capacity should be When the canal capacity is less than the natural channel capacity, it may be economical to build an inverted siphon so the canal crosses the natural channel With siphon crossings, it is not nearly as important to accurately estimate the maximum flow in the natural channel because the structure is for the canal flow In other cases, it may be more economical to provide cross-drainage by building a culvert to accommodate natural flows after the canal is constructed In these cases, the cross-drainage structure does one of the following: 1. Carry water under the canal 2. Carry water over the canal 3. Carry water into the canal Here are the common cross-drainage solutions: 1. Culverts These are often appropriate where natural flows cross a fill section of the canal Culverts may tend to clog with weeds, debris, rock, gravel, and or sediments, especially at or near the inlet 2. Over-chutes These are appropriate where the bottom of the natural channel is higher than the full supply level of the canal For example, over-chutes might be used in a cut section of the canal Canal over-chute & bridge BIE 5300/6300 Lectures 237 Gary P. Merkley
Open-channel over-chutes can carry debris and sediment that might clog a culvert, but pipe over-chutes may be equally susceptible to clogging 3. Drain Inlets II. Alignment With these structures, the flow of the natural channel is directed into the canal These may be appropriate where the natural flows are small compared to the canal capacity, and or when the natural flows are infrequent These may be appropriate when the canal traverses a steep slope, and cross-drainage might cause excessive downhill erosion, compromising the canal These may be less expensive than over-chute or culvert structures, but may require more frequent maintenance of the canal Drain inlets may be problematic insofar as rocks, sediment and other debris can clog the inlet and or fill the canal near the inlet, obstructing the canal flow Align the culvert along natural open channels where possible so that the natural runoff pattern is not disturbed any more than necessary If the natural drainage channel is not perpendicular to the canal, it is best to have a skewed alignment of the culvert One or more bends in the culvert can be used to help follow the natural channel, especially in longer culverts If there is no apparent natural runoff channel, consider using the shortest straight path from inlet to outlet In some cases it may be unnecessary or undesirable to follow a natural channel III. Barrel Profile Knowing the inlet and outlet locations will determine the length and slope of the culvert The invert of the inlet and outlet should correspond approximately to the natural ground surface elevations at the two respective locations -- otherwise, sedimentation and or erosion will likely occur, requiring maintenance However, a compound slope may be needed if: 1. The culvert would not have enough vertical clearance under a canal (about 2 ft for an earth canal, or 0.5 ft for a concrete canal), road, etc.; 2. The slope of the culvert would cause supercritical openchannel flow, which might require a downstream energy dissipation structure (making the design more costly); or, 3. You want to force a hydraulic jump to dissipate energy. Gary P. Merkley 238 BIE 5300/6300 Lectures
The USBR recommends, in general, a minimum slope of 0.005 and a maximum slope of somewhat less than the critical slope (maintain subcritical flow) The minimum slope is imposed in an effort to prevent sediment deposition in the culvert barrel The barrel of the culvert is usually circular (perhaps corrugated pipe) or rectangular The maximum slope is imposed in an effort to avoid the additional cost of an energy dissipation structure at the outlet (channels upstream and downstream of culverts are typically unlined, although there may be some riprap) With a compound slope, the upstream slope is steeper than critical, and the downstream slope is mild, thereby forcing significant energy dissipation through a hydraulic jump in the vicinity of the break in grade, inside the barrel IV. Inlets and Outlets USBR Culvert Inlets Type 1: broken-back transition, appropriate for natural channels with welldefined upstream cross-section (USBR Figs. 7-1 & 7-2) Type 2: suitable for wide natural channels with poorly-defined upstream cross section (USBR Fig. 7-4) Type 3: box inlet, also for use in a poorly-defined natural channel, but allows for a lower barrel invert at the inlet (USBR Fig. 7-5) Type 4: similar to Type 3, but with a sloping invert, allowing for an even lower barrel inlet (USBR Figs. 7-6 & 7-7) USBR Culvert Outlets 1. With energy dissipation structure 2. Without energy dissipation structure There are other USBR standard inlet designs (besides the above four) USBR-type culvert Type 1 Transition (USBR) inlets and outlets are made almost exclusively of concrete Some corrugated metal culverts have a circular or elliptical cross section with smooth metalic inlet and outlet transitions Use standard inlet & outlet designs if possible to save time and to avoid operational and or maintenance problems BIE 5300/6300 Lectures 239 Gary P. Merkley
Type 3 Transition (USBR) Type 4 Transition (USBR) Gary P. Merkley 240 BIE 5300/6300 Lectures
V. Pipe Collars Pipe collars are used to prevent piping along the outside of the barrel and or damage by burrowing animals For culverts under canals, the typical USBR design calls for three collars: one under the center of the upstream canal bank, and two under the downstream canal bank A short path between two adjacent collars means that the collars are too close together and or their diameters are too small The USBR recommends the following for minimum collar spacing: VI. Basic Design Hydraulics X 1.2Y min = (1) Culverts are typically designed for fullpipe flow in the barrel at the design discharge value This means that pressurized pipe flow is impending at the design discharge, but at lower flow rates open-channel flow exists in the barrel The upper limit on barrel velocity is usually specified at about 10 fps, or perhaps 12 fps with an energy dissipation structure at the outlet For full pipe flow without inlet and outlet structures, in which case the culvert is simply a buried pipe, you can use a limit of 5 fps Culvert with collars (USBR) Knowing the design discharge and the velocity limit, the diameter (circular barrels) for full pipe flow can be directly calculated For rectangular barrel sections, you need to determine both width & height Discharge capacity can be checked using the Manning (or Chezy) equation for a circular section running full (again, impending pressurization) For new pre-cast concrete pipe, the Manning n value is about 0.013, but for design purposes you can use a higher value because the pipe won t always be new BIE 5300/6300 Lectures 241 Gary P. Merkley
You can also check the discharge using the Darcy-Weisbach equation, with specified values for upstream and downstream water surface elevations in the inlet and outlet structures, respectively The head loss through a typical inlet structure with inlet control can be estimated as a minor loss by: 2 V hf = (2) K 2g where the coefficient K may vary from 0.05 for a smooth, tapered inlet transition, flush with the culvert barrel, to 0.90 for a projecting, sharp-edged barrel inlet Note that the inlet and or outlet losses may or may not be minor losses when dealing with culverts, especially when the barrel is short For outlet control, the head loss is estimated as in the above equation for inlet control, except that there will also be expansion losses downstream For barrel control, the head loss is the sum of the inlet, barrel, and outlet losses References & Bibliography USBR. 1978. Design of small canal structures. U.S. Government Printing Office, Washington, D.C. 435 pp. Gary P. Merkley 242 BIE 5300/6300 Lectures