Culvert Entrance Loss Coefficients (Ke) and Barrel Manning's n

Culvert entrance loss coefficients

The entrance loss coefficient Ke sets the head lost as flow enters a culvert barrel, and enters the outlet-control headwater calculation. The tables below give Ke for each barrel shape, material and inlet as used by Delta Hydro Studio's culvert calculator, the full HDS-5 Table C.2, and Manning's n for culvert barrels.

Where Ke enters the calculation

Under outlet control, the head needed to push the flow through the barrel is the sum of the entrance loss, the friction loss and the exit loss (HDS-5, Section 3.2):

H = [1 + k_{e} + \frac{2 g n ^{2} L}{R ^{4/3}}] \frac{V ^{2}}{2 g} (SI)

where V is the barrel velocity, L the barrel length, R the hydraulic radius of the full barrel and n its Manning's roughness. The headwater is then the outlet water level plus H, less the barrel's fall. Ke is the only term that depends on the inlet; a square-edged opening loses half a velocity head, a well-rounded or bevelled one a fifth. Use these values with the culvert calculator, which checks both inlet and outlet control and reports which governs.

Ke by barrel and inlet

The values the culvert calculator offers for each shape and material:

Barrel and inlet	Ke
Circular pipe, concrete
Square Edge with Headwall	0.5
Groove End with Headwall	0.2
Groove End Projecting	0.2
Projecting, square-cut end	0.5
Beveled Ring	0.2
Mitered to Slope	0.7
Circular pipe, corrugated metal
Headwall	0.5
Projecting	0.9
Mitered to Slope	0.7
Beveled Ring	0.2
Circular pipe, smooth-wall HDPE
Groove End with Headwall	0.2
Groove End Projecting	0.2
Square Edge with Headwall	0.5
Projecting	0.9
Circular pipe, PVC
Groove End with Headwall	0.2
Groove End Projecting	0.2
Square Edge with Headwall	0.5
Projecting	0.9
Box culvert, concrete
Headwall, Square Edge	0.5
Headwall, Chamfered	0.2
Wingwalls 30-75°	0.4
Wingwalls 10-25°	0.5
Projecting	0.7
Box culvert, corrugated metal
Headwall	0.5
Projecting	0.9

HDS-5 does not list plastic pipe separately. The calculator treats smooth-wall HDPE and PVC like concrete pipe for headwalls and groove ends, and like thin-walled corrugated metal when the end projects from the fill. The box-culvert value of 0.7 labelled Projecting corresponds to the HDS-5 row for wingwalls parallel to the barrel (extensions of the sides) with a square-edged crown.

HDS-5 Table C.2 in full

Type of structure and design of entrance	Ke
Concrete pipe
Projecting from fill, socket (groove) end	0.2
Projecting from fill, square-cut end	0.5
Headwall or headwall and wingwalls: socket end of pipe (groove end)	0.2
Headwall or headwall and wingwalls: square edge	0.5
Headwall or headwall and wingwalls: rounded (radius = D/12)	0.2
Mitred to conform to fill slope	0.7
End section conforming to fill slope	0.5
Bevelled edges, 33.7° or 45° bevels	0.2
Side- or slope-tapered inlet	0.2
Corrugated metal pipe or pipe arch
Projecting from fill (no headwall)	0.9
Headwall or headwall and wingwalls, square edge	0.5
Mitred to conform to fill slope, paved or unpaved slope	0.7
End section conforming to fill slope	0.5
Bevelled edges, 33.7° or 45° bevels	0.2
Side- or slope-tapered inlet	0.2
Reinforced concrete box
Headwall parallel to embankment (no wingwalls), square-edged on three edges	0.5
Headwall parallel to embankment, rounded on three edges to radius D/12 or B/12, or bevelled on three sides	0.2
Wingwalls at 30° to 75° to barrel, square-edged at crown	0.4
Wingwalls at 30° to 75° to barrel, crown edge rounded to D/12 or bevelled top edge	0.2
Wingwalls at 10° to 25° to barrel, square-edged at crown	0.5
Wingwalls parallel (extension of sides), square-edged at crown	0.7
Side- or slope-tapered inlet	0.2

End sections conforming to the fill slope are the precast or prefabricated flared ends supplied with pipe; HDS-5 notes they performed comparably to headwalls in tests. A mitred end is cut to the embankment slope with no flare and loses more.

Manning's n for culvert barrels

Barrel material	Minimum	Typical	Maximum
Concrete	0.010	0.012	0.015
Corrugated Metal (CMP)	0.021	0.024	0.030
HDPE (smooth interior)	0.009	0.010	0.012
HDPE (corrugated interior)	0.018	0.022	0.025
PVC	0.009	0.010	0.011
Steel	0.010	0.012	0.014

The calculator uses the typical value unless you enter your own. Corrugated metal varies with the corrugation profile and whether the invert is paved; use the manufacturer's value for the pipe you specify. For open channels upstream and downstream of the culvert, see the Manning's n table.

Common questions

What is the entrance loss coefficient for a projecting pipe?

HDS-5 Table C.2 gives Ke = 0.9 for corrugated metal pipe projecting from the fill with no headwall, and 0.5 for concrete pipe with a square-cut end (0.2 with a socket or groove end facing upstream). A thin-walled projecting end is the worst common inlet; a headwall reduces the loss to 0.5 for both materials.

Does Ke matter in inlet control?

No. Under inlet control the headwater depends on the inlet geometry through the inlet-control coefficients of HDS-5 Table C.1 (the K, M, c and Y constants), not on Ke. Ke enters only the outlet-control energy balance. A better inlet (bevelled, groove end, tapered) often helps both: it raises inlet-control capacity and lowers the outlet-control entrance loss.

What Manning's n should I use for a culvert barrel?

Concrete pipe and box culverts are usually 0.012 (0.010 to 0.013 depending on the finish and joints), smooth-wall plastic about 0.009 to 0.012, and corrugated metal 0.022 to 0.027 for the common 68 × 13 mm (2⅔ × ½ in) corrugation, more for larger corrugations. Manufacturers and road authorities publish values for specific products; use those where they apply.

How much does the entrance loss add to the headwater?

The loss is Ke times the velocity head in the barrel. At 2.5 m/s the velocity head is 0.319 m, so a square-edged headwall (Ke = 0.5) loses 0.159 m and a groove end (Ke = 0.2) 0.064 m. The difference matters most for short, outlet-controlled culverts where the friction loss is small.

Sources

Schall, J.D., Thompson, P.L., Zerges, S.M., Kilgore, R.T. and Morris, J.L. (2012) Hydraulic Design of Highway Culverts, Hydraulic Design Series No. 5 (HDS-5), 3rd ed. FHWA-HIF-12-026. Federal Highway Administration. Section 3.2 and Appendix C, Tables C.1 and C.2.

Norman, J.M., Houghtalen, R.J. and Johnston, W.J. (1985) Hydraulic Design of Highway Culverts, HDS-5, 1st ed. FHWA-IP-85-15. Federal Highway Administration.

American Concrete Pipe Association (2011) Concrete Pipe Design Manual. Irving, Texas.

The values are the ones Delta Hydro Studio's calculators use. Last reviewed 9 October 2026. Check them against the design manual your project is reviewed under.