FHWA Chart and Scale Numbers

The FHWA chart and scale numbers are required input data. The FHWA chart number and scale number refer to a series of nomographs published by the Bureau of Public Roads (now called the Federal Highway Administration) in 1965 [BPR, 1965], which allowed the inlet control headwater to be computed for different types of culverts operating under a wide range of flow conditions. These nomographs and others constructed using the original methods were republished [FHWA, 1985]. The tables in this chapter are copies of the information from the 1985 FHWA publication.

Each of the FHWA charts has from two to four separate scales representing different culvert entrance designs. The appropriate FHWA chart number and scale number should be chosen according to the type of culvert and culvert entrance. Table 6-6 may be used for guidance in selecting the FHWA chart number and scale number.

Chart numbers 1, 2, and 3 apply only to pipe culverts. Similarly, chart numbers 8, 9, 10, 11, 12, and 13 apply only to box culverts. The HEC‑RAS program checks the chart number to assure that it is appropriate for the type of culvert being analyzed. HEC‑RAS also checks the value of the Scale Number to assure that it is available for the given chart number. For example, a scale number of 4 would be available for chart 11, but not for chart 12.

The following figures can be used as guidance in determining which chart and scale numbers to select for various types of culvert inlets.

Culvert Inlet with Hardwall Culvert Inlet Mitered to Conform to Slope

Table 6‑6 FHWA Chart and Scale Numbers for Culverts

Chart Number	Scale Number	Description
1		Concrete Pipe Culvert
	1 2 3	Square edge entrance with headwall (See Figure 6-10) Groove end entrance with headwall (See Figure 6-10) Groove end entrance, pipe projecting from fill (See Figure 6-12)
2		Corrugated Metal Pipe Culvert
	1 2 3	Headwall (See Figure 6-10) Mitered to conform to slope (See Figure 6-11) Pipe projecting from fill (See Figure 6-12)
3		Concrete Pipe Culvert; Beveled Ring Entrance (See Figure 6-13)
	1(A) 2(B)	Small bevel: b/D = 0.042; a/D = 0.063; c/D = 0.042; d/D = 0.083 Large bevel; b/D = 0.083; a/D = 0.125; c/D = 0.042; d/D = 0.125
8		Box Culvert with Flared Wingwalls (See Figure 6-14)
	1 2 3	Wingwalls flared 30 to 75 degrees Wingwalls flared 90 or 15 degrees Wingwalls flared 0 degrees (sides extended straight)
9		Box Culvert with Flared Wingwalls and Inlet Top Edge Bevel (See Figure 6-15)
	1 2	Wingwall flared 45 degrees; inlet top edge bevel = 0.043D Wingwall flared 18 to 33.7 degrees; inlet top edge bevel = 0.083D
10		Box Culvert; 90-degree Headwall; Chamfered or Beveled Inlet Edges (See Figure 6-16)
	1 2 3	Inlet edges chamfered 3/4-inch Inlet edges beveled 2-in/ft at 45 degrees (1:1) Inlet edges beveled 1-in/ft at 33.7 degrees (1:1.5)
11		Box Culvert; Skewed Headwall; Chamfered or Beveled Inlet Edges (See Figure 6-17)
	1 2 3 4	Headwall skewed 45 degrees; inlet edges chamfered 3/4-inch Headwall skewed 30 degrees; inlet edges chamfered 3/4-inch Headwall skewed 15 degrees; inlet edges chamfered 3/4-inch Headwall skewed 10 to 45 degrees; inlet edges beveled
12		Box Culvert; Non-Offset Flared Wingwalls; 3/4-inch Chamfer at Top of Inlet (See Figure 6-18)
	1 2 3	Wingwalls flared 45 degrees (1:1); inlet not skewed Wingwalls flared 18.4 degrees (3:1); inlet not skewed Wingwalls flared 18.4 degrees (3:1); inlet skewed 30 degrees
13		Box Culvert; Offset Flared Wingwalls; Beveled Edge at Top of Inlet (See Figure 6-19)
	1 2 3	Wingwalls flared 45 degrees (1:1); inlet top edge bevel = 0.042D Wingwalls flared 33.7 degrees (1.5:1); inlet top edge bevel = 0.083D Wingwalls flared 18.4 degrees (3:1); inlet top edge bevel = 0.083D
16-19		Corrugated Metal Box Culvert
	1 2 3	90 degree headwall Thick wall Projecting Thin wall projecting
29		Horizontal Ellipse; Concrete
	1 2 3	Square edge with headwall Grooved end with headwall Grooved end projecting
30		Vertical Ellipse; Concrete
	1 2 3	Square edge with headwall Grooved end with headwall Grooved end projecting
34		Pipe Arch; 18" Corner Radius; Corrugated Metal
	1 2 3	90 Degree headwall Mitered to slope Projecting
35		Pipe Arch; 18" Corner Radius; Corrugated Metal
	1 2 3	Projecting No bevels 33.7 degree bevels
36		Pipe Arch; 31" Corner Radius; Corrugated Metal
	1 2 3	Projecting No bevels 33.7 degree bevels
41-43		Arch; low-profile arch; high-profile arch; semi circle; Corrugated Metal
	1 2 3	90 degree headwall Mitered to slope Thin wall projecting
55		Circular Culvert
	1 2	Smooth tapered inlet throat Rough tapered inlet throat
56		Elliptical Inlet Face
	1 2 3	Tapered inlet; Beveled edges Tapered inlet; Square edges Tapered inlet; Thin edge projecting
57		Rectangular
	1	Tapered inlet throat
58		Rectangular Concrete
	1 2	Side tapered; Less favorable edges Side tapered; More favorable edges
59		Rectangular Concrete
	1 2	Slope tapered; Less favorable edges Slope tapered; More favorable edges
60		ConSpan Span/Rise Approximately 2:1
	1 2 3	0 degree wingwall angle 45 degree wingwall angle 90 degree wingwall angle
61		ConSpan Span/Rise Approximately 4:1
	1 2 3	0 degree wingwall angle 45 degree wingwall angle 90 degree wingwall angle

Culvert Inlet Projecting from Fill Culvert Inlet with Beveled Ring Entrance

Flared Wingwalls (Chart 8) Inlet Top Edge Bevel (Chart 9)

Inlet Side and Top Edge Bevel with Ninety Degree Headwall (Chart 10)

Inlet Side and Top Edge Bevel with Skewed Headwall (Chart 11)

Non-Offset Flared Wingwalls (Chart 12)

Offset Flared Wingwalls (Chart 13)