§ 3.3.2. Straight Crowns

Flow in gutters on straight crown pavements is normally assumed to be uniform, with Manning's Equation being used to determine the flow. However, because the hydraulic radius assumption in the Manning's Equation is not able to adequately describe the hydraulic characteristics of the gutter cross section, modification of the equation is necessary to accurately compute the flow. The modified Manning's Equation is (HEC-22 Manual, FHWA-NHI-10-009):

Q _o = 0.56 (z/n) S _o ^1/2 Y _o ^8/3 (Eq. 3-2)

Where,

Q _o = Gutter discharge, cfs

z = Reciprocal of the crown slope, ft/ft

S _o = Street or gutter slope, ft/ft

n = Roughness coefficient

Y _o = Depth of flow in gutter, feet

The nomograph in Figure 3-1 in Appendix D of this manual provides a direct solution for flow conditions in triangular channels. For a concrete pavement gutter, an n value equal to 0.016 is recommended. For gutters with small slope (less than one (1) percent) where sediment may accumulate, an n value of 0.02 is recommended.

Table 3-1 Minimum Clear Widths for Roadway Design When Gutter is Flowing Full
Roadway Type		Minimum Clear Width (ft)
Local		0
Collector	Residential or Neighborhood	12
	All others	12 in each direction
Arterial	4 lanes one way or 8 lanes divided	24 in each direction
	All others	12 in each direction