**Gutter Flow**

Gutters are the sections of roadway that run adjacent to the curb. Their purpose is to collect and convey surface runoff to drainage inlets and in turn to underground storm sewers. The corresponding spread of water onto the pavement, or top width of flow measured perpendicular to the edge of the roadway, is a primary concern from an analysis perspective. The lateral cross slope of a traffic lane facilitates drainage of incident rainfall to the gutter. Depending on the cross slope, conventional gutters may be grouped as uniform gutter (i.e., has uniform cross slope) or composite gutters (i.e., has multiple cross slopes).

**Uniform Gutter Sections**

Uniform gutters have a shallow, triangular cross section, with a curb forming the near-vertical leg of the triangle as shown in the following figure.

The governing equation for uniform gutters is given as

where *Q* = gutter flow rate (m3/s, ft3/s)

*Kc* = empirical constant (0.376 in SI, 0.56 in English)

*n* = Manningโs roughness coefficient

*Sx* = gutter cross slope (m/m, ft/ft)

*SL* = longitudinal slope of the road way (m/m, ft/ft)

*T* = spread (m, ft)

Spread *T* is related to depth at the curb, *d*, and flow area, *A*, by

Table 3-7: Manningโs n for Street and Pavement Gutters

Type of Gutter or Pavement |
Manningโs n |

Concrete gutter, troweled finish | 0.012 |

Asphalt Pavement: | |

Smooth texture | 0.013 |

Rough texture | 0.016 |

Concrete gutter-asphalt pavement: | |

Smooth | 0.013 |

Rough | 0.015 |

Concrete pavement: | |

Float finish | 0.014 |

Broom finish | 0.016 |

*Adapted from FHWA (2001)*

**Composite Gutter Sections**

Evaluation of composite gutters requires additional consideration of flow in the depressed section. The depression serves to retain more water above inlet entrances and thus increases gutter flow capacity. The relationship between total discharge, *Q*, and depressed gutter flow, *Qw*, can be expressed as

where *Qw and Qs* represent portion of the gutter flows for the sections shown in the figure above (m3/s, ft3/s).

The relationship between *Q* and *Qs* is given as

where *Eo* = ratio of *Qw* to *Q*, or

(71)

where *W * = width of the depressed section (m, ft)

*Sw* = cross slope of the depressed section (m/m, ft/ft)

The slope terms and the width of depression are related through depth of the depression, *a*, as

where *a* is the gutter depression (m, ft) illustrated in the figure given above.