FLOOD08739

• County: Statewide
• Community: Statewide
• Basin: Statewide
• Title: Determination of Roughness Coefficients for Streams in Colorado
• Date: 1/1/1985
• Prepared For: CWCB
• Prepared By: CWCB
• Floodplain - Doc Type: Educational/Technical/Reference Information

Urban Overbanks

Urban development in overbank areas may significantly alter the roughness
characteristics of natural areas, Removal of natural vegetation, grading,
paving, construction of roads, streets with curbs and gutters, buildings, and
fences may completely alter the natural roughness characteristics.

Generally, grading and paving, especially parallel to the direction of
flow, will reduce turbulence and roughness. Conversely, construction of
buildings, fences, and other obstructions can significantly reduce the total
area of flow and increase turbulence and roughness,

Because there are little or no verified data for estimating roughness
coefficients for urban areas containing buildings, two basic approaches have
been used for calculating roughness coefficients for flow in urban areas.
These approaches are: (1) Eliminating that part of the overbank cross
section occupied by buildings and other obstructions, and selecting a
roughness coefficient for the effective area of flow between the buildings;
and (2) using the total area of the overbank cross section for estimating a
roughness value that includes the effect of buildings and other obstructions
(Hejl, 1977).

Using either approach, the following factors need to be considered.
Buildings aligned with each other tend to produce less turbulence; rows of
buildings and fences aligned parallel to the general direction of flow tend to
produce less turbulence than the same structures at an angle to the direction
of flow; chain link fences tend to catch debris resulting in decreased
conveyance; and solid board fences at an angle to the flow can block flow
completely unless they are pushed over by the force of the water, Both
approaches need subjective judgment to evaluate all factors involved,

An objective method was developed by Hejl (1977) for determining the n
values of flooded urban areas by considering the density of buildings on the
overbank and using the n values of the open areas between the buildings. The
steps needed to determine values of Manning's n for urban areas are as follows
(Hejl, 1977):

1. Select cross section in the same manner as would be selected
for nonurbanized areas. The cross sections are subdivided to
separate the main-channel flow from the flow between the buildings
on an overbank. The urban roughness coefficient, n , is applied
only to the subsections that include buildings. Th~ left and right
overbanks are evaluated independently. A subdivided cross section
is shown in figure 13.

2, Estimate the ratio of total width, WT, to a summation of
the widths of individual openings, Wo' for a cross section
perpendicular to the direction of flow through a row of
buildings of average density on the section of flood plain
being evaluated, as shown in figure 13. This estimate can be
made ansite, by aerial reconnaissance, or from a map.

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