FLOOD10355

• County: Statewide
• Stream Name: Colorado River
• Title: Colorado River Basin Probable Maximum Floods, Hoover and Glen Canyon Dams
• Date: 9/1/1990
• Prepared By: US Department of the Interior Bureau of Reclaimation
• Floodplain - Doc Type: Floodplain Report/Masterplan

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3.5 LAG COEFFICIENTS - "C" FACTORS

The lag coefficient, or "C" factor, is a measure of the hydraulic
efficiency of a basin to transmit water. The "C" factor is a
reflection of the overall basin roughness, steepness, and vegetative
cover, It is generally equivalent to 26n, where n is Manning's
roughness coefficient. .

In applying the unit hydrograph approach, the lag coefficient is
used in determining the lag time of the flood hydrograph. Lag time
is defined as the time from the center of unit rainfall excess to
the time that one-half the volume of unit runoff from the drainage
basin has passed the concentration point. Lag time is calculated
using the equation:

LL 0.33
Lag=C( ~)
tfJ.5

A review of previous studies indicates that "C" factors for basins
above Hoover ranged from 1.3 to 5.5. These values were dependent
upon antecedent conditions (with or without snow), as well as
vegetative cover and basin topography. Lag coefficients selected
for use in this study were based upon no snow cover. Reclamation
uses "rule of thumb" guidelines presented in table 3.3 for
estimating "C" values based upon the results of many reconstituted
historical floods in the area. This information was refined based
on the data collected on the field trips (table 3.2), to compute
elevation-weighted "C" values for each subbasin. The values
determined for the subbasins used in this study were presented in
table 3.1.

3.6 TRAVEL TIMES

Travel time is defined as the time needed for a volume of water to
proceed through a stream reach. It is used in routing the flood
hydrograph from an upstream subbasin to a point downstream,

Reclamation office files were examined to determine whether travel
time data were available. The limited amount of data which was
collected was of little value in the analysis. Travel time
information was generally for low or normal flow conditions, not for
extreme flood flows.

USGS flow records were also examined for flow travel time
information. A limited amount of data were found for the San Juan
River at the Farmington, Shiprock, and Bluff stations. Between 1964
and 1977, travel times and flow velocities were determined 11 times.
Flow velocities ranged from 3.1 to 13.3 ft/s between Farmington and
Shiprock for flows between 4~730 and 14,000 ft3/s, and from 5.7 to
8.8 ft/s between Shiprock and Bluff for flows ranging from 6,140 to
13,600 fe/so