FLOOD00787

• County: Denver
• Community: All
• Stream Name: All
• Title: Floodplain Management Tools for Alluvial Fans
• Date: 11/1/1981
• Prepared For: State of Colorado
• Prepared By: FEMA
• Floodplain - Doc Type: Educational/Technical/Reference Information

Sediment Deoosition

flood waters on fans generally carry large quantities of

sediment eroded from both the upstream watershed and from

,the fan itself.
magnitude of the

Deposition results from a reduction in the
velocity of flow and can be caused by

trees, houses. or other obstructions and by a lowered fan
slope. Deposition is least likely in entrenched areas of
fans and most likely upstream of obstructions or where
channel slopes decrease. The costs resulting from sediment
depostion make up a large portion of total damage on most
fans and include the removal of sediment from structures and
the hauling away of sediment from streets and yards.

Scour/Underminina

flood flows that are carrying less sediment than their
.transport capacity will tend to entrain additional material,
resulting in scour. Scour damage consists of the undermining
of structural foundations and the loss of pavement. sod,'and

topsoil from streets and lawns.

The constriction of flood

flow in localized areas on any portion of the fan can result

in scour.

The magnitude of the damage from scour over the

whole fan is not usually very large, but scour damage can be

a major portion of the financial loss at individual sites.

Impact Damaqes

The hazard from high velocity impacts is greatest at the fan
apex and least at the toe since velocity decreases as the
flood flow spreads down the fan. The damages resulting from
high velocities are due to the momentum forces applied to
structural walls blocking the path of the flood channel.
Momentum force hazards will be most severe at the first row
of homes struck by the flood wave and decrease for homes
shielded below. Total loss of a structure can result from

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