FLOOD03462

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Procedure for Approximate Dam Failure Floodplain Mapping
• Date: 1/1/1988
• Prepared For: Association of State Dam Safety Officials
• Prepared By: John W. Van Sciver
• Floodplain - Doc Type: Educational/Technical/Reference Information

COMPUTATION OF PElIK DISCHl\RGE DUE TO FJl.ILURE OF THE Dl\M (OP)

The vast majority of dams are constructed of earth and rock. and the methods
presented in this section primarily relate to calculation of Qp for those
types of structures. (A simplified method for calculation of Qp for concrete
and masonry dams is presented at the end of this section).

For failure of an earth or rock fill dam under normal weather conditions. the
fOllowing general assumptions have been adopted:

1. The dam fails by erosion of embankment materials. initiated by piping.
embankment slides, deterioration of the outlet works. or spillway backcutting.

2. At the time of failure. the water level in the reservoir is at the crest
elevation of the emergency spillway.

3. The dam fails at its maximum section.

The conditions listed above have been found to be typical for dry-weather
failures of earth dams in Colorado. (The peak discharge for overtopping
failure can be calculated by adding base flow to the Qp value.)

The failure hydrograph is assumed to exist at a section immediately downstream
of the dam and to have the characteristics as shown in figure 1.

Q. cfs

Qp

1 Hours

o

.

Base Time Tp

JI

Figure 1: Idealized Failure Hydrograph

The shape is that of an isosceles triangle. having a peak value equal to Qp
(cfs) and base time Tp (Hours). Tp represents the time required to breach the
dam and ful1v drain the contents of the reservoir. The following relationship
is developed from the geometry of the triangle; V is the reservoir volume in
acre-feet (the area inside the triangle), and the constant. 24.2. is a
conversion factor to obtain units of cubic feet per second.

Qp = 24.2 V
TP

Equation 1

2