FLOOD06918

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: The NWS Simplified Dam Break-Flood Forecasting Model
• Date: 5/16/1986
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

DOWNSTREAM ROUTING

The peak outflow discharge determined in the preceding step may be

routed downstream using the dimensionless routing curves in Appendix Ia.

These curves were developed from numerous executions of the NWS DAMBRK Model

and they are grouped into families based on the Froude number associated

with the floodwave peak. To determine the correct family and member curve

that most accurately predicts the attenuation of the flood, the user must

first define certain routing parameters.

Prior to defining the routing parameters, however, the user must first

describe the river channel downstream from the dam to the first routing

point as a prism. To describe the river channel downstream of the dam as a

prismatic channel the user must reduce the topwidth vs. elevation data (see

table 1) for the first two cross-sections. This cross-section data is reduced

(see table 4, p. 1-24) to depth (h) vs. topwidth (B) data by subtracting the

channel invert elevation from the elevation associated with each topwidth at a

given cross-section. From this reduced data, an average cross section may be

determined using the following algorithm.

For each depth (hi)' the average topwidth (Bi) is given by the
relation:

-
Bi

(Bi,1 + Bi,2)
2

(18)

,

Where: hi is the ith depth, i = 1,2,3 ... I (number of topwidths per

X-Section)

Bi'~ is the ith topwidth (corresponding to the ith depth hi) at the

jt X-section where j = 1,2
th
Bi is the average i topwidth

Note: The preceding calculation is unnecessary when depth vs. topwidth
data is available for only ~X-section.

1-13