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

This value for K * is substituted into Eq. (15a) to obtain an averaged
s
submergence correction factor given by the following:

K k _
s

* k-l
K + K
s s

(15a)

2

where the k superscript is the iteration counter and the first iteration value
for Kso is 1.

Note:

In some situations, the correction factor
less than 0.8, in which case it should be

*
K , may be computed to be
s
reset to 0.8 to prevent

over-compensating for submergence in the first iteration.

This correction factor is applied to the breach outflow to compute the
corrected breach outflow as follows:

k
Qb

=

Kk
s

k-l
Qb

(16)

where:

k-l
Qb = Q in the first iteration
bmax

k
The corrected breach outflow (Qb ) is then compared with Qv' and a new

outflow depth (hmaxk) is calculated using Eq. (8) or (11). Also, because

there is decreased flow through the breach, there is less drawdown. Thus,

the head over the weir (~eir) must be recalculated using the relation:

k k-l
hweir = ~eir +

k-l
Qb

_ Q k
b

tf (sec.)

2A (sq.ft.)
s

(17)

Now the ratio of the two new values, hmaxk/hweirk is used in Eq. (15) to
compute a new submergence correction factor.

..

If the new maximum breach outflow computed via Eq. (16) is

significantly different (t 5%) from that computed in the previous iteration,

the procedure is repeated. Generally, within two or

three iterations the Ks value will converge and a suitable value for the

maximum breach outflow (Qb) is achieved which properly accounts for the

effects of submergence.

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