Transcript SMPDBK Model The NWS i lified

The NWS Simplified Dam Break Model
SMPDBK Model
SMPDBK Model Description
The NWS SMPDBK model computes the dam break
outflow from a simplified equation and routes the
outflow base on curves generated with the NWS
DAMBRK model. Flow depths are computed based
on Manning’s equation.
SMPDBK Limitations
 Backwater effects ignored
 No special effects included
(e.g. bridges, dams, constrictions, etc.)
 Error range: 0 - 10% Relative to NWS DAMBRK
(when used within its limitations)
 Rapid, easy, low cost, emergency situations
SMPDBK Model
PEAK OUTFLOW:
Q bmax

C
 Q o  3.1B r 
 t / 60  C / h
d
 f




3
C  23.4
Sa
Br
Qo = Spillway Flow
Br = Breach Width (avg.)
tf = Time of Failure (hour)
hd = Height of Dam
As = Avg. Surface Area of Reservoir in Top Portion of Reservoir
Qbmax corrected for tailwater submergence and dynamic effects
SMPDBK Model
FLOW DEPTH:
Depths computed from Manning Eq. for a known discharge
1.49
2/3
Q
A A B S
n
n2
Sc  77000 1/3
D
The slope (S) is dependent on the upstream and downstream reach:
If both subcritical, use average slope;
If both supercritical, use upstream slope;
If upstream slope is supercritical & downstream slope is subcritical,
hydraulic jump present – compute two depths;
If upstream slope is subcritical & downstream slope is supercritical,
use critical slope (Sc;
SMPDBK Model
ROUTED FLOW:
Routing of peak discharge via empirically derived relationships (using
DAMBRK model) as represented via dimensionless parameters Qp/Qbmax,
X/Xc, V*, and Fr.
Xc 
V* 
X* 
6VOL
A (1  4  0.5
VOL
Ac Xc
X
Xc
m1
)
Fr 
Q 
*
Vc
gA c /B c
Qp
Q bmax
SMPDBK Model
SMPDBK Model
Time to Peak (Tp):
x
Tp  t f 
c
5 2 m 
c  0.682 Vx i  
 3 3 m  1
Vx i 
Dxi 
1.49
n
S Dxi
2/3
h ref
m 1

h ref  f Q* , n, S, A x i , D x i
Manning Equation:
Q
1.49
A D 2/3 S
n
m

Q*  Q x / 2  0.3  
10 


SMPDBK Model
Time to Flooding and Deflooding of Elevation hf:
Q f  a h f 
b
t fld
 Qp  Qf 
t
 tp 
Q Q  f
o 
 p
t defld
 24.2 VOL r
  Qp  Qf 


 tp 
 tf  
 Q Q
Q Q 
p
o
o 

 p