Structures

Report
Structures
MIKE 11
Structures
Structure Types:
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Weirs, spillways
Culverts
Pumps
Reservoir operations
Advanced controllable structures
Dambreak
Bridge module
MIKE 11
General Structure Features MIKE 11
• Structures are located at Q-points
• Flow equations substituted by energy equation
Q
H Q H
Q
General Structure Features MIKE 11
• Upstream and downstream cross sections must exist in database
at a distance < dx-max from the structure, preferably about half a
channel width upstream and downstream of structure
• Valve regulation to allow flow in one direction only - e.g. for flap
gate operation
• Group structures in parallel to describe complex geometries (eg
combined overflow and throughflow). These can be placed at same
Branch, Chainage and differentiated by the ID.
Internal Conditions
MIKE 11
Structures impose internal boundary conditions:
a) due to a control somewhere in the structure Qstr = f (Hu/s)
b) due to energy losses through the structure, Qstr = f (Hu/s, Hd/s )
MIKE 11 looks at both cases and decide which is the governing
mechanism.
Replace momentum equation with control equation (a) or local
energy balance (b).
Upstream Control
MIKE 11
Control somewhere in the structure, Qstr = f (Hu/s)
Egs:
- Weir;
Free flow over the weir
- Culvert; Inlet critical
Outlet critical
Orifice flow at inlet
Upstream Control
Zero flow,
Upstream or inlet controlled
MIKE 11
Downstream Control
Energy losses through the structure, Qstr = f (Hu/s, Hd/s )
Egs:
- Weir;
Drowned flow over the weir
- Culvert;
Drowned flow through the culvert
MIKE 11
Downstream Control
Downstream or outlet
controlled
MIKE 11
Downstream Control
MIKE 11
Qstr = f (Hu/s, Hd/s) comes from energy equation which gives
the headloss as a function of flow.
HU/S
-
HD/S
 Hlost
Hlost is a function of Q and is due to:
• Eddy losses / vortices / turbulence
• Contraction / expansion of streamlines
Head Loss in Structures
HD/S
 Hlost
h2
HD/S
-
h1
HU/S
HU/S
MIKE 11
2
2
2




vs
v1
v2
 h1 
   h2 
  
2g  
2g 
2g

Loss Coefficient, 
MIKE 11
Contributions from inflow and outflow:
A1
A2
As
Note!
J-1
J
J+1
h
Q
h
As < A1 and A2

 1


in



 2


s 


1
A
   1
A
 As 
out 1  
 A2 
2
Total Headloss
Contributions from:
• inflow (note As1, str.area at inlet)
• friction (for culverts, note Asa, average str. area)
• bend (for culverts, note Asa, average str. area)
• outflow (note As2 , str. area at outlet)
• subject to min specified in the HD11 file, default
values page.
MIKE 11
Specifying Loss Coefficients MIKE 11
Defaults:
in = 0.5
out = 1.0
Determine from:
• Flume tests
• Field measurements
• Model calibration
Function of :
• Degree of smoothness of
entry, exit
Free Overflow
MIKE 11
Q = a c Qc
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For culverts and weirs
Qc is tabulated, ac is applied during simulation
Irregular sections: H not horizontal, v not uniform.
To be used when known, otherwise ac = 1
ac > 1, for non-parallel flow (curved streamlines) over
weir as in the case of a sharp-crestred weir
• ac < 1, for side effects.
Weirs
MIKE 11
Culverts
MIKE 11
Culverts
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Rectangular
Circular
Irregular H-B
Irregular h-B
Cross-section DB
MIKE 11
Weir cf. Culverts
MIKE 11
Weirs and culverts are very similar, except:
• Culverts have a length, therefore a friction loss
• Culverts have a length, therefore an outlet critical plus
friction loss control mechanism
• Culverts have a soffit therefore a possible orifice control
mechanism
• Culverts have a bend loss option
Tabulated Structures
MIKE 11
Defined as:
• Qstr = f (Hu/s, Hd/s)
• Hu/s = f (Qstr, Hd/s)
• Hd/s = f (Qstr, Hu/s)
Some pumps can be modelled as a tabulated structure with Qpump
= f (Hu/s, Hd/s)
Local Energy Losses
MIKE 11
• Abrupt change in river alignment
• Gradual change in river alignment,
• User defined energy loss
• Flow contraction loss
• Flow expansion loss
where,  = 0.1 to 0.2
(In)Stability at Structures
MIKE 11
Ensure there is sufficient headloss through the structure. A very small
headloss leads to an ill-conditioned solution  Increase energy loss or
remove structure
Ensure a monotonically increasing Q/h-relation
 Edit the Q/h-relation by hand or change structure geometry
Ensure gradual variation in structure area
 Alter structure area slightly
Also play with Delta, Delhs, Zetamin and Inter1Max in the HD11 file, default
values page
MIKE-11 Bridge Structures
MIKE 11
Bridge Module
MIKE 11
• Simplifies approach to bridges
• Specific menu for including bridges
• Uses recognised approaches for estimating
head loss at bridge structures
Bridge Module - Approach
MIKE 11
• User specified physical bridge parameters
and user selected approach.
• Bridge module pre-calculates a rating table.
• Uses rating table in fully dynamic model
mode to calculate bridge flow impacts
Eight Bridges Types
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FHWA WSPRO
USBPR Bridge Method
Fully Submerged Bridge
Arch Bridge (Biery and Delleur)
Arch Bridge (Hydraulic Research)
Bridge Piers (D’Aubuisson Formula)
Bridge Piers (Nagler)
Bridge Piers (Yarnell)
MIKE 11

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