A. Hydraulic Structures
Transcript of A. Hydraulic Structures
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Buzau, Mures and Crisuri Hydrographic Areas
Hydraulic Structures
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Weirs, spillways
CulvertsPumps
Reservoir operations
Advanced controllable structures
Dambreak structures
Bridge module
Structure Types
And in the
Networkfile
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Structures are located at Q-points
A structure is a contraction/expansion by definition,therefore area of structure must be less than upstream
and downstream areas FOR ALL LEVELS.
QH
QQ H
General Structure Features
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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.
General Structure Features
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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 governingmechanism.
Replace momentum equation with control equation (a) or local
energy balance (b).
Internal Conditions
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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
Full barrel flow with free outflow
Upstream Control
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Zero flow,Upstream or inlet controlled
Upstream Control
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Energy losses through the structure, Qstr = f(Hu/s, Hd/s )
Egs:
- Weir; Drowned flow over the weir- Culvert; Drowned flow through the culvert
Downstream Control
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Downstream or outletcontrolled
Downstream Control
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Qstr= f (Hu/s, Hd/s) comes from energy equation which gives
the headloss as a function of flow.
lostD/S-U/S HHH
Hlost
is a function of Q and is due to:
Eddy losses / vortices / turbulence
Contraction / expansion of streamlines
Downstream Control
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g
v
g
vh
g
vh s
222
22
22
2
11
h1
h2
HU/S
HD/S
lostD/S-U/S HHH
Head loss in Structures
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A1 A
sA2
J-1 J J+1
h Q h
Contributions from inflow and outflow:
2
21
21
11
A
A
A
A sout
sin
Note!
As < A1 and A2
Loss Coefficient,
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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.
Total Head Loss
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Defaults:
in= 0.5
out= 1.0
Determine from:
Flume tests
Field measurements
Model calibration
Function of :
Degree of smoothness of
entry, exit
Specifying Loss Coefficients
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Q = ac Qc
For culverts and weirs
Qc is tabulated, ac is applied during simulationIrregular 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 thecase of a sharp-crestred weir
ac < 1, for side effects.
Free Overflow
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Weirs
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Broadcrested Weir
Level-Width relationship
Weirs
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Special weir
Same geometric input as for a broadcrested weir exceptthat the Q/h relationship can be inserted manually
Weirs
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Weir Formula 1 (Villemonte)
Weirs
W i
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Weir Formula 2 (Honma)
Weirs
C l t
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Culverts
C l t
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Rectangular
Circular
Irregular H-B
Irregular h-B
Cross-section DB
Culverts
C l t Fl C
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Downstream control
Zero flow and Upstream Control
Culverts Flow Cases
C l t M i t
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Culverts - Maintenance
W i f C l t
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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 orificecontrol mechanism
Culverts have a bend loss option
Weirs cf. Culverts
Tab lated St ct es
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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)
Tabulated Structures
Local Energy Loss
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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
Local Energy Loss
(In) Stability in Structures
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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
(In)-Stability in Structures