A. Hydraulic Structures

7/28/2019 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

A. Hydraulic Structures

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