MIKE 11StructuresStructures

MIKE 11StructuresStructures

Structure Types:• Weirs, spillways

• Culverts

• Pumps

• Reservoir operations

• Advanced controllable structures

• Dambreak

• Bridge module

MIKE 11General Structure FeaturesGeneral Structure Features

• Structures are located at Q-points

• Flow equations substituted by energy equation

QQHHQQQQ HH

MIKE 11General Structure FeaturesGeneral Structure Features

• 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.

MIKE 11InternalInternal ConditionsConditions

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).

MIKE 11UpstreamUpstream ControlControl

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

MIKE 11Upstream ControlUpstream Control

Zero flow, Upstream or inlet controlled

MIKE 11Downstream ControlDownstream 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 11Downstream ControlDownstream Control

Downstream or outlet controlled

MIKE 11Downstream ControlDownstream Control

Qstr = f (Hu/s, Hd/s) comes from energy equation which gives the headloss as a function of flow.

lostD/S - U/S H HH

Hlost is a function of Q and is due to:

• Eddy losses / vortices / turbulence

• Contraction / expansion of streamlines

MIKE 11Head Loss in StructuresHead Loss in Structures

g

v

g

vh

g

vh s

222

222

2

21

1

h 1

h 2

HU

/S

HD

/S

lostD/S - U/S H HH

MIKE 11

A 1 A sA 2

J-1 J J+1

h Q h

Loss Coefficient, Loss Coefficient,

Contributions from inflow and outflow:

2

21

21

11

A

A

AA s

outsin

Note!

As < A1 and A2

MIKE 11Total HeadlossTotal 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 11Specifying Loss Specifying Loss CoefficientsCoefficients

Defaults:

in = 0.5

out = 1.0

Determine from:

• Flume tests

• Field measurements

• Model calibration

Function of :

• Degree of smoothness of entry, exit

MIKE 11Free OverflowFree Overflow

Q = ac Qc

• 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.

MIKE 11WeirsWeirs

MIKE 11CulvertsCulverts

MIKE 11CulvertsCulverts

• Rectangular

• Circular

• Irregular H-B

• Irregular h-B

• Cross-section DB

MIKE 11Weir cf. CulvertsWeir cf. Culverts

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

MIKE 11Tabulated StructuresTabulated Structures

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)

MIKE 11Local Energy LossesLocal Energy Losses

• 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

MIKE 11(In)Stability at Structures(In)Stability at Structures

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 11MIKE-11 Bridge StructuresMIKE-11 Bridge Structures

MIKE 11Bridge ModuleBridge Module

• Simplifies approach to bridges

• Specific menu for including bridges

• Uses recognised approaches for estimating head loss at bridge structures

MIKE 11Bridge Module - ApproachBridge Module - Approach

• 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

MIKE 11

• 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)

Eight Bridges TypesEight Bridges Types