Structures

Structures Structure Types: Weirs, spillways Culverts Pumps Reservoir operations Advanced controllable structures Dambreak Bridge module

Structures in NWK11 file Structure Types: Weirs Culverts Regulating Tabulated Energy Loss

General Structure Features Structures are located at Q-points Flow equations substituted by energy equation Q H Q H Q

General 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.

Internal Conditions 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 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 or inlet controlled Upstream Control Zero flow, Upstream or inlet controlled

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

Downstream Control Downstream or outlet controlled

Downstream Control Qstr = f (Hu/s, Hd/s) comes from energy equation which gives the headloss as a function of flow. Hlost is a function of Q and is due to: Eddy losses / vortices / turbulence Contraction / expansion of streamlines

Head Loss in Structures

Loss Coefficient,  Contributions from inflow and outflow: Note! As < A1 and A2

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.

Specifying Loss Coefficients 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 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.

Weirs

Weirs Broadcrested Weir Level-Width relationship

Weirs Special weir Same geometric input as for a broadcrested weir except that the Q/h relationship can be inserted manually

Weirs Weir Formula 1 (Villemonte)

Weirs Weir Formula 2 (Honma)

Culverts

Culverts Rectangular Circular Irregular H-B Irregular h-B Cross-section DB

Weir 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

Regulating Structures Q = f (t) eg pumps, reservoir outlet, historical release breaks branch into two specified in boundary (BND11) editor Q(t) specified in timeseries (DFS0) file

Regulating Structures Q = A Za where: Za is H or Q at location a A is a coefficient given by A = f (Zb) Zb is H or Q at location b Examples: Bifurcation, Qbif = f (Qu/s) Reservoir, Qout,res = f (Qin, Hres) Dummy branch Q = 1  Q = f (Zb)

Regulating Structures Regulating structures are part of standard MIKE 11 HD Some pumps can be modelled as a regulating structure with Q = f (H) For more complex functionality, for example in control cases, the user may need to use a control structure, which is an add-on module, with moveable underflow (including radial gates) and overflow gates, which can be controlled by a hierarchical list of control strategies / control and target points / PID control / logical operands, etc. Pumps are best modelled as a control structure

Tabulated 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)

Local 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

(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-11 Bridge Structures

Bridge Module Simplifies approach to bridges Specific menu for including bridges Uses recognised approaches for estimating head loss at bridge structures

Bridge 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

Eight Bridges Types 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)