Report

Structures MIKE 11 Structures Structure Types: • • • • • • • 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 • • • • • 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 • • • • • 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 • • • • • • • • 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