MIKE 11 offers a number of approaches when modelling flow through bridges. The approach to choose should be based on the assumptions for the different methods and the requirements of the modelling.

The bridge modelling approaches can be divided into pure free flow methods and methods which may be combined with submergence/overflow methods. The pure free flow methods can be further sub-divided into methods for piers and methods for arches.

The methods specially designed for piers are

• Bridge piers (Nagler): An orifice type of flow description with the effect of the piers taken into account through an adjustment factor.

• Bridge Piers (Yarnell): An equation derived from experiments for normal flow conditions in the sub critical flow range. Again the effect of piers is handled through the use of adjustment factors.

The free flow arch methods available are

• Arch bridges (Biery and Delleur): An orifice type of equation is used to describe the discharge through the bridge. The equation is derived under the assumption of a rectangular channel and is based on a single span arch opening. Multiple arch openings are handled by a simple multiplication factor.

• Arch bridges (Hydraulic research method, HR): The HR method is based on laboratory experiments of both single and multi spanned arch bridges in rectangular channels. The method uses tables describing the relation between the blockage ratio, the downstream Froude number and the upstream water level.

The methods that can be combined with both submergence and overflow methods are the following

• Energy Equation: A standard step method where a backwater surface profile is determination is used to calculate the discharge through the bridge. The method takes the contraction and expansion loss for bridges of arbitrary shape into account. The method assumes sub-critical flow and may default to critical flow for steep water surface gradients.

• Federal Highway Administration (FHWA) WSPRO method: The FHWA WSPRO method is based on the solution of the energy equation. Contraction loss is taken into account through the calculation of an effective flow length. Expansion losses are determined through the use of numerous experimentally based tables. The method takes the effect of eccentricity, skewness, wingwalls, embankment slope etc. into account through the use of these tables.

• US Bureau of Public Roads (USBPR) method: The USBPR method estimates free-surface flow assuming normal depth conditions. the method is based on experiments and takes the effect of eccentricity, skewness and piers into account.

The submergence methods available are

• Pressure Flow using the Federal Highway Administration method: Two orifice equation descriptions are used. One for situations when the orifice is submerged downstream and modified equation for situations when only the upstream part of the orifice is submerged.

• MIKE 11 culvert: A standard MIKE 11 culvert description may be chosen for submergence flow. The culvert to be used is specified by the user. The culvert is only active if submergence occurs.

• Energy equation: The flow under the bridge is determined through a standard backwater step method. The flow is assumed to be in the sub-critical range and thus the method may default to critical flow. Both contraction and expansion loss is taken into account.

Overflow methods available are:

• Energy Equation: The flow over the bridge is determined through a standard backwater step method. The flow is assumed to be in the sub-critical range and thus the method may default to critical flow. Both contraction and expansion loss is taken into account.

• Road overflow using the Federal Highway Administration method: The overflow is modelled using a weir equation taking tail water submergence into account through the use of a submergence coefficient. The method may be used for both gravel and paved surfaces.

• MIKE 11 weir: A standard MIKE 11 weir description may be chosen for overflow. The weir to be used is specified by the user. The weir is only active if overflow occurs.

Finally there are two additional bridge types which are not pre-processed prior to the simulation. These bridge types form part of a separate module.