Bridges in ICM are dealt with differently to the empirical Arch Bridge (HR Wallingford, 1988) and US BPR Bridge (US Federal Highway Administration, 1978) approaches in Infoworks RS. The approach in ICM has much more in common with the approach used in HECRAS (USACE, 2008) and also described in the Afflux Estimation System (DEFRA/Environment Agency, 2007). The approach is considered to be much more physically meaningful than the empirical approaches derived from limited experimental data. This blogs provides a description of the types of losses that can be represented within Infoworks ICM.
There are a number of different losses which can be represented in ICM bridge links. These are:-
- Contraction Losses-Based on geometry changes between the upstream link section and the upstream bridge section
- Bridge Inlet Losses-Based on the geometry of the U/S bridge opening
- Friction losses-based on roughness and the length of bridge opening
- Headlosses within the bridge openings
- Pier Losses-associated with any piers
- Bridge Outlet losses-based on the geometry of the D/S bridge opening
- Expansion losses-based on the changes between the D/S bridge section and D/S link section.
Bridge inlet/outlet loss, headlosses and pier losses are all optional.
The contraction and expansion losses are represented as an energy loss through the expansion/contraction reach. It is described by the user-defined coefficient multiplied by the absolute change in velocity head between the pair of sections (ie, upstream section and upstream face in the case of the contraction reach). Ineffective flow areas are represented whereby the areas outside of openings where no flow occurs until overtopping of the road deck occurs is determined. These ineffective flow areas are considered in the contraction/expansion losses.
A new term is added to the dynamic equation of the hydraulic solver. Effectively the term is (k/2g) * d(v2)/dx. The contraction reach is typically described as the reach between the normal width of the channel at the upstream bridge link section and the constricted bridge opening at the upstream bridge face section (with the expansion reach being the opposite at the downstream side of the bridge). Of course, in the case of reverse flows the contraction/expansion reaches flip around.
Typical values for contraction and expansion coefficients for subcritical flow are as follows:-
- No transition losses, contraction=0, Expansion =0
- Gradual Transition, contraction=0.1, expansion=0.3
- Typical bridge section, contraction=0.3, expansion=0.5
- Abrupt transition, contraction=0.6, expansion=0.8.
In general, values for supercritical flow should be lower than subcritical flow.
The inlet/outlet losses are the same as the inlet/outlet losses that can be applied for culverts. These are described in the help topic ‘Representation of Culverts in Infoworks’. The culvert inlet an doutlet properties are defined as Bridge Inlet and Bridge Outlet losses.
The bridge opening allows friction losses to be represented and is modelled using the momentum approach taken in conduits within Infoworks ICM albeit allowing for irregular beds and non-prismatic openings. It also allows other headlosses to be represented if necessary. Typically an upstream headloss can be applied to represent any pier losses. Multiple openings are represented as multiple bridge openings which can all have different properties.
Piers can also be explicitly represented within the opening. The piers are defined as tables of pier width versus elevations together with a roughness value. The piers are subtracted from the width and area of the section geometry, and wetted perimeters added to the section wetted perimeter.
The bridge deck is represented by the deck section which effectively is an irregular weir, in parallel with the bridge.
The blog summarises some of the key aspects as to the way bridges are dealt with within ICM. The use of a physical approach allows a lot more flexibility in the representation of losses and afflux which take place at bridges.
Hydraulics Research Wallingford, (1988), Afflux at Arch Bridges, Report SR182.
US Federal Highway Administration, (1978), Hydraulics of Bridge Waterways, (supersedes the 1958 report).
US Army Corps of Engineers, (2008), HC-RAS River Analysis System-Hydraulic Reference Manual.
DEFRA/Environment Agency, (2007), SC030218/PR Afflux Estimation System-AES User Guide.