Tuflow and estry manual Version 3

3.14.4.2Bridges

Bridge channels do not require data for length, Manning’s n, divergence or bed slope (they are effectively zero-length channels, although the length is used for automatically determining nodal storages – see Section 3.13.2.2). The bridge opening cross section is described in the same manner to a normal channel.

As of Build 2005-05-AN, two bridge solutions are offered using Bridge Flow. Method B is an enhancement on Method A by providing better stability at shallow depths or when wetting and drying. There are also some subtle differences between the methods in how the loss coefficients are applied at the bridge deck as discussed further below.

Bridge structures are modelled using a height varying form loss coefficient rather than fixed contraction and expansion losses. A table (referred to as a BG Table) of backwater or form loss coefficient versus height is required. BG Tables can be entered using .csv files via a 1d_ta (often renamed as 1d_bg) layer (see Section 3.13.3). Alternatively, fixed field formatting can be used anywhere in the .ecf file using BG Data (see Section E.1 for the fixed field formatting rules). Where the loss coefficient is constant through to the bridge deck (eg. no losses such as a clear spanning bridge, or pier losses only), the BG table can automatically be created by specifying a postive non-zero value for the Form_or_Bend_Loss attribute in the 1d_nwk layer (see Table 4 .10). Also note the use of this attribute in applying additional losses to a BG table.

The coefficients may be obtained from publications such as “Hydraulics of Bridge Waterways” (US FHA 1973), through the following procedure. The bridge opening ratio (stream constriction ratio), defined in Equations 1 and 2 of “Hydraulics of Bridge Waterways”, is estimated for various water levels from the local geometry. Alternatively, the bridge opening ratio is estimated with the help of a trial modelling run in which the stream crossed by the bridge is represented by a number of parallel channels, providing a more quantitative basis for estimating the proportion of flow actually obstructed by the bridge abutments. For each level this enables the value of Kb to be obtained from Figure 6 of “Hydraulics of Bridge Waterways”. Additional factors, for piers (Kp from Figure 7), eccentricity (Ke from Figure 8) and for skew (Ks from Figure 10) are obtained. The backwater coefficient input into the table is the sum of the relevant coefficients at each elevation. The velocity through the bridge structure used for determining the head loss is based on the flow area calculated using the water level at the downstream node.

For Method A, the underside of the bridge deck (the obvert) is taken as the elevation when the flow area stops increasing, or the highest elevation in the bridge’s CS data, whichever occurs first. For Method B, the highest (last) elevation in the CS table is always assumed to be the underside of the bridge deck.

For Method A, once the downstream water level is within 10% of the flow depth under the bridge, a bridge deck submergence factor is phased in by applying a correction for submerged decking using a minimum value of 1.5625 (if the specified loss coefficient is greater than 1.5625, this value is applied). Method B does not use the 10% of the flow depth phasing in nor applies a minimum loss coefficient once the bridge deck is submerged (ie. it applies the value as per the specified loss coefficients (BG) table). Method B relies on the user to provide appropriate values at all flow heights.

A calibration factor is available for bridges. For a given flow the backwater (head increment) of a bridge channel is proportional to the factor. It is normally set to 1.0 by default, and modified if required for calibration purposes. This option is presently only available if using the fixed field input BG tables – see BG Tables (1D), but is planned to be made available via the 1d_nwk layer in a future release.

Any wetted perimeter or Manning’s n inputs in the hydraulic properties table are ignored. If the flow is expected to overtop the bridge, a parallel weir channel should be included to represent the flow over the bridge deck, or a BW channel can be specified.