In HEC-RAS models, a lateral structure is often used to establish a connection between 1D river reach and a storage area/2D flow area for flow exchanges. A lateral structure is to be added to the model by assigning a river station value slightly smaller than its immediate upstream cross section (Figure 1).
In Lateral Structure Editor window, HW Position must be selected: Left/Right overbank, or Next to left/right bank station (Figure 2).
For a 1D river reach in HEC-RAS, there is only one single WSE value at a cross section but these WSE values vary along river profile direction. Different HW position settings will determine how a lateral structure is to be “connected” or “correlated” to cross sections, which will in turn impact how headwater elevations (WSE of cross sections) along river profile are interpolated and applied to lateral structure weir equations. Apparently, if the cross sections have the same downstream reach lengths for LOB, Channel, and ROB, a HW position of left/right overbank or next to left/right bank station does not matter since no matter which position is selected, the “connection” between cross sections and a lateral structure is not to make a difference.
However, when the cross section downstream reach lengths for LOB, Channel, and ROB are different, the HW position setting needs to be carefully selected.
As shown in Figure 3, a 1000ft long lateral structure is established between cross section 2189 and 1116 and its HW position is set to Next to right bank station. The lateral structure (Begin, LS Sta=0) is 10ft away from its upstream cross section 2189. The “connection” between cross section 2189/1757/1387/1116 and the lateral structure is explained and shown in Figure 3 and Figure 4.
Now change the lateral structure HW position to Right overbank and due to the downstream reach lengths of ROB are different from those of the channel, the “connection” between the cross sections and the lateral structure is different from the above (Figure 5 and Figure 6).
In the examples above, the “connection” between the cross sections and the lateral structure is different with different HW positions, which in turn impacts the headwater interpolation along the lateral structure, and therefore the modeling results are different (Figure 7).