A proposed highway upgrade required four new bridge crossings over a braided, tidally-influenced distributary channel in the Mekong Delta — a setting where flood behavior is governed as much by tidal stage as by upstream discharge. The transport authority needed defensible design flood levels and scour estimates before finalizing pier locations and span lengths, but the channel's combined fluvial-tidal dynamics meant a conventional single-event flood study risked missing the governing design condition.
GeoVeris built a 2D unsteady hydraulic model of the full 18 km reach using a combined multibeam and LiDAR survey for bathymetry and floodplain topography, then drove it with both design-flood hydrographs and a full tidal boundary derived from harmonic constituents at the nearest gauge. Rather than pairing a single flood event with a single tide condition, we screened dozens of plausible fluvial-tidal combinations to find the governing case at each crossing.
That screening changed the outcome at two of the four crossings, where the client's original single-event assumption understated the design water level by more than 0.2 m. The final model outputs — design water-surface profiles, afflux estimates, and pier-specific scour countermeasure recommendations — were adopted directly into the crossings' preliminary engineering design package.
The engagement also flagged a span-length risk at the preferred alignment early enough in the design process that the authority could extend the span by 15 m rather than relocate a pier, avoiding a substantially more expensive redesign later in the process.
Methodology
Built a 2D unsteady HEC-RAS model of an 18 km reach of a tidally-influenced Mekong distributary channel using a hybrid mesh (12 m cells in-channel, 25 m on the floodplain), with bathymetry from a combined multibeam/LiDAR survey merged with a national 5 m DEM for overbank areas. Applied a compound upstream boundary (design flood hydrographs at 1%, 2%, and 10% AEP) and a downstream tidal water-level boundary derived from harmonic constituents at the nearest tide gauge, run across three spring-neap cycles to capture combined fluvial-tidal peaks. Screened local pier scour at four proposed crossing configurations using HEC-18 methods against the 2D velocity and shear-stress fields, and tested riprap and articulated-mattress countermeasure layouts.
Results & Outcomes
- Delivered design flood water-surface profiles for the 1%, 2%, and 10% AEP events at all four crossing alignments
- Identified a 0.4 m afflux risk at the preferred alignment, prompting a 15 m span extension that avoided a costly pier relocation
- Recommended pier-specific scour countermeasures projected to reduce local scour depth by up to 35%
- Findings adopted directly into the crossing's preliminary engineering design package
Challenges
Spring-tide flood coincidence made it difficult to define a single governing design event, since peak fluvial discharge and peak tidal stage did not consistently align in the historic record. We ran a joint-probability screening across 40 synthetic hydrograph-tide combinations spanning plausible phase offsets to identify the reasonable worst-case pairing for each AEP, rather than relying on a single boundary assumption — this changed the governing design water level at two of the four crossings by more than 0.2 m relative to the client's original single-event assumption.