𝗪𝗵𝘆 𝗪𝗮𝘁𝗲𝗿 𝗗𝗼𝗲𝘀𝗻’𝘁 𝗙𝗹𝗼𝘄 𝗨𝗻𝗱𝗲𝗿 𝗕𝗿𝗶𝗱𝗴𝗲𝘀 𝗶𝗻 𝗬𝗼𝘂𝗿 𝗛𝗘𝗖-𝗥𝗔𝗦 𝟮𝗗 𝗠𝗼𝗱𝗲𝗹

𝗪𝗵𝘆 𝗪𝗮𝘁𝗲𝗿 𝗗𝗼𝗲𝘀𝗻’𝘁 𝗙𝗹𝗼𝘄 𝗨𝗻𝗱𝗲𝗿 𝗕𝗿𝗶𝗱𝗴𝗲𝘀 𝗶𝗻 𝗬𝗼𝘂𝗿 𝗛𝗘𝗖-𝗥𝗔𝗦 𝟮𝗗 𝗠𝗼𝗱𝗲𝗹

𝗟𝗜𝗗𝗔𝗥 𝗔𝗿𝘁𝗶𝗳𝗮𝗰𝘁𝘀 𝗮𝗻𝗱 𝗕𝗿𝗶𝗱𝗴𝗲 𝗥𝗲𝗽𝗿𝗲𝘀𝗲𝗻𝘁𝗮𝘁𝗶𝗼𝗻 𝗜𝘀𝘀𝘂𝗲𝘀 𝗶𝗻 𝗛𝘆𝗱𝗿𝗮𝘂𝗹𝗶𝗰 𝗠𝗼𝗱𝗲𝗹𝗹𝗶𝗻𝗴

If your simulation shows excessive upstream flooding near a bridge, the issue is often not related to hydraulic parameters but to the terrain representation.

A frequent problem when building 𝗛𝗘𝗖-𝗥𝗔𝗦 𝟮𝗗 𝗺𝗼𝗱𝗲𝗹𝘀 from 𝗟𝗶𝗗𝗔𝗥 𝗱𝗮𝘁𝗮 is the inaccurate representation of bridges.

Because the bridge deck blocks LiDAR returns, the ground beneath the structure is often not properly captured. This leads to the 𝗗𝗶𝗴𝗶𝘁𝗮𝗹 𝗧𝗲𝗿𝗿𝗮𝗶𝗻 𝗠𝗼𝗱𝗲𝗹 (𝗗𝗧𝗠) containing artificial elevations or “spikes” under the bridge.

As a result, the model may incorrectly interpret the bridge opening as partially or fully obstructed.

This can lead to:

• Underestimation of flow capacity under the structure
• Overestimation of upstream water levels
• Distorted flood extents
• Artificial backwater effects

For bridges that remain unsubmerged during the event, a practical solution is to correct the terrain before running simulations:

✓ Remove LiDAR artefacts beneath the bridge deck
✓ Reconstruct the actual riverbed geometry
✓ Preserve embankments and abutments acting as hydraulic controls
✓ Ensure hydraulic continuity of the bridge opening

This simple preprocessing step can significantly improve the accuracy and stability of hydraulic simulations.

Before running your next 𝗛𝗘𝗖-𝗥𝗔𝗦 𝟮𝗗 𝘀𝗶𝗺𝘂𝗹𝗮𝘁𝗶𝗼𝗻, carefully inspect the terrain under bridges—small LiDAR errors can strongly control your results.