What Changed After the Initial Review

After the first round of feedback on the topographic survey of the Sena basin hills, several assumptions about the erosion rates on the gypsum formations had to be revised. The initial model, based on regional averages from the 1990s, did not account for the recent acceleration of surface runoff in the northern sectors of Montmartre.

The review focused on three specific transects where the original contour maps showed a discrepancy of more than 1.5 meters in elevation. Field verification using differential GPS revealed that the historical quarry subsidence, not fluvial erosion, was the dominant factor in two of those three areas. This shifts the interpretation of the terrace formation sequence.

For the engineering forestry team, the main takeaway was the need to update the slope stability layers. The old limestone quarries, marked as inactive, still show measurable creep during wet seasons. The native shrub cover—particularly boxwood and hawthorn—is holding the topsoil, but the deeper gypsum layers remain exposed in several gullies.

The revised map set now includes a separate overlay for anthropogenic cavities. This changes the risk assessment for the planned reforestation plots. Instead of uniform planting, the team will use targeted placement of deep-rooted species on the unstable sections.

What seemed like a routine update turned into a substantial correction of the baseline data. The next step is to cross-reference these findings with the groundwater monitoring stations installed last year. That comparison will determine whether the current drainage plan needs adjustment before the autumn rains.