Coastal Highway, Wetlands, and Subsidence Data
A coastal parish in Louisiana proposes a four-lane highway interchange through 120 acres of wetlands to serve a new port. Planners rely on a 1978 USGS topographic map that shows the tract at 4–6 feet above sea level, with marsh symbols and a few drainage canals. A 2023 satellite image shows open water over roughly half of the same tract, with the remaining marsh fragmented. Tide-gauge data for the parish show relative sea-level rise of about 9 millimeters per year; local subsidence estimates are 6–8 millimeters per year. The port authority says the 1978 map proves the land is 'solid' and suitable for construction. A local environmental group says the project will worsen storm-surge flooding. The parish council asks you to evaluate the two claims and recommend what additional information it needs before voting.
1. Evaluate the port authority’s claim that the 1978 map 'proves the land is solid.'
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The port authority’s claim confuses a historical map with a current site assessment. The 1978 topographic map shows elevation and marsh at a particular scale and date; it does not 'prove' that the land is solid today. A contour map with a 5-foot interval may not capture the microtopography of a marsh, and the map’s symbols describe wetland vegetation, not bearing capacity. More importantly, the scenario gives relative sea-level rise of about 9 mm per year and subsidence of 6–8 mm per year. Over 45 years, that is roughly 0.3–0.5 meters of effective elevation loss, which can convert marsh to open water. The 2023 satellite image is consistent with that process. 'Solid' is an engineering conclusion that requires current elevation, soil borings, and hydrologic data, not a 1978 map.
2. Explain why the 1978 map and the 2023 image differ, using the geomorphic and hydrologic processes named in the scenario.
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The two sources differ because the landscape has changed. Relative sea-level rise and subsidence lower the surface relative to the water; canal dredging can increase saltwater intrusion, kill marsh vegetation, and accelerate peat collapse. Once the root mat decays, the marsh can convert to open water relatively quickly. The 1978 map and 2023 image are both representations at different times and scales; the satellite image may also show water beneath vegetation or during a high-water period, so it is not a perfect comparison. But the direction of change is consistent with the measured processes. The difference is not evidence that one source is wrong; it is evidence that the parish is dealing with a dynamic coastal system.
3. Recommend two types of data the parish should collect and explain how each would improve the decision.
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Two useful data sets would be high-resolution LiDAR elevation and a wetland delineation. LiDAR would show current ground elevation and microtopography, allowing the parish to compare the 1978 contours with present conditions and to model storm-surge flooding. A wetland delineation would identify jurisdictional wetlands and the vegetation and hydrology that qualify them, which is necessary for permits and mitigation. Additional useful data would be soil borings for bearing capacity, subsidence measurements, and storm-surge modeling under current and projected conditions. These would not resolve every value question, but they would replace the port authority’s historical-map claim with current evidence.