Novel Quantification of Shallow Sediment Compaction by GPS Interferometric Reflectometry and Implications for Flood Susceptibility

Estimates of flood susceptibility and land loss in the world's coastal regions depend on our knowledge of sea level rise (SLR) from increases in ocean mass and volume, as well as knowledge of vertical land motion. Conventional approaches to the latter include tide‐gauge and Global Positioning System (GPS) measurements relative to well‐anchored monuments few meters below the surface. However, in regions of rapid Holocene sedimentation, compaction of this material can add a significant component to the surface lowering. Unfortunately, this process has been difficult to quantify, especially for the shallowest material above the monument. Here we use a new technique, GPS interferometric reflectometry, to estimate the rate of this process in the Mississippi Delta and the eastern margin of the North Sea. We show that the rate of shallow compaction is comparable to or larger than the rate of global SLR, adding 35% and 65%, respectively, to the rate of relative SLR by 2100. Plain Language Summary Sea level change is influenced by vertical motion of the sea surface as well as vertical motion of the land in coastal areas. Tide gauges and GPS are two conventional approaches to measure coastal vertical land motion (VLM). GPS conventional positioning determines the vertical component of position changes resulting from displacements beneath the monument foundation. However, quantifying shallow VLM that occur above the base of monument has not been possible so far. In regions of rapid Holocene age (roughly 11,500 yr before present) sedimentation such as river deltas and coastal alluvial plains, compaction of this material, if not countered by ongoing sedimentation, can add a significant component to the rate of coastal subsidence. Here, we use a new technique, GPS interferometric reflectometry, to estimate the rate of shallow VLM in two coastal regions with thick Holocene deposits, the Mississippi Delta and the eastern margin of the North Sea. We show that the rate of VLM from shallow compaction is comparable to or larger than the rate of sea level rise. Since many of the world's great coastal cities are built on river deltas with comparable Holocene sections, our results suggest that estimates of flood risk and land loss have been underestimated. Key Points Shallow displacement within the layer between the surface and the base of a GPS monument are measured using the GPS‐IR technique Rates of shallow sediment compaction (SSC) are quantified at GPS sites in two coastal r