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Vice chair presents sea-level rise and subsidence update showing subsidence currently outpacing sea-level rise in Mariner Cove
Summary
Vice Chair Ken Bongren summarized local datasets and projections showing subsidence in Mariner Cove (about 10 mm/year) currently contributes more to relative sea-level change than ocean rise; combined sea-level rise and settlement could expose low spots to multiple feet of water in major storms by midcentury.
Ken Bongren, vice chair of the Corte Madera Flood Control Board, presented a data update for Mariner Cove that combined historic surveys, satellite measurements and state sea-level guidance to show how land subsidence and tides together affect local flood risk.
Bongren said his analysis used three main local sources: the AN West subsidence study (2007), recent satellite-based measurements summarized in a 2024 Science article, and the town’s updated Storm Drain Master Plan prepared by Schaff & Wheeler. “I wanted to have a common understanding for members of the flood board and the town and the community to understand what the historic land surface and sea level has been, what it is now, and what it’s predicted to be in the future,” he said.
Key findings presented: - Measured subsidence: Recent satellite and local survey data show a subsidence rate in the Mariner Cove edge of roughly 10 millimeters per year (about 0.4 inches per year), which currently exceeds the rate of sea-level rise in that location. - Local elevations and tide exposure: Bongren used a surveyed low spot at the Ebb Tide sanitary pump station (surveyed elevation cited at about 7.2 feet) as a reference point. He noted the theoretical maximum storm-plus-tide combination is about a 10-foot tide, meaning large storms could put that low spot several feet underwater today. - Projections: Combining the town’s subsidence estimates and the California Ocean Protection Council’s 2024 sea-level guidance, Bongren showed that in a large storm the surveyed low spot could see water depths of roughly 3 feet today and between about 5.3 and 5.8 feet by 2050 under the intermediate guidance range. By 2150 the combined projections could produce substantially larger depths (Bongren illustrated scenarios up to about 12 feet at that single low spot under high-end assumptions).
Bongren emphasized that subsidence — compression of bay mud beneath filled areas — is the larger contributor to relative sea-level change in Mariner Cove today. He said the AN West 2007 report’s subsidence estimates are corroborated by the Science article’s satellite observations and Schaff & Wheeler’s more recent master-plan surveys.
Board reaction and context: Members thanked Bongren for collating the datasets and asked about applicability elsewhere in town. Staff noted NISAR/satellite data cover larger swaths and that bay-mud thickness correlates with greater settlement; shallower marsh and areas protected by levees show lower subsidence rates. Multiple board members said the update underlined the need to pursue adaptive projects and to plan for flexible, incremental structures rather than one-time hard defenses.
Data sources and availability: Bongren said his slides and underlying tables are available and that he used the NOAA Richmond tide gauge (bedrock-mounted) for long-term tidal comparisons, the AN West subsidence work, the 2024 Ocean Protection Council guidance for sea-level scenarios, and the Schaff & Wheeler Storm Drain Master Plan for local survey and bay-mud thickness estimates.
Ending: The presentation closed with a reminder that near-term adaptation choices should favor solutions that can be incrementally augmented as both subsidence and sea-level rise continue.
