SEDIMENT AND EROSION •
Landforms that separate the marshes from nearby creek mouths, such as artificial levees and embankments, reduce the amount of sediment from the creeks that deposits on the marshes. The North Bothin Marsh tidal channel network is reasonably well-connected to the mouth of ACMdP , however the South Bothin Marsh tidal channel network is disconnected from the mouth of Coyote Creek by a remnant berm along the north bank of the creek and the Bay Trail embankment.
Comparison of LiDAR surveys in 2009 (CA Coastal Conservancy) and 2019 (Marin County) do not show signs of progradation of deltaic tidal marsh or mudflat at the Coyote Creek canal or Arroyo Corte Madera del Presidio mouth. Both creek channels express across the mudflats and connect with the main Richardson Bay thalweg. The LiDAR datasets show signs of deposition in the former dredged channels near North Bothin marsh, slight erosion/ subsidence on the shallow wave-exposed mudflats, and show evidence of shoreline erosion along the bay-facing edge of the berms around the perimeter of North Bothin Marsh. The LiDAR data shows a mix of accretion and erosion/subsidence within the tidal marshes. The trail and perimeter berms show evidence of slight subsidence, but are within the margin of error of the surveys.
Re-suspension of sediments from mudflats during large wave conditions can cause temporary increases in suspended sediment concentrations and may cause episodic pulses of sediment delivery to marshes. PWA (2009) observed visible areas of higher turbidity offshore in Richardson Bay relative to outflow of Coyote Creek during windy conditions while monitoring sediment at Coyote Creek.
TOPOGRAPHIC CHANGE 2009-2019
LiDAR surveys have less accuracy in vegetated areas, and the observed elevation changes in the vegetated tidal marsh areas are within the margin of error of the LiDAR datasets.
This map shows the observed change in ground elevation between 2009 and 2019 based on available LiDAR datasets (CA Coastal Conservancy 2009; Marin County 2019). The mapped elevation changes are subject to the limitations and uncertainties inherent in LiDAR-based topographic surveys, and may exhibit errors and distortions due to vegetation, standing water, and built infrastructure. 52
BOTHIN MARSH OPEN SPACE PRESERVE - EVOLVING SHORELINES - INITIAL PLANNING MEMO