ECOTONE SLOPE
OPPORTUNITIES
• Create ecotone slopes in low-value, weedy terrestrial fills or, in higher-value salt marsh areas, use a phased approach to add resilience while minimizing impacts. • Opportunities for ecotone slope creation exist in areas where new upland fill placement may be otherwise unavoidable; for example, adjacent to potential new trail embankments. • Integration of treated wastewater into an ecotone slope could be explored in coordination with nearby wastewater treatment facilities in Mill Valley. CONSTRAINTS AND CONSIDERATIONS
• There is little terrestrial space for ecotone slope creation at most of the Bothin Marsh Preserve. • Ecotone slopes require more fill and have a larger footprint compared to traditional steepsloped embankments. Several regulatory policies for fill placement in tidal wetlands and waters encourage projects to minimize fill volume and footprint to the extent feasible.
100
• Single-event construction of ecotone slopes in existing high tidal marsh would likely have excessive near-term adverse ecological impacts unless located in existing upland or degraded areas. Create ecotone slopes opportunistically in areas where impacts are already unavoidable (e.g., for trail improvements) or where fill can be justified given anticipated long-term benefits. UNCERTAINTIES
• The nearby Mill Valley Sewage Treatment Plant is a potential source of treated wastewater, but the feasibility of treated wastewater delivery to North Bothin Marsh across Arroyo Corte Madera del Presidio is currently unknown. PROJECT PHASING
• Consider use of methods like gradual construction by staggered/phased thin-lift sediment deposition in pace with sea-level rise, and to minimize ecological impacts.
BOTHIN MARSH OPEN SPACE PRESERVE - EVOLVING SHORELINES - INITIAL PLANNING MEMO