Publication Date
9-2013
Abstract
Humboldt Bay has the highest rate of seal level rise (18.6 inches per century) in California, and because of compaction and subsidence, former tidelands behind dikes surrounding Humboldt Bay are lower in elevation than Humboldt Bay at high tide. In order to adapt to future change in sea level rise, coastal Wildlife Area Managers need to understand vulnerability and risk, because adaptation to sea level rise is a risk management strategy against an uncertain future. The Humboldt Bay Inventory, Mapping, and Sea Level Rise Vulnerability Assessment Project and associated data and reports (Laird 2013 and Powell and Laird 2013) provide an excellent and timely opportunity to rank shoreline vulnerability on State Wildlife Areas located along the Northern California shoreline and Humboldt Bay. The Humboldt Bay shoreline vulnerability rating is a quantitative measure of vulnerability, which uses combinations of shoreline attributes to model mean monthly maximum high water (MMMW) to rank the vulnerability of segments of the shoreline to erosion or overtopping due to extreme tides, storm surges, and future sea level rise. A preliminary shoreline vulnerability analysis of three Wildlife Areas along the Northern Coast of California was conducted, which included Fay Slough Wildlife Area (FSWA), Mad River Slough Wildlife Area (MRSWA), and Elk River Wildlife Area (ELKRWA). Breaching or overtopping of the shoreline on Eureka Slough, Mad River Slough, Fay Slough, Elk River, and Elk River Slough has the potential to flood numerous land uses, infrastructure, and natural and agricultural resources, located within the historic (1870) tidal inundation footprint, which predates the current extensive community and agricultural development surround Humboldt Bay. Quantitative assessment of these potential future risks is timely and will greatly assist the California Department of Fish and Wildlife (CDFW) Land Managers to: (1) Identify areas along the boundary of regional Wildlife Areas that are at risk from shoreline breaching, erosion, and overtopping of water control structures; (2) prioritize, plan, and budget for future infrastructural needs and proactively identify solutions to issues in anticipation of potential effects from sea level rise in the short and long term; and (3) assess overall purpose and ability of particular Wildlife Areas to support their: (a) current natural, agricultural, and cultural resource sites; (b) public recreation management goals and objectives; and (c) agency response capacity to anticipated future changes in the landscape of coastal Wildlife Areas resulting from potential sea level rise. Application of Overall Vulnerability criteria to potentially impacted diked shoreline segments based on information presented herein, indicates that ELKRWA had greatest percentage (86.6%, 1,429.6 ft.) of diked shoreline ranked as Highly Vulnerable, followed by FSWA (69.2%, 2,076.9 ft.), and MRSWA (33.0%, 906.1 ft.). The total length of diked shoreline potentially impacted by sea level rise for all three Wildlife Areas together was approximately 5.7 miles (29,840.3 ft.); and the relationship between vulnerability and elevation of diked shoreline segments was significantly (p <0.000) correlated with overall average elevation of all three Wildlife Areas. Additionally, overall Vulnerability of linear segments of diked shoreline was significantly affected by the type of shoreline surface covering. Mad River Slough Wildlife Area had the highest overall percentage of shoreline fortified with concrete and rock (62.2%, 9,435.4 ft.), followed by ELKRWA (0.3%, 9,235.7 ft.), whereas concrete fortifications were lacking at FSWA. Conversely, ELKRWA and FSWA had the greatest percentage of vegetated shoreline (99.3% [9,299 ft.] and 91.5% [11,105.6 ft.], respectively); whereas FSWA had the greatest percentage of exposed shoreline (7.4%, 826.6 ft.). Finally, sea level rise adaptive planning recommendations are presented for Wildlife Areas vulnerable to sea level rise, which include step-by-step recommendations for conducting a: (1) Sea Level Rise Vulnerability Analysis; (2) Risk Assessment; (3) Adaptation Plan; (4) Review of the Adaptation Plan; (5) Implementation of the Adaptation Plan, and (6) Review, Updating, and Monitoring of the plan.