SanFranciscoBay_Adapt2SeaLevelRise_CostData

In metropolitan regions made up of multiple independent jurisdictions, adaptation to increased coastal flooding due to sea level rise requires coordinated strategic planning of physical and organizational approaches. To better understand specific choices for sea level rise adaptation we examined potential costs to raise current protective infrastructure to future water levels.

Methods
Our analysis is based on four primary steps.

First we reclassified the shoreline infrastructure data from San Francisco Estuary Institute (SFEI). We used the following dictionary to reclassify the data in terms of the Landform / Wall value: {'Berm': 'Landform', 'Channel or Opening': 'Landform', 'Shoreline Protection Structure': 'Landform', 'Embankment': 'Landform', 'Engineered Levee': 'Landform', 'Floodwall': 'Wall', 'Natural Shoreline': 'Landform', 'Transportation Structure': 'Wall', 'Water Control Structure': 'Wall', 'Wetland': 'Landform'}. We used the following dictionary to reclassify the data in terms of the Dynamic / Static value {'Berm': 'Static', 'Channel or Opening': 'Static', 'Shoreline Protection Structure': 'Static’, ‘Embankment': 'Static', 'Engineered Levee': 'Static', 'Floodwall': 'Static', 'Natural Shoreline': 'Dynamic’, ‘Transportation Structure': 'Static', 'Water Control Structure': 'Dynamic', 'Wetland': 'Dynamic'}. Additionally, for some of the shoreline protection structure sites we used Google Earth and site visits to shift their categorization from landform to wall.

Second we conducted a rapid assessment of water exceedance levels by calculating the difference in height between the projected floodwater and the structure. We subtracted the height of every shoreline segment in the SFEI data from the USGS CoSMoS model’s projected future water levels.

Third we generated three potential shorelines for cost comparison. The most bayward line, referred to here as “Shoreline A”, was mapped just using SFEI’s shoreline infrastructure data. We used the “Bayshore_Defense” category, with the values “First line of shoreline defense” or “Wetland on Bay shore” to designate “Shoreline A.” We designated the other two shorelines using an intersection of SFEI’s shoreline infrastructure data and the BAARI data to distinguish the saltwater and freshwater habitat zones. The shoreline referred to as “Shoreline B” uses SFEI’s mapping of saltwater habitat. The shoreline referred to as “Shoreline C” is the most landward of the three we designated, and is based on SFEI’s mapping of freshwater habitat.

Finally, we calculated potential costs based on our review of costs specific to projects in the San Francisco Bay Area, and supplemented these data with cost calculations from the literature where needed. For walls we used $218.0 (2016 USD$) as the middle value for each linear kilometer, and for each meter of raised infrastructure height. For walls we used a high and low value based on + or – $75.0 from our middle value. For landforms we used $8.0 (2016 USD$) as the middle value for each linear kilometer, and for each meter of raised infrastructure height. For landforms we used a high and low value based on + or – $4.0 from our middle value.

We compared 4 different approaches to calculating an approximate cost of raising existing coastal structures: 1) simple without parcel costs, 2) simple with parcel costs, 3) complex without parcel costs, and 4) complex with parcel costs. The “simple” approaches (#1 and #2) used a linear relationship when calculating the cost of raising the height of a levee or a wall. In our “complex” cost estimation approaches (#3 and #4), we applied a levee-growth cost factor that incorporates greater nuances to cost estimation. We also recognized that land costs will be a major factor in the cost of higher levees. Therefore at the county scale we added an additional parcel cost. This parcel calculation was applied to both the simple and complex levee approaches resulting in approaches #2 and #4. The data for this parcel cost is in our county data. Our file called “ModelCode_4Approaches-NoStorm” provides the code for these four different approaches.