Perform a Cost-Benefit analysis on the ZSR

Rationale

Instead of a homogenous solution across a whole town, the zones of shared risk created by tool allow us to design more localized solution by looking at the cost and benefits of protecting each segment. It also also allows us to study different flood defense scenarios possible for specific segment. For an example paper that focus on fine-scaled economic analysis for coastal defense planning, please visit this link. The study shows that by carefully identifying the lowlying segments where the benefits exceeds the cost, the over cost benefit ratio for a town can be increased. Meaning it would be a much more interesting investment.

Workflow

There are 2 ways the economic analysis can be conducted. The first one is by sticking to the default results created by toolbox which consists of seawall, road-raising, or building raising scenarios for each segment. These are default solution and the planners have the option of designing more creative solutions by modifying the shapefiles in the project folder (fp-project_id, sw-project_id, rd-project_id, and sg-project_id). Creative solutions can be created in myriads of ways and are not part of the automated results by the toolbox.

Please note that when adding new creative solutions, fill the SCENDESC() field with one or a combination (in case of hybrid solutions) of prefix variables in Reference. For example, if you’re adding a scenario combining seawall and storm gates, fill the field with sw+sg. While this is not essential for the toolbox to work, it is used later on for reporting purposes which can be done automatically using an R script.

Damages

For each scenario, the economic analysis starts by fetching all buildings at risk of flooding within the floodplain, then the damages to these buildings are calculated for each surge level. The damage to each building starts at zero at a flood depth of -1 m relative to the ground and rises linearly to 100% when flood depth is 8 m above ground. Thus, each building will have an expected damage (percent of the market value) given a specific storm surge value, which is the probability of the storm surge times the damage that is caused by that surge. The present value of this expected damage is then aggregated over the duration of the planning scenarios but constrained to not exceed the building market value. This will ensure that the total damage to a building will not exceed its market value. After that, we aggregate the damages to buildings within a town or coastal segment to obtain the total buildings damages in that area at each storm surge height. Then, we double this damage to include damage to roads, utilities, and public facilities as well as costs of emergency services and clean up to the state and towns (de Moel, Botzen, and Aerts 2013). We assume that this doubled damage constitutes the total direct damages caused by the given storm surge. (Please note that details on where the data on the value of the buildings comes from can be found on the Cleaning section.

Costs

For seawalls, the construction cost of increases linearly with the length of the wall. Because the wall is effectively a triangle with a wide base, the construction cost of the wall also increases with the square of its height as first suggested by (Yohe et al 1995).

Cost estimates for raised roads are developed based on the recently funded project to raise Beach Road in West Haven. The total cost of the project is 8 million USD, and this raised the road 3 feet higher with a length of 2,267 feet. Cost includes tearing up road, relocating utilities, fill, paving, sidewalk, and construction costs. In fact, there are big fixed costs to completely rebuilding a road. We found that the cost to raise a road 1 foot higher is about 578 USD. About 25 USD is for the fill, 500 USD for the walls to contain the side of the road, and the remainder is a contingency fee. In addition, there is a fixed cost per linear foot that includes repaving, sidewalks, traffic control, ground preparation, and moving utilities of about 1795 USD / linear foot. This cost estimate does not consider whether there are constraints about people’s driveways or whether there are inconveniences associated with living next to an elevated road.

Finally for the buildings, in addition to the height, the cost of raising buildings also vary based on the square feet of the building footprints as well as the construction type and foundation of the building (Botzen et al, 2013). Buildings with slab-on-grade foundation are the most expensive to elevate. In this study, we assume that buildings in the study area have masonry construction with a basement, which cost USD 60 per square feet to raise up to 2 feet, 63 USD for 4 feet and 68 USD for 8 feet (values based on 2009 USD).

All defense measures are expected to last 30 years after which they will have to be replaced (Aerts 2018). The maintenance cost over the lifetime of the stuctures is the present value of annual maintenance for 30 years. Annual maintenance is expected to be 0.5% of construction cost per year (Aerts 2018). The interest rate is the current municipal bond rate, 2.5% (United States Treasury Department 2019).

Outputs

The outputs of the Perform CBA tool are saved in a folder named cba-outputs. All files are prefixed with their categories. Here are the list of output files:

  • project-project_id: all scenarios and their economic results. Each row represent a scenario and each column shows the economic results / parameters. If you need help with the column names, please consult the Reference.

  • fp-scenario_id: floodplain of the scenario.

  • sw-scenario_id: seawalls of the scenario.

  • rd-scenario_id: raised-roads of the scenario.

  • sg-scenario_id: storm gates of the scenario.

  • prbldg-scenario_id: protected buildings by the scenario.

  • rsbldg-scenario_id: raised buildings for the scenario.

  • wl-scenario_id: wetlands affected by the scenario.