Applying Principles of Uncertainty within Coastal Hazard Assessments to Better Support Coastal Adaptation
2. Sea-Level Rise Scenarios
3. How Certain are We? Uncertainty is Important
Location of Uncertainty Within Coastal Hazard Assessment
4. Using Uncertainty to Guide Coastal Hazard Assessment
- To avoid risk to new or existing development where, for non-habitable use, the risk of damage from coastal hazards and SLR is low, or the asset can be easily adapted to cope with future SLR. Although there may be high uncertainty around SLR in the long term, because the asset has a short life or low value, and has a functional need to be in the coastal margin, that uncertainty is inconsequential, or can be deliberately ignored. Examples might be a toilet block, a surf-lifesaving lookout, or a culvert supporting a minor access way. Such assets can be easily replaced or relocated, so modeling effort can be kept simple and low-cost. For example, using a simple “building block” model to allow for various coastal hazard sources, or relying on expert judgement or sensitivity testing to decide on an appropriate floor or culvert elevation or setback distance. The assumption in Figure 2 is that hazards are more likely to be accepted for non-habitable short-lived and/or low-value assets, although that decision will be influenced by the planning process, including socio-economic assessment.
- The greatest demands on coastal hazard assessment are for existing, exposed developments, where ongoing adaptation will be required to cope with rising sea level. For avoiding risk to existing development, or for land use intensification or change in land use, the hazard assessment will require sufficient information to inform the decision(s) to be made, and, when intolerable or nuisance risks may emerge (if not already). This will require the use of both present-day statistical uncertainty (where calculable for non-SLR coastal hazards such as storm-tide), plus several SLR scenarios—thus, the hazard assessment is likely to be more complex and costly. Within the DAPP process, the hazard assessment will need to provide enough information to identify vulnerabilities and thresholds, to design adaptation pathways, and to identify trigger points for when to switch pathways before the threshold eventuates.
- To avoid increasing the coastal risk exposure from new development and to test the longevity of the decision in establishing new developments on greenfield land where the logical and statutory requirement is to avoid future hazard (e.g., NZCPS); modeling effort can be kept relatively straightforward, focusing on an upper-range hazard scenario of at least the maximum-likelihood 1% annual exceedance probability (AEP) hazard plus a higher SLR scenario, e.g., the H+ SLR scenario (Section 2), or a higher percentile, e.g., .
5. A Coastal Flood Assessment Case Study to Support Dynamic Adaptive Policy Pathways
- They show land either as ‘in’ or ‘out’ of the hazard area, but provide no information of the gradient in hazard magnitude away from the sea (e.g., a property at the landward edge of the 1% AEP + 1 m SLR area will only be affected towards the end of the 100-year planning timeframe);
- They provide no information on the timing of the emerging hazard;
- They provide no information on the increasing frequency of flooding with future SLR;
- The hazard analysis for the +1 m and +2 m SLR scenarios may not be useful for adaptation planning if flooding begins to occur frequently at lower SLR.
6. Applying the Uncertainty Framework within the DAPP Process
- A community living in the town shown in Figure 3 (or Figure 4, Figure 5, Figure 6 and Figure 7) decides to proceed with a DAPP process before further development occurs. Such planning fulfils the requirement in the NZCPS for risk-based planning. There is also general agreement within the community (established through a community engagement process and council knowledge) that the planning is required, based on existing coastal flooding problems in some areas, plus an existing simple hazard assessment and expert opinion that show increasing flooding depth with SLR.
- The local council, which is responsible for planning to reduce or avoid risk from climate change, commission a detailed coastal hazard study. Based on the uncertainty framework (Figure 2), the hazard study estimates the flood height from a storm-tide with a present-day likelihood of flooding of 1% AEP, plus the upper 95% confidence interval of the 1% AEP estimate. The effect of SLR is assessed by adding 0.1 m increments up to 0.5 m, onto the present-day 1% AEP estimate. A higher SLR of +1 m is also assessed to provide a longer-term scenario consistent with a 100-year planning timeframe, e.g., , and an H+ SLR scenario of +1.9 m by 2150 (Table 1) is assessed for the purposes of risk avoidance for greenfields development within the town.
- The community then meets with the council, and the hazard maps for the various scenarios are presented and explained. The maps form the basis of a discussion whereby the community identifies vulnerable assets, and identifies tipping point scenarios where the depth and frequency of flooding of those assets (i.e., consequences) would become unacceptable if no action were taken, and therefore adaptation is required. Thus, when applying the framework, the consequences have been separated from the likelihood of occurrence, and the community initially makes decisions based primarily on consequence.
- The possible timing of those scenarios is then assessed using Table 1. Thus, given that the likelihood of future SLR scenarios is unknown, Table 1 brackets the possible earliest and latest timing of consequences. There is a clear separation between the statistical uncertainty associated with the storm-tide estimates and the various SLR scenarios, which provides clarity to the decision-making process. Community understanding of the flooding risk can be further enhanced by using images of historical damaging coastal flooding (when available), to provide a visual representation of present-day statistical likelihood.
- The community, with assistance from practitioners, uses the knowledge of the depth, frequency and timing to decide on several pre-determined courses of action (adaptation pathways). Those pathways could include staged alternative strategies such as coastal protection, building modifications, retreat from the coast, and avoidance of greenfields development. Planning provisions to control future development can form supporting strategies to avoid further lock-in of the current pathway. The community identifies potential trigger points, for example, based on a frequency of flooding of a given depth that is not tolerable, which identifies when a switch between pathways needs to occur. They then monitor and review the situation over time using the specified triggers in an iterative fashion as the physical and socio-economic conditions change.
7. Discussion and Conclusions
Conflicts of Interest
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NZCPS Policy 27 I (e) identifying and planning for transition mechanisms and timeframes for moving to more sustainable approaches.
|SLR (m)||RCP8.5 H+ (83rd Percentile)||RCP8.5 (Median)||RCP4.5 (Median)||RCP2.6 (Median)|
|Location||Level 1||Level 2||Level 3||Level 4||Level 5|
|Context ||A clear enough future (with sensitivity)||Alternate futures (with probabilities)||Alternate futures (with ranking)||A multiplicity of plausible futures (unranked)||Unknown future|
|System model ||A single system model||A single system model with a probabilistic parameterization||Several system models, one of which is most likely||Several system models, with different structures||Unknown system model, know we don’t know|
|SLR treatment within coastal-hazard assessment||Present-day MSL, or modest SLR range for the next few decades (≤2050)||Probabilistic SLR trajectories within a single RCP scenario, e.g., ||Rank one RCP SLR scenarios relative to each other, e.g., RCP2.6 now considered unlikely ||Treat all IPCC AR5 RCP scenarios as separate and equally plausible to test pathways||SLR rate at very long timeframes not considered in available literature, e.g., beyond 2150–2200|
|Other hazard source examples||Median, or “best estimate” of AEP, where calculable for non-SLR coastal hazards, e.g., storm-tide||Statistical probabilities, where calculable for non-SLR coastal hazards, e.g., storm-tide||An allowance for increased future storm-tide variability, e.g., ±10%||Geomorphic response to SLR of tidal inlet/spit systems on sand or gravel shorelines|
|Coastal hazard assessment situation||Little uncertainty (or, uncertainty is inconsequential to decision being made, or deliberately ignored)||Statistical probabilities for storm-tide or coastal erosion based on historical observations||SLR scenarios added to storm-tide probability levels||High SLR scenarios added to present-day storm-tide or coastal erosion “best estimates”|
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Stephens, S.A.; Bell, R.G.; Lawrence, J. Applying Principles of Uncertainty within Coastal Hazard Assessments to Better Support Coastal Adaptation. J. Mar. Sci. Eng. 2017, 5, 40. https://doi.org/10.3390/jmse5030040
Stephens SA, Bell RG, Lawrence J. Applying Principles of Uncertainty within Coastal Hazard Assessments to Better Support Coastal Adaptation. Journal of Marine Science and Engineering. 2017; 5(3):40. https://doi.org/10.3390/jmse5030040Chicago/Turabian Style
Stephens, Scott A., Robert G. Bell, and Judy Lawrence. 2017. "Applying Principles of Uncertainty within Coastal Hazard Assessments to Better Support Coastal Adaptation" Journal of Marine Science and Engineering 5, no. 3: 40. https://doi.org/10.3390/jmse5030040