Coastal hazards such as flooding and erosion threaten many coastal communities and ecosystems. With documented

increases in both storm frequency and intensity and projected acceleration of sea level rise, incorporating the

impacts of climate change and variability into coastal vulnerability assessments is becoming a necessary, yet

challenging task. We are developing an integrated approach to probabilistically incorporate the impacts of climate

change into coastal vulnerability assessments via a multi-scale, multi-hazard methodology. By examining the

combined hazards of episodic flooding/inundation and storm induced coastal change with chronic trends under a

range of future climate change scenarios, a quantitative framework can be established to promote more sciencebased

decision making in the coastal zone. Our focus here is on an initial application of our method in southern

Oregon, United States.  (PDF contains 5 pages

Allan, Jonathan

Harris, Erica

Ruggiero, Peter

English

Aquatic Commons

 AN INTEGRATED APPROACH FOR EVALUATING COASTAL VULNERABILITY IN A CHANGING 
CLIMATE 
 
Erica L. Harris,* College of Oceanic and Atmospheric Sciences, Oregon State University 
Peter Ruggiero, Department of Geosciences, Oregon State University 
 Jonathan C. Allan, Oregon Department of Geology and Mineral Industries  
 
 
Abstract 
 
Coastal hazards such as flooding and erosion threaten many coastal communities and ecosystems.  With documented 
increases in both storm frequency and intensity and projected acceleration of sea level rise, incorporating the 
impacts of climate change and variability into coastal vulnerability assessments is becoming a necessary, yet 
challenging task.  We are developing an integrated approach to probabilistically incorporate the impacts of climate 
change into coastal vulnerability assessments via a multi-scale, multi-hazard methodology.  By examining the 
combined hazards of episodic flooding/inundation and storm induced coastal change with chronic trends under a 
range of future climate change scenarios, a quantitative framework can be established to promote more science-
based decision making in the coastal zone.  Our focus here is on an initial application of our method in southern 
Oregon, United States. 
 
Introduction 
 
At present, one of the most widely applied approaches for assessing physical vulnerability to coastal hazards is 
based on the probability of backshore properties being exceeded by the one percent annual chance total water level 
(also known as the 100-year flood).  Since FEMA’s National Flood Insurance Rate Maps are developed using this 
technique, many communities use this as the basis for determining coastal management plans.  However, this 
approach, taken in isolation, is limited in that it does not address long-term trends or the impacts of a variable 
climate, possibly leaving many communities at risk.     
 
The majority of the existing literature on developing physical coastal vulnerability assessments characterizes hazards 
on an individual basis (Ferreira et al., 2006).  However, recent work reveals a shift toward a more integrated 
approach of evaluating multiple hazards simultaneously.  Mendoza et al. (2008) take a multi-hazard approach of 
examining the impacts of both flooding and coastal change at a time scale of a single event.  Ferreira et al. (2006) 
examine the response of shoreline change rates to both individual storm impacts and long-term trends, generating a 
multiple time scale, single hazard approach.  An understanding of the relative importance of these multiple hazards 
occurring on various time scales and their inclusion in methodologies to assess the vulnerability of coastal 
communities and ecosystems is critical for effective, sound management of the coast.  
 
Within this context, the objective of this work is to develop a methodology to enhance coastal vulnerability 
assessments by incorporating the impacts of climate change and variability via an integrated multi-scale, multi-
hazard approach (Figure 1).  Application of this methodology is along a ~90 km stretch of the southern Oregon coast 
between Winchester Bay and Port Orford. 
   
Methodology 
 
As illustrated in Figure 1, our approach conceptually integrates two coastal hazards (flooding and coastal change) 
over individual storm event and long-term (interannual to decadal) time scales under a variety of climate scenarios.   
 
  
Figure 1. Conceptual model showing integration of a varying climate into coastal change and flooding on storm 
event and long-term timescales. 
 
Flooding: Individual Events 
 
The total water level (TWL) governed by hydrodynamic forcing conditions and foreshore beach slope and relative to 
the elevation of backshore geomorphic features is critical to assessing the occurrence of flooding on beaches. For 
our Southern Oregon application we developed a 30-year hourly wave and water level time series (1979-2008) using 
a combination of measured and hindcast wave data and corresponding tide levels recorded by the Charleston tide 
gauge (Figure 2). The hourly two percent exceedance wave run-up values (Stockdon et al., 2007) were calculated 
using the wave data and beach slopes derived from 2008 Lidar data (Figures 2B, C, and D), which were then added 
to the hourly tide levels to produce a 30-year total water level (TWL) time series. The 2008 Lidar data was also used 
to determine other morphological parameters including the dune crest/bluff top and beach/dune juncture. 
 
One approach to assess the vulnerability posed by flooding is to examine the number of hours per year the backshore 
dune crests are met or exceeded by the TWL (Figure 2F)  Another, more traditional, approach is to examine 100-
year TWL elevations (calculated using a peak-over-threshold (POT) extreme value theory approach) and assess 
spatially where these extreme TWL values exceed the dune crests (Figure 2G).  This approach provides a means of 
determining the potential extent of flooding caused by extreme storms.  
 
  
Figure 2. Panels A-D show the physical setting of the study region. Alongshore variability of existing conditions and 
future estimated changes are shown in Panels E-H.  Black lines correspond with initial conditions while dark gray 
and light gray correspond with the 30 and 90 year scenarios respectively.  
Flooding: Long-term Trends  
 
Both sea level rise and increasing storminess have the potential to exacerbate the extent of vulnerable stretches of 
coast.  Here we incorporate these two climate controlled factors into projections of TWLs along our study area 30 
and 90 years into the future.  Using the empirically derived global sea level rise (SLR) predictions in Rahmstorf et 
al., 2007, we examine the impacts (Table 1) of three SLR scenarios (using rates from models projecting low (L) 
rates of SLR, average (M) SLR rates and high (H) SLR rates) for each of the time frames mentioned above on 
coastal vulnerability to flood hazards. We crudely estimate local land level changes along the southern Oregon coast 
(1.7 mm/yr) by subtracting the local sea level rise rate recorded at the Charleston tide gauge (1.3 mm/yr) from the 
present eustatic rate (3 mm/yr).  We assume that land level changes will remain constant for all scenarios and simply 
subtract them from the projected accelerations in rate of global sea level.  The potential for increasing storminess is 
accounted for by incorporating the existing trend in winter wave heights offshore of Oregon of ~ 0.016m/year 
(Ruggiero et al., 2010) into projected future TWL time series created using the same approach described in the 
individual event scale flooding methodology. It should be noted that the increase in winter wave heights was only 
applied for the first 30 years of each new time series. Projecting the future wave climate beyond this time scale is 
unrealistic.  Figure 2F-G shows the results of expected changes for the medium sea level rise scenario in 2040 and 
2100.  
 
 2040 2100 
L M H L M H 
% increase in 100 yr TWL 9 10 11 13 18 24 
  
% increase in Hrs of overtopping 
per year 19 46 53 126 291 615 
% increase in Hrs of impact per 
year 10 21 23 49 103 212 
% increase in Dune erosion 
 
23 27 28  37 56 92 
Table 1. Average percent increases of change are shown for three estimated climate change scenarios (high, 
medium, and low) in 2040 and 2100.  Alongshore variability of bolded medium scenarios is plotted in Figure 2F-H. 
 
Coastal Change: Individual Events 
 
Storms are often responsible for dramatic coastal change and therefore, quantifying dune response to these events is 
critical for vulnerability assessments.  Several numerical models exist to forecast beach and dune changes to large 
storms.  However, these models can be complex and often computationally intensive.  Therefore, we utilized the 
simple geometric foredune erosion model of Komar et al. (1999).  Using the difference between TWL and dune toe 
elevations, and dividing by the beach slope, this model estimates the maximum expected horizontal dune erosion 
(Figure 2H). 
 
Coastal Change: Long-term Trends 
 
While episodic events can drastically alter coastlines on short time scales, it is ultimately long-term trends that 
determine a region’s evolving susceptibility to hazards.  It is therefore important to include these long-term 
observations into vulnerability assessments.  This study incorporates ‘short-term’ shoreline change rates computed 
between 1967 and 2002 using the end-point method (Figure 2E).  Without sophisticated shoreline change modeling 
or sediment budget development, shoreline change rates could conceptually be extrapolated into the future (not 
shown), providing additional estimates of how long-term trends impact relative vulnerability. 
 
Summary 
 
Coastal decision makers are presently operating without sufficient quantitative information regarding the impact 
posed by a changing climate to the vulnerability of both infrastructure and ecosystems.  Typical approaches to 
assessing vulnerability to coastal natural hazards do not account for the effects of long term trends and may leave 
many regions of the coast overly vulnerable. 
 
The analysis described above quantifies coastal vulnerability based on two hazards (flooding and erosion) at 
individual events and long-term time scales.  While these individual components are useful for understanding 
contributing factors to vulnerable areas, when combined (ideally probabilistically) they provide a quantitative 
methodology to assess coastal vulnerability in the face of a changing climate using a multi-hazard, multi-time scale 
approach.   
 
References 
 
Ferriera, O., Garcia, T., Matias, A., Taborda, R., and Dias, J.A.,, 2006. An integrated method for determination of 
set-back lines for coastal erosion hazards on sandy shores. Continental Shelf Research 26, 1030-1044. 
 
Komar, P.D., McDougal, W.G., Marra, J.J., Ruggiero, P., 1999. The Rational. Analysis of Setback Distances: 
Applications to the Oregon Coast. Shore & Beach 67, 41-49. 
 
Mendoza, E.T. & Jiménez, J.A., 2008. Vulnerability assessment to coastal storms at regional scale. Proc. 31st 
International Coastal Engineering Conference, ASCE, Hamburg, 4154-4166. 
 
Rahmstorf, S.,  2007. A semi-empirical approach to projecting future sea-level rise. Science 19 (315), 368-370. 
 
Ruggiero, P., Komar, P.D., Allan, J.C., 2010. Increasing wave heights and  extreme vlue projections: The wave 
climate of the US Pacific Northwest. Coastal Engineering (in press). 
  
Stockdon, H.F., Holman, R.A.., Howd, P.A., Sallenger, A.H., 2006. Empirical parameterization of setup, swash, and 
runup. Coastal Engineering 53, 573-588. 
 
 
Erica L. Harris 
Oregon State University  
College of Oceanic and Atmospheric Sciences 
104 COAS Admin Bldg 
Corvallis, OR 97331 
Ph (541) 737-3251  
eharris@coas.oregonstate.edu  


An integrated approach for evaluating coastal vulnerability in a changing climate

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An integrated approach for evaluating coastal vulnerability in a changing climate

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