Local Tide Height

Evaluation: Some Action Needed /Closely Monitor

Issue Summary:

Local average tide height is rising at a rate of about 1.3 mm/year (5.1 inches in 100 years). This rate is expected to accelerate due to predicted changes in ocean temperatures and ice cap melting.

Why do we care?

Sea level rise will cause more frequent of coastal flooding and shoreline erosion here in the future. Because the project area is so influenced by the ocean and estuary, it is important that predicted scenarios for future sea level rise be clearly understood by the community so we know what, if anything, the community should be doing to prepare.

What’s happening?

Based on 36 years of continuous tidal data, the mean tidal height at the Charleston tide station is rising at a rate of about 1.3 mm per year (Figure 1). This translates to about 5 inches in 100 years which should not be cause for much concern. However, this rate of rising sea levels may accelerate. Scientists are analyzing data from satellites
and tide gauges, building mathematical models to predict just how much we can expect the sea to rise worldwide over the next century. Some estimates predict sea level 2 meters (6.6 feet) higher in 2100 than it is today, which should be cause for alarm (Vermeer and Rahmstorff 2009).

Figure 1. Tide heights plotted from the NOAA National Water Level Observation Network (NWLON) tide station in Charleston for the past 36 years.

We know there’s debate about the causes of sea level rise and whether, and how much its rise will be accelerating. But despite uncertainty surrounding the issue, we feel it’s important for our community, with its proximity to and dependence on the ocean and estuary, to clearly understand the predicted scenarios for future sea level rise and what they may mean for us living here.

To help understand what sea level inundation levels might look like here, the South Slough Reserve science staff mapped several conservative inundation scenarios for the tidally-influenced areas associated with the South Slough and Coastal Frontal watersheds (Figures 2-5). We used a “bathtub ring” model (derived from LiDAR elevation data), recognizing our work will need to be validated with a more sophisticated SLR modeling approach.

Figure 2. Tide heights mapped for the South Slough estuary showing current mean higher high water level (MHHW), current maximum recorded water level, and mean higher high water level given +1, +2 and +3 foot rises in sea level rise.

Figure 1 shows current tide levels and several potential future tidal inundation scenarios. The dark blue area shows the current area in South Slough inundated during an average higher high tide (mean higher high water- MHHW). The light blue area shows the area flooded during the highest recorded high tide to date at the Charleston NWLON tide station (14.94 ft. on January 26, 1983). The yellow, orange and red areas show the areas that would be flooded during an average higher high tide after sea level has risen 1 foot (yellow), 2 feet (orange) and 3 feet (red).

The map shows that many of the developed areas around Charleston may be flooded during higher tides in those 1, 2, and 3 foot sea level rise scenarios (more about Charleston below).
The map also shows how a successive rise in sea level, will likely result in a successive “landward migration” of South Slough’s emergent tidal wetlands, probably resulting in a reduction of the areal extent of those habitats in the estuary.

When thinking about the effect sea level rise will have on emergent estuarine wetlands (including salt marshes, scrub-shrub wetlands and Sitka spruce swamps), it’s important to note that our maps, nor do any SLR models we know of, take into account the degree to which sediment accumulation will raise elevations of current tide flats and salt marshes under new tidal flooding regimes. Just as the emergent tidal wetlands we see in the estuary today were formed after countless years of sediment accumulation in our “drowned river-mouth” estuary (starting with the warming trend that ended the last ice age and re-set after the last Cascadia subduction-zone earthquake), it’s likely that SLR-driven sedimentation will also will raise tide flat elevations ultimately to elevations capable of supporting emergent marsh vegetation.

The key questions at this point are what the SLR rate will be over the next several decades and whether the rate of sediment accumulation can keep pace with the rate of SLR. To our knowledge SLR rates have not yet been modeled specifically for the Oregon coast where local tectonic uplift rates affect local SLR rates. But we do have sediment accumulation information for the South Slough estuary. For example, sediment accumulation rates measured between 10/11/94 and 6/12/07 (12.67 years) at South Slough’s Hidden Creek marsh are as follows:

 Emergent high marsh vertical accretion rate: 3.8 mm/yr (measured using replicated feldspar soil horizon markers)
 Emergent low marsh elevation change rate: 3.0 mm/yr (measured using a surface elevation table (SET) instrument)
 Tide flat vertical accretion rate: 6.6 mm/yr (measured using replicated plastic grid “eggcrate” soil horizon markers)

These data suggest that the local SLR rate will need to accelerate to above 6.6 m/yr (just over 2 feet in 100 years) to outpace local sediment accumulation rates, assuming the sediment accumulation rate does not also rise with the local SLR rate. We are compiling sediment accumulation rates from other sediment dynamics sites in the South Slough estuary to develop a more robust estimate of sediment accumulation rates in the South Slough estuary.

Our maps also do not help us understand what the spatial extent of eelgrass bed change due to the various sea level rise scenarios is likely to be. Efforts are currently under way to predict the effects of SLR on eelgrass, emergent marshes and other intertidal habitats (e.g., Reusser et al. 2009).

The map in Figure 3 shows the extent of average higher high tide flooding in the Charleston area in the same SLR scenarios. Under the +3 feet of inundation scenario, extensive low-lying areas in the Charleston area (except the boat basin fill areas) will likely flood regularly during most higher high tides.

Figure 3. Tide heights mapped for the Charleston area showing current mean higher high water level (MHHW), current maximum recorded water level, and mean higher high water level given +1, +2 and +3 foot rises in sea level rise.

It’s important to note that high water surges caused by storms (predicted to be more frequent and more intense in the future) will accelerate the effects of sea level rise, causing areas to flood many years before rising average high tides flood those areas.
In the coastal frontal area, the steepness of the coastal landscape and limited development appear to limit the effects of future flooding due to SLR on human developments (Figures 5 and 6)(the LiDAR data used to generate these inundation maps did not extend to the part of the Coastal Frontal area that includes Bandon Dunes resort. We will develop those maps as those data become available to us). In addition to coastal erosion, it appears that the main effects of local SLR in the Coastal Frontal area will likely be flooding at the Sunset Bay campground.

Figure 5. Tide heights mapped for the Sunset Bay Beach and Campground area showing current mean higher high water level (MHHW), current maximum recorded water level, and mean higher high water level given +1, +2 and +3 foot rises in sea level rise.

Figure 6. Tide heights mapped for the Five Mile Creek mouth area showing current mean higher high water level (MHHW), current maximum recorded water level, and mean higher high water level given +1, +2 and +3 foot rises in sea level rise.


Globally, sea level is rising at an average rate of about 1.3 mm per year, and many scientists predict sea level rise will increase dramatically in the coming century (Vermeer and Rahmstorff 2009, IPCC 2007).

There appear to be two reasons for rising sea levels. First, land ice is melting and adding more water to the sea. For example, 124 glaciers have melted in Glacier National Park (leaving just 26 named glaciers in the park). Second, the temperature of the sea is increasing and as water heats up, it expands. This process, called “thermal expansion” means that water in the ocean is taking up an increasing amount of space, raising sea levels.

Sea level rise varies across the globe in part because of tectonic activity. The tectonic plates that form our continents are in some regions being pushed up (emerging) and in some places being pushed down (subsiding). In areas where the land is emerging, sea level rise will not be as substantial, and in areas where the land is subsiding, sea level rise will be greater. Coos Bay is an emergent part of the coastline so sea level rise here is not as dramatic as it might be otherwise. But according to Komar (2011), by the mid 21st century, sea level rise is expected to overtake the rate of land emergence.

Why do we care?

A rise in sea level could have a variety of negative effects in Coos County. Property near the coast would experience substantial damage from flooding. As the sea pounds more of our coastline, erosion will increase, meaning the likely loss of beaches and probable undercutting and slumping of coastal cliffs. Recreational use of coast areas would likely be affected. Our port facilities may be damaged in ways that inhibit local commerce.

Local wildlife, too, could be affected by rising seas. Saltwater encroaching farther up the bay flooding marshes would alter the habitat that supports the wide variety of animals we have in the Bay area. Animals like Dungeness crabs, which require healthy eelgrass beds for cover, or salmon, which require tidal wetlands during their juvenile stages for food and cover, may struggle to survive shifts in loss or degradation of critical habitat.

Wetlands also serve as the base of the estuarine food web and limit the severity of storms by acting as barriers between the sea and the land. If these areas are flooded, coastal storms could take a heavier toll on our community’s developed areas. To compound this problem, storminess is predicted to increase with rising seas, and the height of significant waves appears to be increasing (OCCRI 2010).

Although there is some debate in the scientific community about the degree of past and future sea level rise, there is plenty of everyday evidence to indicate the reality of global sea level rise. In some low-lying places, whole communities are preparing to move to higher ground. For example, the citizens in Tuvalu, an island nation halfway between Hawaii and Australia, are moving to New Zealand as part of a gradual evacuation. Communities of Inuits that have lived off the coast of Alaska for centuries are now moving to higher ground on the mainland. And in the case of the island nation of the Maldives, higher ground has been constructed on the island out of landfill. Other places, like the Netherlands, are building huge storm gates designed to protect channels from increasingly devastating storms (Pilkey and Young 2009).

What’s being done?

The deep connection our community has with the sea—commercial, recreational, and cultural—has the potential to be undermined by rising sea levels. Our community needs to define the problem and understand, specifically, what challenges we’re facing so we can plan ahead for the changes we expect to see. Many coastal communities are drafting emergency preparedness plans for tsunamis. Because sea level rise is like a tsunami in very slow-motion, we have time to plan and implement changes that will help our community become better prepared for the local effects of sea level rise.

Literature cited

Intergovernmental Panel on Climate Change (IPCC). 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Komar, Paul D. Allan, Jonathan C. and Ruggiero, Peter. 2011. “Sea level variations along the U.S. Pacific northwest coast: Tectonic and Climate Controls. Journal of Coastal Research. 27(5): 808-823.

Oregon Climate Change Research Institute (OCCRI). 2010. Oregon Climate Assessment Report. K.D. Dello and P.W. Mote (eds). College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR.

Pilkey, Orrin H. and Young, Rob. 2011. The Rising Sea. Island Press/ Shearwater Books. Washington.

Reusser, D. A. et al. 2009. Predicting climate change threats to key estuarine habitats and ecosystem services in the Pacific Northwest. USGS Climate Change and Estuaries project proposal.

Vermeer, Martin and Rahmstorff, Stefan. 2009. “Global sea level linked to global temperature.” Proceedings of the National Academy of the Sciences. 106 (51): 21527-21532.