Casco Bay Past and Present
When discussing future sea-level rise in Maine it is important to consider the shifting of Maine’s coastline from prehistoric times until the present. Over time, Maine’s coastline has shifted dramatically in concert with changing sea-level. Yet sea-level is not a simple measure of the height of water; it is the intersection of a dynamic continent with an ever-changing water system. More than 15,000 years ago, the weight of the Laurentide Ice Sheet that covered much of northern North America depressed the continent. With the glacier's retreat, water was allowed to rush in to such an extent that the sea-level in Casco Bay was 75 meters (246 feet) above today’s level. After the glacier’s retreat, the land rebounded with the coastline receding to 60 meters (197 feet) below to day’s level. This fluctuation began to even out around 11,500 years ago at 20 meters (66 feet) below to day’s level, with a slow sea-level rise measurable since that time.
Maps and map descriptions on this page were authored by Nathan Broaddus in collaboration with Jan Piribeck; the content is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs International License.
King Tide Apical Tidal Pond Extent
Portland’s vulnerability to a rapidly increasing rate of sea-level rise is nowhere clearer than around Back Cove. Because much of this area is the product of a progressive infill process begun almost two centuries ago, most of the area is either at or just above sea-level. Beyond the obvious risks of direct inundation, sea-level rise is also clearly discernible at its intersection with Portland’s combined sewer and storm water system. Because this system drains into Back Cove, rising waters push back through storm drains to create saltwater ponds of varying size throughout the neighborhoods closest to the waterfront. This is of great enough concern to the local government to warrant the development of a specific zoning regulation requiring any future development in this area of Portland to raise the surrounding street by 2 feet to respond to increasing flood risk.
On July 14th, 2014 between midnight and 2:00am, the impact of that month’s predicted King Tide on back flow through the drainage system emptying into Back Cove was recorded. Using a GeoXT Trimble GPS device, two perimeter walks produced two polylines that, when combined, approximated the maximal extent of the tidal pond on Marginal Way. When both polylines were overlain, only the outermost segments were included in the final line, in order to compensate for the impossibility of recording the highest-water mark on all sides of the large pond as it changed constantly over the course of a 1 hour and 15 minute period. The same process was repeated on October 9th to measure the same tidal pond on a different month’s King Tide and develop a sense of month-to-month variation, which proved to be quite significant. The blue area on the map shows the outline of the pond as traced in July, and the magenta area represents the shape of the pond in October. The red outline was traced on December 4th, 2013 using a mobile mapping application.
Compounded Flood Risk In Casco Bay- Portland, South Portland and Chebeague
While the most apparent risks of sea-level rise are in its impact on the lowest neighborhoods closest to the coast, some of its less trumpeted but more dramatic impacts arise when sea-level rise exacerbates the present risk from catastrophic storms. The term “100-year flood” refers to a degree of flooding that has a 1% chance of occurring in any given year. Thus, a 100-year flood may occur more than once in a single year, but averaged over time, a storm of this intensity is likely to only happen once every 100 years. If a storm of this magnitude were to strike Maine at high-tide (at present sea-level), many coastal areas would be significantly impacted. On the Portland peninsula, most of Bayside, East Bayside and Commercial Street would be under water. Sea-level rise from anthropogenic climate change would put a ring of additional land at risk in the event of a 100-year flood at high tide. This ring is not uniform; while it is just a thin slice in some areas, others are quite large additions to developed sites at risk of flooding. The Millcreek area of Knightville in South Portland is not at great risk today in the event of a 100-year storm at high tide, but would be almost entirely underwater if sea-level rise were added as an additional risk factor.
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To define the values used for the rise associated with a high tide, a 100-year-flood, climate change related sea level rise, and the uncertainty inherent in the climate change related sea level rise prediction, a number of different studies were used as sources. Elevation was ultimately visualized using a digital elevation model which expresses the elevation of each pixel as a numeric value. Each pixel was equal to a 4 square meters (2 meters on each side) of ground area, and was accurate to 6 inches of vertical elevation. The DEMs 0 value is equal to the 0 value of the NAVD88 vertical datum.
High tide was represented using the Mean High Water average from the Portland Tidal gauge. Mean High Water is the average of both high tides (one is always higher than the other), averaged over 19 years. This value was chosen for political purposes: if only the Mean Higher High Water (the 19 year average of only the higher of the two daily tides) mark was used, there is a greater chance that many will dismiss the scenario for its extreme unlikelihood. Thus, for broader public acceptance, the average value was used to represent high tide. Because the Portland tidal gauge is the only one in Casco Bay for which historic data is retained and the majority of the locations of interest are located nearby, the Portland value was used to represent the whole of Casco Bay for ease of processing. Mean High Water’s zero mark is 4.21 feet, or 1.283 meters, above NAVD88 zero.
The 100-year storm flooding estimation is based on FEMA’s flood risk mapping values. Though these values are admittedly a product of some degree of political negotiation and are subject to several periods of public comment and contest, they are again more resistant to public dispute because of the public process that they have already weathered. While other studies have estimated slightly higher values, there is a prior precedent for using the FEMA values, including in work done by the Casco Bay Estuary Project. The FEMA 100-year storm sea level rise projection is 8.9 feet, or 2.713 meters.
The value defining climate change related sea-level rise required much more rigorous research. The IPCC (Intergovernmental Panel on Climate Change) Fifth Assessment Report has used scenario modeling techniques to estimate the impact of different likely political choices and population and economic trends. Though the scenarios themselves are not included in the main body of the report, they are available in separate studies online, as referenced in the report’s notes. A variety of consulting groups were each given the underlying assumptions of a scenario and required to create a mathematical model that would enumerate the impacts of that possible future. On average, 8 different firms were contracted for each scenario. The results of each model were averaged to find the statistical mean and a standard deviation. Though it is unlikely that the mean would ultimately be the true value of sea-level rise for each scenario, the standard deviation defines a range in which the mean would fall with a high degree of certainty. The scenarios are each detailed in the attachment IPCC Fifth Assessment Report Scenarios.
Ultimately, the RCP8.5 scenario, which is the most extreme scenario, was chosen for this project. This was for two reasons, one political, one technical. In the first case, scenario RCP8.5 assumes that the status quo will be maintained for 100 years into the future. Greenhouse gas emissions will continue to increase at their current rate, as will population numbers. No price on carbon is established. Slow income growth and little economic convergence between rich and poor countries means that increases in energy efficiency due to technological advances will be limited. A good part of the underlying impetus for this research is to inspire action by relating to the public the implications of doing nothing. In this sense, the RCP8.5 scenario is useful. In the second case, the first three scenarios (RCP2.6, RCP4.5 and RCP6.0) all have means that fall within 6 inches of each other, meaning that all fall within the same vertical unit of measurement on the Digital Elevation Model and will thus be represented by the exact same boundary, even thought they differ one from the other. Because RCP8.5 is the only scenario with a mean that falls in a different vertical unit of measurement, it is really the only scenario for which visualization is useful.
The mean sea-level rise projected by scenario RCP8.5 is 3.167 feet or 0.9653 meters. However, this mean does not describe the uncertainty inherent in this statistically derived value. The mean is framed by a confidence interval of 90%, with .7458 meters on the low end and 1.2060 meters on the high end. Thus, with 90% certainty, one can say that the sea-level rise correspondent with the RCP8.5 scenario will be greater than the lower value and lower than the higher value. Thus for the purposes of visual representation, sea-level rise up to .7458 meters was represented in one color, while the zone between .7458 meters and 1.2060 meters was represented in a different color. While it is possible that the sea-level might reach the upper boundary of this zone, it is intended rather as a representation of an unclear future, with the highest water line falling somewhere in the uppermost zone.
Thus the 100-year-flood value served as a baseline (2.713 meters) to which the high tide value was added (summing to 3.996 meters) to which the lower limit of the 90% confidence interval for climate change based sea-level rise was added (summing to 4.775 meters), finally capped by the upper limit of the 90% confidence interval for climate change based sea-level rise (summing to a total of 5.226 meters above NAVD88 zero). This classified range was overlain on top of mosaicked satellite imagery of Portland, South Portland, and Chebeague Island to give viewers a precise view of the potential compounded flood risk.
3' Sea Level Rise Polyline for the King Tides Trail
Though Mean High Water was used as a baseline for the compounded flood risk mapping of Casco Bay, other research has come to an alternate conclusion and has used the Highest Annual Tide line as a baseline indicator, not for political expediency, but to relate the maximum possible flooding that might occur during a given year and to show how much worse it might be if compounded by climate change related sea-level rise. The Maine Geological Survey (MGS) has used the Highest Annual Tide (HAT) line for its predictions of the impact of sea level rise. The MGS has developed a tool that modifies the HAT line based on predictions from all over Casco Bay to build a locally precise model of HAT.
The use of predicted HAT data by MGS prompted the use of actual observed HAT data for an Envisioning Change project that delineated the potential 3' rise in physical space; the project is called the King Tides Trail. More is said about the project in the King Tides section of this website.
The choice of 3’ as a sea-level rise value does not conform to the choices made in building the Compounded Flood Risk maps; 3’ represents the upper limit of The IPCC v5 Assessment Report’s RCP8.5 scenario, the “do-nothing” scenario. Aside from a single statistical outlier (one model predicted the sea-level rise from scenario RCP8.5 to be greater than 7 feet), 3’ represents the upper limit of the 90% confidence interval. It is represented by the uppermost boundary of the red zone in the compounded flood risk maps (recall, however, that the baseline high tide upon which that is based is the MHW instead of the HAT, and is additionally compounded by flooding from a 100-year storm). But as with the election of the HAT value rather than MHW, the purpose of this line is to leave an impression of the greatest possible impact sea-level rise will have without the impact of any extraordinary flooding. For that purpose, the worst-case scenario is effective in relating the gravity of the choices and action that must be taken in order to avert that possibility.
Both the predicted and observed values were included in the final geodatabase, to which 3’ were added to represent the added risk from 100 years of sea-level rise from the worst-case climate change scenario. The polyline created from the observed value was used to lay physical markers in the field to create the King Tides Trail.