**2. Background**

## **2.1 Prior understanding of interactions between storm responses and SLR**

The relationship between SLR and storm response is still not well understood, as was made clear by Woodruff et al. in a review of studies up to 2012 aimed at dissecting the relationship between SLR and flooding caused by tropical cyclones [6]. Of interest in Woodruff's review were two studies mentioned earlier which applied modeling techniques to investigate storm surge in hurricane conditions under SLR scenarios [1, 2]. Smith et al. [1] was the first to show quantitatively that the relationship between SLR and storm surge is not necessarily linear. In areas with high surge under present conditions, the increase in storm surge under the relative sea level rise (RSLR) scenarios remained linear, with RSLR defined as the cumulative change in vertical height of both land and water [7], but the amplification of surge in areas that typically saw low surge heights was increased by a much larger factor under heightened RSLR scenarios. While not explored in depth by the authors, another important conclusion was a potential plateau effect on the relative impact of SLR on storm surge in certain areas.

Interest in researching the impacts of global SLR and risk management has increased significantly since the NOAA 2012 National Climate Assessment (https:// scenarios.globalchange.gov) wherein 100-year projections of SLR scenarios were produced for the coastal United States. The assessment report acknowledged the uncertainties regarding the relationship between ocean warming, ice sheet and glacier loss, and SLR, and in doing so provided four different SLR projections, with final endpoints ranging from 0.2 to 2.0 m of coastal SLR by 2100. This range formed the basis for the SLR scenarios chosen for many subsequent investigations, including the present Saco and Casco model study. Some recent studies have acknowledged the uncertainties in the 2012 assessment, illustrating the benefits of analyzing the acceleration of flooding, which appeared to be a more precise calculation than measuring acceleration of SLR [3, 8]. These studies assumed zero acceleration of SLR, linearly generalizing the predicted rise to the entire Gulf of Maine.

The most recent modeling efforts of coastal responses to storms have largely focused on risk management and damage estimation under potential SLR scenarios, such as changes in land cover due to increased storm surge resulting from SLR [9]. Passeri et al. offered a good review of such studies looking at changes in coastal structure estimated from secondary SLR impacts, such as increased surge morphing the landscape in shallow areas [10]. The proposed structural impacts of SLR tie back into the efforts to estimate RSLR, as the generalized linear SLR projections did not account for changes in vertical land height or coastal slopes.

Looking specifically at the Saco and Casco domain, groups local to the region have been focusing on the global SLR projections, as RSLR projections, such as those for NYC and Louisiana, are not readily available. Peter Slovinsky of the Maine Geological Survey incorporated the global projections made by these earlier studies into a presentation for the 2015 State of the Bay Conference [11], in which he outlined the steps that coastal communities have been taking in anticipation of future SLR impacts, including ordinance changes, vulnerability assessments, coastal modeling efforts, public outreach, and infrastructure remodeling. He also pointed out how SLR trends in Portland, Maine, such as those discussed by Ezer and Atkinson [3], may indicate accelerated SLR over the past few decades, which would increase the 2100 SLR projections for Portland to be closer to the higher estimates offered by NOAA [12]. At present, focus continues to rest on risk mitigation and community actions in preparation for worst-case scenario future projections. The Saco and Casco storm response study was devised to support this continued effort through

the simulation of two major storm events: The Blizzard of 1978 and the Patriots Day storm in 2007.
