**3. The Saco and Casco model**

#### **3.1 Model setup**

The Saco-Casco model was an implementation of FVCOM that was developed to model complex coastal systems [5]. The finite-volume method takes the advantage of both the finite-element and finite-difference methods. It calculates the transport between elements by evaluating the integral form momentum and mass conservation equations along each element's boundaries [5, 19]. The three-dimensional unstructured grid is specified as the two-dimensional mesh coupled with the terrain-following layers in the sigma coordinate in the vertical. By performing calculations across an unstructured grid, FVCOM allows for high-resolution modeling along complex coastlines that would otherwise be difficult to accurately simulate [5].

The domain defined for the Saco and Casco model covers the coastal waters, including intertidal areas, from Kennebunkport in the south to Sebasco in the north in the Gulf of Maine (see the lower right inset in **Figure 1**). Saco Bay was discretized to the highest resolution of 10 m in areas shallower than 2 meters below the mean sea level, while equivalent depths in Casco Bay were set to 100-m resolution. Resolution in the rest of the domain was determined by depth, expanding to a maximum resolution along the open boundary to match that of NECOFS Gulf of Maine 3 (GOM3) mesh.

The 1/3 arc-second NOAA digital elevation model (DEM) for Portland, Maine [20], was used to specify the bathymetry for Saco Bay and Casco Bay. Through Aquaveo's Surface Modeling Software, the 10-m-resolution DEM was interpolated onto the unstructured triangular mesh developed for this study. Prior to interpolation, the DEM was converted from mean high water (MHW) to MSL to match the rest of the input data for the FVCOM model setup. Additional iterations of the Saco and Casco mesh were developed by integrating LiDAR bathymetry data from the NOAA digital coast system [21]. Specifically, the 2010 USACE NCMP Topobathy and 2014 USACE NAE Topobathy datasets were used, covering the Saco Bay coastline and Scarborough marsh with vertical accuracies of 20 and 10 cm and horizontal accuracies of 75 and 100 cm, respectively.

The Saco, Fore, Presumpscot, and New Meadows rivers were incorporated in the Saco and Casco model mesh. Two USGS gauges in the Saco and Casco domain were used for estimating discharge rates from rivers. Station 01064118 at Westbrook, Maine, for the Presumpscot River provides 15-minute discharge rates and gauge heights recorded from October 2016 and 2007 to present, respectively. Fifteenminute discharge rates and gauge heights for the Saco River are available from station 01066000 at Cornish, New Hampshire, from October 1989 and 2007 to present, respectively.

### *Linear and Nonlinear Responses to Northeasters Coupled with Sea Level Rise: A Tale of Two Bays DOI: http://dx.doi.org/10.5772/intechopen.87780*

Discharge rates for the 2007 event from station 01066000 were applied directly to the model's river forcing for the Saco River. Estimations of freshwater discharge had to be made in all other cases. Regressions were developed between gauge height and discharge rates for stations 01064118 and 01066000 using monthly datasets for February and April in years when both variables were available. Numerous iterations on these relationships were implemented, using a past study on the plume structure of Saco River [22] as a guide to adjust the regression coefficients. The results discussed herein reflect model simulations using the most stable freshwater discharge forcing, with the Saco and Nonesuch rivers using simplified discharge rates of 5.94 and 2.97\*Gh, respectively, where Gh is the observed gauge height in feet at site 01064118. For the Fore, Presumpscot, and New Meadows rivers, discharge rates were estimated at 2.97, 1.48, and 2.97\*Gh, respectively. Only gauge 01064118 was used for the final 1978 simulations, as the regressions built from gauge 01066000 indicated lower than reasonable estimations. For the 2007 event, Gh for these three rivers was also taken from site 1016600, as site 01064118 has no available data for April 2007 and there was no suitable proxy to capture the freshwater discharge event. As gauge heights for 1978 were unavailable, February 2017 observed gauge heights were used as a proxy, as a northeaster occurred at roughly the same time of the year in 2017 as in 1978.

At the time of writing, estimated hindcast discharge rates have been made available at site 01064118 from October 1975 to present and at site 01066000 from May 1916 to present. In comparing our estimated discharge rates to those presented by USGS, the same trends are depicted. Furthermore, the baseline (0 ft. SLR) model has since been rerun upon release of these datasets, which confirms that no noticeable changes are detected when using the modeled discharge vs. USGS predictions.

The Saco-Casco model was initialized and forced at the open boundary with hourly outputs from NECOFS hindcasts gom3\_197802.nc and gom3\_200704.nc (http://www.smast.umassd.edu:8080/thredds/catalog/models/fvcom/NECOFS/ Archive/Seaplan\_33\_Hindcast\_v1/catalog.html). The NECOFS, supported by the Northeastern Regional Association of Coastal and Ocean Observing Systems (NERACOOS) to complement the ocean observing system, is an FVCOM-based ocean model covering the domain between Long Island and Nova Scotia [23]. The NECOFS was configured using the third iteration of FVCOM coupled with the SWAN model, using the output from a larger-scale Weather Research and Forecasting (WRF) model for meteorological forcing. Data from the National Data Buoy Center buoys, NOAA C-MAN stations, river discharge statistics, and satellites were collected to support the development and testing of the NECOFS model. The NECOFS hindcasts used a mesh, labeled the GOM3, which has a peak resolution of 0.3–1.0 km in coastal areas, including the full Saco and Casco domain.
