**4. Bay response to northeasters**

Responses in this study were defined as deviations from the typical circulation patterns seen during non-storm conditions. A storm window (see Section 2.3 above) was chosen for each storm event wherein anomalies were detected and collected for further analysis.

#### **4.1 Casco Bay**

Following the path of the storm winds, we first examine the surface currents entering the model domain from the northeast corner of the model's open boundary. **Figure 5** depicts frames of surface currents during flood and ebb tides prior to and within the1978 event's storm window. From this figure, we can see typical flooding and ebbing currents as strong flows in and out of the bay through the Broad Sound and the passage between the Peaks Island and Long Island. Outflow from the New Meadows River is visible in the upper reach of the estuary during ebb tides. As storm winds reached their peak magnitude, the surface current velocities in New Meadows River, measured at the sites of Lobo 1 and Lobo 2, increased sharply in the southward direction during ebb tides, increasing the reach of the New Meadows river plume into Casco Bay. The most apparent change was the

**Figure 5.**

*Frames of modeled surface currents in northern Casco Bay for the 1978 event before (top) and during (bottom) the storm window during flood (left) and ebb (right) tides. The yellow arrow in the top left panel indicates velocity scale.*

increased northward surface current during flood tides within the storm window, which flowed into the Broad Sound along the east coast of Chebeague Island, circulating counterclockwise around Cousins Island.

Continuing southward (**Figure 6**), the flood tide entered southern Casco Bay mostly through the passage between Long Island and Peaks Island, which circulated counterclockwise to enter Portland Harbor and keep the Fore River plume inside the estuary. During ebb, the Presumpscot River and Fore River plumes joined the outgoing tidal flows to form a strong southward current extending from Portland Harbor to south of Cape Elizabeth. Southward ebbing tidal currents were also strong in the passage between Long Island and Peaks Island. Albeit the flows were strengthened, the general patterns remained during the 1978 events except that the Presumpscot plume was more restricted during flood by the impeding tidal plus storm currents.

Briefly comparing the northern and southern halves of Casco Bay, the more open segment in the north, including Broad Sound and Maquoit Bay and Middle Bay, was less susceptible to storm forcing. The southern Casco Bay showed more noticeable storm responses in Portland Harbor, where the Presumpscot River and Fore River plumes were altered significantly by storm winds.

## **4.2 Saco Bay**

Surface currents increased sharply as they continued south of Casco Bay, colliding with the northern coastline of Cape Elizabeth (**Figure 7**). The increase in current velocity was most evident during ebb tides when storm currents and tidal currents aligned but was also visible during flood tides, overpowering the typical tidal currents. Water carried by the southwestward storm currents was directed clockwise around Cape Elizabeth to split to the north and south of Richmond Island. Even though only a small percentage of the water passed to the north of

*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*

**Figure 6.** *Similar to Figure 5 but for southern Casco Bay.*

**Figure 7.** *Similar to Figure 5 but for Saco Bay.*

Richmond Island, it was enough to cause a reversal in current velocities there compared to the prestorm flood and ebb tides.

Moving on to Saco Bay itself, under calm conditions, currents formed a clockwise circulation with slow northward flows nearshore and southward flows near the opening. Under storm conditions, circulation in Saco Bay was comprised of a complex relationship between storm winds, tidal currents, and freshwater plume dynamics. During flood tides, storm currents turning around Cape Elizabeth surged into the bay, generating a persistent southward flow along the Saco Bay shoreline. This southward flow exited the bay primarily through flooded areas in Biddeford Pool, with some merging back with the open-water southward storm currents via a small channel between Biddeford Pool and Wood Island. During ebb tides, the same southward coastal flow was present, but tidal currents increased the velocity of the Saco River and Nonesuch River plumes, which acted as partial barriers against the storm currents from Cape Elizabeth. As flooding in Biddeford Pool decreased, storm currents exiting the bay increased in the channel between Biddeford Pool and Wood Island.
