**5.1 Impact of SLR on bay structure**

To quantify the impact modeled SLR had on the storm responses, a baseline understanding of how SLR impacted the shapes of Saco Bay and Casco Bay had to be established. As such, inundation maps were generated for the both storm cases under each SLR scenario, where "inundation zone" refers to the subset of the intertidal zone where bathymetric data indicated that the cell had a digital ground relief value above the MHW. **Figure 8** depicts such inundation zone coverage under the baseline (0 ft) and 7-ft. SLR scenarios.

Saco Bay was particularly vulnerable to flooding in response to SLR, specifically in the Scarborough marshes and around the mouth of Saco River. Every beach along *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 8.**

*Inundation maps for the Saco Bay and Casco Bay. Baseline scenario flooding (black) is overlaid by flooding measured in the modeled 7-ft. SLR scenario (black). Inundation cells were identified as the modeled intertidal zone above 0 m MHW.*

the bay was completely flooded by 7 ft. of SLR in both storm events, along with the marshes and communities around Goosefare Brook. In contrast, Casco Bay saw less change in inundation zone coverage (relative to the size of the bay) between the baseline and 7-ft. SLR scenarios, primarily isolated to the localized flooding around Portland, where storm-induced flooding spread most noticeably around the mouths of the Fore River and Presumpscot River. The trends of inundation zone expansion can be seen in **Figure 9**, along with the trends of each cell type (dry, wet, and intertidal) against SLR.

It was expected that the inland expansion of the intertidal zone during the 2007 event would mirror that of the 1978 event with a 1-foot "lag" in SLR scenario, as the peak sea level during the 2007 event was roughly 1 foot lower than that of the 1978 event. This lag is clearly visible in the inundation and dry cell trends in both Saco Bay and Casco Bay. Looking closer at the inundation and dry cells, both bays saw a net increase of roughly 20 km2 in inundation zone coverage from the baseline scenario to the 7-ft. SLR scenario, reflecting an identical drop in dry cell coverage. This 20-km2 change corresponded to an 18.2% reduction in Casco Bay's dry cell coverage versus a 57.1% reduction in Saco Bay's dry cell coverage. Furthermore, these reductions were not the result of continuously linear trends.

Casco Bay saw a linear drop in dry cell coverage from the baseline to 4-ft. SLR scenario for the 1978 event (baseline to 5-ft. SLR for the 2007 event), before

**Figure 9.**

*Cell-state distribution from the FVCOM wetting/drying module vs. SLR during the 1978 and 2007 events in Saco Bay and Casco Bay. Mesh limits were reached in the 7-ft. SLR scenario, causing the zonal distributions in both events to converge.*

dropping at a significantly higher rate until the mesh limitations were reached in the 6-ft. SLR scenario (7-ft. SLR for the 2007 event). This "drop off" point was a result of the peak sea level exceeding roughly 13 ft. above MSL, at which point many of the steep coastal slopes in Casco Bay, mainly around Portland Harbor, were overcome, yielding significantly increased flooding. In contrast, Saco Bay's inundation increased at a slightly exponential rate before slowing down following the 4-ft. SLR scenario (5-ft. SLR scenario for the 2007 event).

The intertidal and wet cells of each bay saw far more complex changes in response to SLR. In Casco Bay, there was a significant difference in behavior of the intertidal zone during the 1978 event when compared to the 2007 event. In the 1978 event, after an initial drop of ~5 km2 , the intertidal zone in Casco Bay saw very little change in size until the 5-ft. SLR scenario, at which point the intertidal zone decreased in size by roughly 5 km2 per 1 ft. of SLR. These drops in intertidal zone coverage were reflected by spikes in wet cell coverage in the 1-ft. SLR and 6-ft. SLR scenarios, resulting from low tides rising above 7.25 and 12.25 ft. above MSL, respectively. For the 2007 event, the wet zone expanded greatly between 2- and 3-ft. SLR, which was accompanied by a sharp decrease in the intertidal zone. The intertidal areas stayed mostly the same between 3- and 5-ft. SLR despite the slight increase of wet zone, which was compensated by the decrease of dry zone. However, between 5- and 7-ft. SLR, the intertidal area expanded largely at the expense of contracting dry zone.

This complex relationship can be better visualized in **Figure 10**. As Casco Bay's coastal slopes are largely characterized by short steps formed by tall shelves, the lower tidal ranges of the 2007 event resulted in low tides being constrained by these *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 10.**

*Sketch of intertidal zones of Casco Bay under the 5- and 7-ft. SLR scenarios. Due to larger tidal ranges in the 1978 event, there was a net loss in intertidal zone coverage, in contrast to a net gain between these scenarios for the 2007 event.*

stairs, limiting the change in wet cell coverage across SLR scenarios. In simulations of the 2007 event, the change in wet cell coverage plateaued after the 3-ft. SLR scenario, while dry cell coverage decreased steeply following the 5-ft. SLR scenario, yielding an overall increase in intertidal zone coverage between the 5- and 7-ft. SLR scenarios. In contrast, simulations of the 1978 event yielded far lower low tides, allowing wet cell coverage to increase following the 5-ft. SLR scenario, resulting in a decrease in intertidal zone coverage.

In Saco Bay, wet cell coverage simply increased linearly alongside SLR for the 1978 event, and the intertidal zone also expanded allowed by the much faster rate of decrease of the dry cell coverage. However, the behavior of the wet cell coverage was more dynamic during the 2007 event, largely explained by the relationship between freshwater discharge and sea level around the Scarborough marshes and Nonesuch River. Referring quickly back to the inundation maps (**Figure 8**), one key distinction between the 1978 and 2007 events was that even though the 2007 had lower peak sea level at Portland, the baseline scenario flooding around the Nonesuch River was higher during the 2007 event than that of the 1978 event, suggesting a positive relationship between discharge from the Nonesuch River and localized flooding along the river's edge. Another anomalous behavior occurred after the 4-ft. SLR scenario, where wet cell coverage in the 2007 event slightly decreased by ~1 km2 , contrary to any expected results. This small drop occurred in the Nonesuch River and is likely attributed to a decrease in minimum sea level in the Nonesuch River following an expansion of the channel between Prouts Neck and East Grand Beach during high tides. To explain further, to stabilize the FVCOM model, a limit of 1.5 m/s had to be placed on currents flowing along this channel, which resulted in elevated sea levels during low tide in the Scarborough marshes and Nonesuch River, as the water was unable to empty out from the marsh during ebb. Once the channel was widened following the 4-ft. SLR scenario, the total volume of water carried under the limited currents was increased enough to lower minimum local water level during low tide. The complexity of the relationship between SLR, estuarine dynamics, and intertidal zone structure highlighted by these results further underscores the limitations of generalized predictions on the effects of SLR on a coastline.

#### **5.2 Impact of SLR on bay circulation**

Given the dynamic changes SLR yielded on the structure of the two bays, it was reasonable to expect consequential changes in nearshore circulation. Looking first at the storm currents themselves, **Figure 11** depicts the rate of change of vertically

averaged mean current speed at points of interest for each storm across SLR scenarios. Temporal means of currents at all 24 sigma layers were taken within the storm windows and then averaged to produce the values reflected in these plots. Negligible changes to storm currents were witnessed in northern Casco Bay with the exception of a slight increase in slow storm currents at Buoy D0301 during the 2007 event (**Figure 11d**), so the other five chosen points of interest reflect impacts of SLR on storm currents affecting the four freshwater plumes in southern Casco Bay and Saco Bay.

Starting in Portland Harbor (**Figure 11a**), storm currents consistently increased alongside SLR in both storm events, albeit at different rates. The CAB 3 site was chosen to observe trends in both the Presumpscot River and Fore River plumes, as the southward flux of freshwater into the bay from Portland Harbor was located in this channel (**Figure 6**). The 1978 event, while yielding far less freshwater discharge than the 2007 event, saw greater southward storm currents at the CAB 3 site throughout the storm window due to extreme wind speeds. These currents initially decreased in response to the localized increase in flooding around Portland Harbor from the baseline to the 1-ft. SLR scenario, as was discussed earlier (**Figure 9**). Following this drop, as Casco Bay's coastline resisted additional flooding, storm currents began to increase with the higher volumes of water directed through this channel in higher SLR scenarios, though this effect was nonlinear and plateaued quickly. The storm currents at the CAB 3 site in the 2007 event saw a smaller, more linear rise alongside SLR, as storm currents were largely dominated by high discharge rates which remained constant in the SLR simulations.

Moving southward, the storm currents turning around Cape Elizabeth saw a proportionate rise in velocity across SLR (**Figure 11b**), pulling greater volumes of freshwater out of Portland Harbor. This increase in current speed was mostly linear and consistent from the 1- to 7-ft. SLR scenarios for the 1978 event, matching the linear rise from the 3- and 7-ft. scenarios in the April 2007 event. Further offshore to the southeast of Cape Elizabeth at the site of buoy 44007 (**Figure 11c**), the 1978 storm currents saw a more complex response to SLR, while the 2007 event saw no changes at all. The minor (<0.01 m/s) change in current speed from 0- to 4-ft. of SLR in the 1978 event was identified as a small response to the sudden drop in current speed from Portland Harbor following the initial flooding in southern Casco Bay. The increase in storm currents at site 44007 from 4- to 6-ft. of SLR resulted from an increase in southward currents between the barrier islands throughout Casco Bay. This rise was followed by a plateau effect as these islands began to flood, decreasing the effect of SLR on currents within the channels. Following the storm currents into Saco Bay, SLR had a much stronger effect on the dynamics of the Saco River (**Figure 11e**) and the Nonesuch/Scarborough River (**Figure 11f**).

Saco River behaved as expected as SLR increased. The sides of the river flooded rapidly as sea levels rose, resulting in drops in the current speed exiting the mouth of the river. Interestingly, during the low-discharge 1978 event, this drop was largely linear following a small initial spike of 0.01 cm/s, while the 2007 event saw an exponential decay in storm currents as SLR increased, suggesting a nonlinear relationship between river discharge and SLR as factors influencing estuarine storm currents. Nonesuch river, which is renamed to Scarborough River as it enters the Scarborough marshes along the western shore of Prouts Neck (see **Figure 1**), saw the most dynamic changes in response to SLR.

Prouts neck and the beaches around the mouth of the Scarborough River proved to be the most resilient land to flooding in Saco Bay, resulting in few changes to the structure of the river until SLR increased from 3 to 4 ft. for the 1978 event (4 to 5 ft. for the 2007 event). Because of this delayed response, water built up in the Scarborough marshes as SLR increased, negating any potential expected drop in *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 11.**

*Vertically averaged mean current speed vs. SLR within the storm windows at selected sites (see Figure 1 for locations) for the 1978 and 2007 events. Temporal averages throughout either storm window reflect the impact of SLR on storm-induced plume dynamics.*

current speeds in the 1978 event and resulting in an increase in current speeds aligning with heightened discharge in the 2007 event. Once these shores started to flood, current speed decreased rapidly with SLR, as the constriction point for discharge from the Nonesuch River widened greatly. To fully explain how these differences in storm current response to SLR impacted circulation in the bays, one must look at the resultant changes to plume dynamics following either storm.

**Figure 12** was created to show the change in minimum surface salinity (ΔS) between the baseline and 7-ft. SLR scenarios. By plotting minimum surface salinities, we were able to analyze the maximum reach of each river plume and how that reach was affected by SLR. In Casco Bay, the increase in mean storm currents exiting the Fore River and Presumpscot River resulted in further extensions of the combined Fore River and Presumpscot River plumes northeastward toward Broad Sound and southward around Cape Elizabeth for the 1978 event in the 7-ft. SLR simulation. For the 2007 event, flux out of these two rivers due to river discharge decreased dramatically with SLR, as the widened rivers allowed storm currents to dominate freshwater discharge. The end result was a net increase in salinities throughout the Portland Harbor area, as the offshore water was mixed higher up the rivers by storm winds under heightened SLR scenarios.

Saco Bay saw even greater variations in minimum salinity in response to SLR between the two storms, attributable mostly to the icing vs. flooding states of the Saco River and Nonesuch River. For the 1978 event, the inundation zones present in higher SLR scenarios were comprised primarily of offshore high-salinity waters, resulting in a net increase in salinity for the floodwater across the beaches of Saco Bay and large parts of Scarborough marshes except in the Nonesuch River plume. The resiliency of the modeled Nonesuch River was largely influenced in these simulations by mesh limitations; due to an instability issue with FVCOM, the mesh boundaries had to be restricted to 2 m above MSL around this river. Because of this limitation, the model likely underpredicted the full-range up-river mixing of higher-salinity waters into the Nonesuch River.

The stronger river discharge estimated for the April 2007 event resulted in plume water around Prouts Neck, more so in the higher SLR scenarios, as flooding allowed plume waters to flow southward to the eastern shore of Prouts Neck. Interestingly, despite the freshwater discharge from the Saco River being higher in

**Figure 12.**

*Map of changes in minimum salinity in response to SLR during the storm windows for the 1978 and 2007 events. Darker colors indicate a decrease in minimum salinity, implying a greater concentration of freshwater with the 7-ft. SLR than the baseline simulation.*

the 2007 event than in the 1978 event, the waters just north of Biddeford Pool and around Wood Island saw a large increase in minimum salinity as SLR increased. The reason for this change was the increased SLR resulted in a more northward shift of the Saco River plume that flooded around the mouth of Saco River and the beaches to the north, while the eastward current velocities directed toward Wood Island and Biddeford Pool decreased (**Figure 11e**), hence the higher minimum salinity for the 2007 event at 7-ft. SLR.
