Maps: Including Impact of Sea Level Rise

Journal of Marine Science and Engineering

Article Stormtools Design Elevation (SDE) Maps: Including Impact of Sea Level Rise

Malcolm L. Spaulding 1,*, Annette Grilli 1, Chris Damon 2 , Reza Hashemi 1, Soroush Kouhi 1 and Grover Fugate 3

1 Ocean Engineering, University of RI, Narragansett, RI 02882, USA; [email protected] (A.G.); [email protected] (R.H.); [email protected] (S.K.) 2 Environmental Data Center, University of RI, Kingston, RI 02881, USA; [email protected] 3 Rhode Island, Coastal Resources Management Council, South Kingstown, RI 02879, USA; [email protected] * Correspondence: [email protected]; Tel.: +401-782-1768

Received: 16 February 2020; Accepted: 14 April 2020; Published: 18 April 2020

Abstract: Many coastal communities in the US use base flood elevation (BFE) maps for the 100-year return period, specified on Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRMs), to design structures and infrastructure. The FIRMs are increasingly known to have serious problems in accurately specifying the risk coastal communities face, as most recently evidenced during hurricanes Harvey and Irma in 2017 and Florence and Michael in 2018. The FIRM BFE maps also do not include the impact of sea level rise, which clearly needs to be considered in the design of coastal structures over the next several decades given recent National Oceanic and Atmospheric Administration (NOAA) sea level rise (SLR) projections. Here, we generate alternative BFE maps (STORMTOOLS Design Elevation (SDE) maps) for coastal waters of Rhode Island (RI) using surge predictions from tropical and extratropical storms of the coupled surge-wave models from the US Army Corp of Engineers, North Atlantic Comprehensive Coast Study (NACCS). Wave predictions are based on application of a steady state, spectral wave model (STWAVE), while impacts of coastal erosion/accretion and changes of geomorphology are modeled using XBeach. The high-resolution application of XBeach to the southern RI shoreline has dramatically increased the ability to represent the details of dune erosion and overtopping and the associated development of surge channels and over-wash fans and the resulting landward impact on inundation and waves. All methods used were consistent with FEMA guidelines for the study area and used FEMA-approved models. Maps were generated for 0, 2 ft (0.6 m), 5 ft (1.5 m), 7 ft (2.1 m), and 10 ft (3.1 m) of sea level rise, reflecting NOAA high estimates at various times for the study area through 2100. Results of the simulations are shown for both the southern RI shoreline (South Coast) and Narragansett Bay, to facilitate communication of projected BFEs to the general public. The maps are hosted on the STORMTOOLS ESRI Hub to facilitate access to the data. They are also now part of the RI Coastal Resources Management Council (CRMC) risk-based permitting system. The user interface allows access to all supporting data including grade elevation, inundation depth, and wave crest heights as well as corresponding FEMA FIRM BFEs and associated zones.

Keywords: coastal flooding; inundation; and waves; flood insurance rate maps (FIRMs); coupled wave and surge modeling; base flood elevation

1. Introduction In most coastal states in the US, state and municipal building codes use the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRM) generated during Flood Insurance

J. Mar. Sci. Eng. 2020, 8, 292; doi:10.3390/jmse8040292 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2020, 8, 292 2 of 18

Studies (FIS) (see FIS for Washington County, RI, as an example [1]) to specify the flooding zones (and the associated estimates of inundation depths and wave heights used in the design of structures and infrastructure. The latter are normally expressed in the form of Base Flood Elevation (BFEs), relative to the North Atlantic Vertical Datum (NAVD88) datum, and represent the sum of the still water elevationStudies level (FIS) (SWEL) (see FIS for due Washington to 100-yr County, flooding RI, as plus an example the controlling [1]) to specify wave the flooding crest heightzones (Figure1). The controlling(and wavethe associated crest heightestimatesrepresents of inundation andepths estimate and wave of height the uppers used in end the design of the of wavestructures spectra and is and infrastructure. The latter are normally expressed in the form of Base Flood Elevation (BFEs), 1.12 times therelative significant to the North wave Atlanti heightc Vertical (Hs). Datum The ( FIRMSNAVD88) also vertical include datum, 500-yrand represent levels the (0.2% sum of annual the chance), but they representstill water flooding elevation only, level without(SWEL) due the to associated100-yr flooding wave plus environment.the controlling wave FEMA crest furtherheight delineates the flood-impacted(Figure 1) area. The incontrolling terms ofwave VE, crest AE, height and represents X zones. an The estimate VE zoneof the hasupper a waveend of the crest wave height greater spectra and is 1.12 times the significant wave height (Hs). The FIRMS also include 500-yr levels (0.2% the 3 ft (0.9 m),annual the chance), AE zone but they has represent crest heights flooding lessonly, thanwithout 3 ftthe (0.9 associated m), and wave the environment. X zone is FEMA where flooding occurs fromfurther the 500-yr delineates event, the flood but-impacted not the area 100-yr in terms event. of VE, The AE, AEand X zone zones. is The often VE zone divided has a wave in two sections, one where thecrest wave height crest greater height the 3 ft (0.9 is above m), the AE and zone the has other crest height belows less 1.5 than ft(0.45 3 ft (0.9 m). m), and The the dividing X zone line is the is where flooding occurs from the 500-yr event, but not the 100-yr event. The AE zone is often divided Limit of Moderatein two sections Wave, Actionone where (LIMWA) the wave crest and height the zoneis above between and the other 1.5 (0.45m) below 1.5 and ft (0.45 3 (0.9 m). The m) feet is often referred to asdividing the Coastal line is the A L zone.imit of M Anyoderate area Wave that Action can (LIMWA) be flooded and the during zone between the 100-yr 1.5 (0.45m) event and is defined as the Special Flood3 (0.9 m Hazard) feet is often Area referred (SFHA). to as the Coastal A zone. Any area that can be flooded during the 100- yr event is defined as the Special Flood Hazard Area (SFHA).

Figure 1. FEMA flood zone map definitions. Dashed red line is the 1% inundation level and the Figure 1. FEMA flood zone map definitions. Dashed red line is the 1% inundation level and the dashed dashed blue line is the base flood elevation (BFE). These can be referenced to as either grade elevation blue line is theor baseNAVD88 flood benchmarks. elevation Zone (BFE). definitions These are can provided be referenced at the top of to the as figure either (VE, grade AE, LIMWA, elevation and or NAVD88 benchmarks.X) Zone. definitions(http://www.r3coastal.com/home/coastal are provided at the top-hazard of- theanalysis figure-mapping/coastal (VE, AE,- LIMWA,flood-hazard and- X). (http: //www.r3coastal.commapping, /accessedhome/coastal-hazard-analysis-mapping on 11 February 2020). (Definition of /thecoastal-flood-hazard-mapping flood zones are provided at , accessed https://www.fema.gov/flood-zones, accessed on 16 April 2020.) on 11 February 2020). (Definition of the flood zones are provided at https://www.fema.gov/flood-zones, accessed on 16These April BFEs 2020.) are also included in the widely used in American Society of Civil Engineers ASCE 7–16 [2] (https://asce7hazardtool.online/, accessed on 16 October 2018), which sets the minimum These BFEsdesign areloads also for structures included, and in are the often widely required used or reference Americand in state Society and municipal of Civil building Engineers ASCE codes. Strong concerns have been voiced about the FEMA FIRMs being outdated and unreliable, 7–16 [2](https:particularly//asce7hazardtool.online after flooding in /,Texas accessed and onLoui 16sian Octobera from 2018),hurricanes which Harvey sets theand minimumIrma design loads for structures,(https://www.bloomberg.com/graphics/2017 and are often required or referenced-fema-faulty- inflood state-maps/ and, accessed municipal on 5 February building 2020 codes.). Strong concerns haveThese been were voiced highlighted about in a the recent FEMA review FIRMs by the Office being of outdated Inspector General and unreliable, (OIG, 2017) [ particularly3] and after earlier by an in-depth technical review by National Research Council (NRC) (2009) [4] of the methods flooding in Texasused in generating and Louisiana the maps. from While hurricanesthe FIRMs were Harvey explicitly anddeveloped Irma for ( https:setting //floodwww.bloomberg.com insurance / graphics/2017-fema-faulty-flood-mapsrates, their use for design is being increasingly/, accessed questioned on 5 February since they 2020). do not address These sea were level highlighted rise in a recent review(SLR) by, thewhich O ffiis projectedce of Inspector to be very General significant (OIG, (several 2017) meters) [3 ]over and the earlier next 100 by yrs an. Design in-depth of technical structures that don’t explicitly consider the risk from SLR put property owners, communities, review by National Research Council (NRC) (2009) [4] of the methods used in generating the maps. 2 While the FIRMs were explicitly developed for setting flood insurance rates, their use for design is being increasingly questioned since they do not address sea level rise (SLR), which is projected to be very significant (several meters) over the next 100 yrs. Design of structures that don’t explicitly consider the risk from SLR put property owners, communities, governmental entities, insurance companies, businesses, and financial institutions at unnecessary risk. The goal of this paper was to generate BFE maps (called STORMTOOLs Design Elevation (SDE) maps) which explicitly consider SLR for coastal communities in Rhode Island (RI) for both open coastal J. Mar. Sci. Eng. 2020, 8, 292 3 of 18 areas, where waves and erosion are important (southern coast), and for a protected bay (Narragansett Bay), where surge amplification dominates. The maps were developed for RI using methods that meet the FEMA guidelines [5] for coastal mapping in the region and employ models approved by FEMA [6] for use in such investigations. This strategy was used to facilitate their adoption by coastal states and communities. The methods used in generating the maps also addressed several of the fundamental weaknesses of the FEMA FIRMs for the study area [7,8]. These SDE maps are an integral building block in the RI Coastal Resources Management Council (CRMC) risk-based permitting system that resulted from adoption of the Beach Special Area Management Plan (SAMP) in July 2018. The CRMC has also recently integrated the STORMTOOLS Design Elevation (SDE) Maps into their online Coastal Hazard Application process, which is a coastal hazard, risk-based permitting tool. Given its maturity and widespread use in the state, STORMTOOLS, including the SDE, is currently being migrated to the ESRI Hub format.

2. Approach and Study Area The maps were developed under the STORMTOOLS initiative [9], whose goal is to provide access to a suite of coastal planning tools (numerical models, maps, data sets, etc.), available as a Geographic Information Systems (GIS)-based web service that allows widespread accessibly and applicability at high resolution for user-selected coastal and inland areas of interest. They are called STORMTOOLS Design Elevation (SDE) maps in recognition of their use to support the design of coastal structures and infrastructure. The maps are also an integral part of the STORMTOOLS Coastal Environmental Risk Index (CERI) [10–12] which assesses the risk that structures and infrastructure face from storm surges, including flooding and the associated wave environment, in the presence of sea level rise (SLR), and shoreline erosion/accretion, based on estimating damage to structures in the impacted area. The study area selected for the development of the SDE maps were the coastal communities of RI including both the southern RI shoreline (South Coast) and Narragansett Bay. These areas were selected for application given the relatively low-lying topography of the towns and the number of residential/commercial/municipal structures at risk from SLR and storm surge. They include all coastal areas in RI. Figure2 shows the density of structures at risk for 100-yr storm, plus 7 ft (2.1 m) of SLR for RI. The locations of the structures were based on the RI E911 emergency response database and partially verified with parcel-level data from the coastal towns. The Narragansett Bay and southern RI coastline study areas are outlined by the red and blue ovals, respectively. The division into open coastline and bay areas is in recognition of the fact that flooding dynamics are dominated by the amplification of surge levels with distance up the bay, with waves and shoreline erosion being secondary, while waves and shoreline erosion are critically important along the southern RI coast with very limited surge amplification.

2.1. The 100-Yr Water Levels Including Effects of SLR Following the approach presented in Spaulding et al. [9], the 100-yr water levels for storm surge for the study area were estimated using predictions of unstructured grid, coupled, hydrodynamic, and wave models (ADvanced CIRCulation model- STeady state spectral WAVE model/Wave Model ADCIRC- STWAVE/WAM) for 1050 synthetic tropical storms and 100 historical extratropical storms performed as part of the US Army Corp of Engineers, North Atlantic Comprehensive Coast Study (NACCS) [13]. The results of the simulations and return period analyses are archived at approximately 1000 save points principally located near the coast of the entire state. The water level for the upper 95% confidence limit (CL), for the surge plus tidal case, was employed to address uncertainty in the estimates. Spaulding et al., 2016 [9] provided additional details, including a comparison of the NACCS-based return period analysis to that based on historical observations at the National Oceanic and Atmospheric Administration, National Ocean Survey (NOAA NOS) water level stations at Newport (#8452660) and Providence (#8454000), RI [14]. The analysis showed that the surge water levels were approximately J. Mar. Sci. Eng.comparison2020, 8, 292 of the NACCS-based return period analysis to that based on historical observations at 4 of 18 the National Oceanic and Atmospheric Administration, National Ocean Survey (NOAA NOS) water level stations at Newport (#8452660) and Providence (#8454000), RI [14]. The analysis showed that linearly amplifiedthe surge with water distance levels were from approximately the mouth linearly to the amplified head with of Narragansett distance from the Bay mouth (factor to the of 1.35), but head of Narragansett Bay (factor of 1.35), but were approximately constant along the southern RI were approximatelyshoreline. constant along the southern RI shoreline.

Figure 2. DensityFigure of2. Density structures of structures inundated inundated with with 100-yr 100-yr storm storm surge surge plus plus 7 ft 7(2.1 ft m (2.1) sea m) level sea rise level (SLR rise) (SLR) for Narragansettfor Bay, Narragansett RI, and theBay, southern RI, and the coast southern of RIcoast based of RI onbased STORMTOOLS. on STORMTOOLS The. The red red and and blue ovals show ovals show the Narragansett Bay and southern RI shoreline study areas, respectively. The green oval the Narragansettshows Baythe location and southern of Charlestown RI shoreline, RI, and the study yellow areas,oval the respectively. towns of Barrington, The greenWarren ovaland shows the location of Charlestown,Bristol, RI, and the yellowRI. oval the towns ofstudy Barrington, Warren and Bristol,areas RI. study areas (http://(www.beachsamp.orghttp://www.beachsamp.org/wpcontent/uploads/2016/11/Map_SLR7_100YR_Edited_11_15.jpg/wpcontent/uploads/2016/11/Map_SLR7_100YR_Edited_11_15.jpg, , accessed, 26a Octoberccessed, 26 2017). October 2017).

In the interest of simplicity, it would be ideal if the surge water level in the presence of SLR could In the interestbe approximated of simplicity, by the simple it would addition be of ideal the SLR if value the surge to the 100 water-yr flood level levels in (without the presence SLR) (the of SLR could be approximatedso-called by linear the superposition simple addition method) of. To the evaluate SLR this value linear to superposition the 100-yr method flood, simulations levels (without SLR) (the so-calledwere linear performed superposition for a much higher method). resolution To evaluategrid, based thison the linear original superposition NACCS grid, but method, that more simulations accurately captured the nearshore bathymetry and topography in RI. The ADCIRC-SWAN were performed(Simulating for a WAves much Nearshore higher resolution) hydrodynamic grid,-wave based model on, used the in original this analysis NACCS, was the grid, same butas that more accurately capturedthat used thein the nearshore NACCS [13] bathymetry. It is described and in detail topography and its application in RI. The and validation ADCIRC-SWAN to selected (Simulating WAves Nearshore)wind-forced hydrodynamic-wave storm events in the RI study model, area are used presented in this in Torres analysis, et al., was2018 [1 the5]. Simulations same asthat used in were performed with this higher resolution model for NACCS tropical storm #492. This storm was the NACCS [13]. It is described in detail and its application and validation to selected wind-forced selected since it gave approximately the same water level as the 100-yr event (95% confidence limit) storm eventsat in the the NOAA RI study NOS areastations are at presentedNewport and in Providence Torres et, al.,RI, as 2018 the [estimated15]. Simulations values based were on performed with this higherobservations resolution. model for NACCS tropical storm #492. This storm was selected since it gave approximately the same water level as the 100-yr4 event (95% confidence limit) at the NOAA NOS stations at Newport and Providence, RI, as the estimated values based on observations. Figure3a shows the model-predicted water levels (WL) vs. time for the south coast of RI (Charlestown, RI) (green oval in Figure2) and at the Newport and Providence NOAA NOS stations. The amplification of the surge height, with distance up the bay, is clearly evident. Surge levels along the southern RI coastal line (not shown) are very close to the values at Charlestown, RI. Figure3b shows the NACCS-predicted water level vs. time at NACCS save point #8741 (point closest to Newport), the results of simulations with the higher resolution grid for the 100-yr event, the 100-yr simulation results with 6.6 ft (2 m) of SLR simply added (linear approximation), and results of the high-resolution simulation where sea level rise was accounted for by changing the original bathymetry used as input to the coupled hydrodynamic and wave model. The 2-m SLR case was selected for simulation since it was at the center of the range of SLR values of principal interest and was sufficiently large to clearly show the impact of nonlinearity on the model predictions. The latter case was based on the full simulation results and, hence, included any nonlinear (NL) interactions between surge and increased water depth J. Mar. Sci. Eng. 2020, 8, 292 5 of 18 due to SLR. Figure3c is a repeat of 2b but at the Providence station. Results of the linear and nonlinear simulation using the high-resolution model at the Charlestown, RI, location are shown in Figure3d. Table1 shows the maximum water level at Newport and Providence, RI, from NACCS storm #492 from the NACCS save point closest to these locations (Save Point #8741- Newport; #10395-Providence) based on the original simulations, as a result of the application of ADCIRC-SWAN high-resolution model without SLR, with 6.6 ft (2-m SLR) simply added (linear superposition), and with 6.6-ft (2-m) SLR added to the initial bathymetry and nonlinear(NL) simulations performed. The high-resolution (lower limit of 5 to 10 m) model was in excellent agreement with the original NACCS simulation for the peak water levels at the Newport and Providence locations. The linear (L) superposition method predicted slightly larger peak water levels than for the full nonlinear (NL) model, about 3.2% at Newport and 5.7% at Providence. Figure4 shows the spatial structure of the peak water level for the linear superposition method, the full nonlinear results, and the percent difference between the two for the 6.6-ft (2-m) SLR case. The percent difference between the two increased with distance up the bay. There was, in essence, no difference along the southern RI shoreline (Figure3c).

(a) (b)

Figure 3. (a) Water level vs. time for NACCS storm #492 at Providence, Newport, and Charlestown, Figure 3. (a) Water level vs. time for NACCS storm #492 at Providence, Newport, and Charlestown, RI; (b) ADCIRC-SWAN-predicted water level vs. time for NACCS storm #492 at Newport without RI; (b) ADCIRC-SWAN-predictedsea level rise (SLR), with 6.6 ft water(2 m) of levelSLR simply vs. timeadded foron, and NACCS with 2-m storm SLR added #492 to at the Newport initial without sea level risebathymetry (SLR), with and simulations 6.6 ft (2 m)run ofwith SLR the full simply model, added (c) same on, as in and (b) but with at the 2-m Providence SLR added station; to the initial bathymetryand and (d simulations) results of linear run (superposition) with the fulland nonlinear model, simulation (c) same at as Char inlestown. (b) but at the Providence station;

and (d) resultsTable of 1. linear Maximum (superposition) water level at Newport and nonlinear and Providence simulation from NACCS at Charlestown. storm #492 for the save point closest to these locations (NACCS save point #8741-Newport, #10395-Providence) based on the original simulations, as a result of the application of ADCIRC-SWAN high-resolution model (Torres et al., 2018) [15] without SLR, with 2-m SLR simply added, and with 2-m SLR added to the initial bathymetry and nonlinear simulations performed.

Max Water Elevation (m) NACCS at Save ADCIRC-SWAN ADCIRC_SWAN + 2m ADCIRC_SWAN Non-Linear Point w/o SLR SLR with 2m SLR 8741/10395

Newport 3.43 3.38 5.38 5.21 Providence 5.46 5.48 7.48 7.05

J. Mar. Sci. Eng. 2020, 8, 292 6 of 18

Table 1. Maximum water level at Newport and Providence from NACCS storm #492 for the save point closest to these locations (NACCS save point #8741-Newport, #10395-Providence) based on the original simulations, as a result of the application of ADCIRC-SWAN high-resolution model (Torres et al., 2018) [15] without SLR, with 2-m SLR simply added, and with 2-m SLR added to the initial bathymetry and nonlinear simulations performed.

Max Water Elevation (m) NACCS at Save Point ADCIRC-SWAN ADCIRC_SWAN ADCIRC_SWAN 8741/10395 w/o SLR + 2m SLR Non-Linear with 2m SLR Newport 3.43 3.38 5.38 5.21 Providence 5.46 5.48 7.48 7.05

(a) (b) (c)

Figure 4. PeakFigure water 4. Peak level water for level NACCS for NACCS storm storm #492 #492 forfor Narragansett Narragansett Bay for Bay (a) forlinear (a sup) linearerposition superposition method, (b) full nonlinear simulation results, and (c) percent (%) difference between the two for 6.6- method, (b) fullft (2- nonlinearm) SLR. simulation results, and (c) percent (%) difference between the two for 6.6-ft (2-m) SLR. These simulations show that the amplification factor decreased slightly with distance up the bay These simulations(Figure 4a,b), showwith no that change the amplificationalong the southern factor RI shoreline. decreased In the slightly interest with of developing distance up the bay conservative design elevation maps, the SLR value of interest was, therefore, simply added to the (Figure4a,b),100 with-yr nosurge. change along the southern RI shoreline. In the interest of developing conservative design elevation maps, the SLR value of interest was, therefore, simply added to the 100-yr surge. 2.2. Waves Including Effects of SLR 2.2. Waves IncludingThe analysis Effects ofof SLRwave heights (and the associated impact of coastal erosion) necessary to determine BFEs was performed using two different approaches. For Narragansett Bay, where waves The analysisare quite of wavelimited,heights wave heights (and were the estimated associated by applying impact STWAVE of coastal [16,17] erosion) for a grid necessary covering the to determine BFEs was performedbay area. For using the southern two di RIff sherentoreline approaches., where waves and For coastal Narragansett erosion are important Bay, where, simulations waves are quite were performed using the geomorphological model XBeach to determine the eroded dune profiles limited, wave heights were estimated by applying STWAVE [16,17] for a grid covering the bay area. [18]. The profile was then used to generate an eroded digital elevation model (DEM), which was then For the southernused as RI input shoreline, to STWAVE where for simulations waves of and 100 coastal-yr wave conditions, erosion arewith important,and without SLR. simulations The were performed usingmethod thes for geomorphological each location are provided model in more XBeach detail below. to determine the eroded dune profiles [18].

The proﬁle was2.3. Narragansett then used Bay to generate an eroded digital elevation model (DEM), which was then used as input to STWAVE for simulations of 100-yr wave conditions, with and without SLR. The methods Waves heights were estimated in Narragansett Bay for extreme storm events associated with an for each locationannual are probability provided of exceedance in more of detail 1% (100 below.-year storm) using the Steady State Spectral Wave model (STWAVE), a phase-averaged wave model [16,17]. STWAVE was forced along the open boundary to 2.3. Narragansettthe south Bay (RI Sound) with significant wave heights of 33 ft (10 m), a period of 20 sec, and a direction of 165 degrees referenced to north (N). These values were extracted from the NACCS save point wave Waves heightsdatabase werelocated estimatedat the mouth inof Narragansettthe bay. Wind forcing Bay of for 35 extremem/sec (80 mph) storm (100 events yr) was associatedassumed with an annual probabilityaligned with of exceedance the central axis of of 1% the (100-yearlower bay at storm)180 degrees using to maximize the Steady the fetch State along Spectral the lower Wave model east and west passages. STWAVE predictions employed a 15-m grid to optimize accuracy and (STWAVE), acomputational phase-averaged efficiency wave and mapped model on [16 a, 1716.4].-ft STWAVE (5-m) grid. wasThe DEM forced used along the NOAA the National open boundary to the south (RIOcean Sound) Survey with bathymetry signiﬁcant merged wave with 2011 heights Laser ofImaging, 33 ft Detection, (10 m), and a periodRanging ( ofLIDAR 20 sec,) data and and a direction of 165 degreeswas referencedavailable via RI to Geographic north (N). Information These System values (RI were GIS) (http://www.rigis.org/data/topo/2011, extracted from the NACCS save point wave databaseaccessed located on 16 atJanuary the mouth2017). The of DEM the ha bay.d a horizontal Wind forcingresolution ofof 33 35 ft m(10/ secm) across (80 mph)the ocean (100 yr) was and 3.3 ft (1 m) on land. The vertical accuracy on land was approximately 0.5 ft (15 cm), based on a assumed aligned with the central axis of the lower bay at 180 degrees to maximize the fetch along 7

J. Mar. Sci. Eng. 2020, 8, 292 7 of 18 the lower east and west passages. STWAVE predictions employed a 15-m grid to optimize accuracy and computational eﬃciency and mapped on a 16.4-ft (5-m) grid. The DEM used the NOAA National Ocean Survey bathymetry merged with 2011 laser imaging, detection, and ranging (LIDAR) data and was available via RI Geographic Information System (RI GIS) (http://www.rigis.org/data/topo/2011, accessed on 16 January 2017). The DEM had a horizontal resolution of 33 ft (10 m) across the ocean and 3.3 ft (1 m) on land. The vertical accuracy on land was approximately 0.5 ft (15 cm), based on a comparison to the blind control points used during the 2011 LIDAR survey and subsequent evaluations of the DEM using certiﬁcates of elevation made available for selected towns in the state. Simulations were performed for a no sea level rise as well as for four SLR scenarios: 0, 0.6, 1.5, 2.1 m, and 3.04 m (0, 3, 5, 7, and 10 ft). These SLR values spanned the likely range for RI (Newport and Providence) based on NOAA high estimates from mid-century through 2100 for the bay. They are available via the US Army Corp of Engineers (USACE) SLR calculator (http://corpsmapu.usace.army.mil/rccinfo/slc/slcc_calc.html, accessed on 18 October 2018). NOAA high estimates were selected by RI Coastal Resources Management Council to specify the SLR values for their newly adopted risk-based permitting system.

2.4. Southern RI Shoreline (South Coast) Schambach et al., 2017 [18] applied the geomorphological model XBeach to a section of the southern RI shoreline in Charlestown, RI (see Figure2 for location, green oval). XBeach is a fully coupled hydrodynamic (wave action and horizontal mean flow) and morpho-dynamics (change in geomorphology and sediment transport) model that dynamically simulates processes that occur during four erosion regimes, as defined by Sallenger [19] (swash, collision, over-wash, and inundation). A detailed description of the model is provided in Harter and Figlus [20]. Additional details on XBeach applications to the general study area and detailed sensitivity studies to the initial conditions, seasonal variabilities in storms, and sea level rise are provided in [21,22]. The model was applied to the Charlestown, RI, study area (approximately 6.2 miles (10 km) along shore and 4.4 miles (7 km) cross shore) including the dune and adjacent coastal pond using a nominal grid resolution of 33 ft (10 m). The bathymetry/topography was provided using a merge dataset which included NOAA bathymetric data and 2011 Northeast LIDAR data for the land area, described above. The land cover vegetation was obtained from RI GIS database. The offshore boundary condition was specified in terms of water level and wave conditions during the event of interest. The lateral boundaries assumed no flow and no along shore pressure gradient. The model was applied and validated to tropical storm Irene, which impacted RI on 11 August 2011. Time series of water levels and waves during the storm were generated by Torres et al., 2018 [15] using the high resolution ADCIRC-SWAN models described above. The model was calibrated using the wave asymmetry and skewness parameter and resulted in a 6% mean relative error between simulated and measured subaerial eroded volumes from four cross-transects (beach profiles) where measurements were made shortly (within several days) before and after the storm. The model was then applied to predict the shoreline erosion for the 100-yr storm event. The forcing for the event was based on the NACCS database with simulations performed using NAACS storm #457 as the 100-yr synthetic design storm (SDS). This SDS was selected based on the similitude of the return period of its water level and wave elevation to the NACCS-estimated 100-yr significant wave height and surge elevations, at the closest NACCS save points from the grid offshore boundary. The predicted median eroded volume was 46 m3/m for the entire barrier beach, in good agreement with FEMA’s estimates, based on their protocol [6] of 50 m3/m in their most recent Washington County FIS [1]. Model performance showed similar mean post-storm reductions in dune crest heights to those generated by FEMA. The model showed large spatial variations in along shore eroded volumes, with the highest values where breaching and surge channels developed and the lowest where healthy back dune vegetation was present. Model results were broadly consistent with the databased approach performed as part of an earlier study to estimate storm damage to structures in the study area under the Coastal Environmental Risk Index (CERI) initiative [10]. J. Mar. Sci. Eng. 2020, 8, 292 8 of 18

In the current study, the method outlined by Schambach et al., 2018 [18] was applied to the entire southern RI coastline (Figure2, blue circle). Time series of water level and waves, necessary as input on the offshore boundary, were similarly based on the 100-yr SDS. Values of water elevations and wave spectral parameters were extracted at many NAACS save points to accurately define the energy spectra placed at the offshore boundary conditions of the computational grid. Four separate domains were used to accurately represent the study area. A JOint North Sea WAve Project (JONSWAP) wave energy spectrum was employed with a significant wave height varying between 23 to 29.5 ft (7 to 9 m) from Napatree Point, Westerly, to Point Judith, South Kingstown, RI, to capture the sheltering effect of Block Island on the southwestern RI shoreline. The model-predicted changes in bathymetry and topography were then used to generate a revised (eroded) digital elevation model (DEM) that reflected the 100-yr storm event. The STWAVE model was then used,were employing used to accurately the revised represent DEM, the study to simulatearea. A JOint the North wave Sea WAve conditions Project (JONSWAP in the flood) wave inundated area. energy spectrum was employed with a significant wave height varying between 23 to 29.5 ft (7 to 9 The offshore surge level boundary conditions for STWAVE were provided by simulations of the 100-yr m) from Napatree Point, Westerly, to Point Judith, South Kingstown, RI, to capture the sheltering storm eventeffect as of noted Block Island above. on the southwestern RI shoreline. The model-predicted changes in bathymetry and topography were then used to generate a 3. Resultsrevised (eroded) digital elevation model (DEM) that reflected the 100-yr storm event. The STWAVE model was then used, employing the revised DEM, to simulate the wave conditions in the flood The SDEinundated maps area. (BFE The maps offshore with surge SLR) level are boundary shown conditions separately for forSTWAVE the Narragansett were provided Bay by and southern RI shorelinesimulations (South of Coast) the 100- areas.yr storm event as noted above. The division3. Results into two separate viewers was done for convenience in organizing the results and based on theThe diff SDEerent maps methods (BFE maps used within SLR) their are generation,shown separately as for described the Narragansett above. Bay Each and is presented separatelysouthern below. RI shoreline (South Coast) areas. The division into two separate viewers was done for convenience in organizing the results and 3.1. SDE Mapsbased foron the South different Coast, methods Southern used RIin Shorelinetheir generation, as described above. Each is presented separately below. Figure5 shows the user interface for the SDE maps for the southern RI shoreline. The map selected 3.1. SDE Maps for South Coast, Southern RI Shoreline in this case was 100-yr surge with no SLR (highlighted blue tab at the top left of the interface). BFE maps for thisFigure caseare 5 shows shown the inuser the interface figure, for as the is SDE the maps legend for the to thesouthern left. RI All shoreline. water levelsThe map are provided in selected in this case was 100-yr surge with no SLR (highlighted blue tab at the top left of the interface). ft, referencedBFE maps to NAVD88. for this case Theare shown highest in the water figure, as levels is the legend in this to casethe left were. All water immediately levels are provided along the shoreline (about 30in ft) ft and, referenced decreased to NAVD88. with distanceThe highest inland, water levels as o inff shorethis case waves were immediate are dissipatedly along the by breaking and friction. Mostshoreline of the (about wave 30 ft dissipation) and decrease occurredd with distance immediately inland, as offshore waves of the are coastaldissipated dune by system. BFE breaking and friction. Most of the wave dissipation occurred immediately offshore of the coastal dune heights decreasedsystem. BFE rapidly heights with decrease distanced rapidly landward with dista asnce alandward result of as wave a result breaking of wave breaking and frictional and dissipation. Frictional dissipationfrictional dissipation. was highest Frictional in dissipation the presence was highest of dune in the vegetationpresence of du andne vegetation more limited and more in its absence. limited in its absence.

Figure 5. User interface for the STORMTOOLS Design Elevation (SDE) Maps, southern RI coast line Figure 5. User(South interfaceCoast), no SLR for. The the case STORMTOOLS selected was the 100 Design-yr event Elevation with no SLR (SDE). Blue star Maps,, Napatree southern Point, RI coast line (South Coast),Westerly no, SLR.and red The star case, Point selected Judith, South was theKingstown 100-yr. The event units with on the no tab SLR. are Bluein ft of star, SLR Napatree. Point, Westerly, and(https://crc red star,- Point Judith, South Kingstown. The units on the tab are in ft of SLR. (https://crc- uri.maps.arcgis.com/apps/MapSeries/index.html?appid=3ba5c4d9c0744392bec2f4afb6ee22 uri.maps.arcgis.com/apps/MapSeries/index.html?appid=3ba5c4d9c0744392bec2f4afb6ee228. Accessed 8. Accessed on 16 October 2018). on 16 October 2018). 9

J. Mar. Sci. Eng. 2020, 8, 292 9 of 18

Figure6 shows SDE maps for 0, 5 ft (1.5 m), and 10 ft (3 m) of SLR. Maps are available for 2 (0.6 m) and 7 (2.13 m) ft, but not shown here. These maps were assembled from individual images from the map viewer. In this case, the background map was based on the topographic map used in Figure5. Background maps in the BFE map interface were user selected with six options (imagery, imagery with labels, streets, topography, and dark and light gray). The 100-yr ﬂooding map showed the expected increase in BFE due to SLR. The BFEs for the SLR cases reﬂected the increase due to the increase in surge depth plus the increase in the controlling wave height given the greater underlying surge depth.

Figure 6. Impact of SLR on Narragansett Bay SDE maps, cases shown are 0, 5 ft, and 10 ft (0, 1.5, and Figure 6. Impact3 m) of (upper, SLR middle, on Narragansett and lower panels, Bay respectively, SDE maps, note cases highlighted shown tab) are SLR. 0, 5(Note ft, andthat the 10 ft (0, 1.5, and 3 m) (upper,background middle, map and in these lower figures panels, used the respectively, topographic map note shown highlighted earlier. Background tab) SLR. maps (Note were that the background mapuser inselected.) these. ﬁgures used the topographic map shown earlier. Background maps were

user selected.).To help gain insight into the maps, Figure 7 shows the surge inundation levels (upper panel) and associated wave heights (lower panel) for the no-SLR case. The sum of the two is the BFE shown To help gainin Figure insight 5. into the maps, Figure7 shows the surge inundation levels (upper panel) and associated wave heights (lower panel) for the no-SLR case. The sum of the two is the BFE shown in Figure5.

J. Mar. Sci. Eng. 2020, 8, 292 10 of 18

Figure 7. Surge inundation levels (relative to NAVD88) (upper panel) and the wave crest height (lower Figure 7. Surge inundation levels (relative to NAVD88) (upper panel) and the wave crest height panel) for southern(lower panel) coast for southern of RI forcoast 100-yr, of RI for no-SLR100-yr, no case.-SLR case.

The ﬁgureThe shows figure shows that thethat surgethe surge levels levels (upper panel) panel) were were approximately approximately constant (14 constant ft, 4.3 m) (14 ft, 4.3 m) along the coastalong the and coast increased and increase slightlyd slightly with with distance inland. inland. The lat Theter was latter due wasto local due wind to forc localing wind forcing over the flood inundated area. The lower panel shows the predicted wave heights for the same area. over the ﬂoodThe waves inundated were quite area. large The(14 tolower 16 ft, 3.65 panel to 4.3 m) shows immediately the predicted along the coast wave and heightsdecreased forvery the same area. The wavesrapidly were quitewith distance large inland (14 to, as 16 the ft, waves 3.65 topropagate 4.3 m)d immediatelyover the dunes. Locations along the of breaches coast and in the decreased very rapidly withdune distance and the associated inland, over as the-wash waves fans into propagated the adjacent ponds over were the c dunes.learly evident. Locations Wave heights of breaches in the on the backside of the coastal ponds were generally quite low (1.5 ft, 0.5 m) if the dunes remained dune and theintact. associated If the dunes over-wash were over-washed fans,into the wave the heights adjacent could ponds increase were to 6 ft clearly (1.8 m). evident. Wave heights on the backside ofThe the user coastal had the ponds ability wereto zoom generally into an area quite of interest low, (1.5either ft, by 0.5 using m) the if thezoom dunes feature remainedor intact. If the dunesentering were the over-washed, street address or the latitude wave-longitude heights of the could location increase of interest. to Figure 6 ft (1.8 8 shows m). the 100-yr, no-SLR case for the Charlestown, RI, area, with Ninigret and Green Hill Ponds as the focus area. The The usermap hadshows the the abilityfloodingto levels zoom relative into to an NAVD88, area of while interest, the insert either highlights by using the ability the zoom to feature or entering theinte streetrrogate address the map and or determine latitude-longitude the BFE at a location of the of location interest; in of this interest. case the FigureBFE was8 17. shows0 ft the 100-yr, no-SLR case(5.2 for m) the. Charlestown, RI, area, with Ninigret and Green Hill Ponds as the focus area. The map shows the flooding levels relative to NAVD88, while the insert highlights the ability to interrogate the map and determine the BFE at a location of interest; in this case the BFE was 17.0 ft (5.2 m).

Figure 8. Flooding in Charlestown, RI, (see green oval in Figure2) in the vicinity of Ninigret and Green Hill PondsFigure for8. Flooding 100-yr, in no-SLR Charlestown, case. RI The, (see insert green oval shows in Figure the ability 2) in the of vicinity the point of Ninigret interrogation and tool to Green Hill Ponds for 100-yr, no-SLR case. The insert shows the ability of the point interrogation tool determine theto determine BFE at the the userBFE at location the user location selected. selected.

The interface also allowed the user to compare and contrast the FEMA FIRMs and SDE maps. As an example, selecting the same area in Charlestown, RI, as shown in Figure 8 (see green oval in Figure 2 for its location along the coast), Figure 8 shows the SDE BFE map for this area in the upper panel and the FEMA FIRM BFE map in the lower panel. The FEMA BFE values were typically 0.9 to 1.3 m (3 to 4 ft) lower than the SDE maps. The FEMA maps also lacked the level of detail provided in the SDE maps. The unusual anomaly in the FEMA map (high water level) for Green Hill Pond was a result of the 1-D transect method used to generate the wave heights and its subsequent spatial interpolation to a 2-D map. (FEMA transect lines 20 and 21 shown in the figure. Results must be interpolated between these two transect lines.) A detailed presentation and discussion on the sources of the differences are provided in Spaulding et al., 2017 [7]. To highlight the utility of the interrogation tool, Table 2 provides a comparison of the SDE (no- SLR) to FEMA BFE and to the SDE estimates for various SLR values at the location of the Charlestown Breachway Bridge (yellow dot in Figure 8). SDE value (15.6 ft (4.75 m) = 13.5 ft surge and 0.8 ft wave) was predicted to be 3.6 ft (1.1 m) higher than FEMA estimate with no-SLR (12 ft, 3.65 m). (Note: FEMA maps round to the nearest whole number.) The impact of SLR raises the BFE by simply offsetting the flood inundation level but by also by increasing the wave height (e.g., for 10 ft (3.05 m) of SLR case water level was increased to 23.1 ft (7 m) and the wave height to 6.1 ft (2 m)). The values for other SLR cases are provided Table 2.

J. Mar. Sci. Eng. 2020, 8, 292 11 of 18

The interface also allowed the user to compare and contrast the FEMA FIRMs and SDE maps. As an example, selecting the same area in Charlestown, RI, as shown in Figure8 (see green oval in Figure2 for its location along the coast), Figure9 shows the SDE BFE map for this area in the upper panel and the FEMA FIRM BFE map in the lower panel. The FEMA BFE values were typically 0.9 to 1.3 m (3 to 4 ft) lower than the SDE maps. The FEMA maps also lacked the level of detail provided in the SDE maps. The unusual anomaly in the FEMA map (high water level) for Green Hill Pond was a result of the 1-D transect method used to generate the wave heights and its subsequent spatial interpolation to a 2-D map. (FEMA transect lines 20 and 21 shown in the figure. Results must be interpolated between these two transect lines.) A detailed presentation and discussion on the sources of the differences are provided in Spaulding et al., 2017 [7]. To highlight the utility of the interrogation tool, Table2 provides a comparison of the SDE (no-SLR) to FEMA BFE and to the SDE estimates for various SLR values at the location of the Charlestown Breachway Bridge (yellow dot in Figure9). SDE value (15.6 ft (4.75 m) = 13.5 ft surge and 0.8 ft wave) was predicted to be 3.6 ft (1.1 m) higher than FEMA estimate with no-SLR (12 ft, 3.65 m). (Note: FEMA maps round to the nearest whole number.) The impact of SLR raises the BFE by simply offsetting the flood inundation level but by also by increasing the wave height (e.g., for 10 ft (3.05 m) of SLR case water level was increased to 23.1 ft (7 m) and the wave height to 6.1 ft (2 m)). The values for other SLR cases are provided Table2.

Figure 9. Comparison of SDE (upper panel) vs. FEMA FIRM (lower panel) BFE maps for the 100-yr, Figure 9. Comparisonno-SLR case. ofYellow SDE dot (upper indicates panel) the location vs. of FEMA the comparative FIRM (lower analysis panel)shown in BFE Table maps 2. for the 100-yr, no-SLR case. Yellow dot indicates the location of the comparative analysis shown in Table2. Table 2. Comparison of surge depth, wave height, and SDE values (ft, m) at two selected locations vs. In addition1% to water the informationlevel with sea level presented rise (SLR). here (BFEs from the SDE maps, with varying SLR and FEMA FIRMs),Location: we also includedRte 103 Bridge the option to access maps of FEMA-designatedCharlestown Breachway zones Bridge (V, A, LIMWA, and SFHA) (Figure 1) forWarren, both FEMA RI FIRMs and SDE maps, transectsCharlestown, along RI which FEMA made wave estimates, the variousSurge*components Wave ofBFE* the SDEBFE* BFE includingSurge* bothWave surge BFE* and waveBFE* crest height (ft) Height (ft) (m) (ft) Height (ft) (m) (see example in Table2), and SDE-based(ft) water depths relative to local(ft) grade (this metric, depth of inundation,FEMA is FIRM diﬀerent than BFEs, which are12 typically3.66 referenced to NAVD88). 14 4.27 SDE 0 SLR 14.3 1.6 15.9 4.85 13.4 0.6 14.0 4.27 SDE 2 SLR 16.5 1.8 18.3 5.58 15.5 1.7 17.2 5.24 SDE 3 SLR 17.7 1.2 18.9 5.76 16.4 2.1 18.5 5.64 SDE 5 SLR 18.9 2.2 21.1 6.43 18.4 3.3 21.7 6.61 SDE 7 SLR 20.7 2.6 23.3 7.1 20.5 4.5 25.0 7.62 SDE 10 SLR 23.9 3.0 26.9 8.2 23.4 6.1 29.5 8.99 * Referenced to NAVD88.

In addition to the information presented here (BFEs from the SDE maps, with varying SLR and FEMA FIRMs), we also included the option to access maps of FEMA-designated zones (V, A, LIMWA, and SFHA)(Figure 1) for both FEMA FIRMs and SDE maps, transects along which FEMA made wave estimates, the various components of the SDE BFE including both surge and wave crest height (see

J. Mar. Sci. Eng. 2020, 8, 292 12 of 18

Table 2. Comparison of surge depth, wave height, and SDE values (ft, m) at two selected locations vs. 1% water level with sea level rise (SLR).

Location: Rte 103 Bridge Charlestown Breachway Bridge Warren, RI Charlestown, RI Surge* Wave BFE* BFE* Surge* Wave BFE* BFE* (ft) Height (ft) (m) (ft) Height (ft) (m) (ft) (ft) FEMA FIRM 12 3.66 14 4.27 SDE 0 SLR 14.3 1.6 15.9 4.85 13.4 0.6 14.0 4.27 SDE 2 SLR 16.5 1.8 18.3 5.58 15.5 1.7 17.2 5.24 SDE 3 SLR 17.7 1.2 18.9 5.76 16.4 2.1 18.5 5.64 SDE 5 SLR 18.9 2.2 21.1 6.43 18.4 3.3 21.7 6.61 SDE 7 SLR 20.7 2.6 23.3 7.1 20.5 4.5 25.0 7.62 SDE 10 SLR 23.9 3.0 26.9 8.2 23.4 6.1 29.5 8.99 * Referenced to NAVD88.

3.2. SDE Maps for Narragansett Bay

Providedexample below in isTable a similar 2), and overviewSDE-based water of the depths Narragansett relative to local Bay grade SDE (th mapsis metric that, depth is comparable of to that above forinundation, the South is different Coast. than BFEs, which are typically referenced to NAVD88). Figure 10 shows the user interface for the SDE maps for Narragansett Bay. The map selected in this 3.2. SDE Maps for Narragansett Bay case was 100-yr surge with no SLR (highlighted blue tab at the top left of the interface). BFE maps for Provided below is a similar overview of the Narragansett Bay SDE maps that is comparable to this case are shownthat above in for the the figure, South C asoast. is the legend to left. All water levels are provided in feet, referenced to NAVD88. TheFigure highest 10 shows water the user levels interface in this for the case SDE were maps found for Narragansett at the upper Bay. The end map ofselected Narragansett in Bay and were a resultthis case of was the 100 amplification-yr surge with no of SLR the (highlighted surge height blue tab with at thedistance top left of the up interface). the bay BFE (see maps Figure 4 as an for this case are shown in the figure, as is the legend to left. All water levels are provided in feet, example) [9].referenced Wave heights to NAVD88. in the The bay highest are generally water level quites in this limited, case we givenre found the at protectedthe upper end nature of of the bay and limited fetchNarragansett distances. Bay and The were large a result waves of the atamplification the mouth of the of thesurge bay height propagating with distance fromup the obayffshore (29.5 ft (9 m) significant(see Figure wave 4 height,as an example) 20-sec [9] peak. Wave period) heights in decrease the bay are quite generally rapidly quite in limited magnitude, given the as they enter protected nature of the bay and limited fetch distances. The large waves at the mouth of the bay the east and west passages of lower Narragansett Bay. This transition from large offshore waves to propagating from offshore (29.5 ft (9 m) significant wave height, 20-sec peak period) decrease quite much smallerrapid fetch-limitedly in magnitude local as they wind-generated enter the east and waves west passages was included of lower Narragansett in the STWAVE Bay. This methodology used for modelingtransition waves from large for offshore the SDE waves maps. to much smaller fetch-limited local wind-generated waves was included in the STWAVE methodology used for modeling waves for the SDE maps.

Figure 10. User interface for the STORMTOOLS Design Elevation Maps, Narragansett Bay. The case Figure 10. User interface for the STORMTOOLS Design Elevation Maps, Narragansett Bay. The case selected was the 100-yr event with no SLR. (https://crc- selected wasuri.maps.arcgis.com/apps/MapSeries/index.html?appid=9b85db9b7aaa400cac1a3cb404a96 the 100-yr event with no SLR. (https://crc-uri.maps.arcgis.com/apps/MapSeries/index. html?appid=be89b85db9b7aaa400cac1a3cb404a96be8, accessed on 16 October 2018). , accessed on 16 October 2018).

Figure 11 shows SDE maps for 0, 5 (1.52 m) ft, and 10 ft (3.05 m) of SLR. Maps are available for 2 (0.61 m) and 7 (2.13 m) ft, but not shown here. These maps were assembled from individual images from the map viewer. The 100-yr flooding maps showed the expected increase in BFE due to SLR. The BFEs for the SLR cases reflected the increase, due to the increase in surge depth plus the increase in the controlling wave height given the greater underlying surge depth. The wave contribution was generally quite small in the bay, given the small wave heights.

J. Mar. Sci. Eng. 2020, 8, 292 13 of 18

Figure 11 shows SDE maps for 0, 5 (1.52 m) ft, and 10 ft (3.05 m) of SLR. Maps are available for 2 (0.61 m) and 7 (2.13 m) ft, but not shown here. These maps were assembled from individual images from the map viewer. The 100-yr ﬂooding maps showed the expected increase in BFE due to SLR. The BFEs for the SLR cases reﬂected the increase, due to the increase in surge depth plus the increase in the controlling wave height given the greater underlying surge depth. The wave contribution was generally quite small in the bay, given the small wave heights.

Figure 11. Impact of SLR on Narragansett Bay SDE maps, cases shown are 0, 5 ft (1.52 m), and 10 ft (3.05 m) SLR (upper, middle, and lower panel, respectively.

To help gain insight into the maps, Figure 12 shows the surge inundation levels (upper panel) and associated wave heights (lower panel) for the no-SLR case. The sum of the two is the BFE shown in Figure 10. The figure shows that the surge levels (upper panel) increased with distance up the bay from 13 ft (4 m) along the coast to about 18 ft (5.48 m) at the head of the bay, or an amplification of 1.38. This was consistent with the results of the application of the16 high-resolution hydrodynamic model to the bay shown in Figure4a,b. Note there is little cross-bay variation in the amplification. The lower panel of Figure 12 shows the predicted wave heights for the same area. The waves were quite large at the mouth of the bay (10 ft, 3 m) but decreased as one moved northward in the bay with values typically The figure shows that the surge levels (upper panel) increased with distance up the bay from 13 J. Mar. Sci. Eng. 2020, 8, 292ft (4 m) along the coast to about 18 ft (5.48 m) at the head of the bay, or an amplification of 1.38.14 This of 18 was consistent with the results of the application of the high-resolution hydrodynamic model to the bay shown in Figure 4a,b. Note there is little cross-bay variation in the amplification. The lower panel of 3 ft (1 m) or lower.of It Figure is noted 12 shows on southward-facingthe predicted wave heights shorelines for the same inside area. The the waves bay we thatre quite are exposedlarge at the to mouth of the bay (10 ft, 3 m) but decreased as one moved northward in the bay with values typically long fetches along theof east 3 ft and(1 m) westor lower. passages It is noted of on Narragansett southward-facing Bay, shorelines wave heightsinside the canbay that reach are 6exposed ft (1.8 to m). Erosion at these locationslong wasfetches quite along limited, the east and given west the passages low waveof Narragansett heights Bay, and wave limited heights length can reach of shoreline 6 ft (1.8 exposed to the large waves.m). Erosion at these locations was quite limited, given the low wave heights and limited length of shoreline exposed to the large waves.

Figure 12. Surge inundation levels (relative to NAVD88) (upper panel) and the wave crest height Figure 12. Surge inundation levels (relative to NAVD88) (upper panel) and the wave crest height (lower panel) for Narragansett Bay for 100-yr, no-SLR case. (lower panel) for Narragansett Bay for 100-yr, no-SLR case. Figure 13 shows the 100-yr, no-SLR case for the towns of Barrington, Warren, and Bristol as a Figure 13 showsfocus the 100-yr,area (Figure no-SLR 2, yellow case oval) for. This the area towns was selected of Barrington, to provide a higher Warren, resolution and view Bristol, given as a focus area (Figure2, yellowits very oval). high density This of area structures was selected (see Figure to 2) provide subject to acoastal higher flooding resolution. The upper view, panel given shows its the flooding levels relative to NAVD88, while the lower panel highlights the ability to interrogate the very high density of structuresmap and determine (see Figure the BFE2 at) subjectthe location to of coastal interest; flooding.in this case the The BFE upper was 14.7 panel (4.5 m) showsft. A close ther flooding levels relativelook to NAVD88,at the map whileshows that the lowerthe south panel-facing highlights coastal areas the, within ability the tomap interrogate window, ha thed BFE map and determine the BFEamplitudes at the location higher than of interest;those immediately in this landward. case the This BFE was was due 14.7 to the (4.5 largest m) ft.wind A fetch closer being look along the central axis of the bay, with a wind direction toward the north. at the map shows that the south-facing coastal areas, within the map window, had BFE amplitudes 18 higher than those immediately landward. This was due to the largest wind fetch being along the central axis of the bay, with a wind direction toward the north. The interface also allowed the user to compare and contrast the FEMA FIRMs and SDE maps. As an example, Figure 14 shows the SDE BFE map for this area in the upper panel and the FEMA FIRM BFE map with associated transect lines in the lower panel. The FEMA BFE values were typically 2 (0.6 m) to 3 (0.9 m) ft lower than SDE maps. The FEMA maps also lacked the level of detail provided in the SDE maps. A detailed presentation of the sources of the differences for this general location in the bay are provided in Spaulding et al., 2017 [8]. Table2 shows a comparison of the FEMA BFE and the SDE with varying sea level rise (SLR) at end of the Route 103 Bridge, Warren, RI. The location is shown by the yellow circle in Figure 14 (lower panel). The SDE value for no-SLR is 15.6 ft (1.45 m) (14-ft surge elevation and 1.6-ft wave) compared to FEMA FIRM value of 12 ft (3.65 m). SDE values increased with increasing SLR, reaching a peak value of 27.6 ft (8.4 m) for the 10-ft (3-m) SLR case. The change in BFE was principally caused by the increase in surge from SLR with a very small change in wave height. J. Mar. Sci. Eng. 2020, 8, 292 15 of 18

Figure 13. Flooding in the vicinity of Barrington, Warren, and Bristol, RI, for 100-yr, no-SLR case. The Figure 13. Floodingpoint interrogation in the tool vicinity was used of to Barrington, determine the BFE Warren, at the location and Bristol, selected. RI, for 100-yr, no-SLR case. The point interrogation tool was used to determine the BFE at the location selected. The interface also allowed the user to compare and contrast the FEMA FIRMs and SDE maps. As an example, Figure 14 shows the SDE BFE map for this area in the upper panel and the FEMA FIRM BFE map with associated transect lines in the lower panel. The FEMA BFE values were typically 2 (0.6 m) to 3 (0.9 m) ft lower than SDE maps. The FEMA maps also lacked the level of detail provided in the SDE maps. A detailed presentation of the sources of the differences for this general location in the bay are provided in Spaulding et al., 2017 [8]. Table 2 shows a comparison of the FEMA BFE and the SDE with varying sea level rise (SLR) at end of the Route 103 Bridge, Warren, RI. The location is shown by the yellow circle in Figure 14 (lower panel). The SDE value for no-SLR is 15.6 ft (1.45 m) (14-ft surge elevation and 1.6-ft wave) compared to FEMA FIRM value of 12 ft (3.65 m). SDE values increased with increasing SLR, reaching a peak value of 27.6 ft (8.4 m) for the 10-ft (3-m) SLR case. The change in BFE was principally caused by the increase in surge from SLR with a very small change in wave height.

Figure 14. Comparison of SDE (upper panel) vs. FEMA (lower panel) BFE maps for the 100-yr, no- Figure 14. ComparisonSLR case. The of FEMA SDE transect (upper lines panel) are shown vs. FEMAin the FIRM (lower maps. panel) BFE maps for the 100-yr, no-SLR case. The FEMA transect lines are shown in the FIRM maps. 4. Conclusions 4. Conclusions A state-of-the-art method to generate BFEs including the effects of SLR (SDE maps) was developed and applied to protected, low-lying coastal communities inside Narragansett Bay and to A state-of-the-artthose along method the wave- toimpacted generate, erosion BFEs-prone including southernthe RI shoreline effects of(South SLR Coast) (SDE. The maps) approach was developed 19 and applied toimplemented protected, use low-lyingd FEMA-approved coastal models communities and guidelines inside, but addresse Narragansettd weaknesses Bay identified and to in those along the wave-impacted,the develop erosion-pronement of the FEMA southern FIRMS for RIthe study shoreline area. The (South strategy Coast). to develop The the approachmaps was based implemented on extensive state-of-the-art hydrodynamic and wave modeling studies performed as part of the US used FEMA-approvedArmy Corp modelsof Engineers and (ACOE guidelines,) North Atlantic but addressed Coast Comprehensive weaknesses Study identified(NACCS) simulations in the development of the FEMAwith FIRMS a fully for coupled the studyhigh-resolution area. surge The strategyand wave model to develop (ADCIRC the-SWAN) maps with was and without based the on extensive state-of-the-arteffects hydrodynamic of SLR. Application and of waveXBeach modelingto the southern studies RI coast performed line was next performed as part of to thedetermine US Army Corp the geomorphology of the eroded shoreline and dune system and then STWAVE was applied to the of Engineers (ACOE)XBeach-modified North topography Atlantic to Coast predict Comprehensive the impact of SLR on Studystorm wave (NACCS) heights for simulations areas along the with a fully coupled high-resolutionsouthern RI shoreline surge. andUse of wave XBeach model, after extensive (ADCIRC-SWAN) validation and withsensitivity and testing, without dramatically the effects of SLR. Application ofincreased XBeach the to ability the southern to provideRI high coast-resolution line was representation next performed of the erosion to determine and overtopping the geomorphologyof the of the erodeddune, shoreline to include and the ability dune to system delineate andthe locations then STWAVEof surge channels was and applied associated to over the-wash XBeach-modified fans in the presence of SLR. STWAVE was independently applied to predict wave conditions for topography toNarragansett predict theBay impactwith wave of forcing SLR onat the storm open waveboundary heights from the for NACCS areas. The along underlying the southern RI methodology and its application have been documented in the publications provided in the reference list by the authors. The approach taken in developing the maps was conservative in that the assumptions and decisions made in the mapping methodology were used to address the uncertainty in the predictions. The two most important assumptions were (1) the use of the upper 95% confidence

J. Mar. Sci. Eng. 2020, 8, 292 16 of 18 shoreline. Use of XBeach, after extensive validation and sensitivity testing, dramatically increased the ability to provide high-resolution representation of the erosion and overtopping of the dune, to include the ability to delineate the locations of surge channels and associated over-wash fans in the presence of SLR. STWAVE was independently applied to predict wave conditions for Narragansett Bay with wave forcing at the open boundary from the NACCS. The underlying methodology and its application have been documented in the publications provided in the reference list by the authors. The approach taken in developing the maps was conservative in that the assumptions and decisions made in the mapping methodology were used to address the uncertainty in the predictions. The two most important assumptions were (1) the use of the upper 95% confidence level value for the surge water levels compared to FEMA’s approach, which uses the 50% confidence level, and (2) the assumption that linear superposition of SLR values on 100-yr surge can be used to represent surge levels in the presence of SLR. As shown in the paper, this was a conservative assumption since fully coupled simulations showed the surge level predicted by nonlinear methods gave values slightly lower than the linear superposition approach. To the authors’ knowledge, this is the first example of the development of BFE maps which explicitly considered the impact of SLR in the coastal waters and used state-of-the-art models. The Narragansett Bay study area was impacted by the amplification of the 100-yr surge as it progressed up the bay (amplification of about 1.38, between Newport and Providence), but experienced relatively low wave heights (typically less than 3 ft (1 m)), given the protected location from offshore waves. Wave heights were largest when the fetch distances were greatest. This was observed on the south-facing portions of the coast, given that winds that give the largest wave heights come from the south. Sea level rise was shown to increase the 100-yr surge levels and slightly increase the wave heights. The methods employed here assumed that SLR had no additional impact on the surge level beyond raising the mean water level by the selected value of the SLR. The southern RI coastline (South Coast) showed very little variation in the 100-yr surge height from Napatree Point to Point Judith. Wave heights were very large (23 to 33 ft (7 to 10 m), significant wave heights, 20-sec peak period) along this coastline as it borders on the open ocean with strong winds and unlimited fetch distances. The presence of the large waves resulted in substantial nearshore erosion with overtopping or breaching of the dunes and flooding of the adjacent coastal ponds, while large waves broke at the top of the eroded dune line and the decrease in dune height allowed waves with substantial heights to propagate into the adjacent ponds.

Author Contributions: Conceptualization: M.L.S.; methodology, M.L.S., A.G., R.H., and S.K.; software; A.G., C.D., S.K.; validation: A.G.: formal analysis, A.G.; resources, G.F.; data curation: C.D.; writing—original manuscript: M.L.S.; writing—review and editing- All; visualization: C.D.; supervision and project administration: M.L.S., A.G.; funding acquisition: M.L.S., G.F. All authors have read and agreed to the published version of the manuscript. Funding: The development of SDE maps for the state of RI was supported by Housing and Urban Development (HUD), Hurricane Sandy Coastal Development Block Grant (CDBG) Disaster Recover–Catalog of Federal Domestic Assistance (CFDA) #14.269 (Grant #B-13-D5-49-0001) and administered through the State of Rhode Island, Executive Office of Commerce, Office of Housing and Community Development (OHCD). Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Acronyms

ADCIRC ADvanced CIRCulation model ASCE American Society of Civil Engineers BCP Blind Control Points BFE Base Flood Elevation CERI Coastal Environmental Risk Index CI Conﬁdence Interval CRMC RI Coastal Resources Management Council DEM Digital Elevation Model J. Mar. Sci. Eng. 2020, 8, 292 17 of 18

ESRI Supplier of Geographic Information System (GIS) software FEMA Federal Emergency Management Agency FIRM Flood Insurance Rate Maps FIS Flood Insurance Study GIS Geographic Information System HUD Housing and Urban Development JONSWAP JOint North Sea WAve Project L Linear LIDAR Laser Imaging, Detection, and Ranging LIMWA limit of moderate wave action MSL Mean Sea Level NACCS USACE, North Atlantic Comprehensive Coastal Study NAVD88 North Atlantic Vertical Datum, 1988 NL Nonlinear NOAA NOS National Ocean and Atmospheric Administration- National Ocean Survey OIG Oﬃce of Inspector General OHCD Oﬃce of Housing and Community Development RI GIS Rhode Island- Geographic Information System SFHA Special Flood Hazard Area SDE STORMTOOLS Design Elevation maps SDS Synthetic Design Storm SLR Sea Level Rise STWAVE STeady state spectral WAVE model SWEL Still water elevation level STORMTOOLS tools in support of storm analysis SWAN Simulating WAves Nearshore URI University of Rhode Island USACE US Army Corp of Engineers WL water level XBeach nearshore wave and geomorphological model

References

1. FEMA. Flood Insurance Study, Washington County, RI, FEMA Flood Insurance Study Number 44009CV001B; FEMA: Washington, DC, USA, 2012. 2. ASCE. Minimum Design Loads and Associated Criteria for Buildings and Other Structures, ASCE /SEI 7-16; ASCE: Reston, VA, USA, 2017; ISBN 978-0-7844-7996-4. 3. Office of Inspector General (OIG). FEMA Needs to Improve Management of Its Mapping Program; Report OIG-17-110; Department of Homeland Security: Washington, DC, USA, 27 September 2017. 4. NRC. Mapping the Zone: Improving Flood Map Accuracy; Committee on FEMA Flood Maps; Board on Earth Sciences and Resources/Mapping Science Committee; National Research Council; National Academies Press: Washington, DC, USA, 2009; ISBN 978-0-309-13057-8. 5. FEMA Approved Models. Available online: https://www.fema.gov/coastal-numerical-models-meeting- minimum-requirement-national-flood-insurance-program (accessed on 17 January 2017). 6. Federal Emergency Management Agency (FEMA). Atlantic Ocean and Gulf of Mexico Coastal Guidelines Update; FEMA: Washington, DC, USA, 2007. 7. Spaulding, M.; Grilli, A.; Damon, C.; Fugate, G.; Oakley, B.A.; Isaji, T.; Schambach, L. Application of state of art modeling techniques to predict flooding and waves for an exposed coastal area. J. Mar. Sci. Eng. 2017, 5, 10. [CrossRef] 8. Spaulding, M.; Grilli, A.; Damon, C.; Fugate, G.; Isaji, T.; Schambach, L. Application of state of art modeling techniques to predict flooding and waves for coastal area with a protected bay. J. Mar. Sci. Eng. 2017, 5, 10. [CrossRef] J. Mar. Sci. Eng. 2020, 8, 292 18 of 18

9. Spaulding, M.L.; Isaji, T.; Damon, C.; Fugate, G. Application of STORMTOOLS’s simplified flood inundation model, with and without sea level rise, to RI coastal waters. In Proceedings of the ASCE Solutions to Coastal Disasters Conference, Boston, MA, USA, 9–11 September 2015. 10. Spaulding, M.L.; Grilli, A.; Damon, C.; Crean, T.; Fugate, G.; Oakley, B.A.; Stempel, P. STORMTOOLS: Coastal Environmental Risk Index (CERI). J. Mar. Sci. Eng. 2016, 4, 54. [CrossRef] 11. Spaulding, M.L.; Grilli, A.; Damon, C.; Crean, T.; Fugate, G. Developing the RI Coastal Environmental Risk Index (CERI) to Inform State and Local Planning and Decision Making: Application to the Communities along the Southern RI Shoreline; Report Prepared on Behalf of the RI; Coastal Resources Management Council: South Kingstown, RI, USA, 2019. 12. Grilli, A.; Spaulding, M.L.; Damon, C.; Becker, A.; Menendez, J.; Stempel, P.; Crean, T.; Fugate, G. Application of the Coastal Environmental Risk Index (CERI) to Barrington, Bristol, and Warren, RI; Report Prepared for RI; Coastal Resources Management Council: South Kingstown, RI, USA, 2018. 13. Cialone, M.A.; Massey, T.C.; Anderson, M.E.; Grzegorzewski, A.S.; Jensen, R.E.; Cialone, A.; Mark, D.J.; Pevey, K.C.; Gunkel, B.L.; McAlpin, T.O.; et al. North Atlantic Coast Comprehensive Study (NACCS) Coastal Storm Model Simulations: Waves and Water Levels; MS 39180-6199, Report: ERDC/CHL TR-15-44; Coastal and Hydraulics Laboratory U.S. Army Engineer Research and Development Center: Vicksburg, MS, USA, August 2015. 14. Zervas, C. Extreme Water Levels of the United States, 1893–2010; NOAA Technical Report NOS CO-OPS 067; NOAA National Ocean Survey: Washington, DC, USA, 2013. 15. Torres, M.; Hashemi, R.; Hayward, S.; Spaulding, M.; Ginis, I.; Grilli, S. The role of hurricane wind models in accurate simulations of storm surge and waves. ASCE J. Waterw. Port Coast. Ocean Eng. 2019, 145, 04018039. [CrossRef] 16. Smith, J.M.; Sherlock, A.R.; Resio, D.T. STWAVE: Steady-State Wave Model User’s Manual for STWAVE, Version 3.0; ERDC/CHL SR-01-01; U.S. Army Engineering Research and Development Center: Vicksburg, MS, USA, 2001. 17. Massey, T.C.; Anderson, M.E.; McKee-Smith, J.; Gomez, J.; Rusty, J. STWAVE: Steady State Spectral Wave Model. User’s Manual for STWAVE, Version 6.0; US Army Corp of Engineers, Environmental Research and Development Center: Vicksburg, MI, USA, 2011. 18. Schambach, L.; Grilli, A.; Grilli, S.; Hashemi, R.; King, J.W. Assessing the impact of extreme storms on barrier beaches along the Atlantic coastline: Application to southern Rhode Island coast. Coast. Eng. 2018, 133, 26–42. [CrossRef] 19. Sallenger, A.H. Storm impact scale for barrier islands. J. Coast. Res. 2000, 16, 890–895, ISSN 0749-0208. 20. Harter, C.F.; Figlus, J. Numerical modeling of the morphodynamics of low lying barrier island beach and foredune system inundated during Hurricane Ike, using XBeach and CSHORE. Coast. Eng. 2017, 120, 64–74. [CrossRef] 21. Grilli, A.; Spaulding, M.L.; Oakley, B.A.; Damon, C. Mapping the coastal risk for the next century, including sea level rise and changes in the coastline: Application to Charlestown RI, USA. Nat. Hazards 2017, 88, 389–414. [CrossRef] 22. Al Naser, N.; Grilli, A.; Grilli, S.; Baxter, C.; Bradshaw, A.; Maggi, B. Land use and mitigation effects on barrier beach erosion in storms: Case study in Rhode Island. Coast. Eng. Proc. 2018, 108. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).