Over the past decade, several coastal communities in southeast Florida have experienced a significant increase in flooding frequency, which has caused significant disturbance to property, commerce, and overall quality of life. The increased flooding frequency reflects the contribution of global, regional, and local processes that affect elevation difference between coastal communities and rising sea level. In a recent project, funded by the state of Florida, we monitor coastal subsidence in southeast Florida using GPS and InSAR observations, in order to evaluate the contribution of local subsidence to the increased coastal flooding hazard. Preliminary results reveal that subsidence occurs in localized patches (\u3c 0.02 km2) with magnitude of up to 3mmyr1, in urban areas built on reclaimed marshland. These results suggest that contribution of local land subsidence affect only small areas along the southeast Florida coast, but in those areas coastal flooding hazard is significantly higher compared to non-subsiding areas

Fiaschi, Simone

Oliver-Cabrera, Talib

Wdowinski, Shimon

English

DigitalCommons@Florida International University

Florida International University FIU Digital Commons Department of Earth and Environment College of Arts, Sciences & Education 4-22-2020 Land subsidence contribution to coastal flooding hazard in southeast Florida Shimon Wdowinski Florida International University, swdowins@fiu.edu Talib Oliver-Cabrera Florida International University Simone Fiaschi University College Dublin Follow this and additional works at: https://digitalcommons.fiu.edu/earth_environment_fac Recommended Citation Wdowinski, Shimon; Oliver-Cabrera, Talib; and Fiaschi, Simone, "Land subsidence contribution to coastal flooding hazard in southeast Florida" (2020). Department of Earth and Environment. 72. https://digitalcommons.fiu.edu/earth_environment_fac/72 This work is brought to you for free and open access by the College of Arts, Sciences & Education at FIU Digital Commons. It has been accepted for inclusion in Department of Earth and Environment by an authorized administrator of FIU Digital Commons. For more information, please contact dcc@fiu.edu. Proc. IAHS, 382, 207–211, 2020https://doi.org/10.5194/piahs-382-207-2020© Author(s) 2020. This work is distributed underthe Creative Commons Attribution 4.0 License.Open AccessTenthInternationalSymposiumonLandSubsidence(TISOLS)Land subsidence contribution to coastal floodinghazard in southeast FloridaShimon Wdowinski1, Talib Oliver-Cabrera1, and Simone Fiaschi21Department of Earth and Environment, Florida International University, Miami, FL 33199, USA2UCD School of Earth Sciences, University College Dublin, Dublin, IrelandCorrespondence: Shimon Wdowinski (swdowins@fiu.edu)Published: 22 April 2020Abstract. Over the past decade, several coastal communities in southeast Florida have experienced a significantincrease in flooding frequency, which has caused significant disturbance to property, commerce, and overallquality of life. The increased flooding frequency reflects the contribution of global, regional, and local processesthat affect elevation difference between coastal communities and rising sea level. In a recent project, funded bythe state of Florida, we monitor coastal subsidence in southeast Florida using GPS and InSAR observations,in order to evaluate the contribution of local subsidence to the increased coastal flooding hazard. Preliminaryresults reveal that subsidence occurs in localized patches (< 0.02 km2) with magnitude of up to 3 mm yr−1, inurban areas built on reclaimed marshland. These results suggest that contribution of local land subsidence affectonly small areas along the southeast Florida coast, but in those areas coastal flooding hazard is significantlyhigher compared to non-subsiding areas.1 IntroductionSeveral coastal communities in southeast Florida have beenperiodically subjected to flooding events, which have beeninduced by heavy rain, high tide, and storm surge. The fre-quency of the flooding events has increased over the past twodecades causing property damage, transportation problems,an overall impact on daily life. Our recent study of floodinghazard in Miami Beach has shown that flooding frequencyin the city doubled during the years 2006–2013 compared tothe previous eight-year period of 1998–2005, mainly due toan increased number of high tide events (Fig. 1) (Wdowinskiet al., 2016).The increased flooding frequency reflects the contributionof global, regional, and local processes that affect elevationdifference between coastal communities and rising sea level.Along the US Atlantic coast, the increasing coastal flood-ing has occurred mainly due to higher sea level, but has alsobeen affected by land subsidence. In order to evaluate thecontribution of land subsidence to coastal flooding hazardin southeast Florida, we began a new subsidence monitoringproject, which is supported by the Florida Office of Insur-ance Regulation. The monitoring relies on two geodetic tech-niques, GPS and Interferometric Synthetic Aperture Radar(InSAR), as well as on visual field observations. The twogeodetic techniques provide observations of surface changesover time with different spatial and temporal resolutions and,hence, complement one another. The project supports theconstruction of four continuous GPS (cGPS) stations, whichare currently being constructed in four locations in southeastFlorida. In addition, the project supports InSAR data pro-cessing of both archive and current data.This study describes the geodetic monitoring project,which aims at quantifying the spatial extent and rates ofcoastal subsidence in southeast Florida. We first present fieldobservations indicative of land subsidence in various loca-tions along the southeast Florida coastline. Based on the ob-served subsiding areas, we selected locations for construct-ing four continuous GPS stations, which are described in thegeodetic monitoring section on the study. We also describethe planned InSAR monitoring analysis as well as presentingpreliminary InSAR results of archived ERS-1/2 data. Basedon the preliminary InSAR results, we discuss the contribu-tion of local land subsidence to coastal flooding hazard insoutheast Florida.Published by Copernicus Publications on behalf of the International Association of Hydrological Sciences.208 S. Wdowinski et al.: Land subsidence contribution to coastal flooding hazardFigure 1. (a) “Sunny sky” flooding in Miami Beach. (b) Annualflooding occurrence in Miami Beach between 1998–2013 indicat-ing a significant increase in tide flooding events (green) since 2006(modified after Wdowinski et al., 2016).2 Coastal subsidenceSubsidence in Florida typically occurs at the local scale dueto soil oxidation, sediments compaction, and sinkhole activ-ity. Regional scale subsidence due to Glacial Isostatic Ad-justment occurs in many sections of continental US, but isnegligible in Florida (Sella et al., 2007; Kargar et al., 2016).Also, the tectonic stability of the Florida peninsula suggestsnegligible tectonic-induced subsidence in Florida.In southeast Florida subsidence occurs mainly due to sed-iment compaction, as urban development took place, in part,on reclaimed marshland. Marshland subsidence is a natu-ral process that is often compensated by sediment accretionduring inundation events (Nicholls, 2004). However, in re-claimed marshlands, inundation prevention and lack of andsediment supply result in land subsidence. Differential sub-siding urban areas often result in structural damage to build-ing and structures, which can be used as proxies for land sub-sidence.Along the southeast Florida coast, we observed structuraldamage to buildings in several coastal parks, including Math-eson Hammock, Morningside, and Haulover, as well in theKovens Conference Center at the Biscayne Bay Campus ofFlorida International University (Fig. 2). All four locationswere constructed between 1930–1960 on reclaimed marsh-land. The observed structural damage in these locations sug-gest that local land subsidence is an active process reducingthe elevation of some section of the southeast Florida coast.3 Geodetic monitoringIn order to quantify the distribution and rate of coastal sub-sidence along the southeast Florida shoreline, we begana geodetic monitoring project, which is supported by theFlorida Office of Insurance Regulation. The monitoring re-lies on two geodetic techniques, GPS and InSAR, whichcompliments one another. Continuous GPS measurementsprovide high temporal 3-D positioning observations at lim-ited number of observation points with respect to an externalreference frame. Whereas, InSAR provides high spatial reso-lutions observations of surface changes in line of sight (LOS)between the satellite and the surface, with respect to an arbi-trary internal reference point. InSAR-based subsidence mea-surements are of higher quality in urban settings and aridenvironments than in vegetated areas, due to the scatteringbehaviour of each land cover. By using both measuring tech-niques, we will obtain detailed information of subsidenceprocess in four selected locations and less detailed, but withvery good spatial coverage of land subsidence along most ofthe urban sections of southeast Florida coastline.3.1 Continuous GPSPrecise GPS measurements require the installation of cGPSstations in coastal subsiding areas and monitoring the subsi-dence over a period of at least 3–4 years. This project sup-ports the installation of four cGPS stations along the south-east Florida shoreline. Because during the writing of this pro-ceedings the four cGPS stations are still in the planning stage,we describe here the site selection criteria and related activi-ties, as well as the planned monitoring activities.The first year of the project was devoted to site selectionand obtaining permits to install the four cGPS stations. Ap-parently, this task was found to be much more difficult thananticipated, because most of the urban sections of southeastFlorida coastline is privately owned. All of our attempts toinstall cGPS stations in backyards of private homes alongthe shorelines failed. Thus, we needed to select cGPS sitelocations on public land, which is a long process requiringpermits by the local authorities. During the first year of theproject, we identified fifteen possible sites on public land, aswell as on private schools. Some locations were found to beunsuitable in terms of physical conditions (small plots, toomany trees, or building plans for the area), whereas in somelocations we were not able to obtain the required permit. Bythe end of the process, we identified four sites for cGPS sta-tion installation.The selected locations include two sites in the DeeringEstate, one in Morningside Park, and one in Haulover Park(Fig. 2a). The two sites in the Deering Estate are located onProc. IAHS, 382, 207–211, 2020 proc-iahs.net/382/207/2020/S. Wdowinski et al.: Land subsidence contribution to coastal flooding hazard 209Figure 2. (a) Location map of our study area in southeast Floridabased on a ©Google Map imagery. The map shows the locationof documented coastal subsiding areas and planned GPS sites. In-sert shows the location of the study area with respect to state ofFlorida. (b) Subsidence-induced structural damage to a building inMatheson Hammock Park. (c) Observed subsidence along seawallin Morningside Park. (d) Subsidence-induced structural damage atthe entrance to the Kovens Conference Center at the Biscayne BayCampus of Florida International University (FIU). (e) Observedsubsidence along seawall in Haulover Park.two different geological settings in two sides of the countypark. The eastern site, located near the shoreline on peat soil,will be used to measure coastal subsidence. The western site,located on a limestone ridge, will be used as a control pointto measure the stability of houses and infrastructure built onlimestone. The two sites in Morningside and Haulover parksare located on unconsolidated sediments and will be used formonitoring coastal subsidence (Fig. 2c and e).As soon as permits will be issued for the four sites,the cGPS stations will be conducted and start operating.The station construction and data downloading, archivingand processing will be conducted by UNAVCO (https://www.unavco.org/data/gps-gnss/gps-gnss.html, last access:26 February 2020), which operate more than 1500 cGPS sta-tions, including the Plate Boundary Observatory (PBO) andthe Continuously Operating Caribbean GPS ObservationalNetwork (COCONet).3.2 InSAR analysisThis geodetic monitoring project also support InSAR dataprocessing of both current and archived data. The analysis ofcurrent, Sentinel-1 data, did not reveal yet significant results,because the observation time span (2015–2019) is too shortfor obtaining velocity measurement with 1–2 mm yr−1 un-certainty level (Havazli and Wdowinski, 2017). Actually, theeffective Sentinel-1 observation period for southeast Floridais even shorter (only 3 years), because systematic data ac-quisition every 12 d repeat path began in September 2016. Inorder to obtain subsidence measurements at the 1–2 mm yr−1uncertainty level, Sentinel-1 time series should extend for aperiod of at least five years (Havazli and Wdowinski, 2017).We will continue processing Sentinel-1 data after obtainingsufficiently long data span (> 5 years).InSAR data analysis of archived ERS-1/2 data yielded sig-nificant results with the desired accuracy of 1–2 mm yr−1(Fiaschi and Wdowinski, 2019). The ERS-1/2 data wereacquired during 1993–1999 and cover a total period of7 years. We processed the data using the Small Base-line Subset (SBAS) algorithm (Berardino et al., 2002) with24 acquisitions and 95 interferometric pairs. Data analysisincludes multi-looking (1 × 5), topography phase removal(1 arcsec SRTM), ESA’s Precise Orbits, Goldstein adaptivefilter (Goldstein and Werner, 1998), co-registration using theDelaunay Minimum Cost Flow (MCF) algorithm (Costan-tini, 1998), and Atmospheric Phase Screen removal by apply-ing spatial (low-pass) and temporal (high-pass) filters. Moredetails of data analysis are provided in Fiaschi and Wdowin-ski (2019).Our ERS-1/2 InSAR analysis focused on subsidencewithin the city of Miami Beach, which have been subjectedto periodic flooding (Fig. 1). Miami Beach is a densely pop-ulated barrier island, roughly 10 km long and 2.5 km wide atits widest point (Fig. 3). Our InSAR time series results reveala patch-like pattern of coherent velocity observations locatedproc-iahs.net/382/207/2020/ Proc. IAHS, 382, 207–211, 2020210 S. Wdowinski et al.: Land subsidence contribution to coastal flooding hazardFigure 3. Vertical velocity map over Miami Beach from 1993–1999 and displacement time series (from Fiaschi and Wdowinski, 2019).(a) vertical velocity map obtained with the SBAS technique. The black circles mark the location of the extracted displacement time series.Red lines mark the location of roads affected by flooding during 1998–2012 (data from Wdowinski et al., 2016). Base image source: Esri,DigitalGlobe, GeoEye, Earth Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community. (b) Verticaldisplacement time series of the four selected points.mainly over the built environment. The results indicate thatmost of the city (∼ 97 %) was stable during the 1993–1999observation period (green in Fig. 3a). Several localized sub-siding areas (< 0.02 km2) were detected mostly in the westernand eastern parts of the city (yellow in Fig. 3a). The detectedsubsidence rate is generally in the 1–3 mm yr−1 range, withuncertainty level of 0.6–0.8 mm yr−1. Uncertainties are cal-culated for each pixel, based on their multi-temporal coher-ence values.4 Discussion and conclusionsIncreasing flooding frequency in several coastal communi-ties along the southeast Florida coastline resulted in dam-age to property, commerce, and overall quality of life. Theincreased flooding frequency reflects increasing rate of rel-ative sea level rise, mostly due to ocean dynamic contribu-tions, but possibly also due to coastal subsidence. In orderto evaluate the contribution of land subsidence to the coastalflooding hazard, we began a geodetic monitoring project thatuses both cGPS and InSAR observations. The project sup-ports the construction of four cGPS stations that will be con-structed on subsiding public land along the southeast Floridacoastline. The project also supports InSAR monitoring of theurban environment in southeast Florida. Preliminary resultsreveal that subsidence occurs in localized patches with mag-nitude of 1–3 mm yr−1, mainly in urban areas built on re-claimed marshland. These preliminary results suggest thatland subsidence affect only small areas along the southeastFlorida coast. However, in these areas coastal flooding haz-ard is significantly higher compared to non-subsiding areas.As part of the project, we will also conduct InSAR time anal-ysis of Sentinel-1 data, which have been acquired systemati-cally since 2016.Proc. IAHS, 382, 207–211, 2020 proc-iahs.net/382/207/2020/S. Wdowinski et al.: Land subsidence contribution to coastal flooding hazard 211Data availability. GPS data will be archived by UNAVCO (https://www.unavco.org/data/gps-gnss/gps-gnss.html, UNAVCO DataArchive Interface Version 2 (DAI v2), 2020). ERS-1/2 data are pro-vided by ESA through the EOLi-SA (Earth Observation Link) clientfor Earth Observation Catalogue Service (https://esar-ds.eo.esa.int/oads/access/, ESA Online Dissemination, 2020)Author contributions. SW initiated the study and wrote themanuscript, SW and TOC carried the field survey including GPSsite survey, SF conducted the InSAR analysis, SF and SW inter-preted the InSAR results.Competing interests. The authors declare that they have no con-flict of interest.Special issue statement. This article is part of the special is-sue “TISOLS: the Tenth International Symposium On Land Sub-sidence – living with subsidence”. It is a result of the Tenth Inter-national Symposium on Land Subsidence, Delft, the Netherlands,17–21 May 2021.Acknowledgements. The authors thank UNAVCO and particu-larly John Galetzka for helping selecting and constructing the CGPSstations. This is contribution number 952 from the Southeast En-vironmental Research Center in the Institute of Environment atFlorida International University.Financial support. This research has been supported by theFlorida Office of Insurance Regulation.ReferencesBerardino, P., Fornaro, G., Lanari, R., and Sansosti, E.: A new algo-rithm for surface deformation monitoring based on small base-line differential SAR interferograms, IEEE T. Geosci. Remote,40, 2375–2383, 2002.Costantini, M.: A novel phase unwrapping method based on net-work programming, IEEE T. Geosci. Remote, 36, 813–821,1998.ESA Online Dissemination: The archive provides a high-performance infrastructure for storing and providing accessto ESA EO data, available at: https://esar-ds.eo.esa.int/oads/access/, last access: 26 February 2020Fiaschi, S. and Wdowinski, S.: Local land subsidence inMiami Beach (FL) and Norfolk (VA) and its contribu-tion to flooding hazard in coastal communities along theUS Atlantic coast, Ocean Coast. 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Res.Lett., 34, 1–6, 2007.UNAVCO Data Archive Interface Version 2 (DAI v2): Data of in-terest to you using a variety of filters (Metadata, Spatial, andTemporal), available at: https://www.unavco.org/data/gps-gnss/gps-gnss.html, last access: 26 February 2020Wdowinski, S., Bray, R., Kirtman, B. P., and Wu, Z. H.: Increasingflooding hazard in coastal communities due to rising sea level:Case study of Miami Beach, Florida, Ocean Coast. Manag., 126,1–8, 2016.proc-iahs.net/382/207/2020/ Proc. IAHS, 382, 207–211, 2020

Land subsidence contribution to coastal flooding hazard in southeast Florida

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Land subsidence contribution to coastal flooding hazard in southeast Florida

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