il

The inte rtidol zone, where coostol wetlands provide

the first buffer to storm energy ond seo levef rise, hos

not troditionolly been o focus of geodetic surveying.

However, the incredsing vulnerobility of coostlines to

e nvi ro n m e nto I ch a n ge, i n cre o si n g P oP u I oti o n densities

neor the coast, ond increosing relionce on codstal goods

and services have oll increosed the need for occurote

i nte rti d ol e I evotions.

By Galen Scott and Phil ippeHensel, National Geodetic Survey,National Oceanic & AtmosphericAdministration (NOAA), USA

Figure I : The deep rod SET as deployed on top of a SET benchmark in a coastal salt

marsh. (Photo courtesy of D.R. Cahoon, US Geological Survey).

lmportance of Verticallnfrastructure

Surface elevation is a cri t ical struc-tural component of low-lying coastalareas, where slight changes in eleva-tion can mean the difference betweenextensive wetland habitats and openwater. Wetland plants, which struc-ture the intert idal community, havevery specif ic tolerances to f lood-ing frequency and duration, both ofwhich are dependent on elevation.Maintaining elevation with respectto sea level is critical to coastalwetland survival, especial ly where

up-slope migration is constrained.Global sea level r ise, local geologicalsubsidence and human activi t ies,including hydrological alterat ions,changes in sediment supply, coastaldevelopment and subsurface f luidwithdrawal, are among the factorsthat may contribute to elevation lossin coastal wetlands. With the rapidloss of coastal wetlands worldwide,we need to careful ly monitor theelevations of existing coastlines withrespect to sea level, understand theprocesses leading to elevation changeand predict the outcome of possiblemitigation scenarios.The first step isto obtain accurate elevations.

Lack of Vertical Control

Vertical control in coastal wetlandshas been problematic for severalreasons.Whereas shipborne sensorsprovide gravimetric data offshore,and land-based measurements definegravity on stable uplands, gravitymeasurements in wetlands are morediff icult to obtain.As a result, geoidmodels are not well defined in thecoastal zone.The National GeodeticSurvey (NGS) is currently seeking tocol lect new gravity measurementsalong coastal zones to improve geoidmodels and allow for more accurate(orthometric) heights from GPSobservations. In addition, the nationalnetwork of passive survey controlmarks ('benchmarks'), established byNGS over the past decades, rarelyextends into the intertidal zone.Theintertidal zone is often dynamic andunstable, yet i ts elevation is cr i t icalto sustaining its environmental value.

Geodesy on the Water's EdgeApplications of Accurate Heights in the Coastal Zone

Advances in vertical positioning nowenable the application of geodetictools to the intert idal zone, wherecentimetre-level changes in elevationcan have dramatic consequences oncoastal resources. Determining accur-ate elevations within coastal wet-lands improves the understanding ofprocesses affecting wetland dynam-ics and land loss, and the monitor-ing of elevation change relative tosea levels. Now more than ever, theknowledge of accurate elevations inthe coastal intertidal zone is essentialto informed decision making in thecoastal zone.

16 - Moy 2007 Hydro INTERNAI/ONAL

Figure 2: By tying both SET bench marks and tide stations

to the National Spatial Reference System, an accurate esti-

mate of relative sea level rise is possible.

the Louisiana SpatialReference Center. isproviding accurateheights for southernLouisiana in the areasaffected by HurricanesKatrina and Rita. Over300 benchmarks havebeen re-surveyed,and the i r pub l ishedelevations updated.New gravity meas-urements have gen-erated an improvedgeoid model. Sixteencontinuously operat-ing reference stationshave been establ ishedto enhance the GPS-

derived capabil i t ies of the NSRS.AnRTK-GPS network has also beenestablished in south-east Louisiana.Aspart of this project, state, local andprivate agencies including the coastalparishes of Louisiana have beentrained in obtaining accurate heights(elevations). Accu rate elevations areessential data for the rebui lding oflevees and other infrastructure, flood-p la in mapping and issuance of homeelevation certifi cates.

Sarface Elevation Tables

Surface Elevation Tables (SETs) arepor tab le measur ing ins t rumentsdeployed atop wetland vert icalbenchmarks (F igure l ) , a l lowing mi l -limetre-level changes in surface eleva-t ion to be measured over t ime. SETsintegrate both surface and subsur-faceprocesses affecting elevation changedown to the depth of the benchmark.However, SETs do not convey infor-mation about elevation with respectto sea level.Tide gauges have trad-i t ional ly been used toestimate sea level; how-ever, since these gaugesare t ied to uplandelevations, they do notinclude elevation changewithin coastal wetlands.Therefore, NGS is devel-oping guidel ines for sur-veys to be performedthat wi l l provide NSRSorthometric heights onSET benchmarks. so thatwetland surface eleva-t ion measurements canbe related to local sealevel (Figure 2). Usingthese guidel ines, the ver-

tical motion of the whole SET bench-mark can also be measured, providingestimates of deep subsidence.

Providing Vertical Control onSET Benchmarks

NGS has joined with federal andnon-federal partners to provideaccurate heights on SET benchmarks.In co-operation with the Universityof New Orleans, the US GeologicalSurvey and the Louisiana Depart-ment of Natural Resources. NGS hasrecently provided accurate heightsfor over 50 SET benchmarks insouth-eastern Louisiana. which wil lbe used to understand processes ofelevation dynamics in this subsidingcoastl ine, as well as serving as sen-tinels for the effects of sea level rise(Figure 3).To provide these heights,NGS developed a level adapter to fital l known types of SET benchmarks.This adapter allows a SET benchmarkto receive a GPS antenna, enablingit to be surveyed by a static GPSreceiver, a critical step in establishingknown elevations in a wetland arearegardless of the proximity to uplandbenchmarks. This also al lows eleva-tions to be transferred to nearby SETbenchmarks.

Results of Tying SETBenchmarks to NSRS

Two important results are obtainedby tying SET benchmarks to NSRS.First, mil l imetre-level measurementsof elevation change are taken out ofa relat ive vert ical frame and placedwithin a known + 2cm f ixed abso-lute elevation scale.Wetland surfaceelevation processes such as sedimentdeposit ion, biological production

NGS is working with partners toestabl ish a network of intert idalbenchmarks and elevation productsin coastal wetlands of the Gulf andAtlantic coasts of the USA.

Height Modernisation

A height modernisation programmewas developed by NGS to update thevert ical (height) component of theNational Spatial Reference System(NSRS) using the latest GPS-derivedtechnolog ies. Wi thout modern isedheights, i t is dif f icult to assess andmanage risk in areas prone to flood-ing or other natural hazards. Thiswas the case in the aftermath ofHurricane Katrina, where up-to-dateheights were not available in portionsof the affected states. Traditionally,NSRS benchmarks were posit ioned6y expensive and t ime-consum-ing spir i t level l ing; recent advances,inc lud ing h igh-accuracy GPS andGPS combined wi th laser leve l l ing,have al lowed rapid, highly accurateand precise vert ical measurements,especial ly in non-tradit ional environ-ments such as dynamic and remotecoastal wetlands. Height modernisa-t ion is therefore the primary vehicleto bring vertical control to the inter-tidal zone.lt can provide updated ref-erence elevations for both t ide sta-tions and coastal wetlands, making itpossible to relate wetland elevationsto local sea level.

Louisiana Height Modernisation

The coastal zone of Louisiana isexperiencing rapid land loss. NOAA,in co-operation with the US FederalEmergency Management Agency and

Hydro INTERNAI/ONAL

Figure 3: SET sites updated with vertical control in south-

eastern Louisiana (photo courtesy D.J. Reed, University

of New Orleans).

G

Moy 2007 - 17

IGil

Figure 4: Protocols for tying SET benchmarks to the National Spatial Reference System. Clockwise from top:

locating top of SET benchmark pipe; level l ing to top of pipe; instal l ing tr ibrach adapter; tr ibrach instal led; level-

l ing to top of tr ibrach; instal l ing CPS antenna; instal l ing RTK-CPS base stat ion; obtaining elevation using RTK.

Artwork courtesy Bob Leonard, NOAA NCS.

and peat formation are a function ofhydrology,which is affected by surfaceelevation. Therefore, knowing wherean elevation trajectory is in relat ionto water leve l is most impor tant ini ts in terpreta t ion. Second, when theelevat ion o f the SET benchmark isre-taken at a later time, a vertical vel-oc i ty o f the benchmark is obta ined.This velocity relates to processesoccurr ing be low the benchmark, andtherefore complements data fromthe SET, which integrates processesoccurr ing over the depth o f thebenchmark (F igure 2) . Wi th these

two elevation products, a completepicture of coastal sediment elevationdynamics is poss ib le .

Expanding Vertical Control

A de ta i l ed se t o f gu ide l i nes w i l lbe publ ished to enable any user toobtain vertical control on any existingor fu ture SET benchmark accord ingto str ict NGS standards with knownver t ica l er ror (F igure 4) . NGS is a lsoconducting an in-depth analysis of thevert ical error of the dif ferent typesof SET technology, so that users

can expect cer ta in bounds on er rordepending on ins t rument , so i l andhabi ta t type. NGS a lso a ims to ass is tthe coastal research and managementcommuni t ies by develop ing othere levat ion products , such as t ies toloca l t ide s ta t ions, spat ia l ly in tens ivel< inemat ic GPS in suppor t o f d ig i ta lelevation models, and the adaptationof remote sens ing technolog ies tointert idal elevations. NGS is currentlydevelop ing t r ia l programmes wi th inthe Nat iona l Estuar ine ResearchReserve System, a network of 27 pro-tected reserves throughout the USAand i ts ter r i tor ies , ded icated to theresearch of estuarine ecosystems.I

Biographies of the Authors

Galen Scott received a master 'sdegree in Env i ronmenta l Sc ience andPolicy from Johns Hopl<ins Universityin 2003. He jo ined NOAA's Nat iona lGeodet ic Survey as a pres ident ia lmanagement fel low, worl<ing on stra-teg ic p lanning and t rans la t ing thesc ience of geodesy for managementpurposes. Scot t now heads a teamthat provides infrastructure, tools andt ra in ing to t ie together geodet ic andtidal datums in support of appl icat ionsat the land-water interface.Phi l ippe Hensel obta ined a PhD inOceanography & Coastal Science in1998 from Louisiana State Univer-s i ty and ho lds master 's degrees inStatist ics and Estuarine Ecology. Hehas worked extensively on wetlandelevat ion dynamics and is cur rent lyhelping to develop guidelines for prod-ucing coastal elevation products forNOAA's National Geodetic Survey.

Em o il : go I e n.scott@n o o o. gov

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