Review of coastal process impacts of Emergency Pare Street revetment

2June2020

Competency MyfullnameisWillemPieterdeLange,andIamaSeniorLecturerintheSchoolofScience,Division

of Health, Engineering, Computing and Science, University of Waikato. I hold the following

qualificationsfromtheUniversityofWaikato:

• BSc(1981)inComputerScienceandEarthSciences;

• MSc(Hons)(1983)withfirstclasshonoursinComputerScienceandEarthSciences;

• DPhil(1989)withathesisonwave-inducedsedimenttransport.

MytrainingwasinEarthandOceanSciences,particularlyinsedimentologyandcoastalprocesses,

andComputerScience,particularlynumericalmodelling,dataanalysisandvisualisation.MyMSc

thesisexaminedtsunamihazardintheBayofPlenty,andinvolvednumericalmodellingoftsunami

generation,propagationandimpactsalongthecoastlineandwithinTaurangaHarbour.MyDPhil

thesisdealtspecificallywiththebehaviourofdredgespoilusedtorenourishPilotBayBeach,Mt

Maunganui, but also compared the performance of the two numerical models (2DD and DHI

System 21). My DPhil also assessed the contribution of wave-induced sediment transport,

particularly over the flood tidal delta (Centre Bank) and intertidal flats, to the total sediment

transportwithintheHarbour.

IamamemberoftheCoasts,Oceans,PortsandRiversInstitute(COPRI)oftheAmericanSociety

of Civil Engineers (ASCE), the Coastal Education and Research Foundation (CERF), The

Oceanographic Society (TOS), and the NZ Coastal Society and NZ Society for Earthquake

EngineeringtechnicalgroupsofEngineeringNewZealand.

Since1984,mostlyat theUniversityofWaikato, Ihaveundertakenresearch into fundamental

coastalprocessesandmanagementpertainingtoNewZealandestuariesandthecoast.Myresearch

hasconcentratedonthecharacterizationandmitigationofcoastalhazards,includingtsunamis,

storm surges, meteo-tsunamis, extreme waves, waterspouts, sea level variations, climate

fluctuations, and coastal erosion. I have also participated in and reviewed various lifelines

vulnerabilityassessmentsformajorurbanareasaroundNewZealand.

Ihavepublishedmorethan200peer-reviewedscholarlypublicationsincluding110journalpapers

andbookchapters,and107conferencepapers.Ihavealsosupervisedmorethan150graduateand

postgraduate student theses to completion including 30 PhD projects. My teaching has been

focused on coastal geomorphology, processes, engineering and management. Until the

establishmentofaBECivildegreeattheUniversityofWaikato,theengineeringcomponenthas

not includedthedesignofstructures; insteaddealingwithconstrainingdesignparametersand

impactassessment.

Review 4SightConsultinghaveprepareda retrospectiveconsentapplicationonbehalfof theGisborne

DistrictCouncilfortheemergencyrepairandrestorationofcoastalprotectionstructuresandland

fronting22,24and26PareStreet,WainuiBeach.Emergencyrepairswererequired following

storm damage to existing structures and erosion during an event on 6 September 2019. The

consent application is for a 3-year term to provide time for amore permanent solution to be

determinedandapproved.

AppendixDintheapplicationsummarisestheexpectedcoastalimpactsofthetemporarystructure

(note that theapplicationalsorefers to thisAppendixasEandH).Inmyopinion, theanalysis

presented in Appendix D is correct and identifies that the primary impact at this site is the

reflection of wave energy during storm events associated with a lower sand level. The main

mitigationmeasuresavailablearetominimisethesteepnessofthestructureandincreaseenergy

dissipation.Inthiscase,4SightConsultinghavealsoraisedconcernsaboutthestabilityofstructure

duetothesmallsizeofrockunitsusedandthesteepnessoftherevetment.Inmyopinion,these

concernsarevalid.

ExaminingasequenceofimagesofthelocationavailableinGoogleEarth(Figures1-3),itisclear

thatsandlevelsarehighlyvariableasidentifiedintheapplication.Thesandlevelisafunctionof

wave conditions; includingwave steepnessandwave approach angle. There appears to bean

additionalprocessactingbetweentheWainuiStreamand24PareStreet,however.Dischargefrom

theWainuiStreamappearstobeimpactingonbeachvolume,andhencesandlevelfrontingthe

affectedproperties.Therearetwomainfactors:

• ThedischargefromWainuiStreamduringfloodeventsappearstopushsedimentoffshore,

asdemonstratedbythelargerchannelcrossingtheupperbeachinFigure1;and

Figure 1 – Google Earth image of the Wainui Stream andaffectedPareStpropertieson4February2019.Thestream

channelhasbeendisplacednorthwardsanderodedtheupper

part of the beach as far north as 24 Pare Street (concrete

path/stepsdescendingtobeach).Thegreaterchannelwidth

anddepthareprobablytheconsequenceofaflood.Whenthe

imagewastakenthechannelhasbeenmigratingsouthward.

• The direction of discharge from Wainui Stream varies with the dominant longshore

transport direction. The available images on Google Earth indicate a predominantly

southwardsdisplacementofthechannel(Figure3);however,theseimagesmaynotbe

representative.Whenthechannelisdisplacednorthwards,thechannelabutsthecoastal

protectionstructuresandlowersthesandelevationattheirbase(Figure1).Figure2taken

duringtheconstructionofthetemporaryrevetmentindicatesthatthesedimenteroded

bythechannelinFigure1hadn’tbeenreplacedbeforethestormon6September.Thisis

alsoindicatedbyFigure1intheapplicationshowingtheloweredsandlevelon30August

2019infrontto26PareStreet.

Figure2–GoogleEarthimageoftheWainuiStreamandaffectedPareStpropertieson12September2019.Thestreamchannel

has migrated further south since Figure 1, and has been

significantlyinfilledwithsediment.Theupperbeachfronting24-

28PareStreetisdepletedofsedimentrelativetoFigure1.The

rockstockpileusedfortheemergencyrepairispresent,andno

sandhasbeen“borrowed”fromthemouthofWainuiStreamfor

therestorationoftheerosionoftheaffectedproperties.

Figure3 –GoogleEarth imageof theWainui StreamandaffectedPareStpropertieson27December2019.The streamchannelhas

been displaced southwards and has increased in size relative to

Figure2.MostGoogleEarthimagesshowthestreamchannelinthis

general position. The volume of sand in the upper beach has

increasedsinceFigure2.

Inmyopinion,theWainuiStreamcontributedtoerosionaffectingtheexistingstructuresfronting

24-28PareStreet,byloweringthesandvolumeintheupperbeachbystreamerosion.Theuseof

the mouth of Wainui Stream as a sand borrow area, while understandable as an emergency

response,isnotadvisableasitrepresentsalossofsandvolumefromthisareathatwillbereplaced

bycoastalprocessesfromelsewhereinthebeachsystem.Giventhecurrentunderstandingthat

sedimentsupplytothebeachislimited,sandborrowpitswithinthebeachsystemwillresultin

erosionsomewhereonWainuiBeach.

Summary Inconclusion, inmyopinion,theassessmentofcoastal impactsassociatedwiththisemergency

structureisvalidandreasonable.However,inmyopinion,theexcavationofsandfromthemouth

of WainuiStreamshouldbeavoidedinfutureduetotheinfluencethestreamhasonlowering

beachelevationsinfrontoftheaffectedproperties.