Full Text
An EqualO pportunity Em ployerM /F/V /H S EN T V IAELEC T R O N IC M AILN O V EM BER 3,2020 N ovem ber3,2020 File N o.18.0171674.04 M r.D avid H.K now lton,P C ity Engineer/D P S D irector C ity ofS alem ,M A 98W ashington S treet,2nd Floor S alem ,M assachusetts01970 R e: P relim inary D esign S um m ary Letter-Executive S um m ary C olum busAvenue S eaw allR econstruction P roject(EEA#258-2020-3) S alem ,M assachusetts D earM r.K now lton: G ZAG eoEnvironm ental,Inc.(G ZA)ispleased to subm itthisExecutive S um m ary to our sum m ary letterofprelim inary design progressdated O ctober21,2020forthe C olum bus Avenue S eaw allR econstruction P rojectto the C ity ofS alem (C lient).T hisExecutive S um m ary w asprepared atthe requestof,and in response to,ourconversationsw ith you. T hisExecutive S um m ary issubjectto the term sand conditionsofouroriginalagreem entand the Lim itationsofourprelim inary design sum m ary letterdated O ctober21,2020.Elevations provided are referenced to the N orth Am erican V erticalD atum 1988(N AV D 88). W e thankyouforthe opportunity to w orkon this projectand look forw ard to ourcontinued involvem ent.Ifyouhave any furtherquestions orrequire any additionalinform ation,please do nothesitate to callD avid S m ith at781-278-4806orem ailatD avid.sm ith@ gza.com . V ery truly yours, G ZAG EO EN V IR O N M EN T AL,IN C . D avid A.S m ith S eniorP rojectM anager Enclosed: Executive S um m ary ---PAGE BREAK--- N ovem ber3,2020 P relim inary D esign S um m ary Letter– Executive S um m ary C olum busAvenue S eaw allR econstruction P roject,S alem ,M A P rojectN o.18.0171674.04 P age I 1 An EqualO pportunity Em ployerM /F/V /H BAC K G R O U N D G ZAw asretained bythe C ityofS alem to provide design and perm itting servicesforthe replacem entofthe existing C olum bus Avenue seaw all. G ZA understands that the C ity w ould like to replace the existing seaw allw ith consideration to raise the height of the seaw allfor greater protection and resilience to w ave surge/flooding conditions.In addition,the C ity w ould like to im prove coastalresiliency ofthe area by im plem enting an im proved living shoreline adjacentto the w allin areasofexisting and deteriorated saltm arsh habitat. G ZA’sscope ofservicesincludesinspection,survey,design,perm itting services,and bid solicitation phase services for the proposed C olum bus Avenue S eaw all R econstruction P roject located w ithin Juniper C ove, S alem , M assachusetts.T he prelim inary design sum m ary letterdated O ctober21,2020,presentsexisting conditions,site investigations and assessm ent,coastalw ave and flood analysis,and the proposed project design progress perform ed to date. EXIS T IN G C O N D IT IO N S T he C olum busAvenue seaw allisan olderfieldstone and granite blockm approxim ately474linear feet long,located along the northw estern portion of Juniper C ove in S alem ,M assachusetts betw een the properties of44 C olum bus Avenue and 30 Bay V iew Avenue.T he seaw allprovides foreshore protection to;the public roadw ay (C olum busAvenue),public sidew alk,utilities,and residentialdw ellings.T he seaw allis fronted by the publicly accessible ‘S teps Beach’and an area of salt m arsh vegetation along the southw est portion of the beach area.T he seaw allvaries in elevation from approxim ately 10.1 feetN AV D 88 atthe southw estern cornerto 8.5feetN AV D 88atthe northeastern corner. N eighboring private w allson each end are atthe sam e approxim ate elevation asthe adjacentC olum busAvenue seaw all. T he seaw allhas never had the benefit of any ongoing,periodic preventative m aintenance program but has received spotrepairs from tim e to tim e especially afterthe dam age thatoccurred during the 2018 N or’easters. T he seaw allisvulnerable to the ever-increasing severity ofcoastalstorm sand higherw aterlevelsthan previously experienced. If left as-is the structure is likely to experience additionaldegradation and potentially failure com prom ising the roadw ay,public access,utilitiesand residentialdw ellings. S IT E IN V ES T IG AT IO N S AN D AS S ES S M EN T G ZAhasperform ed investigationsto docum entthe existing conditionsofthe site and to assistin the developm ent of proposed reconstruction and restoration designs including an updated topographic survey of the site, inspectionsofthe seaw and lim ited ecologicalsurvey ofthe saltm arsh habitatarea. T he C olum bus Avenue seaw allw as observed to have m inorto advanced defects and deterioration.T he seaw allisparticularly susceptible to failure,due to age,existing deteriorated condition,lackofconsistent m aintenance,and lackofproperstone sizing and design. T he saltm arsh area w asobserved to be and degraded.Im provem entsand enhancem ent ofthe saltm arsh area w illhelp to stabilize the shoreline and reduce erosion,attenuate w aves,and provide habitatforplantand anim alspecies. ---PAGE BREAK--- N ovem ber3,2020 P relim inary D esign S um m ary Letter– Executive S um m ary C olum busAvenue S eaw allR econstruction P roject,S alem ,M A P rojectN o.18.0171674.04 P age I 2 C O AS T ALW AV E AN D FLO O D AN ALYS IS G ZAhas perform ed a m etocean and w ave analysis for the projectsite to determ ine design param eters for the proposed reconstructed seaw alland living shoreline m arsh restoration.T able S tillw aterelevations (flood levelw ithout w ave height),totalw ater level(flood levelw ith w ave height),and w ave runup elevation (m axim um w ater levelreached by w aves and sw ash along the seaw all) for the 10-year,50-year,100-year recurrence intervalflood events.T hatis,a 10-yearflood eventstatistically has a 10-percentchance ofoccurring in any given year. T able 1:S um m ary ofFlood Elevations S torm EventR eturn P eriod S tillw aterElevation (S W L)(ft,N AV D 88)1 T otalW aterLevel(T W L) (ft,N AV D 88)2 R unup Elevation Along S eaw all(ft,N AV D 88)3 10-year(10-percent) 8.4 8.4 to 10.7 8.4 to 12.8 50-year(2-percent) 9.4 10.7to 11.9 11.9to 14.2 100-year(1-percent) 10.0 11.2to 13.2 12.7to 15.0 1FEM AS tillw aterelevations 2T otalW aterLevel(T W L)equalsthe S tillw aterelevation plusw ave heights 3C alculated by EurO top M anual(2018) T he existing seaw allvaries in elevation from approxim ately 8.5 feetN AV D 88 atthe northeastern cornerto 10.1 feet N AV D 88 at the southw estern corner. T he existing seaw allprovides som e flood protection against the S tillw aterelevationsofa flood eventbutprovideslesserflood protection againsthigh w aterplusw ave heightand w ave runup/overtopping along the seaw all. R ELAT IV E S EALEV ELR IS E T he relative sea levelrise atthe projectsite w asestim ated using the U S AC E sea levelrise calculatorand the N O AA sea levelrise projection atN O AABoston tide station.Assum ing the N O AAInterm ediate sea levelrise projection, sea levelrise forthe year2070,relative to 2020 is approxim ately 2 feet.S ea levelrise w illincrease the w ave and flood risksatthe site. BR EAK W AT ER AN ALYS IS Atthe entrance to JuniperC ove there is an existing deteriorated breakw aterapproxim ately 750 feetseaw ard of the seaw all.T he breakw aterextendsapproxim ately 120feetperpendicularfrom existing bedrockoutcropsatthe northern shoreline ofJuniperC ove,atthe approxim ate property of72Bay V iew Avenue.T he existing breakw ater w asobserved to have a crestatapproxim ate elevation 0feetN AV D 88and the arm orstonesw ere observed to be raveled in som e areasw here itappearsthe bedrockcore ofthe breakw aterw asexposed. S ignificantw ave heights w ere estim ated forincreased breakw aterheights.T able 2 presents the estim ated w ave heightreductionsalong the seaw allforthe 10-year,50-year,and 100-yearrecurrence intervalflood events. ---PAGE BREAK--- N ovem ber3,2020 P relim inary D esign S um m ary Letter– Executive S um m ary C olum busAvenue S eaw allR econstruction P roject,S alem ,M A P rojectN o.18.0171674.04 P age I 3 T able 2:S um m ary ofW ave HeightR eductions along S eaw all1 S torm EventR eturn P eriod Breakw aterHeight5feet (El.5ft-N AV D 88) Breakw aterHeight8feet (El.8ft-N AV D 88) Breakw aterHeight10 feet(El.10ft-N AV D 88) 10-year(10-percent) 0to 0.5 feet 0to 1.4 feet 0to 1.2 feet 50-year(2-percent) 0.1to 0.3feet 0.4 to 0.7feet 0.5 to 0.7feet 100-year(1-percent) 0to 0.4 feet 0.4 to 1.3feet 0.5 to 1.4feet 1S ignificantw ave ere estim ated using the S W AN (S im ulating W AvesN earshore)m odel If the breakw ater crest w as raised 5 feet,the w ave heights at the seaw allw ould be reduced by less than approxim ately 0.5 feetforthe 10-year,50-year,and 100-yearrecurrence intervalflood events.Ifthe breakw ater crestw as raised 10 feet,the w ave heights atthe seaw allw ould be reduced by less than approxim ately 1.5 feet forthe 10-year,50-year,and 100-yearrecurrence intervalflood events. P R O P O S ED P R O JECT D ES IG N T he reconstructed seaw allisproposed w ith an increased heightranging from approxim ately 1.5feetto 3feet,up to a consistentelevation of11.5 feetN AV D 88,roughly atthe approxim ate top ofthe existing planter/pillars.T he reconstructed seaw allw illprovide im proved flood protection S tillw ater elevations up to a 500-year recurrence intervalflood event(elevation 11.4 feetN AV D 88)and w illprovide increased flood protection against high w aterplus w ave heightand w ave runup/overtopping along the seaw all.An elevation of11.5 feetN AV D 88 w aschosen to balance the desired levelofprotection w ith cost,practicality,and aesthetics. T he proposed saltm arsh enhancem entdesign aim sto stabilize the shoreline and reduce erosion,attenuate w aves, and provide habitat for plant and anim alspecies. A healthy salt m arsh m ay reduce w ave heights by up to approxim ately 30to 40percentasshow n from the M etocean and W ave Analysis. T he proposed projectseaw allreconstruction and living shoreline saltm arsh enhancem entdesigns w illprovide a holistic im provem ent w ith increased shoreline stabilization and coastalstorm /flood protection and resiliency. S pecific site im provem entsofthe proposed projectinclude: R econstructed seaw allw ith large cutgranite stone w allfounded on a reinforced concrete footing to provide increased protection to the roadw ay,public w alkw ay,dw ellings,utilities,and otherlandside features. Am ore and shoreline area lessprone to degradation from w ave surge/flooding conditions and design elem dissipate w ave energy. Increased protection and flood prevention w ith a reduced access opening,flood protection barrier,landing im provem ents,and beach accessim provem ents. P rom otion of nature-based and naturalelem ents to provide a m ore resilient shoreline able to provide increased storm dam age prevention and flood control. M inim ized m aintenance to the seaw alland associated costs. R econstruction w ithin sam e w allfootprint.