HomeMy WebLinkAboutSconset Presentation 2014-4-30_201405061122529368Temporary Baxter Road Stabilization ProjectTemporary Baxter Road Stabilization ProjectSupplemental Information for 4/30/14 HearingSupplemental Information for 4/30/14 HearingSupplemental Information for 4/30/14 HearingSupplemental Information for 4/30/14 HearingSiasconset Beach Preservation FundSiasconset Beach Preservation FundandandandandTown of NantucketTown of NantucketImage courtesy of George Riethof and the ‘Sconset Trust
Engineering Basis of DesignEngineering Basis of Design
Planting PlanPlanting PlanAmerican beachgrass will be planted first. American beachgrass in use along Baxter Road and on north shore of NantucketNantucket. American beachgrass specifically recommended by CZM for coastal banks.Fertilizer will not be used if required as part of an Order of Conditionsas part of an Order of Conditions.Upper 5 to 7 feet will be reserved for swallows.Minor addition of sand needed at 93Minor addition of sand needed at 93 and 97 Baxter Road (areas outside of sand delivery locations) to prepare a planting bed for the Aibh b hiAmerican beachgrass by smoothing deeper rills and gullies.Use of erosion control blanket or jute netting proposedjute netting proposed.No equipment on beach needed; access will be from the top.67 Baxter Road
Planting PlanPlanting PlanBeachgrass will be planted first.Woody vegetation will be planted after the bank surface is stabilized. Cjihb dAmerican Beachgrass (AmmophilaCommon juniper has been removed.(Ammophila breviligulata)Bayberry (Myrica pensyvanica)Bearberry (Arctostaphylos uva-ursi)Beach Heather Beach Plum (Prunus maritima)4444(Hudsonia tomentosa)
Failure CriteriaFailure CriteriaBased on Maintenance, Monitoring, and Mitigation Requirements1Failure to provide the required sand mitigation volume (currently specified1.Failure to provide the required sand mitigation volume (currently specified as 22 cy/lf/yr) on an annual basis (defined as April 1 through March 31). 2. Failure to conduct the required shoreline and post-storm monitoring.3. Failure to repair and/or replace the geotextile tubes in a timely manner p/p g yfrom storm-related or other types of damage.4. Excessive change in updrift or downdrift beach cross section that can be attributed to the project. Determination will consider: (a) Whether areas i di l dj h j di i f hi i iimmediately adjacent to the project are eroding in excess of historic erosion rates; (b) The ongoing erosion of the adjacent unprotected sections of coastal bank and how the volume of this erosion compares to the sand mitigation volumemitigation volume If failure occurs, an appropriate course of action will be determined, and may include: a change in the placement of the template sand, an increase in the volume of mitigation sand, or if those adjustments are not effective in correcting the problem, system removal.If removal is necessary, geotextile will be removed and discarded; sand will be spread on beach.
Temporary Nature of ProjectTemporary Nature of ProjectGeotube project proposed for most threatened section of Baxter Road and is an “interim” project.Applicants seek an Order of Conditions for 3 years, with the understanding pp y , gthat this can be extended three times in one year intervals.Expect that this requested Order of Conditions will be replaced by a subsequent Order for an expanded project to protect all of Baxter Road it ill b t d t t t l f il f th bl ffor it will become moot due to a total failure of the bluff.Removal of the “interim” geotube project is proposed only if there is a “failure” under the Order, or if a long-term, expanded coastal engineering structure replacing it is not approved.engineering structure replacing it is not approved.Image courtesy of George Riethof and the ‘Sconset Trust
Construction NarrativeConstruction NarrativeNarrative:Detailed narrative provided in 4/25/14 submittalNarrative:Detailed narrative provided in 4/25/14 submittal.Returns:15-foot circumference geotextile, jute, or coir tubes will be field-fitted at a 45 degree angle between the geotubes and the bluff. No e tio i to bl ff o o ed Botto ele tio of +3 to +4No excavation into bluff proposed. Bottom elevation of +3 to +4 MLW to avoid need for shoring; top equal to height of third geotube if project is limited to 3 tubes currently in place. Constructed via skid steer (and possibly excavator) using dry pit sand (no slurry)steer (and possibly excavator) using dry pit sand (no slurry).
Updated AsUpdated As--Built PlanBuilt PlanBuilt PlanBuilt PlanAs-built plan updated after April pp p2014 shoreline survey.Updated beach contours and top of bank position shown.Seaward edge of tier 2 geotube surveyed; sand template surveyed.Sand template shown.Sand template shown.
Shoreline Monitoring ProgramShoreline Monitoring ProgramSpring surveys (toe of bank to5MLW)Spring surveys (toe of bank to -5 MLW)Fall surveys (toe of bank to ~3,000 feet offshore or -25ft to -35ft contour, whichever is less)whichever is less)Top of bank monitored along profiles in project area and w/in 300’ of ends
Monitoring ProgramMonitoring ProgramPost-Storm Reports:Following significant storms: pggPhoto-documentation of geotubes and sand templateEstimate of volume lost from sand templateEstimate of beach level in front of geotubes. If replenishment is required,Estimate of beach level in front of geotubes. If replenishment is required, estimated volume of replenishment and schedule.•Identification of the location of any exposed geotextile.•Identification of any repair required to the geotextile.•Visual observation of the ends of the tubes to determine if flanking is occurring.Reports to be submitted within 3-7 days of end of a storm.Analysis of Results (Annual Report):•Review beach transect data to estimate accretion and erosion distances at each monitored transect.•Calculate accretion and erosion volumes at transect locations•Calculate accretion and erosion volumes at transect locations.•Compare top of bank locations and estimate bank retreat over the previous 12 months.•Calculate bank volume loss in the project area and 300 feet north and south.pj•Submit data and computations to the Conservation Commission for review with a recommendation for changes to nourishment program if required.
Sand Nourishment ProgramSand Nourishment ProgramProvide the initial cover of 22 cy/lf•Provide the initial cover of 22 cy/lf during and/or immediately following construction.•Annually in April starting in 2014:Annually in April starting in 2014: Provide additional sand and/or adjust the existing template sand to obtain a minimum of 2’ cover th t b t t t thover the geotubes to protect them from UV degradation. The volume of any sand placed in April will be recorded and counted towards the annual 22 cy/lf requirement. •Annually in November starting in 2014:Place an additional volume of sand to ensure a substantial sand cover (10-15 cy/lf ) is available at the fonset of the winter storm season. Throughout the winter, place additional sand on an as-needed basis (when half the vertical height of the lowest geotube is exposed.) If the balance of the 22 cy/lf volume is not placed in its entirety during the 2014-2015 winter, the balance ofnot placed in its entirety during the 20142015 winter, the balance of the sand will be placed by March 31, 2015.•Provide delivery tickets on an annual basis.
Sand Nourishment ProgramSand Nourishment ProgramSand Template is e pected to beSand Template is expected to be available to the littoral system.•Sand template is located seaward of the toe and is more available during glower wave energy events.•Sand cover is loose sand that is more erodible than native sand in the coastal bankthe coastal bank.During March 26, 2014 storm, ~1.5-2.5 cy/lf contributed by sand cover.Terraces contributed only 0.25 cy/lf.yy/Wave runup will be higher in larger storms.50 Year Run-upPost-storm reports will estimate sand contribution from the project area vs. 10 Year Run-up20 Year Run-up50 Year Runuppjunprotected areas and indicate if template needs to be adjusted.
Public AccessPublic AccessSBPF supports adding access from the top of the bluff to the top of the erosion protection system with additional stairs but believes it issystem with additional stairs but believes it is premature. SBPF believes additional stairs can be permitted as part of long-term protection project.Bh tl iddiHik’Beach access currently provided via Hoick’s Hollow, stairs at 65/67 Baxter and to the south, and at Codfish Park.
Regulatory ComplianceRegulatory ComplianceLimited Project:Revised NOI Form submitted to reflect DEP feedback that project is not considered a Limited Project.Enforcement Order: To be resolved through NOI process and most recent submittals.Stormwater Drainage Pipe:Described in written narrative. SBPF requesting permission to leave it disconnected and abandon it in placerequesting permission to leave it disconnected and abandon it in place unless and until an approval is granted at some time in the future, with a requirement to remove it if becomes exposed.Use of concrete plugs in geotube portholes:The Applicants are ilfhi fhlljrequesting approval for this as a component of the overall project, which was not described in advance but is a standard method for sealing geotubes in this setting. Excavation of the coastal beach below the Mean High Water mark:Excavation of the coastal beach below the Mean High Water mark:Previously submitted information demonstrates that work was not occurring seaward of the Mean High Water mark. Placement of sand on the face of the coastal bank outside the sand deli er areasOnl sand placement o tside the sand deli er areasdelivery areas:Only sand placement outside the sand delivery areas was at 93 Baxter Road. The Applicants are requesting approval of the existing sand placement at 93 Baxter Road.
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