Design Features Beach Nourishment o Berm & Dune dimensions based on storm damage reduction goals (B/C Analysis) o Sediment Compatibility o Equilibrium Profile o Nourishment Cycle Optimization = f(Background erosion rate, volume of nourishment, & Mob/Demob costs)
Engineering Challenge – “Sand” Where can we find a large volume of sand….close by (good borrow source)? Does it match the native beach sand (sediment compatibility)? How will we transport it (hydraulic, mechanical, by- pass)? Cost (“who’s gonna pay for it”)?
Planform Adjustments Equilibrium Adjustment…how long? How long will the sand last (short-term & long-term)? Winter & summer changes. Erosion is not a “sink” (where does it go then)? How often…can this be optimized? function of: o Planform adjustment o Background erosion rat o Volume of nourishment o Mob/Demob costs
96’ Position This paper reiterated FEMA’s policy of not considering the effects of beach nourishment projects in coastal flood hazard analysis. Nourished beaches are therefore treated as temporary shoreline disturbances or uncertified coastal projects that are not likely to withstand the 1- percent-annual chance flood. Mapping Partners are required to use pre-nourishment topographic data in all coastal flood hazard analyses and mapping.
FEMA’s policy was established on the basis that: Beach nourishment projects attract additional and denser development into areas that are the Nation’s most vulnerable to multi-hazards (e.g., wind, flooding, and erosion); The scientific and engineering uncertainties in the longevity of nourishment projects that produce constructed beaches; & The uncertainty and tentative nature of the flood protection and of the funding to maintain nourishment projets (NRC, 1995; Davidson et al., 1996). Communities may, however, receive credit for erosion management activities, including beach nourishment, in the form of reduced insurance premiums through the Community Rating System.
Remove the Beach Nourishment…no problem! Many communities have had ongoing beach nourishment projects for a decade or more. As a result, it is becoming increasingly problematic to find and use pre-nourishment beach topography. When the pre-nourishment data are not available for use in mapping, Mapping Partners must “remove” the nourishment project from present-day topographic data.
If What …? For purposes of the National Flood Insurance Program (NFIP) FEMA does not want to invest resources to remove beach nourishment data from transects or topography if altering the topography does not affect flood hazard analysis results or mapping. 08’ Study: …evaluate whether beach nourishment affects Base Flood Elevations (BFEs) and flood hazard zone designations in order to determine if revisions to the present policy are warranted.
Impact Criteria: A significant change in flood zone designations is defined as a shift in the Zone VE/Zone AE boundary of more than 50 feet or transects for which the pre-nourishment profiles were all Zone VE and the post-nourishment profile showed a Zone VE/Zone AE boundary. A significant change in BFE refers to a difference (positive or negative) between the pre-nourishment landward-most BFE and the post-nourishment landward-most BFE of 0.5 feet or greater. Conclusions: There were seven transects [20%] among the various study sites for which flood zone designation and BFEs did differ between the pre-nourishment profile and the post-nourishment profile results. That does not include the six transects [17%] for which the landward-most BFE differed between profiles because of rounding to the nearest whole foot. For twenty-two transects [63%], neither the flood zone designation nor the landward-most BFE differed between the pre-nourishment and post-nourishment profile results.
Conclusions Detailed coastal analysis showed that for three out of eight study sites, changes in beach topography related to beach nourishment did not affect either flood zone designations or BFEs landward of the beach. If changes in BFEs from rounding to the nearest whole foot were not considered, five out of eight study sites showed no significant changes in flood zone designations or BFEs. [So….63% of flood zones would be impacted] Detailed coastal analysis showed that for three out of eight study sites, changes in beach topography related to beach nourishment did not affect either flood zone designations or BFEs landward of the beach. If changes in BFEs from rounding to the nearest whole foot were not considered, five out of eight study sites showed no significant changes in flood zone designations or BFEs. [So….63% of flood zones would be impacted] Based on the results of this study, it can be concluded that there are certain types of beaches and nourishment projects (not all) for which either pre-nourishment or post-nourishment beach topography can be used in an NFIP flood insurance study (FIS) without impacting the identification of the coastal flood hazards. and Dimensions & Elevations w.r.t. T-SWEL?
Summary: There are four major factors that control whether flood hazard analyses for a beach will be affected by changes in topography as a result of beach nourishment: 1.The type of beach nourishment project conducted; 2. Profile elevations relative to local 1-percent-annual-chance SWEL; 3. The elevation of the crest of the eroded dune profile or shore protection structure; and 4. Effective FEMA guidelines and regulations
Recommendations It is recommended that FEMA revise the current policy to permit the use of topographic information that may include the effects of beach nourishment projects, if certain criteria are met. 1.Permit use beach nourishment topography, if certain criteria are met. 2.Use the decision-making charts (Figure 1) to determine if criteria is met for FIS. A.If the criteria are not met, conduct pre-nourishment topography and the post-nourishment comparison. B.For significant changes in flood zone designations or BFE, must have a FEMA-approved sustainability plan, which documents financial and physical means, political will, and future provisions. 3.Develop a methodology for the analysis of episodic erosion of beach berm features.
Conclusions 1.Berm only – no change beyond PFD. 2.Note, risk is reduced by berm further seaward of dune (waves break further away). 3.Engineered dune can significantly reduce BFEs and move AE/VE boundary inland.
Damage Comparison – Nourished vs. Unnourished 1.Comparative analysis based on percent damage of protected (Carolina Beach & Wrightsville Beach) vs. unprotected (Kure Beach & Topsail Beach) areas. 2.Total damages based on FIA claims, BPAS / communities total property damage (surge and wave).
Damage Observations: Kure Beach: Structural damage to development along the oceanfront of Kure Beach was extensive, as no structure escaped damage and several were totally destroyed. Carolina Beach: Structural damage behind the beach fill portion of the protection project fronting Carolina Beach was limited to that caused by wind, rain, and flooding from the sound side. Wrightsville Beach: The storm damage reduction project, with its dune at elevation 13.5 feet above NGVD, was generally overtopped along its entire length north of the Oceanic Pier (located approximately 3,200 feet north of Masonboro Inlet)…and….a considerable amount of dune erosion also occurred north of the Oceanic Pier. Even though the dune was eroded and generally overtopped, none of the ocean front development received any substantial damage due to wave impacts or storm surge. This lack of wave or surge related damage was attributed to the width of the beach above NGVD that existed prior to the storm.
Topsail Beach Damages: Hurricane Bertha, which hit the area in mind July 1996, severely weakened the dunes. Hurricane Fran, which occurred eight weeks later, completed the job, destroying all of the manmade dunes along Topsail Beach and North Topsail Beach. Hurricane Fran also destroyed most of the natural dunes with the exception of approximately 2.5-miles of shoreline in the Town of Surf City where the landward portions of the dunes still remain. Hurricane Fran caused extensive erosion of the shoreline, rendering a majority of the ocean front lots unbuildable and caused extensive structural damage to ocean front structures as well as structures located on the second and third rows from the ocean.
Perhaps one of the most telling statistics is the number of structures destroyed. This is because the structures were largely destroyed by erosion and wave runup. Looking at damages (for those submitting FIA claims) as a percent of value, Wrightsville Beach and Carolina Beach had the lowest percentage of the property value damaged.
USACE Conclusions: · The areas protected by Corps of Engineers shore protection projects (Wrightsville Beach and Carolina Beach) received less damage as a percent of total property value than did the unprotected areas (Kure Beach, Topsail Beach, Surf City and North Topsail Beach). · While differences in physical storm parameters (winds, storm surge and waves) were observed from Kure Beach to North Topsail Beach, the differences were not large enough to explain the differences in damage. If anything, storm parameters showed the most severe part of the storm hit Wrightsville Beach and the less severe part of the storm hit Topsail Island. · The areas with existing wide beaches and a frontal dune system, either natural or manmade, experienced less storm damage.