Navigate Up
Sign In
User Login

IndividualStory

Accounting for Wave Attenuation Over Flooded Marsh and Agricultural Land
Brian Wardman and Brad Hall, Northwest Hydraulics Consultants (NHC)
The San Francisco Bay region contains extensive tracts of marsh and agricultural lands which have the potential to dissipate, or reduce, offshore waves as the waves propagate over them. The primary model used to calculate overland wave propagation over these, and other, land types is the WHAFIS (Wave Height Analysis for Flood Insurance Studies) transect-based model. Although the WHAFIS model has been used in FEMA flood studies for over three decades, the WHAFIS model only includes default plant parameters for common Atlantic Coast vegetation types. In order to enhance the WHAFIS model’s ability to more accurately dissipate waves over vegetation specific to the Pacific Coast and San Francisco Bay, NHC completed field reconnaissance to identify the predominant vegetation species and measure their respective physical characteristics. NHC used the results of their field study to develop WHAFIS input parameters for the predominant vegetation types.
 

Natural marsh vegetation in the San Francisco Bay Area is generally classified as either salt or brackish. Salt marshes are characterized by the presence of pacific and smooth cordgrass (or their hybrid) in the low marsh zone, pickleweed in the middle marsh zone, and bulrush in the high marsh zone. Brackish marshes, which are more common where salinity levels are relatively low, are characterized by tule and cattails in the low marsh zone, and salt grass in the middle and high marsh zones. Diked agricultural lands in the Bay Area are typically used to cultivate oats. Numerous other plant species also exist in these regions; however, due to their more limited aerial extent, it is not expected that the other vegetation types would contribute significantly to wave dissipation.
 


The field reconnaissance measured and classified the following plant characteristics for each vegetation type noted above: stem diameter (at the base, mid-stem, and top of the plant), stems per square foot, plant height, and frontal area ratios (FAR). The FAR is an estimate of the ratio of the frontal area of the leaves to the frontal area of the stem. WHAFIS uses these parameters to calculate the area blocked by the vegetation as a wave passes over the marsh. Additional details on the WHAFIS calculations are found in the WHAFIS documentation.     
 
 
Although NHC originally completed their field research for use within the north San Francisco Bay study area (north of the Richmond-San Rafael Bridge), their recommended WHAFIS parameters for Pacific coast vegetation types have been reviewed and adopted for use within the entire CCAMP San Francisco Bay Area Coastal Study area. The parameters are also under review for potential use within marsh dominated areas of the Open Pacific Coast study area.
 

 Coastal Beat Story Archive

 
collapse Year : 2012 ‎(7)
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=8'>Risk Map Local</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=6'>FEMA Leverages LiDAR</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=14'>FEMA’s CCAMP Studies and Our Coast, Our Future</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=18'>Region IX to Conduct First Flood Risk Review Meeting for CCAMP</a>
collapse Year : 2013 ‎(19)
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=27'>FEMA Partners with Oceanweather and Scripps Institution of Oceanography to Bring Modeling Expertise to CCAMP OPC Study</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=29'>FEMA Region IX Holds Meetings for the California Coastal Analysis and Mapping Project / Open Pacific Coast Study</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=33'>Primary Frontal Dune Coastal High Hazard Area Mapping Requirements</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=47'>FEMA Holds South Bay Workshop to Kick-off Detailed Analysis in the South Bay Counties</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=50'>Translating Coastal Flood Hazard Modeling Results into Floodplain Mapping</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=60'>Terrain Modeling in FEMA’s California Coastal Flood Studies</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=62'>Join FEMA’s Community Rating System Program Using California’s Statewide Floodplain Management Activities</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=54'>Coastal Flood Processes Along the California Coast</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=56'>FEMA’s Annual Risk Awareness Survey: Findings from Previous Surveys and the Focus for the 2013 Survey</a>
collapse Year : 2014 ‎(9)
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=64'>E386 Residential Coastal Construction</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=68'>Engaging Stakeholders to Help Communicate Impacts of BW-12</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=70'></a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=74'>California Coastal Storm History Part Two – Ventura County</a>
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=76'>Redelineation: What does it mean for me?</a>
collapse Year : 2015 ‎(2)
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=78'>FEMA increases community access to draft floodplain mapping data </a>
collapse Year : 2016 ‎(6)
<a href='http://www.r9map.org/Pages/EbulletinStory.aspx?storyID=85'></a>
1 - 40Next
 

 Other Stories

 
expand 
expand Arizona
Educating Maricopa County on the Power of Water,
Discovery Process, Thursday, February 23, 2012
expand California
Discovery Process, Thursday, March 1, 2012
NFIP Substantial Improvement and Substantial Damage Course, Wednesday, August 29, 2012
Santa Barbara County and Incorporated Areas Countywide Flood Insurance Rate Map,
FEMA RELEASES PRELIMINARY FLOOD MAPS FOR SOLANO COUNTY , Thursday, January 31, 2013
Join FEMA’s Community Rating System Program Using California’s Statewide Floodplain Management Activities,
expand Coastal Studies
Discovery Process, Tuesday, January 10, 2012
Betty the Prepared Dog, Saturday, April 28, 2012
expand Hawaii
Tsunami: Learning from Experience in Hawaii,
Public Outreach Meeting for FEMA Flood Insurance Rate Map, Wednesday, August 8, 2012
expand Nevada
Clark County Flood Facts,
expand Region Wide
Be Prepared for a Flooding Event in your Community Today!,
Watershed University , Friday, June 1, 2012
Know Your Line: Be Flood Aware, Thursday, May 2, 2013
1 - 30 Next
  • The Summary of Map Actions (SOMA) Report: SOMAs Deciphered

    Many community officials and floodplain managers are called upon to review a Summary of Map Actions (SOMA) report, but how familiar are you with this document, and the process of developing it? Did you know that the ability to revise or amend effective Flood Insurance Rate Map (FIRM) panels is always available? Revisions and amendments to flood hazard information may be accomplished through FEMA’s Letter of Map Change (LOMC) process, without having to re-publish the FIRM. When a Flood Insurance Study (FIS) update or restudy requires a revised FIRM panel to be re-published, the existing LOMCs that applied to the old effective FIRM panels are superseded by the newly effective FIRM panels, and LOMCs that have remained valid are revalidated. As part of a map revision, FEMA publishes a Preliminary and Final SOMA report. The SOMA records a complete list of all previously issued LOMCs, and their valid or superseded status, for each community affected by revised FIRM panels. The SOMA assists community officials in maintaining the most up-to-date information on their community’s FIRMs. (Read more…)

    More...
  • Flood Insurance Rate Maps and Flood Insurance Studies: From Preliminary to Effective

    ​FEMA flood hazard maps inform communities about the local flood risk. Flood hazard mapping is an important part of the National Flood Insurance Program (NFIP), as FEMA regulatory products provide the basis for regulating development and determining flood insurance requirements under the NFIP regulations. FEMA maintains and updates regulatory flood hazard data through Flood Insurance Rate Maps (FIRMs) and Flood Insurance Study (FIS) reports.

     
    Following the data acquisition and data development phases of the flood insurance study process, FEMA distributes one set of the preliminary FIRM panels, the preliminary FIS Report, and the preliminary Summary of Map Actions (SOMA) to community CEOs. FEMA also posts PDF versions of the preliminary FIRM panels and FIS Report, as well as the DFIRM database containing the GIS shapefiles, to the Map Service Center (MSC), where they are available for public download. (The preliminary FIRM panels and the preliminary DFIRM database are archived on the MSC when the panels, FIS report, and database become effective.) The steps that follow, referred to as the post-preliminary process, include the public comment and appeal period, community and public meetings, the Letter of Final Determination, and community adoption/compliance requirements. 
     
    From July 2015 through November 2015, FEMA distributed preliminary data to ten coastal counties as part of the Open Pacific Coast Study and three counties as part of the San Francisco Bay Area Coastal Study. For more information about what goes into a flood map, go to this infographic on FEMA’s website. (Read more…)

    More...
View RSS feed



Powered by BakerAECOM