APPENDIX B- 15.04.080 GENERAL FLOODPLAIN CONSTRUCTION STANDARDS              

 Use of flood resistant materials  15.04.080 B6

Portions of buildings below the base flood elevation (BFE) are often constructed entirely out of concrete, which is considered a flood resistant material. It is also a common building practice to frame up from a concrete stem wall with wood construction to create a garage/storage space below the elevated first floor. Since garage spaces typically utilize sheetrock to achieve the necessary fire separation, construction of this type results in the use of materials subject to flood damage.

In order to comply with FNSB guidelines for flood resistant materials,  the use of untreated wood and sheetrock to cover wall members below the BFE is prohibited. The preferred design alternative (other than concrete walls) will be the use of pressure treated heavy timber construction (6"x10" horizontal, 8"x8" vertical) and pressure treated frame members. The ceiling can be protected with sheetrock if the first floor above the protected ceiling is one foot above the BFE and the sheetrock is less than one foot below that elevation. Cement board may be used as a substitute for sheetrock.

The area of a building below the BFE may only used for building access, parking and storage. No living space is permitted below the BFE.  All mechanical and electrical devices servicing the building shall be located at or above BFE.

The Federal Emergency Management Agency (FEMA) guidelines for Flood Damage Resistant Materials are contained in Technical Bulletin 2, August 2008. This publication is available for review or reproduction upon request. This publication is also available on the Web. (http://www.hawaii-county.com/pubwks/TB2-08.pdf)

All construction below the BFE is susceptible to flooding and must consist of flood-resistant materials. In order to adequately determine if flood-resistant materials are required, applicants proposing construction in Special Flood Hazard Areas shall provide  existing ground elevations for at least one corner of the proposed development.

The BFE shall be shown on the elevation drawings for the proposed structure. The BFE will be established by this department and conveyed to the applicant for incorporation into the building plans unless the applicant wishes to develop their own BFE using commonly accepted engineering practices. 

Basements/CRAWLSPACES

The FNSB Floodplain Ordinance requires that the lowest floor, including basement, be elevated at or above the BFE. The National Flood Insurance Program defines a basement as "any area of the building having its floor subgrade (below ground level) on all sides."

Applicants proposing construction in flood prone areas will need to be aware of final interior and exterior grade levels of the proposed structure.  Subgrade enclosures can incur significant flood insurance penalties if not constructed properly.

Openings to equalize hydrostatic flood forces 

The FNSB Floodplain Ordinance requires that all fully enclosed areas below the lowest floor that are usable solely for parking, building access, or storage such as crawl spaces shall have a minimum of two openings having a total net area of not less than one square inch for every square foot of enclosed area. The bottom of all openings shall be no higher than one foot above grade. Openings may be equipped with screens, louvers or other coverings or devices provided that they permit the automatic entry and exit of floodwaters. 

The vents should be placed on opposing walls to allow the entry and exit of floodwaters. Detailed information about FEMA’s flood venting requirement may be found in Technical Bulletin 1.  This publication is also available on the Web. (http://www.fema.gov/library/viewRecord.do?id=1579)

Electrical gear and equipment 

All electrical, heating (including fuel tank), ventilation, plumbing and air conditioning equipment that is permanently affixed to a structure and which may be subject to floodwater damage shall be elevated at the BFE or higher unless otherwise constructed to prohibit the entry of flood waters.  FEMA has published a document titled Protecting Building Utilities from Flood Damage that gives specific guidance on proper construction technique.  This publication is also available on the Web. (http://www.fema.gov/library/viewRecord.do?id=1750&fromSearch=fromsearch) 

ONSITE WASTE DISPOSAL SYSTEMS 

Onsite waste removal systems shall be designed to minimize or eliminate the infiltration of flood waters into the disposal systems and discharges from the systems into floodwaters; and shall be located to avoid impairment to them or contamination to them during flooding.  An anti-back flow valve is a common and effective means to insure septic effluent is contained. 

PLACEMENT OF MANUFACTURED HOMES 

All new construction and substantial improvements, including the placement of prefabricated buildings and manufactured homes, shall be designed and adequately anchored to prevent flotation, collapse or lateral movement of the structure resulting from hydrodynamic and hydrostatic loads, including the effects of buoyancy. 

Fill/Encroachment Guidelines 

Proposed developments cumulatively may not increase base flood heights more than one-foot anywhere in the identified floodplain.  This applies only to floodplains with BFEs but without identified floodways. (15.04.050 D) 

All watercourse alterations or modifications must not reduce the carrying capacity of the stream or increase BFEs.  The applicant must submit an analysis that compares existing channel capacity with proposed capacity.  Floodway regulations apply for alterations within a designated floodway. (15.04.050 C) 

 APPENDIX C – “NO-RISE” ANAYLSIS PROCEDURES 

Section 60.3(d)(3) of the National Flood Insurance Program (NFIP) requires that the FNSB  prohibit encroachments, including fill, new construction, substantial improvements, and other development within the adopted regulatory floodway unless it has been demonstrated through hydrologic and hydraulic analyses performed in accordance with standard engineering practice that the proposed encroachment would not result in any increase in flood levels within the city during the occurrence of the base (100-year) flood discharge. 

In most cases, the “No-Rise Certificate” must be supported by technical data based upon the standard step-backwater computer model utilized to develop the 100-year floodway shown on the effective Flood Insurance Rate Map (FIRM) and the results tabulated on the Flood Insurance Study (FIS).