Volume 24, Number 9  ●  August 2, 2016

 

Fairfax County Floodplain Studies May No Longer Be Valid – New RPA Plans Affected

Since 2004, the National Oceanic Atmospheric Administration (NOAA) has been updating precipitation frequency estimates across the country, which is utilized by a common method of hydrologic analysis developed by the National Resources Conservation Service (NRCS).  Since this updated precipitation frequency data can result in larger 100-year peak discharges, this can result in higher flood elevations that might render previously approved floodplain studies invalid.  Fairfax County has issued a Technical Bulletin to address how the updated data affects the validity of previously approved floodplain studies and Resource Protection Area (RPA) plans that have a mapped floodplain component.   

Background

Various hydrologic computational methods may be utilized to compute the 100-year peak discharge necessary in performing a floodplain analysis to determine 100-year flood elevations and resulting flooding limits.  In 1961 and 1977, respectively, the National Weather Service (NWS) published precipitation data collected across the country in Technical Paper-40 (TP-40) and Hydro-35.   Various common and widely accepted hydrologic methods utilized this NWS data, which until recently, was the best available data. 

Since 2004, NOAA has been compiling updated precipitation and storm distribution data.  This new data is being published in Atlas 14 as it becomes available region by region.  Currently, updated data is available for the entire country except for TX, MT, WY, ID, WA and OR.

What Does This Mean For Floodplains and RPAs in FAIRFAX COUNTY?

Fairfax County identifies acceptable hydrologic methods in Section 6-0801 of the Public Facilities Manual (PFM) that may be utilized for various analyses.  The application of one particular method over another is generally based on available data and the size of the watershed being analyzed.   However, per the PFM, NRCS methodology is the preferred method for all size watersheds.  Therefore, it has been common for floodplain studies to utilize NRCS methodology when a hydrologic analysis was required to develop 100-year peak flood flows.  In such cases, the hydrologic analysis was performed utilizing the prior NWS precipitation data.  With updated precipitation data available in Atlas 14 for Virginia, Fairfax County has issued Technical Bulletin 16-02, Impact of National Oceanic and Atmospheric Administration (NOAA) Atlas 14 Rainfall Data on use of Existing Floodplain Studies to address how such updated data affects the validity of previously approved floodplain studies and RPA plans.  

For any development in Fairfax County, the location of floodplains and RPAs must be identified on the plans in order to conform to the County’s floodplain regulations and the Chesapeake Bay Preservation Ordinance (CBPO).  Since a major floodplain (a floodplain with a drainage area of 360 acres or more) is a buffer component of determining an RPA in Fairfax County, any new RPA plan utilizing floodplain data from a previously approved floodplain study must include a statement that identifies whether the prior approved study is still considered valid (per the Technical Bulletin). 

How do you determine whether a prior approved floodplain study is still valid?

Step 1:    Did the prior floodplain study utilize NRCS peak discharge methodology?

If NO – prior study remains valid.

If YES – go to Step 2.

Step 2:  Re-evaluate the NRCS hydrology utilizing the updated NOAA Atlas 14 rainfall data.

If use of NOAA Atlas 14 results in reduced 100-yr peak flow rates, then the prior study remains valid.

If use of NOAA Atlas 14 results in greater 100-yr peak flow rates, go to Step 3.

Step 3:  Perform a new hydraulic analysis to determine effect on flood elevations.

If greater 100-year peak flows result in increases in flood elevations that are equal to or less than 0.1 foot, the prior study remains valid

If greater 100-year peak flows result in increases in flood elevations greater than 0.1 foot, the prior approved floodplain is no longer valid and a revised floodplain study is required. 

What about previously approved RPA plans in FAIRFAX COUNTY?

In Fairfax County, a site plan for any proposed development project submitted for review/approval prior to May 20, 2016, and for infill lot projects, a prior-approved RPA delineation study remains valid.  For a proposed development project submitted for site plan review/approval after May 20, 2016 (excluding infill lot projects), a prior-approved RPA delineation study should be re-evaluated and updated to address the requirements of LTI 16-02.  Any new or additional encroachment of an existing structure that results from extension of the RPA buffer created by the procedures outlined in LTI 16-02 will not require additional mitigation.

How are OTHER LOCALITIES incorporating the updated NOAA Atlas 14 data into the plan approval process?

Regarding RPAs, Fairfax is the only locality in Northern Virginia that incorporates the floodplain as a buffer component in determining the RPA.  So RPAs in other Northern Virginia localities in the Chesapeake Bay watershed will not be affected by the updated NOAA Atlas 14 data.  However, although most localities do not require previously approved floodplain studies to be re-evaluated for other plan approvals, most require that all new hydrologic analyses incorporating NRCS methodology utilize NOAA Atlas 14 data.

Below is a list of some Northern Virginia localities and their requirements regarding the use of the new NOAA Atlas 14 data:

LOUDOUN COUNTY:

Loudoun County has developed regression equations to be used for all new hydrologic computations.  All plan submissions that incorporate a new hydrologic analysis or depends on prior approved hydrologic analysis results must utilize county specific regional equations or be re-evaluated using county specific regional equations, respectively.  

PRINCE WILLIAM COUNTY:

For all plan submissions, all new hydrologic calculations based on NRCS methodology must use the updated Atlas 14 data.  The validity of previously approved hydrologic calculations that utilized NRCS with older rainfall data needs to be discussed on a project-by-project basis with the floodplain administrator.

CITY OF ALEXANDRIA AND ARLINGTON COUNTY:
(THIS IS ALSO THE GENERAL RULE FOR MOST OTHER LOCALITIES)

For all plan submissions, previously approved hydrologic and floodplain analyses remain valid until a floodplain is impacted.  If the impact is significant enough to require new hydrologic calculations and such calculations are based on NRCS methodology, the analysis should use the updated Atlas 14 data.  Otherwise, previously approved hydrologic calculations remain valid. 

 

For help in determining whether a previously approved floodplain affecting your site is still valid, or for general information regarding floodplains and RPAs, please contact Mike Marsala, Frank Graziano, or Ben Rosner.   


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