NFPPR –B. HYDROLOGY AND HYDRAULICS
12-15-14
Recommendation Explanation/rationale
- Federal programs should incorporate future-conditions hydrology and cumulative impacts into flood risk determinations. Such future-condition hydrology should incorporate the impacts of climate change and cumulative impacts of permitted flood-fringe filling and watershed land use changes, . This would include maps produced under the NFIP Flood insurance studies, and should institute sSimple alternative methods for considering future-condition hydrology, such as using the 0.2% annual chance peak discharge in place of 1% annual chance peak discharge in urban areas or using 125% of the 1% annual chance peak discharge, in lieu of detailed analysis to determine a future condition 1% annual chance discharge, should be instituted. FEMA would show the resulting future conditions floodplain boundary as an advisory flood boundary. All federal projects should use future conditions in planning, design and construction to avoid loss of level of protection and adverse impacts on other properties [FEMA, MitFLG, DOT, EPA, NOAA, USACE]
All federal projects should use future conditions in planning, design and construction to avoid loss of level of protection and adverse impacts on other properties
- Consider using 95% upper confidence interval (instead of 50%) for flow values used in FIS studies to account for uncertainties in determining regulatory discharges in a non-stationary meteorological environment. [FEMA]
- Use the “Full Conveyance Floodway”, as discussed in mapping section, to designate regulatory floodways instead of current procedures. In lieu of and until such standard is in place, FEMA should consider designating the BFE to that calculated for the “with floodway” conditions as a minimum national standard and also encourage states to adopt such “full conveyance floodway” concept as a higher standard floodway and also Since floodplain storage is currently not normally considered when floodway or floodplain limits are determined, the NFIP should either require consideration of storage loss in determining the floodway/floodplain or require a FEMA should promote no net loss floodplain storage requirements. [{FEMA]
Communities could apply for an alternative to this process through measures such as offsetting compensatory storage for any floodplain development
- Engineering models must be properly calibrated to topography, historic flood events and, where available, the stage-discharge relationship at USGS gaging stations to reduce the uncertainty associated with the model results before such models can be deemed accurate and acceptable. [FEMA]
- Guidelines and quality assurance protocols must be established for performing and evaluating all engineering and flood models, including the unsteady and two-dimensional models.
- Promote the use of Unsteady State models to account for floodplain storage and uncertain flow paths, and to improve the accuracy of the results. 2-D modeling should also be promoted for use in appropriate situations.
NFPPR Combined comm rec and commentsPage 1 of 2 draft 12-1501-14