NFPPR –B. HYDROLOGY AND HYDRAULICS

12-15-14

Recommendation Explanation/rationale

  1. 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]
/ Incorporating reasonably expected future hydrology (likely changes that are unmitigated) into all federal programs is necessary as a national security and sustainability measure. This should include maps produced under the NFIP flood insurance studies as required by the 2012 NFIP reform legislation. FEMA wcould show the resulting future conditions floodplain boundary as an advisory flood boundary for insurance purposes.They should identify assumptions for hydrologic estimates, especially those accounting the impacts of climate change and cumulative impacts of permitted flood-fringe filling and watershed land use changes; that were used for future conditions analysis.
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
  1. 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]
/ Studies have shown that contrary to current assumptions, the meteorological conditions are not stationary. In addition, frequency estimation errors may lead to underestimation of regulatory discharges. Utilizing the 95% upper confidence limits is a prudent way to address uncertainties regarding regulatory flows and to protect life and property.
  1. 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]
/ This would prevent the BFE from increasing due to encroachments and avoid transferring the responsibility from those that cause the problem to those that suffer the consequences.
Communities could apply for an alternative to this process through measures such as offsetting compensatory storage for any floodplain development
  1. 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]
/ Many engineering models for Flood maps have never been verified or calibrate, so it is not surprising significant errors are found while the map is being used or when restudies take place
  1. Guidelines and quality assurance protocols must be established for performing and evaluating all engineering and flood models, including the unsteady and two-dimensional models.
[FEMA. ACWI, MitFLG, USACE] / Better guidance is needed for the use of emerging unsteady state and 2-D modeling techniques to assure uniformity and appropriateness for a regulatory environment. This would include reporting requirements appropriate for these modeling techniques, and the development and review of floodway boundaries derived from such models.
  1. 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.
/ Unsteady state and 2-D models produce more accurate results and their use should be promoted, especially where circumstances require such representation of the system.

NFPPR Combined comm rec and commentsPage 1 of 2 draft 12-1501-14