ASFPM--NFPPR –2015

B. HYDROLOGY AND HYDRAULICS

12-26-14

Recommendation Explanation/rationale

  1. (a) Federal programs should incorporate future-conditions hydrology and cumulative impacts of watershed and hydrologic changes 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.
(b)Simple 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 the future condition 1% annual chance discharge, should be instituted.
(c)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, DOI, 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 could show the resulting future conditions floodplain boundary as an advisory flood boundary for insurance purposes.
Some communities or states require hydrology to be calculated based on some expected future scenario, such as the expected development conditions in the community development plan, or the expected development in the next x years (20 to 50) if it increases impervious surface by more than x%
Too often structural or development projects have been constructed or improved on one side of the river only to raise flood elevations on the other side of the river or upstream or downstream. Such impacts must be mitigated or flooding easements purchased before construction occurs.
  1. Consider using the 95% upper confidence interval (instead of the current50%) for flow values used in Flood map studies to account for uncertainties in determining regulatory discharges in a non-stationary meteorological environment. [FEMA, ACWI- SOH, mapping partners]
/ Studies have shown that contrary to current assumptions, the meteorological conditions are not stationary. In addition, watershed development, frequency estimation errors or blocking of bridges, etc., usually 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. (a) Use the “Full Conveyance Floodway”, as discussed in mapping section A-8, to designate NFIP regulatory floodways (instead of current proceduresthat allows an artificialrise (one foot for NFIP) in flood levels(and a variable amount (down to zero rise) from state to statechosen by that state) to ensure that new development does not result/cause an increase in flood elevations.
(b)When full conveyance floodways are not adopted, the flood elevations calculated for the “with floodway” condition in the Flood insurance study floodway data tableshould be designated as the regulatory BFE for that community; and the community should be required to obtain easements from any existing development impacted by the resulting increase in flooding allowed. In addition, any new development in the floodway should not be allowed to cumulatively increase flood elevations or floodwater velocity or reduce floodwater storage either within the floodway or within the fringe areas.
(c)In lieu of and until the standard in (a) is in place, The NFIP should consider designating the BFE calculated for the “with floodway”conditions as a minimum national standard and also encourage states and communities to adopt such “full conveyance floodway” concept as a higher regulatory standard.
See A--8
[FEMA, TMAC, mapping partners] / The one foot rise allowed by the NFIP results in a significant loss of conveyance areas because the floodway width decreases from 32% to 68% and velocity increases from 16% to 62%reporthere
This provision would prevent the BFE from increasing due to encroachments into what is called flood fringe but is really floodway; and avoid transferring the responsibility from those that cause the problem to those that suffer the consequences
Under current procedures the flood insurance study allows the floodway to be pinched in until the flood level rises by a pre-determined amount (default one foot or in some states a smaller threshold); but while that map allows development to occur in the pinched in area that is designated as flood fringe, it is actually floodway. However, the community is not required to adopt that higher elevation, guaranteeing that those who build to the BFE will experience a higher elevation of flooding once that pinched in area is developed. The same is true for velocity, it may be increased because development is allowed in the pinched in area, but no consideration is given to that increase
This would provide incentives for communities to adopt full conveyance floodways or regulations that would require that the adverse impacts of new development on existing development or floodplain habitat would be mitigated.
4. Engineering models used to produce NIFP maps must be properly calibrated tohistoric flood eventsby using the stage-discharge relationship at USGS gaging stations;or where gage data is unavailable to historic high water marks to reduce the uncertainty associated with the model results before such models can be deemed accurate and acceptable.[FEMA, mapping partners] / Streams gages and high water marks HWMs) on streams with no gages document historic flood events.
The reason calibrated maps can be deemed accurate is that while the uncertainty can never be reduced to zero, it can then be quantified. In instances where engineering models have not been calibrated or validated against historic flood events – the uncertainty is unknown, as is the accuracy.
Many engineering models for Flood maps have never been verified or calibrated, sowhen owners of buildings that have never flooded are required to purchase flood insurance the focus shiftsto questioning the accuracy of the maps instead of avoiding flood hazard areas or mitigating flood risks,.
  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 2015 ASFPM Rec/rationalePage 1 of 3 H&H Section Bdraft 12-26-14