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Floodplain Development And Floodway Guidance


The purpose of this guidance is to provide aid in those situations when floodplain development is proposed and the intent of the project is to meet the "no-rise" and/or "no adverse effect" criteria by using the effective hydraulic model, but not to revise the flood maps. Under those conditions the analysis can be reviewed at the local or state level, and the Federal Emergency Management Agency (FEMA) need not be involved. If changes to the maps are required, such as a change in base flood elevation (BFE), or the extent of the special flood hazard area (SFHA), or the location of the floodway, then a request must be submitted to FEMA for a map change.

"No-Rise" versus "No Adverse Effects"

There is a two-tiered system of technical evaluation for proposed development in the floodplain. All proposed floodplain development must meet the "no adverse affect" criteria, while proposed floodway development must also meet the "no-rise" criteria. For streams with detailed studies/AE-zones, the 100-year floodplain has been divided into two zones, the floodway and the floodway fringe. The floodway is that area that must be kept open to convey flood waters downstream. The floodway fringe is that area that can be developed in accordance with FEMA standards as adopted in the local law.

"No-Rise" Any proposed development within the floodway requires a hydraulic analysis to demonstrate "no-rise." "No-rise" is defined as a 0.00-feet difference in the computed BFE's at each modeled cross-section. The determination of "no-rise" would normally be demonstrated by comparing the natural run, base flood elevations (BFE) as listed in the output of the Pre-project and Post-project Models. See Table 1 - Comparison of BFEs, for an example. Using such a table, the differences stemming from each successive modeling step can be clearly seen, then the reasons for those differences can be adequately justified and documented.

Table 1-Comparison of BFE's
profile Section Label Model Section # Effective Model Duplicate Effective Model Dup-Eff Pre-project Model Pre-Dup Post-project Model Post-Pre
F 5235 1325.33 1325.33 0.00 1325.33 0.00 1325.33 0.00
5265 1325.38 1325.38 0.00 1325.38 0.00 1325.38 0.00
5330 1325.48 1325.48 0.00 1325.48 0.00 1325.48 0.00
G 5510 1325.82 1325.83 0.01 1325.86 0.03 1325.86 0.00
5580 1325.77 1325.79 0.02 1325.78 0.00 1325.78 0.00
5620 1325.87 1325.88 0.01 1325.90 0.03 1325.90 0.00
H 5820 1326.49 1326.46 -0.03 1326.63 0.16 1326.63 0.00
6225 1327.00 1326.97 -0.03
6306 1326.95 1326.89 -0.06
6387 1326.90 1326.82 -0.07
6468 1326.86 1326.80 -0.05
6549 1327.15 1327.34 0.19
I 6630 1327.19 1327.27 0.08 1327.48 0.21 1327.60 0.12
7540 1329.09 1329.29 0.20 1329.48 0.19 1329.62 0.14

When attempting to use the procedure described below to satisfy the "no-rise" requirement, the entire proposed development should be added to the Post-Project model, regardless of whether it is in the floodway. The no-rise analysis with appropriate mitigation should be completed, as appropriate, prior to starting a no adverse effects analysis.

"No Adverse Effects" The local Flood Damage Prevention law defines "adversely effected" as any physical damage to an adjoining or other property. It is not the intent of the National Flood Insurance Program regulations to require a detailed engineering analysis for all developments within the floodway fringe. However the local floodplain administrator may require a "no adverse effect" analysis for larger projects that can potentially result in physical damage to other property.

One method of determining if an adverse effect would occur is to compare the BFE's as listed in the output of the Pre-Project and Post-Project Models. When attempting to use the procedure described below to satisfy the "no adverse effects" requirement, both the mitigated floodway development and the proposed floodway fringe development should be added to the Post-Project Model. Then any rise in BFE can be attributed to the portion of the proposed development that falls in the floodway fringe, which by definition should be less than 1-foot. In any area where there is an increase in BFE, the lowest adjacent grades (LAG) at all potentially affected structures in the area could be surveyed, and if a structure's LAG was below the BFE resulting from the proposed development, then it could be assumed that water would enter and damage that structure and/or its contents. Any such identified adverse effects would then be mitigated as a condition of the floodplain development permit. There is a wide variety of technical publications available from FEMA and the US Army Corps of Engineers that discuss the range of mitigation techniques that are available.

Ultimately, it is the responsibility of the local Floodplain Administrator (FPA) to determine what form of technical evaluation is acceptable. However, it should be noted that NYSDEC has Regional Floodplain Coordinators who are available to provide advice and technical support for specific floodplain development situations. In general, if the FPA has any doubt whether the "no-rise" and no adverse effect criteria can be met, then the applicant should be required to conduct a hydraulic analysis using the effective model. Such a hydraulic analysis, when properly developed, is considered to be both definitive and defensible.


In general, the authority for requiring a hydraulic analysis to satisfy the "no-rise" criteria stems from 44 CFR Part 60.3(d)(3) which states that where a regulatory floodway has been designated the community shall: "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 practices that the proposed encroachment would not result in any increase in flood levels within the community during the occurrence of the base flood discharge." Specific State authority to require a "no-rise" analysis (for State-owned and State-funded projects, only) stems from 6 NYCRR Part 502.4(b). Local authority stems from Article 36 of the Environmental Conservation Law (ECL), as well as various provisions in the applicable local law for flood damage prevention, which are based on FEMA minimum standards, and require technical evaluations for "no-rise" and "no adverse effect." While a hydraulic analysis is considered an option for satisfying the "no adverse effects" criteria for proposed development solely in the floodway fringe, it may be unreasonable to require such an analysis for anything but a large development with a large quantity of fill.

Hydraulic Modeling Software

Historically, HEC-2 was the most commonly used software for hydraulic modeling in flood studies. FEMA now encourages the use of the software package known as HEC-RAS for new flood studies. The conversion of effective studies from HEC-2 into HEC-RAS is also encouraged. However, it should be noted that FEMA does accept models developed using other software packages. A list of accepted hydraulic modeling software packages can be viewed at:

It should be noted that there is a difference between a hydraulic model and the software package used to develop a model. A software package generally includes the algorithms used to perform the calculation, along with any associated input and output interfaces. A "model" is the input file containing the flow rate and channel geometry information for a specific study reach, which is used with the software to simulate how that reach would respond to a specific set of flood conditions.

Recommended Procedures

The following is a summary of recommended procedures for use in updating an effective study to reflect changed and/or proposed conditions. By using this procedure, the effects of model changes at each step can be properly documented and isolated, so that ultimately, only the effects of the proposed development are considered.

Special Circumstances

From time to time, situations such as changed flow conditions or undocumented, man-made changes are discovered. As the available period of record grows for gauge data, or as better hydrologic techniques are developed and applied, significant changes in flows can be encountered during the modeling process. Ideally, any man-made modifications to the floodplain will have been documented through the map revision process. However, significant undocumented, man-made changes are sometimes also discovered during the modeling process.

Care should be taken to incorporate these conditions into the analysis in an acceptable manner. When either of these conditions is encountered, the appropriate map revisions must be requested by the community. However, if these undocumented changes are not the result of a previous action on the part of the developer, then the developer should not be penalized for circumstances that are outside of his/her control. The developer can then either wait for the required map revisions to be implemented (a process which may take several months, or more), or the developer can proceed with a two-scenario approach.

The first scenario would use the Corrected Effective Model to represent the pre-project conditions without the flow changes and/or undocumented, man-made changes included, and would serve to meet the requirements of the current regulatory standard in effect in the community. The second scenario would convert the Corrected Effective Model into a Pre-Project Model by incorporating the flow changes and/or undocumented, man-made changes that have been discovered. The second scenario would serve to meet the requirements of a theoretical, "corrected" version of the regulatory standard. Both scenarios would be developed in parallel based on the same Effective, Duplicate Effective and Corrected Effective Models, and each scenario would be carried through the Post-Project Conditions Model and the Post-Project with Compensatory Actions Model, as appropriate. Both scenarios must separately satisfy the "no-rise" and "no adverse effect" criteria.

Effective Model The effective model is the basis of the current regulatory standard in effect for a community, and serves as the starting point for the modeling of any proposed development in the floodplain. Ideally, the Effective Model will include the "natural run" (resulting in the 10-, 50-, 100-, and 500-year water surface profiles), which is used to define the extent of the floodplain, and the "encroachment run," which is used to divide the floodplain into the floodway and the floodway fringe. It should be noted that the Effective Model often includes more cross sections than are shown on the Flood Insurance Rate Map (FIRM), Flood Boundary and Floodway Map (FBFM) and Flood Insurance Study (FIS). If an FIS has been published for the community and base flood elevations are shown on the FIRM covering the site, the effective model is likely to be available, at no charge from this office. If we don't have it, then we can provide the procedures for requesting that information from FEMA. We can also provide procedures on what to do if the Effective Model cannot be recovered.

Duplicate Effective Model Copies of the hydraulic model used in the effective FIS, referred to herein as the Effective Model, must be obtained and then reproduced on the requester's equipment/software, thus creating the Duplicate Effective Model. The same equipment and software should be used for all succeeding modeling steps. The documentation of this step is required to assure that the Effective Model's input data has been transferred correctly to the new equipment/software and to assure that the revised model will be integrated into the effective data to provide a continuous water surface profile both upstream and downstream of the revised reach. The differences in the water-surface elevations between the Effective and Duplicate Effective Models must be fully documented and thoroughly explained. The entire Effective Model may not need to be recreated. The Duplicate Effective Model must merge with the effective water-surface profile, to within +/- 0.5-feet at the upstream and downstream ends of the revised reach, in compliance with 44 CFR Part 65.6(a)(2).

Differences between the Effective and Duplicate Effective models can often be attributed to computational differences between succeeding versions of the model software, such as: (1) differences in bridge/culvert modeling routines, (2) method of conveyance calculations, (3) critical depth default, and (4) floodway computations.

The HEC-RAS User's Manual and the HEC-RAS Hydraulics Reference Manual provide details on computational differences between the HEC-2 and HEC-RAS and guidance on simulating HEC-2 results; such manuals should be consulted to explain the differences between the Effective and Duplicate Effective Models. The HEC-RAS software and manuals can be downloaded from the internet at:

Another useful reference is FEMA's "HEC-RAS Procedures for HEC-2 Modelers," dated April 2002. This publication can be downloaded from the internet at:

Corrected Effective Model The Corrected Effective Model corrects any modeling errors that exist in the Duplicate Effective Model, adds any additional cross sections to the Duplicate Effective Model required to represent the proposed project, incorporates natural changes in the floodplain (such as stream meandering), and incorporates more detailed topographic information than that used in the Effective Model.

An error could be a technical error in the modeling procedures (such as improperly addressed or ignored warning and error messages), or any development in the floodplain that occurred prior to the effective date of the study, but was not incorporated into the effective model. Changes in structures or vegetation would be reflected in updated n-values. Changes in the floodplain geometry, such as cut or fill, would be reflected in the cross-section. The Corrected Effective Model must not reflect any man-made physical changes since the date of the effective model.

All cross-sections that are required to represent the proposed development should be added during this step. The number and location of cross-sections in the model should remain constant for all remaining steps. The cross-section should represent all significant breaks in slope along the section. Ideally, the collection of new cross-section information will be supervised and certified by a surveyor, engineer or architect who is licensed in New York State. However, cross-sections compiled from available information may be acceptable, if suitably justified and documented. When adding cross-sections, the roughness coefficients (Manning's n-values) for the reach should remain the same, unless a reasonable explanation of why they should be changed is included in the supporting data. The cumulative reach lengths should also remain the same, unless justified and documented.

Note: If it becomes necessary to revise the floodway run, the effective encroachment stations must be retained for those cross-sections that were in the Effective Model. Ideally, encroachment stations for any new cross-sections that are added, should be developed by performing a linear interpolation between the cross-sections immediately upstream and downstream, as included in the Effective Model. If the Effective Model is not available, then the encroachment stations can be approximated by scaling the effective floodway width off of the FIRM or FBFM. Estimated encroachment stations should be confirmed by completing a floodway run, and ensuring that the surcharge (the rise in water surface elevation between the natural and encroachment runs) is no more than 1-foot anywhere along the profile. The estimated encroachment stations must be adjusted until this 1-foot surcharge criteria is met.

Pre-Project Conditions Model Under the second scenario of a two-scenario approach, the Corrected Effective Model is modified to reflect any changed flow conditions and/or undocumented, man-made changes that have occurred within the floodplain since the effective study was adopted, but prior to the construction of the project for which the revision is being requested. If no changed flow conditions or undocumented, man-made changes have been identified, then this model would be identical to the Corrected Effective Model.

Post-Project Model The Pre-Project Conditions Model (or Duplicate Effective or Corrected Effective Model, as appropriate) is revised to reflect post-project conditions. This model must incorporate any physical changes to the floodplain since the effective model was produced as well as the effects of the project. Depending on the circumstances of the project, the applicant may be requested to mitigate to an earlier condition, as represented by the Corrected Effective or Duplicate Effective Models. Output values should be limited to two decimal places. In some cases, the proposed development may result in "no-rise," but will result in increases in water velocity. Such increases may also require mitigation.

Post-Project with Compensatory Actions Model If a rise results from a proposed development, then subsequent model runs should be developed to represent the impacts of any proposed compensatory action and demonstrate compliance with the "no-rise" and "no adverse effect" requirements. A list of potential compensatory actions has been included in FEMA's "Procedures For Compliance With Floodway Regulations," dated May 1990. Copies of this document can be obtained from this office. Any proposed compensatory actions involving the alteration of the stream channel or within 50-feet of the stream banks should be reviewed by the appropriate DEC Regional Permit Administrator for compliance with 6 NYCRR Part 608, Use and Protection of Waters.

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