Requirements for Channel Modifications in Unnumbered A Zones
The attached forms are provided to help clarify the data requirements for engineering studies within FEMA Unnumbered (Approximate) A Zones. The forms address engineering requirements necessary to evaluate the impact of proposed developments on flooding. This form is to be used for structural and non-structural activities that modify channels located with Special Flood hazard Areas (SFHA’s) as defined by the Richland County Special Purpose Flood Damage Prevention Resolution (Regulations) as adopted May 28, 1987.
Building standards for protection of structures from flood damage are not addressed by the forms.
The USDA Forest Service publication referenced in the forms and the presentation (General Technical Report RM-245, Stream Channel Reference Sites: An Illustrated Guide to Field Technique) can be downloaded via the Internet at the following address: http://www.stream.fs.fed.us/publications/PDFs/RM245E.PDF
Note: Fill out one form for each flooding source studied
Describe the limits of the reach being studied or submit a workmap with the study area clearly highlighted. If the reach has been identified by FEMA as a Special Flood Hazard Area, a highlighted FIRM or FHBM should be attached depicting the area of study.
Copy of highlighted FIRM or FHBM attached? Yes No
MODELS REQUIRED - FEMA ZONE A – NO BFE PROVIDED
Purpose of study:
Determine Base Flood Elevation data for “Subdivisions and Large Developments” (as defined by Regulations Section 5.2-6)
Indicate models submitted below and complete Section 3. Full input and output listings along with files on diskette for items A, B, and C listed below and a summary of the source of input parameters used in the models must be provided. The summary must include a description of any changes made from the Existing or Pre-Project Conditions model to the Revised or Post-Project Conditions model. Delineation of a floodway is recommended, but may not be required.
The Existing or Pre-Project Conditions model represents existing conditions at the time the flood study was conducted.
B. Revised or Post-Project Conditions Model Natural Floodway
(File name) (File name)
The Existing or Pre-Project Conditions model is revised to reflect revised or post-project conditions. This model must incorporate any physical changes to the floodplain resulting from the project. When the request for approval is for the proposed project this model must reflect proposed conditions.
C. Other Please attach a sheet describing all other models submitted along with the file names.
Perform an impact analysis for alteration of a watercourse
Computer model not required. Complete Section 3.
Please contact Richland County for information on data requirements for flood studies completed for purposes other than those listed above.
ALTERATION OF WATERCOURSE
Field Determination of Watercourse Geometry:
The horizontal extent of the watercourse must be determined by field observation. It is recommended that bankfull stage, as described in the USDA Forest Service General Technical Report RM-245, Stream Channel Reference Sites: An Illustrated Guide to Field Technique, be used to define the watercourse. Provide a site plan clearly indicating the location of cross-sections where the watercourse geometry has been determined.
Provide a plot of all cross-sections used to determine the bankfull watercourse. Any of the following bankfull indicators that are present, as described in the above referenced report, should be clearly labeled on the cross-section:
4) Change in Bank Materials 5) Bank Undercuts 6) Stain Lines
The following information should be determined at multiple cross-sections to ensure proper evaluation of the channel bankfull geometry:
1. Bankfull width 2. Bankfull depth/stage 3. Water surface elevation at bankfull stage
Note: Provide a summary table for the information listed above. If bankfull indicators disagree, please attach an explanation for how bankfull determination was made.
Flood Carrying Capacity:
Does the proposed work encroach upon the bankfull watercourse determined above?
If “No”, the proposed work will not diminish the flood carrying capacity of the watercourse.
If “Yes”, an engineering analysis must be performed that determines the following information for each cross-section:
Bankfull water surface elevation (as determined above) Bankfull discharge capacity
Using the existing conditions bankfull discharge capacity, compute the water surface elevation for the proposed channel configuration.
Note: Provide a summary table and supporting calculations for the information listed above.
Does the proposed channel configuration convey the bankfull discharge without increasing the water surface elevation?
If “Yes”, the proposed work will not diminish the flood carrying capacity of the watercourse.
If “No”, the proposed work will diminish the flood carrying capacity of the watercourse.
Maintenance of Watercourse:
Does the proposed channel design incorporate concepts/elements intended to prevent increased erosion or sedimentation?
If “Yes”, please attach plans and an explanation describing how the design elements work to prevent increased erosion or sedimentation.
If “No”, has a maintenance agreement been signed between the permit requestor and an authoritative government entity, so that the flood carrying capacity of the altered or relocated watercourse will not be diminished?
Note: It is recommended that a maintenance agreement between the permit requestor and the appropriate government entity be required that indicates the specific maintenance responsibilities of the requestor.
Riverine Hydraulic Analysis Form
(Models required - FEMA Zone A - no BFE provided) updated 08/05/2010