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SECTION 10: FLOOD HAZARD EVALUATION
- Detailed Information
- Waiver of Requirement
- General. Land subject to being flooded
by a flood of one hundred (100) year frequency as defined by Title 76,
Chapter 5, MCA, or land deemed to be subject to flooding by the Council,
shall not be subdivided for building or residential purposes, or other
uses that may increase or aggravate flood hazards to life, health or
welfare, or that may be prohibited by state or local floodplain or floodway
- Procedure. If any portion of a proposed
subdivision is within two thousand (2,000) horizontal feet and less
than twenty (20) vertical feet of a watercourse draining an area of
twenty-five (25) square miles or more, and no official floodplain or
floodway delineation (study) of the stream have been made, the subdivider
shall provide in detail the calculated 100-year frequency water surface
elevations and/or 100-year floodplain boundaries. This detailed information
shall be performed by a licensed professional engineer experienced in
this field of work.
- Submission of Report: This information shall be submitted at the
time of preliminary plat application. This information may be submitted,
upon the request of the Commission, to the Floodplain Management
Section, Water Resources Division, Department of Natural Resources
and Conservation (DNRC) for review and concurrence.
- Detailed Information. The detailed
information to be submitted, shall include the following:
- Certification: Certification by a registered professional engineer.
- Overall Plan View: An overall scaled plan view (project map)
with identified scale for vertical and horizontal distance showing
- Floodplain boundaries.
- Location of property.
- Cross sections.
- Bridges or other contractions in the floodplain.
- USGS gauging stations (if any).
- Benchmark(s): The location and elevation of a temporary benchmark(s)
established within the subdivision and referenced to mean sea level
with appropriate elevation adjustment.
- Cross Sectional Information:
- Cross section elevations and stations should be determined
at points representing significant breaks in ground slope and
at changes in the hydraulic characteristics of the floodplain
(e.g., points where ground cover, soil, or rock conditions change).
Elevations must be reported in NAVD 88 or NGVD 29 datum.
- Each cross section shall cross the entire floodplain. The
cross section alignment should be perpendicular to the general
flow of the watercourse (approximately perpendicular to contour
line). Occasionally, wide floodplains require a dog-leg alignment
to be perpendicular to the anticipated flow lines. Shots should
be taken at the water’s edge and measurement taken (if
elevation shots can not be taken) to determine the channel bottom
shape. Cross sections shall be accurately located on a USGS
7½-minute quad sheet.
- The number of cross sections needed, and the distance between
cross sections will vary depending on the site, the slope of
the watercourse, the slope of the channel, and the hydraulic
characteristics of the reach. A minimum of four cross sections
are required over the entire reach with at least two cross section
at the property where the elevations are desired. Additional
cross sections must be taken at bridges, control structures,
or natural constrictions in topography.
**NOTE: Photogrammetric methods may be used in lieu of cross
sections whenever appropriate, and when the use has been reviewed
and approved in advance by the City Engineer.
- Bridges: Descriptions and sketches of all bridges within the
reach, showing unobstructed waterway openings and elevations.
- Water Surface: Elevation of the water surface is to be determined
by survey as part of each valley cross section.
- Supporting Documentation: Engineering report of computer computations,
calculations, and assumptions that may include:
- Hydrology (Research of published hydrology or calculations
showing how hydrology was derived).
- Input Files (hardcopy and on diskette).
- Output Files (diskette only).
- Waiver of Requirement. The Council
shall waive this requirement where the subdivider contacts the Water
Resources Division, NRCS, and that agency states in writing that data
indicates that the proposed subdivision is not in the flood hazard area
as defined in this Section.