FLOOD00875

• County: Grand
• Community: Granby
• Title: Floodplain Information Report - Town of Granby
• Date: 3/1/2002
• Prepared For: Granby
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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In comparing hydrology results with the previous drainage report for the Middle and High School
Campus at the campus outfall, the previous study calculated a 100-year discharge of 91 cfs while
the FIR calculated a loo-year discharge of 70 cfs. The difference is due to different hydrology
methodologies and slightly different drainage basin sizes. If and when drainage improvements
for this area are designed, further hydrology calculations will be necessary to calculate required
capacities of drainage facilities.

Hvdraulic Analvsis
Normal depth calculations were performed at several representative cross-sections to estimate the
extent of the lOO-year flow. Roughness characteristics were estimated using typical figures from
available publications and are based on field investigations of the sites. In addition, the
estimated flood prone areas were field verifiel;! and modified as deemed necessary based on field
judgement. Two types of flow areas have been shown on Figure 3.

~ Flood Prone Areas with loo-year Flow Depth Less than one foot
~ Flood Prone Areas with loo-year Flow Depth Greater than or equal to one foot

Because the drainage basins for the flood prone areas are less than 1 square mile, FEMA and
CWCB do not consider these flood prone areas as floodplains. There are also several areas of
minor localized flooding that may not be shown on the maps. However, delineation of these
flood prone areas will assist the Town in its planning efforts.

For future studies, an approximate loo-year flood elevation at a particular location may be
obtained by using the l00-year peak flow value (as provided in this report) in conjunction with a
hydraulic analysis similar to the analysis used above. These calculations may be performed using
FEMA's Quick-2 computer program, the Corps of Engineers' HEC-2 computer program,
F1owmaster, or other acceptable hydraulic methods. Channel and floodplain cross-sections may
be obtained from detailed topographic mapping or from field surveys. Cross-sections obtained
from USGS quadrangle maps are not advised.

Floodplain Mappin!!
The floodplains shown herein for Granby depict areas subject to loo-year flooding based on
approximate technical methods. The map does not show flood profiles, base flood elevations, or
sufficient detail in order for Town officials, engineers, developers, and land owners to make
floodplain detenninations with any certainty. The Colorado Water Conservation Board can be
contacted to request technical assistance for approximate floodplain analyses as needed.

Granby should regulate the approximate floodplains on a case-by-case basis. FEMA requires
that a detailed floodplain analysis must be performed by a professional engineer and submitted
by the developer or development proponent for development projects involving more than 5
acres or more than 50 units (whichever is more restrictive).

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