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<br />Roaring Fork River
<br />Floodplain In/annation Report
<br />
<br />Town of Basalt, Eagle & Pitkin Counties, Colorado
<br />
<br />Roaring Fork River
<br />Floodplain Information Report
<br />
<br />Town of Basalt, Eagle & Pitkin Counties, Colorado
<br />
<br />the buildings and general flow patterns, When setting up the HEC-RAS model for the South Side split,
<br />the flow lrace animation was run to show areas of high velocity and flow concentralion,
<br />
<br />The SMS flow trace animation and velocity distributions allowed the team to more accurately locate flow
<br />concentrations between buildings and where the flow was effective in Ihe floodplain, The animalion also
<br />facilitated cutting cross sections and adjusting Manning's "n" values to account for the two dimensional
<br />flow characteristics,
<br />
<br />SECTION 5 - INTERPRET A TION AND USE OF REPORT DATA
<br />
<br />5.1 Flood FreQuencv and Discharl!e
<br />
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<br />The 10, 50, 100, and 500'year flood evenlS were used as the flood frequencies for this floodplain analysis,
<br />Thus, the data developed in Ihis report will be compalible not only for regulation purposes and State of
<br />Colorado H. B. 1041 designalions, but are also for FEMA flood insurance rale sludies,
<br />
<br />The 500-year flood event is important in making the public aware Ihat floods larger than the 100.year
<br />flood can and do occur. The 500-year flood event can also be used for regulating developments within
<br />the floodplain,
<br />
<br />5.2 Flood Elevations
<br />
<br />Figure 6 - Flow trace animation snapshot created from the solution of the two-dimensional flow field using
<br />
<br />The flood frequency elevation and discharge data table, Table 3, lists the 10,50, 100, and 500,year flood
<br />elevations at selected reference points (cross-section locations), Base flood elevation contours are shown
<br />on the mapping for the 100-year flood, The flooded area sheets give the plan view of the flooded area on
<br />a contour base map, and the high water elevations for the lOO'year flood can be inlerpolated from this
<br />information, The flood profile plates show the streambed elevation and Ihe high water elevations for all
<br />four frequency floods, Also, the cross section figures in the Appendix of this report show a graphical
<br />representation of the high water elevations at valley cross sections throughout the study reach,
<br />
<br />4.2.2 River Oaks Subdivision
<br />The River Oaks Subdivision, located on the north bank (right descending bank) between the Lower Basalt
<br />Bypass Bridge and Hook's Bridge was an area that was originally mapped outside of the lOO'year
<br />floodplain mapping by Wright McLaughlin Engineers the 1978, Although the undeveloped land, as it was
<br />during the time of mapping, was shown to be outside of the floodplain, the land was only marginally
<br />above the IOO-year floodplain, usually less than one foot,
<br />
<br />The flood profiles may be used in areas where controversy arises over the lOO-year flood boundary on the
<br />flooded area sheets, Since the flood profile plates give the elevation and distance or stationing from a
<br />known point, the high water elevations can be surveyed on the ground to alleviate any discrepancies on
<br />the base map,
<br />
<br />Floodwater will inundate the River Oaks Subdivision and flow northerly, Flow into the subdivision has
<br />been classified as "ineffective flow" since it becomes hydraulically disconnected from the main channel
<br />and will not be flowing in the direction of the channel. It is estimated that the IOO-year flood will overtop
<br />Willits Lane over a distance of 320 feet, with a maximum depth of 1.65 feel. The low point in the
<br />roadway is elevation 6483,04, while flooding is estimated to be elevation 6484,69 at Willits Lane, The
<br />anticipated flow that will enter the Basalt Industrial Park is estimated to be 120 cfs thaI must be managed
<br />in the local drainage system, This flow will be ineffective and shallow sheet flow (Zone 'X'), The Basalt
<br />Induslrial Park is inundated by the 500'year flood,
<br />
<br />Table 4 shows the Floodway data, The communities of the Town of Basalt, Pitkin County and Eagle
<br />County support a "Zero-Rise" flood way rather than the national standard of a "One-Foot Rise" f1oodway,
<br />The exhaustive studies of this river in the Roaring Fork and Fryingpan Rivers Multi-Objective
<br />Planning Project by the Colorado Water Conservation Board in 1999, and this Floodplain Information
<br />Report have concluded that the one,foot rise that was allowed over the past 25 years has already been
<br />used in many areas, In other words, the residential and commercial development, the construction of new
<br />bridges and roads, as well as other encroachments of levees and fill material have forced the elevation of
<br />the flood way to rise past the allowable one foot in many areas, The inherent channel instability on the
<br />Roaring Fork River is a further significant hazard that is not considered in current floodplain modeling,
<br />Therefore, by adopting a zero rise f1oodway, the communities are simply enforcing the spirit of the
<br />original regulation, Furthermore. Ihe threat to life and property by flooding has been shown to be a
<br />genuine cause for concern, Because of the severe danger of flooding and cumulative rise in the flood
<br />elevations over the past 25 years, it is the zero rise flood way that the Town of Basall, Pitkin County and
<br />Eagle County have endorsed for the following summarized reasons:
<br />1. Protection of life and property from high velocity floodwaters on this Sleep channel,
<br />2, No adverse impact to flood conveyance due to past changes that have occurred,
<br />3, Channel instability which is not considered in floodplain modeling, and
<br />4, Added measure of conservatism due to modeling and mapping inaccuracies,
<br />
<br />Due to the development of the River Oaks Subdivision and apparent risk to flooding after the 1995 flood,
<br />the area was studied in greater detail using 17 surveyed cross sections, This new study showed the
<br />channel bed to be nearly 4-feet higher than the 1978 Wright McLaughlin Engineers study in the reach of
<br />the river adjacent to River Oaks Subdivision (cross section 30 through 46,1), This may be the result of
<br />channel aggradation upstream of the bridge and/or mapping inaccuracies in the 1978 study resulting from
<br />aerial interpretation,
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