FLOOD09354

• County: Douglas
• Community: Douglas County
• Basin: South Platte
• Title: South Platte River Post Wildfire Floodplain Study, Hayman Burn Area, Douglas County
• Date: 11/24/2004
• Prepared For: CWCB and Douglas County Public Works
• Prepared By: Anderson Consulting Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

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II. TOPOGRAPHIC BASE ,\IAP A;;D FIELD SliRVEYI;;G

2.1 Topographic Base .\Iap

The primary source of information for the South Platte River topographic base map was
prepared by Spectrum ~1apping LLC and provided by Douglas County in the fonn of an ortho-
rectified aerial photograph and contours at a 1:.[001 interval. The topography was prepared based
on aerial photography taken on January 8. 2004. The above-water LiDAR data provided by the
County was supplemented \'.ith bathymetric field survey data provided by Douglas County and
prepared by ICON Engineering. Inc.. as described in Section 2.2. Additional cross section
infonnation was provided by the USGS. as discussed in Section 4.3.
The topographic base map was prepared using the North American Datum of 1983
(NAD83) as the horizontal datum. and the North American Vertical Datum of 1988 (NA VD).
The grid coordinate system selected for the work map is State Plane Colorado Central. Douglas
County chose these horizontal and vertical datums. as well as the identified coordinate system. to
be in compliance with FEMA requirements and to maintain consistency between the floodplain
maps that will be prepared as part of this study and floodplain information recently prepared as
pan of the Douglas County DFIRM.
A portion of this 2-foot contour mapping can be seen on the floodplain and floodway
work maps included in this report. The topographic information shown on the work maps is only
a portion of the entire set of contours provided by the County. Due the large amount of data
associated with the entire topographic coverage. the work maps show only a relatively narrow
corridor along the South Platte River. The complete set of LiDAR-based topography can be
found on the disk included with this report.

2.2 Field Surveying

In an effort to define detailed bridge geometry and channel cross sections (including
bathymetr:'). tield surveying of the South Platte River was completed by ICO>J" Engineering in
support of the current project in November 1003 and June 2004.
The Held sun:ey'ing efforts included the inventory of all bridges and culverts along the
study reach, For each bridge. this inventor:' included top of deck elevation. low chord elevation.
bridge length. bridge opening dimensions. and pier \\idth and location. A surveyed channel
cross section was also obtained at each bridge, It is noted that three bridges located in the
Wigwam area. at the upstream end of the study reach. were not surveyed due to property
accessibility issues. Documentation of the bridge inventor:' data can be found in Appendix A.

COCWCB03_South PlancJeport_doc

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