BOARD01485

• Board Meeting Date: 11/19/2001
• Description: Flood Section - Floodplain Designations and Approvals
• Board Meetings - Doc Type: Memo

Staff recommendation: Staff recommends that the Board: 1) designate and approve the 100-year .
detailed floodplain information for the Roaring Fork River contained in said report within the Town
of Basalt and within unincorporated Pitkin County, and 2) authorize staff to prepare a floodplain
resolution to be signed by the Director and transmitted to Basalt, Eagle County,. and Pitkin County,
This action is recommended in ()rder to meet statutory req*irements.

2) Agenda Item 2.b .
Floodplain Resolution Number: FPR 01-555
Affected Community: Adams County, City and County of Denver
Name of study to be formally acted on: "Flood Hazard Area Delineation for Lower Box Elder
Creek Watershed" dated September 2001 - prepared by Wright Water Engineers under contract
with the Urban Drainage & Flood Control District
Studied Streams: 1) Lower Box Elder Creek, a tributary to the South Platte River. 2) Bear Gulch,
a tributary to Boxelder Creek, and 3)Hayesmount Creek, a tributary to Boxelder Creek.

Technical Issues: The Boxe1der Creek watershed is a long, narrow drainage basin that has its
headwaters in El Paso County at the Palmer Divide. The basin is approximately 100 miles in
length, and drains directly to the South Platte River east. of Greeley in Weld County. The creek
passes through Denver International Airport (DIA) property and unincorporated areas of Adams
County, which are the focus of the subject study. Thd floodplain of lower Boxelder Creek was
previously studied by approximate techniques only, and therefore its accuracy and usability was
marginal. Development in and around DIA was the primary reason why the District made the .
decision to study lower Box Elder Creek in detail.

Wright Water Engineers (Consultant) was under contract with the Urban Drainage & Flood Control
District (District) for the completion of the floodplain stUdy for Lower Box Elder Creek. Hydrology
for the most upstream portion of the basin (approxi,mate1y 200 square miles) was previously
developed in a study entitled ''Upper Box Elder Creek :Outfall Systems Planning Study Hydrology
Report" by CH2:MHill in 1994. The Consultant completed new hydrologic analyses for the lower
part of the basin under existing and future development watershed conditions, The only reservoir
within the study area is Bootleg Reservoir, which is a state jurisdictional. dam owned by the
Henry1yn Irrigation District. The dam was breached in the early 1980's because of safety concems
by the Colorado State Engineer's Office.

Hydraulic calculations and floodplain delineations wete completed using accepted state-of-the-art
practices. The hydraulic analyses were done for approximately 53 stream miles of Box Elder
Creek, Bear Gulch, and Lower Hayesmount Creek. A floodway analysis \\ras also completed for
the study.

'Cilnlnluuity reSp9nse: 'tliePiSffict transrmTtooa1~tter dated .Octoberl~-zoor requestmg the
CWCB to review and designate the subject floodplain. information, District staff were notified by
letter dated October 18, 2001 of the proposed CWCB action at the November 19-20, 2001 Board
meeting. The District staff is very familiar with CW~B designation procedures and therefore did .
not request a meeting to discuss the proposed action. Adams County and the City & County of

Flood Protection. Water Project Planning and Financing. Stream and Lake Protection
Water Supply Protection. Conservation Planning

4