South Boulder Creek Flood Mitigation Plan

From the Boulder City Council Hotline, Feb 3 2017. From Mary Young

Sender: Young, Mary

Hello Colleagues on council and planning board as well as hotline subscribers, Deb Gardner and I both serve on the Urban Drainage and Flood Control District board of directors. After yesterday’s meeting we approached Ken MacKenzie, the executive director with some clarifying questions about CU south. Below is their response along with contacts regarding a potential BVCP groundwater policy. Thank you.

Mary Dolores Young
Boulder City Council

From: Shea Thomas 
Date: February 2, 2017 at 10:43:16 AM MST
To: Mary Young 
Cc: "Deb  Gardner" 
Subject: RE: South Boulder Creek/ CU South Campus

I was the UDFCD project manager for the South Boulder Creek Mitigation Plan and would be happy to answer any questions you have about the process and the resulting report. It was an extensive process, stretching over a period of nearly 6 years starting in 2010 and ending in 2015, and included many public meetings.

We analyzed the following 9 alternatives with the goal of reducing flood risk to the approximately 700 structures in the 100-year floodplain:

  1. Status Quo (no improvements)*
  2. High Hazard Zone floodproofing (no regional improvements, only floodproofing two structures in the High Hazard Zone)*
  3. Regional Detention at US36 (large basin on south side of US36, also additional detention and pipes/channels to the north)*
  4. Regional Detention near Hwy 93 (large basin outside of city, also requires additional detention and pipes/channels to the north)
  5. Distributed Regional Detention (smaller basin on south side of US35, also requires additional detention and pipes/channels to the north)*
  6. Mainstem flow containment (also includes West Valley improvements)
  7. Dry Creek No. 2 Ditch pipeline (capturing flow and conveying it in a very large pipe)
  8. Bear Canyon Creek pipeline* (capturing flow and diverting it to Bear Canyon Creek)
  9. Nuisance level protection only (doesn’t reduce risk of flooding in major events)

Based on recommendations from WRAB and Open Space, we proceeded with evaluating the alternatives with asterisks further (1, 2, 3, 5, 8). We took into consideration Benefit-Cost Analyses, water quality impacts, natural environment protection, threatened and endangered species impacts, open space land impacts, and other City policies when analyzing the alternatives. The result was a recommendation of Alternative 3, which includes:

  • A large berm and basin on the south side of US36
  • A detention basin on the south side of Arapahoe Avenue within Flatirons Golf Course
  • A small detention basin near Manhattan Middle School
  • A small detention basin near the intersection of Foothills Parkway and Baseline Road
  • Modification of the concrete wall along Baseline Road east of Foothills Parkway
  • Improvements to the New Anderson Ditch
  • A diversion structure on the Wellman Canal east of Foothills Parkway
  • Improvements to Dry Creek No. 2 Ditch

The most important element is the berm and basin south of US36. We worked with representatives from CU to understand their plans for the land that would be impacted during a very large storm and to best accommodate the flood pool. We worked extensively with Open Space to minimize any impacts to designated open space property. We coordinated with CDOT’s team designing the improvements to US36 to understand the ultimate configuration of the roadway with regard to the proposed berm. The recommended plan would remove flood risk from 411 structures currently in the 100-yr floodplain.

You can download the final report here: Final South Boulder Creek Major Dranageway Plan – Alternatives Analysis Report

As for a recommendation for groundwater engineers, I would start with Leonard Rice Engineers or HRS Water Consultants. I haven’t worked with either personally and UDFCD doesn’’t typically need to engage a groundwater engineer, but individuals in our office have dealt with these firms for other reasons so I feel comfortable pointing you in their direction.

Please feel free to contact me with any other questions you have.

Shea Thomas
Watershed Services Manager