To: Members of the Castle Rock Water Commission
From:
Mark Marlowe, P.E., Director of Castle Rock Water
Title
Discussion/Direction for the 6400 East Tributary at Paintbrush park Phase 1 Project Alternative Analysis
Town Council Agenda Date: March 3, 2020
Body
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Executive Summary
Castle Rock Water is seeking Town Council direction regarding design alternatives for the 6400 East Tributary at Paintbrush Park Phase 1 Project. The purpose of this project is to stabilize a section of 6400 East Tributary by implementing recommended Master Plan improvements. Complicating matters is the existence of a retention pond within the project limits that can either be rebuilt to current design standards or removed entirely to return the drainageway to historic conditions. This pond was originally constructed for agricultural purposes prior to development. Environmental restrictions prevented the developer from eliminating the pond at the time of development. The pond has since been used for passive recreation including fishing. Swimming, wading and access on the ice is prohibited due to potential risks. Current requests for use include stocking the pond with fish by Colorado Parks and Wildlife and community fishing events hosted by Castle Rock Police.
A decision is required to remove or reconstruct the pond prior to proceeding to final design and construction. The pond is currently at risk due to active erosion and headcutting along the spillway. The “do nothing” alternative is not an option given the unstable vertical banks, the imminent breach of the dam and known regulatory issues the current pond presents related to water rights and dam safety. The cost to reconstruct the dam is $1,857,000 (see Attachment A), and the cost to remove the dam is $550,000 (see Attachment B). This project is funded through the Stormwater Enterprise. The retention pond does not provide any Stormwater function or benefit to water quality, conveyance or flood control. To preserve the pond would be entirely recreational and aesthetic.
This project was originally scheduled for Construction in 2019, however significant regulatory hurdles have caused delays in the schedule. In the meantime, Phase 2 improvements are now under design including the stabilization of 6400 East Tributary downstream of the retention pond. This project is planned to be constructed concurrently with Phase 2 in the fall of 2021.
Notification and Outreach Efforts
Internal discussions have been held between Castle Rock Water and Parks and Recreation Department representatives. Additionally, a presentation was made to the Meadows Neighborhood Company HOA at a recent board meeting.
Discuss open house comments, letter to Mayor and any other feedback to date from individuals of which we are aware
At the direction of council, Town staff may conduct a Town-wide online survey of residents to solicit feedback on the two proposed project alternatives.
Project details and updates will be provided on the Town website, CRgov.com once an alternative has been selected and detailed design is underway.
Discussion
Selection of an engineering design consultant was initiated on November 5, 2018, with Anderson Consulting Engineers, Inc. selected to perform an alternatives analysis for the project. The original intent of the project was to incorporate repairs and upgrades to the existing retention pond as part of the stream stabilization effort. However, during the alternatives analysis several issues came to light that required attention before a decision could be made on the direction of the project. These issues consisted of the following:
• The cost to stabilize the existing overflow to protect and maintain the existing pond exceeds the cost of conventional stabilization. The cost to stabilize the embankment is estimated at approximately $755,000.
• The existing pond has an embankment (dam) that exceeds ten feet in height, which places it under the regulatory control of the State Engineers office as a Jurisdictional Dam. As such, full reconstruction of the dam is necessary to comply with current design criteria, and periodic inspection and maintenance will be needed once the facility is completed to satisfy State Dam Safety Program requirements. The cost to reconstruct the dam is estimated at approximately $870,000.
• Currently, the Town does not own any water rights to permanently retain the impounded water in the pond. Therefore, the Town will be required to go through Water Court to obtain the necessary legal right to retain the water and develop an Augmentation Plan to offset the water loss to downstream users who have priority rights. The cost to secure the necessary water right is estimated at approximately $230,000.
As part of the alternatives analysis report, Anderson provided the following table listing pros and cons of each option.
|
Alternative |
Pros |
Cons |
|
Maintain Existing Pond |
• Keeps Pond • New Embankment • 10-year flow through Primary Outlet • Maintains Perceived Aesthetics/Amenities |
• Cost • Large Outlet Facility • Emergency Spillway • Enhanced Inspection and Maintenance • Needs Water Rights • Higher Risk • State Engineer’s Office Oversight |
|
Remove Dam and Stock pond and Rehabilitate Channel |
• Cost • Ease of Construction • No Outlet Facility • No Emergency Spillway • Low Public Risk |
• No Water Quality • Lack of “Pond Aesthetics” • Loss of Perceived Community Amenity |
Staff recommends proceeding with removal of the dam and restoration of the original channel. The master plan recommendations suggested the draining of the pond and conversion to a water quality capture volume facility. The cost to complete this alternative was less than those presented here, however it was determined that this alternative is not desirable nor required given other water quality projects in the area. Removal of the dam is the next most consistent recommendation to the master plan. It holds to the mission of the plan for preservation and stabilization of the drainageway. Stabilization improvements provide water quality benefit and safety adjacent to the public park and clubhouse.
Budget Impact
Construction cost estimates for the two alternatives under consideration are as follows:
• Rebuild existing pond - $1,857,000
• Remove pond and rehabilitate channel - $ 550,000
The current construction budget for Phase 1 improvements is $353,000. Although an additional $200,000 to cover the cost of removing the dam and rehabilitating the channel can reasonably be absorbed by the capital fund, an additional $1.5 million is a significant increase in cost resulting in the need for a rate increase in 2021 to Stormwater Rates. Discussion with the Parks and Recreation Department indicate that funding is not available in the general fund at this time to offset this cost increase. Discussions with the HOA Board indicated there was not an interest from the HOA to participate in the costs to keep the pond.
Staff Recommendation
Staff recommends Town Council consideration to remove the dam and restore the channel to original condition. Public outreach efforts will proceed during final design to ensure residents are informed of this direction, assuming council support.
Attachments
Attachment A Rebuild Existing Pond Exhibit
Attachment B Remove Pond and Rehabilitate Channel Exhibit