HomeMy WebLinkAbout2.b. Danbury Way Improvement Project, City Project 453, Rain Gardens and Water QualityG:\ENGPROJ\453\Council Items\20151020 CCWS Rain Gardens and Water Quality.docx
EXECUTIVE SUMMARY
City Council Special Work Session: October 20, 2015
AGENDA ITEM: Danbury Way Improvement Project, City
Project 453, Rain Gardens and Water
Quality
AGENDA SECTION:
Discussion
PREPARED BY: Patrick Wrase, PE, Director of Public
Works / City Engineer AGENDA NO. 2.b.
ATTACHMENTS: Maps APPROVED BY: ddj
RECOMMENDED ACTION: Presentation and discussion
BACKGROUND
Authorization for plans and specifications was approved by the city council at the July 21, 2015 meeting.
Design is currently being completed. At the meeting, City Council asked for further information regarding
rain gardens and water quality improvements.
With the preliminary design for the feasibility report water quality improvements were evaluated and
proposed. On Danbury Way, there are five stormwater discharge locations. Two of these discharge points
are proposed to have stormwater treatment structures installed to improve water quality. These two
locations were selected because the stormwater discharges to a Preserve Wetland. The other three
locations are proposed to have the city standard sump manholes prior to discharge. After completion of
the feasibility report, and at the request of the city council, rain gardens were evaluated in addition to the
stormwater treatment structures and sump manholes.
Throughout the site, there are potentially 15 locations that would be good candidates for rain gardens. The
attached map shows the approximate locations. The rain gardens would offer improvement in water
quality where installed, in addition to the stormwater treatment structures and sump manholes. A typical
rain garden removes up to 70% of Total Suspended Solids (TSS) and 60% of Total Phosphorus (TP).
This compares to 35% TSS and 15% TP for a stormwater treatment structure. The cost of the rain
gardens on this project is estimated at $8,000 each and require approximately $500 each in annual
maintenance expenditures. The stormwater treatment structures are estimated to cost $40,000 each and
require $100 each annually in maintenance expenditures. It is important to note that the stormwater
treatment structures can be designed to treat all runoff from the project area whereas the rain gardens for
this particular site can only be configured to treat approximately 30% of the runoff, if all 15 potential sites
are developed.
If the Council believes that the additional water quality benefit of the rain gardens is worth pursuing, staff
recommends that the property owners adjacent to the potential locations should be contacted as soon as
possible to determine if they are agreeable to having rain gardens within their property. While there are 15
potential locations for rain gardens, staff does not anticipate all 15 being incorporated into the project.
Based on past experience with rain gardens, staff believes that the willingness from residents could be
minimal. Additionally, the rain gardens will require specialized annual maintenance. In other
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communities, the maintenance of the rain gardens has been required to be performed by the property
owner. An option would be to have City staff perform the maintenance which would require additional
staff time and associated expenditures.
SUMMARY
Staff would like to initiate a discussion with the council to determine if rain gardens should be included
within the Danbury Way reconstruction project and if so whether the maintenance of the rain gardens will
be a resident or City responsibility.