HomeMy WebLinkAboutGUARDIAN SELF STORAGE - PDP190020 - - VARIANCE REQUEST (2)11-18-20
Terracon Consultants, Inc. 1901 Sharp Point Drive, Suite C Fort Collins, Colorado 80525
P [970] 484 0359 F [970] 484 0454 terracon.com
November 16, 2020
City of Fort Collins – Utilities Department
700 Wood Street
Fort Collins, Colorado 80521
Re: Engineering Justification Memo
Fort Collins Stormwater Criteria Manual Variance Request
Guardian Storage
Northeast Corner of South College Avenue and Fossil Creek Parkway
Fort Collins, Colorado
Terracon Project No. 20205090
To whom it may concern:
Issue Identification: Current design for stormwater management on the property includes a
sand filter basin planned on the southeastern portion of the Guardian Storage project site.
Previous subsurface exploration performed by others on the site identified comparatively shallow
groundwater (reportedly near elevation 4,940 feet) that may extend above the invert of the sand
filter basin underdrain line planned at elevation 4,938.82 feet. Section 4.1.2 (Item 4) of Chapter
6 within the Fort Collins Stormwater Criteria Manual (dated December 2018) indicates the
groundwater level must be documented to be at least two feet (2’) below reservoir bottom during
the high groundwater period of the calendar year. The project includes design of an underdrain
system below the proposed sand filter basin to permanently lower groundwater levels. In order
to allow for gravity outfall of the permanent underdrain system, a variance is requested to reduce
the amount of separation between the invert of the sand filter basin invert and groundwater levels.
Proposed Alternate Design: We believe design and construction of the proposed permanent
underdrain system will provide at least 1 foot of permanent separation between the invert of the
sand filter basin outlet and nearby groundwater levels. The design will include underdrain lines
buried within trenches that extend around the majority of the sand filter area. The underdrain
system will run parallel to stormwater lines planned for the project for a portion of the alignments
and within separate trenches for portions of the alignment. We anticipate the underdrain line will
consist of 4-inch diameter, perforated pipe. Drain rock will be used around the buried underdrain
lines consisting of ASTM C33 No. 57 or 67 rock with the perforated pipe and drainage rock
protected from soils along the perimeter of the trench and trench backfill materials using a filter
fabric. Slopes of the underdrain lines will be comparatively shallow (on the order of 0.25 to 0.4
percent) to provide gravity drainage towards the outlet which will be a concrete box structure
planned as part of the stormwater sewer system for the project in the south end of Snead Drive.
We have attached conceptual exhibits depicting the proposed design.
Comparison to the Subject Criteria or Standard: We believe the proposed underdrain system
will permanently lower groundwater levels to at least 1 foot below the invert of the proposed sand