FLOOD02900

• County: Adams
• Community: Commerce City
• Basin: South Platte
• Title: Lower First Creek and Direct Flow Area 0055 Major Drainageway Planning
• Date: 5/1/2002
• Prepared For: UDFDC
• Prepared By: Turner Collie & Braden Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

portions of First Creek with a defined channel, and along irrigation canals and ditches. Figure II-2A
shows the general location of the wetlands within the lower First Creek study area. Additionally the
mining pits east of the South Platte River are declared waters of the US. Channels have been
aligned, where possible, to avoid conflicts with wetlands.

According to the Wetland Assessment of the First Creek Watershed prepared by ERO Resources
Corporation the Fulton Ditch, O'Brian Canal, and Burlington Ditch probably are not regulated by the
US Army Corps. of Engineers. However, this may change in the future and the status of the ditchcs
should bc determined before any activities occur.

The 26.2 How Pit is designated a wildlife habitat mitigation site as part of a Section 404 permit.
Any impacts to the site would have to comply with the Corps' permit conditions.

Before any project activities occur that include thc potential dredging or filling into waters of the
U.S., wetlands should be delineated following the Corps guidelines. The jurisdictional wetlands
described in the Wetlands section of the will require a U.S. Army Corps of Engineers Section 404
Permit before any activities occur.

H.

GROUNDWATER

Review of the logs for existing wells on file with the State Engineers Office shows that thc depth to
ground water varies from I-foot to over 100-feet below the ground surface. The shallowest
groundwater was shown to be in the vicinity ofthe RMA's off-post groundwatcr treatment system.
It is possible that shallow groundwater could be a problem for construction and maintenance of
drainage improvements. Before construction begins it is recommended that selective drilling be
conducted along the alignment to determine the depth to groundwater so that the impacts can be
minimized during the preliminary design.

Two monitoring wells in RMA's long-term groundwater monitoring program (wells 37074 and
37083) are located within or near the downstrcam toe of the proposed dam. They will need to be
protected or replaced by the UOFCD. If the existing wells cannot be retained, they must be closed
properly according to Colorado water well construction and abandonment regulations (2 CCR 402-
2).

Five monitoring wells in RMA's long-term groundwater monitoring program (37081, 37318, 37323,
37369, and 37389) are located within the area of the 100-year water surface. The well top of casing
(TOe) elevations are lower than the maximum water surface elevation. One ofthe wells (37369)
would be inundated by the estimated water surface elevation associated with a 10-year storm event.
Watertight well seals should be provided by thc UOFCD to prevent surface water from entering the
wells during storm events retained by the proposed dam. RMA personnel will be responsible for
installing the seals. All 5 wells have 4-inch diameter (ID) PVC casings.

Between 6 and 10 private water wells are located within the area of the 100.year water surface. At
least some of these wells still are used. Any remaining water wclls should be located and their status

evaluated before final construction plans are completed. The wells should be sealed or closed to
prevent surface water from entering the wclls during storm events. Of the six wells where
information is available, four are completed in the Arapahoe Formation, one is completed in the
Denver Formation, and one is an alluvial welL Completion depths range from about 30 feet for the
alluvial well to 575 feet for an Arapahoe welL

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I.

UTILITIES

Ten and l6-inch high pressure gas lines, owned by Public Service Company of Colorado, run
through the study area. The approximate location of these gas lines and other utilities is shown on
Figure II-2B. The alignment generally runs along 96th Avenue to Havana Street and north in Havana
approximately 3500-feet. It then runs in a northwesterly direction. The gas lines are located in a 50.
feet wide easement. They are buried 24 to 48-inches deep. The gas lines may need to be lowered
for culvert improvements along 96th Avenue, east of Havana Street, for the arsenal North Drainage
and for the reach of First Creek from the South Platte River to Brighton Road

South Adams County Water and Sanitation District has water and sanitary sewer lines in the study
area. A review of thcir system maps shows that the sanitary sewer is generally greater than 8-feet
deep. The water lines are 4 to 5-feet deep. Water lines will need to be lowered to cross drainage
improvements.

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