FLOOD06961

• County: Jefferson
• Community: Littleton
• Stream Name: Massey Draw and SJCD South
• Basin: South Platte
• Title: Conceptual Design Report for Massey Draw and SJCD ‎(South)‎ Mjor Drainageway Planning Update
• Date: 2/1/2006
• Prepared For: Jefferson County
• Prepared By: Olsson Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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8 City of Littleton

Massey Draw and SJCD ISouth) Major Drainageway Pianning Update
Conceptual Design Report
February 2006

Culvert Improvements: The Jefferson County criterion for culvert overtopping does not allow
overtopping during the 5-year storm. Twelve inches of overtopping is allowed during the 100-year
event on local and collector streets. On arterial streets, a minimum clearance between the energy
grade line and the bridge low chord, or culvert crown, is required during the 100-year event. The
minimum clearance varies from 6 inches to 2 feet, depending on the size of the upstream watershed.
The project sponsors agreed that this minimum clearance criterion was too restrictive for evaluating
existing culverts. It was agreed that culvert improvements that eliminated the culvert overtopping
during the 1 OO-year storm would be evaluated. Advantages and disadvantages of culvert
improvements include the following:

Advantaaes
· Roadway overtopping can be reduced or eliminated, increasing safety for motorists and
citizens (damage incurred from overtopping can also be reduced or eliminated).
. Inadvertent detention upstream of embankments can be reduced or eliminated, reducing the
width of the floodplain upstream of the culvert.

sediment basin to remove sediment from the stream flow is difficult to estimate not knowing the
gradation of the sediment. Because of the unknowns related to sizing and effectiveness, sediment
basins were not further evaluated.

Wetland bottom channels were considered as a means to filter out sediments and nutrients during low
flows. Because of the wide bottom width of this type of channel, only those reaches that do not already
have extensive wetland bottoms and that are not already developed as parks, green belts, or golf
courses were considered.

The advantages and disadvantages of wetland bottom channels are:

Advantaaes
. Enhanced water quality and reduced sediment loads.
. Wildlife habitat creation.
. Floodplain storage.
. Channel stabilization.

Disadvantaaes
. Culvert replacement can be expensive for a small amount of benefit.
. Elimination or reduction of inadvertent detention can increase peak flows downstream of the
improved culvert.

Disadvantaaes
. Cost to obtain the necessary land.

Easement / Propertv Acauisition: Along drainageway reaches that cross private property, the area
needed for drainage easements was identified. The easement area was estimated by using the 100-
year floodplain limits from the hydraulic analysis. Properties where structures were shown to be
flooded in the hydraulic model were identified for possible acquisition, Following are advantages and
disadvantages of easement and/or property acquisition:

Status Quo: The status quo or "do nothing" alternative was considered for each reach. In some
reaches, potential improvements were not recommended based on a combination of cost, level of
benefit achieved, and feasibility. The advantages and disadvantages of maintaining the status quo
are:

Advantaaes
. Low/no cost.

Advantaaes
. Floodplain preservation.
. Elimination of private ownership of impacted property.
. Easements and acquisitions provide access for maintenance,
. Easements and acquisitions help ensure continued floodplain function by restricting building
within the floodplain,

Disadvantaaes
. Continued channel degradation.
. Possible high operation and maintenance costs.
. Property flooding.
. Culvert overtopping.
. Decreased public safety.

Disadvantaaes
. Cost of obtaining easement or property.

Deer Creek Golf Course and Meadow Ranch Alternatives: Six alternatives to prevent structure
flooding in the Meadow Ranch area along Deer Creek Golf Club were evaluated, The alternatives
evaluated include culvert improvements, levee construction, channel construction, and regional
detention.

Reaional Water Qualitv Facilities: Regional facilities to enhance stormwater quality were evaluated.
The water quality capture volume and estimated land area needed for extended dry detention basins
were estimated. The land requirement was considered to be excessive and extended detention basins
were not considered any further, Water quality settling ponds are being constructed on SJCD (South)
and North Tributary on the Centennial Water and Sanitation District's South Platte Reservoir property.

5,3 Alternative Evaluation

Other means to enhance stormwater quality that were considered include sediment basins and
wetland channels. If the channels are stabilized and the watershed is fully developed, it is believed
that the primary source of sediment would be street sand and gravel deposited into the drainageway at
each storm sewer outfall. There is not a good method to estimate the quantity of sand and gravel that
could be deposited into the drainageways from the streets. Furthermore, the effectiveness of a

Improvements were identified and evaluated to alleviate the identified problems along each of the
reaches. Because improvements have been constructed along Massey Draw downstream of
Wadsworth and improvements have been constructed along SJCD (South) downstream of South
Platte Canyon Road, these reaches were not included in the alternatives evaluation. The alternative
improvements that were evaluated include channel stabilization and improvements, floodplain