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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Massey Draw and SJCD ISouthl M~or Drainageway Planning Update
Conceptual Design Report
February 2006

City of Littleton

5.0 Alternatives Development and Evaluation

. Eliminating vertical banks improves public safety,
. Eliminating mosquito habitat.

5.1 Alternate Process

Improvements were identified and evaluated to alleviate the identified problems along each of the
reaches. Because improvements have been designed and constructed along Massey Draw
downstream of Wadsworth and improvements have been approved and 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 preservation, regional detention, culvert improvements, easement and/or
property acquisition, and regional water quality facilities. Following are excerpts from the alternative
evaluation process.

Disadvantaaes
. Vertical banks are habitat to some wildlife species,
. High construction and land acquisition cost.
. Possible impacts to wetlands and habitat.

Floodplain Preservation: Reaches that are not already developed as parks, green belts, golf courses,
or are encroached upon by development were considered for floodplain preservation, Improvements
along these reaches would be minimal and would include low-flow grade control and select bank
stabilization.

5.2 Alternate Plans

Advantages and disadvantages of floodplain preservation include the following:

Grade Control: Along reaches where channel degradation was observed, opportunities to stabilize the
low flow channel with check or drop structures and select bank stabilization were evaluated, The
number of check structures was estimated by assuming a 0.4 percent stable slope and a 3-foot
maximum drop, In locations where the degradation was more severe, or an additional drop was
needed to improve an existing, shallow downstream culvert, a drop structure 6 to 8 feet high was
considered. Advantages and disadvantages of channel stabilization include the following:

Advantaaes
. Preserve floodplain storage and conveyance.
. Enhance storm water quality.
. Preserve wetlands and wildlife habitat.
. Preserve open space.

Advantaaes
. Flatter slopes and lower velocities within reaches can be obtained, thus reducing erosion,
. Preventing or reducing the formation of incised channels with vertical banks (these channels
pose a safety hazard and can impact public and private property).
. Preventing or reducing channel migration, which can lead to new flow paths that reduce usable
open space or potentially damage structures close to the new flow path.
. Reduced sediment load and enhanced water quality.
. Elimination of mosquito habitat.

Disadvantaaes
. Easement / land acquisition cost.

Reaional Detention: Regional detention was evaluated along Massey Draw to reduce flooding, the
culvert overtopping at Carr Street and the flooding immediately downstream. Along SJCD (South) and
its tributaries, regional detention was evaluated at four existing ponds to determine whether they
should be formally accepted as regional detention,

Advantages and disadvantages of regional detention include the following:

Disadvantaaes
. Due to the number of grade control or drop structures or extent of stabilization often needed,
the cost can become prohibitive.
. Certain stabilization measures may be considered aesthetically unattractive.
. Possible land or easement acquisition costs are high if easements do not exist.
. Possible impacts to wetlands and habitat.

Channel Improvements: Reaches were identified where channel improvements may be warranted to
increase capacity, realign the flow path away from structures, stabilize the channel banks, or to
accommodate drop structure or culvert improvements,

Advantaaes
. Regional detention can potentially attenuate downstream peak flows to reduce downstream
property flooding, eliminate the need for improving road crossings, and/or reduce the
magnitude of other downstream improvements.
. Reduced peak flows can reduce the amount of land required for easements
. Regional detention is generally considered to be more effective with operation and
maintenance more consistent and economical for regional facilities as opposed to on-site
detention facilities.
. Regional detention facilities can provide opportunities for open space, recreation, enhanced
property values, and wildlife habitat.

Advantages and disadvantages of channel improvements include the following:

Disadvantaaes
. Land requirements to provide effective peak flow reduction may exceed available open space,
particularly in developed areas.
. Local jurisdictions must often acquire the land on which regional detention is operated.
. Regional detention facilities over a certain size are considered jurisdictional and are subject to
requirements of the State Engineers Office,

Advantaaes
. Stabilizing the stream banks reduces erosion, enhancing storm water quality.
. Eliminating / reducing structure flooding and / or street overtopping.

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