FLOOD10390

• County: Boulder
• Community: Boulder
• Stream Name: Fourmile Canyon
• Basin: South Platte
• Title: Fourmile Canyon Creek - Major Drainageway Planning - Phase A Report - Alternatives Analysis
• Date: 6/1/2000
• Prepared For: UDFCD, Boulder
• Prepared By: Love & Associates
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

Potential increased erosion and deposition due to increased base flows associated
with channel invert exvavation.

uration

lain Confi

Maintain Existin

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Alternative 3 -

with the benefit of enhancing the ecosystem long-term. This in turn will have greater short-term
adverse impacts on neighbors due to increased setbacks and the removal of more trees and shrubs
than in the 50-year alternative. Phasing flood improvements construction, however, will help to
reduce those impacts on both neighbors and wildlife habitat. Construction of drop structures, as in
the 50-year alternative, will have some short-term impacts on the aquatic habitat.

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Environmental Benefits

Because no improvements to existing conditions, other than regular channel
proposed, this alternative has no notable benefits to the long-term health of the ecosystem.
sediment removal recommended for maintaining existing flood conveyance may provide some water
quality and aquatic habitat benefits.

This alternative has no quantifiable short-term environmental impacts, nor impacts to neighbors
Existing habitats are preserved in their current condition. It is noted, however, that the affects on
other tributaries due to continued flood flow spills from Fourmile Canyon Creek and the
environmental impacts associated with flood improvements on those streams, were not evaluated in
this study. To avoid adverse impacts to aquatic and terrestrial habitat, selected sediment and debris
removal should be performed in an environmentally sensitive manner, as described below.
Vegetation management and weed control will still be needed to halt continued degradation of the
native ecosystem

maintenance are
Selected

Environmental Detriments

Due to channel width constraints, the channel invert may be excavated several feet in some reaches to
provide lOa-year flood flow capacity. Lowering the channel invert may cause a groundwater draw
down effect near the stream channel. As a result, the potential exists for adjacent wetlands, riparian
areas, or wells dependent on the existing groundwater table to be adversely affected. On the other
hand, by excavating to or below groundwater, perennial flows may be created in portions of the
stream which currently flow only intermittently. While these flows may benefit wildlife and provide
for construction of adj acent wetlands, significant changes in the low flow may increase erosion and
subsequent sedimentation. Measures designed to prevent potential adverse impacts related to channel
invert excavation are included in the design and management recommendations below.

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Greatest opportunity to expand riparian terrace width and enhance habitat
Least preservation of existing riparian vegetation
Preservation of notable vegetation and habitats
Opportunity to establish some meanders via selected active channel excavation and
reconstruction

lOa-year improvements alternative evaluation:

The following summarizes the

Environmental Benefits

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maintaining the existing floodplain configuration alternative evaluation:

The following summarizes

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Environmental Benefits

No short-term adverse impacts to riparian vegetation or wildlife habitat
Select sediment removal may provide some water quality and aquatic habitat
benefits

Environmental Detriments

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impacts

No benefits to the health, value, and function of the ecosystem
Flood flow spills to Wonderland Creek and potential environmental
associated with improvements due to spills remain

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selected

structure

impacts associated

and

outlets

VIa

Opportunity to provide water quality features at storm sewer outlets
Opportunity to enhance aquatic habitat in 'poor' aquatic habitat
active channel excavation and reconstruction
Opportunity to enhance aquatic habitat and water quality via grade control
construction, removal of barriers, and sediment removal
Flood flow spill to Wonderland Creek and potential environmental
with improvements due to spill are eliminated
Opportunity to recharge groundwater on riparian terraces, at storm
sediment basins.

sewer

areas

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into consideration for design of

DESIGN AND MANAGEMENT RECOMMENDATIONS

The following information and recommendations should be taken
flood improvements and long-term management of the ecosystem.

7.6

Greatest short-term impacts to existing riparian vegetation, and to habitat
neighbors
Greatest short-term impacts to aquatic habitat
Potential adverse impact to wetlands, riparian vegetation, and wells due to drop in
ground water associated with invert excavation in some reaches.

and

Environmental Detriments

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7-11

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