Recommended Plan
6.1 Plan Description
The recommended plan, shown in Figure 6-1, is described below.
6.1.1 Reach 1 Recommended Plan
Although cost estimates showed the Minimum Crossing Improvement and Maintenance alternative to be more expensive, this plan is recommended at West 72nd Avenue. The easements needed to implement the 100-yr Conveyance Alternative are not feasible given the residential lots in this area.
The channel grading in the Woodland Valley No. 7 subdivision associated with the 100-yr Conveyance Plan is included in the recommended plan. The residential buildings currently in the regulatory 100-year floodplain can be removed by the proposed project.
6.1.2 Reach 2 Recommended Plan
The Minimum Crossing Improvement and Maintenance Plan is recommended for Reach 2. While the other two alternative plans remove additional buildings from the floodplain, the additional benefits associated with those alternatives do not justify the added cost. The Minimum Crossing Improvement Plan will address the Zone X shaded flood hazard area by preventing the spill flows that creates the shallow flooding at Alkire Street. This alternative will not have any impact on the proposed Ward Road expansion trail or roadway crossing at Leyden Creek.
6.1.3 Reach 3 Recommended Plan
The recommended plan for Reach 3 includes the 100-yr Conveyance Plan crossing structure at Indiana Street and the Croke Canal, but does not include the overflow channel west of Cameo Estates. The overflow channel was not included as it would not directly mitigate any flood hazards.
At the Croke Canal, the proposed improvement will convey the 100-yr discharge underneath the canal, minimizing the comingling flows, eliminating the need for the passive weir structure. During final design, an additional flood control purge structure should be investigated to determine the feasibility between the City of Arvada and the Croke Canal representatives.
6.1.4 Reach 4 Recommended Plan
No improvements are recommended at Leyden Lake. Improving Leyden Lake to contain the 100-yr design storm only impacts a short reach of Leyden Creek immediately downstream of the spillway. The first roadway crossing at Indiana Street is not affected in size if any improvements to Leyden Creek are proposed. Although no roadway crossings would be impacted, the Farmers’ High Line Canal would be impacted. Any flows that are discharged over the emergency spillway are intercepted by the Farmers’ High Line Canal. Flows through the primary spillway do not influence the Farmers’ High Line Canal.
The Minimum Crossing Improvement and Maintenance Plan is recommended plan for Quaker Street. The benefit cost analysis determined the additional work associated with channelization of Leyden Creek through this stretch was not equivalent to the additional cost. Any trail crossing near Quaker Street will be further evaluated during Conceptual Design.
The Minimum Crossing Improvement and Maintenance Alternative is also recommended at West 82nd Avenue. The additional benefit of the 100-year Conveyance alternative is removing the trailhead from the floodplain. Since it is not feasible to assume the trailhead would be heavily used during the 100-year event, removing the trailhead from the floodplain is not a priority.
Blue Mountain Road is only access road for Blue Mountain Estates, creating the need to provide access during the 100-year event. After comparing the cost associated with each alternative, the Minimum Crossing Improvement and Maintenance Plan was selected for Blue Mountain Estates. This plan eliminates roadway overtopping at Blue Mountain Estates, and the grouted boulder structure will ensure the majority of the discharge overtopping the railroad will remain tributary to the historic flow path in the Coal Creek Watershed.
The final design of elevating Blue Mountain Road should investigate any impacts to the structures downstream of Blue Mountain Road.
Table 6-1 : Recommended Cost Estimate Summary
| Reach | Selected Plan | Capital | Easement/ROW | Eng/Admin/Legal | Contingency | Total Capital Cost | 50-year O&M Cost |
|---|---|---|---|---|---|---|---|
| Reach 1 | Minimum Crossing and Maintenance | $2,686,870 | $24,288 | $806,062 | $671,718 | $4,188,938 | $29,925 |
| Reach 2 | Minimum Crossing and Maintenance | $920,731 | $59,698 | $276,218 | $230,183 | $1,486,830 | $27,300 |
| Reach 3 | 100-yr Conveyance | $1,134,068 | $17,884 | $340,220 | $283,517 | $1,775,689 | $0 |
| Reach 4 | Minimum Crossing and Maintenance | $7,109,199 | $353,731 | $2,132,760 | $1,777,300 | $11,372,990 | $128,764 |
| Total Cost | $11,850,868 | $455,601 | $3,555,260 | $2,962,718 | $18,824,447 | $185,989 |
Table 6-2 : Jurisdictional Cost Estimate Summary
| Jurisdiction | Capital | Easement/ROW | Eng/Admin/Legal | Contingency | Total Capital Cost | 50-year O&M Cost |
|---|---|---|---|---|---|---|
| City of Arvada | $7,964,835 | $266,812 | $2,389,452 | $1,991,210 | $12,612,309 | $131,319 |
| Jefferson County | $1,611,792 | $85,562 | $483,536 | $402,948 | $2,583,838 | $58,065 |
| CoA / Jeff Co | $2,470,136 | $118,980 | $741,040 | $617,534 | $3,947,690 | $41,125 |
6.2 Water Quality Impacts
There is no regional water quality improvements proposed for the Leyden Creek watershed. New developments, such as Leyden Rock, and Leyden Ranch West, will handle water quality on a site specific basis.
Eroding channel banks also can lead to degradation in water quality throughout a watershed. By implementing the proposed stable channel slopes, less erosion will occur, improving the water quality in the watershed.
6.3 Operations and Maintenance
Culvert maintenance for all proposed improvements was assumed to occur once every two years. Channel maintenance for proposed improved channels, and all proposed hydraulic structures was assumed to be performed once every five years.
6.4 Environmental and Safety Assessment
Implementation of the recommended plan will reduce flood hazards at selected roadway crossings. These changes will also improve emergency access during flood events. With exception to at 75th Place, culvert capacity up to the 100-year event has been provided.
ERO assessed the study area for potential wetlands and waters of the U.S., threatened and endangered species, and general wildlife use. Below is a summary of the resources found at the study area and recommendations or future actions necessary based on the current site conditions and federal, state, and local regulations. ERO’s entire assessment can be found in APPENDIX E - Wetland and Riparian Inventory (PDF version only).
6.4.1 General
Although there are some regulated resources in the study area, none of them are unique or regionally important. Aside from potential depletions associated with new water bodies, ERO has not identified any significant regulatory constraints on the proposed projects. Because of their dynamic natures, site conditions and regulations should be reconfirmed by a qualified consultant prior to conducting construction activities within the watershed.
6.4.2 Wetlands and Other Waters of the U.S.
Leyden Creek, most of its tributaries, and abutting wetlands are likely waters of the U.S. Any activity in the proposed alternatives that would require placing dredged or fill material into Leyden Creek, tributaries, or wetlands must comply with Section 404 of the Clean Water Act. Many of the activities in the proposed alternatives would provide a net benefit to the stream, particularly reestablishing open reaches and grading the channel.
6.4.3 Threatened and Endangered Species
There is no suitable habitat present for any listed threatened or endangered species, but depletions associated with new water bodies would likely require consultation on potential depletions associated with the water bodies and their potential effects on downstream threatened and endangered species on the Platte River in Nebraska. If construction activities are likely to take place outside of the Preble’s Denver metro block clearance zone, it is recommended that a threatened and endangered species letter to the U.S. Fish and Wildlife Service be submitted.
6.4.4 Migratory Birds and Other Wildlife
If any active nests are found in the study area, any work that would destroy the nests should not be conducted until the birds have abandoned the nests. If possible, ground-clearing activities should occur outside of the April 1 through August 31 migratory bird breeding season. If construction activities would occur during the migratory bird breeding season, a nest survey should be conducted immediately prior to construction.