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Johnson LaneArea Drainage Master Plan
Floodplain Management Association
September 6, 2018
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•FEMA Cooperative Technical Partner (CTP) grant•Douglas County
JLADMP
Project Funding
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• Data Collection• Topographic Mapping (LiDAR + Survey)• Watershed Assessment
• Geology• Historical Flow Paths
• Flood Hazard Assessment• Hydrologic Modeling• Hydraulic (2D) Modeling
• Flood Hazard Classification• Flood Mitigation Alternatives
• On-Site/Local• Regional
Johnson Lane ADMP Major Project Elements
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• Lot-Split Development (“Wildcat”)• Minimal drainage infrastructure• High sedimentation• Changing flow paths• Minimal public
right-of-way
Challenges
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Flood Hazard Assessment
• Hydrologic Modeling (HEC-HMS)• Four Storm Events
• 25-year, 24-hour storm• 100-year, 6-hour storm• 100-year, 24-hour storm• July 2015 storm estimate
•Hydraulic Modeling (2D)• 10-foot grid element (4M total grids)
• Sedimentation Engineering
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Verification of Existing Condition Results
• USGS gage records for Johnson Lane Wash• Resident flooding experience• Past flood damage reports• USGS Regional Regression
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Flood Hazard Assessment
• Sediment Engineering• Collected 20 samples• Washes with high sediment transport capacity
• Identify need for sediment basins
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Buckbrush Wash
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Flood Hazard Classification
• Identify specific hazard areas for:• Pedestrians• Vehicles• Buildings
• Inundation depth
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• Individual Lot Management Plan• Individual Lot Retention• Collector Road All-Weather Access• Off-Highway Vehicle Impacts• Roadside Ditch Lining• Regional Alternatives
• Contour Trenching• Off-Site Detention
Flood Mitigation Alternatives
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Regional Mitigation Alternatives
•On-site mitigation limited impact• Community-wide solutions
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Regional Mitigation Alternatives: Contour Trenching
• Contour Trenching• Trap runoff• Interim solution
• Pine Nut Mountain Assessment• 100-year, 6-hour storm
• 64% reduction Romero Wash• 62% reduction Skyline Wash• 23% reduction Stephanie Wash• 0% reduction Chowbuck Wash
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Regional Mitigation Alternatives: Off-Site Detention
Sediment/Detention Basins w/ Collector and Conveyance Channels
Regional Systems• Unnamed Wash A• Hot Springs-Buckbrush• Pine Nut North• Pine Nut South• Johnson Lane Wash Dam
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Johnson Lane Wash Alternatives
• Investigated several locations for detention basins
• Detention volume required very large, deep basins
• Most viable alternative was a dam structure upstream of Mackay St. alignment
• Designed for ½ PMF and full PMF
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Southeast 100-Year Sediment Basin
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JL Wash Dam PMF
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Prioritization Ranking Metrics
• Downstream protected area (total # structures)• Buildings removed from flooding risk• Percent flow reduction• Construction/maintenance cost• Reduction in economic losses (HAZUS)
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Initial Prioritization
Priority System Design Level Construction Cost
1 Pine Nut North 25-Year $880,000
2 Johnson Lane Wash Dam PMF $4,900,000
3 Hot Springs-Buckbrush 100-Year $8,020,000
4 Pine Nut South 25-Year $1,130,000
5 Unnamed Wash A 25-Year $240,000
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