Johnson LaneArea Drainage Master Plan

Floodplain Management Association

September 6, 2018

•FEMA Cooperative Technical Partner (CTP) grant•Douglas County

JLADMP

Project Funding

• 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

• Lot-Split Development (“Wildcat”)• Minimal drainage infrastructure• High sedimentation• Changing flow paths• Minimal public

right-of-way

Challenges

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

Verification of Existing Condition Results

• USGS gage records for Johnson Lane Wash• Resident flooding experience• Past flood damage reports• USGS Regional Regression

Flood Hazard Assessment

• Sediment Engineering• Collected 20 samples• Washes with high sediment transport capacity

• Identify need for sediment basins

Buckbrush Wash

Flood Hazard Classification

• Identify specific hazard areas for:• Pedestrians• Vehicles• Buildings

• Inundation depth

• 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

Regional Mitigation Alternatives

•On-site mitigation limited impact• Community-wide solutions

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

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

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

Southeast 100-Year Sediment Basin

JL Wash Dam PMF

Prioritization Ranking Metrics

• Downstream protected area (total # structures)• Buildings removed from flooding risk• Percent flow reduction• Construction/maintenance cost• Reduction in economic losses (HAZUS)

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

Questions?

Mike Kellogg, PG, CFM, GISPmike@jefuller.com

Richard Waskowsky, PErichard@jefuller.com