GREEN CITY, CLEAN WATERS Dan O’Rourke, CDM Smith Jason Cruz, Philadelphia Water Chris Bergerson, Philadelphia Water Stephen White, Philadelphia Water Taylor Heffernan, Philadelphia Water Matthew Gamache, CDM Smith Laurie Kellndorfer, CDM Smith Investigation of Infiltration in Unsaturated Urban Fill Material VUSP Pennsylvania Stormwater Symposium October 15, 2015
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GREEN CITY, CLEAN WATERS
Dan O’Rourke, CDM SmithJason Cruz, Philadelphia Water
Chris Bergerson, Philadelphia WaterStephen White, Philadelphia Water
Taylor Heffernan, Philadelphia WaterMatthew Gamache, CDM Smith
Laurie Kellndorfer, CDM Smith
Investigation of Infiltration in Unsaturated Urban Fill MaterialVUSP Pennsylvania Stormwater SymposiumOctober 15, 2015
Philadelphia or Fill-adelphia?
• Urban fill covers much of Philadelphia
• Often exceeds the depth of green stormwater infrastructure
• What happens when we infiltrate into it?
Fill Areas
3from USGS OFR 224 (2000)
The Importance of Over-Excavation
Contact between fill and silty clay (orange)
Poor Fill Quality
• Brick fill
Groundwater Flow Modeling
• 3D regional and site scale models (DYNFLOW; www.dynsystem.com)
• Mounding analysis by USGS (1D, using Hantush equation)
Length of Trench = 137 ftDrainage Area = 15,500 ft2
8’
Bedrock (assumed to be impermeable)
Model Set-Up (Tree Trench)
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Fixed head at lateral boundaries
No-flow along bottom
Infiltration at set of nodes (distribution pipe)
after Gamache et al (2015)
Scenarios
14
BuildingFoundation
Tree Trench
Water Table
Tree Trench with Liner
BuildingFoundation
Tree Trench, No Liner, Gravel Patch
BuildingFoundation
after Gamache et al (2015)
Tree Trench
15
BuildingFoundation
Under baseline conditions, the tree trench
drains twice the design flow without excessive mounding or basement infiltration. Increases in water content/saturation is simulated around the trench in all directions.
after Gamache et al (2015)
Tree Trench with Liner
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With a liner in place, the tree trench retains water and lateral increases in water content/saturation are minimized. However, infiltration is restricted,
relative to the base case.
BuildingFoundation
after Gamache et al (2015)
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In this instance, the simulated gravel patch acts as a conduit for flow to the building foundation. Brick fill would produce the
same effect.BuildingFoundation
Tree Trench, No Liner, Gravel Patch
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This conduit effect is exacerbated with lower
permeability soils
BuildingFoundation
after Gamache et al (2015)
Calibration Check
• Use observation wells within GSI
• Approximately 50 systems are actively being continuously monitored by Philadelphia Water
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Monitoring Data: April 2014 Event
• Compare observation well data from 6 different tree trenches
• Two dimensional modeling can be used for GSI siting, particular in areas of shallow water table or poor fill quality– Can also be used for design to determine infiltrated volume
• Permeable conduits can be problematic– Need good characterization
• Reasonable match to measured observation well data• Need to correlate with: