GREEN CITY, CLEAN WATERS...GREEN CITY, CLEAN WATERS. Dan O’Rourke, CDM Smith. ... (distribution...

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)

• Saturated zone

8after Gamache et al (2015)

SWMS 2D

• USDA Code (finite element)• Solves Richards equation for saturated

and unsaturated groundwater flow• Requires:

– Saturated hydraulic conductivity (Ks)– Residual saturation (θr)– Maximum saturation (θs) = porosity (φ)– Coefficient in soil water retention

equation (α)– Exponent in soil water retention

equation (η)

9after Gamache et al (2015)

Applications of 2D Modeling

• Provide guidance on siting GSI– Evaluate how infiltrated water migrates within the

subsurface• risk of lateral flow to adjacent infrastructure

– Evaluate extent of water table mounding

10

Tree Trench 2D Simulations

• Test 3 scenarios:– General tree trench– General tree trench with a liner– Conduits

• Very permeable areas within fill– Gravel, brick fill, etc.

• Set-up based on actual tree trench

11

10’

7’Trench GravelKh = Kv = 1000 ft/day

FillKh = Kv = 16 ft/day

Native Aquifer Sediments Kh = 10 ft/dayKv = 1 ft/day

6’ 5’

Tree Trench

Length of Trench = 137 ftDrainage Area = 15,500 ft2

8’

Bedrock (assumed to be impermeable)

Model Set-Up (Tree Trench)

13

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

16

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)

17

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

18

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

19

Monitoring Data: April 2014 Event

• Compare observation well data from 6 different tree trenches

20

N

after Gamache et al (2015)

Rain Event

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Observed Data

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Observed Data

Observed Data

24after Gamache et al (2015)

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15’

4’

Tree Trench

10’

Length of Trench = 100 ftDrainage Area = 15,000 ft2

9’

Trench GravelKh = Kv = 1000 ft/d

FillKh = 0.50 ft/dKv = 0.25 ft/d

Native Aquifer Sediments Kh = 10 ft/dayKv = 1 ft/day

26

Simulation Results

27

Simulation Results

Summary & Next Steps

• 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:

– Residual saturation – Simulated run-off testing– Groundwater monitoring wells

28

Water Table Mounding

29

Green City, Clean Waters

www.phillywatersheds.org

www.phillywatersheds.org/ltcpu

Dan O’Rourke, P.G.CDM Smith110 Fieldcrest Ave, #8, 6th Fl.Edison, New Jersey 08837orourkede@cdmsmith.com