Groundwater Mounding Between Subsoil Drains, two case studies applying the draft IPWEA groundwater separation guidelines. Presented by Alex Rogers from JDA at Engineers Australia WA, June 2017

1. Groundwater Mounding Between Subsoil Drains
- Application of IPWEA (2016)
Alex Rogers Principal Engineering Hydrologist
Gregorio Serafini Engineering Hydrologist
Min Li Engineering Hydrologist
1

2. PRESENTATION OF TWO CASE STUDIES
•Vertical slice model of urban development; and
•Detailed 3D groundwater model of POS.
2

3. IPWEA (2016) Separation Distances for
Groundwater Controlled Urban Development
Section 3.4 Rainfall
“A 30 year daily timestep rainfall record is to be used to develop a
probability density function from which the required level of service can
be selected. This data should be sourced from the Department of Water.
Rainfall Predictions to be used as modelling inputs should be based on
the Department of Water’s future median scenario, as outlined in the
Selection of future climate projections for Western Australia (DoW, 2015).”
3

4. Future Climate Projections for Western Australia
(DoW, 2015)
Figure 1: Scenarios at Perth Airport, projected mean annual rainfall to 2100 for the wet, median and dry scenarios (DoW, 2015)
4
𝟕𝟔𝟎 − 𝟕𝟎𝟎
𝟕𝟔𝟎
= 𝟖% 𝐫𝐞𝐝𝐮𝐜𝐭𝐢𝐨𝐧 "𝐚𝐧𝐨𝐦𝐚𝐥𝐲"

5. Future Climate Projections for Western Australia
(DoW, 2015)
5

6. CASE STUDY #1
• Site is located south of Perth;
• Native soil is 1 m of silty sand overlying clay. Saturated hydraulic
conductivity of native soil estimated to be 1 m/d;
• Proposed development has spacing of 70 m between roads / subsoil
drains;
• Soakwells will be used to infiltrate the first 15 mm of rainfall on lots;
• Sand fill to be brought to site; and
• Groundwater expected to sit close to subsoil drain invert for most of
the year.
6

7. CASE STUDY #1
Assumptions and Parameters
• Baseline rainfall from Bureau of
Meteorology Website
• Effective porosity: 0.2
• Clean sand fill above subsoil drains
saturated hydraulic conductivity (Ka)
• 1 m/day
• 5m/day
• 10m/day
• Site specific rainfall anomalies to 2030
from Department of Water
• Net uniform recharge rate: 70%
• 1m native sand below subsoil drains
with saturated hydraulic conductivity
(Kb) of 1m/day
7Example in Red

8. IPWEA (2016) Specification Separation Distances for
Groundwater Controlled Urban Development
Note 1:300mm of coarse sand applied to anticipated
garden areas in the rear of lots above the 50% AEP mound.
Note 2: 150mm of coarse sand applied to anticipated
garden areas in the rear of lots above the 50% AEP mound.
*Classification of soils types is based on Table A1 of
AS1726-1993 Geotechnical site investigations
SOIL TYPE SEPARATION DISTANCE
Gravel
Coarse 150 mm
Medium 150 mm
Fine 200 mm
Sand
Coarse 300 mm
Medium 450 mm
Fine 650 mm
Table 2: IPWEA (2016) Draft Specifications Table 3: Separation distance for turfed open space based
on typical soil types*
INFRASTRUCTURE
AEP
MOUNDING
(%)
SEPARATION
CRITERIA
(mm)
Drainage (infiltration systems & devices)
Underground 50 0
Surface 50 300
Private Gardens
Residential lots 400 to 800 m2
50 Note 1
Residential lots <400 m2 50 Note 2
Public Open Spaces (Turfed)
Sport
Local 50 Table 3
Neighbourhood 20 Table 3
District 20 Table 3
Regional 10 Table 3
8Example in Red

9. CASE STUDY #1 – Sketch of Groundwater Mounding
between Parallel Subsoil Drains
Example: Residential Lots < 400 m2, 70m Drain Spacing.
9
Ka (Hydraulic Conductivity
above subsoil)
Kb (Hydraulic Conductivity
below subsoil)
Finished Surface

10. 0
0.5
1
1.5
2
2.5
2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 2039 2040 2041 2042 2043 2044 2045
MoundingHeight(m)
CASE STUDY #1 - RESULTS
Ka =1m/d
Ka =5m/d Ka =10m/d
10Example in Red (Ka=5m/d) Median Rainfall 2016 – 2045 = 694mm/yr
50% AEP Mounding: 0.80m2017: 0.80
0
0.5
1
1.5
2
2.5
2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 2039 2040 2041 2042 2043 2044 2045
AnnualMaximaMoundingHeight
(m)
Ka =1m/d
Ka =5m/d
Ka =10m/d
50% AEP Mounding: 0.80m

11. CASE STUDY #1 – Separation for 2030 Median
Climate Scenario for Residential Lots
Example: Residential Lots < 400 m2, 70m Drain Spacing.
0.95m
total
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Ka (Hydraulic Conductivity
above subsoil)
Kb (Hydraulic Conductivity
below subsoil)
Mounding: 0.80m
Finished Surface

12. 0.96
0.62
0.51
1.21
0.80
0.66
3.25
1.91
1.52
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
0 2 4 6 8 10
50%AEPMoundingHeight(m)
Saturated Hydraulic Conductivity of Imported Sand Fill (m/d)
CASE STUDY #1 – 50% AEP Mounding vs. Saturated
Hydraulic Conductivity of Imported Sand Fill
50m Spacing
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0.96
1.21
3.25
0.62
0.8
1.91
0.51
0.66
1.52
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 50 100 150 200 250
50%AEPMounding(m)
Spacing (m)

13. CASE STUDY #1 – Impact of Recharge Distribution on
Groundwater Mounding
Groundwater Mounding Modelling Scenarios for single year
• Uniform Rainfall Recharge
Scenario 1: Uniform recharge over the lot
• Non-Uniform Rainfall Recharge
Scenario 2: Soakwell at the back and front of the lot
Scenario 3: Soakwell at the front of the lot only
Scenario 3 gives the lowest groundwater mounding heights.
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14. CASE STUDY #1 – Assessment of Recharge Distribution
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15. CASE STUDY #2 – The Avenue – Oval Subsoil Design
15

16. IPWEA (2016) Specification Separation Distances for
Groundwater Controlled Urban Development
Note 1:300mm of coarse sand applied to anticipated
garden areas in the rear of lots above the 50% AEP mound.
Note 2: 150mm of coarse sand applied to anticipated
garden areas in the rear of lots above the 50% AEP mound.
*Classification of soils types is based on Table A1 of
AS1726-1993 Geotechnical site investigations
SOIL TYPE SEPARATION DISTANCE
Gravel
Coarse 150 mm
Medium 150 mm
Fine 200 mm
Sand
Coarse 300 mm
Medium 450 mm
Fine 650 mm
Table 2: IPWEA (2016) Draft Specifications Table 3: Separation distance for turfed open space based
on typical soil types*
INFRASTRUCTURE
AEP
MOUNDING
(%)
SEPARATION
CRITERIA
(mm)
Drainage (infiltration systems & devices)
Underground 50 0
Surface 50 300
Private Gardens
Residential lots 400 to 800 m2
50 Note 1
Residential lots <400 m2 50 Note 2
Public Open Spaces (Turfed)
Sport
Local 50 Table 3
Neighbourhood 20 Table 3
District 20 Table 3
Regional 10 Table 3
16Example in Red

17. CASE STUDY #2 – The Avenue – Oval Subsoil Design
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18. CASE STUDY #2 – The Avenue – Clay Subgrade
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19. CASE STUDY #2
Assumptions and Parameters
• Armadale baseline rainfall
• Site specific rainfall anomalies to 2030
from Department of Water
• Net recharge rates:
• Oval and School – 90%
• Oval Services – 65%
• Waste Water Treatment Station – 10%
• Effective porosity: 0.2
• Clean sand fill above subsoil drains with
a saturated hydraulic conductivity (Ka)
of 5m/day
• Underlying clay subgrade modelled as
impermeable
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20. CASE STUDY #2 – The Avenue – Oval Subsoil Design 1
20

21. 21
CASE STUDY #2 – The Avenue – Oval Subsoil Design 1

22. CASE STUDY #2 – The Avenue – Oval Subsoil Design 2
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23. CASE STUDY #2 – The Avenue – Oval Subsoil Design 2
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24. CASE STUDY #2 – The Avenue – Oval Subsoil Design 2
20% AEP Mounding
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25. Conclusions
• These case studies provide an example for application of the DoW (2015) Future
Climate Scenarios to groundwater modelling estimation of water mounding
between parallel subsoil drains as recommended in IPWEA (2016) guidelines.
• If the IPWEA clearance guidelines were applied there is the potential to reduce
groundwater clearance and minimise imported fill.
• High importance of soakwell near front of lots, and near subsoil.
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