Mounding AnalysisEXAMPLES

Groundwater

Changi Wu/Lisa SchaeferNJDEP Division of Water Quality

SWMDR Training Day 2October 31, 2019

Introduction

• Demonstrate simple analysis• Demonstrate stretching the time interval• Demonstrate applicability to drainage pipes• Demonstrate calculating impacts to nearby

structures

Presentation Goals

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Examples

1. No adverse hydraulic impact to groundwater2. Adverse hydraulic impact to groundwater and

adjustment of infiltration period3. Underground perforated pipe4. Drywell

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Scenarios:

No adverse hydraulic impact to groundwater scenario

• A major development located in the coastal plain proposes an infiltration basin, whose bottom footprint measures 50 ft by 50 ft, to infiltrate 5,000 cf of runoff generated by the Water Quality Design Storm (WQDS). o The maximum depth of ponding water is designed to be

2 ft o Soil testing was performed in accordance with Appendix Eo The soil testing report shows that the Seasonal High Water

Table (SHWT) is 7.5 ft below the 6 in thick bottom sand layer of the basin

o The tested soil permeability rate of the most restrictive soil horizon below the basin is 4 in/hr

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No adverse hydraulic impact to groundwater scenario

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Step #1: Calculate the duration of infiltration period

Duration of infiltration period, t (hours)

= / /

Infiltration area = 50 ft 𝑥 50 ft 2,500 sf

Recharge Rate = ½ Tested Infiltration rate = 0.5 𝑥 4 in/hr 2 in/hr

Therefore, t

= , /, /

= 12 hr

No adverse hydraulic impact to groundwater scenarioStep #2: Prepare the inputs for the spreadsheet

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Parameter Value

Recharge rate (R) 2 in/hr (4 ft/day)

Specific yield (Sy) 0.15

Horizontal hydraulic conductivity (Kh) 20 in/hr (40 ft/day)

½ length of basin (x direction) 25 ft

½ length of basin (y direction) 25 ft

Duration of infiltration period (t) 12 hr (0.5 days)

Initial thickness of saturated zone (hi(0)) 10 ft

No adverse hydraulic impact to groundwater scenarioStep #2: Input Section of the Hantush Spreadsheet

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No adverse hydraulic impact to groundwater scenarioStep #2: Results Section of the Hantush Spreadsheet

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SHWT

Adverse hydraulic impact to groundwater and adjustment of drain time scenario

• A major development located in the coastal plain proposes an infiltration basin, whose bottom footprint measures 88 ft by 88 ft, to infiltrate 15,000 cf of runoff generated by the 2-year design storm. o The maximum depth of ponding water is designed to be

2 ft

o Soil testing was performed in accordance with Appendix E

o The soil testing report shows that SHWT is 7.5 ft below the 6-in thick bottom sand layer of the basin

o The tested soil permeability rate of the most restrictive soil horizon below the basin is 4 in/hr

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Adverse hydraulic impact to groundwater and adjustment of drain time scenarioStep #1: Calculate the duration of infiltration period

Duration of infiltration period, t (hours)

= / /

Infiltration area = 88 ft 𝑥 88 ft = 7,744 sf

Recharge rate = 0.5 𝑥 4 in/hr = 2 in/hr

Therefore, t = , /

, / = 11.62 hr

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Adverse hydraulic impact to groundwater and adjustment of drain time scenarioStep #2: Prepare the inputs for the spreadsheet

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Parameter Value

Recharge rate (R) 2 in/hr (4 ft/day)

Specific yield (Sy) 0.15

Horizontal hydraulic conductivity (Kh) 10 in/hr (20 ft/day)

½ length of basin (x direction) 44 ft

½ length of basin (y direction) 44 ft

Duration of infiltration period (t) 11.62 hr (0.48 days)

Initial thickness of saturated zone (hi(0)) 10 ft

Adverse hydraulic impact to groundwater and adjustment of drain time scenarioStep #2: Results Section of the Hantush Spreadsheet

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SHWT

Adverse hydraulic impact to groundwater and adjustment of drain time scenarioStep #3: Adjust the inputs

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Parameter Value

Recharge rate (R) 1 in/hr (2 ft/day)

Specific yield (Sy) 0.15

Horizontal hydraulic conductivity (Kh) 20 in/hr (40 ft/day)

½ length of basin (x direction) 44 ft

½ length of basin (y direction) 44 ft

Duration of infiltration period (t) 23.24 hr (0.96 days)

Initial thickness of saturated zone (hi(0)) 10 ft

Adverse hydraulic impact to groundwater and adjustment of drain time scenarioStep #3: New Results

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Adverse hydraulic impact to groundwater and adjustment of drain time scenarioA Comparison of Mounding Effects Caused by Different Recharge Rates

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0.000

1.000

2.000

3.000

4.000

5.000

6.000

7.000

8.000

9.000

10.000

0 20 40 60 80 100 120 140

---- R = 2.0 in/hr @ 11.62 hrs; ̶̶̶̶̶ ̶̶̶̶̶ ̶̶̶̶̶ R = 1.0 in/hr @ 23.24 hrs

.

SHWT (7.5 ft below sand layer)

Infiltration Basin

Sand Layer

Underground perforated pipe scenario

• A site is located in the coastal plain. A 500 LF, 36-inch diameter underground perforated pipe is proposed to store and infiltrate 12,000 cf of runoff. o The proposed duration of infiltration period is 24 hro The building has a basement floor located just above the

groundwater table. The center of the underground pipe is 20 ft away from the building

o Soil testing was performed in accordance with Appendix Eo The soil testing report shows that Seasonal High Water

Table (SHWT) is 2 feet below the pipeo The tested soil permeability rate of the most restrictive soil

horizon below the basin is 8 in/hr16

Underground perforated pipe scenario

Step #1: Calculate the duration of infiltration period

Infiltration area 500 ft x (36 in /12 in/ft) = 1,500 sf

Recharge rate = 0.5 𝑥 8 in/hr = 4 in/hr

Duration of infiltration period, t (hours)

= /

/

Therefore, t

= , /, /

= 24 hr17

Underground perforated pipe scenario

Step #1: Prepare the inputs for the spreadsheet

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Parameter Value

Recharge rate (R) 4 in/hr

Specific yield (Sy) 0.15

Horizontal hydraulic conductivity (Kh) 20 in/hr

½ length of basin (x direction) 1.5 ft

½ length of basin (y direction) 250 ft

Duration of infiltration period (t) 24 hr

Initial thickness of saturated zone (hi(0)) 10 ft

Underground perforated pipe scenario

Step #2: Results Section of the Hantush Spreadsheet

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1.800

1.600

1.400

1.200

1.000

0.800

0.600

0.400

0.200

0.000

Underground perforated pipe scenario

Step #2: Results Section of the Hantush Spreadsheet

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Drywell scenario

• A site is located outside the coastal plain. A 7 ft high by 5 ftdiameter drywell is proposed to store and infiltrate 137.4 cf ofroof runoff from a single-family home for the Water QualityDesign Storm.o The proposed duration of infiltration period is 42 hro The house has a basement floor located just above the

groundwater table. The center of the drywell is 10 ft awayfrom the house.

o Soil testing was performed in accordance with Appendix Eo The soil testing report shows that Seasonal High Water

Table (SHWT) is 2 ft below the bottom of the drywello The tested soil permeability rate of the most restrictive soil

horizon below the basin is 4 in/hr

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Drywell scenario

Step #1: Prepare the inputs for the spreadsheet

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Parameter Value

Recharge rate (R) 2 in/hr (4 ft/day)

Specific yield (Sy) 0.15

Horizontal hydraulic conductivity (Kh) 2 in/hr (4 ft/day)

½ length of basin (x direction) 2.5 ft

½ length of basin (y direction) 2.5 ft

Duration of infiltration period (t) 42 hr

Initial thickness of saturated zone (hi(0)) 10 ft

Drywell scenario

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Step #2: Results Section of the Hantush Spreadsheet

SWHT

Centerline of the drywell

Drywell scenario

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Step #2: Results Section of the Hantush Spreadsheet

Any Questions?

Bureau of Nonpoint Pollution ControlDivision of Water Quality401 East State StreetPO Box 420, Mail Code 401-2BTrenton, NJ 08625-420Tel: 609-633-7021www.njstormwater.org

Changi Wu Lisa SchaeferChang.I.Wu@dep.nj.gov lisa.schaefer@dep.nj.gov

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