Hydraulic Design of Permeable Pavements: Do They Work

1
Hydraulic Design ofPermeable PavementsDo They
Work?
H1

• Bill Hunt
• Bio Ag Engineering Department
• North Carolina State University

2
Hydraulic Design
H2

• Purpose to reduce runoff by increasing
  infiltration
• Calculate ability of pavement to store
  infiltrate Water

3
Hydraulic Design
H3

• Two Part Process
• Ability of water to exfiltrate gravel layer
• Ability of water to infiltrate surface layer

4
Hydraulic Design
H4

• Exfiltrating Gravel Base Layer
• Select Design Storm
• Determine Water Storage Capacity of Pavement
• Select Exfiltration Time

5
Hydraulic Design
H5

• Exfiltrating Gravel Base Layer
• Calculate Drawdown Time
• Compare Actual Drawdown Time with Design
  Exfiltration Time

6
Hydraulic Design
H6

• Exfiltrating Gravel Base Layer
• Select Design Storm
• At a minimum trap first flush 1.0
• For many permeable pavements in Eastern NC a 2.0
  storm can be captured.

7
Hydraulic Design
H7

• Exfiltrating Gravel Base Layer
• Determine Water Storage Capacity (Storage)
• Depends upon
• 1. Depth of Gravel Base Layer (Tgravel -
  determined in Structural Design)

8
Hydraulic Design
H8

• Exfiltrating Gravel Base Layer
• Determine Water Storage Capacity
• Depends upon
• 2. the porosity, n, of the gravel. Porosity for
  Size 57 stone or coquina varies from 30-40

9
Hydraulic Design
H9

• Exfiltrating Gravel Base Layer
• Determine Water Storage Capacity
• Formula Storage Tgravel X n

10
Hydraulic Design
H10

• Exfiltrating Gravel Base Layer
• Select Exfiltration Time
• At maximum, 48 hours. It is best if under 20
  hours ( day) (DT Design Time)

11
Hydraulic Design
H11

• Exfiltrating Gravel Base Layer
• Calculate Drawdown Time
• Drawdown Storage / Infiltration Rate
• Infiltration Rates for typical soils given on
  Page One of Hydraulic Design Worksheet.

12
Hydraulic Design
H12

• Exfiltrating Gravel Base Layer
• Calculate Drawdown Time

13
Hydraulic Design
H13

• Exfiltrating Gravel Base Layer
• Compare Actual Drawdown Time with Design Time
• Is Drawdown lt DT?
• If so, Exfiltration Objective Met.
• If not, re-evaluate.

14
Hydraulic Design
H14

• EXAMPLE Exfiltrating Gravel Base Layer
• Select Design Storm 2.0
• Determine Water Storage Capacity
• (from Structural Design Handout 8)
• Storage 8 X 0.30 2.4
• STORAGE IS GREATER THAN DESIGN STORM

15
Hydraulic Design
H15

• EXAMPLE Exfiltrating Gravel Base Layer
• Select Design Exfiltration Time 20 hours

16
Hydraulic Design
H16

• EXAMPLE Exfiltrating Gravel Base Layer
• Calculate Drawdown Time
• Assuming a Loamy Sand Soil (Union
  Point) (Infiltration Rate 2.5 inches/hour)
• Drawdown 2.4 / 2.5 inches/hour
• Drawdown 1 hour

17
Hydraulic Design
H17

• EXAMPLE Exfiltrating Gravel Base Layer
• Compare Actual Drawdown Time with Design Time
• Drawdown 1 hour ltlt Design Time (20 hrs)
• Hydraulic Design is easily sufficient

18
Hydraulic Design
H18

• Infiltrating Surface Layer
• Find Surface Layer Soil Infiltration Rate
• Calculate Effective Rainfall to Match
  Infiltration - this is Rate for Runoff

19
Hydraulic Design
H19

• Infiltrating Surface Layer
• Find Surface Layer Soil Infiltration Rate
• Use Table 1 from Handout. Often soil at surface
  will be sand (Infiltration Rate 8 in/hr)

20
Hydraulic Design
H20

• Infiltrating Surface Layer
• Calculate Effective Rainfall (Eff RF)
• Establish of soil versus of impermeable
  concrete
• Eff RF Rainfall Rate / of Permeable Area

21
Hydraulic Design
H21

• Infiltrating Surface Layer
• Calculate Effective Rainfall (Eff RF)
• The Rainfall Rate (P) at which
• Eff RF Infiltration - is the Rainfall Rate
  where Runoff will be produced.

22
Hydraulic Design
H22

• EXAMPLE Infiltrating Surface Layer
• Find Surface Layer Soil Infiltration Rate Sand
  Used (8 inches/hour)

23
Hydraulic Design
H23

• EXAMPLE Infiltrating Surface Layer
• Calculate Effective Rainfall Concrete Pavers are
  typically 60 impervious, 40 pervious. Eff RF
  Infiltration Rate 8 inches/hour P / 0.40 or
  2.5 P.
• P 8/ 2.5 3.2 in/hr

24
Hydraulic Design
H24

• EXAMPLE Infiltrating Surface Layer
• Therefore the rate of rainfall (P) which will
  produce runoff is 3.2 in/hr. Rather Intense.

25
Calculating a Rational C
H25

• FIVE PART PROCESS
• Measure Runoff from Parking Lot (Volume)
• Measure Rainfall from Rooftop (Depth)
• Determine Watershed Boundaries (Area)

26
Calculating a Rational C
H26

• FIVE PART PROCESS
• Multiply Rainfall Depth by Watershed Area gives
  Potential Runoff (Volume)
• Compare Volume of 1 to Volume of 4

27
Measuring Runoff
H27
Q 0.5 h 0.5
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Rainfall at Kinston
H28

• From July 1999 to May 2000
• 22 events greater than 0.50
• 10 events gt 1.00
• 3.5 Hurricanes
• Total Rainfall from these noted storms gt 43

29
Can You Guess a Rational C?
H29

• A 0.80
• B 0.50
• C 0.30

30
Can You Guess a Rational C?
H30

• C 0.30

31
Sample Runoff Events
H31
See attached Handout for Complete List
32
Hurricane Dennis Runoff
H32
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Cross Section of Pavement
H33