LID vis‐à‐vis Detention SCM Ability to Meet Stream Erosion Standards

Erica D. Tillinghast, EITNCSU Masters Student

Geosyntec Consultants, Inc.

Outline• Background and Research Objective

• SCM Design Standards to Limit Stream Erosion for Piedmont North Carolina

• Increasing Stream Geomorphic Stability Using LID Practices and/or Wet Ponds in Chapel Hill

• Summary and Final Recommendation

Outline• Background and Research Objective

• SCM Design Standards to Limit Stream Erosion for Piedmont North Carolina

• Increasing Stream Geomorphic Stability Using LID Practices and/or Wet Ponds in Chapel Hill

• Summary and Final Recommendation

Impacts of Urbanization

• Increased impervious surfaces

• Higher stormwater runoff flowrates

• Higher stormwater runoff volumes

LEADING TO EROSION

Factors of Erosion

• Type of sediment within stream

• Decrease of incoming sediment

• Stream power

Class Name Ds (mm) τc  (Pa)SandVery Course >1 0.47Coarse > 0.5 0.27Medium > 0.25 0.194Fine > 0.125 0.145Very Fine > 0.0625 0.11

SiltCoarse > 0.031 0.083Medium > 0.016 0.065

Stormwater Control Measures

Stormwater Control Measures

Current SCM design standards:

1. Peak flow attenuation2. Volume reduction3. Enhancement of water 

quality

Do NOT consider erosionalprocesses of receiving streams

Impact of SCMs on StreamsLess than 1‐year Storm: 31 mm fell within 8 Hours

Critical Discharge

Art Museum in natural state (5% impervious)

Art Museum 36% impervious

Art Museum with wet pond at outlet

PCSWMM• Used to model urbanized watersheds

• Spatially distributed model

• Model LID practices and wet ponds

• Long‐term continuous simulations

• Non‐linear reservoir routing

Research Objective

• Use rural reference streams to :1. Develop unit critical discharge model2. Annual allowable erosional hour standard3. Annual allowable volume of eroded bedload standard

• Analyze hydrologic impact of LID practices and structural detention SCMs in urbanized watershed

• Ability to meet developed erosional standards

Outline• Background and Research Objective

• SCM Design Standards to Limit Stream Erosion for Piedmont North Carolina

• Increasing Stream Geomorphic Stability Using LID Practices and/or Wet Ponds in Chapel Hill

• Summary and Final Recommendation

Procedure: Reference Streams

Reference streams:– Parts of a stream that have developed a stable dimension, pattern, and profile

– Used in stream restoration projects to restore disturbed streams to natural conditions and create state of dynamic equilibrium (Rosgen 1996)

Procedure: Reference Streams

y = 0.0035x1.5048R² = 0.86

0

0.5

1

1.5

2

2.5

0 20 40 60 80

Unit C

ritical Discharges (L/s/hectare)

d65 (mm)

Results: Unit Critical Discharge

τc = γ RcSQc = (1/n)A*Rc2/3 S1/2

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.02 0.04 0.06 0.08 0.1 0.12

τ crit(kPa)

τbkf (kPa)

d50d60d65d75d85

1:1 Line

Results: Unit Critical Discharge

79 % of d50 below 0.1 L/s/ha51 % of d60 below 0.1 L/s/ha6 % of d65 below 0.1 L/s/ha

54 % of d85 represented sub‐bankfull flows

Results: Unit Critical Dischargey = 0.0035x1.5048

R² = 0.86

0

0.5

1

1.5

2

2.5

0 20 40 60 80

Unit C

ritical Discharges (L/s/hectare)

d65 (mm)

Results: Erosional Standards

‐2.5

‐2

‐1.5

‐1

‐0.5

0

0.5

1

1.5

2

‐0.5 0 0.5 1 1.5 2

Log(AA

EH)

Log(d65)

Results: Annual Allowable ErosionalHours

Log(AAEH) = ‐1.26Log(d65) + 1.21R2 = 0.40

‐4

‐3.5

‐3

‐2.5

‐2

‐1.5

‐1

‐0.5

0

0 0.5 1 1.5 2 2.5

Log(AV

)

Unit Critical Discharge (L/s/ha)

Results: Annual Allowable Volume of Eroded Bedload

Log(AV) = ‐ 0.64 (Qc) ‐ 1.52R2 = 0.66 

Results: Erosional Standards

• Unit Critical Discharge Equation:Qc=0.0035(d65)1.5048

•Annual Allowable Erosional Hours Standard:Log(AAEH)= ‐1.26Log(d65)+1.21

•Annual Allowable Volume of Eroded Sediment:Log(AV)= ‐0.64(Qc)‐1.52

House Creek Watershed

Results: House Creek Watershed

Results: House Creek Watershed

Conclusions

• Current SCM design standards fall short of protecting stream geomorphic stability

• Unit critical discharge, Qc=0.0035(d65)1.5048, and erosional standards, Log(AAEH)= ‐1.26Log(d65)+1.21 and Log(AV)= ‐0.64(Qc)‐1.52, can be applied to urbanized watersheds

Outline• Background and Research Objective

• SCM Design Standards to Limit Stream Erosion for Piedmont North Carolina

• Increasing Stream Geomorphic Stability Using LID Practices and/or Wet Ponds in Chapel Hill

• Summary and Final Recommendation

Tanyard Branch

Methods: Gathering Data

Methods: Calibration Data

White = ResidentialBlack = DowntownBlue = UNC Campus

Results: Erosional Standards

d65(mm) = 21.2 mmQc = 0.0035(d65)1.5048Log(AV) = ‐0.64(Qc) ‐1.52Log(AAEH) = ‐1.26(d65) +1.21

Results: Cost

Results: Alternative Outlet Structure 

20.32 cm Orifice

1.52 m 

0.48 m

1.52 m

Overflow Weir

17.78 cm Orifice

1.52 m 

0.457 m

1.22 m

Overflow Weir

25.4 cm Orifice

0.31 m

Current Design Alternate Design

Modeled erosional hours from 19.6 to 15.4 hrs/ha/yr

Modeled volume of eroded sediment from 0.06 to 0.07 m3/m/ha/yr

Conclusions• LID practices with a wet pond maximized storage and infiltration within watershed

• Wet ponds extended erosional hours but decreased volume of eroded bedload

• Altering wet pond outlet structure increased stream stability

• Highly impervious watershed was incapable of meeting strict stream erosion metrics

Outline• Background and Research Objective

• SCM Design Standards to Limit Stream Erosion for Piedmont North Carolina

• Increasing Stream Geomorphic Stability Using LID Practices and/or Wet Ponds in Chapel Hill

• Summary and Final Recommendations

Future Work

• Incipient motion analysis with urban reference streams

• Apply unit critical discharge model, erosionalstandards, and alternative outlet structure to multiple, diverse watersheds

• Analyze impact of time

Conclusions

• SCM design standards need to consider stream erosional processes

• Developed Qc, AAEH, and AV standards can be used in urbanized watersheds

• LID practices with wet ponds and alternate outlet structure increased stream stability 

Acknowledgements

• Dr. William Hunt (Advisor)• Dr. Gregory Jennings (Co‐Advisor)• Dr. Sankar Arumugam (Committee)• NCDENR• Patricia D’Arconte• NC State BAE Stormwater Team• Aaron Poresky (Geosyntec)

Additional Information

• Tillinghast, E.D., Hunt, W.F., Jennings, G.D. (2011). “Stormwater Control Measure (SCM) Discharge Design Standards to Limit Stream Erosion for Piedmont North Carolina.” Journal of Hydrology. Accepted 18 September 2011.

• Contact: etillinghast@geosyntec.com

Questions?

Conclusion

• Higher geomorphic stability in stream, the higher cost of project

• Under‐sized wet pond provided minimal mitigation in terms of eroded sediment and nitrogen and phosphorous reduction

• Unless full‐sized wet pond chosen, need additional nitrogen removal 

Modeling ProgramsSWMM 5.0 MOUSE SWAT

Pros •Spatially Distributed Model•Sub‐Hourly Time Scales•Hydraulic routing•Groundwater/Baseflow•Urbanized Settings•Ponds/Wetlands•Planning and Preliminary Design•Long‐term Continuous simulations

•Spatially Distributed Model•Sub‐Hourly Time Scales•Hydraulic routing•Groundwater/Baseflow•Ponds/Wetlands•Urbanized Setting•Long‐term Continuous simulations

•Continuous Modeling 

Cons •Too complex to be used by general public or non‐modeling planners

•Widely Used OUTSIDE U.S.•COST $5,000 for license•Too complex to be used by general public or non‐modeling planners

•Large –Scale Watersheds•Daily Time‐Steps•Not For Single‐Event Storms•Rural/Agriculture Watersheds•Hydrologic Response Units•BMPs not spatially represented

Results: Flooding

Storm Event As-Is Restored Channel Under-Sized Wet Pond Required Sized Wet Pond100-year, 24-hour Yes Yes Yes50-year, 24-hour Yes Yes Yes25-year, 24-hour Yes Yes No10-year, 24-hour Yes Yes No5-year, 24-hour Yes No No2-year, 24-hour Yes No No1-year, 24-hour No No No

Results: Nutrient ReductionScenario % Reduction Nitrogen % Reduction PhosphorousExisting 0 0

Under-Sized Wet Pond 11 14

Required Wet Pond 39 52Residential 4 5

UNC Campus 4 5Downtown 4 5

Residential+ Under-Sized Wet Pond 15 19

UNC Campus+Under-Sized Wet Pond 15 19

Downtown+Under-Sized Wet Pond 15 19

Results: Altering Wet Pond

Orifice Size

Wet Pond Size Increase of

Original Value

Modeled Erosional

Hours (hrs/ha/yr)

Modeled Volume

Sediment Transport

(m3/m/ha/yr)

Modeled Erosional

Hours (hrs/ha/yr)

Modeled Volume

Sediment Transport

(m3/m/ha/yr)

Modeled Erosional

Hours (hrs/ha/yr)

Modeled Volume

Sediment Transport

(m3/m/ha/yr)1 21.3 0.51 20.6 0.29 20.2 0.282 21.6 0.63 20.8 0.27 20.3 0.283 21.9 0.55 21.1 0.25 20.7 0.265 22.7 0.51 21.6 0.23 21.2 0.24

10 23.6 0.44 21.9 0.17 21.6 0.19

5.08 cm 7.62 cm 10.16 cm

Results: Impact of Wet Pond

Location

Modeled Erosional Hours

(hrs/ha/yr)

Empirical Allowable Erosional Hours

(hrs/ha/yr)

Modeled Volume Eroded

Sediment (m3/m/ha/yr)

Empirical Allowable

Volume Eroded Sediment

(m3/m/ha/yr)Institutional (Wet Pond) 89 11.2 0.99 0.49Institutional (No SCM) 37 11.2 1.81 0.49Institutional (Natural) 14 11.2 0.11 0.49

Results: Ecological Benefits

Scenario Ecological ServicesExcavation of

SedimentSewer Line Exposure

(In 30 Years?)1) Existing 9 9 Yes

2) Under-Sized Wet Pond 6 6 No3) Required Wet Pond 2 2 Removed

4) Residential 8 8 No5) Residential+ Under-Sized

Wet Pond 4 4 No6) Residential + UNC

Campus 7 7 No7) Residential + UNC

Campus+Under-Sized Wet Pond 3 3 No

8) Residential + UNC + Downtown 5 5 No

9) Residential + UNC + Downtown+Under-Sized Wet

Pond 1 1 No

Tanyard Branch

Land area = 0.3 ha

Sewer Lines

White = ResidentialBlack = DowntownBlue = UNC Campus

Results: Magnitude of Critical Flow Rates

Location

D65

Critical Discharge

(cms)

1-year return period (cms)

2-year return period (cms)

5-year return period (cms)

10-year return period (cms)

UT to Varnals 0.02 0.01 0.41 2.56 4.73UT to SW Beaverdam 0.04 0.66 1.82 5.07 5.22

Landrum Creek 1.04 0.29 1.77 9.46 17.19Mid Pines 0.10 1.09 2.75 8.85 11.23

UT to Lake Wheeler 0.03 0.40 1.09 3.26 4.23

80% of 33 cross‐sections below 1‐year storm

94% of 33 cross‐sections below 2‐year storm

Impacts of UrbanizationStable, Rural Reference Stream

0

5

10

15

20

25

30

35

40

0 2 4 6 8 10 12

τ(Pa)

Q (cms)

Bankfull

Impacts of UrbanizationIncised, Urban Stream

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30 35 40

τ(Pa)

Q (cms)

Bankfull

Procedure: PCSWMM

SWMM RAIN

SWMM RUNOFF

SWMM EXTRAN

SWMM TRANSPORT

SWMM STATISTICS

Input rainfall data Creates surface runoff Stage‐discharge relationships 

Routes flows through basins/sub‐catchments

Flow frequency data

House Creek Watershed

Results: Stream Classification

Urban Reference Stream Tanyard Branch Percent DifferenceBankfull Cross-Sectional Area (m2) 2.19 8.1 -270

Bankfull Discharge (cms) 3.49 18.15 -420Width (m) 4.61 7.62 -65Depth (m) 0.46 1.28 -178

Geomorphic Characteristics of Rural Reference Streams

Stream LocationRosgen Stream

TypeDrainage Area (ha)

Bankfull Cross-Sectional Area

(m2)

Bankfull Width

(m)

Bankfull Mean

Depth (m)

Average Channel

Slope

D50

(Reach Wide)

Fork Creek Upstream B4c 700 4.13 5.85 0.43 0.005 0.9Landrum Creek C3 544 2.53 5.30 0.46 0.008 26.1

Mid Pines E5 337 3.34 7.04 0.49 0.005 1.3Morgan Creek C4 2124 6.28 10.49 0.61 0.006 9.3

Sals Branch E5 78 1.44 3.32 0.43 0.008 1.7Sandy Creek E5 673 3.59 6.37 0.58 0.006 1.6

Terrible Creek C5 596 2.55 5.85 0.43 0.005 1.8UT Ledge C5 906 0.92 4.94 0.18 0.003 0.8

UT to Cane Creek E5 233 2.78 5.52 0.52 0.009 1.7UT to Lake Janette C5 26 2.17 5.76 0.37 0.007 0.8UT to Lake Raleigh E5b 26 1.09 3.44 0.30 0.036 0.1UT to Lake Wheeler E4 104 1.62 3.23 0.49 0.006 2.6UT to Polecate Creek E3 129 0.85 2.80 0.30 0.015 22.6UT to Sandy Creek #2 E5 259 1.64 3.66 0.46 0.004 0.9UT to SW Beaverdam E4 78 1.67 3.78 0.46 0.014 3.8

UT to UT Billy's Creek E5 26 1.27 3.20 0.37 0.015 0.6UT to Varnals E5 104 1.50 4.21 0.37 0.017 0.4

Watershed Land Uses of Reference Streams

Stream Location % Impervious % Water % Forest % Pasture % Other1

Fork Creek Upstream 0.9 0 74.4 12.7 12Landrum Creek 0.9 1.9 75.6 14.7 6.9

Mid Pines 5.4 0.7 51.4 18.9 23.6Morgan Creek 0.7 0.5 69.8 19.3 9.7

Sals Branch 8.3 0.7 83 0 8Sandy Creek 0.6 0.7 43.2 43.4 12.1

Terrible Creek 12.6 0.3 30.4 4.6 52.1UT Ledge 0.6 1.5 42.3 18.9 36.7

UT to Cane Creek 1.6 1.5 44.8 38.7 13.4UT to Lake Janette 20.9 0 7.4 2.6 69.1UT to Lake Raleigh 0.4 0 95.1 1.8 2.7UT to Lake Wheeler 5.5 0 51.7 8.9 33.9UT to Polecate Creek 1.3 0.7 55.2 28.8 14UT to Sandy Creek #2 0.6 0.7 43.2 43.4 12.1UT to SW Beaverdam 14.8 0 2.8 0 82.4

UT to UT Billy's Creek 0 0 61.8 23.3 14.9UT to Varnals 0.1 0 94.8 3.5 1.6

Land Use

House Creek: Highway

House Creek: Golf Course

House Creek: Art Museum

Incipient Motion Analysis

1) τc = τ*c (γs – γ) D

Where:τc = critical bed shear stress (Pa)τ*c = dimensionless Shields parameter γs = unit weight of sediment (N/m3)γ = unit weight of water (N/m3)

D = diameter of sediment (m)

2) τc = γ RcS3) Qc = (1/n)A*Rc2/3 S1/2

Meyer‐Peter Muller (1948) Transport for Sand Streams 

q*= 8(τ*‐ τ*c)1.5Where:τ*c = critical dimensionless shear stress (assume same values as did for Equation 1) τ*= dimensionless shear stress parameter

Where:H = depth of flow (m)S = stream slope (m/m)D = size of sediment (m)ρs = density of sediment (g/m3)ρ = density of water (g/m3)

Parker (1979) for Gravel Bed Streams

q*= * 4.5

*3

( 0.03)11.2

 

Whereq*= dimensionaless volumetric bedload transport rateτ*= dimensionless shear stress parameter

Volumetric Bedload Transport (Einstein, 1950)

q*=( 1)

b

s

q

D g D

 

Whereqb = volumetric bedload transport rate (m3/m/s)q*= dimensionaless volumetric bedload transport rateD = size of sediment (m)ρs = density of sediment (g/m3)ρ = density of water (g/m3)

Tanyard Branch

Chapel Hill, NC

9 Scenarios in Tanyard BranchScenario Areas Treated Description

1 None Existing condition2 Entire watershed (68 hectares) with 

additional 1.2 hectaresUndersized wet pond at outlet

3 Entire watershed (68 hectares) with additional 2.4 hectares

Full‐size wet pond at outlet

4 Residential area only (24 hectares) 41 cisterns and 56 rain gardens5 Residential area + under‐sized wet 

pond (25.2 hectares)48 cisterns, 63 rain gardens, under‐sized wet 

pond from scenario 26 Residential + UNC campus  (36 

Hectares)41 cisterns, 56 rain gardens, 4 green roofs 

(0.49 hectares), and 7 permeable pavements (2.45 hectares)

7 Residential + UNC campus + under‐sized wet pond (37.2 hectares)

48 cisterns, 63 rain gardens, 4 green roofs (0.49 hectares), 7 permeable pavements (2.45 hectares), and under‐sized wet pond 

from scenario 28 Residential + UNC campus + downtown 

(68 hectares)41 cisterns, 56 rain gardens, 10 green roofs 

(1.01 hectares), and 13 permeable pavements (6.5 hectares)

9 Residential + UNC campus + downtown + under‐sized wet pond (69.2 hectares)

48 cisterns, 63 rain gardens, 10 green roofs (1.01 hectares), 13 permeable pavements (6.5 hectares), and under‐sized wet pond 

from scenario 2

Added Drainage Under‐Sized Wet Pond

Added Drainage Full Sized Wet Pond

Stream Characteristics of TanyardBranch

Stream Characteristic ValueEntrenchment Ratio 12Width-Depth Ratio 9.1Bank Height Ratio 2.26

Sinuousity 1.2d50 (mm) 14

Slope 0.013

Ecosystem Services

Biome Area (ha x 106)Water

RegulationWater Supply

Waste Treatment

Food Production Recreation

Total Value per ha ($/ha/yr)

Lakes/Rivers 200 5,445 2,117 665 41 230 8,498

Ecosystem Services (1994 US$/ha/yr)

Art Museum

Golf Course

Highway

SWMM

Particle Size Moved at Bankfull

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.02 0.04 0.06 0.08 0.1 0.12

τ crit(kPa)

τbkf (kPa)

d50

d60

d65

d75

d85

1:1 Line

1:1 Line1:1 Line

Outlet Structure (Under‐Sized Wet Pond)

10.16 cm Diameter

Orifice

1.83 m

0.457 m

1.52 m Storage

Overflow Weir

Description DimensionsDraw-Down Orifice 10.16 cm Diameter4 Rectangular Weirs 1.83 m by 0.457 m

Rectangular Overflow Weir 1.83 m by 1.83 m

Modeling LID Practices in PCSWMM

0

20

40

60

80

100

120

0.1 1 10 100 1000 10000

Percen

t Cum

ulative

Median Diameter (mm)

Tanyard Branch

Under‐Sized Wet Pond

Full Sized Wet Pond

Wet Pond Cross‐Section