An Investigation of Boundary Shear Stress and Pollutant Detachment From an Impervious Surface During Simulated Urban Sto

1. An Investigation of Boundary Shear Stress and Pollutant Detachment From an Impervious Surface During Simulated Urban Storm Runoff C.P. Richardson1 and G.A. Tripp2 1Associate Professor of Environmental Engineering, Department of Civil and Environmental Engineering, New Mexico Tech, Socorro, New Mexico 87801 2Graduate Research Assistant, Department of Mineral Engineering, New Mexico Tech, Socorro, New Mexico 87801

2. Significance of the Problem • Urban Stormwater Runoff • Large areas of impervious or semi-impervious surfaces • Major non-point source of pollutants previously deposited during dry weather • Runoff quantity typically high volume and relatively short duration

3. Significance of the Problem • National Urban Runoff Program (19 cities) • 33 % lake contamination from runoff • 10 % river contamination from runoff • Several priority pollutants found in at least 10 % of samples collected • e.g. #121 phenanthrene; #30 lead; #51 chloroform; #5 lindane; #23 arsenic

4. Modeling Background • Stormwater Water Quality Models • Two-stage process • Pollutant accumulation on catchment surfaces during dry weather periods • Pollutant washoff during rainfall and subsequent runoff.

5. Modeling Background (cont’d) • Pollutant Washoff is the Critical Stage • Transport limited process governed by rainfall and runoff characteristics • Dependent upon overland flow shear stress (Nakamura, 1984) • Dependent upon raindrop and runoff energies (Vaze and Chiew, 2003)

6. Modeling Background (cont’d) • Typical Modeling Approach • Estimate pollutant washoff empirically by a first-order relationship (exponential) • Washoff rate depends linearly on the available accumulated pollutant mass, on the rainfall intensity, and/or the overland flow runoff rate (Alley, 1981; Millar, 1999)

7. Modeling Background (cont’d) • Storm Water Management Model (SWMM) • Algorithm uses exponential relationship between pollutant washoff and runoff volume (Huber and Dickinson, 1988) • This type of model lacks a physical basis for pollutant detachment from the impervious surface

8. Modeling Background (cont’d) • Previous Research • Mass flux of pollutants from a pervious surface is a function of boundary shear stress (Richardson and Parr, 1988) • Pollutant mass flux increased linearly as the product of shear velocity and the square root of boundary permeability increased

9. Research Objective • Two-fold Objective • Examine rates of pollutant detachment from an impermeable surface for various chloride compounds and determine their relationship to boundary shear stress • Quantify a washoff coefficient under varied hydraulic conditions for different chloride compounds and, if possible, to identify controlling factors

10. Research Methodology • Plexiglass Laboratory Flume • 2.44 m long by 20.3 cm wide • Impermeable test section • 1.14 m long by 20. 3 cm wide • Beach sand surface 0.4 to 0.8 mm • Simulated overland flow and rainfall • Rainfall module 1.0 m above flume

11. Research Methodology (cont’d) • Plexiglass Laboratory Flume (cont’d) • Flowmeters • Applied overland flow and rainfall • Boundary Shear Stress (Re versus f) • Lory depth gauges • Flush-mounted hot film anemometer

12. Research Methodology (cont’d) • Tracer Chemicals • Four inorganic chloride salts • NaCl, KCl, LiCl, and CaCl2 • Spray applied to test section/air dried • Fixed Cl areal density at t = 0 • Chloride analysis of flume effluent • Orion specific-ion electrode

13. Research Methodology (cont’d) • Overland Flow Experiments • 2.27, 3.78, and 6.06 Lpm • Laminar flow regime as Re • Simulated Rainfall Experiments • 1.89, 3.78, and 6.06 Lpm overland flow • Rainfall intensity 6.86 cm/hr • Laminar flow regime as per Re

14. Description of Model • Mass Flux N = dP/dt = - kSfYP • dP/dt = pollutant mass flux off the impervious surface [M/L2T] • k = washoff coefficient based only on pollutant characteristics [L-1T-1] • Sf = friction slope or slope of the water surface profile [L/L]

15. Description of Model (cont’d) • Mass Flux (cont’d) • Y= average runoff flow depth [L] • P = areal pollutant density [M/L2] • Friction Slope Sf = fV2/(8gY) • V = average velocity (L/T) • g = acceleration of gravity (L/T2) • f = friction factor (unitless)

16. Description of Model (cont’d) • Boundary Shear Stress  = fV2/(8g ) •  = unit weight of water [M/L2T2] • Actual Mass Flux N = CQ/A = CR • C = chloride concentration [M/L3] • Q = flow rate [L3/T] • A = area of the impervious surface [L2] • R = rate of runoff [L/T]

17. Description of Model (cont’d) • Unitless Mass Flux dN*/dt* = -{kDv/g}N* • Dv =V*2Y/3V • Vertical momentum transfer coefficient • V* = shear velocity (L/T) = V (f/8) • k = washoff coefficient = -3mVg/V*2Y • m = slope of unitless semi-log plot • m = -kDv/g

18. Description of Model (cont’d) • Alternate Mass Flux N = dP/dt = - wRP • w = washoff coefficient [L-1] • P = areal pollutant density [M/L2] • R = rate of runoff [L/T] • Load Characteristic Curve • YF = {[1 – exp(-wVF)]/ [1 – exp(-wVT)]} • Derived from the mass flux equation

19. Description of Model (cont’d) • YF = {[1 – exp(-wVF)]/ [1 – exp(-wVT)]} • YF = fraction of total chloride load for a given runoff event [dimensionless] • VF = cumulative runoff volume up to a specified runoff time [L] • VT = total runoff volume for a complete runoff event [L]

20. Description of Model (cont’d) • Washoff Coefficient w • Catchment specific and varies with pollutant type; however, no physical basis • Positive values of w can only produce convex load characteristic curves • Decreasing concentrations of a constituent with increasing time after runoff event starts (Alley, 1981)

21. Description of Model (cont’d) • Washoff Coefficients (w versus k) • k = wR/ • For a given rate of runoff (R) and constant unit weight of water (), the boundary shear stress () of the impervious surface is constant and, thus, k is linearly proportional to w

22. Salt Q (Lpm) Y (cm) Re f V* (cm/s) NaCl 2.27 0.47 197 0.355 0.88 3.78 0.50 341 0.192 1.05 6.06 0.56 533 0.115 1.16 NaCla 1.89 0.48 165 0.433 0.79 2.27 0.57 335 0.314 1.16 6.06 0.60 535 0.188 1.37 CaCl2 2.27 0.46 197 0.272 0.76 3.78 0.48 336 0.153 0.95 6.06 0.53 533 0.095 1.07 KCl 3.78 0.49 321 0.150 0.87 LiCl 3.78 0.50 317 0.115 0.76 Hydraulic Parameters

23. Results and Discussion • Frictional Resistance of Test Surface • Laminar flow regime (Re < 900) • Smooth surface theoretical relationship • f = 24/Re • Parallel to theoretical relationship • Higher boundary shear stress

24. Flume Friction f versus Re

25. Results and Discussion • Frictional Resistance (cont’d) • Test surface became progressively less rough as sand was removed during runoff • Surface roughness phenomenon, however, was accounted for in the normalization procedure for mass flux

26. Results and Discussion • Observed Chloride Mass Flux • Unitless mass flux versus time plots • Normalized to flow-related parameters, including flow depth, velocity, shear velocity via a vertical momentum transport coefficient • Plots for a given chloride salt should collapse to a single line

27. Salt Q (Lpm) Dv (m2/s)*106 k (m-1s-1) w (mm-1) NaCl 0.6 2.90 1348 0.065 1.0 2.71 972 0.040 1.6 2.58 1197 0.038 NaCla 0.5 2.94 3894 0.181 1.0 4.38 2527 0.127 1.6 4.21 2608 0.114 CaCl2 0.6 2.18 708 0.025 1.0 2.11 705 0.024 1.6 2.08 441 0.018 KCl 1.0 1.97 1337 0.038 LiCl 1.0 1.52 1830 0.039 Washoff Coefficients (k and w)

28. NaCl Mass Flux w/o Rainfall

29. CaCl2 Mass Flux w/o Rainfall

30. Monovalent Mass Flux at 3.78 Lpm w/o Rainfall

31. Results and Discussion • Normalization Procedure • Non-flow-related factors may have been operative as there was not complete coalescence of all the runoff data for a given chloride salt • Aqueous solubility, molecular weight, molecular diffusivity, heats of solution, and cation ionic radius were examined

32. Results and Discussion • Monovalent versus Divalent Chloride Salt • Divalent chloride salt CaCl2*H2O behaved significantly different than the monovalent salt NaCl at same runoff rate • Much lower washoff coefficient and slower mass flux from the test surface

33. Mono- versus Divalent Mass Flux at 3.78 Lpm w/o Rainfall

34. Results and Discussion • Washoff Coefficient k • Akan (1987) describes the washoff coefficient k as depending only on the pollutant characteristics • Chloride detachment of monovalent salts ( NaCl, KCl, and LiCl) was similar • In general, higher overland flow rates produced lower washoff coefficients

35. Results and Discussion • Simulated Rainfall with Overland Flow • Washoff coefficient, k, was much higher for the runs with superimposed simulated rain compared to those without rainfall • Casts some doubt on the postulate of Nakamura (1984) and Akan (1987) that pollutant detachment rate is a function of pollutant characteristics and not influenced by hydraulic conditions

36. NaCl Mass Flux at 3.78 Lpm

37. Results and Discussion • Higher Mass Flux with Rainfall • Raindrops retard the runoff flow because a transfer of momentum is required to accelerate the drops from zero velocity in the horizontal direction up to the velocity of overland flow • Produces higher friction factor and increased shear at the test surface

38. NaCl Mass Flux w/ Rainfall

39. Results and Discussion • Simulated Rainfall with Overland Flow • Rainfall intensity herein was constant • Rainfall-induced turbulence over test section appeared less dominant with increasing overland flow rates • e. g. increasing overland flow rates may cause the rainfall effect to become less pronounced

40. Results and Discussion • Washoff Coefficient w • Varied over an order of magnitude with simulated rainfall runs being the highest • Range from 0.018 to 0.18 mm-1 • Typical washoff coefficient value in simulation models is 0.18 mm-1 (Alley, 1981 and Millar, 1999) • i.e. a 12.7 mm/hr runoff event removes 90 % of the pollutant in 1 hr

41. Results and Discussion • Washoff Coefficient (k versus w) • Recall that by equating the two mass flux models • k = wR/ • For constant hydraulic conditions, k is proportional to w • Linear relationship observed • r2 = 0.86

42. Comparison of Washoff Coefficients (k versus w)

43. Conclusions • Washoff coefficients were similar for each monovalent chloride compound (NaCl, KCl, and LiCl) at the same rate of runoff • Detachment rates for the divalent chloride compound CaCl2*H2O was approximately one-half the monovalent NaCl • In general, the washoff coefficient decreased as the rate of runoff increased

44. Conclusions • Not possible to completely normalize the data for different flow rates in the dimensionless mass flux versus dimensionless time semi-log plots • Used a derived average vertical transport coefficient based on a momentum and mass transfer analogy for laminar flow • Non-flow-related factors possible

45. Conclusions • Washoff coefficient significantly increased with simulated rainfall superimposed on overland flow • Increased boundary shear stress • Effect may be reduced at higher overland flow with constant rainfall intensity

46. Recommendations • Perform additional experiments under varied hydraulic conditions using overland flow and overland flow with superimposed simulated rainfallin order to clarify if the washoff rate is a function of only pollutant characteristics

47. Recommendations • Evaluate additional salt compounds with a common cation and different anions to determine if washoff coefficients are correlated with any chemical and physical property, e. g., LiBr and LiCl, or CaCl2 and CaBr2

48. Recommendations • Examine detachment rates between various monovalent and divalent compounds, such NaCl and CaCl2, or NaBr and MgBr2 • Include more complex substances as tracers, such as typical organics found in runoff • Fertile grounds for research into pollutant detachment rates

49. Acknowledgments • Funding for Research Provided by • New Mexico Tech Research Council