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SCOPE & EFFECT EQUATIONS Chapter 7

SCOPE & EFFECT EQUATIONS Chapter 7. Chapter Objectives. Explain Background Behind Drainage Systems Describe Hydraulic Conductivity Relating to Drainage Describe 4 Equations for Determining Affect of Water Management on Wetlands Adjust L e for a Wetland at the Base of a Slope

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SCOPE & EFFECT EQUATIONS Chapter 7

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  1. SCOPE & EFFECT EQUATIONSChapter 7

  2. Chapter Objectives • Explain Background Behind Drainage Systems • Describe Hydraulic Conductivity Relating to Drainage • Describe 4 Equations for Determining Affect of Water Management on Wetlands • Adjust Le for a Wetland at the Base of a Slope • Explain How State Drainage Guides Are a Tool for Understanding Water Management Systems

  3. Typical application of observation wells, Scope and Effect equations, DRAINMOD ENDOSATURATION

  4. For Wetlands with a g.w. source. S&E does not apply directly, flow intercepted , back drainage systems off multiple times the lateral effect distance.

  5. Hydraulic Conductivity Where K and T Are the Hydraulic Conductivity and Thickness of Each Layer (flow being evaluated is horizontal flow only, so flow is by layers)

  6. Effective Radius of Tile

  7. Equations for Determining the Affect of Water Management on Wetlands • Ellipse • Hooghoudt (used in DRAINMOD) • van Schilfgaarde • Kirkham - removal of surface water (used in DRAINMOD)

  8. Permeability Classes Permeability should not be used for Saturated Hydraulic Conductivity. Use Hydraulic Conductivity determined by: measurements, algorithms using texture (MUUF, ROSETTA, Soil Water Characteristic Triangle).

  9. Values from ROSETTA Class Averageshttp://www.ussl.ars.usda.gov/models/rosetta/clssaval.htm Texture Drained f Ksat Class Vol@30 cm 0-30 cm (cm) (cm/cm) (cm/hr) Clay 0.317 0.0106 0.615 C Loam 0.413 0.0138 0.341 Loam 0.248 0.0083 0.502 L Sand 1.231 0.0410 4.383 Sand 1.318 0.0439 26.779 S Clay 0.470 0.0157 0.473 S C L 0.470 0.0157 0.549 S Loam 0.761 0.0254 1.595 Silt 0.131 0.0044 1.823 Si Clay 0.397 0.0132 0.401 Si C L 0.192 0.0064 0.463 Si Loam 0.074 0.0025 0.760

  10. Use of Ellipse Equation - The equation has been used to design drainage and water supply systems across the United States. - It is a steady state equation - basically removes the rain that falls at a constant rate. - It is used to determine approximate economical spacings and depths of agricultural drain tubing and ditches for ag. crops assuming that the water table should be lowered below the root zone within 24 to 48 hours. - It has also been used to determine the hydrology of a wetland that has been modified by drainage measures.

  11. Ellipse Equation Assumptions • Soil is Homogeneous and Has a Hydraulic Conductivity, K • Drains are Evenly Spaced a Distance “S” Apart • Impermeable Layer Underlies Drain at Depth “a” • Rain Is Falling or Irrigation Water Applied at Rate, “v”

  12. Limitations of Ellipse Equation • Does Not Apply When Vertical K Exceeds Horizontal K • Assumes Homogeneous Soils • No Direct Factor for Time • Assumes Surface Water is Removed • Tile in Good Repair with an adequate outlet

  13. Ellipse Equation S = [(4K) (m2 + 2am) / q]1/2 K and q must be in same units, e.g. in/hr, ft/day S will be in units of a and m Le=1/2(S)

  14. Ellipse Equation S = [(4K) (m2 + 2am) / q]1/2 For wetland hydrology determination, q is evaluated as water that must be removed by drainge in lowering water table (drained volume) divided by the time to remove. Example: lower water table from surface to 12”below in 14 days. Drained volume is 0.01” (WT 0” to -12”) q = 0.01 in/14 days = .00071 in/day

  15. Ellipse Equation S = [(4K) (m2 + 2am) / q]1/2 For wetland hydrology determination, Sometimes a term called drainable porosity (f) is used. “f” is drained volume divided by depth water table was lowered. Drained volume is 0.01” (WT 0” to -12”) f = 0.01 in/12 in = .00083 (dimensionless) Therefore to get q from f q=(f x depth drained)/time=(.00083 x 12 in)/14 days=0.00071 in/day(same as on previous slide)

  16. Figure 1: Example Using The Ellipse Equation m=d-c S = [(4K) (m2 + 2am) / q]1/2

  17. Review Results • Here are 4 important points to consider in evaluating whether equation is being applied correctly: • Is the Time For the Saturation to be Removed an important point? • Is the assumption of Steady State Conditions valid? • Can a Reasonable Drainage Coefficient be calculated or assumed? • Was an Equivalent Hydraulic Conductivity Value, K, calculated if multiple soil layers are involved?

  18. Hooghoudt Equation K1 = weighted hydraulic conductivity above the drainage feature, in/hrK2= weighted hydraulic conductivity below the drainage feature, in/hrNote: K1 and K2 do not have to be different (also uses an equivalent depth that will be discussed later)

  19. van Schilfgaarde Equation m = height of water table above the center of the drain at midplane after time t, ftm0= initial height of water table above the center of the drain at t=0, ftt = time for water table to drop from m0 to m, daysa = depth from free water surface in drainage feature to impermeable layer, ftf = drainable porosity of the water conducting soil, dimensionlesss = water trapped on the surface by soil roughness, ft s=0.0083 ft (0.1 in) would be typical Note: set s=0 if unsure of appropriate value

  20. van Schilfgaarde EquationData Requirements • This equation requires information on Hydraulic Conductivity • A Drainable Porosity Value (f) is needed, and can be determined most readily by an application of the MUUF soils program (which is used with the DRAINMOD computer program), ROSETTA, or SPAW Soil Water Characteristic routine. f is calculated for depth drained, not a set value (ex. 60 cm used for agricultural production). • The time to Remove the Saturation and the Height of the Water Table After Drainage are specified in the NFSAM and COE criteria

  21. van Schilfgaarde EquationLimitations • This is a Non-Steady State Equation. Assumptions of equation are appropriate for many parts of US where the rainfall is sporadic rather than constant • Equation does not yield a reasonable solution when the Drain Rests on the Impermeable Layer; that is, when “a” = 0 • Equation must use Equivalent Depth instead of Actual Depth to give the best results

  22. van Schilfgaarde EquationLimitations (continued) • Surface water must be removed from a site in order to apply this equation correctly. The Surface Water my be removed by a Ditch, Natural Ground Slope, or the Surface Intake of a Tile

  23. van Schilfgaarde EquationLimitations (continued) • Equations for “de” below allow one to convert Actual Depth to the Impermeable Layer to Equivalent Depth de = a ;for a/S’<0.3 1 + a/S’ [(8/pi) ln(a/re) - 3.4] de = S’ pi ;for a/S’<0.3 8[ln(S’/ re) - 1.15

  24. van Schilfgaarde EquationFactors Affecting Accuracy • van Schilfgaarde equation does include a Parameter for Time, so it can be used to Compare How Much Water is Removed in 7 days vs. Removal in 14 days • Time Period selected does affect the results noticeably • Drainable Porosity (f) affects the results of the equation, but “f” Values that are similar in magnitude result in Minor Differences in Spacing

  25. van Schilfgaarde EquationProcedure • Use with the known depth, “a”, in place of “de” to determine Estimated spacing, S’ • Use Estimated spacing, S’ in Appropriate Equation to determine Equivalent Depth “de” , which replaces “a” in the van Schilfgaarde Equation for final computations

  26. van Schilfgaarde EquationProcedure (continued) • Use “de” to determine the Spacing, S, in the van Schilfgaarde Equation • Compare the Estimated S’ to S, if they are within 10% of each other, the Difference Can Be Assumed to be Negligible. If the Difference is More Than 10%, use the Calculated S Value as S’, Repeat Calculations until the S’ and S Values are Within 10%

  27. Figure 2: Pothole With TileDrainage System

  28. Kirkham’s Equation • Kirkham’s Equation for Parallel DrainsFor removal of ponding only

  29. Kirkham’s Equation

  30. Definition of Terms for Kirkham’s Equation

  31. Kirkham’s Equation • Kirkham’s Equation for a Single Drain Q = 2(pi)K (t + d - re) ln(2d/re)

  32. Kirkham’s Equation

  33. Figure 3: Adjusting for a Wetland at the Base of a Slope

  34. Figure 4- Locating a tile w/probe

  35. Analyzing the correct problem

  36. d Non-Typical application of Scope and Effect equations, DRAINMOD Observation wells must be installed appropriately: EPISATURATION Many wetlands are created by this type hydrology: a shallow depth of saturation to fragipan (may be <12”) and/or ponding on surface.

  37. ?? Care must be taken in applying Scope and Effect equations, DRAINMOD to this situation. Which water table is being analyzed?How do you install observation wells and interpret data? EPISATURATION

  38. The factors affecting the accuracy 1. the value assumed for the drainage coefficient has a significant effect on the results of the equation 2. minor variability in K does not change the drain spacing much 3. the depth to the impermeable layer affects the resultant spacing most significantly when the K is high, such as with sandy soils 4. assumes no rainfall event with in the same time period (multiple events), can be incorporated in drainage rate. 5. assumes no Evapotranspiration (can be incorporated in drainage rate)

  39. Differences between Wetland Hydrology Determination and No-Impact Evaluation Wetland Determination No Impact existing system new system ex. 12 in., 14 days ex. 0.12 in., 90 days Le = 95 ft Le = 295 ft minimal wetland fully functional wetland

  40. 93’

  41. Note: with drain at 3’ (lowering WT 0.5’ from previous)Le is only 7’ more. Major impact is ET.

  42. Ellipse Example Problem 1. Soil Water Properties 2. Problem

  43. End

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