1. Drainage Setback Tables Minnesota Wetlands Conference January 30, 2013
Megan Lennon
Dennis Rodacker
State Soils Specialist
Senior Wetland Specialist
Board of Water and Soil Resources

2. Acknowledgement Greg Larson, BWSR Dr. Joel Peterson, UW River Falls
Sonia Jacobsen & Engineering staff, NRCS

3. 4/1/2017
Drainage
Anything that decreases the input or increases the output of water can cause a drainage impact The challenge concerns determining if a decrease or increase is acceptable!!

4. Guidance Goals 480 acres 196,000 linear feet of tile
Determine acceptable level of drawdown
Measure wetland impacts related to drainage projects
480 acres
196,000 linear feet of tile

5. Methods of drainage Most common: Tiling Ditching Also:
pumping from high capacity wells
Surface water diversions
encirclement

6. Methods of drainage
Most common:
Tiling
Ditching
Also:
pumping from high capacity wells
Surface water diversions
encirclement
Setback tables provide guidance to avoid wetland impacts

7. A Brief Background 5 Common drainage equations Hooghoudt
van Schilfgaarde
Kirkham
Ellipse
Skaggs

8. 4/1/2017
Lateral Effect
The distance on each side of a tile or ditch in its longitudinal direction where the ditch or tile has an influence on the hydrology
Zone of Influence
Tile or ditch through a wetland Le
Note: This is a plan view

9. 4/1/2017
Drainage Setback
The minimum distance--in feet-- from the wetland boundary to the centerline of the tile line or toe of the ditch bank necessary to minimize adverse hydrologic impacts to adjacent wetlands
Setback distance
Wetland boundary
Note: This is a plan view

10. van Schilfgaarde Equation
S – drain spacing
de – effective depth from drain to impermeable layer
m0 – initial water table height above drain
m – water table height after time t
t – time to drop water table from m0 to m
f – drainable porosity
K – Saturated hydraulic conductivity

11. van Schilfgaarde Equation
S – drain spacing
de – effective depth from drain to impermeable layer
m0 – initial water table height above drain
m – water table height after time t
t – time to drop water table from m0 to m
f – drainable porosity
K – Saturated hydraulic conductivity
Notoriously difficult to obtain!

12. Old NRCS Hydrology Tools

13. ND- Drain program Run drainage equations using ND- Drain
Lateral Effect
Problem: Drainable porosity input

14. Sensitivity of inputs The effects are cumulative
Ksat: a 10% increase in Ksat results in a 5% increase in LE
f: a 10% increase in f results in a 5% decrease in LE
Time: A 10% increase in T results in a 5% increase in LE
The effects are cumulative

15. The New Way! County soil data specific tables Consistent values
MN NRCS Setback tables
County soil data specific tables
Consistent values
Relieves uses need to research & generate drainage estimates
Generates (f) via pedotransfer function
Organics are literature based
Model water table drawdown

18. Purpose of BWSR guidance
Companion to NRCS setback tables
Supplemental info on background & assumptions
A tool for wetland managers and regulators to assess impacts

19. BWSR Guidance How to Use Identify wetland boundary
Overlay drains on map
Determine drain depth
Determine setback distance for each soil type*
Delineate a setback corridor for drain
* If drain crosses more than 1 soil type, compute a weighted average setback

20. Example 1 - ID wetland boundary
539

21. Example 1- overlay drains on map
Proposed pattern tile project

22. Example 1- determine setback distance

23. Example 1- delineate setback corridor

24. Example 2 - ID wetland boundary
252
468

25. Example 2 - overlay drains on map
New pattern tile installation

26. Example 2 - determine setback distance

27. Example 2 - delineate setback corridor
252
468

28. Example 2 - determine setback distance for 2nd soil

29. Example 2 - delineate setback corridor

30. Weighted Average Calculation

31. Example 2 – Weighted Average
Unknown distance
43 ft

32. Weighted Average Calculation

33. Weighted Average Calculation

34. Weighted Average Calculation

35. Weighted Average Calculation

36. Example 2 - weighted average setback corridor

37. When to use the tables
Assess loss of wetland hydrology via tile or ditch
Determine setback to minimize impact to wetland hydrology
Potential wetland restoration

38. Setback tables are no panacea
Surface water diversions
Encirclement
Volume considerations in ditch maintenance

39. User Cautions Verify soils on site Setbacks are approximations
Organic soils are problematic
Extreme water holding capacity
Organic over sand is a barrier
Soils are variable
Soil maps are approximate
Do not overrule evidence of hydrology on site
Verify soils on site

40. Regulatory Aspects

41. Use of the Drainage Setback Tables for Regulatory Purposes.
Consistent Results for Rule Implementation
Pre-guidance drainage impact numbers were highly variable, which led to inconsistent rule implementation
Guidance provides consistent decisions from LGU to LGU, and agency to agency
Provides a frame work to implement wetland regulation
Provides predictable permitting process

42. Use of the Drainage Setback Tables for Regulatory Purposes.
Drainage Guidance is Using The Best Available Information
Gives justification for decisions by both regulators and applicants alike

43. Use of the Drainage Setback Tables for Regulatory Purposes.
Tables Provide Ease of Use for Applicants/LGUs/TEPs
Reduces complicated concepts and math to usable tables and predictable results

44. Where it May Prove Useful
Pre-Project Analysis
Existing and estimated lateral effects for ditch maintenance
Assess viability of a wetland restoration project
Installation of Ag Drainage to Avoid, Minimize or Account for Wetland Impact
Wetland Restoration Projects
Understanding how drain is affecting wetland
Credit allocation
Wetland Delineations

45. Take home messages
Setback values are institutionally accepted & provide consistent implementation
Guidance using best available information
Okay to use drainage equations
Engage all parties to establish mutually agreeable procedures

46. We want your comments and suggestions

47. Questions ?