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Drainage Setback TablesMinnesota Wetlands ConferenceJanuary 30, 2013
Megan LennonState Soils SpecialistBoard of Water and Soil Resources
Dennis RodackerSenior Wetland SpecialistBoard of Water and Soil Resources
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Acknowledgement
•Greg Larson, BWSR•Dr. Joel Peterson, UW River Falls•Sonia Jacobsen & Engineering staff,
NRCS
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3
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!!
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Guidance Goals1. Determine acceptable
level of drawdown2. Measure wetland
impacts related to drainage projects
480 acres196,000 linear feet of tile
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Methods of drainage
Most common:• Tiling• Ditching
Also: pumping from high capacity wells Surface water diversions encirclement
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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
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A Brief Background
5 Common drainage equations•Hooghoudt•van Schilfgaarde•Kirkham•Ellipse•Skaggs
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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
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Drainage SetbackThe 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
Note: This is a plan view
Setback distance
Wetland boundary
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1
e0
e00e2
)2dm(m
)2d(mmln
f
)t(t9KdS
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
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1
e0
e00e2
)2dm(m
)2d(mmln
f
)t(t9KdS
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!
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Old NRCS Hydrology Tools
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ND- Drain program
• Run drainage equations using ND- Drain
• Lateral Effect• Problem: Drainable
porosity input
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Sensitivity of inputs• 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
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The New Way!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
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Purpose of BWSR guidance
•Companion to NRCS setback tables•Supplemental info on background &
assumptions•A tool for wetland managers and
regulators to assess impacts
http://www.bwsr.state.mn.us/wetlands/delineation/Drainage_setback_guidance.pdf
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BWSR GuidanceHow to Use
1. Identify wetland boundary2. Overlay drains on map3. Determine drain depth4. Determine setback distance
for each soil type*5. Delineate a setback
corridor for drain
* If drain crosses more than 1 soil type, compute a weighted average setback
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Example 1 - ID wetland boundary
539
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Example 1- overlay drains on map
Proposed pattern tile project
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Example 1- determine setback distance
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Example 1- delineate setback corridor
Setback corridor
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Example 2 - ID wetland boundary
468
252
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Example 2 - overlay drains on map
New pattern tile installation
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Example 2 - determine setback distance
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Example 2 - delineate setback corridor
468
252
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Example 2 - determine setback distance for 2nd soil
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Example 2 - delineate setback corridor
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Weighted Average Calculation
setback
soilin Distance power Drawing
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Example 2 – Weighted Average
43 ft
Unknown distance
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Weighted Average Calculation
%33ft 130
ft 43 468 SMUpower Drawing
setback
soilin Distance power Drawing
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Weighted Average Calculation
67% 33% - 100% 252 SMUpower Drawing
%33ft 130
ft 43 468 SMUpower Drawing
setback
soilin Distance power Drawing
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Weighted Average Calculation
ft 360
252) (SMU soilin Distance 67%
67% 33% - 100% 252 SMUpower Drawing
%33ft 130
ft 43 468 SMUpower Drawing
setback
soilin Distance power Drawing
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Weighted Average Calculation
ft 284.2 43 241.2 setback Weighted
ft 241.2 252) (SMU soilin Distanceft 360
252) (SMU soilin Distance 67%
67% 33% - 100% 252 SMUpower Drawing
%33ft 130
ft 43 468 SMUpower Drawing
setback
soilin Distance power Drawing
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Example 2 - weighted average setback corridor
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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
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Setback tables are no panacea
•Surface water diversions
•Encirclement•Volume
considerations in ditch maintenance
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User Cautions
• 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
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Regulatory Aspects
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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
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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
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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
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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
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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
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Questions ?