Drainage Setback Tables Minnesota Wetlands Conference January 30, 2013 Megan Lennon State Soils Specialist Board of Water and Soil Resources Dennis Rodacker Senior Wetland Specialist Board of Water and Soil Resources
Mar 31, 2015
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
Acknowledgement
•Greg Larson, BWSR•Dr. Joel Peterson, UW River Falls•Sonia Jacobsen & Engineering staff,
NRCS
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!!
Guidance Goals1. Determine acceptable
level of drawdown2. Measure wetland
impacts related to drainage projects
480 acres196,000 linear feet of tile
Methods of drainage
Most common:• Tiling• Ditching
Also: pumping from high capacity wells Surface water diversions encirclement
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
A Brief Background
5 Common drainage equations•Hooghoudt•van Schilfgaarde•Kirkham•Ellipse•Skaggs
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
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
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
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!
Old NRCS Hydrology Tools
ND- Drain program
• Run drainage equations using ND- Drain
• Lateral Effect• Problem: Drainable
porosity input
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
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
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
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
Example 1 - ID wetland boundary
539
Example 1- overlay drains on map
Proposed pattern tile project
Example 1- determine setback distance
Example 1- delineate setback corridor
Setback corridor
Example 2 - ID wetland boundary
468
252
Example 2 - overlay drains on map
New pattern tile installation
Example 2 - determine setback distance
Example 2 - delineate setback corridor
468
252
Example 2 - determine setback distance for 2nd soil
Example 2 - delineate setback corridor
Weighted Average Calculation
setback
soilin Distance power Drawing
Example 2 – Weighted Average
43 ft
Unknown distance
Weighted Average Calculation
%33ft 130
ft 43 468 SMUpower Drawing
setback
soilin Distance power Drawing
Weighted Average Calculation
67% 33% - 100% 252 SMUpower Drawing
%33ft 130
ft 43 468 SMUpower Drawing
setback
soilin Distance power Drawing
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
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
Example 2 - weighted average setback corridor
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
Setback tables are no panacea
•Surface water diversions
•Encirclement•Volume
considerations in ditch maintenance
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
Regulatory Aspects
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
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
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
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
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
We want your comments and [email protected]
Questions ?