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

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

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!!

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

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

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

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

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

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 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

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

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 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

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

Example 2 - ID wetland boundary 252 468

Example 2 - overlay drains on map New pattern tile installation

Example 2 - determine setback distance

Example 2 - delineate setback corridor 252 468

Example 2 - determine setback distance for 2nd soil

Example 2 - delineate setback corridor

Weighted Average Calculation

Example 2 – Weighted Average Unknown distance 43 ft

Weighted Average Calculation

Weighted Average Calculation

Weighted Average Calculation

Weighted Average Calculation

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 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

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 suggestions Megan.Lennon@state.mn.us

Questions ?