drainage setback tables minnesota wetlands conference january 30, 2013 megan lennon state soils...

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 suggestionsMegan.Lennon@state.mn.us

Questions ?