SWROA Training May 2013 Potential Retention / Detention Techniques for Surface Mine Sites

SWROA TrainingMay 2013

Potential Retention / Detention Techniques for Surface Mine Sites

Disclaimer

The following presentation was prepared by, and represents the views and opinions of:

R. B. (Barry) Doss, PE, PS, MBA

President, Doss Engineering, Inc.

38CSR2 5.6.a Storm Water Runoff

5.6.a. Each application for a permit shall contain a storm water runoff analysis.

Provided, however, an exemption may be considered on a case by case basis for mining operations with permitted acreage less than 50 acres. Furthermore, haulroads, loadouts, and ventilation facilities are excluded from this requirement.

The storm water runoff analysis shall include the following:

5.6.a.1 An analysis showing the changes in storm runoff caused by the proposed operation(s) using standard engineering and hydrologic practices and assumptions.

5.6.a.2. The analysis will evaluate pre-mining, worst case during mining, and post-mining (Phase III standards) conditions.

The storm used for the analysis will be the largest required design storm for any sediment control or other water retention structure proposed in the application.

5.6.a.2. (cont) The analysis must take into account all allowable operational clearing and grubbing activities.

The applicant will establish evaluation points on a case-by case basis depending on site specific conditions including, but not limited to, type of operation and proximity of man-made structures.

5.6.a.3. The worst case during mining and post-mining evaluations must show no net increase in peak runoff compared to the pre-mining evaluation.

5.6.b. Each application for a permit shall contain a runoff-monitoring plan which shall include, but is not limited to, the installation and maintenance of rain gauges. The plan shall be specific to local conditions. All operations must record daily precipitation and report monitoring results on a monthly basis and any one (1) year, twenty-four (24) storm event or greater must be reported to the Secretary within forty-eight (48) and shall include the results of a permit wide drainage system inspection.

5.6.c. Each application for a permit shall contain a sediment retention plan to minimize downstream sediment deposition within the watershed resulting from precipitation events.

Sediment retention plans may include, but are not limited to decant ponds, secondary control structures, increased frequency for cleaning out sediment control structures, or other methods approved by the Secretary.

Land Disturbance and Man-Made Systems(Includes Surface Mines & Their Drainage Requirements)

Can Increase Runoff Volume and Peak Discharge by Removal of Forest Cover and Vegetation Change in Infiltration Rates and Runoff Characteristics Compacted, Impervious, or Semi-Impervious Surfaces Removal of Natural Drainways Replacement w/ Engineered Channels and Drainways Disturbance in Multiple Sub-Watersheds Extended Project Life or Duration (time) Permanent Land Use Change(s)

Land Disturbance and Man-Made Systems

Historically, Design Concepts Have Favored The Rapid Removal of Storm Water Runoff From, or Around, Disturbed Areas Get and Keep The Water Off The Project Area

Usually This Was Accomplished by: Channelization Diversions Drainage Control Designed to Discharge

Quickly and Minimize Impounded Volume

Land Disturbance and Man-Made Systems

Engineered Systems, Without Detention / Retention Controls in Place, Can Result in:

Increased Peak Flow, And/or Reduced Time to Peak of Runoff Hydrograph

Fundamental Principles of Storm Water Management (All Project Types)

Reduce During- & Post-Development Peak Flow Rates to be Equal or Less Than The Pre-Development Runoff Peak Flow Rate

First, Utilize All Available and Practicable Measures to Control and/or Reduce Runoff Volume and Runoff Characteristics

Then, Control Remaining Quantity of Excess Storm Water Runoff by Temporary Storage Retention, Wet Storage Detention, Dry Storage

Fundamental Principles of Storm Water Management

Storm Water Runoff ManagementWhy No Net Increase?

Reducing Peak Flow & Storing Storm Water Runoff Will Reduce Frequency and Extent of: Downstream Flooding Potential Property Damage Soil Erosion and Scouring Sedimentation and Release of Pollutants

It’s Also A Requirement For All Surface Mine Permit Applications, and

It’s Prudent and In The Public Interest

Storm Water Runoff ManagementWhy No Net Increase?

What Can We Learn From Plaintiff’s Attorneys and The Media? Floods Will Continue to Occur and They Will

Be Somebody’s Fault It Never Rained Like That Around Here Before

They Put That Coal Mine In

Storm Water Runoff ManagementWhy No Net Increase? There are Tremendous Costs in Defending

Litigation, Even If Unfounded There are Tremendous Costs (Public and

Private) Involved in Cleaning Up Property Damage Regardless of the Cause

Risk of Loss of Life is Unacceptable and Must be Minimized

Examples of Flooding and Damage

Storm Water Runoff ManagementProfessional Engineer’s Role We As Engineers:

Must Conduct Thorough and Diligent Reviews Propose Prudent and Adequate Designs, and Only Certify Those Project Plans Which Meet

the Requirements of the Regulations Engineers, In The Fulfillment of Their

Professional Duties, Shall Hold Paramount the Safety, Health, and Welfare of the Public

Measures to Control and Reduce Runoff Volume and Characteristics Reduce Overall Disturbed Acreage Limit Incremental Disturbance by W/S Design Project Features to Realistically Lower Runoff

Curve Numbers (CN) Reduce Compaction of Backfill & Regrade Practice Reforestation - FRA Approach Introduce Attenuating Features in Landscape Revegetate Timely Reestablish Native Species and Buffers Reduce Grades and Slopes When Possible

Both Channels and Regrade Area

Storage of Storm Water Runoff

There Are No Magic Bullets The Designer Must Provide Either:

Increased Retention Additional Wet Storage From Existing or

Modified Structure Delayed Flow Discharge, Outlset Control

or, Detention Structures Dry Ponds or Embankments Located On or Below the Project Area

All Sites Different, Detailed Modeling Required

General Concept of Retention

Wet Storage, Similar to Our Sediment Pond

General Concept of Retention

Staged Discharge and Storm Water Release

Typical Surface Mine Sediment DitchStandard 0.125 Acre Foot Design

Typical Surface Mine Sediment DitchDecant Proposed at 60% Cleanout Elev

Typical Sediment Ditch BarrierOverflow Between Cells at 100% C/E

Sediment Ditch BarrierOutlet Control Via Notched Overflow

Typical Surface Mine Sediment PondStandard 0.125 Acre Foot Design

Typically-Sized Sediment PondDecant Proposed at 60%

Pond EmbankmentTypical Open Channel Spillway

Open Channel Embankment SpillwayOutlet Control to Increase Retention

Over-Sized Sediment PondDecant Proposed at 40% Cleanout

Over-Sized Sediment Pond (150%)Decant Proposed at 40% Cleanout Example:

40 acres of disturbance X 0.125 ac-ft/ft Requires 5 ac-ft pond that must be cleaned at

60% or 3 ac-ft of accumulated sediment Increase total pond volume by 150% For a 7.5 ac-ft pond, 3 ac-ft equals 40% 2.5 ac-ft storm water storage gained below n/p

DEP can approve such designs if included in the SWROA & Section P, and justified with demonstration water quality can be met

Over-Sized Sediment DitchDecant Proposed at 40% Cleanout

Example of Downstream Retention Embankment

Structure Design Considerations

On-Bench Sediment Ditches are Most Common Form of Sediment Control On WV Mine Sites

Increasing Ditch Volume or Size Is Not Necessarily Difficult, But Must Be Planned & Coordinated into Mine Plan

Increasing Ditch Sizes Can Affect Backfill Storage Volume, May Need to Be Addressed in AOC Model

Outlet & Dewatering Controls in Perimeter Sediment Ditches Can Be Difficult to Maintain

The “True Dip” of Seam Outcrop Can Foil the Best Laid Plans

Increasing In-Stream Pond Sizes Can Be Effective Form of Retention

Topography Is Often Limiting Factor Construction & Maintenance Costs Can Vary

Widely, Location Specific Outlet Control by Narrowing Spillways

Generally Increases Discharge Velocities Increased Embankment Heights

Understand MSHA & Dam Control Regs

Detention Structures Work Best In On-Bench Areas, or Flatter Areas Below Project

Stream Impacts From Retention / Detention Structures May Require 404 Authorization Can Be NWP 21 Eligible

Detention Dams Can Create Wetland Features Wetlands in Mountainous Topography

Generally Less Effective Then Ponds at Retention

Can Yield Water Quality Benefits

Discussion ExampleSWROA Pond

Discussion ExampleSWROA Detention Basin

Discussion ExampleDetention Embankment

Discussion ExampleSWROA Pond

Discussion ExampleOn-Bench Wildlife Pond

Discussion ExampleOn-Bench Wildlife / Agriculture Pond

Potential Issues w/ Off-SiteRetention / Detention Facilities

Potential Problem Areas to Consider During Design Storm Water Quantity Control Facilities Release Timing Safety Maintenance

Release Timing Can Be Critical Structures Are Intended to Reduce Peak

Discharge and Increase Flow Duration While This May Be Desired Result for

Individual Tributary, Shifting of Peak Flow Time and Duration Can In Some Instances Cause Adverse Effects Downstream

May Need to Model Receiving Stream or Main Tributary – Particularly If Discharging into lower Portions of Larger Watersheds

Safety Structures Should Be As Simple As Possible

Lessen Chances for Murphy’s Law and Tampering at Off Site Facilities

Evaluate Potential for Unintentional Clogging, Debris Buildup, Blockage

Off Site Impounding Structures Can be Attractive Nuisance Consider Signage, Restricted Access, Fencing

Maintenance Attenuating Structures Can Be More

Problematic Than Simple Sediment Control Potential for Clogging, Debris Buildup, &

Malfunction Can Be Higher Inspection and Monitoring Should Be

Accelerated In Early Stages of Use Inlet and Outlet Devices, Particularly

Standpipe or Riser Structures, Deteriorate Over Time - May Need Periodic Replacement

Comments on SWROA Submissions

Include Well-Written Narrative Clearly Explain Assumptions & Sources

SedCad Printouts Alone Won’t Do It Identify Any Practices & Controls That Were

Added During the SWROA Process Be Sure SWROA Plan Matches All Other

Applicable SMCRA Permit Sections Take Care in Selecting “Worst Case”, Can

Require More Than One Case Per W/S Propose In-Pit Storage w/ Great Caution

Discussion Example - Do You Know What Your Worst Case Will Be?

Discussion ExamplePost-Mining Case

Discussion Example (or How Many Valley Fills Are In This Picture?

Discussion ExampleIn-Pit Storage?

Discussion Example

What Can We Learn From Other Activities & Industries?

What Can We Learn From Others?Retention, Retention, Detention …

Questions / Comments ?

SWROA Training May 2013 Potential Retention / Detention Techniques for Surface Mine Sites

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SWROA Training May 2013