Presentation on theme: "SWROA Training May 2013 Potential Retention / Detention Techniques for Surface Mine Sites."— Presentation transcript:
SWROA Training May 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:
38CSR2 5.6.a Storm Water Runoff 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.
38CSR2 5.6.a Storm Water Runoff 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.
38CSR2 5.6.a Storm Water Runoff 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.
38CSR2 5.6.a Storm Water Runoff 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.
38CSR2 5.6.a Storm Water Runoff 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.
38CSR2 5.6.a Storm Water Runoff 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 Management Why 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 Its Also A Requirement For All Surface Mine Permit Applications, and Its Prudent and In The Public Interest
Storm Water Runoff Management Why No Net Increase? What Can We Learn From Plaintiffs Attorneys and The Media? Floods Will Continue to Occur and They Will Be Somebodys Fault It Never Rained Like That Around Here Before They Put That Coal Mine In
Storm Water Runoff Management Why 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
Storm Water Runoff Management Professional Engineers 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
Open Channel Embankment Spillway Outlet Control to Increase Retention
Over-Sized Sediment Pond Decant 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 Ditch Decant Proposed at 40% Cleanout
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
Structure Design Considerations 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
Structure Design Considerations 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 Example On-Bench Wildlife / Agriculture Pond
Potential Issues w/ Off-Site Retention / Detention Facilities Potential Problem Areas to Consider During Design Storm Water Quantity Control Facilities Release Timing Safety Maintenance
Potential Issues w/ Off-Site Retention / Detention Facilities 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
Potential Issues w/ Off-Site Retention / Detention Facilities Safety Structures Should Be As Simple As Possible Lessen Chances for Murphys 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
Potential Issues w/ Off-Site Retention / Detention Facilities 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 Wont 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 W orst Case Will Be?