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## Applications to Stormwater Management

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**Applications to Stormwater Management**Presented by: Mr. Brian Oram, PG, PASEOWilkes UniversityGeoEnvironmental Sciences and Environmental Engineering DepartmentWilkes - Barre, PA 18766570-408-4619 http://www.water-research.net**Nearly 50% of Soil is Space or Space Filled with Water**• Water – 25+ % • Air – 25 + % • Pore Space Makes Up 35 to 55 % of the total Soil Volume • This Space is called Pore Space Therefore, soil can be used as a storage system, treatment system, and transport media.**Soil Properties Critical To Stormwater Management**• Soil Texture • Porosity and Pore Size • Water Holding Capacity • Bulk Density • Aggregate Stability • Infiltration Capacity • Hydraulic Conductivity !!! Just to Name a Few Properties !!!!**Types of Pores**Macropores (> 1,000 microns)-Large Mesopores (10 to 1,000 microns)-Medium Micropores (< 10 microns)- Small Source: http://www2.ville.montreal.qc.ca**Key Points on Soil Pores**Under gravity, water drains from macropores, where as, water is retained in mesopores and micropores, via matrix forces. Coarse-textured horizons (e.g., sandy loam) tend to have a greater proportion of macropores than micropores- but they may not have more macropores than finer textured soils. Soils with water stable aggregates tend to have a higher percentage of macropores than micropores. Proportion of micropores tends to increase with soil depth, resulting in greater retention of water and slower flow of water with depth.**Water Holding Capacity**Please Do Not Use Sand in a Bio-Retention System !**Bulk Density, Porosity, and Texture**Sands – Tend to have higher bulk density and lower permeabilityPlease do not use sands in Bio-Retention systems!**How can a**silt loam have more macropores than sand? Answer: More Water Stable Structures Source: Brady, Nyle, C. “ The Nature and Properties of Soils” (1990).**Better Structural Development More**Macropores Source: Brady, Nyle, C. “ The Nature and Properties of Soils” (1990).**Water Stable Aggregates I**Aggregates on left are more water stable, i.e., aggregate stays together and do not separate into the its components, i.e., three soil separates. Water Stable Aggregates**Water Stable Aggregates – IIThe Classic Photo**Source: Brady, Nyle, C. “ The Nature and Properties of Soils” (1990)Great Desk Reference Text !!!!**Hydrologic Soil Terms**• Infiltration - The downward entry of water into the immediate surface of soil or other materials. • Infiltration Flux (or Rate)- The volume of water that penetrates the • surface of the soil and expressed in cm/hr, mm/hr, or inches/hr. The rate of infiltration is limited by the capacity of the soil and rate at which • water is applied to the surface. It is a volume flux of water flowinginto the profile per unit of soil surface area (expressed as velocity). • Infiltration Capacity (fc)- The amount of water per unit area of time that water can enter a soil under a given set of conditions at steady state. • Cumulative infiltration:Total volume of water infiltrated per unit area of soil surface during a specified time period. Horton Equation, Philip Equation, Green- Ampt Equation**Infiltration Rate (Time Dependent)**Steady Gravity Induced Rate Infiltration with Time Initially High Because of a Combination of Capillary and Gravity Forces f = fc +(fo-fc) e^-ktfc does not equal K Final Infiltration Capacity(Equilibrium)- InfiltrationApproaches q - Flux Density**Infiltration Rate**Decreases with Time 1) Changes in Surface and Subsurface Conditions2) Change in Matrix Potential and Increase in Soil Water Content and Decrease in Hydraulic Gradient3) Overtime - Matrix Potential Decreases and Gravity ForcesDominate - Causing a Reduction in the Infiltration Rate 4) Reaches a steady-state conditionfc – final infiltration rate**Infiltration Rate Function of Slope & Texture**Source: Rainbird Corporation, derived from USDA Data (Oram,2004)**Infiltration Rate Function of Vegetation**Source: Gray, D., “Principles of Hydrology”, 1973.**Infiltration Rate Function of Horizon A, B, Btx, Bt, C, RC/R**Testing - Areas Fractured Rock Source: On-site Infiltration Testing - Mr. Brian Oram, PG (2003) and FX. Browne, Inc. (Lansdale, PA)**Infiltration (Compaction/ Moisture Level)**Site Compaction – Can Significantly Reduce Surface Infiltration Rate**Rain Drop Impact Bare Soil**Destroys Soil AggregatesDisperses Soil SeparatesSeals Pore SpaceAids in Loss of Organic Material Creates a Surface Crust Source: (D. PAYNE, unpublished)http://www.geographie.uni-muenchen.de**Percolation Rate**Percolation -Downward Movement of Water through the soil by gravity. (minutes per inch) at a hydraulic gradient of 1 or less. Used and Developed for Sizing Small Flow On-lot Wastewater Disposal Systems. On-lot Disposal Regulations (Act 537) has preliminary Loading equations, but for large systems regulations typically require permeability testing. Also none as the Perc Test, Soak-Away Test (UK) Not Directly Correlated to or a Component of Unsaturated or Saturated Flow Equations**Comparison Infiltration to Percolation Testing**Percolation Testing Over Estimated Infiltration Rate by 40% to over 1000% * Source: On-site Soils Testing Data, (Oram, B., 2003)**Darcys Law- Saturated FlowVertical or Horizontal**Volume of discharge rate Q is proportional to the head difference dH and to the cross-sectional area A of the column, but it is inversely proportional to the distance dL of the flow path and coefficient K is called the hydraulic conductivity of the soil. The average flux can be obtained by dividing Q with A. This flux is often called Darcy flux qw .**Flux Density or Hydraulic Conductivity (Ksp)**Flux Density (q): The volume of water passing through the soil per unit cross-sectional area per unit of time. It has units of length per unit time such as mm/sec, mm/hour, or inches/ day (q = -K(ΔH/L )) Actually the term is volume/area/time= q = Q/At Hydraulic Conductivity (Ksp)quantitative measure of a saturated soil's ability to transmit water when subjected to a hydraulic gradient. It can be thought of as the ease with which pores of a saturated soil permit water movement . Side by Side (Pagoda, J, 2004)**Goals of the Field Method**• Field Measurement of the Flux Density (qw) and calculate hydraulic conductivity – qw = Ksp (dh/dl) • Field Measurement of Hydraulic Conductivity (Ksp)**Single Rings Infiltrometers**Cylinder - 30 cm in Diameter- Smaller Rings Available.Drive 5 cm or more into Soil Surface or Horizon.Water is Ponded Above the Surface- Typically < 6 inches. Record Volume of Water Added with Time to Maintain a Constant Head. Measures a Combination of Horizontal and Vertical Flow**ASTM Double Rings Infiltrometers**Outer Rings are 6 to 24 inches in Diameter (ASTM - 12 to 24 inches)Mariotte Bottles Can be Used to Maintain Constant HeadRings Driven - 5 cm to 6 inches in the Soil and if necessary sealed Very Difficult to Install and Seal – ASTM Double Rings in NEPA**Significant Effort is Needed to Install and Seal Units**ASTM requires documentation of the Depth of the Wetting Front Potential Leaking Areas**Other Double Rings Small Diameter**3” and 5” Double Ring in Flooded Pit 6” and 12” Double Ring**Infiltration Data- Double Ring Test**Note: Ring Diameter – 26 cm (Oram 2005)**Cumulative Infiltration**(cm) Infiltration Rate –cm/hr Steady-State Rate (slope)0.403 cm/hr Fc = Ultimate Infiltration Capacity (approx.0.47 cm/hr)**Estimated Methods- Based on Grain Size**C- Factor Hazen Method Applicability: sandy sediments • K = Cd102 • d10 is the grain diameter for which 10% of distribution is finer, "effective grain size" - where D10 is between 0.1 and 0.3 cm • C is a factor that depends on grain size and sorting**Guelph and Amoozegar Borehole Permeameters**$ 1500 each Field Testing (Oram, 2000) Photo Source:http://www.usyd.edu.au**Measuring Hydraulic Conductivity**12-inch/ 6-inch Double Ring Constant or Falling Head Permeameter- Homemade - $ 15.00 Side by Side Testing Mr. Brian Oram and Mr. Chris Watkins, 2003.**Constant Head Borehole Permeameters**Talsma Permeameter-Could be Homemade $ 50.00 Retail ($ 300.00) Modified Amoozegar- Could be Homemade – $30.00- Retail ($ 200.00) Side by Side Testing by Mr. Brian Oram and Mr. John Pagoda, 2004**Measuring Infiltration Rateto Estimate / Calculate the Flux**Density • Infiltrometers- Yes ! • Single ring- May Not Be Advisable – Multiple tests required • Double ring- Yes ! - May be difficult in rocky and stony areas (i.e., Most of the Poconos !) • Smaller Double Ring in Flood Pit – Yes ! • Flooded Infiltrometers – Yes ! • Adoption of a Strict Double Ring ASTM Method – Likely not appropriate, but method should be used as a guide by professionals. • Cased Borehole Permeability Test – ASTM Method– Yes ! (Minimum diameter casing 4 inches) with bentonite packing of annular space – Maximum Pipe Height is a function of soil conditions.**My Recommendation and Opinion !Please Do NOT Use a**Conventional Percolation Rate or Percolation Test for Developing Engineering Design !**Percolation Testing**• Does not directly measure permeability or a flux velocity. • Has been used to successfully design small flow on-lot wastewater disposal systems, but equations and designs have a number of safe factors. • Results may need to be adjusted to take out an estimate of the amount of horizontal intake area. • Without Correction Percolation Data over-estimated infiltration rate data by 40 to over 1000 % with an adjustment for intake area error could be reduced to 10 to 200% (Oram, 2003) , but infiltration rate can overestimate saturated permeability by a factor of 10 or more (Oram, 2005). • May need to consider the use of larger safety factors and equations similar to sizing equations used for on-lot disposal systems. Safety factors of 50% reduction may not be enough !!**SU**MMA RY • Borehole Permeability Testing can be a Suitable Method. • Falling Head , Constant Head, and Quasi Constant Head Methods would be suitable. • Permeability Data for Specific Site should be calculated using Geometric Average. • Equations and Methods Based on Darcy’s Law and the result is a value for Ksp or qw. • Do not recommend estimating permeability based on particle size distribution – Ok for preliminary desktop evaluations if data is available – Not for Final Design ! • Laboratory permeability testing is possible, but it may be difficult to get a representative sample and account for induced changes. May be Ok for Preliminary Evaluations.**The Hydrologic Cycle**Discharge Zone Recharge Zone Where is the Project Site ?**Save Your Client – MoneyNone Structural Development**Practices • Maintain Soil Quality and Maximize the Use of Current Grading to Minimize Loss of O, A, and upper B horizons. • Minimize Compaction, Maximize Native Vegetation, and Use Good Construction Practices • Consider Hydrological Setting and Existing Hydrological Features in Site Design and Layout Answer: New Development/ Construction Practices and New and Updated Ordinances and Planning Documents !**Infiltration System ApproachIndividual Infiltration BMP**Units Soil: Tunkhannock Series Soil had stratified sand and gravel lenses Water Table > 8 feet Open Voids (Gravel and Cobbles) 3 to 6 feet Ksp Field Measured 1 to 10+ inches per hour Reported Permeability > 6 inches per hour Design Used a Ksp of 0.5 inch per hour (50% reduction) Note- A few sections of the site had permeability of 0.1 inch per hour**Infiltration Unit Configuration**Installed: Sump and Grass Swale Prior to Unit and Geotextile within unit to capture large organic material Concrete – Open bottom perforated tank not filled with gravel for storage. Conceptual Design by: Malcolm Pirnie (Scranton, PA) and Brian Oram (October 2004), Anticipated Installation 2007.**Sizing Calculations- Areas 0.5 in/hr**• Impervious Area Roof and Driveway– 3500 ft2 • Design Storm – 1.3 inch • Volume of Water to Recharge- 2840 gallons (379 ft3) • Design Loading- Based on Field Measured Soil Permeability- 0.5 inch per hour or 0.5 in3/in2.hour = 7.481 gpd/ft2 • Minimum Recharge Period – 72 hours (PADEP Recommended) • Recharge Volume per day – 945 gpd • Minimum Recharge Area- (945 / 7.481) =126 ft2 • Internal Tank Storage – 3 ft * 8 ft perforated Concrete Tank, plus 3+ foot perimeter and subsurface aggregate storage to generate a minimum surface area of 150 ft2. • Additional Gravel Layer was added to Meet System Storage Requirement. Primary Limiting Factor is Not Recharge Capacity but Providing Detention Storage or Storage in the System !