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 !