Infiltration Testing For Stormwater BMPs: What Would Mickey Mouse Do? CCLC Turning A New Leaf November 3, 2017 David J. Hirschman Hirschman Water & Environment, LLC www.hirschmanwater.com dave@hirschmanwater.com
Objectives: Provide a deeper understanding of the pros and cons of various methods for measuring infiltration for stormwater management practices. Discuss a risk-based approach so that infiltration testing can be costeffective, relevant, and not just a numbers-game.
C mon Mousekateers... Let s Do An Infiltration Test!
Terminology Soil infiltration: Rate at which stormwater enters the soil. Influenced by soil structure, compaction, organic matter, moisture content, and other physical characteristics at the soil surface. Soil permeability Rate at which stormwater flows through the soil. NOTE: Infiltration and Permeability are often used interchangeably in many reference materials. Soil Hydraulic Conductivity: Hydraulic conductivity (K) of a given soil can be related to its infiltration and permeability. The rate at which water enters the soil, infiltration, under optimal conditions, starts very fast and then declines and eventually approaches a constant rate of entry. This constant rate of infiltration is... technically defined as the saturated hydraulic conductivity (Ksat). Tested at soil property that is most limiting to soil movement
Overview of a Rain Garden
Hydrologic Soil Groups (HSGs)
Importance of soil in pollutant uptake and runoff reduction Pollutant Mitigation Physical Chemical Biological Runoff Reduction Infiltration Storage Slow Release Increased evapotranspiration Photo Credits: http://www2.stetson.edu/~wgrubbs/datadriven/langmuir/langmuirwtg.html;
Soil Compaction Decreases Water storage Infiltration rate Ability to grow vegetation Increases Ponding time Overflow volume Can be avoided in critical areas Site Planning (don t pave over the soils that are most suitable for stormwater control) Photo Credit: http://www.constructionequipmentguide.com/case-adds-high-performance-650l-to-crawler-dozer-line/13495/
Infiltration Testing for Stormwater BMPs Purpose of Test: Determine whether an infiltration practice is feasible Determine need for an underdrain system for bioretention, permeable pavement, etc. Understand soils at depth in location
Virginia DEQ 2011: Appendix 8-A, Infiltration & Soil Testing Test pit OR soil boring (1 per 1,000 square feet of practice area) At bottom of practice + 2 feet deeper Pre-soak for 24 hours Fill 24, measure drop over 1 hour Repeat 3X
MD Stormwater Design Manual, Appendix D.1 (2000, revised 2009) Initial Feasibility USDA Soil Classification Infiltration: Similar to VA DEQ 2011
Simple Perc (Falling Head) Test
Appropriate Test for Infiltration BMPs? Perc Test??? - At what depth? - What size hole? Source: Jay Dorsey
Virginia DEQ 2013: Appendix 8-A, Infiltration & Soil Testing Initial site evaluation/screening Soil profile (pit) Infiltration Field test Tube Permeameter Method (ASTM D 2434); Double-Ring Infiltrometer (ASTM D 3385); Basin flooding test for bedrock; or other constant head permeability tests that utilize in-situ conditions and accompanied by a recognized published source reference. Bottom of practice + 4 feet deeper or double ponding depth Seasonal High Water Table
Double Ring Infiltrometer Source: http://www.hilbec.com/stormwater.htm
Table 8A.2 Number of Soil Profiles and Infiltration Tests Required (Virginia DEQ, 2013) Area of Practice # of Soil Profile # of Infiltration Explorations (Permeability) Tests Up to 2,500 ft 2 1 2* 2,500 ft2 to 5,000 ft 2 2 3 5,000 ft2 to 7,500 ft 2 2 4 7,500 ft2 to 10,000 ft 2 2 5 Greater than 10,000 ft 2 Add 1 soil profile and 2 infiltration tests for each additional 5,000 ft 2 of practice Linear practices should add 1 additional soil profile for each 100 linear feet of practice, and 1 additional infiltration test for each additional 50 linear feet of practice. *Micro scale applications with a small footprint (<500 ft 2 ), such as a downspout disconnection (Design Specification No. 1) require only one infiltration test per location.
WSA Rainscaping Manual, Appendix B, Soil Assessment Soil sample at bottom of practice: feel, ball, ribbon test Check compaction Perc test (similar to Virginia DEQ 2011) Water table indicators
WSA Rainscaping Manual, Appendix B, Soil Assessment
Oregen State Univ. Sea Grant: Testing Soil for Rain Gardens Dig a hole to rain garden depth Pour water in & time Repeat 3X Ball & ribbon test
Review of 24 States, Counties, Cities (Larry Johnson & Vincent Day) Double-ring Infiltrometer (ASTM 3385) = 11 Constant Head Borehole Permeater (USBR 7300-89) = 11 Single-ring Infiltrometer = 8 Falling-head Borehole = 8
From Larry Johnson, Vincent Day It is very important to adhere to hydraulic conductivity testing methods that are recognized published standards or are based on high-quality published academic or controlled field studies and are verified by adequate testing under actual field conditions. We have not found that the falling-head borehole method we are discussing here meets either of the above conditions... This method is thought to provide a very nonconservative infiltration rate. Stormwater infiltration systems require extensive planning, engineering, testing, construction, and maintenance costs. If these systems fail after construction, the environmental and reconstruction costs can be very significant. Unfortunately, these systems often do often fail for various reasons.
Appropriate Test for Infiltration BMPs? Bore Hole/ Perc Test? Ponded Ring Infiltrometer Test 3-Dimensional Flow ~1-Dimensional Flow Source: Jay Dorsey
Single Ring Infiltrometer Test Equipment Source: Jay Dorsey
Excavate to Proposed BMP Depth Source: Jay Dorsey
Prepare Test Surface Source: Jay Dorsey
Install Infiltration Rings Bucket Press Source: Jay Dorsey
Install Infiltration Rings Drop Hammer Source: Jay Dorsey
Wet Surface and Bring to Ponding Depth Source: Jay Dorsey
Record Volume Infiltrated While Maintaining Constant Depth Source: Jay Dorsey
If Possible Inspect Ring X-section Source: Jay Dorsey
Describe Soil Profile from Surface to (at least) 3 Feet Below Proposed Depth Source: Jay Dorsey
Post-Processing Make Correction for Flow Divergence and Capillarity Source: Reynolds et al (2002) Soil Texture Structure Categories for Visual Estimation of Alpha-star (Reynolds, 2008a; Table 76.1) Coarse - Coarse and gravelly sands; may also include some highly structured soils with large and numerous cracks and biopores. Loamy - Most structured and medium textured materials; include structured clayey and loamy soils, as well as unstructured medium sands. {Includes most topsoil} Fine - Porous materials that are both fine textured and massive; include unstructured clayey and silty soils, as well as fine structureless sandy materials. Compacted - Compacted, structureless, clayey or silty materials such as landfill caps and liners, lacustrine, or marine sediments, etc.
Source: Vincent Day, True North Environmental, Johnson Meter
Jon Roller, Ecosystem Services, using Amoozemeter, Dr. Amoozegar, NCSU https://www.nrcs.usda.gov/wps/portal/ nrcs/detail/nh/home/?cid=nrcs144p2_0 15725
Engineering Standards require a # to inform yes/no decisions
Infiltration Testing Thoughts: A Risk-Based Approach? Depends on size, drainage area, sophistication of practice AND risks associated with failure (e.g., residential yard vs. elementary school or public library) Cost and need for outside professional will be driving factors Soil scientist can tell a lot by examining soil with or w/out a final # -- chasing a # using a Mickey Mouse test may not provide good results
Infiltration test sophistication, costs, drainage area, ID risks.
Native Root Depths