Infiltrating Stormwater

Guidance to stormwater designers about assessing a potential stormwater infiltration site to determine the rate and volume of stormwater that can be expected to infiltrate into the soil.
Infiltrating Stormwater - Articles

Updated: August 24, 2017

Infiltrating Stormwater

Infiltration is the process by which water ponded or flowing over a soil surface is absorbed into the soil profile. Designing systems to infiltrate stormwater into the soil requires an in depth understanding of the soil, its texture and structure, and its water-flow characteristics. Stormwater infiltration systems may be on or at the soil surface and easy to get to and construct/create. Some stormwater infiltration systems are located under the soil surface, hidden from sight. These underground systems may also serve as the floor of underground stormwater detention units. Many designers have little or no knowledge of the physics of soil-water movement, which is where infiltration is typically taught at university.

The latest version of Chapter 102 of the Pennsylvania rules and regulations states that the net change in runoff from developed sites, up to and including the 2-year/24-hour storm event when compared to pre-construction runoff volume, is to be infiltrated, thus producing no off-site runoff impact to the downstream watershed.

This new requirement added to the traditional Post Construction Stormwater Management Plan (PCSM Plan) considerations has greatly complicated the work needed to develop, design and get approval of PCSM Plans. One of the most difficult issues faced by stormwater designers is coming up with appropriate ways to capture and store the site's runoff and facilitate its infiltration into the soil under the site.

Soil

Soil is the weathered and fragmented outer layer of the earth's land surface. Soil is formed by disintegration and decomposition of rocks by physical and chemical processes and its properties are influenced by the activities and accumulated residues of numerous biological species. These soil formation processes usually produce layers, called horizons, of unconsolidated porous media with varying physical and chemical properties. On most areas being developed, the uppermost soil layer, known as the "topsoil" or "A Horizon" is usually removed and stockpiled or trucked off-site. This topsoil layer (A Horizon) typically is high in organic matter, often has a granular structure, is lower in clay content and darker in color than the underlying soil layers, and has larger pores, which can take-in and transport water rather rapidly.

Under the A Horizon (topsoil) is the subsoil, or B Horizon. The B Horizon is usually higher in clay content and lighter in color, has smaller amounts of organic matter, has a more well-defined structure, and smaller pores, which transport water more slowly than the A Horizon.

Under the subsoil, or B Horizon is the C Horizon. The C Horizon is the slowly deteriorating parent material. The C Horizon often has similar characteristics to those found in the B Horizon, except the pores are smaller, the structure is often better developed, contains almost no organic matter, and water transport can be very slow.

When planning for stormwater infiltration, the designer must take into account the water flow characteristics of all of the soil layers/horizons under the infiltration surface. Stormwater infiltrated into the A Horizon will most likely enter the soil rather quickly, thus disappearing from the soil surface in a few hours. Often times this stormwater, having been infiltrated into the A Horizon, will be stored in the A Horizon waiting to be percolated downward into and through the B and or C Horizons, where the water is transported more slowly.

On development sites, where the A Horizon has been removed, exposing the B Horizon, stormwater infiltration is most often limited by the slower water flow characteristics of the B and C Horizons. In addition, if the B horizon has been compacted or puddled during the construction process, moving stormwater through this layer may be nearly impossible. Therefore, the area(s) that are planned for future stormwater infiltration, should be set aside and fenced before construction starts so these soils remain in their natural condition and the A horizon is left undisturbed.

Soil Water Movement

The soil's particle sizes (percentages of sand, silt, and clay yields the soil's textural classification) and the structure greatly influences how rapidly water will enter or move through a soil and how much water the soil profile can be expected to hold or store. There are several terms used to convey the idea of a soil's ability to transport water. The first of these is infiltration. Infiltration is the process of moving water standing or flowing over the surface of the soil into the soil profile. As shown in Figure 1, infiltration is a time dependent process that starts at infinity at the time when the soil is initially wetted and then declines in the rate of entry into the soil until it reaches steady-state. This long-term steady-state rate of water movement into the soil through the soil surface is the sustainable infiltration rate. This sustainable infiltration rate for a soil is usually referred to by one of two names depending on the discipline of the user. If you are talking to a soil scientist, this sustainable infiltration rate will most likely be referred to as the "Saturated Hydraulic Conductivity", or Ksat. If you are talking to a geologist or a civil engineer, this sustainable infiltration rate will most likely be referred to as the "Permeability". This sustainable infiltration rate can also be identified as the "K" in Darcy's Law (see insert box) when the driving gradient is one(1), where "I", the infiltration rate, equals "Ksat" the saturated hydraulic conductivity, or "K" the permeability.

Figure 1. The infiltration process.

When Chapter 102 states that all runoff for up to the 2-year runoff event is to be infiltrated, they are implying that the designer will show that the entire site's runoff volume (up to the 2-year runoff event) can be expected to enter the soil at the sustainable infiltration rate during the period the runoff is available at the soil surface. This then leads to the most difficult part of infiltrating stormwater. Identifying or determining the soil's saturated hydraulic conductivity or permeability. These are terms that are used in science and engineering, but are rarely measured or cataloged or published. This is because a soil's ability to infiltrate water can change greatly depending on how the soil is treated. If the soil surface is driven over or walked on and compacted, the soil's permeability1 may decrease greatly. If the soil is too wet when the infiltration surface is shaped/constructed the soil structure will be destroyed and the permeability will decrease greatly. If it rains on a bare, unprotected soil surface the soil's permeability may decrease more than an order of magnitude.

Darcy's Law relates the rate of water movement to the soil's permeability and the driving head as II = KK dddd dddd, where I = infiltration rate, K = the soil permeability, and dH/dz = the driving head pushing the water into the soil.

Another consideration is identifying and determining the permeability of the soil at the depth where the runoff water will be applied to the soil, the infiltration surface. If you were to consult the NRCS Soil Survey, you would find that each horizon of a soil may (and probably does) have a different permeability. For most soils the A horizon has the fastest permeability. The permeability's of the B and C horizons are usually slower. For design purposes, it is very important that you know the permeability of the soil located at the depth where the runoff will be introduced into the soil.

Another major issue related to infiltrating stormwater is that once the infiltration area has been identified and designed, it must be built/constructed by the contractor. Contractors need supervision because the timing and method of constructing the infiltration surface is extremely important. I recommend consulting Creating an Infiltration Surfacefor more information on forming/constructing the infiltration surface.

So, important considerations for a successful infiltration system must be (a) will the soil in the proposed infiltration area absorb the stormwater runoff at an acceptable rate, (b) how much (what depth) stormwater can the soil accept and store, and (c) how long (how many days) will it take for this stormwater stored in the soil profile to percolate down through the lower soil layers.

Let's try to address each of these issues, one at a time.

What Is The Infiltration Rate of Your Soil?

Ever since the current version of Chapter 102 was released, designers have been struggling with "How can I determine the infiltration rate of this soil?" Many methods are available for evaluating the water movement properties of a soil. The Pennsylvania Association of Professional Soil Scientists (PAPSS) lists the following methods of estimating a soils ability to infiltrate or transport water:

  • Double ring infiltrometer
  • Single Ring Infiltrometer
  • Constant-head well permeameter
  • Falling-head well permeameter
  • Cased-borehole
  • Falling Head Test
  • Constant Head Test
  • Basin Flooding Test

Each of these soil evaluation methods has been shown to yield soil-water flow rates for the soil horizon being evaluated, not the whole soil profile. Each method requires application of the proper equipment, methodology, and it should be conducted by a person who is experienced in using the method. Some designers have turned to other methods such as the Percolation Test as described in Chapter 73 of the Pennsylvania rules and regulation and used to size many the on-lot wastewater systems used in Pennsylvania. The Perc Test is not a recommended method for determining a soil's permeability.

The results from all of these methods are limited to the soil horizon that was evaluated. As was mentioned above, a well-designed stormwater infiltration system requires permeability values for each horizon within the soil profile. And these permeability values should be determined for the soil as it will be receiving the stormwater; after it has been created, shaped and located. In most cases the construction process used to create the infiltration surface will have compacted or greatly disturbed the soil. New soil maybe brought in. Equipment may drive over the area. In most cases the soil is no longer natural.

Soil Morphological Evaluation

For many on-lot disposal systems in Pennsylvania, the absorption area size is determined based the soils morphological characteristics. This evaluation must be conducted by a Certified Professional Soil Scientist (see papss.org) and is conducted by evaluating the soil characteristics found in several pits dug in the soil.

Three soil characteristics must be determined for the site (taken as the most restrictive conditions found in several pits);

  1. the soil's texture
  2. the soil's structural class
  3. the degree or grade of the structural development.

These three parameters should be evaluated for each of the soil's horizons, thus yielding the soil characteristics for the whole soil profile. You will know which horizons can store water, which horizons have rapid permeabilities, and which horizons have very limited permeabilities.

Soil scientists have correlated the rate at which water will move through each soil to these three soil morphological characteristics. Table 1 gives the sustainable infiltration rates (permeabilities) for each combination of these soil characteristics. Note from Table 1 that by knowing the soil's texture (left column) and the shape and grade of the soil's structure, the soil's "Infiltration Rate" in units of inches/day can be obtained. Also note that soils with certain combinations of texture and structure (especially those with platy structure) cannot be expected to infiltrate water.

Let's demonstrate how these soil characteristics can be used to establish a soil's acceptability for infiltrating stormwater.

Example 1: A site is found to have a 10-inch deep A horizon with a SL (sandy loam) and a weak (1) prismatic (PR) structure and a 20-inch deep B horizon of silt loam (SIL) soil with (M) massive structure, what is the expected infiltration rate? From Table 1 this A horizon has a sustainable infiltration rate of 1.1 in/d, and the B horizon has an infiltration rate of 0.3 in/d. The soil evaluation shows that you can infiltrate stormwater at about 1.1 in/d until the A horizon is filled with what should be about one-half to one-inch of stormwater if the A Horizon is 10 inch deep. With the pores within the A horizon filled with stormwater, the effective infiltration rate will be limited by the B horizon, which has an infiltration rate of 0.3 in/d.

There is one more very definite benefit to have the proposed soil profile evaluated by a soil scientist. In addition to texture and structure, the morphological evaluation will also reveal any layers in the soil profile that are unusually dense (a fragipan or plow pan), or rock parent material, or have excessive large open pores (such as gravel). In on-lot wastewater evaluations these types of layers are called limiting zones. If dense, slowly permeable layers exist within a soil profile that we desire to use for infiltrating stormwater, the presence of this one layer may render the site unacceptable.

Soil's Water Storage Capacity

Consider a soil, saturated with water. By definition, every pore is filled with water. If we leave this saturated soil undisturbed for a period of, let's say, one day, some of the water will drain from the soil. This drainage water is removed from the soil by the force of gravity. After the drainage water has been drained away, some of the larger soil pores will now be partially filled with air because some of the water has drained out. Following 24 hours of gravitational drainage, a soil has reached what soil scientists call "field capacity". The water remaining in the soil at (or below) field capacity can only be removed by plants taking up the water via their roots or by evaporating the water from the soil.

The pore space in the soil at field capacity is the storage volume you have available for the storage of stormwater entering the soil. In sandy soils, the drainable porosity, DP may be 10 to 20% of the total volume. In clayey soils the drainable porosity may be as small as 0.5 to 1%. The drainable porosity of most healthy A horizons is in the 3 to 6% range. B and C horizons have more clay and are rarely disturbed, so they have lower drainable porosities; most likely in 1 to 2% range.

Example 1 con't: To continuing the example above. If this soil has 10 inches of undisturbed A horizon (DP = 5%) above 20 inches of B horizon (DP = 1%), the soil profile will be able to store a total depth of about [10x0.05 + 20x0.01 =] 0.5 + 0.2 = 0.7 inches of infiltrated water. After the storage within the A Horizon has been satisfied, the infiltration rate of this soil is only 0.3 in/d. So, to take this example a bit further, if you build a BioRetention Cell (Rain Garden) over this soil and design the Cell to receive and pond 6 inches of stormwater runoff, it will take about 15 hours (0.7/1.1) to infiltrate the water needed to fill the A & B Horizons. This leaves 5.3 inches of water standing in the Cell, which will now infiltrate at 0.3 in/d or it will take almost 16 days (5.3/0.3) for this water to be removed from the Cell and taken into the underlying soil. Since, in Pennsylvania we get on average 0.1 inches of rain every 5 days and 0.5 inches of rain every 13 days, this BioRetention Cell will probably remain wet and ponded with some water continuously. This is unacceptable.

To be more realistic, let's develop a more workable solution for this example. Since the B Horizon has a permeability (sustainable infiltration rate) of only 0.3 in/d, it would be better to remove the top 4 feet of soil under this BioRetention Cell and replace it with a mixture of sand and hard-wood mulch. The sand will have an infiltration rate of 2.6 in/d (Table 1) and a drainable porosity of about 25%. Now look at Example 2.

Table 1. Infiltration rates as a function of a soil's morphological evaluation.

Soil TextureSoil Structure
Type
Soil Structure
Grade
Infiltration Rate
(in/d)
COS,S,LCOS,LSSG-2.6
FS,VFS,LFS,LVFSSG-1.5
COSL,SLM-1.0
COSL,SLPL10.8
COSL,SLPL2,30.0
COSL,SLPR/BK/GR11.1
COSL,SLPR/BK/GR2,31.6
FSL,VFSLM-0.8
FSL,VFSLPL1,2,30.0
FSL,VFSLPR/BK/GR11.0
FSL,VFSLPR/BK/GR2,31.3
LM-0.8
LPL1,2,30.0
LPR/BK/GR11.0
LPR/BK/GR2,31.3
SIL,SIM-0.3
SIL,SIPL1,2,30.0
SIL,SIPR/BK/GR11.0
SIL,SIPR/BK/GR2,31.3
SCL,CL,SICLM-0.0
SCL,CL,SICLPL1,2,30.0
SCL,CL,SICLPR/BK/GR10.5
SCL,CL,SICLPR/BK/GR2,31.0
SC,C,SICM-0.0
SC,C,SICPL1,2,30.0
SC,C,SICPR/BK/GR10.0
SC,C,SICPR/BK/GR2,30.5

Adapted from Tyler, 2000.

The abbreviations used in the first three columns of Table 2 are:

Soil Texture: COS = coarse sand, S = sand, LCOS = loamy coarse sand, LS = loamy sand, FS = fine sand, VFS = very fine sand, LFS = loamy fine sand, VLFS = loamy very fine sand, COSL = coarse sandy loam, SL = sandy loam, FSL = fine sandy loam, VFSL = very fine sandy loam, L = loam, SIL = silt loam, SI = silt, SCL = sandy clay loam, CL = clay loam, SICL = silty clay loam, SC = sandy clay, C = clay, SIC = silty clay

Structure, Type (formerly Shape): PL = platy, PR = prismatic, BK = blocky, GR = granular, SG = single grain, M = massive

Structure, Grade: 0 = structure less, 1 = weak, 2 = moderate, 3 = strong

Example 2: With the natural soil replaced with 4 feet of sand (S) (SG), the design infiltration rate is 2.6 in/d. Now the 6 inches of stored water will infiltrate in 2.3 days (6/2.6) and will fill the bottom 24 inches (6/0.25) of the sand. This infiltrated stormwater can either be left to infiltrate into the underlying soil (may not be the best idea because of the very slow permeability of the underlying soil) or drained from the sand layer with a subsurface drainage system.

Rules of Thumb

Before we close this discussion about infiltrating stormwater, let me give a few water management rules that can help you design successful stormwater systems.

  • Since infiltrated stormwater does not just disappear after it infiltrates, it is important to limit the depth of water you expect to infiltrate as a result of each runoff event. Based on much experience, I recommend that you NEVER plan to infiltrate more than a 6-inch ponded depth of runoff from any one runoff event.
  • Geotextiles have become an important inclusion in many earthmoving and building projects. If you are considering the infiltration of stormwater, the placement of a geotextile within the soil profile should be avoided. If you wish to move water into and downward through the soil, NEVER place a geotextile in the water's path. The fabric will clog and slow or stop infiltration.
  • Lastly, construction activities must be scheduled for times when the soil is relative dry; no recent rains, no standing water, no soils too wet for tillage. Even if the soil is quite dry, NEVER allow equipment onto the infiltration surface. These infiltrating surfaces should be shaped and created by hand so the natural soil structure and permeability are preserved.

Summary

Infiltrating stormwater is a great idea and one that is long overdue. Designing and constructing a functional stormwater system that can reliably infiltrate the first flush (usually assumed to be the first inch of runoff), or even the 2-year storm requires careful planning, proper design, and construction. In reality, infiltrating stormwater runoff, as required by Pennsylvania can be a very difficult process. It requires a detailed understanding of the entire soil profile and information about how the soil will transport and store water. A certified soil scientist should always be used to evaluate the soils.

Prepared by Albert R. Jarrett, Emeritus Professor of Agricultural Engineering

For additional assistance contact your County Extension Agent.

1 From this point forward, permeability will be used for both permeability and saturated hydraulic conductivity.