Developing an Effective Infiltration Design for Clay Soils

Transcription

1 Developing an Effective Infiltration Design for Clay Soils A Presentation to the Pennsylvania Stormwater Management Symposium William C. Lucas

2 INFILTRATION Infiltration is precipitation that enters the soil/root zone, instead of running off. What are the benefits of infiltration? After conversion of forested areas to development, runoff volumes in small events that erode stream banks increase by roughly 5-20 times. Resulting bank erosion generally is considered among most insidious impacts of urban runoff, causing increased TSS loads and smothering spawning areas. Therefore, by reducing runoff volumes in small events, infiltration is very beneficial. Volume reduction from infiltration also reduces corresponding nutrient and TSS loads (except NO 3 ).

3 RECHARGE Recharge is that portion of the infiltrated volume that passes below the root zone. What are the benefits of Recharge? Recharge increases base flow, keeping streams cooler and fuller during the most critical drought conditions, thus enhancing habitat. Cooler temperatures increase dissolved oxygen (DO), reducing stress on fish and organisms. Meets recharge requirements where applicable. Best attained by infiltration trenches.

4 Recharge vs. Infiltration Recharge is typically accomplished by nonvegetated facilities. Pros: Provides greatest recharge volume for a given volume of rainfall. Cons: Requires pretreatment to ensure no clogging or introduction of contaminants (see Parmer et al 1995; Pitt et al, 1996). Performance is determined by inherent subsurface soil properties, so infiltration rates are not augmented by plant/microbial activity. Often below grade, so remediation of failures expensive. Relatively expensive to construct per unit volume of rainfall recharged.

5 Recharge vs. Infiltration Infiltration typically accomplished by vegetated facilities. Cons: Provides less recharge volume for a given volume of rainfall. Pros: Provides own pretreatment, so minimal clogging, or introduction of contaminants. Performance is augmented by plant/microbial activity, so infiltration rates can be much greater than inherent subsurface soil properties. Easily accessible, so remediation of problem areas relatively straightforward. Relatively inexpensive to construct per unit volume of rainfall infiltrated.

6 Recharge vs. Infiltration Choice depends upon site specific objective. If space is limited, and recharge needs are more urgent, then an infiltration trench would be preferable. Space is still needed for pretreatment. If reduction in runoff volumes and TSS loads more urgent, then vegetated facility is preferable. In particular, bioretention facilities, which not only provide detention, but also very substantial TSS and metals reductions, are a most helpful alternative. Often, a mix of facilities will be provided in a project, depending upon the site constraints.

7 Centralized vs. Distributed If possible, a holistic, integrated and distributed approach is preferred. Linear vegetated/trench systems can integrate conveyance, treatment, detention and infiltration. By being placed throughout the uplands, as opposed to the lowest part of the site, conditions are better for infiltration. Extending the footprint of the facilities allows for variability over the natural range of site conditions. As a result, a holistic approach is not only more effective, it is often less expensive.

8 Centralized vs. Distributed If possible, a discrete centralized approach should be avoided. Separate systems are then required for conveyance, while pretreatment and detention still required. By being placed in the lowest part of the site, conditions are generally poor for infiltration. Restricting the facility footprint to less favorable location is more risky in terms of site conditions. Therefore, it is not only less effective, it is often more expensive.

9 Infiltration Hydraulics Effects of Compaction (Pitt 1987, OCSCD, 2001, Saxton and Rawls, 2004). Compaction substantially reduces infiltration rates. This is especially pronounced in sandy soils, where rates have been shown to decline from HSG A to D. Compaction greatly inhibits the growth of plants, since roots cannot extend through the soil. This further reduces infiltration potential. Under extreme conditions, compaction not alleviated by freeze/thaw cycles- Chariot wheel tracks from roman times are still visible in England. Therefore, MUST avoid compaction by using only excavators when constructing BMPs. Pans and track loaders are to avoided at all costs. Even low ground pressure dozers are not recommended.

10 Infiltration Hydraulics Effects of Organic Matter (Saxton and Rawls, 2004). Organic matter (OM) content can substantially increase infiltration rates, primarily due to decreased bulk density. This is due to the fact that soils high in OM cannot be compacted as much as soils with less OM. OM also increases field capacity in sandy soils by approximately 10% total volume, tripling available water. OM promotes the microbial community, contributing to soil aggregate formation. Intact mineral topsoils are typically 1-2% OM, while soils disturbed in development can be substantially less. Soils can be amended to an OM content of 5-10%, but then nutrient and DOC leaching losses can be substantial.

11 Infiltration Hydraulics Effects of Vegetation (Ralston, 2004). Vegetation is remarkably effective in restoring and/or enhancing infiltration rates. Rates in undisturbed vegetated areas can be several orders of magnitude higher than underlying soil. Vegetation roots penetrate confining layers, and provide habitat for worms and other burrowing fauna that create macropores, opening up soil structure. Root exudates promote growth of microbes and mycorhizzae fungi, which add organic matter, promoting increased uptake of nutrients and metals. Root turnover promotes the formation of macropores. The deeper the root growth, the better, so native grasses (to 100cm) are much better than turf (to 15cm).

12 Infiltration Hydraulics Pedotransfer Functions (PTF) (Saxton and Rawls, 2004). Preceding effects of compaction and organic matter can greatly affect the hydraulic properties of soils. Given %porosity (=bulk density,=compaction), % sand, %silt, %clay, and % OM, PTF equations can predict soil properties affecting infiltration rates and water retention. These parameters include saturated hydraulic conductivity (K sat ), field capacity (θ 33 ), wilting point (θ 1500 ), and suction wetting tension (Ψ). The SPAW model at Saxton s web site is recommended to be used to obtain K sat. Even when field tests available, SPAW provides more conservative results without having to use a safety factor (typically at least 2). Results in close agreement with pit tests.

13 Infiltration Hydraulics Infiltration rate estimation (Bouwen et al, 1999; Massman and Butchart, 2000). Phased Construction- most expensive, but most accurate, since actual long term infiltration rate and interaction with groundwater is determined. Pit flooding test- expensive, but fairly accurate, since wetting suction/edge effects minimized, and can address groundwater interactions. Single Ring Infiltrometer inexpensive, but overstates by 2-10x, depending upon diameter and texture class, and does not take into account interactions with groundwater. Percolation Test inexpensive, but even less accurate, depending upon operator and location. Simple Texture Class- least expensive, and least accurate, since soil profile is disturbed, and density and organic matter not addressed.

14 Infiltration Hydraulics Mounding/Geometry (Guo, 1998; Bouwen et al, 1999; Li et al, 1999;, Livingston, 2000). The greater the area of recharge of a given volume, the lower the height of mounding. Studies suggest most of the volume flows out of the sides, instead of out the bottom. Depth to groundwater should be at least twice the width of the facility (not just 2 feet). Therefore, designs with more side to bottom ratio would be more effective. This means that trenches (with more sides) or swales (with more area) are more effective than basins in avoiding mounding. Best to model mounding with MODFLOW if a real concern.

15 Infiltration Modeling Routing Methods. Best to use continuous simulation modeling (such as SWMM or HEC-HMS) using Green-Ampt equation, but can be difficult to get data to run complex model. Need extensive period of rainfall data disaggregated into event sized time steps (~10 min). Using S av as the average suction wetting tension (-Ψ 0 /2), Head (H), and length of wetting front (L), the following describes the infiltration dynamics during an event: f ( t) = K sat H ( t) + L( t) + L( t) S av H(t) L(t)

16 Infiltration Modeling Current Routing Methods (NJDEP, 2004; HydroCAD, 2004). GSR-32 method is an acceptable recharge model based upon easily available datasets. Required for recharge design in NJ. However, has substantial problems in more complex sites in terms of the receiving area, nor does it route infiltration or runoff reduction as an explicit output. With standard level pool pond routing, infiltration can be modeled in facilities based upon design saturated infiltration rate times wetted area at each time step, summed over course of routing event through pond. Horizontal area used for routing, since oscillations would occur once full and top exfiltrates. Therefore, this method a conservative approach, since area of sides is not accounted for. Routing set up to account for storage in pipes and stone voids.

17 Infiltration Design Example Project is resort in New Hope, PA. Substantial runoff problems due to compaction by foot traffic of lawn around pool area. Lawn surrounding pool area was to be hardscaped. Runoff was to be managed using distributed infiltration practices.

18 Infiltration Design Example Project Setting: Soils derived from underlying Triassic shale. Local Penn- Readington soils are noted for very poor infiltration rates. However, once the clayey surface horizons of these soils are penetrated, the C horizon at the fractured shale interface can have more rapid infiltration rates. Proposed facilities are installed at depth at least several feet below existing grade. Infiltration tests indicated that the infiltration rates in the subsoil of the project area were nearly 2 /hr. Pretreatment not required since source area has no vehicle traffic, so low TSS, metals or other potentially hazardous pollutants.

19 Infiltration Design Example Infiltration practices involve stone underdrains, stone trenches and a seepage pit.

20 Infiltration Design Example 6 stone underdrain layer underneath the pavers. Runoff enters along edge at walls. Stone trenches with perforated collection pipes located along the retaining walls.

21 Infiltration Design Routing Routing Diagram: Runoff from source subarea is routed into two-stage stone beds, with secondary flows (in dashed red) collected by dummy infiltration reach. Infiltration rate is 0.95 inch/hour. Primary flows then directed to seepage pit. Site runoff from seepage pit is Point of Analysis.

22 Infiltration Design Routing 2-Year Event: Note how all runoff entering the stone beds is infiltrated, with no outflow to seepage pit. 10-Year Event: All runoff entering the stone beds is still infiltrated, with no outflow to seepage pit.

23 Infiltration Design Routing 100-Year Event: A substantial volume is infiltrated from beds, but now a short pulse of outflow to seepage pit. 100-Year Event: Short pulse into seepage pit largely unattenuated due to small surface area and volume.

24 Infiltration Design Routing 10-Year Event, at ¼ inch/hour infiltration rate: Note much longer time to drain, and surface discharge from seepage pit. However, volume released is still less than 10% of runoff.

25 Infiltration Design Conclusions 2-Year Event: No runoff predicted or observed. 10-Year Event: No runoff predicted or observed. 10-Year Event: Short pulse into seepage pit represents very small volume of 442 cu. ft., compared to 5,607 cu. ft. infiltrated. Also substantial attenuation of peak flow, with very short duration.