Site soil variability – site with spatially heterogeneous soils (vertically or horizontally) as determined from site investigations are more difficult to estimate average properties for resulting in a higher level of uncertainty associated with initial estimates. Depth to seasonal high groundwater/impervious layer – groundwater mounding may become an issue during excessively wet conditions where shallow aquifers or shallow clay lenses are present. Table VII.3: Suitability Assessment Related Considerations for Infiltration Facility Safety Factors Consideration High Concern Medium Concern Low Concern Assessment methods (see explanation below) Use of soil survey maps or simple texture analysis to estimate short-term infiltration rates Direct measurement of ≥ 20 percent of infiltration area with localized infiltration measurement methods (e.g., infiltrometer) Direct measurement of ≥ 50 percent of infiltration area with localized infiltration measurement methods or Use of extensive test pit infiltration measurement methods Texture Class Silty and clayey soils with significant fines Loamy soils Granular to slightly soils Site soil variability Highly variable indicated from site assessment or limited soil borings collected during site assessment Soil borings/test pits indicate moderately homogeneous soils Multiple soil borings/test pits indicate relatively homogeneous Depth to groundwater/ impervious layer <5 ft below facility bottom 5-10 ft below facility bottom >10 facility bottom Localized infiltration testing refers to methods such as the double ring infiltrometer test (ASTM which measure infiltration rates over an area less than 10 sq-ft, may include lateral flow, and do not attempt to account for heterogeneity of soil. The amount of area each test represents should be estimated depending on the observed heterogeneity of the soil. Extensive infiltration testing refers to methods that include excavating a significant portion of the proposed infiltration area, filling the excavation with water, and monitoring drawdown. The excavation should be to the depth of the proposed infiltration surface and ideally be at least 50 to 100 square feet.
4 What is the infiltrating surface area of the proposed BMP? ABMP sq-ft 5 What land use activities are present in the tributary area (list all) 6 What land risk category is applicable? L M H 7 If M or H, what pretreatment and source isolation BMPs have been considered and are proposed (describe all): 8 What minimum separation to mounded seasonally high groundwater applies to the proposed BMP? See Section (circle one) 5 ft 10 ft 9 Provide rationale for selection of applicable minimum separation to seasonally high mounded groundwater: 10 What is separation from the infiltrating surface to seasonally high groundwater? SHGWT ft 11 What is separation from the infiltrating surface to mounded seasonally high groundwater? Mounded SHGWT ft 12 Describe assumptions and methods used for mounding analysis: 13 Is the site within a plume protection boundary (See )? Y N N/A 14 Is the site within a selenium source area or natural plume area (See )? Y N N/A 15 Is the site within 250 feet of a contaminated site? Y N N/A
dominant nutrient, the appropriate mix would include a higher proportion of vegetated area such as 80% vegetated, 20% open water. For dry extended vegetated detention basins, vegetated swales, and filter strips, select a variety of plant species that are drought tolerant, but can also survive periodic inundation. Provide an irrigation system, if necessary, for plant establishment and maintenance. Biotreatment BMPs shall incorporate amended media and soils designed for the intended function of the BMP. Select amended media for use in bioretention systems that is effective at removing pollutants of concern, can absorb and evapotranspirate runoff, and where appropriate, can facilitate infiltration. Select and soils that will not potentially leach pollutants, specifically dissolved nutrients and metals in some cases. Amend soils in dry extended detention basins, swales, and filter strips to provide suitable soils for supporting plants, which can absorb and evapotranspire runoff and where appropriate facilitate infiltration. Design wet detention basins (wet and constructed stormwater wetlands using soils that support growth of attached plants. BMPs hydraulics shall be designed to maximize pollutant removal functions. For all biotreatment BMPs, design inlets or overland flow entry to BMPs to prevent scour or re-entrainment of pollutants. Provide maximum flow path distance between outlet and inlet and with sufficient length to width ratio to limit short circuiting. For constructed stormwater wetlands and wet detention basins, provide the storage capacity for the DCV in the wet pool at a minimum. Seasonal constructed stormwater wetlands and seasonal wet detention basins should not be used unless is a reasonable expectation that tributary land uses will provide dry weather flows during seasonally wet period to maintain vegetation and prevent stagnant water. For constructed stormwater wetlands and wet detention basins designed to be continually wet (opportunities may be limited in Orange County), ensure that a low-flow source of is present to maintain vegetation and prevent stagnant conditions. Design features shall allow for monitoring of drawdown such as depth markers and monitoring ports. For bioretention systems, provide media contact time sufficient for pollutant removal, with upper limitations on contact time to avoid leaching of retained pollutants. Traditional media should generally be designed in the range of 2 to 12 inches per hour, while specialized media can be effective for many pollutants of concern at much higher flowrates (residence times on the order of several
volume reductions through infiltration and transpiration during entire monitoring studies. Total volume reductions during a study were calculated based on comparison of the total inflow volume and outflow volumes measured over the duration of each study (including multiple – up to 65 - storm events). As these analyses utilized long-term observed volume reductions over a series of storm events, they provide a valid comparison to the capture efficiency and volume reduction criteria contained in this TGD that were developed upon hydrologic simulations and summaries. Table XIII.1: Volume Reduction Summary of Biotreatment BMP Categories in the International Stormwater BMP Database Source: International Stormwater Best Management Practices (BMP) Database, Technical Summary: Volume Reduction. January 2011. values provide a benchmark for comparing the performance of LID BMPs (infiltration, harvest and use, and evapotranspiration) against the performance of LID biotreatment BMPs, which under some circumstances, may provide a similar level of retention plus offer other pollutant treatment mechanisms. This analysis shows that while LID biotreatment BMPs are not designed to fully retain the DCV, they are capable of providing substantial volume reductions, on the order of half of the water that is captured and managed. This analysis further shows that a well designed LID biotreatment BMP that has been designed to capture 80 percent of average annual storm water runoff and has been designed to achieve maximum feasible volume reduction would be expected to achieve total long term volume reduction on the order of 40 percent of long term runoff volume. This means that a designer, faced with a LID retention BMP with a performance of 40 percent or less could substitute the LID retention BMP with a LID biotreatment BMP that is capable of carrying 100 percent of the DCV without impairing the overall performance of the site’s system of BMPs. This is because roughly 40 percent of the DCV will be incidentally infiltrated or evapotranspirated by the LID biotreatment BMP – roughly equal or better than the low-performing LID retention BMP. Therefore, it is appropriate to designate 40 percent retention as a threshold for eliminating the mandatory selection and use of a specific LID retention measure in favor of using LID bioretention BMPs that achieve a comparable or greater level of retention for the system as a whole. This threshold must not be used to reduce the site’s overall level of retention.
nR = porosity of reservoir fill; 0.35 may be assumed where other information is not available dR = depth of reservoir, ft If the area required is greater than the selected pavement area, adjust reservoir depth and recalculate required area until the required area is achieved. Configuration for Use in a Treatment Train Permeable pavement may be preceded in a treatment train by HSCs in the drainage area, which would reduce the runoff volume to be infiltrated by the permeable pavement Permeable pavement areas can be designed to be self-retaining to lessen the pollutant and volume load on downstream BMPs. Additional References for Design Guidance SMC LID Manual (pp 84): Los Angeles Unified School District (LAUSD) Stormwater Technical Manual, Chapter 5: City of Portland Stormwater Management Manual (Pervious Pavement, page 2-40) San Diego County LID Handbook Appendix 4 (Factsheets 8, 9 & 10): City of Santa Barbara Storm Water BMP Guidance Manual, Chapter 6: County of Los Angeles Low Impact Development Standards Manual, Chapter 5: INF-7: Underground Infiltration Underground infiltration is a vault or chamber with an open bottom that used to store runoff and percolate into the subsurface. A number of vendors offer proprietary infiltration products that allow for similar or enhanced rates of infiltration and subsurface storage while offering durable prefrabricated structures. There are many varieties of proprietary infiltration BMPs that can be used for roads and parking lots, parks and open spaces, single and multi-family residential, or mixed-use and commercial uses. Feasibility Screening Considerations Infiltration bains shall pass infeasible screening criteria to be considered for use. Underground infiltration galleries pose a potential risk of groundwater contamination; pretreatment should be used. Opportunity Criteria Soils are adequate for infiltration or can be amended to provide an adequate infiltration rate. Appropriate for sites with limited surface space. Can be placed beneath roads, parking lots, parks, and athletic fields. Potential for groundwater contamination can be mitigated through isolation of pollutant sources, pretreatment of inflow, and/or demonstration of adequate treatment capacity of underlying soils. Infiltration is into native soil, or depth of engineered fill is ≤ 5 feet from the bottom of the facility to native material and infiltration into fill is approved by a geotechnical professional.