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From page 61... ... 61 Confirmation of BMP Selection Through Prioritized Analyses and Investigations This chapter provides guidance on prioritized analyses and site investigations that may be necessary to confirm or revise preliminary selection of infiltration approach, BMP types, and locations. This is Step 3 in the overall decision-making process. Read the entire page →
From page 62... ... 62 Stormwater Infiltration in the Highway Environment: Guidance Manual Confirmation Required – BMP design and function requires these data Confirmation Optional – BMP design/compliance does not depend on this information, but may be useful to support design and/or compliance Key Analyses and Investigations to Confirm BMP Selection 1a) Tentatively Select a Full Infiltration BMP and Confirm Full Infiltration Feasibility 3a) Read the entire page →
From page 63... ... Confirmation of BMP Selection Through Prioritized Analyses and Investigations 63 • Evaluate ability to better control infiltration properties through the construction phase of the project. – Potential issues. Read the entire page →
From page 64... ... 64 Stormwater Infiltration in the Highway Environment: Guidance Manual No Are BMP sizing and depth feasible? (draw down, performance) Read the entire page →
From page 65... ... Confirmation of BMP Selection Through Prioritized Analyses and Investigations 65 2. Refine site investigations to attempt to support Full Infiltration BMPs. Read the entire page →
From page 66... ... 66 Stormwater Infiltration in the Highway Environment: Guidance Manual 3.2 Design Infiltration Rate Design-level infiltration testing is required for all Full Infiltration BMPs and for Partial Infiltration BMPs that are being designed to meet a specified infiltration performance. Design-level infiltration testing is used to confirm preliminary planning infiltration thresholds and develop design infiltration rates. Read the entire page →
From page 67... ... Confirmation of BMP Selection Through Prioritized Analyses and Investigations 67 If the capacity of the infiltration receptor may limit reliable infiltration, the project team should consider reasonable mitigation approaches. Example mitigation approaches are as follows: • Adapt site design to locate BMPs in areas with fewer limitations (e.g., more permeable soils, greater separation to groundwater) Read the entire page →
From page 68... ... 68 Stormwater Infiltration in the Highway Environment: Guidance Manual of these factors is critical to confirm feasibility. Where it appears that the combination of available space and infiltration rate could be marginal, then Full Infiltration may not be feasible even if preliminary feasibility criteria are met. Read the entire page →
From page 69... ... Confirmation of BMP Selection Through Prioritized Analyses and Investigations 69 • Example 4: Match the long-term volume of surface runoff that is estimated to have occurred in the pre-project condition. • Example 5: Reduce the frequency of discharge from the site by a certain percentage compared with the developed condition without BMPs. Read the entire page →
From page 70... ... 70 Stormwater Infiltration in the Highway Environment: Guidance Manual Infiltration Engineered soil medium thickness Optional stone storage layer thickness Overflow Mulch Inflow via surface flow or pipe inlet Energy dissipation stone Perforated underdrain Optional upturned elbow Ponding depth Figure 11. Example schematic design profile appropriate for use in feasibility analysis. Read the entire page →
From page 71... ... Confirmation of BMP Selection Through Prioritized Analyses and Investigations 71 3.3.3 Example Criteria Sizing and performance criteria should typically be derived from local requirements or guidance. Example criteria include the following: • The storage volume should be recovered via drawdown in not less than 48 hours or a locally acceptable alternative to ensure adequate long-term performance. Read the entire page →
From page 72... ... 72 Stormwater Infiltration in the Highway Environment: Guidance Manual • Recommendations must be supported by and include documentation of soil investigations and infiltration testing if performed by the geotechnical engineer. • Recommendations must identify any construction-phase oversight or monitoring required. Read the entire page →
From page 73... ... Confirmation of BMP Selection Through Prioritized Analyses and Investigations 73 The following topics may need to be assessed further to determine feasibility. Hydrogeologic Conditions. Read the entire page →
From page 74... ... 74 Stormwater Infiltration in the Highway Environment: Guidance Manual 3.5.2 Underlying Criteria At a minimum, the groundwater quality analyses and recommendations should address the following feasibility criteria at the BMP locations: • The selected infiltration BMPs (including the use of pretreatment and soil amendments) provide adequate pollutant attenuation to avoid unacceptable impacts to groundwater quality. Read the entire page →
From page 75... ... Confirmation of BMP Selection Through Prioritized Analyses and Investigations 75 • Hydrologically isolate areas of the site that have higher risk of stormwater contaminants so that infiltration can be more feasibly applied to a lower-risk area. • Consider pretreatment or soil amendment if the following criteria are not met [adapted from (Washington State Department of Ecology 2012) Read the entire page →
From page 76... ... 76 Stormwater Infiltration in the Highway Environment: Guidance Manual Alternate project delivery could give the DOT more control over construction-phase decisions and site management. Alternative project delivery options could include the following: • Development of special specifications or special contract provisions within a standard designbid-build process. Read the entire page →
From page 77... ... Confirmation of BMP Selection Through Prioritized Analyses and Investigations 77 3 Would infiltration pose significant risks for groundwater quality that cannot be reasonably mitigated to an acceptable level? (Section 3.5 and Appendix D) Read the entire page →
From page 78... ... 78 Stormwater Infiltration in the Highway Environment: Guidance Manual regulatory authorities and technical experts to determine site-specific requirements. As a starting point, example feasibility criteria have been summarized in Section 3.2 through 3.5 and can a serve as a reference in the absence of local criteria. Read the entire page →

From page 61...

... 61 Confirmation of BMP Selection Through Prioritized Analyses and Investigations This chapter provides guidance on prioritized analyses and site investigations that may be necessary to confirm or revise preliminary selection of infiltration approach, BMP types, and locations. This is Step 3 in the overall decision-making process.