3.2 Steps for Design of Infiltration Facilities

This section provides a step-by-step process for designing infiltration-based LID BMPs, including bioretention and permeable pavement. The 7-step process is adapted from Section 3.3.4 of Volume III of the 2012 Stormwater Management Manual for Western Washington (Ecology 2012b), with updates as needed for eastern Washington.

STEP 1. SELECT A LOCATION

Identifying a location for the infiltration-based BMP based on the ability to convey flow to the location and the expected soil conditions and locations based on preliminary soils and subsurface hydrology evaluation (Chapter 2: Planning for LID). Conduct a preliminary check of local jurisdiction infiltration feasibility criteria and Site Suitability Criteria (SSC) in Section 6.3 of the 2004 SWMMEW.

STEP 2. PERFORM PRELIMINARY BMP SIZING

Estimate the geometry of the infiltration BMP using an approved modeling method listed in Chapter 4 of the 2004 SWMMEW. For infiltration facilities sized to meet treatment requirements, the BMP must successfully infiltrate the 6-month, 24-hour design storm. Flows in excess of this level can bypass the infiltration facility.

For infiltration facilities sized to meet the flow control standard, the BMP must infiltrate a sufficient amount of the influent stormwater runoff such that any overflow/bypass meets the allowed peak flow discharge rate.

STEP 3. DEVELOP TRIAL INFILTRATION FACILITY GEOMETRY
Use the preliminary infiltration rate developed from the soils and subsurface hydrology evaluation (Chapter 2: Planning for LID) to develop the trial facility geometry. If infiltration rates are not available during this step, use a default infiltration rate of 0.5 inches per hour. Use this trial facility geometry to help locate the facility and for planning purposes in developing the geotechnical subsurface investigation plan.

STEP 4. COMPLETE MORE DETAILED SITE CHARACTERIZATION STUDY AND CONSIDER SITE SUITABILITY CRITERIA
Information gathered during initial soils and subsurface hydrology investigations is necessary to know whether infiltration is feasible. More detailed evaluation may be needed during the design phase to evaluate the suitability of the site for infiltration, establish the infiltration rate for design, and evaluate slope stability, foundation capacity, and other geotechnical design information needed to design and assess constructability of the facility. See Chapter 2: Planning for LID, for more detailed discussion of soils and subsurface hydrology evaluation.

STEP 5. DETERMINE THE DESIGN INFILTRATION RATE

Estimate the design (long-term) infiltration rate as follows:
Use the Large Scale or Small Scale PIT method (or other local-approved method) as described in Appendix B: Evaluation Soil Infiltration Rates to estimate a measured (initial) saturated hydraulic conductivity (Ksat). Alternatively, for sites underlain with soils not consolidated by glacial advance (e.g., recessional outwash soils), the measured Ksat may be estimated using grain size distribution analysis.
Assume that the Ksat is the measured (initial) infiltration rate for the facility.
Adjust this rate using the appropriate correction factors, as described in Chapter 2: Planning for LID.

STEP 6. SIZE STORMWATER FACILITIES
Use an approved modeling method from Chapter 4 of the 2004 SWMMEW to evaluate whether the facility can infiltrate the 6-month, 24-hour design storm if sizing a treatment facility. If sizing a facility to meet the flow control requirement, use an approved modeling method Designing for LID 27 to document that the total of any bypass and overflow meets the applicable flow control standard. Size conveyance facilities in accordance with local jurisdiction requirements.

STEP 7. COMPLETE FINAL DESIGN

After LID BMPs have been sized, the design team can complete the site design. This step entails updating the preliminary site plan to represent final sizing and facility layout and confirming that the site goals are met. In many instances, the sizing of the BMPs and the completion of the site plan will involve several iterations.

OPTIONAL STEP: INTEGRATE PERFORMANCE MONITORING INTO DESIGN
Performance monitoring allows for measurement and direct understanding of how the LID facilities are performing and compare that with design expectations.These findings can provide valuable feedback and lessons learned to help continually improve future design guidelines and standards, as well as construction practices.

If performance monitoring is desired, then the design of LID BMPs and associated hydraulic structures should consider proposed locations for monitoring equipment early in the design process. For example, locations of flow monitoring gauges may influence design of stormwater pipes (material type, size, and slope), as well as conveyance structures and access for maintenance.

The monitoring data can be used to validate design assumptions, inform future design standards updates, and evaluate the LID site performance based on physical, on-site measurements. If the observed site performance was not meeting goals, adaptive management strategies could be implemented. For example, level spreaders could be installed or fixed to maintain disperse flows, curb cut inlets could be modified if needed to improve capture of stormwater flows, orifice controls could be added or adjusted, etc.


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