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79 4.1 Overview Surveys and interviews with DOTs (see Project Summary Report) have provided the research team with insights into the challenges DOTs face in design, construction, and O&M of infiltra- tion BMPs. The following are highlights from these surveys and interviews: â¢ The most common causes of failure were related to (1) incomplete information about the site leading to inadequate design assumptions and (2) compaction or clogging of BMPs during the construction phase of the project. â¢ The most challenging design issues were (1) the remaining uncertainty in long-term, full- scale infiltration rates, even after conducting thorough investigation and (2) challenges with providing enough space for BMPs. A wide range of other factors were also identified. â¢ Respondents commented on the large number of factors that must be adequately considered; a single missed factor can result in premature BMP failure. â¢ Respondents identified challenges associated with maintenance planning and implementa- tion, particularly where the performance and survivability of the BMP depends solely on infiltration rate. The uncertainty in maintenance requirements is a significant barrier to the use of infiltration BMPs. â¢ Respondents also emphasized the importance of consulting with O&M personnel during each phase of design to ensure BMPs are selected and designed in a way that can be maintained. â¢ Finally, respondents identified several considerations that apply to cold and arid climates (see Appendix I). This chapter contains three key approaches for improving the design, construction, and O&M of infiltration BMPs: â¢ Evaluate potential failure modes of the proposed BMPs as part of design and construction plans. This can identify approaches to reduce the risk of these failures occurring or reduce the consequences if failures do occur. Appendix J presents several case studies of infiltration BMP failures. â¢ Conduct a realistic assessment of the uncertainty in site conditions, construction methods, and future O&M. This assessment can be used to support development of designs that are more likely to remain operable within this range of uncertainty (i.e., are more resilient). â¢ Evaluate O&M requirements and methods as part of the design process and seek input from O&M staff regarding system design. This can support BMP designs that can be more effi- ciently operated and maintained at an acceptable cost. C H A P T E R 4 Key Considerations for Design, Construction, and Maintenance of Infiltration BMPs
80 Stormwater Infiltration in the Highway Environment: Guidance Manual 4.1.1 Role of BMP Selection in Managing Uncertainty and Reducing Risk If data are not available at the time of design to ensure feasibility, then BMPs should be selected and designed so that they do not depend on uncertain design parameters (e.g., a certain infiltration rate) for the system to remain operable, or additional investigation should be under- taken during the construction phase to achieve the necessary level of confidence. The former is strongly recommended when it complies with infiltration objectives. BMP design is an extension of the BMP selection process. New information will often become available during the design process, such as better understanding of soil or groundwater char- acteristics, that can influence infiltration BMP selection. Designers should assess new informa- tion as it becomes available to determine if it justifies selection of a different BMP type or design variant. In addition to better information about the site, the design phase may also yield more information about project phasing, construction methods, and project delivery method. These can influence the suitability and feasibility of BMP types, which could also require reassessment of the selected BMPs and locations. In summary, the design process should include feedback loops for confirming or revising BMP selection. 4.1.2 Using Chapter 4 The concept of the planning tracks used in Steps 2 and 3 is carried through this chapter. Table 25 identifies key design, construction, and O&M considerations that apply to each track. Track numbering refers to the planning and design tracks described in Section 3.1. Based on the track selected in Step 3, the designer should consult Table 25 to determine the con- siderations that apply. This chapter supports designs of various levels of complexity. The BMP selection process described in Steps 1 through 3 ensures that BMPs are selected to be compatible with site con- ditions and available data. With appropriate BMP selection, the design team can typically rely on a normal level of design complexity and use standard construction methods. This requires (1) appropriate analyses to develop designs, (2) design provisions to mitigate risk and allow for O&M, (3) appropriate construction specifications to mitigate construction-phase impacts, and (4) post-construction monitoring. In some cases (specifically Track 1b), the project team may be compelled to include Full Infiltration BMPs despite the presence of marginal conditions or residual uncertainty in the as-built condition of BMPs. In these cases, it may be necessary to use more complex approaches for design and construction, such as more adaptable designs, design contingencies, and more rigorous controls on construction phasing and methods. This may also require more rigorous post-construction monitoring. This chapter focuses on guidance for common design, construction, and O&M issues. Addi- tional design guidelines are provided in fact sheets in Appendix A. Design details will vary by local standards, designer preference, and other factors. Designers will need to consult local design guidance in complement to this Guidance Manual to develop complete and acceptable design and construction documents. 4.2 Soil and Media Clogging and Associated Design Decisions Clogging is an inherent process in infiltration and filtration BMPs. In most cases, the rel- evant question is not whether clogging will occur, but rather how frequently it will occur and what will be required to restore infiltration rates when it does occur. The rate at which an infiltration or filtration BMP is expected to clog is a function of several factors, including the following:
Key Considerations for Design, Construction, and Maintenance of Infiltration BMPs 81 â¢ Sediment loading from the drainage area â¢ BMP footprint relative to sediment loading â¢ Soil and media characteristics â¢ Presence of vegetation and type of vegetation â¢ Use of pretreatment â¢ Road salting and sanding Some factors can be beyond the control of the project designer (e.g., sediment concentrations in roadway runoff, footprint available for BMPs, and soil properties), but the remaining project design decisions can have a large effect on clogging risks, including the following: Routing of Non-Roadway Runoff. Areas with disturbed or otherwise erosive soils can con- tribute large sediment loads. Disturbed soils should be remediated via erosion control when Planning and Design Track Example BMPs Key Design, Construction, and O&M Considerations 1a. Full Infiltration in Favorable Conditions BMP Selection: BMPs have been selected to fully infiltrate a specific design volume and solely rely on infiltration. Design and Construction Approach: Design and construct to preserve favorable conditions. Consider the benefit of adaptable designs to provide resiliency to unexpected conditions. BMP 04 Permeable Shoulders BMP 05 Bioretention w/o Underdrains (optionally with Capped Underdrain) BMP 07 Infiltration Trenches BMP 08 Infiltration Basins BMP 09 Infiltration Galleries â¢ Long-term soil clogging and maintenance cycles (pretreatment and other design approaches) (Sections 4.2, 4.3, 4.5) â¢ Construction phasing and site management (Section 4.6) â¢ Adaptable BMP designs (Section 4.4) (optional) â¢ Post-construction monitoring (Section 4.9) 1b. Full Infiltration in Marginal Conditions BMP Selection: Project objectives require Full Infiltration to be attempted, despite limitations. Full infiltration BMPs are tentatively selected. Design and Construction Approach: Design and construct to preserve or improve conditions. Improve certainty in site conditions through the construction phase. Design BMPs to be adaptable or have a backup plan. Include contingencies allowing for designs to be adapted based on construction-phase information. BMP 03 Media Filter Drain with Adaptable Underdrains BMP 05 and 06 Bioretention without and with, respectively, Capped/Adaptable Underdrains Other Full Infiltration BMPs (less preferable because of lower ability to adapt) All the above plus the following: â¢ Greater need for adaptable designs supported by construction or post- construction testing (Section 4.4) â¢ Greater need for controls on construction phasing and methods (Section 4.6). â¢ Greater need for post-construction monitoring (Section 4.9) 2a. Partial Infiltration with Supplemental Media Filtration BMP Selection: BMPs have been selected to provide incidental infiltration but would be operable without any infiltration. Treatment processes rely primarily on filtration. Design and Construction Approach: Preserve infiltration capacity using feasible construction-level controls. Design to mitigate clogging risks. Underdrains BMP 03 Media Filter Drain with Underdrain BMP 06 Bioretention with â¢ Long-term media clogging and maintenance cycles (Sections 4.2, 4.3, 4.5.3) â¢ Construction phasing and site management (Section 4.6), especially approaches to avoid sediment loading to filtration media â¢ Post-construction monitoring (Section 4.9) 2a. Partial Infiltration with Positive Overland Drainage BMP Selection: BMPs have been selected to provide incidental infiltration but would be operable without any infiltration. Treatment processes rely primarily on overland flow. Design and Construction Approach: Preserve infiltration capacity using feasible construction-level controls. Design and construct to promote effective treatment. BMP 01 Vegetated Conveyance/Swale BMP 02 Dispersion/Filter Strip â¢ Avoidance of excess compaction (Section 4.6.2) â¢ Soil decompaction/amendment (Section 4.6.3) â¢ BMP vegetation establishment (Section 4.6.4) â¢ Post-construction monitoring (Section 4.9) Table 25. Key design, construction, and O&M considerations by planning and design track.
82 Stormwater Infiltration in the Highway Environment: Guidance Manual within the DOT ROW. Where open space or off-site areas drain through the same drainage system as roadway runoff, a key design decision is whether to hydraulically separate these or provide a high level of pretreatment so that they do not contribute loads to infiltration or filtra- tion BMPs. Note that stream protection criteria may require coarse sediment supply areas (e.g., naturally erosive areas that produce stream bed sediment) to be passed through to streams to help maintain natural stream processes. Level of Pretreatment Provided. Pretreatment can include vegetated filter strips, swales, forebays, manufactured devices (with varying treatment performance), or filtration cells. Each has a different level of effectiveness for sediment removal. Generally, more effective controls will require more upfront costs as well as greater costs for O&M for the pretreatment system but will require less maintenance of the infiltration system. Designers should consider the tradeoffs between pretreatment costs and the long-term cost of O&M. BMP Footprint and Design Depth. Sediment loading per unit area of BMP surface is a useful metric to estimate the time to clog. A shallower BMP will have greater surface area than a deeper BMP with the same volume. It will therefore have lower sediment load per unit area. In project settings with adequate space, designers should evaluate options with a shallower ponding depth and broader footprint to reduce the frequency of maintenance cycles. Surface versus Subsurface Infiltration. Infiltration systems that are exposed to the atmo- sphere (surface systems) are exposed to a greater range of weathering processes (wind, rain, drying, insects, etc.) that can help break up sediment layers that may form. Similar processes may be less present in subsurface systems. Also, surface systems often support vegetation (inten- tional or incidental), which can reduce clogging risk. Designers should use surface systems whenever practical. Vegetation. Vegetated systems have been found to sustain higher long-term infiltration rates than unvegetated systems (Hart 2017). This is believed to be due to root action, root swelling and shrinking, soil soaking and drying (which is enhanced via root transpiration pro- cesses), and the role of plants in a biologically viable root zone (e.g., a soil stratum that supports insects, worms, fungus, and microbes). Vegetation can also serve to prevent the formation of a less permeable crust of fine sediment on soil surface and provide more pathways for water to enter the soil surface. Designers should consider soil amendments and plantings to support vegetation as a means of improving the longevity of BMPs. Use of Sacrificial Soil Layer Over Underlying Soil. See description in Section 4.5.1. Outlet Control versus Media Control of Filtration BMPs. See description in Section 4.5.3. The BMP Clogging Risk Assessment Tool (Appendix F) is designed to support rapid evalu- ation of these factors to support assessment of relative risks associated with different design alternatives. Figure 13 provides an overview of the inputs and outputs of this tool, and Figure 14 shows example results from the tool. Documentation of inputs, algorithms, results, and inter- pretation is provided as part of the notes within the tool and the supporting user guide. 4.3 Selection of Pretreatment BMPs Pretreatment BMPs can extend BMP lifespan by reducing the rate of sediment accumulation and associated clogging (Section 4.2 and Appendix F). Use of pretreatment BMPs may also be necessary to avoid potential impacts to groundwater quality (see Appendix D and Chapter 3). Table 26 contains potential pretreatment BMPs, classifies how well these BMPs address clog- ging and groundwater protection, and describes appropriate uses. Designers can use this table to support selection of pretreatment options based on project-specific factors.
Key Considerations for Design, Construction, and Maintenance of Infiltration BMPs 83 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0 1 2 3 4 5 6 7 8 9 10 Ap pl ie d Lo ad (l b/ ft2 ) In fil tra tio n R at e (in /h r) Time (yrs) MaintenanceInfiltration Rate (in/hr) Applied Load (lbs/ft2) Initial Infiltration Rate (in/hr) Figure 14. Example results from roadside BMP Clogging Risk Assessment Tool for infiltration rate reduction and applied load over time. Figure 13. Overview of BMP Clogging Risk Assessment Tool (Appendix F). 4.4 Adaptable Design Approaches for Infiltration BMPs There are cases in which adequate confidence cannot be achieved in investigation and design of infiltration BMPs, but stringent infiltration objectives create an incentive to attempt to achieve Full Infiltration. This are mostly cases for which the full-scale, reliable, long-term infiltration rate cannot be estimated with confidence prior to construction activities occurring. Examples include the following: â¢ Inability to reliably translate small-scale tests to full-scale operation â¢ Difficulty predicting post-construction bulk density and permeability of amended soils â¢ Inability to access the proposed BMP location infiltration surface prior to construction (e.g., permitting or excavation requirements) â¢ Inability to protect the infiltration area from construction impacts and uncertainty about the ability to fully remediate those impacts
84 Stormwater Infiltration in the Highway Environment: Guidance Manual In these cases, the use of adaptable design approaches (e.g., infiltration BMPs with a built-in contingency plan) can have an important role. This option is recommended anytime Full Infil- tration BMPs are proposed. Adaptable designs can also allow final confirmatory testing to be conducted after the finish grade of the BMP has been reached. This allows more reliable infiltration testing methods to be used. Appendix B provides guidance on the testing methods that are applicable for confirmatory testing in BMPs. 4.4.1 Adaptable Design Options An adaptable design approach includes predefined contingencies in the BMP design that can be made based on new information obtained from infiltration testing during or following Pretreatment Approach or BMP Type Description Sediment Removal Performance Groundwater Protection Performance Appropriate Uses Settling chambers or sacrificial forebay At least 10% (preferably 20%) additional volume beyond the required BMP size set aside for pre-settling Moderate Negligible Where land use is low risk or in combination with other approaches Catch basin insert baskets or screens Systems intended to strain coarse solids from stormwater as it enters catch basins Negligible Negligible For trash and larger debris and solids control only; no significant benefit for clogging or groundwater quality Sacrificial mulch layer Mulch layer provided on the surface of vegetated systems with commitments to yearly maintenance (periodic replacement of layer) Moderate Limited Bioretention systems where clogging risk is low Sacrificial sand layer A course sand layer above the infiltrating surface with a filtration rate 5 to 10 times higher than underlying soil; ability and commitment to replacement of layer Moderate Negligible Non-vegetated surface or subsurface systems where sand layer can be removed and replaced Amended media layers An engineered bioretention soil media layer installed in the surface of a bioretention BMP or infiltration basin to pre-filter sediment and treat other pollutants Moderate to High Medium to High Bioretention or infiltration systems (see Section 4.4.2) Proprietary pretreatment devices A system with an approved General Use Level Designation for pretreatment by Washington State Technology Assessment ProtocolâEcology (TAPE) program or equivalent Moderate Limited Underground or surface systems with adequate head for pretreatment device and low to moderate clogging risk from expected TSS levels Non- proprietary treatment control BMPs Treatment BMPs such as swales or media filters High Medium to High Where clogging risk and groundwater risks are elevated Proprietary treatment devices A system with an approved General Use Level Designation for basic treatment by Washington State TAPE program or equivalent High Medium to High Where clogging risk and groundwater risks are elevated Source: Adapted from Orange County, California, Technical Guidance Document (Orange County Public Works 2017). Table 26. Pretreatment options and descriptions.
Key Considerations for Design, Construction, and Maintenance of Infiltration BMPs 85 construction. Examples of adaptable design features are described in Table 27. This table only includes Full Infiltration BMPs. BMPs that provide Partial Infiltration and have supplemental discharge pathways do not depend as much (if at all) on an understanding of underlying infil- tration rate. 4.4.2 Permitting and Compliance Demonstration of Adaptable Designs Permitting and compliance demonstrations are typically conducted before construction. Therefore, adaptive approaches can have specific considerations. Potential approaches to sup- port permitting of adaptive designs include the following: â¢ Clear identification of the construction-phase testing required and the thresholds at which a contingency design element or alternative would be activated â¢ If the contingency plan involves changing the type of facility with respect to applicable regula- tions (e.g., conversion from infiltration to treatment), presentation of calculations describing how the system will still conform to applicable sizing criteria if the contingency is activated. Table 27. Potential adaptable design features for Full Infiltration BMPs. Infiltration BMP Type Potential Contingency Design Elements in Marginal or Uncertain Conditions BMP 03: Media Filter Drains â¢ Include elevated underdrain in design, but leave underdrain capped unless needed. BMP 04: Permeable Pavement Shoulders â¢ Provide a contingency to construct a wider gravel reservoir depending on infiltration testing following construction of road base. â¢ Provide supplemental inlets to route water into subbase in the event the surface of the permeable pavement clogs. â¢ Provide a supplemental drainage pathway for the storage reservoir to ensure drainage if underlying infiltration rates decline. â¢ Provide contingency for the use of supplemental downstream BMPs. BMP 05: Bioretention without Underdrains â¢ Design with a capped underdrain and outlet riser such that the underdrain can be opened and converted to a bioretention BMP with underdrains. This contingency could be activated during construction or at any time after construction when need has been determined. â¢ Design with a plugged or capped orifice at the floor of the basin that could allow conversion to a dry detention basin. (Note: the suitability of a detention basin to meet water quality treatment requirements may vary by state or project.) BMP 07: Infiltration Trench â¢ Provide a contingency to construct a larger or deeper footprint, if feasible, based on construction-phase testing. â¢ Or have plans for an alternative BMP within the footprint (e.g., media filter with underdrain). BMP 08: Infiltration Basin â¢ Include an optional biofiltration media layer and underdrain system that can allow conversion to bioretention BMP with underdrains if needed. â¢ Provide a contingency to construct a larger footprint. â¢ Include means to switch to an extended detention basin with Partial Infiltration. BMP 09: Infiltration Galleries â¢ Pretreat influent using an acceptable treatment BMP (e.g., bioretention, proprietary treatment device) to reduce clogging potential and allow any water not infiltrated to have already been treated to applicable standards. â¢ Provide a contingency to construct a larger footprint or deeper gallery (storage).
86 Stormwater Infiltration in the Highway Environment: Guidance Manual This could potentially include changing the size of the facility. This may require primary and contingency calculations to be included in the project permitting and design documents. â¢ For projects requiring state or federal environmental clearance (e.g., environmental impact reports), disclosure and evaluation (in environmental documents) of both the primary and contingency plans The ability to use adaptive approaches may also require modifications to standard project delivery processes so that changes to the compliance approach can be enacted during the con- struction phase of the project. 4.4.3 Whole Lifecycle CostâBenefit Evaluation of Adaptable Designs Contingency elements to support an adaptable design may require greater upfront cost associated with design, permitting, and construction. For example, installing underdrains and engineered media in a basin adds considerable cost, but it also provides the ability to operate the system as either an Infiltration or Partial Infiltration BMP. If site conditions clearly support Full Infiltration, then the cost of these design elements would not be justified. The decision to use these designs may depend on lifecycle costâbenefit calculations. These calculations can be supported by several NCHRP tools, including the following: â¢ Whole Lifecycle Cost and Performance Tools: NCHRP Report 792 â¢ Volume Reduction Tool: NCHRP Report 802 â¢ Roadside BMP Groundwater Mounding Assessment Guide and User Tool: Appendix C â¢ BMP Clogging Risk Assessment Tool: Appendix F Appendix G provides a hypothetical case study example of how these tools can evaluate the use of contingency underdrains in a bioretention basin. 4.5 Other Design Approaches to Extend Design Life or Improve Resiliency 4.5.1 Sacrificial Soil Layers The rate of clogging of infiltration or filtration BMPs can determine maintenance intervals. With all else being equal, a system that starts with a higher infiltration can tolerate more clogging before requiring maintenance than a system starting with a lower infiltration rate. A sacrificial soil or media layer consists of a layer of material (sand, soil, engineered media) placed over the top of the less permeable underlying soil to serve as an embedded pretreatment layer (see Figure 15). Because of its higher permeability, more sediment can be loaded on this Sacrificial Soil Layer (6 to 12â) Underlying Soil Figure 15. Schematic illustration of a sacrificial soil layer.
Key Considerations for Design, Construction, and Maintenance of Infiltration BMPs 87 layer before it approaches the limiting rate of the underlying layer. Additionally, if this material is significantly coarser than incoming sediment, it is more likely that the depth filtration processes will prevail in the media rather than surface filtration (also known as cake filtration). Materials can generally accept greater loading when depth filtration prevails. Conceptual specifications include the following: â¢ The permeability is 5 to 10 times higher than that of the underlying soil. â¢ The layer depth is 6 to 12 in. â¢ The coefficient of uniformity (D60/D10) is approximately 1.5 (i.e., fairly uniform material). â¢ The median particle size is 0.5 mm to 2 mm. â¢ The BMP is designed to allow for periodic raking and removal of the sacrificial layer. â¢ Sacrificial layers are loosely placed and lightly compacted using low ground pressure equip- ment. A target bulk density of approximately 80 lb per ft3 is recommended. The design should allow for approximately 10% settlement of the sacrificial layer. 4.5.2 Compost Soil Amendments Amending soils with compost can alter soil characteristics to allow it to absorb, infiltrate, and retain more water to help reduce runoff volume and velocity, filter pollutants, increase the quality and quantity of vegetation, and reduce erosion potential more effectively than soils without soil amendments. Compost and fertilizers are common soil amendments that must be completely mixed into the soil to function properly. Amending soils with compost (and optionally with sand) can have similar effects as a sacrifi- cial soil layer but can also provide other functions including the following: â¢ Improving the ability of soils to attenuate and retain stormwater pollutants â¢ Improving plant growth, which can have the effect of reducing susceptibility to clogging Conceptual specifications include the following: â¢ Rototill 2 to 4 in. of compost into soil to a minimum depth of 6 in. (12 in. preferred). Sand can also be used as an amendment to improve the drainage rates of amended soils. Sand should be free of stones, stumps, roots, or other similar objects larger than 5 mm. â¢ Specify and source compost that is mature, stable, and weed free derived from waste materials including yard debris, wood wastes, or other organic materials (not including manure or biosolids), and meeting standards developed by the US Composting Council or equivalent. â¢ Design access to the BMP to allow for maintenance of the compost (and sand) layer. â¢ After amendment, loosely compact to approximately 80 lb per ft3. â¢ Where used on slopes, revegetate promptly following amendment and apply temporary erosion and sediment control practices to minimize soil loss. 4.5.3 Passive Outlet Control for Bioretention with Underdrains Bioretention with underdrains (BMP 06) can be an effective BMP for maximizing inciden- tal infiltration while also providing treatment; however, filtration BMPs can be susceptible to clogging. There are two fundamentally different ways to control filter bed hydraulics for bioretention systems with underdrains. The traditional approach has been to specify the saturated hydraulic conductivity of the bioretention soil media (BSM) to within a given range (e.g., âmedia controlâ) and adjust BSM properties (e.g., fine particle content) to achieve this range. Actual infiltration rates of media are highly variable and sensitive to the degree of fines in the mix; the degree of
88 Stormwater Infiltration in the Highway Environment: Guidance Manual mixing during blending; compaction during installation; weathering and breakdown of media materials; types and maturity of plants; amount of clogging from particulates in runoff; and other factors. An alternative, passive, non-proprietary design approach (e.g., outlet control) involves a flow control outlet (e.g., orifice) affixed to the underdrains of the bioretention system as the primary hydraulic control in the system (see examples in Figure 16). This approach can improve per- formance and alleviate several vulnerabilities. Benefits of this approach include the following: â¢ BSM can be specified with a wider range of hydraulic conductivity. This reduces overall system sensitivity to BSM hydraulic conductivity, mixing methods, placement methods, plant growth, and other factors. â¢ BSM can be specified with a higher initial hydraulic conductivity, which allows the system a greater factor of safety before clogging (more void space for captured material) begins to reduce system flow rates. â¢ Outlet control is inherently adjustable to adapt system operations as needed. This approach can improve the lifespan of bioretention systems. It can be compatible with an adaptable design approach (see Section 4.4). The BMP Clogging Risk Assessment Tool (Appendix F) can be used to assess the longevity benefits of this approach. This effectively allows a higher starting flowrate to be used as Fig- ure 17 shows schematically. 4.6 Construction Site Management and Phasing to Reduce Impacts to Infiltration and Filtration BMPs Infiltration and filtration BMPs are susceptible to sedimentation and compaction during or immediately following construction activities. These issues are among the most common causes of failure of infiltration and filtration BMPs. Several construction-phase approaches can be used to reduce these risks or remediate them if they occur. Low flow orifice control in end cap Primary orifice control in standpipe Figure 16. Example outlet control configuration for bioretention with underdrains.
Key Considerations for Design, Construction, and Maintenance of Infiltration BMPs 89 4.6.1 Construction Phasing to Reduce Sediment Risk In accordance with federal and state rules, construction sites must implement effective erosion and sediment control BMPs. This Guidance Manual does not cover how these should be done; however, with respect to infiltration and filtration BMPs, several common-sense approaches should be followed. â¢ Keep infiltration and filtration BMPs off-line (i.e., not receiving flow) during the construction phase until the site has achieved final stabilization. Temporary erosion and sediment control BMPs are not adequate to prevent loading of fine sediment that can clog infiltration facilities. Final stabilization refers to a well-established vegetation layer or local equivalent constituting full stabilization. â¢ Provide erosion and sediment BMPs at the top of the BMP embankment to protect the BMP from sediment-laden water. This can also delineate the BMP so that construction crews do not enter it with heavy equipment. Note, depending on sediment loading and texture, this may not be adequate to prevent clogging. See previous bullet. â¢ If possible, construct infiltration and filtration facilities during later phases of site construc- tion to prevent sedimentation and damage from construction activity. After installation, pre- vent sediment-laden water from entering inlets and pipes draining to infiltration systems. If this is not possible, runoff from the construction site should be diverted away from the BMP to reduce clogging risk. â¢ Avoid using infiltration areas as construction-phase sedimentation ponds if possible. Where site constraints require infiltration areas to be used as sedimentation ponds, the initial excava- tion of the sedimentation pond should stop 2 ft before reaching the final grade of the infiltra- tion BMP. Final excavation to the finished grade should then occur after all disturbed areas draining to the BMP have been stabilized or protected. A sacrificial impermeable liner can also be considered. This liner would be removed after construction is complete. As a last resort, material could be removed and decompaction techniques used to recover infiltration rates (more applicable to partial or incidental infiltration systems). â¢ Place filtration media after the site has been fully stabilized and most construction activities have ceased (unless applied as a sacrificial layer to be removed later). Fi ltr ati on R at e Too slow for drainage Lifespan (outlet controlled) Initial treatment Cumulative Loading (i.e., increasing fines content) Initial uncontrolled K of media Too slow for drainage Lifespan Too fast for treatment Fi ltr ati on R at e Cumulative Loading (i.e., increasing fines content) Traditional Media Control Outlet Control Figure 17. Schematic illustration of the lifespan benefits of outlet control.
90 Stormwater Infiltration in the Highway Environment: Guidance Manual â¢ If local climate prevents pervious areas from being stabilized prior to commissioning of infil- tration facilities, then route these areas separately so they do not pass through the infiltration BMP until they are stabilized. â¢ Stabilize the side slopes of BMPs so they do not contribute sediment to the infiltrating surface. As observed in a case study provided by Minnesota DOT, the side slopes and upper areas of an infiltration basin experienced erosion that led to clogging of the lower portion of the infiltration basin (Figure 18 and Figure 19). For basins with large side slopes, excavate to an intermediate grade (2 ft above finish grade) to begin stabilization of side slopes, then excavate to the final grade after side slopes have been stabilized. 4.6.2 Construction Vehicle and Traffic Management Soil infiltration rates are affected by compaction. Several approaches can reduce the potential for compaction of infiltration areas during construction: Figure 18. Erosion of side slopes and basin floor in Pine Bend infiltration basin. Figure 19. Sedimentation in floor of Pine Bend infiltration basin after construction.
Key Considerations for Design, Construction, and Maintenance of Infiltration BMPs 91 â¢ Restrict heavy equipment and traffic from traveling over the proposed location of infiltration BMPs. â¢ Use construction fence and temporary erosion and sediment control BMPs to demarcate areas proposed for infiltration BMPs. â¢ Avoid routing traffic over the location of future BMPs. For example, a case study provided by Massachusetts DOT found that infiltration failure (very slow drainage) was associated with areas of the median that were used for a temporary traffic detour route (highway traffic was routed from one side of the divided highway to the other over the median to support phased bridge replacement) (see Figure 20) (Personal communication with Henry Barbaro of Massachusetts DOT 2015). â¢ Avoid excavation of infiltration BMPs when soils are wet or when it is raining. Soils are more sensitive to compaction when wet. Additionally, soil smearing can reduce infiltration rate and inhibit vegetation growth, which can jeopardize establishment and operation. A best practice to help recognize these conflicts would be to include outlines of infiltration BMPs as part of the underlying base map that is used for all sheets in the design and construc- tion-phasing sheet set. This could help make designers aware of construction-phase conflicts. 4.6.3 Remediation of Construction Impacts Where construction-phase sedimentation or compaction cannot be avoided, soils should be remediated to restore infiltration properties to the extent possible. â¢ In the case of compaction, remediation could include tilling or other forms of decompaction. A decompaction depth of 6 to 18 in. is recommended depending on the severity of impacts. â¢ For siltation, remediation should include over excavation and removal of soils. A depth of 6 to 18 in. is recommended. â¢ If smearing has occurred, this can be remediated by scarifying and regrading when soils have lower moisture content. â¢ A sacrificial soil layer or compost amendment could also be integrated with these strategies. A sacrificial impermeable liner is another option. â¢ Construction-phase infiltration testing should be used to demonstrate that infiltration rates have been adequately restored. Methods appropriate for confirmatory testing are described in Appendix B. Figure 20. Photos from site visit during construction of I-195 infiltration swales, Swansea, Massachusetts.
92 Stormwater Infiltration in the Highway Environment: Guidance Manual 4.6.4 BMP Vegetation Establishment Vegetated systems are most susceptible to declines in permeability during the period imme- diately after construction and before plants have been able to establish root structures. Several practices can be used to help mitigate these risks: â¢ Where possible, develop construction phasing to allow time for plant establishment before BMPs are brought on-line to receive stormwater inflow. This also allows time to grow plants from seed or start with smaller plantings to reduce cost and improve plant survival. Note that early construction of the facility may also require special provisions to limit construction- phase impacts. â¢ Consider temporary irrigation to improve the rate of vegetation establishment. â¢ Include contract provisions related to percentage of vegetative cover. 4.6.5 Role of Project Delivery Model in Construction Site Management and Phasing Construction-phasing and construction methods can influence the success or failure of infil- tration and filtration BMPs. DOT project managers should review this guidance and determine whether applicable approaches and project controls can be implemented with one of the DOTâs standard project delivery approaches. Key questions include the following: â¢ Does the delivery method allow the DOT to specify phasing? For example, can the DOT specify the timing and order of BMP construction? â¢ Is construction-phase testing required and are design contingencies needed? If so, does the delivery method support this? â¢ What is the maximum bonding period allowed by the delivery method? Is this enough to ensure that full vegetation and stabilization of the site have occurred? â¢ Does the delivery method allow for performance-based standards? For example, if impacts are unavoidable, does the delivery method allow the DOT to specify the minimum perme- ability of the restored soil? Does the delivery method allow for the DOT to require a certain minimum vegetative cover of graded slopes before constructing BMPs? If these answers are âno,â then the project team may justify consideration of alternative project delivery methods or special specifications. 4.7 Design to Facilitate BMP O&M Infiltration BMPs, like all structural stormwater BMPs, will require regular maintenance and inspection to remain operable. As a best practice, the designer should consult with O&M staff beginning in the design phase. This can result in designs that are simpler to maintain. In some cases, additional design complexity and cost can simplify maintenance. These can also make BMPs less susceptible to performance declines or nuisance conditions if there are lapses in main- tenance. The measures in this section result in BMP designs that control O&M costs. 4.7.1 Design Approaches to Control Maintenance Costs There are two primary ways to reduce maintenance costs and complexity. Both are strongly recommended. First, using the approaches described in Sections 4.2 through 4.5, develop BMP designs to increase BMP lifespan and allow BMPs to be adapted to remain operable in adverse conditions.
Key Considerations for Design, Construction, and Maintenance of Infiltration BMPs 93 This tends to result in BMPs that require less maintenance or simpler maintenance. Approaches include following: â¢ Utilize design approaches that limit clogging risk (Section 4.2). â¢ Select effective pretreatment BMPs (Section 4.3). â¢ Develop adaptable designs (Section 4.4). â¢ Use other design approaches to extended BMP life (Section 4.5). Second, BMPs should be located and designed so that they can be readily accessed, evaluated, and maintained by means of the following: â¢ Provide access roads, as applicable, such that portions of the BMP requiring maintenance can be accessed with appropriate equipment without damaging other parts. â¢ Provide pretreatment systems or sacrificial areas (e.g., the initial cell in a multi-cell system) that are intended to concentrate the spatial extent of pollutant accumulation and mainte- nance activities. Design these areas to be readily accessible for maintenance. â¢ Locate systems in areas accessible for maintenance (e.g., not underneath a structure or other site feature). â¢ Locate systems where maintenance access does not require lane or ramp closures whenever possible. â¢ Design systems that can be maintained using readily available maintenance equipment when- ever possible. â¢ Locate systems in areas that will not require permits and costly mitigation to perform main- tenance activities. â¢ Develop maintenance protocols that establish the system as a treatment system and limit any future interpretations as a jurisdictional area (e.g., habitat). Developing an O&M Plan (see Section 4.8) for the facility and consulting with O&M person- nel during the design phase can ensure that these factors are considered. 4.7.2 Design Approaches to Allow for Inspection and Verification BMPs will require regular inspection to verify that they are working properly. Designers can include the following design elements to better accommodate inspection and verification: â¢ Provide inspection ports for observing underground components that require inspection and maintenance; a diameter of at least 6 in. is recommended to accommodate a range of water level measurement equipment. â¢ Install level measurement posts in BMP components that trap and store sediment, trash, and debris so that inspectors can determine how much of the BMP capacity is utilized. â¢ Include a drain plug or valve at the bottom of the surface pool to allow dewatering for inspec- tion and O&M. This can avoid the need to dewater a basin by pump prior to maintenance. Ideally, allow the drain plug or valve to be activated without requiring personnel to enter the ponded water. â¢ Provide a landscape plan sheet in the O&M plan that clearly identifies expectations for vegetation coverage, size, and type. This supports inspectors who assess conditions and determine the need for maintenance. â¢ Provide signage indicating the location and boundary of the BMP. In general, the designer should assume that the BMP will be in a failed or clogged condition when O&M needs occur. If the system is clogged, consider how it will be accessed for mainte- nance and whether it can be drained without a pump. Consider whether there would be any safety issues associated with inspecting or remediating the failed BMP.
94 Stormwater Infiltration in the Highway Environment: Guidance Manual 4.8 O&M Manual The development of a facility-specific O&M manual may be beneficial for communicating maintenance needs to the entity that will be responsible for maintaining the facility. 4.8.1 O&M Manual Contents Table 28 provides suggestions for the contents of a facility-specific O&M manual. An O&M manual should document the specific aspects of the facility that should be consulted when performing inspections and maintenance. If an agency has standard guidelines available for maintenance of certain elements, these can be incorporated as attachments or references to complement the facility-specific details and support consistency across facilities. 4.8.2 O&M Activities O&M activities vary in their frequency and intensity through the lifecycle of the BMP. For a consistent description of activities, the following definitions can be used in the development of O&M manuals. Suggestions for O&M Manual Content Rationale and Guidance Description of the final constructed BMPs and key design sheets In preparing the O&M manual, it should be assumed that the full set of design drawings may not be available to O&M crews. The O&M manual should serve a stand-alone purpose. O&M exhibitâadapted design sheet(s) showing only the features and callouts relevant for field crews performing O&M Identification and contact information of the responsible party(ies) for inspection and maintenance A responsible party should be identified, and contact information must be included. Identification of the required qualifications and any training required for personnel who will perform inspections and maintenance Where certain activities require specific training or qualifications, the required qualifications must be clearly identified. Identification of the funding mechanism and associated supporting information to demonstrate adequacy of funding to cover anticipated and potential expenses The O&M manual should demonstrate adequate funding and the source of funding. Description of any unusual, excessively costly, or hazardous O&M activities required for the proposed BMPs Such activities need to be fully disclosed so that the acceptability of these activities can be evaluated by the entity accepting maintenance responsibility. Regular inspection activities, frequency, and documentation requirements These are core elements of an effective and complete O&M manual. Description of routine and planned maintenance activities, frequency (if scheduled) or triggers (if initiated based on inspection findings), and documentation requirements Description of foreseen rehabilitation activities; anticipated frequency; triggers for conducting activities; and the planning, approval, and documentation process required to conduct rehabilitation Process for identifying, diagnosing, and correcting issues resulting from damage, unusual wear, unforeseen conditions, etc. Spill response; notification requirements; and plans, materials, and responsibilities Table 28. Suggested O&M manual content, rationale, and guidance.
Key Considerations for Design, Construction, and Maintenance of Infiltration BMPs 95 Routine O&M Activities. These are activities conducted at regularly scheduled intervals to sustain long-term performance of each BMP, including inspections and normal upkeep. This category also includes activities conducted on an as-needed basis, prompted by inspections, to correct conditions that are anticipated to occur with normal operation of a BMP. Rehabilitation Activities. These are activities conducted to replace or rehabilitate system components at the end of their usable life. The O&M manual should seek to estimate the expected design life and the triggers for when a system has reached the end of its usable life. Corrective Activities. These activities are conducted to resolve major issues that were not anticipated. Because these were not anticipated, it is not possible for an O&M manual to have pre-defined remedies. Rather, the O&M manual should establish a process for identifying a major issue that requires correction, diagnosing the issue and its underlying causes, determin- ing the appropriate corrections, obtaining applicable permits, and appropriately documenting any changes to the design as a result of its correction. Emergency Response Activities. These activities include the DOTâs response to emergen- cies, including spills. For DOTs, these emergencies responses often require specialized materials and equipment and applicable notifications. 4.8.3 Phases of Maintenance Maintenance needs can change over time commensurate with plant establishment, media conditioning, and stabilization of the watershed. It may be appropriate to define one or more of the following periods. Immediate Post-Construction (2 months to 1 year after construction or major rehabilita- tion). During this phase, the system is stabilizing and there may be limitations to placing the system into full service. After initial construction, the contractor may still be under warranty to maintain the system. Short Term (2 to 3 years after construction or major rehabilitation). This is a period when plants are establishing and initial system conditioning processes (e.g., media settling, soil struc- ture development) are occurring. During this period, more frequent inspections may be needed. Additionally, maintenance activities can be more frequent and intensive, depending on the needs of the BMP. This regime may also need to be reinstated if major replanting occurs at any point in the facility lifespan. Long Term (after end of short-term phase). This period begins after vegetation has been fully established and typical functions have been adequately observed. The intention of the long- term maintenance period is to provide sufficient and sustained maintenance to keep the BMP operating within acceptable ranges while avoiding unnecessary costs. Observations during the short-term period may result in updates to frequencies or activities associated with long-term maintenance. Applicable phases should be explicitly defined in O&M manuals especially when maintenance needs are initially uncertain or are expected to evolve over the life of the facility. 4.8.4 Example Outline of O&M Manual This section provides an annotated outline of an example O&M manual. This outline generally follows Oregon DOT guidelines, and it could serve as a reference for developing an appropriate
96 Stormwater Infiltration in the Highway Environment: Guidance Manual local outline. Oregon DOT policies call for a draft O&M manual to be prepared as part of the design phase and finalized after construction. â¢ Cover Content is intended to provide a quick orientation to the facility â Facility ID number â Brief description of facility â Ground-level picture of facility â Vicinity map â Watershed map â¢ Identification â Provides more detail to accurately identify the facility â¢ Facility Contact Information â Identifies the DOT maintenance contact â¢ Construction â Identifies the dates of original construction and subsequent modification of the facility â Identifies the designer of record and contractor who performed each phase of construction â¢ Storm Drain System and Facility Overview provides a summary of the facility and the related piping connections, including the following: â Tributary area â Storm drain connections to facility â Facility type and design features â Treated discharge point â Bypass/overflow points â¢ Maintenance Equipment Access and Special Features of Facility â Identifies each discrete feature, the equipment access to the facility, and any special issues associated with maintaining each component â¢ Facility Hazardous Materials (HazMat) Spill Features and Response Protocol â Describes the role of facility features in HazMat spill containment â Identifies spill response protocols â¢ Maintenance Requirements summarizes inspection and maintenance requirements, includ- ing the following: â Inspection schedule and observations â Conditions that indicate need for maintenance â Estimated schedule of periodic maintenance â¢ Waste Material Handling â Describes special waste handling requirements â¢ Appendices â Operational Plan: This is an O&M-specific sheets set consisting of approximate 3 sheets that serve as an efficient reference for O&M crews, including the following: ï¿½ Location (e.g., mile points, left or right side of highway), footprint, and type of facility ï¿½ Location of facility components such as flow splitter manhole, forebay, pollution control manhole, flow spreaders, and outlet flow control structure ï¿½ Facility component details (e.g., flow splitter manhole, flow control manhole, forebay) with notes explaining operational functions and how the stormwater drains in and out, flow arrows that illustrate stormwater drainage paths, and any other operational notes needed to assist personnel who maintain the facility ï¿½ Location of maintenance access to facility ï¿½ Footprint of drainage piping and stormwater flow path into and out of the facility â Selected plan sheets â Manufacturer O&M documents (if applicable) â Standard drainage facility guidelines (if applicable)
Key Considerations for Design, Construction, and Maintenance of Infiltration BMPs 97 4.9 Post-Construction Monitoring Monitoring of constructed infiltration BMPs can address a range of questions and current data gaps. Monitoring can serve a range of purposes: Guidance Improvement. DOTs can use results from monitoring to assess the reliability of site investigation methods, evaluate the applicability of feasibility criteria, and assess the influ- ence of BMP design approaches on performance and maintenance requirements. This can be used to improve guidance. Maintenance Planning. DOTs can use results from monitoring to determine maintenance needs, forecast maintenance events, improve maintenance cost estimates, and evaluate material disposal requirements. This can improve the reliability of whole lifecycle cost estimates in the future. It can also inform BMP selection and design. Performance Evaluation. DOTs can use results from monitoring to evaluate and demon- strate performance. This can be used for compliance and crediting purposes and to inform design improvements. Impact Assessment. DOTs can use results to determine if designs result in impacts related to groundwater quality and geotechnical issues. This can be used to identify needed remedial activities and improve guidance and criteria for future projects. Table 29 introduces several monitoring questions that may be relevant to infiltration BMPs and identifies the purpose or purposes these questions may serve. This table also identifies potential study elements to inform these questions. The Urban Stormwater BMP Performance Monitoring Guide (Geosyntec Consultants and Wright Water Engineers 2009) provides guidance on monitoring BMP performance, includ- ing hydrologic monitoring; water quality, groundwater quality, and soil monitoring; statisti- cal analysis, and other topics (http://www.bmpdatabase.org/Docs/2009%20Stormwater%20 BMP%20Monitoring%20Manual.pdf). This document also includes references to numerous resources for monitoring plan development. Purposes of Monitoring Potential Monitoring Questions G ui da nc e Im pr ov em en t M ai nt en an ce Pl an ni ng Pe rfo rm an ce Ev al ua tio n Im pa ct A ss es sm en t Potential Study Elements Are BMPs operating as designed? X X X â¢ Maintenance inspections/drawdown observations â¢ Flow monitoring â¢ Water level monitoring What is the long-term volume reduction and capture efficiency performance of the BMP? X X â¢ Flow monitoring â¢ Water level monitoring What is the treatment performance for water that is treated? X X â¢ Flow monitoring â¢ Influent/effluent water quality monitoring How does performance compare with design-phase analyses? X X â¢ Flow monitoring â¢ Water level monitoring Table 29. Potential monitoring questions, purposes, and study elements. (continued on next page)
98 Stormwater Infiltration in the Highway Environment: Guidance Manual Purposes of Monitoring Potential Monitoring Questions G u id an ce Im p ro ve m en t M ai n te n an ce P la n n in g P er fo rm an ce E va lu at io n Im p ac t A ss es sm en t Potential Study Elements Water level monitoring How reliable were the methods used to estimate design infiltration rate? Did methods have a high or low bias compared with full-scale performance? X â¢ Water level monitoring â¢ Water temperature monitoring1 Does the BMP need to be maintained? X â¢ Maintenance inspections â¢ Water level monitoring Is there a trend in performance that can be used to forecast the timing of maintenance? X X â¢ Periodic drawdown observations or long- term water level monitoring â¢ Water temperature monitoring1 How often are different types of maintenance required? X â¢ Maintenance tracking system What is the cost of conducting maintenance? X X â¢ Maintenance tracking system What is the lifespan the BMP? X X â¢ Maintenance tracking system How do variations in BMP design influence performance? X â¢ Side-by-side studies or meta-studies2 How do variations in BMP design influence maintenance requirements and costs? X X â¢ Side-by-side studies or meta-studies 2 What is the response between sediment inflow and decline in permeability? X X â¢ Water level monitoring â¢ Water quality and flow monitoring â¢ Water temperature monitoring1 â¢ Drawdown observations over time How effectively are pollutants removed from stormwater before reaching the groundwater table? X X â¢ Stormwater quality monitoring â¢ Vadose and groundwater quality monitoring How do pollutant concentrations change with depth from ground surface? X X â¢ Stormwater quality monitoring â¢ Vadose and groundwater quality monitoring Are there detectible changes in groundwater quality resulting from infiltration? X X â¢ Stormwater quality monitoring â¢ Vadose and groundwater quality monitoring â¢ Upstream and downstream monitoring How does infiltration affect the local groundwater table (e.g., mounding)? X X â¢ BMP water level monitoring â¢ Groundwater level monitoring Is mounding associated with a decline in infiltration rate? X X X â¢ BMP water level monitoring â¢ Groundwater level monitoring â¢ Water temperature monitoring1 What is the geotechnical zone of influence of the BMP (e.g., is there elevated moisture in response to infiltration)? X X â¢ BMP water level monitoring â¢ Groundwater level monitoring (array) â¢ Soil moisture monitoring (array) What is the pollutant level in the surficial soil or media? Does this require special disposal? How often should maintenance be done? X â¢ Soil quality monitoring 1 Water temperature affects water viscosity which affects hydraulic conductivity. Water temperature monitoring is recommended as part of monitoring studies in which changes in infiltration rate are relevant to answering study questions. 2 Meta-studies refers to analysis of compiled studies to evaluate overall relationships between study parameters and performance. These studies may not permit isolation of parameters. Table 29. (Continued).