Site-Specific Solutions

June 6, 2007

About the author: Karl L. Dreher, PE, is supervising transportation engineer for the Division of Environmental Analysis, Caltrans, and Timothy B. Sobelman, PE, is supervising transportation engineer chief, Office of Storm Water Management, Caltrans. James Sullivan, PE, and Daniel Peterson, PE, are project managers, Metcalf & Eddy|AECOM. Dreher can be reached at 916/653-3352. Sobelman can be reached at 916/653-5747. Sullivan can be reached at 203/741-2830. Peterson can be reached at 916/929-3124.


With more than $2 billion in transportation construction work completed each year, the California Department of Transportation (Caltrans) is intent on improving mobility across California. One challenge in doing so is complying with their National Pollutant Discharge Elimination System (NPDES) permit.

Caltrans’ highways and highway-related properties, facilities and activities are served by extensive storm drain systems that, in urban areas, are covered explicitly in the federal storm water regulations. The permit requires Caltrans to develop a statewide storm water management plan (SWMP) that establishes a programmatic approach for achieving compliance with state and federal regulations, and to address discharges of storm water and authorized non-storm water to drainages and to surface water bodies in the state.

For discharges that are typical of roadway runoff from paved surfaces, the SWMP provides an approach for the control of pollutants to the maximum extent practicable (MEP) as required by the federal Clean Water Act. MEP is the process of evaluating the selected best management practices (BMPs) based on legal and institutional constraints, technical feasibility, relative effectiveness and cost/benefit ratio. The overall goal of the department’s storm water program is to integrate appropriate storm water control activities into ongoing department activities, thus making control of storm water pollution a part of normal routine business practices.

Caltrans has developed a project planning and design guide (PPDG) that acts as a one-stop toolbox for project engineers to utilize when evaluating a project for storm water compliance. The project engineer (PE) is responsible for preparation of the conceptual design and final design of all proposed projects. The PPDG provides the necessary references and design guidance for the incorporation of storm water quality controls into projects during the planning and design phases. The PPDG addresses key regulatory, policy and technical requirements by providing direction on the procedures to implement the storm water BMPs into the design of all Caltrans projects. The key objective of this PPDG is to provide the overall process for selecting and designing BMPs within the Caltrans planning and design processes.

Approved BMPs

Clatrans has created a systematic approach to identify, evaluate, approve and integrate all BMPs into everyday business practices. This process creates a toolbox of pollutant-based treatment BMPs that can be used for transportation projects and activities. The use of BMPs from the toolbox ensures that BMPs the department develops or approves are cost-effective, efficient and appropriate for the transportation infrastructure.

Caltrans has been evaluating BMPs for effectiveness in removing pollutants from roadways and facilities since 1996. Evaluations range from literature searches to full-scale pilot testing. The status or findings of these research evaluations are made available to the public on the department’s website. To date, the BMP development process has resulted in the approval of a wide range of BMPs that are now available for implementation into projects. The SWMP identifies both permanent and temporary BMPs that have been approved for statewide application. The approved BMPs fall into the following three categories.

Design pollution prevention (DPP) BMPs. These BMPs are technology-based, non-treatment controls selected to reduce post-construction pollutant discharges. They were developed to prevent downstream erosion, stabilize disturbed soil areas and maximize vegetated surfaces. Where an increase in paved surfacing results in a more-than-negligible increase in total or peak runoff discharges, an evaluation is performed to determine if any adverse effects will result. If increased runoff results in an increased potential for downstream erosion or sedimentation in channels, Caltrans will implement design pollution prevention BMPs.

Construction site BMPs. Construction site BMPs are applied during construction activities to reduce the pollutants in storm water discharges throughout the construction phase of a project. Construction site BMPs provide both temporary erosion and sediment control. The department develops a construction site BMP strategy on every project by including details and specification requirements in the project’s contract documents. By doing this, the PE provides adequate information to the contractor for the preparation of a water pollution control plan (WPCP) or storm water pollution prevention plan (SWPPP) to be able to address the potential storm water impacts during construction. The WPCP or SWPPP is site specific and prepared and implemented by the contractor.

Treatment BMPs. During the planning and design process of new facilities and reconstruction of existing facilities, the PE considers and, as appropriate, incorporates treatment BMPs into the project. The treatment BMPs listed in Figure 1 (see page 30) are considered for certain proposed new construction and major reconstruction projects discharging directly or indirectly to receiving waters.

These approved treatment BMPs are considered to be technically and fiscally feasible. The department’s experience has generally determined these BMPs to be constructible, maintainable and effective at removing pollutants to the maximum extent practicable, provided the appropriate siting and design criteria are satisfied. The SWMP requires Caltrans to evaluate whether a project must consider incorporating treatment BMPs. When considering the incorporation of treatment BMPs into a project, a site-by-site determination of treatment BMP feasibility is required. This evaluation is conducted by the PE and is assisted by storm water coordinators who have been assigned to the area in which the project is planned. These coordinators are familiar with any pollution-control requirements identified within the project limits.

TDC approach

The targeted design constituent (TDC) approach is the department’s statewide design guidance to address the “primary pollutants of concern” as identified in the PPDG. When a PE has determined that treatment BMPs must be considered for incorporation into the project design, the TDC approach is utilized to select the most appropriate BMPs that will address the site-specific water quality issues for that project.

A TDC is a pollutant that has been identified during departmental runoff characterization studies to be discharging with a load or concentration that commonly exceeds allowable standards and that is considered treatable by currently available department-approved treatment BMPs. Some TDCs include phosphorus, nitrogen, total copper, dissolved copper, total lead, dissolved lead, total zinc, dissolved zinc, sediments and general metals (unspecified metals).

A project must consider treatment to target a TDC when an affected water body within the project limits (or with the sub-watershed) is on the 303(d) list for one or more of these constituents. Infiltration devices, being the approved treatment BMP capable of treating all the constituents, should be considered as the desired treatment BMP for all watersheds for projects that are required to consider treatment BMPs. However, if infiltration devices cannot be incorporated, or if the proposed infiltration device(s) cannot accept all of the runoff needing treatment, biofiltration, detention devices, multi-chambered treatment train, media filter (Austin Sand filter and Delaware filter), and wet basins must be considered based on the TDC approach. The remaining Caltrans-approved treatment BMPs—dry weather diversion, gross solids removal devices and traction sand traps—are applicable for specific situations as described in the PPDG.

The TDC approach leads the PE through a decision-making process to select a treatment BMP, first based on effectiveness and then, secondly, on cost. Each TDC has a separate priority list of treatment BMPs that are to be considered.

The order for choosing BMPs is based on data from previous Caltrans pilot studies and on other outside performance data for these types of BMPs. These BMPs are grouped based on approximate equal effectiveness. There are typically four groups of effectiveness for each TDC. Group 1 is considered the most effective and Group 4 is considered the least effective at treating the TDC. Those BMPs evaluated but not considered to be effective at all are not shown in the rankings, and therefore not considered for implementation.

Within each group, the BMPs have been re-ordered based upon total life-cycle cost (excluding right-of-way) with the lowest cost BMP listed first. This approach allows the PE to select and design treatment BMPs that are most applicable for the pollutants impacting the water bodies within the project limits. If there are no 303(d)-listed water bodies or TDCs present for a particular project, the PE will evaluate and consider treatment BMPs ranked for general-purpose pollutant removal.

As owner and operator of the single-largest network of impervious surface in the state of California, and probably the U.S., Caltrans is in a unique position to mitigate the effects of urban runoff pollution from developed land. However, as caretakers of publicly funded infrastructure, it is tasked with approaching this mitigation in a sound, fiscally conservative manner. The TDC approach and its systematic design guidelines allow the design staff to make prudent decisions that satisfy all stakeholders.

Download: Here