Designing Highway Stormwater BMPs for TMDL and NPDES Compliance
Last updated August 31, 2026
Highway stormwater is regulated differently from site stormwater. A corridor has many outfalls, limited right of way, and a permit obligation that increasingly arrives as a TMDL wasteload allocation. This five-hour course covers how to design treatment practices that can actually meet that allocation. It begins with how to read the allocation: the pollutant it names, the units it is written in, and where compliance is measured. From there it covers choosing a performance metric, sizing practices by volume or flow rate, and selecting a practice that removes the specific constituent in the permit, including the common practices that export phosphorus and nitrogen rather than remove them. Later sections address siting within the right of way, what manufactured treatment device certifications do and do not prove, monitoring that can demonstrate a load reduction, maintenance planning, and the options available when treatment at the outfall cannot meet the allocation. The material draws on TRB Special Report 360, Managing Highway Stormwater Quality: Driving Progress, the Washington State DOT Highway Runoff Manual, the International Stormwater BMP Database, and the Washington State Department of Ecology TAPE protocol. It suits engineers working in transportation drainage, water resources, and environmental compliance.
Learning Objectives
Interpret a TMDL wasteload allocation and identify the constituent, units, and points of compliance that will govern the design.
Choose between mass-based and concentration-based performance metrics and recognize why percent concentration removal misreads volume-reducing practices.
Select and size treatment practices for a named constituent using unit treatment processes and performance data, including the practices that export phosphorus and nitrogen.
Account for certification limits, monitoring requirements, and maintenance provisions so a facility delivers the performance it was credited with at design.
BasePDH | Course 020 | 5 Professional Development Hours
Designing Highway Stormwater BMPs for TMDL and NPDES Compliance
1. The Allocation, the Permit, and Where Compliance Is Measured
A wasteload allocation arrives at a highway agency as a number written into a discharge permit, and by the time it arrives, the decisions that determine whether it can be met have largely been made elsewhere. Three features of that number govern the design work that follows: the pollutant it names, the units it is expressed in, and the location at which conformance will be judged. Reading those three correctly is the first design act on a project carrying a wasteload allocation, and it precedes opening a stormwater manual to the fact sheet for whichever practice fits the available right of way.
1.1 What a TMDL allocates, and which side highway runoff falls on
A total maximum daily load has two parts. It is the calculation of the maximum amount of a pollutant a water body can receive and still meet water quality standards, and it is the determination of how much of that amount each contributing source may discharge [1]. The second part is what reaches a designer.
The allocation structure is written as:
TMDL = ΣWLA + ΣLA + MOS + RC (optional)
| Term | What it covers |
|---|---|
| ΣWLA | Sum of wasteload allocations, assigned to point sources covered by a discharge permit |
| ΣLA | Sum of load allocations, assigned to nonpoint sources not covered by a Clean Water Act permit |
| MOS | Margin of safety, set aside for uncertainty in the TMDL process |
| RC | Reserve capacity, optional, set aside for future development and discharges within the TMDL boundary |
Highway runoff falls on either side of the first two terms depending on permit coverage. It is a wasteload allocation when it is covered by an NPDES permit and a load allocation when it is treated as nonpoint and carries no NPDES coverage [1]. Reserve capacity has historically been uncommon in the TMDLs that reach state departments of transportation [1]. The distinction between the two allocation types is not a labeling convenience: load allocation reductions assigned to nonpoint sources are implemented voluntarily, and when USEPA establishes or approves a TMDL that allocates to nonpoint sources, it must determine that there is reasonable assurance those reductions will actually be achieved [1]. A wasteload allocation carries the enforceability of the permit it is written into.
The units are mass. A maximum daily load is computed from discharge volume multiplied by pollutant concentration, and TMDLs calculate loads and allocations for a daily period by definition [1]. That definition sits somewhat uneasily against practice. The same report notes that the daily load may be derived from longer-term data, monthly, seasonal, or annual, where pollutant variability or accuracy argues for it, and separately that TMDLs are typically written for annual loads, though other criteria occur including peak concentrations or the duration of a concentration above a threshold [1]. The time base of an allocation therefore has to be read off the document that established it rather than assumed from the name of the instrument.
The regulatory status of the receiving water sets what is coming. Under Section 303(d), a water body on the monitoring and evaluation list is Category 3b and could be listed as impaired in the future; a water body with a completed TMDL that still does not meet the standard is Category 4a; a water body that is impaired and needs a TMDL is Category 5 [1]. States update the impaired-waters list every two years and generally combine the 303(d) and 305(b) lists into a single Integrated Report, with a triennial review of the water quality standards themselves subject to public comment [1]. A highway segment discharging to a Category 3b water has a design horizon; a segment discharging to a Category 5 water has a schedule.
One jurisdictional boundary limits all of this. Clean Water Act jurisdiction covers surface water, and groundwater sits outside it [1]. Some states close that gap themselves. Washington combines NPDES permits with State Waste Discharge permits because groundwater is jurisdictional under the state Water Pollution Control Act, RCW 90.48.030 [1]. The consequence for infiltration practice is direct and appears again in Section 6: a practice that removes a discharge from the surface water pathway has not necessarily removed it from regulation.
1.2 The three forms the requirement arrives in
A wasteload or load allocation reaches a permittee in one of three forms: a load reduction target expressed in pounds, a percent reduction, or an assigned action, such as a requirement to provide water quality treatment at a named discharge location [1]. TMDLs developed by state environmental agencies with delegated USEPA authority, or by USEPA directly, usually specify an absolute load limit, a percentage load reduction, or both, and those are then integrated into permit conditions [1].