Managing the Storm
Stormwater Pollution Management

California Department of Transportation
Storm Drain System Inventory

Douglas C. Brice, Senior GIS Analyst, Burns & McDonnell, San Francisco, California
Danielle Starring, Assistant Environmental Engineer, Burns & McDonnell, San Francisco, California
 

Twenty-five years ago, only a third of the nation’s waters were safe for fishing and swimming. Wetland losses were estimated to be 460,000 acres annually. Agricultural runoff resulted in the erosion of 2.25 billion tons of soil and the deposition of large amounts of phosphorus and nitrogen into many waters.

In response to growing public concern for this serious and widespread water pollution, in 1972 Congress enacted the Clean Water Act (CWA), the first comprehensive national clean water legislation. The CWA is the primary federal law that protects our nation’s waters, including lakes, rivers, aquifers and coastal areas.

A result of the Clean Water Act was the creation of the National Pollutant Discharge Elimination System (NPDES) permit program. The NPDES program was created to control water pollution by regulating point sources that discharge pollutants into surface waters of the United States. A point source generally refers to a conveyance such as a pipe or man-made ditch. In California, the EPA has delegated the administration of the NPDES program to the State Water Resources Control Board (SWRCB) and the nine Regional Water Quality Control Broads (RWQCBs). Since its introduction in 1972, the NPDES permit program has been responsible for significant improvements to our nation’s water quality.

The NPDES permitting system is of particular importance to the California Department of Transportation (Caltrans). Under these federal regulations, many Caltrans properties and facilities fall under the jurisdiction of the NPDES permitting system. Every time it rains, water washes materials from highways, streets, and gutters into the storm drain system. Highways typically contain vehicle exhaust products, brake and tire materials, oil and grease, litter and other materials that get flushed through the storm drain system and eventually into our rivers, lakes, and the ocean. With Caltrans maintaining approximately 15,000 miles of highway, 12,000 bridges and more than 230,000 acres of right-of-way, the potential for a serious pollution problem is great.
Trash strewn storm drain inlet.  

To address this issue, Caltrans has developed a Storm Water Management Plan (SWMP). The SWMP serves as a framework for meeting NPDES permit requirements for Caltrans stormwater discharge and guides all Caltrans activities related to stormwater control and treatment. As part of the SWMP, Caltrans has initiated a Storm Drain System Inventory (SDSI). The SDSI documents all stormwater features along the Caltrans right-of-way. In addition to the highway right-of-way, the project includes inventorying highway-related facilities, including maintenance stations, park-and-ride lots, rest areas, vista points, toll plazas and inspection stations. The completed inventory will give Caltrans engineers a clear vision of existing storm water features and allow them to identify sites where pollution prevention devices could be employed.


Project Overview

In order to accomplish this challenging task, Caltrans has enlisted the help of Burns & McDonnell to perform the inventory in both District 4 (San Francisco Bay Area) and District 12 (Orange County). Stormwater inlets, outfalls, discharge points, drainage areas, and facilities were inventoried using a combination of sub-meter GPS units, pen-based computers, laser range finders, and digital cameras.


Under the first phase of the NPDES stormwater permit, Caltrans was required to conduct the inventory in areas where the population surrounding the highway within a one-mile radius exceeded 100,000 people, as directed by the 1996 U.S. Census. These areas were classified as having municipal separate stormwater sewer systems (MS4) and considered to have the highest risk for violating the NPDES permit. Based upon this selection criteria, the SDSI encompassed 900 highway miles in the San Francisco Bay Area as well as 310 miles in Orange County (Southern California).
 
Map of project study area.
Features Inventoried

The SDSI project involved Burns & McDonnell inventorying all stormwater features found within the Caltrans right-of-way; these included inlets, outfalls, discharge points, tributary drainage areas (TDAs) and facilities.


Inlets
 

Inlets are typically defined as structures that convey stormwater from the surface into a drainage system to an outfall and/or discharge point. Since the inlet serves as the point of entry into the system, it is important to know how it is being impacted by the surroundings. An inlet can be flush to the surface, adjoined to a curb, elevated above grade or open culvert pipe. The type of surface the stormwater is draining from as well as what kinds of grates are being used will let Caltrans know where potential sources for contaminant entry may occur.

Example of storm drain inlet.  

Outfalls

Outfalls are typically defined as the end point of a pipe that daylights within the Caltrans right-of-way and then releases to a discharge point. There are several different types of pipe material that Caltrans uses including asphalt, corrugated metal, reinforced concrete, vitrified clay, and various plastic grades. Field crews inventory the type, size, and surrounding surface information to better characterize where stormwater is being channeled within the right-of-way. Additional information regarding the presence of dry flow, local vegetation, and accessibility will enable Caltrans to determine where best management practices (BMPs) should be implemented, if they do not already exist.

  Example of storm drain outfall.
Discharge Points
 

Discharge points are usually the end point of a pipe, culvert, channel, swale, or ditch that discharges from the right-of-way into a stormwater conveyance system, manholes, junction boxes or inlet that connects downstream to a storm sewer system. Since the stormwater leaving the right-of-way is Caltrans' responsibility under the NPDES permit, field crews collected as much information as possible about the point and surrounding area. Similar to outfalls, pipe type, size, site vegetation, site accessibility, visibility from the highway, and the presence of a best management practice information was inventoried. In addition to this, field crews collected data regarding where the stormwater is discharging to and if any illegal discharges were found in the area.

Example of storm drain system discharge point.  

In many cases discharge points were classified as MS4. An MS4 is a storm sewer that is owned and operated by the local municipal entity. With an MS4 connection, Caltrans is discharging their stormwater into the municipal system for conveyance to a receiving water body or treatment facility. Since the inventory was conducted mainly in these urbanized areas, many discharge points were classified simply as MS4 connections. In these cases field crews concluded that discharge points were underground piping leaving the right-of-way. This could be verified by the use of as-built drawings or by observing the piping directions in the discharging inlets.


TDA

A tributary drainage area (TDA) is typically considered the area that drains stormwater directly to the referenced structures. It is an area that is defined both by a legal boundary, the right-of-way, and a hydraulic boundary, which is some feature of the terrain that guides the flow of water. Once the inlets, outfalls, and a likely discharge point were collected, field crews were able to determine the extent of the drainage area. TDAs were drawn in PenMap based on field observations and as-built information where available. When delineating a boundary line, it was important to be aware of the surrounding slope of the land, the surface grade, and the characteristics concerning the discharge point.
  Example of TDA.

The right-of-way runs along the entire length of the highway, and was the primary factor when considering a TDA boundary. As the crews advanced along the right-of-way, they were cognizant of where TDA boundaries were linked to specific discharge points. Beginning from the discharge point, field crews traced back to all the inlets and outfalls that direct water to that point. The limit of the area that feeds into these structures forms the hydraulic boundaries of the TDA. TDAs were assumed to be polygons, and extended out to the right-of-way line on both sides of the highway. In most cases TDAs were linked to each other. The only exception to this was where the highway had no drainage conveyance system in place and no evidence of stormwater leaving the right-of-way, an occurrence known as sheet flow.


Field crews collected data about the land encompassed in the drainage area, including surface type, approximate average slope, and percentage that is pervious or exposed to infiltration of stormwater. Crews also looked for the presence of pump stations, as this indicates that stormwater is being channeled to specific places for treatment or discharge into a receiving waterbody.


Facilities

Under the NPDES stormwater permit, Caltrans was also responsible for the drainage in the facilities they operate and/or maintain. These areas are mainly comprised of maintenance stations, park & rides, weigh stations, vista points, and rest areas. There are hundreds of these facilities that are located along the highways, as well as neighboring cities. In an effort to control drainage, facilities can have inlets, outfalls, and discharge points. Field crews were required to collect all of these point features, delineate appropriate TDAs, and collect the perimeter of the facility. Field collected information was very similar to a TDA, including surface type and percentage of land that is pervious to stormwater. These facilities could have discharge points that were associated with the highway, but in most cases were self-contained or connected directly into the local municipal system.


Equipment Overview

Burns & McDonnell had to carefully develop a strategy to perform the data collection. Due to the complex nature of the data being collected, a computer system was needed that was portable, could be used in the field, and had the power and ability to integrate various pieces of equipment.

After reviewing several options, PenMap software running on Via field computers was chosen. PenMap is a real-time surveying, mapping, navigation and GIS data collection software that allows the user to view and edit maps and related data while in the field. PenMap gave Burns & McDonnell the ability to integrate all the data collection instruments, including laser range finders, GPS receivers, and digital cameras. Storm drain features were inventoried using a combination of Global Positioning System (GPS), pen-based field computer, laser range finder and digital cameras. The field computer combines a wearable PC with customizable software to provide field crews the full-function power of a laptop. PenMap allows the user to integrate the many types of data collection tools, and organize them into one information system.

System components used in field inventory.  

The GPS receiver unit provides real-time sub-meter accuracy, with GPS, WAAS, satellite differential, and beacon capabilities.The integration and flexibility of these technologiesmeans that there is less equipment to carry. The system provides two-way data flow between the field and GIS, facilitating the data collection and data maintenance processes. By taking advantage of these real-time features, the need for post-processing GPS data is removed, which further closes the gap between fieldwork and GIS. Digital cameras were used for the collection of field photographs. These photographs were then directly linked to the feature information being collected. A laser range finder (LRF) was used for distance measurement, as well as other surveying and mapping uses. The Class 1 eye safe LRF, with an integrated digital compass and inclinometer, accurately reported the distance, bearing and inclination to the selected feature. In previous inventory activities, these devices were cumbersome and very not transportable. This setup allowed for the integration of all the data collection tools into one portable field system.
  Field crew member performing inventory.

Field Collection Process
 

The advantage of using integrated system components is the ability to perform the field inventory within a single software environment. The PenMap software allows for the collection of real-time GPS data then permits the user through a series of customized forms to add additional attributes from a list of values. Taking advantage of the use of forms with pre-defined value selections builds data consistency and standardization.

The software’s flexible and highly customizable interface and forms allowed for the creation of specialized features and attribute forms needed for the stormwater inventory. The PenMap software’s import utility allowed for both raster and vector data to be imported and displayed, while the export utility permitted field collected data to be saved and exported directly into Esri Shapefile format.

Discharge point inventory form within PenMap software.  

GIS Processes
 

As the data was being collected in the field, it was periodically exported from PenMap into Esri shapefile format. From there it was fed into a comprehensive GIS-based quality assurance program. The customized program, which ran in Esri’s ArcView 3.2, assigned unique IDs to each collected feature and performed various data validation checks. The program analyzed the data on the individual TDA basis and treated the features within that TDA as a storm drain system within itself (inlets drain to outfalls, outfalls drain to discharge point, TDA encompasses the entire area).

After running through the QC process, data was organized and merged by highway number and feature type. As the inventory for each county within the Caltrans districts was completed, each feature was attributed with a series of overlays to provide additional fields of information (city, county, hydrologic unit/sub unit, soil type, etc.).

 
  Example of GIS-based QC program running in ArcView 3.2.
Pollution Prevention
 

The SDSI gives Caltrans a powerful tool in stormwater management. The completed project will allow Caltrans engineers to visualize the data not so much as individual features but instead see each TDA as a stormwater system itself. The inventory tracks the flow of water from where it enters a storm drain inlet to the receiving water body. The data will help Caltrans review existing storm features for adequacy and system maintenance as well as serve as a helpful tool for planning highway construction. The delineated TDAs will allow engineers to assign runoff coefficients and compute volume calculations, while the discharge points will show where stormwater leaves the Caltrans right-of-way.

 
Example of Bio Swale Best Management Practice.  

Once the question of ‘where’ has been answered, the task of pollution prevention can be addressed. As part of SWMP, Caltrans is currently attempting to identify discharge points and outfalls along the highways where BMPs can be utilized. A BMP is a technique, measure or structure used to control the quantity and improve the quality of storm water runoff. Specially designed filtration devices, retention ponds, and bioretention areas are all examples of BMPs that serve to reduce the amount of contaminants leaving the Caltrans right-of-way and entering our waterways. The continued utilization of BMP technology is an important component of Caltrans’ comprehensive effort to preserve and improve water quality in California.

Summary

Since the Clean Water Act was enacted in 1972, the quality of America’s waters has steadily improved. The cooperative effort of federal, state, and local governments to implement pollution control programs has gone a long way towards preserving healthy water quality. Stormwater system inventories, such as the one described, that harness the power and flexibility of today’s spatial and information technologies provide a powerful tool in the management and treatment of stormwater runoff and the pollutants it carries.

References
 
Storm Water Management Program.2001. Storm Water Management Plan. California Department of Transportation. Sacramento, CA.

Office of Wastewater Management.2002. Water Permitting 101. Office of Water. United States Environmental Protection Agency. Washington D.C.

Environmental Program.2002.Storm Drain System Inventory User’s Manual. California Department of Transportation, Sacramento, CA.