A Test Case for Improving a Highly Urbanized Watershed

July 22, 2014

Dig just below what appear to be conventional street improvements at the intersection of 43rd Street and Logan Avenue in San Diego, and you will discover new and not-so-conventional stormwater management infrastructure. Dig a little deeper still, and you will learn that this new site is part of the city of San Diego’s rapidly increasing investment in managing what have become ever more demanding and ever more costly regulatory requirements.

The 43rd and Logan intersection sits in a heavily urbanized segment of the Chollas (pronounced choy-as) Creek watershed. Chollas has been singled out by regional regulators for a stringent series of total maximum daily load (TMDL) limits targeting reductions in heavy metals and elimination of diazinon–a sad remnant of an era when the casual overuse of pesticides was common.

Though the Chollas TMDLs were among the first applied in San Diego, other local waterways are under similar regulatory controls. Most of the TMDLs have timelines for success ticking down from 20 years following their adoption. The city had been working to craft an accelerated response to the regulations when negotiations for a new comprehensive discharge permit got underway.

Modifying the filter system to take advantage of the design improvements required altering the existing concrete structures and installing Rainstore3 units.

The second approach was much more experimental. Seeking a design that would fit within the 5-foot-wide parkway landscaping strip, without changing the look and function of this space, the solution took the form of a series of filtration vaults built below pavers set at sidewalk grade. This preserved the pedestrian use of the space and allowed room for other improvements below and above grade, including street trees and a bus stop. Getting the system to perform optimally required a sequence of offsite experiments.

Offsite Testing of Pretreatment Mechanisms
To increase the life of the filtration media and to prevent premature clogging, it was necessary to include a mechanism for pretreatment that would remove larger sediment and debris from the runoff before routing the flow to the media filters. The first design used a series of shallow bays to trap material, with easy access under steel lids to vacuum or shovel accumulated debris on a regular basis. Through the first rainy season it was found that this design clogged too readily. There was a need to modify the system with a more robust treatment mechanism and verify that it would work. Because it is expensive and time consuming to experiment in the public street right of way, the city chose to perform full-scale experimentation at an offsite facility.

Working with its lead consultant, the city was able to establish a temporary outdoor testing laboratory at a university. Through literature review and brainstorming exercise, a number of alternate design concepts were developed. A methodical process was used to narrow down the list to five concepts that would be physically tested as full-scale mockups. Synthesized storm runoff allowed simulating the effects of half of a rainy season in just a few days. A concept could be run to point of failure and then, with a quick change of parts, the same test could be applied to the next concept. Examining the mode of failure of one concept informed the research team on what changes to try for the next test.

Careful thought went into specifying a debris mix that would best simulate actual conditions. This involved literally coming up with quantities of cigarette butts, leaf litter, and every other imaginable debris component to be dosed into the test water. Published studies provided a basis for the initial “design mix,” which was then refined using debris quantification from local street sweeping and catch basin cleaning studies.

The outcome of the offsite experimentation was a final design concept that performed reliably under simulated conditions. The simulation tests showed that the system provides effective pretreatment and should require no more than two maintenance cycles per year. The next test will be under actual rain conditions at the site, which will begin next rainy season.

Column Experiments to Test Media Filter Designs
Modifications to the media filter design were also explored, taking advantage of the most recent advances in technology. A performance concern with the original filter design was the slow rate of filtration. This limits the capture of storm runoff events that have a very narrow peak–a common hydrograph shape for the region. A scan through the most recent literature found a study of optimized filtration media mixes for rapid flow systems. This University of Alabama study had percolated simulated storm runoff through 4-inch-diameter columns of different combinations of filtration materials ranging from granular activated carbon to surface modified zeolite, and compared their effectiveness in removing dissolved metals and other pollutants.

The study findings were exciting to the city’s research team. It was possible to get effective removal of dissolved metals at very high treatment rates using a depth of media as small as 14 inches–a discovery that could be applied toward improving the filter design for the 43rd and Logan site. Again, before investing in costly modifications at the actual test site, the city chose to run simulation tests at a local university laboratory. The environmental engineering laboratory at San Diego State University ran a series of 4-inch-diameter column tests using the exact configuration of existing and proposed filter designs, and using suspended sediment concentrations that mirrored that of the effluent quality from the pretreatment vault experiments. This allowed another simulation of long-term performance, in this case determining how many hours of flow the filter can process before requiring maintenance.

The modified design was found to treat more than four times the flow rate of the existing design. It also maintained a high flow rate after many hours of flow. This translated into less frequent media replacement. Because of the labor involved in removing pavers to perform filter maintenance, a maintenance cycle of three or more years is desirable for this type of system to be considered cost effective. The new media mix takes advantage the earlier optimization study, using a four-component mix of sand, granular activated carbon, peat, and zeolite.

Photos show the sequence of construction for the filtration units. The 10-foot-long units can be grouped or separated to create space for street trees and utilities.

Tackling the Practical Issues
Some of the most valuable lessons to be gained from a pilot installation are learning how to get through all of the practical matters of site constraints and project approvals. Without taking a few test cases through the process, it is unknown whether ideas on paper can be successfully built. Determining early on what is practical for the city to build and maintain is a key element of the pilot study program. A typical street has limited real estate and multiple demands, utilizing nearly every square foot of available space above grade and a considerable amount of the space below grade. There is also a need for street infrastructure to be extremely reliable and low maintenance. San Diego has about 2,800 miles of streets, open to the public at all times. And, again like most municipal agencies, San Diego has a lean staff of personnel able to respond to the maintenance and repair needs of the street network. That is why infrastructure design for within the street right of way is highly standardized, and getting approval for a nonstandard design requires communicating with multiple entities.

One major concern of the city’s engineering and operations staff was the potential risk of water infiltrating soils around other infrastructure. The soils around the project have low permeability, so any attempt to encourage downward percolation of water could result in water finding a preferential pathway to locations near the project where soil saturation would be undesirable, such as under street subgrades or around utilities. It was decided that both designs would be enclosed by an impervious lining to avoid this risk. In future installations, the same design concept could be applied without the lining if site-specific soil conditions allow it.

Another concern was the competition for space with underground utilities. This was addressed with careful space planning, but it remains a challenge for all future installations. Local codes specify separation distances for different utilities. The media filters competed for space with conduits and vaults for underground electrical and communication utilities. These were pushed to the edge of the road because of the required separation distances needed for water and sewer utilities. In the final configuration, these infrastructure types were nestled within the space like tight-fitting puzzle pieces.

By compartmentalizing the media filter into 5-by-10-foot units, it was possible to take a modular approach to arranging these boxes in among other competing uses of underground space. Shifting the spacing created room for street trees, street lamp foundations, and utility service connections. With street grades ranging from 3% to 6%, it was feasible to distribute untreated water and collect treated water across multiple units through a series of weirs and connecting pipes. The variation in street grades across the city will be one challenge to overcome in replicating the design at other street locations.

Modifying the design to take advantage of the column-tested performance enhancements required being able to loosely loft in the filtration media and also to provide more than a foot of free space above the media bed. Research into available products found the Rainstore3 system to provide a suitable solution. By placing these inside the filter units, the Rainstore3 could support the load of the paver bed, creating the necessary non-load-bearing void space. If the concept proves to work well, the city will explore further modification to integrate the filtration media into the Rainstore3 system so that the entire block can lifted as a unit when it is time to replace the filtration media.

It was a much easier process designing the bioretention pond for approvals. Using the available space outside the street right of way eliminated the challenges of crowded utilities and compatibility with pedestrian use. It also provided enough square footage to be able to treat sizeable storm flows with a very modest percolation rate. Because of the ability to maintain the surface, a separate pretreatment device was not needed. The design uses many of the city’s standard designs with slight modifications, thus having the advantage of familiarity to engineering and operations personnel. However, making it attractive to the community required considerable investment in landscaping, and the costs for ongoing maintenance of the landscaping offset some of the benefits.

At other pilot study locations, the city is experimenting with a bioretention pond design within the parkway landscaping strip. This provides easier access for maintenance than is the case for the media filters placed below pavers, but it has the disadvantage of interrupting the pedestrian use of the parkway space. One of the biggest concerns raised by members of the public concerned the depressed bioretention surface between the sidewalk and the parked cars. Through the continued study of various design options, the city will be able to determine which designs provide the best tradeoffs among costs, effectiveness, and community acceptance.

The pilot studies focus heavily on science and technology, but an interesting discovery is the importance of understanding what people want in their communities. This has an influence on the ability to fund maintenance. As the city becomes more densely urbanized, more value is being placed on the role of streetscapes as functional and aesthetic public spaces. There was strong community desire for a beautified 43rd and Logan streetscape, but the maintenance to support it could be established only through a patchwork of agreements between the city and other entities. Looking further into more unified mechanisms for keeping these spaces looking nice could help the program in the future.

Continuing the Study With the Help of a Grant
The city was recently approved for a grant from the California Water Resources Control Board, which will provide enough funds for a thorough assessment of the pollutant removal performance. A detailed study plan has been developed for two rainy seasons of monitoring starting in the fall of this year. Flow rate measurements and water-quality samples will be collected from the inlet and outlet of the BMPs for eight storm events, providing enough samples to establish statistical rigor. Pre-project sampling had found the street runoff at that location to contain significant concentrations of copper and zinc, two of the metals identified in the TMDL for Chollas Creek. Filtration through the bioretention soil and the filtration media is expected to reduce copper and zinc concentrations, but there is uncertainty as to how much. Once the load reduction performance is quantified, the city can refine estimates of how many treatment control BMPs such as this are needed to meet the load reduction mandate.

The monitoring will also assess performance for a suite of other pollutants. The other watersheds in the city’s jurisdiction each have unique pollution concerns. The test results will determine if these two solutions will be effective at meeting targets for other watersheds. In addition to dissolved metals, the pre-project monitoring for this site also found the runoff to contain significant concentrations of bacteria and organophosphates. The source of these two pollutants is uncertain, but the streets receive runoff from surrounding residences, which may have pets and may be using ant control pesticides. The study will focus on treatment control effectiveness alone, but other studies are looking at the effectiveness of source controls.

Appearance, function, and ease of maintenance are all important considerations.

A quality, statistically robust study such as this is expensive. It is questionable whether the study would have moved forward without the help of the grant. Grants to study the effectiveness of stormwater practices can have a broader geographic benefit than just funding brick-and-mortar projects–for instance, encouraging other cities to use this test design if it proves to be effective. In this case, the city was willing to put up the brick-and-mortar funds, thus providing a study site and reducing project risk for the granting agency. This delicately carved agreement should yield benefits for both agencies.

The Road Ahead
Ideally, new design concepts such as the ones featured here would be tested for 20 years or more before investment is made in broader implementation, but regulatory timelines don’t allow that. Concurrent with testing and evaluating the featured project, additional capital improvement projects are coming online to support additional pilot testing and to make a dent in the required load reductions. The fiscal ability to support the growing program is becoming a real concern. The city of San Diego has brought the concern to the forefront of discussion by issuing a comprehensive Watershed Asset Management Plan, which estimates future costs of both regulatory compliance and performing necessary maintenance and upgrades to drainage and flood control infrastructure.

The city funds the majority of its stormwater pollution prevention and flood risk mitigation efforts with general fund money. This is a limited and competitive fund, and the compliance and maintenance needs outlined above far outstrip the fund’s ability to meet the stormwater system needs. Consequently, the city is being forced to explore options to increase its existing and relatively small storm drain fee.

The city is also exploring improved bond funding mechanisms for capital improvement projects including stormwater system upgrades and looking at ways to create greater standardization of designs for stormwater treatment infrastructure.

Finally, the city is working with the local development community to implement the new permit’s requirement for redevelopment projects to upgrade their onsite treatment controls as a means of reducing pollution from existing neighborhoods. The city is exploring allowances for offsite improvements to be provided in lieu of meeting some of the onsite requirements. This option could be used to fund green street projects and other pollutant controls in the watershed, although there could be significant administrative costs in managing such a program.

These time-sequence photos show the simulation test to determine how a pretreatment system design will respond to debris-laden runoff. The simulated storm runoff is entering from the top of each image.

With the current numeric load reduction mandates, and additional TMDL requirements anticipated for the foreseeable future, the city is exploring a broad range of treatment control practices and other nonstructural solutions to its water-quality improvement needs.

The city of San Diego is responding to the daunting load reduction mandates by breaking up the process into a series of manageable pieces. There remains much to learn from the 43rd and Logan project and other test cases throughout San Diego, but a process of exploration, discovery, and program refinement is already taking shape. Gradually digging deeper with each pilot installation, each study outcome, and each recalibration of projected costs yields more precise answers to what investments the city will need to make in upcoming years. Discovering a process that works is good news for San Diego and for any municipal agency challenged by more stringent stormwater regulations. 

About the Author

Jim Nabong

Jim Nabong, P.E., is an associate civil engineer with the city of San Diego Transportation and Storm Water Department.

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