A new series from Jensen Precast and its Water Resources Division, Ask an Engineer touches upon complicated topics and queries that have been asked of our Engineering Team members. Much like a Frequently Asked Questions (FAQ) section, Ask an Engineer will address questions that our Engineers face more often than other inquiries. This Ask an Engineer session is with our Stormwater Systems Division Director, Walt Stein P.E., who answers this common question about Stormwater Treatment Units:

How do you keep a stormwater full capture device from filling up?
Watch this video to hear Walt’s question, or read below for his answer in blog format.

Often times Walt Stein, P.E., Stormwater Division Manager for Jensen Water Resources, is asked questions very similar to the one he was faced with recently:

How do you keep a stormwater full capture device from filling up?

Stein’s simple answer: “You can’t.”

Stein’s detailed explanation: You cannot predict what the land use activity is, nor the generation amount of trash and debris of a given catch in an area. In addition, you can’t predict how Mother Nature turns on the pump that may pick up and mobilize the larger particles that make up the trash and debris in the watershed.

In order to establish better control over what is considered “Full Capture,” Stein says Regional Boards have specified a more intense half-inch per hour intensity storm event, as ‘the’ storm event that picks up and mobilizes the gross solids, trash and debris. Regional Boards then will give a typical ‘rough definition’ that measures up to defining that any device picking up anything bigger than 5mm during a more intense storm event is a successful device.

Stein says, “What you have is a variety of trash and debris, ranging from neutrally buoyant to settleable materials to floatable materials, and who knows how much on a given watershed, how much of that material will be out there. Certainly in some land use activities you’re going to have a huge amount of gross solids; Mother Nature is going to turn on the pump and mobilize the materials.”

Stein continues, proof in Full Capture can be shown particularly down in the Los Angeles area, where 80% of the gross solids are picked up in the first 3 hours of the first storm of the wet season and then not so much at the end of that wet season. He said, “In California we have a long dry period, and you get those first couple storm events. If they’re powerful enough they’ll pick up and mobilize possibly as much as 60-80% of the trash and debris that’s been collecting and sitting out on that watershed during the dry season.”

Enter the Jensen Deflective Separator (JDS). Stein says, “What you want to do is have a design like the JDS system that retains what it captures, so that when it does fill up, it’s not burping the floatables. The JDS isn’t releasing the previously captured solids in its sump…the design aspects of no release of previously captured material is important so that you garnish to yourself credit for capturing all those gross solids.” A win-win!

Stein concludes that while there are plenty of designs out in the marketplace that actually store the material in the treatment flow path until a higher-intense storm event picks up and mobilizes previously captured material, there are no devices that burp the large debris. The JDS system, however, covers this activity in its inherent design. Which is why the New Jersey Corporation of Advanced Technology (NJCAT) recently verified the JDS system as a Total Suspended Solids full capture device. This NJCAT Verification is one of the most rigorous examinations for Stormwater treatment technologies in North America that includes in depth peer review.

This NJCAT Verification follows the recent certification of the JDS unit’s 100 % Full Capture trash and debris from the California Water Resources Control Board.  The JDS unit was the only Certified stormwater treatment to conduct full scale trash capture tests and verify 100% retention of previously capture trash and debris.  Simply put the JDS unit will not scour out or release previously captured TSS or any trash or debris.