Regulations for the project required a New Hampshire Alteration of Terrain permit, which is typical for large developments in New Hampshire. The town of Salem also has a 50-year storm freeboard requirement for detention systems, requiring detention basins to be somewhat larger than the calculated volume.
The consulting engineer, Tighe & Bond, initially looked at a concrete solution for volume control, but it was far too expensive once budget pricing came in. They also investigated other solutions such as large plastic chambers for storage and found that corrugated metal pipe was more economical for the volume being stored. Using a perforated pipe system with a liner and stone between the barrels could reduce the footprint even further. In addition, CDS hydrodynamic separators were utilized upstream as pretreatment to the detention system. While looking at approved technologies for quality control, Tighe & Bond determined that Contech’s Jellyfish Filter was the best solution for treating this large volume and flow. Contech was able to assist with the sizing, hydraulic analysis, and fine-tuning of the complete treatment train.
Three perforated underground stormwater detention systems made from 1,986 LF of 54”, 1,358 LF of 84” and, 831 LF of 84” Aluminized Steel Type 2, 4 CDS hydrodynamic separation systems, and 5 Jellyfish Filter systems were installed. Contech worked closely with the site contractor and held pre-construction meetings to ensure an understanding of the products and best practices to install them efficiently. The local Contech Sales Engineer made frequent visits to the site to answer questions that might arise.
Contech delivered the first detention system in February 2020, during a mild winter, and installation commenced at the beginning of March. Installation of both systems went smoothly and systematically. The installation team was familiar with installing corrugated metal pipe detention systems, so they were off and running after the pre-construction meeting. The complete treatment train was fully installed by the end of April 2020.