Located in an urban setting between a large retail complex and a large office building, the project involved both a pump-around and a temporary stream diversion to accommodate the high flows of Noonday Creek. Additionally, a 21’ tall earthen dam and associated overtopping structure were constructed while having to work around an existing 36" sanitary sewer line.
In order to regulate flow, reduce downstream channel velocities and pass the design flow of Noonday Creek, the following key components needed to be included in the design: principal outlet structure design, dam overtopping protection, channel protection, bridge pier protection and velocity dissipation.
For this public bid project, contractors were allowed to submit designs based upon flow, velocity and design year water surface elevations shown on the plans. The chosen solution utilized 14,934 units of 24-in A-Jacks® and 4,036 units of 48-in A-Jacks. A-Jacks are concrete armor units designed to interlock into a flexible, highly permeable matrix. The ability of the A-Jacks system to dissipate energy and resist the erosive forces of flowing water allows it to protect against scour and erosion. In order to manage the design flow and velocity of stormwater before its downstream release between the two bridge piers of the 4-lane, 450 foot long Chastain Meadows Parkway Bridge, A-Jacks were utilized along a primary bend of the creek, in the stilling basin and in between the two aforementioned bridge piers.
In addition to A-Jacks, various sizes of ArmorFlex® mats were installed — 12,024 square feet of CL 40 Mats, 54,078 square feet of CL 60T Mats and 14,036 square feet of 85L Mats. Florence & Hutcheson, the engineer on the project, selected ArmorFlex due to their experience in using them on past earthen dam projects.
ArmorFlex is an interlocking articulated concrete block system with individualized units of uniform shape, size and weight used for hard armor erosion control. The blocks have specific tested hydraulic capacities and are laced longitudinally with cables to provide ease-of-handling and rapid installation. Rip-rap was considered as an alternative solution but, overall, the cost of the A-Jacks and ArmorFlex provided more cost savings for the owner, Cobb County Water System.