Ellicott City in Howard County, Maryland experienced flood-of-record events in 2016 and 2018, resulting in significant damage to the community and loss of life. Subsequently, Howard County has approved a tunnel project to redirect surface flows underground to reduce flooding potential in the historic downtown area. As part of the design process, a physical hydraulic model study was commissioned to evaluate stormwater diversion and drop structure designs integral to the project. Two 1:12 reduced-scale physical models were constructed evaluate the system design. The models included interception chambers designed to convey 500-yr storm flows, a 17-foot diameter plunge-type drop shaft, a rectangular drop shaft with two inlets, deaeration chambers, a 40-foot construction shaft, an adit, and about 500 feet of 18-foot diameter conveyance tunnel. The models were used to evaluate conveyance capacity of the drop shafts and tunnel, overall hydraulic characteristics, deaeration chamber performance, and tunnel flow features over a wide range of flow scenarios. The model test program resulted in improved deaeration chamber designs, properly sized drop shafts, and simplification of several major components. The changes improved the overall performance of the system and resulted in significant construction cost savings for the project.
