Active Hydromodification Control

The state of the practice for hydromodification management for new development is to mimic long-term predevelopment site hydrology. The theory is that if the predevelopment distribution of in-stream flows is maintained, then the baseline capacity to transport sediment, a proxy for the geomorphic condition, will be maintained as well. A popular method of mimicking the predevelopment flow regime is via flow duration control (FDC), which can be achieved by routing postdevelopment runoff through structural stormwater BMPs such that runoff is stored and slowly released to match predevelopment flow rates and durations. Storage requirements for FDC tend to be much larger than for surface water treatment, particularly when the BMPs are small, distributed LID facilities with simple outlet structures. This paper describes a recently invented (patent pending) methodology and programming logic for active stormwater controls to optimize sizing and design of hydromodification control BMPs to achieve FDC (also known as active hydromodification control) and presents one case study to demonstrate benefits. An actively controlled BMP outlet consists of a modulating valve, orifice, or pump in place of a passive stagnant orifice. With the use of active stormwater controls and the methodology described, benefits are three-fold. First, new BMPs can be optimized to be smaller and, thus, more feasible to implement. Second, existing stormwater facilities designed for flood control or other management objectives can be retrofitted to provide hydromodification control. Third, when utilized with real-time flow and water-level monitoring equipment and data, the flow release logic can be adaptively adjusted based on calibrated data without physical retrofit of the BMP's outlet.