Quantity control performance metrics for stormwater management facilities

There are multiple design objectives for stormwater management facilities, including:

• Hazard protection, which is intended to manage peak flows and velocities for the purpose of protecting people and property from flooding/erosion hazards;
• Quality treatment, which manages sediment, pollution, and temperature to protect public health, habitats, and aquatic/terrestrial resources;
• Volume reduction, which manages stormwater in a way that mimics pre-development conditions to preserve the natural environment; and
• Environmental flow maintenance, which manages the intensity, duration, and frequency of flows over a wide operating range.

As stormwater management has evolved, hazard protection remains the highest priority from a risk management perspective and facility design methodologies continue to be used that achieve a specified level of protection in response to a rare occurrence of rainfall (e.g., a design storm event with fixed duration and 100-year return period intensity). For all other design objectives, event-based methods are not appropriate; continuous hydrologic simulation must be used for analysis and design (e.g., a rainfall record comprising many years of storm events with a wide range of durations and intensities). Likewise, financial incentives for implementing stormwater management facilities (e.g., with stormwater user fee credits) are also biased towards event-based methods, which can limit the return on investment. Typical credit criteria for quantity control include the peak runoff rate and total volume as performance metrics. However, in order to properly plan, build, and reward facilities that address the full suite of design objectives, a wide range of flow conditions must be considered, which is only possible with continuous simulation. 

This paper presents quantity control metrics that incorporate continuous simulation results and can be used to assess facility performance by comparing the relative position between two baselines: natural/undisturbed conditions (i.e., the desired upper baseline that characterizes runoff from a site that has not been previously developed) and existing/uncontrolled conditions (i.e., the undesired lower baseline that characterizes runoff from a previously developed site without any stormwater management controls). For example, if the runoff response from a proposed facility matches or exceeds natural conditions, then 100% stormwater management control is implied, and if the response matches or falls below existing conditions, then 0% control is implied. A methodology using the EPA SWMM model is presented for quantifying various levels of quantity control related to flow duration, flow frequency, and flow volume and this is demonstrated for a variety of stormwater management facilities.