Flow-Duration Hydrograph Analyses for Assessing LID Performance

The sizing of stormwater management facilities across North America largely relies on event-based hydrologic modeling methods, which target specific rainfall characteristics, namely:

• Peak rainfall intensities expressed according to a return period (e.g., X-year storm event), generating peak flows that affect the conveyance capacity of collection systems (e.g., pipes, ditches, and culverts); and
• Total rainfall volumes expressed according to a unit-area depth (e.g., Y-inch storm event), generating runoff volumes that affect the storage capacity of detention facilities (e.g., ponds and LID facilities).

A systems approach to stormwater management requires consideration of both the rainfall stimulus as well as the runoff response, as indicated by impacts to the receiving watercourses and waterbodies. Further, continuous simulation offers a greater diagnostic tool for assessing impacts, since it can describe the full range of runoff response characteristics, compared to a limited snapshot view using design storm events. Even though a stormwater facility may achieve its design objectives for flood control and quality treatment, there are a number of downstream impacts that need to be considered, including:

• Geomorphology (i.e., streamflows and sediment loads that shape the watercourse and affect bank stability);
• Hydrogeology (i.e., baseflows, seepage, and groundwater recharge);
• In-stream water chemistry (i.e., sediment circulation, nutrient cycling, temperature regulation); and
• Aquatic and terrestrial resources (i.e., fisheries, benthic habitat, wildlife, vegetation, and soils that are affected by in-stream velocities, water levels, and inundation periods as well as upland soilwater content).

An analysis of continuous simulation results can be used to quantify downstream impacts, particularly through identifying exceedance/deficit durations with respect to threshold values of flow hydrograph ordinates (e.g., flowrate, depth, or velocity). In California, a regulatory permitting process has been adopted for identifying potential hydrograph modification (known as “hydromodification”) based on flow-duration curves that indicate geomorphologic changes due to development.

This paper presents a new methodology that extends California’s hydromodification criteria for flow-duration exceedances to encompass a broader range of downstream impacts, and is demonstrated for two case studies that illustrate LID facilities applied under vastly different site conditions, including:

• Construction of a new infiltration gallery on a site where soils are conducive to infiltration; and
• Construction of a rainwater harvesting system on a site where soils do not allow infiltration.