GENERAL STORMWATER REQUIREMENTS
All projects must treat and manage all stormwater on site using green stormwater infrastructure where feasible and meet the hydrologic performance standards shown in Table 5-6 as determined by a qualified professional.
Table 5-6 Stormwater Rate and Volume Management Targets
Downstream Context
All projects must review the downstream receiving waters that the project drains to and determine 1) any known water quality impairments or pollutants of concern and 2) the context (i.e., combined sewer, marine waters, wetland, other) shown in Table 5-6. The project must then implement a stormwater management plan that meets the hydrologic performance standard and does not contribute to the identified water quality impairments or pollutants of concern. See additional requirements under General Stormwater Requirements and Water Quality below in these Stormwater Clarifications.
The downstream receiving water is the water body that the municipal conveyance network drains to or the first body of water that the watershed of the site drains to.
Wetlands
The project must comply with the requirements indicated for the wetland downstream context in Table 5-6 if a category 1 or 2 depressional wetland is located downstream of the project and the Project Area is greater than 5% of the tributary basin of the wetland. If the Project Area constitutes 5% or less than the tributary basin of the wetland, its downstream context is the next receiving body downstream of the wetland.
Protection of many wetland functions and values depends on maintaining the wetland’s hydroperiod. This means maintaining the annual fluctuations in water depth and duration as closely as possible. A hydroperiod is the seasonal occurrence of flooding and soil saturation experienced by a wetland and includes the depth, frequency, duration, and seasonal patterns of inundation and saturation.
Marine
The Marine context applies to sites in coastal watersheds that drain to a marine water body either directly or via a separated (not combined) storm drain system with marine outfall.
Historic and Pre-development Hydrology
The historic hydrology of a site reflects its condition in an undeveloped state. The historic discharge rate is based on the original ecological land cover conditions (i.e., forested, grassland, etc.), which, for purposes of this Imperative, may be represented by the most comparable land cover options available within the stormwater hydrologic model. Pre-development hydrology has the same meaning for purposes of this Imperative.
Modeling
Peak Discharge Rate Control
To show compliance with the peak runoff rate control requirements, project teams must use a rainfall-runoff calculation tool or hydrologic model that, at a minimum, accounts for rainfall intensity based on local rainfall data, drainage areas, runoff coefficients from land use types within the Project Area, and times of concentration. Local rainfall data must reflect, at a minimum, the 24-hour precipitation depths for the two-year, 10-year, 25-year, and 100-year recurrence intervals.
The modeling must produce:
- Peak discharge data for the two-, 10-, 25-, and 100-year storm events; and
- A unit hydrograph showing the rate of flow over time, in order to calculate the volume of runoff for each event.
Calculations must be provided demonstrating the peak discharge is managed to the level specified in Table 5-6.
The Santa Barbara Urban Hydrograph Method meets these model requirements, but more complex models may also be used to demonstrate compliance. For all models used, the data sources must be cited and all variables identified.
Volume Management
For contexts with a stormwater volume management requirement, project teams must use a continuous simulation model, such as either the Hydrological Simulation Program – FORTRAN or Storm Water Management Model SWMM, or a spreadsheet-based mass balance model that accurately simulates soil moisture, surface runoff, interflow (water that returns laterally to the surface), base flow (water that continues to flow between precipitation events), and other watershed processes using a precipitation record of sufficient time length (at least 40 years) to accurately calculate average annual stormwater volumes for the site.
Scaling for Climate Change Resiliency
To account for climate change, the design precipitation depths shall be increased by a minimum of 15% or as recommended for expected changes in peak rates in the year 2100 in a regional analysis by a governmental agency or research organization with expertise in the hydrologic effects of climate change. See Resources for suggested references within the US.
Stormwater Retention
The stormwater volume that falls as precipitation on the site and either is reused for potable or non-potable uses, infiltrates into the ground, or leaves the site through evapotranspiration but does not leave the site as piped or overland flow will be considered retained. Stormwater held in a pond or vault and then leaving at a slower rate is detained and not retained.
INTERIOR TYPOLOGIES
Projects that are registered under ILFI’s Interior Typology are exempt from the listed stormwater requirements, as they are out of scope.
MUNICIPAL STORMWATER CONNECTION
Flows leaving the site in compliance with the thresholds specified in Table 5-6 5-6} may go through a municipal stormwater connection if all of the following conditions are met:
- The water is deposited in the same water body where it would have flowed naturally based on pre-development hydrology;
- The water is treated and managed according to the levels shown in Table 5-7 5-7};
- Any pumping energy is accounted for in the energy budget of the project; and
- The municipal stormwater system is not combined with the municipal sewer system.
QUALIFIED PROFESSIONAL
The involvement at a fundamental level of personnel with knowledge and practical experience in stormwater management design, hydraulic and hydrologic modeling, and watershed ecology will be a significant factor in the success of the stormwater management effort. Training and expertise in environmental engineering, stormwater management, landscape architecture, or comparable is necessary for evaluating the pre-development stormwater models, designing and implementing the stormwater control measures, and monitoring the runoff water quality during the performance period.
WATER QUALITY
The project team must demonstrate that stormwater leaving the site (via overland flow or piped flow) in compliance with requirements in Table 5-6 5-6} is treated to support downstream habitat using green infrastructure and meets the standards shown in Table 5-7. If green infrastructure is not feasible, teams may propose alternative solutions, which must be preapproved through a Request for Ruling.
Table 5-7 Stormwater Treatment Requirements
Project teams may demonstrate appropriate treatment via providing technical specifications guaranteeing the reductions outlined in Table 5-7, or they may provide test results from the stormwater runoff demonstrating the required reductions.
There are a variety of stormwater control measures that project teams may consider implementing. Most also provide some level of stormwater treatment (see Table 5-8). Project teams can use these strategies individually or in combination to meet the Imperative requirements.
Table 5-8 Stormwater Control Measures – a Partial List
