Stormwater Quality Design Manual (May 2007) - City of Sacramento ...

Runoff Reduction Control Measure Fact SheetInterceptor TreesHow do interceptor trees protect water quality?Interceptor trees are ideal for all projects, including those where space is limited, in which trees can beplaced along street frontages and in common space. Urban areas with higher numbers of trees exhibithydrology more similar to natural conditions compared to urban areas without a tree canopy. Treesintercept storm water and retain a significant volume of the captured water on their leaves and branchesallowing for evaporation and providing runoff reduction benefits. For example, a large oak tree canintercept and retain more than 500 to 1,000 gallons of rainfall in a given year (Cappiella, 2004). While themost effective Interceptor Trees are large canopied evergreen trees, deciduous trees can also provide abenefit. For example, a leafless Bradford pear will retain more than one half the amount of precipitationintercepted by an evergreen cork oak (Xiao et al., 2000).The shade provided by trees keeps the ground under the trees cooler, thereby reducing the amount of heatgained in runoff that flows over the surface under the trees. This attenuation of heat in storm water helpscontrol increases in stream temperatures. On slopes, tree roots hold soil in place and prevent erosion.Planning and Siting ConsiderationsCheck with the local permitting agency about requirements for trees located in public utility easements.A tree permit may be required to plant, prune or remove such trees. Also, consultation with an arborist isrecommended for selecting and locating appropriate tree species for the unique site conditions.New trees• Select trees from a list of approved species established by the permitting agency (see Table INT-1for examples, but check with appropriate agency for verification). Native species and those with alarger canopy at maturity are generally preferred, depending on available space for root andcanopy.• Select tree species based on the soils found on the site, available water, and aesthetics. Soil inplanter areas may be amended to satisfy species requirements. Consult a landscape architect orarborist to ensure suitability of species for site conditions and design intent.• Do not plant monocultures of same family, genus and/or cultivar. Do not plant trees too closetogether.• Interceptor trees should be incorporated into the site’s general landscaping plan, but treesdesignated for storm water credits must be clearly labeled on plans submitted for local agencyapproval and other planning submittals.• Do not place trees near structures that may be damaged by the growing root system. Theseinclude, but are not limited to, overhead utilities and lighting, underground utilities, signage,septic systems, curb/gutter and sidewalks, paved surfaces, building foundations and existing trees.Utilize approved root barriers (deflectors) when trees are planted close to infrastructure, per thelocal permitting agency standards.Existing trees• New landscaping under existing trees must be carefully planned to avoid any grade changes andany excess moisture in trunk area, depending on tree species. Existing plants which arecompatible as to irrigation requirements and which compliment the trees as to color, texture andform are to be saved.• Grade changes greater than six inches within the critical root zone should be avoided. Also, soilcompaction and texture in the drip-line area greatly affect tree survival.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsINT-2 May 2007