A Fresh Look at the Hidden Opportunities in Urban Runoff

version 1.0

by Owen E. Dell


berm A low, often linear or curvilinear, mound of soil used to contain or direct runoff water.

biofiltration The use of natural systems or biologically-based artificial systems to remove pollutants from runoff water.

biomimicry “Biomimicry is a new science that studies nature’s models and then imitates or takes inspiration from these designs and processes to solve human problems, e.g., a solar cell inspired by a leaf.” Janine Bevins, Biomimicry: Innovations Inspired by Nature (1997) (

bioswale A vegetated drainage channel which accepts, absorbs and treats runoff water, graywater or effluent water, using natural biological systems and processes.

carbon sequestration The storage of carbon in the tissue of living plants.

cistern A holding tank for water, often harvested rainwater.

continuous trench rootzone enhancement A method of providing an adequate root environment for urban street trees by filling a linear planting pit running from tree to tree with an engineered soil mix.

cut curbing A variation on conventional concrete curb-and-gutter systems that penetrates the curb at intervals along its length to allow some of the runoff water to soak into adjacent soil rather than draining into a storm drain.

decomposed granite The product of the weathering of granitic parent rock material, decomposed granite is a granular, compactable soil type that is commonly used for paths and other paved surfaces. Decomposed granite (“d.g.”) is much more permeable than conventional paving materials and can usually be installed at a much lower cost.

dragontooth curbing A variation of cut curbing that alternates short sections of concrete curbing with short sections of open voids that allow water to drain into an adjacent planter bed or other open soil area.

dry streambed An unlined continuous swale running along a natural or artificial flow line, filled with rounded stones of various sizes and sometimes with gravel, that slows runoff water and allows it to percolate into the soil. Dry streambeds are sometimes equipped with percolation zones concealed beneath the rocks.

ecoroof An engineered system of impermeable membranes, low-density growing medium and low-maintenance plantings that covers most or all of the roof of a building. The ecoroof provides absorption of rainwater, biofilters pollutants, reduces urban flooding problems, creates oxygen and sequesters carbon. Various kinds of ecoroofs are used worldwide and are especially prevalent in Europe.

effluent tax A tax on runoff water which has been directed from private property onto public property. The tax would help pay for the costs of managing this water, much as sewer charges and trash pickup charges are borne by the generator of the material rather than by society at large.

engineered soil Specially mixed and graded fill soil intended to serve a particular purpose such as combining structural support for vehicles with a favorable rootzone for street trees.

evapotranspiration (“ET”) The combination of evaporation of water from the soil and transpiration of water through plants. Data on evapotranspiration, usually measured in inches per day or per week, is used to monitor and regulate the application of irrigation water. Evapotranspiration is measured at regional weather stations (such as those of California’s “CIMIS” system), and ET data is transmitted to irrigation managers or directly to automated irrigation controllers, which then adjust water applications to replace the amount of water used in the previous measuring period.

graywater All previously-used household water except for toilet water, which is referred to as “blackwater.”

greenwaste Vegetative material such as lawn clippings, prunings and other plant tissue. No longer considered waste, greenwaste is used as feedstock in compost and as mulch.

groundwater Water from rain and snow that accumulates underground in the water table.

groundwater recharge The practice of returning storm and irrigation water to the groundwater supply by the use of various kinds of infiltration devices such as ponding areas, percolation beds, etc.

impermeable paving Solid paving materials such as concrete and asphalt which shed water rather than absorbing it. Impermeable paving contributes to urban flooding, water pollution and environmental degradation, and has a damaging effect on the rootzone of adjacent trees.

mulch Any loose, non-living material, usually vegetative in origin, used to cover the soil in a planted area for the purposes of conserving water, reducing weed growth, improving absorption of water by the soil, nourishing the soil foodweb, protecting roots, buffering changes in soil temperature, reducing surface erosion and providing an attractive, often walkable, surfacing material. Mulches such as tree chips, processed greenwaste and ground bark are usually applied 3-5 inches thick. Inorganic materials such as crushed rock, gravel or decomposed granite can also be used as mulch but lack the important biological advantages of organic mulch.

pave-and-pipe An engineering-based system of delivering water directly into natural waterways via a system of impermeable roads and other paving and a system of catchments and hard pipes that daylight into nearby watercourses. Pave-and-pipe approaches are characterized by an assumption that water is a waste-product rather than a valuable resource.

percolation The slow absorption of water into the soil. Percolation rates (measured in inches per hour) vary, with sandy soil being the fastest to absorb water and clay soil the slowest.

percolation bed A widely-spreading subsurface layer or zone of highly pervious material such as crushed rock that improves the absorption of water into the soil and facilitates groundwater recharge.

percolation chamber An underground pit of varying dimensions, filled with a highly pervious material such as crushed rock, which catches runoff water and allows it to percolate into the soil.

permeable paving Any traffic-bearing surface that also allows water to penetrate into the subsoil, either through cracks in otherwise impermeable material or directly through the material itself. Examples are paving blocks, pervious concrete, turf block, decomposed granite, crushed rock, gravel or soil pavement.

pervious concrete A “no-fines” concrete paving material that consists of 3/8″ pea gravel and portland cement, without the usual addition of sand or other fine material. Placed over an engineered substrate, pervious concrete absorbs 100 percent of the water that falls on its surface. Pervious concrete technology was developed in Florida and is used extensively there for public roadways and other paved areas.

pervious pavement See “permeable paving.”

ponding zone A low point in the landscape that collects runoff water, allowing it to percolate into the soil and recharge the ground water. Differs from a percolation bed or chamber in that a ponding zone usually consists of native soil only, without any imported gravel.

retention grading Directing runoff to specific locations on the site, where it can be treated in bioswales, directed into the soil via ponding areas or percolation zones, or otherwise held on the site. This is the opposite of the conventional approach of directing water off the site as quickly as possible.

root-path trench rootzone enhancement A system of concealed underground trenches leading from the planting pit of one tree to the planting pit of an adjacent tree in an urban street tree planting. The trenches are filled with a loose soil mixture and equipped with a continuous strip drain. Roots can expand into these trenches, improving the health of the trees.

runoff Rainwater, melted snow or urban water that flows across the surface of the soil and into watercourses and bodies of water.

siltation The deposition of suspended particles of soil by the movement and action of flowing water.

soil foodweb “The community of organisms living all or part of their lives in the soil.” (Soil Biology Primer, Soil & Water Conservation Society) The soil foodweb protects the watershed by decomposing organic compounds and pollutants, storing nitrogen and other nutrients, and enhancing soil porosity.

soil paving The incorporation of various proprietary additives to natural soil that results in a semi-solid or solid, traffic-tolerant surface.

stormdrain inlet A catchbasin, curb drain or other opening at a low point in a water catchment area that directs runoff water into a drainage pipe.

stormwater storage chamber See “percolation chamber.”

structural soil See “engineered soil.”

subsurface filter bed See “percolation bed.”

swale A low-lying trench or shallow ditch with gently-sloped sides that directs runoff water from surrounding areas to a low point in the landscape.

Tree City, U.S.A. To be recognized by the National Arbor Day Foundation as a Tree City USA, a community must meet four standards: a tree board or department, a tree care ordinance, a community forestry program with an annual budget of at least $2 per capita , an Arbor Day observance and proclamation.

Turf Block™ A cellular concrete paving block with openings for grass or other plantings. Other systems use recycled plastic honeycomb-like panels instead of concrete.

water harvesting The practice of catching and storing rainwater for landscape irrigation or potable use.

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