Sustainable Building Technical Manual - Etn-presco.net

POROUS ASPHALTFigure 1Porous asphalt has been proven in the field in hundreds of pavements of everytype, from driveways to highways. 6 The distinctive porous surface layer is constructedwith an “open-graded” aggregate, which leaves the voids between largeparticles unfilled by small particles. An open-graded stone base holds water untilit infiltrates the underlying soil. Compared to impervious asphalt, porous asphaltis better-draining, has better traction and visibility in wet conditions, and producesless glare and vehicular noise.POROUS ASPHALT SECTION6.5 cm porous asphalt surface5 cm of 1.25 cm stoneReservoir base courseof 5 cm stoneControl compaction of subgradeto prevent reduction in soil porositySource: Edmund Thelen and L. Fielding Howe, Porous Pavement (Philadelphia: Franklin Institute Press, 1978)Drainage of Concentrated RunoffIf drainage controls are implemented at the beginning of site planning, they can be integratedeconomically in the overall development. Detailed information on hydrologicanalysis and design can be found in some of the references at the end of this chapter.❑ Consider disconnecting pre-existing downspouts and storm sewers from sanitarysewers.– Discharge downspouts into an earthern depression or gravel-filled pit for infiltration.❑ Moderate and treat runoff from roofs and unavoidable impervious pavements,and, to the degree possible, return it to its natural path in the soil.– Design every conveyance, pool, and drainage basin to match the requirements of itsspecific location and drainage area.– Disperse runoff from impervious surfaces over adjacent vegatative soils with levelspreaders, which change concentrated stormwater flow to sheet flow.– Convey concentrated runoff in vegetated swales, not structural gutters or pipes.When runoff contacts vegetation and porous soil, its volume and velocity arereduced, and pollutants are filtered. Compared to closed structural systems, this opendrainage system increases vegetative variety, reduces need for irrigation water, andreduces drainage velocity and erosion. In addition, it decreases downstream peakflow and runoff volume, increases infiltration, supports wildlife habitat, symbolizesinteraction with nature, and requires little single-purpose maintenance (Figure 2).– Stabilize soil and reduce scouring velocity. Rock or timber checkdams, linings of sodand erosion-control fabrics, and bioengineering with quickly rooting riparian plantsare effective where preexisting stream channels are unstable, or where new swalesmay cause erosion after grading. Swales with broad bottoms reduce velocity.– Moderate discharge through use of constructed pools and wetlands along drainagecourses. Formed by excavation of a shallow reservoir and installation of a low damand controlled flow outlet, such pools and wetlands can temporarily store stormflows and improve water quality by allowing for settling, filtration, and biodegradationof pollutants. 7 Effective water treatment is a result of providing sufficient areafor the water body and providing diverse depth zones for different plant habitats andbiophysical processes.