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. Porous Pavements: A New Approach To An Old Problem Raymond S. Rollins, USAE Waterways Experiment Station As we continue to cover landscape with impermeable surfaces such as roofs and pavements, we pay an increasing social cost. These impermeable surfaces increase water runoff causing problems such as increased flow in traditional waterways with resulting erosion and flood problems, reduced groundwater recharge, and heightened pollution levels in our water systems. To combat these problems, we have undertaken a number of regulatory efforts such as controlling discharge rates from sites, mandating on-site storage levels, limiting the amount of an area that can be covered with impermeable surfaces, or even basing tax rates on covered area quantities. We can also approach this problem by building a porous pavement surface that essentially allows water to pass through the surface with as little hindrance as possible. Thus our manunoth quantities of paved surfaces (parking lots, highways, storage yards, etc.) would not be contributors to the runoff issue, or at least their contribution would be reduced. However, the conceopt of a porous pavement is heresy to a traditional civil engineer such as myself After all, the traditional civil engineer is taught that "there are only three things wrong with pavements: water, water, and water." There are several technical issues that must be resolved to insure that we can actually make the concept of porous pavements a viable concept, and we must develop concepts of how we:an make these porous pavements effective. . First, we must have a pavement that can provide the necessary permeability while also continuing to provide adequate structural support and satisfactory riding characteristics (e.g., skid resistance and smoothness). There are three potential surfacing that we will examine: porous concrete (also sometimes called "no-fines conrete"), asphalt concrete porous friction courses, and proprietary, precast concrete products. Each of these will offer certain advantages and disadvantages that must be matched to the specific problems of the site. The next issue we must examine concerns maintenance and long-term permeability of the porous surface. It is relatively easy to achieve high initial permeability in a surface, but we must also design for reduced future permiability due to siltation and clogging. Alternatively, we might be able to do some rehabilitative maintenance to reestablish the original permeability levels. Once we can achieve and maintain surface permeability in the pavement surface, we absolutely must control the subsurface water so that it does not cause detrimental damage to the integrity of the pavement system. To accomplish this will require careful assessment of the final disposal of the collected water (e.g., allow to rapidly percolate into a sand subgrade or selection of a scheme for disposal over less permeable clay subgrades). This has major design implication on the requirements for and cost of materials used in the pavement and for the subgrade strength for the pavement . This presentation will summarize current knowledge and limitations on how to design and use porous pavements as a method of surface water control.