08-15-1983 Planning Packet - Laserfiche WebLink

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y Clerk/ t attached ution passed , 19 78 ERMIT on y of Orono. Porous pavements and urban hydro!•I*y W7xlrcme urbanization in the U.S. JQin the past 30 years has cre ated serious environmental pres sures on urban land development. Specifically, the hydrology of a developing area is severly im pacted because of two principal factors: (1) The percentage of area made impervious through urban ization, and (2) the land served by storm sewers. “Traditionally, the goal of a stormwater management system has been to dispose of stormwater quickly,” said Richard Field, chief of the Storm and Combined Sewer Program of the U.S. Environmental Protection Agency (EPA). “Runoff is usually conveyed to storm sew ers, draining more rapidly than from areas previously covered by natural vegetation.” Stormwater Management This method of stormwater man agement increases peak runoff, peak velocities, reduces lag time between precipitation and runoff, and degrades water quality. In creases in peak flow can have neg ative impacts downstream auch as stream-bed and bank erosion with downstream sedimentation, stream-bed and bank erosion with associated downstream sedimenta tion and increases in frequency and intensity of downstream flooding. The decrease in lag time results in less ground water recharge and decreased low flows in stream channels,” he said. Many communities have en acted legislation directing devel opers to reduce excess runoff asso ciated with land development. Past stormwater management methods emphasized quicx elimination of stormwater. By contrast, the goals of Innovative stormwater manage ment are to maintain natural runoff levels by providing for filtration, runoff velocity, extending lag time 43 RURAL AND urban ROADS/JULY. 1963 and reducing runoff volume. Porous pavements have poten tial as a technology for maintaining predevelopment runoff levels by allowing infiltration and ground- water recharge. By acting as an attenuation and runoff reduction device, these pavements can de- New urban land pressures put a squeeze on roads and drainage. crease the volume and peak flow rate and increase lag time. Porous asphalt pavements have been used in highway and airport runway construction. This cpen- graded asphalt mix has been re ferred to as “plant mix”, “seal coat ”, “open-graded mix”, “gap- graded mix”, “popcorn mix”, or “porous friction course ”. Material consists of an open-graded asphalt mixture with a high percentage of weight of aggregate larger than a No. 4 sieve having a 0.0187 in. aperture. The material is laid to a thickness of 0.75 to 1 in. The resulting pavement has a course surf.'.ce texture and a high void space-ratio. The course surface texture provides pressure relief channels to remove water under exesss pressure between the pave ment and vehicle tire. Temporary storage Is minimal because rainfall is diverted in nearby drains. Also, the high void space-ratio provides channels for dissipation of pressure and flow under the tire. Consequently, hydrostatic pres sure cannot build up in the film of surface water under a tire and hydroplaning potential is elimi nated. The friction coefficient be tween tire and pavement is almost equivalent to the coefficient under dry conditions. California, Nevada, New Mexi co, Utah and Louisiana highway agencies have been using plant mix seal-coats for 10 years because of their safety aspects. Applications to airport runways was initiated in 1967 at Farnbor- ough, England by the British Royal Air Force, two European airfields and the U.S. Air Force. Since 1947, the California Highway De partment open graded base courses under converitional surfac ing to provide rapid drainage in problem areas. The Franklin Institute Research Laboratories, under EPA sponsor ship, investigated porous bases and porous subbases in conjunction with thicker applications of plant mix seal coat. This experiment was to Investigate the potential of deliv ering water to the subbase rather than removing it to a stormwater collection system. Also evaluated were a variety of conventional and unconventional materials. The most promising ones were tested to determine their physical and eco nomic feasibility as porous pave ment materials. Of those tested, the open-graded asphalt concrete was selected as the most suitable because of its superior characteris tics, low cost and ability to be laid by conventional paving. Other porous pavement types studied included concrete lattice blocks. Analysis Findings An analysis by the Franklin Insti tute found the cost of concrete pavement with storm sewers to be higher than the cost of an equiva lent porous asphalt installation without storm sewer relief. Howev er. installations will have to consid er the need for some storm sewer relief, until more field data on 1 yr ^ t I 1 I 5 i ! porous pavi economics arc very “s governed b cost and pr stones and ability of i user will b< nomic eval the Installa Initial p< tions in D< nia proved al and dra installation and Roch( instrumcnl provide gr benefits ar Pavemen Each pc unique an proved by after a Ion a coopera city of fi hopes to proaches residentia ers latitud plex cond is essentiz The ne changes a ent chanc ities is pa porous p< tunatcly, tices hole ments ha from a ru volume) has watc come a c< ers and p To dc’ rous pav contribut abilitv w volume I base. Ch age basir the soil, will impj runoff ar pavemer soil. Sto can be lii nomic lir