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FGEISStormwaterSection 4.14I 1 1 1 I Fi n p 1 SECTION 4.14 STORMWATER REPORT (SUMMARY) I 1 A. P 1 Stormwater Management Report For The Great Escape Park Area C Revised 5/29 Introduction This report addresses stormwater associated with future development at The Great Escape within Park Area C, west of US Route 9 and east of the Adirondack Northway. Stormwater calculations were conducted using the method prescribed in the USDA Soil Conservation Service Technical Release No. 20 (TR-20), "Urban Hydrology for Small Watersheds." The design storm studied was a fifty (50) year, 24 hour Type II event having a rainfall of 4.8" as per Figure 2, Sheet 5 of 6, Appendix B of the above referenced manual. Also the 2, 10, 100-year storm events were evaluated. B. Topography and Slope Slopes on the Project site range from nearly level to steep (0 to 25%). The topography varies from an elevation of 400 to 508 feet above mean sea level (msl). Site topography was illustrated previously on Figures 2-18, 2-19 and 2-20, "Existing Storm Drainage 1 of 3, 2 of 3 and 3 of 3. Much of the amusement park in Park Area A and a portion of Park Area C have been regraded during the last thirty years. The remaining undisturbed slopes vary from 0-15%. The steeper slopes are associated with the Hinckley soil generally found throughout the project site. Permanent areas of undisturbed Hinckley soils are confined to the undeveloped portions of Park Area A. The least sloping areas on the site are found along the US Route 9 corridor near the existing parking lots, the Glen Lake Wetland area, and the Rush Pond Fen. Slopes become steeper to the south near the former zoological park, to the north behind the Coach House Restaurant, in limited areas around the Rush Pond Fen, and in the extreme eastern portions of the developed amusement park (Park Area A). Slopes in these areas reach a maximum steepness of 25%. C. Geology The project is located north of Albany within the Hudson Valley lowland where the terrain is wide and flat and covered with glacial lake deposits. Immediately north of the project is the physiographic province known as the Adirondack Low Mountains, which surrounds the Adirondack Mountain Peaks and has a local relief of under 1,000 feet above mean sea level (msl). The last glacial advance in this area began approximately 50,000 years ago and ended about 10,000 years ago. This recent geologic event established the character of the Adirondack landscape as viewed today. As the glaciers advanced in a southerlydirection and subsequentlyubsquently retreated north, eroded materials were deposited. On the project site, those deposits consist of light brown gravels and sands in the form of a glacial outwash plain. Bedrock lies at an average depth of 345 feet below the surface. Although numerous wells have been drilled in the area, there has been no recovery of bedrock material with the exception of one occurrence during the installation of a well in a limestone area approximately one mile north of the project site. t. D. Soils IFigure 3-1, "Soils Map," illustrates the types of soils found on the project site, and Table 3-1, "Soils Suitability for Construction," summarizes the important characteristics of these soils. According to the Warren County Soil and Water Conservation District (SWCD), the soils which underlie Park Area A (the existing amusement park east of US Route 9) are from the Oakville soil family, which are nearly level, deep, well drained soils formed in fine sand sediments on outwash plains. Oakville soils are a brown to yellow brown fine sand. The depth to bedrock and to seasonal high water table exceeds six feet, and permeability, or the rate of water movement through the soil, is rapid. The SWCD data states that these soils are well suited for recreational uses. Park Area B, west of the Northway around Rush Pond, is dominated by relatively low slopes associated with the wetland complex and is underlain by Carlisle Muck. The Carlisle Muck is a level, very poorly drained organic soil in wet areas that receives runoff from surrounding higher landscapes. Limited areas of steep slopes are confined to a ridge of Hinckley -Plainfield complex steep soils. No construction activities are planed for this area. The soils in Park Area C, the US Route 9 corridor east of the Northway, are Hinckley, Hinckley -Plainfield and Oakville. These soils are suitable for most construction activities. Hinckley soils consists of a dark gray to dark brown gravelly to cobbly sandy loam. The depth to bedrock and to the seasonal high water table exceeds six feet. The Hinckley -Plainfield soils are similar, with inclusions of Plainfield loamy sand. The Hinckley -Plainfield soils are steeply sloped and will require controlled grading and erosion control management practices during construction (see Table 3-1). Hinckley - Plainfield soils are a deep, excessively well -drained soil on terraces and benches in valleys and on flat to undulating plains. The soil has a high content of sand, gravel and cobblestones. The seasonal high water table is at a depth of more than six feet, and bedrock is generally at a depth of more than five feet. Well logs from the Hinckley - Plainfield soils in the immediate area indicates that bedrock depth to be over 300 feet below grade. 1 Table D-1 Soils Suitability for Construction Park Area A Soil Fill Source Local Roads —Streets Commercial Buildings Hinckley S S S Saprist U U UOakvilleSSS B Hinckley No construction planned in this area Plainfield No construction planned in this area Hinckley No construction planned in this area Carlisle Muck No construction planned in this area C Hinckley S S S Plainfield S S S Oakville Wareham Area was previously filled Area was previously filled Area was previously filledSapristAreawaspreviouslyfilledAreawaspreviouslyfilledAreawaspreviouslyfilled S=Satisfactory U=Unsatisfactory The soils map indicate Wareham (Wa) and Saprist (Sa) along the Rush Pond outlet. These areas were previously modified by filling. Soil test bores for construction of the pedestrian bridge indicate water table depth as much as 7 feet below grade. This area is however subject to seasonal surface flooding. The soils in the Park Area A were assessed with regard to recreation uses because this is the portion of the site containing the actual amusement park and attendant rides and attractions. The soils in Park Area C were assessed in DGEIS Table 3-1, "Soils Suitability for Construction," for suitability for development of local road and streets and for commercial buildings because these are the uses proposed in Park Area C. Park Area of the parking lots. Painted cross walks will be used on the ring road for any necessary pedestrian crosswalks. E. Existing Conditions Park Area C, which lies west of the developed portion of the amusement park, is bordered by US Route 9 to the east, the Adirondack Northway to the west, wooded property to the north, and an existing miniature golf course to the south. The 62.8 acre site includes existing gravel parking areas utilized by the park, an existing ice cream stand with associated motel and cabins, the Adirondack Coach House Restaurant with associated asphalt parking lot, a real estate office, a single family residence, and the Samoset Motel with cabins which lies on the northern portion of the site. The site includes 3.5 acres of wetland and approximately 20 acres of wooded areas interspersed throughout the site. The summary of the current land use covers is found below in Table E-1, "Existing Condition Summary." L k r I Table E-1 Existing Condition Summary Land Characteristic Acres Forest 20.0 29 Paved or Impervious 18.6 27 Wetlands 3.5 5 Landscape lawns 26.8 39 Total 68.9* 100 The stormwater analysis includes lands not under the control of Great Escape. F. Proposed Conditions This DEIS presents a stormwater analysis of Park Area C, the most affected area of the Project with respect to new construction and stormwater management needs. If this analysis were to include the entire watershed in which Park Area C resides, impacts of the Project would be minimized. A watershed approach would dilute nutrient levels due to the presence of the Rush Pond area, with its low levels of development, and the Glen Lake Fen, which acts as a nutrient attenuation zone. Thus, a more responsible approach, such as the one used here, focuses just on the area to be developed. The proposal for Park Area C is to improve the existing parking lots and add more capacity. The proposed improvements will increase the number of parking spaces to approximately 4,000. All lots will be paved and striped and will be interconnected by an internal loop road. The area will also include a 180-200 room hotel and possibly a conference center. The hotel will be developed near the site of the Adirondack Coach House Restaurant. C k Land Characteristic Proposed Forest 8.5 12.4 Paved or Impervious 35.5 51.5 Wetlands 3.5 5.0 Landscape (Lawns) 21.4 1 31.1 Total 68.9 1 100 The proposal for managing stormwater within Park Area C is to collect the runoff from the proposed impervious areas (pavements and buildings) within a series of catch basins. The runoff will then be directed through a series of drywells and detention basins to control and treat the first flush of runoff, infiltrate as much stormwater back into the ground as possible, and reduce the peak rate of runoff to existing levels. The concept is to move stormwater within the individual subcatchments to the same point of discharge (the existing stream which runs through the park) that currently exists. The system moderates the velocity and volume of stormwater to levels that are similar to existing conditions. The means to moderate the velocity and volume is to recharge stormwater by first discharging to a recharge device such as drywells and detention 1 basins. The runoff will then be released into the wetland areas and ultimately the culvert under US Route 9. Currently, the site is divided into thirteen (13) subcatchment areas which may be considered small watersheds. Subcatchments one (1) and three (3) drain to existing catch basins along US Route 9 and are then directed under the highway to a large wetland east of Park Area C. Subcatchments five (5), six (6) and eleven (11) drain to an existing system along US Route 9 and are directed, via pipes to the large culvert and stream which runs under US Route 9 and through the park to the east. Subcatchments four (4), seven (7), eight (8), nine (9), ten (10) and thirteen (13) all drain to existing wetlands within the site. These areas eventually drain to the same culvert and stream which runs through the park to the east. Subcatchments one (1), three (3), five (5), six (6) and eleven (11) will receive no additional flows from the proposed development. With the regrading of the site the existing system along US Route 9 in this area will experience a reduction in flows. For the existing drainage scheme, see Figures 1 a, b and c. The proposed drainage system is broken down into 124 subcatchment areas which will drain through a total of 96 drywells and 12 detention basins. For the proposed drainage designs, see Figures 2a, b & c. Supporting calculations may be found in Section 4.0. The system is complex due to the reliance on recharge in the natural sand and with overflow collection with discharge to ponds. The ponds will detain stormwater, allow recharge and discharge during large prolonged rainfall events. The stormwater system is modeled in one segment for the existing conditions and in three portions for the post development conditions. The schematic illustrates the various components of the system. A hexagon represents subcatchments that collect stormwater and discharge the stormwater to the next phase of the system. A reach is represented by a square which includes conveyance systems such as pipes, ditches or other channels. A pond is represented by a triangle which includes detention ponds and dry well, recharge devices. A link is represented by the misshapen box and represents connection between various portions of the system. Links are used to combine the entire system into a single model since a link can be a single discharge number. I U n 11 1 P LI I H Cl The solid arrow represents an outflow while the dash arrow is an overflow weir or a secondary outfall from a subcatchment, reach or pond. The size of the segment is related to the ability of computer model to assemble necessary data points. The schematic is an illustration of the functional components of the stormwater system for each area at the Great Escape. Each of the schematics has been broken apart into the major segment and labled to indicate the overall general area such as yellow or blue parking lots and corresponds to the detail site plans. The subcatchments are drainage areas that are connected by reaches pipes) and ponds (ponds or drywells). The 2-year and 50-year event discharges for the area are listed. The key release point is the culvert under Route 9 in front of The Great Escape near the admission gates. Eventually, all stormwater discharges from the Area C past this location, whether it is in the Route 9 NYSDOT drainage or in the proposed system. This is the location at which the determination of the overall discharge volume is made for the site. (See attached sheets runoff totals Area A, B, Q. Below is a summary of all the evaluated storms: Storm Existing Condition Proposed Condition 2 0.60 CFS 2.15 CFS 10 5.02 CFS 6.80 CFS 50 11.62 CFS 10.99 CFS 100 20.16 CFS 12.98 CFS Conclusions The proposed system of catch basins, drywells and detention basins will reduce the post - development runoff to below existing levels. The peak current runoff for a 50 year storm leaving the site through the existing stream is 11.68 CFS. The proposed peak runoff is 11.01 CFS or a reduction of 0.67 cubic feet per second. This analysis is based on HydroCAD. The following page shows the possible flood displacement as a result of the parking lot fills below the 405 foot elevation. The fill may displace four inches of water vertically upstream and the above indicates 1.2 inches for a total of possible five inches of additional flooding upstream. 1 11 r] The table below identifies changes in water elevations as it relates to various stormwater events including the 100 year flood. Wetland Elevation Entering Box Culvert 2 year Pre 401.7 3 feet 4 inches Post 402.0 10 year Pre 402.1 3 feet 4 inches Post 402.4 50 year Pre 402.5 2 feet 2.4 inches Post 402.7 100 year Pre 402.9 1 feet 1.2 inches Post 402.8 Evaluation of Flood Plain Raising the elevation of the parking fields is necessary to provide proper stormwater management. This fill will displace potential flood storage volume. The effect of this displacement is minor, as presented below. 1. The flood plain elevation of 405'f would inundate ±12 acres of existing lands, to average/assumed depth of 3'. Total potential flood storage displaced by parking fill is 36 acre feet or 12 ac x 43,560 x 3 = 1,568,160 cu. ft. 2. The flooding impact areas below the parking fill is Glen Lake — 320 ac and Glen Lake Fen — 156 acres, on Great Escape property - 200f acres total. The displacement of 36 acre feet spread over the downstream areas would be; 320 ac + 200 ac = 520 ac 36 acre ft (displacement) 520 ac (impact areas) or 82 inch (less than 7/8") rise in water level. 3. The flooding impact of upstream backup into Rush Pond must also be considered. All the displacement effect of the 12 acre parking on the 113.6 acre Rush Pond is; 36 ac ft (displacement) 113.6 ac (Rush Pond Area) _ .31 ft or 3 3/4 inches rise in water level. 7 4. Assuming that both upstream and downstream areas are affected equally; 520 acres (downstream) +113.6 acres (upstream) = 633.6 acres 36 ac feet = .0568 ft or .68 inch (less than 11/16"). 9080wr09.doc 8 fl I F P I 1 BEIGE LOT 76 82 83 y 0.00 GFS 84 f74 a o.00 HOTEL PARKING 1 SOUTH OF BUILDING 0•o GFS 3.sb GFS GREEN LOT 77 Paz S7 62 \& WETLAND BROWN NORTH 32 \ St ----62 y \j 68 69 T 13,p8c ETLAND 33 7® 5 41R 59 y 73 75 " y ie I 87 80 61 Iira c ee 42 - CJ 73 89 4 Z• 5 tr=s 98 E--- e 1 o. 8cf5 . ee X CULVERT UNDER RO TE 9BO C 4%6Y7 FS EL+H .JroJ- KEY: ctSTlKS (0.00) 2 YEAR TOTAL 0.00) 50 YEAR TOTAL 0 SUBCATCHMENT REACH POND LINK J 140T TO wAa IPA WII M.Y su.uwu. PINK EAST WHITE 3.4a CFS 71 87 y I 85 B6 96 l1 PINKLOT (SOUTH) ® xi 98 O s45 7®) id 97 79 91 Ca ppKy 1 1 ` q. 30 c, MARTHA' S LOT Iy / 0 u4 O o.59 cFs Oy 1 T " yQ z.8 cFg n / 0 ( ® 0.01 G r I•bl C O n 95 x9e a 88 O0.00 RANG 1, 75c LAVENDER LOT 9. 27 cF // 7 7q4CS x ACCESS ROAD © Ay BE' RNEEN MAROON O ' 8 LAVENDER LOTS ,a ACCESS ROAD MAR N O IU IiJ •© I.q2 cF5 C9•Ij FS SUBCATCHMENT aREACH LINK 2 I V c?5 i, i a.yr, cFS 84 BROWN LOT T O 93 F® SOUTH 86 94 FtH9 r\ 2'14c 5 ® T 82 83 85 1. 89 q.sacc Is PINK WEST a F— 99 WETLAND ACCESS ROAD OOQ91cf ld d E— Y 92 BLACK LOT 93 98 E ® T c 0. 00 L FS i 4.90 wF5 O0 t—OF „P TA OON LOT e O iLIGHT GREEN LOT KEY; 0. 00) 2 YEARATOTAL M ( 0. 00) 50 YEAR TOTAL POND I ! LINK NOT To 9G18 Pevleel . GD i — Dnnf i EAST OF YELLOW LOT (NORTH END) OSUBCATCHMENT REACH e FS' POND HOTEL & WESTERN PARKING PARKING KEY: 0.00) 2 YEAR TOTAL 0.00) 50 YEAR TOTAL LINK wu y eu.uc.. it a I I 1 11 PROJECT NAME: GREAT ESCAPE PEDESTRIAN BRIDGE LOG OF BORING NO. 3 PROJECT Lake George, New YorkLOCATION: OFFSET - ON CLIENT: Ryan -Biggs Associates UNMUMM ova SIVE STR IM (TSs) 2 3 4 5 o gE illH 4 wA 0 Nw a 1 N ww Ea SS N aH N yA a o DESCRIPTION OF MATERIAL PIM= COxm ' LI IIID X — A l0 20 30 40 50 PENNETRRATION (m'0"g/F0O2) 10 40 50 SURFACE ELEVATION Fine to medium san ,trace to some s1 dark brown and brown, moist, medium dense (SM) 2 SS Fine to coarse sand, some gravel, trace silt, brown, moist, loose (SP) uncontrolled fill cj 3 SS Fine sand and silt, trace to some organics, dark brown, moist, loose (SM)(ML) organic 83.6 4 SS Fine sand and silt, trace organics, gray, moist, loose (SM)(ML) 5 PA SS Fine sand, some silt, gray, moist, medium dense (SM) 1 0 PA 1 6 SS END OF BORING AT 17.0 FEET 2 2 3 WATER LEVEL at 7.0' while sampling VCH FILE NO. 5693 VERNON HOFFMAN PE SOIL i FOUNDATION ZUGINEBRING 118 SOUTH FERRY STREET 12305SCHEIJ( 51e) 2-0207 FAx382-1035) DATE 10/01/99 STRATIFICATION LINES REPRESENT APPROXIMATE BOUNDARY LINES BETWEEN SOIL TYPES: IN —SITU, TRANSITION MAY BE GRADUAL. I 1 I u Id PROJECT NAME: GREAT ESCAPE PEDESTRIAN BRIDGE LOG OF BORING NO. 4 PROJECT Lake George, New York LOCATION: OFFSET - ON CLIENT: Ryan -Biggs Associates Uu=frn= OoemaasSIVE STREOM (TSF) 2 4 5 z o H tW-7 W P4 a a W 1 a W U) H SS cWi aEA H to A a U DESCRIPTION OF MATERIAL PLASTIC I LI In x • — l0 20 30 5040 STANDARD PENETRATION ( sLa s/Foox 10 20 0 40 50SURFACEELEVATION Fine to medium sand, some gravel and silt, dark brown, moist, medium dense (SM) topsoil 2 SS Fine to medium sand and gravel, trace to some silt, light brown, moist, medium dense SM GM) 5 3 SS Fine to medium sand, trace to some silt, brown, moist, medium dense (SM) 4 SS Fine to medium sand, trace to some silt, brown, wet, medium dense (SM) PA Fine to coarse sand, some gravel, trace to some silt, brown, wet, medium dense (SM) 1 5 SS PA Fine to medium sand, trace to some silt, brown, moist to wet, medium dense (SM) 1 6 SS PA Fine sand, some silt, brown, moist to wet, medium dense (SM) 2 7 ssI L PA Medium to coarse sand, trace to some silt, brown, moist to wet, medium dense (SM-SP) 8 SS 2 END OF BORING AT 25.0 FEET 3 WATER LEVEL at 13.0' while sampling VCH FILE NO. 5693 VERNON HOFFMAN PE SOIL s FOUNDATION MGINZERnrz 118 SOUTH FERRY STREET SCHENECTADY NY 12305 518) 382-0207 FAY 382-10351 DATE 10/01/99 STRATIFICATION LINES REPRESENT APPROXIMATE BOUNDARY LINES BETWEEN SOIL TYPES: IN -SITU, TRANSITION MAY BE GRADUAL. 17 I 1 1 1 PROJECT NAME: GREAT ESCAPE PEDESTRIAN BRIDGE LOG OF BORING NO. 5 PROJECT Lake George, New YorkLOCATION: OFFSET - ON CLIENT: Ryan -Biggs Associates UVOMMOM 0CHnWSrVR STD (TSF) 2 4 5 H x H W a+ w A 0 w a aw a 1 w a aw a 07 H SS wFDA aka aH to co a a o U a DESCRIPTION OF MATERIAL F TTICo7AM LI D to X20 0 40 50 STANDARD (soars/sooT) PENETRATION 10 40 501 SURFACE ELEVATION ine to medium sand, trace to some silt, brown, moist, medium dense (SM) uncontrolled fill 2 SS Fine to medium sand, some gravel, trace to some silt, yellow and brown, moist, loose SM uncontrolled fill cj 3 ss Fine sand and silt, dark gray, moist to wet, loose (SM)(ML) slightly organic 4 SS Fine sand, some silt, dark gray, moist to wet, loose (SM) 5 PA SS Fine to medium sand, trace to some gravel and silt, dark gray, moist to wet, loose (SM) 1 PA Fine sand, some silt, gray, wet, medium dense (SM) 1 6 SS PA 2 7 SS PA 8 SS 2 END OF BORING AT 25.0 FEET 3 WATER LEVEL at 7.0' while sampling VCH FILE NO. 5b93 VERNON HOFFMAN PE SOIL i FOUNDATION FNGINSSRING 118 SORTS FERRY STREET1205SCHFai( 518) 3 2- 12305 207518) 382-0351FAY382-1035] DATE l0/01/99 STRATIFICATION LINES REPRESENT APPROXIMATE BOUNDARY LINES BETWEEN SOIL TYPES: IN -SITU, TRANSITION MAY BE GRADUAL. I 1 P-1 it 1 United States Office of Water EPA 832-F-99-023 Environmental Protection Washington, D.C. September 1999 Agency A` -.EPA Storm Water Technology Fact Sheet Porous Pavement DESCRIPTION Porous pavement is a special type of pavement that allows rain and snowmelt to pass through it, thereby reducing the runoff from a site and surrounding areas. In addition, porous pavement filters some pollutants from the runoff if maintained. There are two types of porous pavement: porous asphalt and pervious concrete. Porous asphalt pavement consists of an open -graded coarse aggregate, bonded together by asphalt cement, with sufficient interconnected voids to make it highly permeable to water. Pervious concrete consists of specially formulated mixtures of Portland cement, uniform, open -graded coarse aggregate, and water. Pervious concrete has enough void space to allow rapid percolation of liquids through the pavement. The porous pavement surface is typically placed over a highly permeable layer of open -graded gravel and crushed stone. The void spaces in the aggregate layers act as a storage reservoir for runoff. A filter fabric is placed beneath the gravel and stone layers to screen out fine soil particles. Figure 1 illustrates a common porous asphalt pavement installation. Two common modifications made in designing porous pavement systems are (1) varying the amount of storage in the stone reservoir beneath the pavement and (2) adding perforated pipes near the top of the reservoir to discharge excess storm water after the reservoir has been filled. Some municipalities have also added storm water reservoirs (in addition to stone reservoirs) beneath the pavement. These reservoirs should be designed to accommodate runoff from a design storm and should provide for infiltration through the underlying subsoil. APPLICABILITY Porous pavement may substitute for conventional pavement on parking areas, areas with light traffic, and the shoulders of airport taxiways a runways, provided that the grades, subsoils, drainage characteristics, and groundwater conditions are suitable. Slopes should be flat or very gentle. Soils should have field -verified permeability rates of greater than 13 centimeters (0.5 inches) per hour, and there should be a 1.2 meter (4-foot) minimum clearance from the bottom of the system to bedrock or the water table. ADVANTAGES AND DISADVANTAGES The advantages of using porous pavement include: Water treatment by pollutant removal. Less need for curbing and storm sewers. Improved road safety because of better skid resistance. Recharge to local aquifers. The use of porous pavement may be restricted in cold regions, and regions or regions with high wind erosion rates, and areas of sole -source aquifers. The use of porous pavement is highly constrained, requiring deep permeable soils, restricted traffic, and adjacent land uses. Some specific 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Sign Posted to Prevent Resurfacing and Use of Berm Keeps Off -site Runoff Asphalt is Vacuum Swept, Abrasives, and to Restrict and Sediment Out, Provides Followed by Jet Hosing to Truck Parking Temporary Storage Keep Pores Open Q Posted Overflow Pipe Filter Fabric Lines Sides of Reservoir to Prevent Sediment Entry Porous Perforated Pipe Discharges Only When 2-Year Storage i Volume Exceeded Stone Reservoir Drains in 48 - 72 Hours Undisturbed Soils with a Feld Capacity> 0.27 IncheslHour Preferably 0.50 Inches/Hour J // 1 Observation Well Gravel Course or 8- Inch Sand Layer Source: Modified from MWCOG, 1987. FIGURE 1 TYPICAL POROUS PAVEMENT INSTALLATION disadvantages of porous pavement include the following: 0 Some building codes may not allow for its installation. Many pavement engineers and contractors lack expertise with this technology. • Anaerobic conditions may develop in underlying soils if the soils are unable to Porous pavement has a tendency to become dry out between storm events. This may clogged if improperly installed or impede microbiological decomposition. maintained. Porous pavement has a high rate of failure. There is some risk of contaminating groundwater, depending on soil conditions and aquifer susceptibility. Fuel may leak from vehicles and toxic chemicals may leach from asphalt and/or binder surface. Porous pavement systems are not designed to treat these pollutants. As noted above, the use of porous pavement does create risk of groundwater contamination. Pollutants that are not easily trapped, adsorbed, or reduced, such as nitrates and chlorides, may continue to move through the soil profile and into the groundwater, possibly contaminating drinking water supplies. Therefore, until more scientific data is available, it is not advisable to construct porous pavement near groundwater drinking supplies. 1 I In addition to these documented pros and cons of porous pavements, several questions remain regarding their use. These include: Whether porous pavement can maintain its porosity over a long period of time, particularly with resurfacing needs and snow removal. Whether porous pavement remains capable of removing pollutants after subfreezing weather and snow removal. The cost of maintenance and rehabilitation options for restoration of porosity: DESIGN CRITERIA Porous pavement - along with other infiltration technologies like infiltration basins and trenches - have demonstrated a short life span. Failures generally have been attributed to poor design, poor construction techniques, subsoils with low permeability, and lack of adequate preventive maintenance. Key design factors that can increase the performance and reduce the risk of failure of porous pavements (and other infiltration technologies) include: Site conditions; Construction materials; and oxygen demand. Some key factors to increase pollutant removal include: Routine vacuum sweeping and high pressure washing (with proper disposal of removed material). Drainage time of at least 24 hours. Highly permeable soils. Pretreatment of runoff from site. Organic matter in subsoils. Clean -washed aggregate. Traditionally, porous pavement sites have had a high failure rate - approximately 75 percent. Failure has been attributed to poor design, inadequate construction techniques, soils with low permeability, heavy vehicular traffic, and resurfacing with nonporous pavement materials. Factors enhancing longevity include: Vacuum sweeping and high-pressure washing. Use in low -intensity parking areas. Restrictions on use by heavy vehicles. Installation methods. • Limited use of de-icing chemicals and sand. These factors are discussed further in Table 1. PERFORMANCE Porous pavement pollutant removal mechanisms include absorption, straining, and microbiological decomposition in the soil. An estimate of porous pavement pollutant removal efficiency is provided by two long-term monitoring studies conducted in Rockville, MD, and Prince William, VA. These studies indicate removal efficiencies of between 82 and 95 percent for sediment, 65 percent for total phosphorus, and between 80 and 85 percent of total nitrogen. The Rockville, MD, site also indicated high removal rates for zinc, lead, and chemical Resurfacing. Inspection and enforcement of specifications during construction. Pretreatment of runoff from offsite. Implementation of a stringent sediment control plan. OPERATION AND MAINTENANCE Porous pavements need to be maintained. Maintenance should include vacuum sweeping at least four times a year (with proper disposal of 1 1 TABLE 1 DESIGN CRITERIA FOR POROUS PAVEMENTS Design Criterion Guidelines Site Evaluation Take soil boring ;3 a depth of at least 1.2 meters (4 feet) below bottom of stone reservoir to check for soil permeability, porosity, depth of seasonally high water table, and depth to bedrock. Not recommended on slopes greater than 5 percent and best with slopes as flat as possible. Minimum infiltration rate 0.9 meters (3 feet) below bottom of stone reservoir: 1.3 centimeters (0.5 inches) per hour. Minimum depth to bedrock and seasonally high water table: 1.2 meters (4 feet). Minimum setback from water supply wells: 30 meters (100 feet). Minimum setback from building foundations: 3 meters (10 feet) downgradient, 30 meters (100 feet) upgradient. Not recommended in areas where wind erosion supplies significant amounts of windblown sediment. Drainage area should be less than 6.1 hectares (15 acres). Traffic conditions Use for low -volume automobile parking areas and lightly used access roads. Avoid moderate to high traffic areas and significant truck traffic. Avoid snow removal operations; post with signs to restrict the use of sand, salt, and other deicing chemicals typically associated with snow cleaning activities. Design Storm Storage Volume Highly variable; depends upon regulatory requirements. Typically design for storm water runoff volume produced in the tributary watershed by the 6-month, 24-hour duration storm event. Drainage Time for Design Storm Minimum: 12 hours. Maximum: 72 hours. Recommended: 24 hours. Construction Excavate and grade with light equipment with tracks or oversized tires to prevent soil compaction. As needed, divert storm water runoff away from planned pavement area before and during construction. A typical porous pavement cross-section consists of the following layers: 1) porous asphalt course, 5-10 centimeters (2-4 inches) thick; 2) filter aggregate course; 3) reservoir course of 4-8 centimeters (1.5-3-inch) diameter stone; and 4) filter fabric. Porous Pavement Placement Paving temperature: 240• - 260• F. Minimum air temperature: 50• F. Compact with one or two passes of a 10,000-kilogram (I 0-ton) roller. Prevent any vehicular traffic on pavement for at least two days. Pretreatment Pretreatment recommended to treat runoff from off -site areas. For example, place a 7.6-meter (25-foot) wide vegetative filter strip around the perimeter of the porous pavement where drainage flows onto the pavement surface. Source: Field, 1982. removed material), followed by high-pressure 5. U.S. EPA, 1981. Best Management hosing to free pores in the top layer from clogging. Practices Implementation Manual. Potholes and cracks can be filled with patching mixes unless more than 10 percent of the surface 6. U.S. EPA, 1992. Stormwater Management area needs repair. Spot -clogging may be fixed by for Industrial Activities:. Developing drilling 1.3 centimeter (half -inch) holes through the porous pavement layer every few feet. Pollution Prevention Plans and Best Management Practices. EPA 833-R-92- 006. The pavement should be inspected several times during the first few months following installation 7. Washington State Department of Ecology, and annually thereafter. Annual inspections should 1992. Stormwater Management Manual for take place after large storms, when puddles will the Puget Sound Basin. make any clogging obvious. The condition of adjacent pretreatment devices should also be ADDITIONAL INFORMATION inspected. Andropogon Associates, Ltd. COSTS Yaki Miodovnik The costs associated with developing a porous 374 Shurs Lane Philadelphia, PA 19128 pavement system are illustrated in Table 2. Estimated costs for an average annual maintenance Cahill Associates Thomas H. Cahill program of a porous pavement parking lot are 104 S. High Street approximately $4,942 per hectare per year ($200 West Chester, PA 19382 per acre per year). This cost assumes four inspections each year with appropriate jet hosing Center for Watershed Protection and vacuum sweeping treatments. Tom Schueler 8391 Main Street REFERENCES Ellicott City, MD 21043 1. Field, R., et al., 1982. "An Overview of Fairland Park, Maryland Porous Pavement Research." Water Ken Pensyl Resources Bulletin, Volume 18, No. 2, pp. Nonpoint Source Program 265-267. Water Management Administration Maryland Department of the Environment 2. Metropolitan Washington Council of 2500 Broening Highway Governments, 1987. Controlling Urban Baltimore, MD 21224 Runoff A Practical Manual for Planning and Designing Urban BMPs. Fort Necessity National Battlefield National Park Service 3. Metropolitan Washington Council of 1 Washington Parkway Governments, 1992. A Current Assessment Farmington, PA 15437 ofBest Management Practices: Techniques for Reducing Nonpoint Source Pollution in Massachusetts Highway Department a Coastal Zone. Clem Fung Research and Materials Group 4. Southeastern Wisconsin Regional Planning 400 D Street Commission, 1991. Costs of Urban Boston, MA 02210 Nonpoint Source Water Pollution Control Measures, Technical Report No. 31. Morris Arboretum Robert Anderson 9414 Meadowbrook Avenue Philadelphia, PA 19118 Washington Department of Ecology Linda Matlock Stormwater Unit P.O. Box 47696 Olympia, WA 98504-7696 The mention of trade names or commercial products does not constitute endorsement or recommendation for the use by the U.S. Environmental Protection Agency. For more information contact: Municipal Technology Branch U.S. EPA Mail Code 4204 401 M St., S.W. Washington, DC, 20460 oMTB Excekrce n congixxe ttragh qMrrkl taunts+ soMM MUNICIPAL TECHNOLOGY BRAN HU