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Environmental
NRMCA Pervious Concrete Contractor Certification
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Hydrologic Design of Pervious Concrete Part 1 w Part 2 w Part 3 w Part 4 w Part 5
Hydrological Design Concepts and Issues
Hydrological analysis can be a complex process and a detailed review is beyond the scope of this report. Ferguson (1994) and Viessmann and Lewis (2003) provide additional information on hydrology and stormwater management. An overview of the characteristics of primary interest in the design of pervious concrete pavement systems, including a brief discussion of the terminology and analytical tools commonly used in hydrology, is provided for several reasons. Not only must the effect of a pervious concrete pavement system on runoff be assessed quantitatively, but the solution must address the needs of key decision makers, using the terminology, values, units and methodologies with which they are familiar. Many key decision makers represent permit granting agencies and often have a technical background in environmental or water quality and quantity issues.
Complete hydrological analysis, when required, must be conducted by a registered professional engineer (PE) or other design professional. In many practical cases, however, detailed analysis may not be required. Tables D1 and D2 in Appendix D show typical characteristics of pervious concrete pavement systems for various situations.
3.1 Runoff Characteristics
An important factor in site development is often the amount of excess surface runoff that can be tolerated for a specific site, area, or watershed. Estimating the volume and rate of runoff is a key part of the hydrologic design. Excess surface runoff is the amount of rain which falls less that amount intercepted by ground cover, that held in depression storage (the small to moderate sized “birdbaths” and “mud puddles” which occur with all surfaces), or that which infiltrates into the soil. Excess storm water runoff will occur with virtually all natural groundcover for any rainfall event of practical interest. With impervious surfaces runoff accumulates more rapidly and more pollutants can wash into streams than with vegetated surfaces.
Once precipitation begins, rain will build up in excess of that caught on vegetation or in small depressions and begin to flow overland in sheets. The overland flow quickly becomes channelized and the flow will continue into streams and creeks, then downstream into rivers and larger bodies of water. As runoff from the more distant part of the watershed area accumulates, the quantity and speed of the water in the channel increases. After the rain ends, the runoff subsides. A graph, the runoff hydrograph, which shows the rate of runoff over time at some particular point of interest such as a culvert location, has the typical shape shown in Figure 3. The rain itself may be shown as “falling” from the top of the graph. The peak discharge of the hydrograph is shown in Figure 3 as Qp, normally in cubic feet per second in US customary units, or cubic meters per second in metric units. The volume of runoff is the area under the curve, often converted to acre-ft or m3.
Urbanization results in a shift of the runoff hydrograph as shown in Figure 4, due to the increase in impervious surface which promotes faster runoff and more rapid accumulation. The peak flow of the hydrograph not only increases but occurs sooner. In addition, the area under the curve increases; that is, there is more runoff, since there is less infiltration than with impervious surfaces. Structural BMPs such as detention or retention ponds are intended to reduce the peak runoff by holding some portion of the runoff for some period of time; infiltration of some part of the runoff into the soil may also occur.
A common goal of hydrologic analysis of smaller watersheds, such as residential developments or a shopping center, is the design of an “outlet structure,” such as a channel (swale), storm sewer, or culvert, to carry the excess runoff in a particular rainfall event (design storm) without flooding. The design of the outlet structure is often based on the peak discharge the structure is intended to handle. The design of retention or detention structures, such as pervious concrete pavement systems or ponds, however, is based on the volume which must be captured. Both types of design require determination of the hydrologic characteristics of the watershed, selection of an appropriate design storm, and application of the appropriate design method.
3.2 Hydrologic Characteristics of the Watershed: Infiltration and Runoff
Two important hydrologic characteristics of a watershed are the amount of runoff which can be anticipated from different areas and the amount of infiltration, that is, the amount of precipitation which will soak into the soil for some given rainfall. Both factors are related to the soil type. A sandy soil will tend to have more infiltration and less runoff. An area with a tight clay will tend to have less infiltration and more runoff.
Runoff is also affected by the slope of the land and the type and extent of vegetation. Estimation of runoff characteristics relies heavily on empirical data and methods. The values used vary with the design method and have been published, but the specific values used in the design of a specific site can vary significantly between different, experienced practitioners. Ferguson (1994) provides a detailed and informative discussion of infiltration. Selection of values is discussed in more detail in Part 4.
The infiltration rate of a soil will vary with the amount of moisture already in the soil, the antecedent moisture condition (AMC). Using the steady state infiltration rate (see Figure 5) is reasonable and conservative for pervious concrete pavement systems. The rate is approximately constant within about an hour for the types of soils where infiltration is an important part of the design.
Using the steady state value makes the design less sensitive to assumptions regarding prior rainfall and AMC. This approach also means that performance in service will often exceed design characteristics. Values of typical infiltration rates are published in several sources, but professional judgment is required in selecting an appropriate value for hydrologic design of pervious concrete pavement systems.
Soil type is one of the most important factors affecting the rate of infiltration. Soils can be classified for hydrologic purposes as HSG (Hydrologic Soil Group) A (sand, loamy sand, or sandy loam), HSG B (silt loam or loam), HSG C (sandy clay loam), or HSG D (clay loam, silty clay loam, sandy clay, silty clay, or clay). The soil horizon with the highest infiltration capacity is HSG A; the infiltration rate is lowest for HSG D. The location of the water table can also affect infiltration significantly. A high water table will impede infiltration, even in sand.
Table 1A. Infiltration Rates Based on General HSG Classifications (SCS 1986)
Table 1B. Infiltration Rates at One Hour Based on General Soil Types (ASCE 1949)
Various estimates of the infiltration rate for different soil horizons have been developed (see Figure 6, for example). Table 1A shows values given in TR-55 (SCS 1986). The values in Table 1B are derived from ASCE Manual of Engineering Practice (American Society of Civil Engineers 1949), summarized by Viessmann and Lewis (2003). The values in both tables are very similar.
Guidance on the selection of an appropriate infiltration rate to use in design is provided in texts, Natural Resources Conservation Service (NRCS, previously the Soil Conservation Service, SCS) soil surveys, and ASCE guidelines. The designer must consider, however, several limitations when selecting infiltration values from published data (Malcom 2002). First, NRCS values are for natural soils which, even for a specific soil type, can vary significantly. Further, the infiltration rate of natural soils decline with depth, so the published data from NRCS are, at best, average values over large, minimally disturbed, surface areas. In addition, moving soil during construction often, in effect, turns the natural soil “upside down.” This soil is then re-compacted prior to construction. Considering these factors, and the values in Table 1B (ASCE), initial estimates can be established as indicated in Table 2.
These values are appropriate for preliminary designs and feasibility studies, but may need to be adjusted based on site investigation. In many cases they are sufficiently accurate for permit application review, to make decisions on the technical viability of a proposed pervious concrete paving system (Malcom 2002), and can be used in final designs if the system is sufficiently robust.
The design of pervious concrete paving systems in soils with substantial silt and clay content or a high water table should be approached with some caution. It is important to recall that runoff is relatively high in areas with clayey soils or clayey-silts, even with natural ground cover, and properly designed and constructed pervious concrete pavement systems can provide a positive benefit in many situations. In very tight, poorly draining soils, lower infiltration rates can be used for feasibility studies, but the “drawdown” time, that is, the time needed for captured runoff to drain out of the pervious concrete pavement system through infiltration of the soil subgrade, may limit some applications. This topic is discussed in more detail in Part 5.
Pervious concrete pavement systems may be used for active mitigation even with very tight, non draining soils when designing the system as a detention rather than retention device, although additional structural details must be provided. In these situations, since the soil will take in very little runoff anyway, regardless of the cover, the intent is to simply reduce the peak flow by holding the runoff for some period of time. Infiltration is not considered a critical feature of the design since virtually all of the captured runoff will be released directly into natural channels or the storm sewer system. With the inclusion of additional subsurface storage devices the peak flow can be reduced significantly. This approach may be required when riparian rights are an issue.
Table 2. Recommended Infiltration Rates for Preliminary Design and Feasibility Studies
In areas where the clay layer is relatively thin and close to the surface, it may also be possible to provide water flow through an impervious soil layer into underlying permeable strata by drilling through any impervious layers and installing a well. The well shaft should be lined with a geotextile filter fabric and filled with stone. These wells, sometimes referred to as injection wells, connect the pervious concrete to the pervious strata. Additional analysis with additional elements are typically required in these situations, especially if the water table is close to the surface, and may require the services of a geotechnical engineer as well as a hydrologist. These features are likely to be economically viable only where permeable strata exist at reasonable depths.
3.3 Permeability and Storage of the Pervious Pavement System
Design of pervious concrete pavement systems must consider two possible conditions. Surface runoff in excess of the desired quantity must not occur in the design rainfall event due to:
1. Low permeability of the pervious concrete, or 2. Inadequate storage provided in the pervious concrete system.
Permeability is, in general, not a limiting or critical design feature. The permeability of the pervious concrete and any underlying base course will be much higher than the steady state infiltration rate of almost all soils as long as the pavement surface is adequately maintained. A moderate-porosity pervious concrete pavement system will typically have a permeability of 3.5 gal/ft2/min (143 L/m2/min), which is equivalent to an infiltration rate in excess of 340 in./h (8600 mm/h),* more than 100 times the infiltration rates of most natural, saturated sands. The exfiltration rate of captured runoff from the pervious concrete pavement system into the underlying subgrade is controlled by the soil infiltration. Permeability of the pervious concrete pavement should be retained by routine maintenance in service, which may consist of periodic (annual or semi-annual, for example) vacuuming.
[*Note: Since hydrological engineers and technical personnel at many permitting agencies are more familiar with the types of units discussed above, the designer of a pervious concrete pavement system may elect to use these types of units to ensure good communication. To convert from inches of rainfall per hour to the typically used units of gallons per square foot per minute for the passage of water (permeability) through pervious concrete, the designer may multiply the value in in./h by 0.0104 to obtain the required flow in gal / ft 2 / min. A simpler conversion factor of 0.01 can be used for almost any practical purpose since the input values are rarely known with enough precision to justify a more accurate conversion factor (In metric units, to convert from mm/h to units of L / m2 / min, divide by 60).].
3.3.1 Storage Capacity
The total storage capacity of the pervious concrete pavement system includes the capacity of the pervious concrete pavement, plus that of any base course used, and may be increased with optional storage features such as curbs or underground tanks. The amount of runoff captured should also include the amount of water which leaves the system by infiltration into the underlying soil. All of the voids in the pervious concrete will not be filled in service because some may be disconnected, some may be difficult to fill, and air may be difficult to expel from others. It is more appropriate to discuss effective porosity, that portion of the pervious concrete which can be readily filled in service.
If the pervious concrete has 15% effective porosity, then every inch (25 mm) of pavement depth can hold 0.15 in. (3.8 mm) of rain. Thus, a pervious concrete pavement 4 in. (100 mm) thick with 15% effective porosity can hold up to 0.6 in. (15 mm) of rain.
An important source of storage is the base course. Compacted, clean stone (#67 stone, for example) used as a base course has a design porosity of about 40%; a conventional aggregate base course, with a higher fines content, will have a lower porosity (on the order of 20%). From the example above, if 4 in. (100 mm) of pervious concrete with 15% porosity were placed on 6 in. (150 mm) of clean stone, the nominal storage capacity would be 3.0 in. (75 mm) of rain:
The effect of the base course on the storage capacity of the pervious concrete pavement system is significant.
A third potential source of storage is available with curbed pavement systems. Where curbs are provided for traffic control, edge-load carrying capacity, or safety, and the accumulation of standing water is permitted, the depth of water impounded by the curb will also provide storage capacity. A design incorporating ponded water up to the depth of the curbs is not normally included at mercantile establishments or other areas anticipating significant foot traffic or public exposure during an intense storm. This feature may be included, however, in applications such as low-use or low-traffic parking areas, particularly with well draining soils where the impoundment will be brief. This feature would also not normally be used if an extended impoundment time is anticipated in an area which is also subject to freezing.
When used, a curb provides essentially 100% porosity, so the height of the curb adds directly to the storage capacity of the pavement system (see Figure 7) in a flat area. To continue the example above, the total storage capacity of the pavement including 4-in. high curbs will be 7 in. (175 mm):
Additional storage capacity can also be obtained by adding underground storage devices or tanks. These “cistern” type applications are often used to store water for purposes other than simple runoff control.
3.3.2 Effects of Slope
A critical assumption so far is that the entire system is level. If the slab is not level, and the rainfall intensity is greater than the infiltration rate of the soil, the upper portion of the slab will not be filled and the rainfall will quickly run to the lowest part of the slab (see Figure 8). Once the lower part is filled, the rain will run out of the lower end of the pavement rapidly due to the high permeability of the pervious concrete, limiting the beneficial effects of the pervious concrete.
The effective volume, expressed as a percent of the nominal volume of a pervious concrete pavement with a slope greater than d/L, can be shown to be:
where d and L are the width and length of the slab (respectively, in consistent units), and s is the slope (Equation 1 is valid only with s> d/L). [*Note: Equation 1 is not exact unless the length of the slab is the map length rather than the surface length. The error is negligible, however, unless the slope exceeds 12%. Equation 1 is not applicable for slopes less than d/L; see Appendix E for additional information]. For example, for a 6 in. (150 mm) deep, 100 ft (30.5 m) long slab with a 1% slope, the % Vol is only 25% of the nominal volume of the pervious concrete without considering the effects of a base course.
% Vol = (6 in. / 12 in./ft) / [(2) (0.01) (100 ft)] = 25%.
% Vol = (150 mm / 1000 mm/m) / [2 (0.01) (30.5 m) = 25%.
These reductions in useable volume can be significant and indicate two important features in the design of pervious concrete pavement systems. Pervious pavements should not be constructed with crowns and should be as level as possible. When the pervious concrete pavement is not level, and the anticipated rainfall rate exceeds the infiltration rate, which is the case for all soils except deep, very clean sands, the depth of the pervious system must be increased to meet the desired runoff goals. It is often the base course thickness that is increased due to economic considerations.
The needed storage capacity can also be provided by a relatively deep recharge bed of clean stone located beneath the downstream end of the pavement. The effects of non-uniform saturation on axle-load carrying capacity of the pavement must be considered with this type of structure, however. When a slope is unavoidable and a highly localized recharge bed or well is used (see Figure 9), the design implications of the recharge bed on site hydrology must be closely examined by the designer of record. For pervious concrete pavement systems which are very long, it may be necessary to use terracing or include intermittent “check dams” to increase the storage volume (see Figures 10A and 10B).
3.3.3 Effective Storage Capacity — Recovery Through Infiltration
Soil infiltration can significantly affect the amount of useful storage in a pervious concrete pavement system over time. The net storage capacity of the pavement system is dynamic.
The amount of runoff held at any one time is a function of the storage capacity of the pervious pavement and base course (based on porosity and geometry), the runoff entering the pavement system (both rain falling on the pervious concrete and runoff from adjacent surfaces), and runoff accumulated from previous rain during the storm, less infiltration into the soil (the “exfiltration” from the pavement system). Soil infiltration drains the system so as to restore some part of the storage capacity during the storm and to remove the rainfall captured by the system after the storm. A hydrologic model developed to predict the behavior of a pervious concrete pavement system should include both the effects of runoff accumulation and the positive benefits of infiltration on recovery of storage capacity during the storm.
Figure 11. Infiltration of rainfall into the soil increases the effective storage capacity of the pervious concrete pavement system.
An example can demonstrate system behavior (see Figure 11). As shown above, a 6-in. (150-mm) thick pervious concrete pavement with 15% porosity can hold about 0.9 in. (23 mm) of runoff. Assume that the pervious concrete has accumulated 0.2 in. (5 mm) of rain and that, during the next hour, an additional 0.8 in. (20 mm) of runoff will flow into the pavement. This would lead to 0.1 in. (2.5 mm) of runoff flowing off of the pervious concrete if no infiltration were to occur (Figure 11A). If the pervious concrete was placed on a loamy sand with an infiltration of 0.5 in. (13 mm) per hour, a net inflow of only 0.3 in. (7 mm) would occur (0.8 in. (20 mm) inflow minus 0.5 in. (13 mm) outflow). Instead of 0.1 in. (3 mm) of runoff from the system, the pervious concrete would have a net positive storage capacity of 0.4 in. (10 mm) remaining (the total capacity of 0.9 in. (23 mm) less the sum of the 0.2 in. (5 mm) already accumulated and the net 0.3 in. (8 mm) inflow) (Figure 11B).
Evaporation of stormwater in the pervious concrete pavement system after the storm will also contribute to storage capacity recovery. Estimates of the quantity and rate of evaporation have not been fully established for pervious concrete. Neglecting this effect is both computationally convenient and conservative.
3.4 Design Storms
Runoff is also affected by the nature of the storm itself; clearly a heavier rain results in more runoff. Storms have a distribution, or pattern, of rainfall intensities, often starting and ending with lower intensities, with the maximum intensity often occurring at some point after the storm has begun. Different sizes of storms will result in different amounts of runoff and the selection of an appropriate design storm is important. Larger storms occur less often on average and storms are typically designated based on their return period. For example, a storm which occurs on average once in 20 years is designated a “20-year storm” and will be larger (more rainfall is produced in the same period of time) than a “10-year storm.”
3.4.1 Selection of the Appropriate Return Period
Selection of the appropriate return period is important because it establishes the quantity of rainfall which must be considered in the design. Often, the design storm is chosen by local authorities, such as city or county water boards. Storms of interest in hydrologic design of small watersheds are typically the 2-year storm and the 10-year storm. The 2-year storm is often used as the “service load” storm for the watershed for water quality purposes. The 10-year storm has traditionally been used in the design of storm water collection systems (Veissman and Lewis 2003; Malcom 1986).
One of the primary purposes of pervious concrete paving systems is water quality, so pervious concrete pavement systems are often designed to capture a 2-year storm. When flood control is a major issue, the 10-year storm may be used as the design load for the system. Performance should be checked in both storms. A pervious concrete paving system integrated into a storm water collection system designed for the 10-year storm can easily result in the use of smaller pipes and culverts, resulting in cost savings, especially for new construction (Malcom 2002).
Other storms, such as the 20-year, 50-year, and 100-year storms are generally used when analyzing much larger basins for flood control. Local jurisdictions may also require analysis of smaller system behavior in storms with these longer return periods when restricting post-development peak discharge in new construction. The methodology discussed in Part 5 is also appropriate for use with these larger design storms.
3.4.2 Design Storm Characteristics
3.4.2.1 Duration-Depth-Frequency
There are several aspects of precipitation characteristics to consider in the hydrologic design of flood control or water quality features in small watersheds. The total volume of precipitation for a given duration and return period can be estimated based on Duration-Depth-Frequency charts, tables, or maps. Estimates of the maximum rainfall expected in depth (inches or mm) for a given duration (such as the 20-minute, 1-hour, 2-hour, or 24-hour storm) in a given return period (such as 2 years, 10 years, 100 years, etc.) are available for different locations. The National Oceanic and Atmospheric Administration (NOAA) Atlas 14 is currently being updated to replace previous NOAA Atlas maps and estimates (2004). Rainfall estimates for many areas are available online at http://hdsc.nws.noaa.gov/hdsc/pfds/. For example, in one location in the mid-Atlantic region, 3.6 in. (90 mm) of rain is expected to fall in a 24-hour period, once every 2 years, on average. The 24-h rainfall amount is used both for retention or detention structures and in the Curve Number Method described in Part 4.
The 24-h rainfall amount for the return period of interest, such as the 2-year storm, is not distributed uniformly. A typical rainfall will often start out with lighter rainfall, with the heaviest rain occurring sometime after the storm has begun. The distribution, or pattern, of rainfall within the storm varies by location. Areas in the northwestern US, (temperate rainforest regions) will have a different pattern of rainfall than areas more exposed to subtropical storms or “nor’easters.” Specific rainfall distributions or patterns to be used in hydrological design are discussed in more detail in Section 4.4, describing the NRCS design methodology.
3.4.2.2 Intensity-Duration-Frequency
Small watersheds are “sensitive” to (that is, they tend to flood in) short, intense storms. Designs of flood control structures in small watersheds based on the Rational Method use Intensity-Duration-Frequency (IDF) values. In general, the rainfall intensity (the rate of rainfall) will be more intense the shorter the rainfall period. For example, in a 1-hour storm, the rate of rainfall may be 1.5 in./h (about 38 mm/h), while in a 15-minute storm the rate of rainfall in that same location will be higher, perhaps 3.2 in./h (about 80 mm/h). Although the total amount of rain falling in that 15 minute time period would only be about 0.8 in. (about 20 mm), the rainfall could accumulate rapidly enough to cause flooding if the outlet structures could not handle the flow (volume per unit time) of runoff occurring during that relatively short period. The Rational method approach and IDF values are described in more detail in Part 4.
IDF curves or charts are available for many locations. A typical chart is shown in Figure 13. In this chart, the rainfall intensity for a storm with a duration of 20 minutes which occurs once every 10 years on average (20-min, 10-year storm), is about 4.7 in./h (about 12 cm/h).
3.5 Water Quality
Water quality issues for small watersheds have become increasingly important. Pervious concrete paving systems can form an important part of current storm water discharge plans required for Municipal Separate Storm Sewer Systems (MS4) permits by improving water quality, reducing peak discharge and increasing base flow. The EPA’s BMP Summary (US EPA 1999) lists a number of structural BMPs, including: infiltration systems (infiltration basins and porous pavement), detention systems (including basins and underground vaults), retention systems (wet ponds), constructed wetland systems, filtration systems, media filters and bio-retention systems, vegetated systems (such as grass filter strips and vegetated swales), minimizing directly connected impervious surfaces, and miscellaneous and vendor supplied systems (including oil-water separators or hydrodynamic devices).
The primary goals of structural BMPs are to control flow, (i.e. reduce the peak discharge and volume of runoff), and to reduce pollutant loadings (US EPA 1999). While flow control is traditionally related to flood control, it is also strongly related to overall water quality because a reduction in runoff volume means more infiltration and a reduction in peak discharge results in lower stream velocities and erosion. Infiltrating more of the runoff means that rain is returned to the water table and the base flow of streams is maintained at higher levels, improving habitats and maintaining desirable ecosystems.
Another contribution to water quality provided by pervious concrete paving is a reduction in the temperature of stormwater runoff or discharge. Water temperature is an important measure of water quality (EPA 1999) and pervious concrete paving systems not only capture that part of the runoff warmed by flowing over initially hot pavements, but they also can reduce the heat island effect, which is common with asphalt pavements.
Pervious concrete paving systems also capture a portion of the pollutants before they flow into the receiving waters. The source of much of the material washing into streams, rivers, and eventually into ground water, can be classified as either an excess of intentionally applied materials such as fertilizers and nutrients, pesticides, and road salts, or accidentally or casually applied materials such as gasoline and petroleum products from drips, spillage, and tire abrasion, plus other residue such as litter, spills, animal waste, and fine dust. Some of these are quickly picked up and carried by runoff, while others, including relatively insoluble products such as grease and low volatile content oils, may not be. Another source of concern with water quality has been poor stewardship practices such as ineffective or un-enforced control of runoff on bare earth, often from sites under development. Lack of effective controls has resulted in significantly increased sediment loads in some areas.
Often, although not always, the initial storm water runoff will carry a higher concentration of pollutants than runoff that occurs later, after the surface has been washed off by the rain. This part of the runoff with a higher pollutant load is termed the first flush. In more arid areas, with long periods between rains, a seasonal first flush may need to be considered. One of the common goals of mitigation is to capture the first flush of runoff, particularly when dealing with small catchments, or drainage areas. While capturing the first flush of an area is often desirable, the disposal of the first flush and cleaning of the catch basin after removing the first flush so that is does not wind up in rivers and streams can be problematic and expensive.
The first flush may not be observed for several reasons. First, larger areas rarely show a first flush since a steady stream of the first flush of areas farther and farther away from the outlet arrive over time. Part of the difficulty in assessing the first flush is the combination of travel time and dilution effects occurring in larger areas. Second, the first flush may not be apparent if pollutants are not easily washed away or dissolved. Third, differences in pollutant load over time may be difficult to detect if the supply of pollutants is essentially continuous; an example of this situation is the supply of sediment from bare, easily eroded ground.
Adoption of specific types of mitigation devices and features depends on the use of the site, the types and quantities of pollutants anticipated, the estimated runoff, and site characteristics. A lack of sufficient data in many areas, variations from place to place, and seasonal variations have resulted in the use of relatively simple rules of thumb for selecting or approving certain types of mitigation features.
As a crude rule of thumb, the first flush is often considered to occur during the first 30 minutes to one hour for small sites such as parking lots (Veissman and Lewis 2003). If pervious concrete is present, analysis indicates that the first hour of rain will generally be captured. Thus, it is reasonable to assume that, as a minimum, that part of the runoff with the highest pollution load will be captured. Pervious concrete systems can thus provide an effective tool to capture the first flush, including trapping floatables, such as plastic bottles, paper or foam cups, and snack wrappers on the surface where they can be removed during routine maintenance rather than discharged into the storm sewer. These items can significantly detract from the aesthetic effects of receiving waters.
It is believed that pervious concrete pavements will carry the soluble “first flush” pollutants into the pores of the concrete and additional rain will carry the pollutants further into the system, where they will be held until infiltrated, rather than becoming a part of the runoff stream. Compounds contributing to biological oxygen demand (BOD) and chemical oxygen demand (COD) should then undergo natural filtering and purification such that the water reaching the ground water table will be of roughly the same quality as that moving through similar in-situ soils. Greases and low-volatile content oils, such as drips from vehicles, will be typically adsorbed onto the surface of the pervious concrete or, at worst, in the pores of the pervious concrete. This is expected to result in negligible effects on porosity and permeability of the pervious concrete, although this is an area in which additional research is needed.
The effect of the total suspended solids (TSS), including the grit and fines in the runoff, carried into the pervious concrete pavement system have not been fully established and additional research is warranted. Sedimentation in the concrete paving system may result in a slight loss of storage capacity. A simple analysis indicates that storage capacity may be minimally affected, as long as the pervious concrete paving system is protected from wash-off during construction activities, however. The TSS tends to be about 1,000 pounds per acre per year (0.112 kg/m2/year) (USEPA 1999) (Wurbs and James 2002) from commercial areas and less for most other types of urbanized sites except construction. For a 20-acre (80,000-m2) shopping site, a pervious concrete pavement system designed to be an active mitigation structure may occupy 40% of the total area draining into the pavement. The TSS deposited in the pavement will be less than 152 in. (12 mm) in depth in 20 years of service, resulting in only a few percent loss in storage capacity. Clearly, additional storage should be included in any design where sedimentation is expected to be high; an extra inch (25 mm) of aggregate base would supply sufficient storage capacity to more than offset volume losses due to sedimentation in this example.
The effects of sedimentation on permeability may be more significant. The surface of pervious concrete is typically denser than the bulk due primarily to compaction operations during construction; sedimentation of larger particles (sands) may be concentrated at the surface such that flow into the pervious concrete is reduced. Studies (MCIA 2002) have indicated that permeability may be largely restored by routine maintenance operations (Valavala, Haselbach, and Montes 2006).
Rules of thumb concerning sedimentation of conventional ponds are not appropriate and may be misleading since the “footprint” of a pervious concrete paving system is so much larger than that of conventional water quality ponds. It is important to note, however, that construction can contribute significantly higher amounts of sediment and so a pervious concrete paving system must be protected during construction. Additional research on the effects of sedimentation on permeability and porosity would be useful.
Reference: Leming, M.L., Malcom, H.R., and Tennis, P.D., Hydrologic Design of Pervious Concrete, EB303, Portland Cement Association, Skokie, Illinois, and National Ready Mixed Concrete Association, Silver Spring, Maryland, USA, 2007, 72 pages. |
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