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Benefits
Design
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Hydrologic Design of Pervious Concrete Part 1 w Part 2 w Part 3 w Part 4 w Part 5
Examples and Discussion
In this section, application of the design methods presented in Part 4 are examined and the implications of the analysis for several common situations are discussed. Broad conclusions are drawn regarding pervious concrete pavement system design needs in many situations. Some of the limitations of the Rational Method with this type of system are also examined.
Tabular results of the analysis are provided and discussed in Section 5.5. Values of both Equivalent CN and total runoff are provided. The total runoff values provide important hydrologic information and may be needed for permit applications. The total runoff value is also used to assess the often significant benefits of impoundment. Sensitivity of the solution to the values of initial estimates used in the design, which may not be known with great precision, is assessed by examining changes in the total runoff with changes in the values of initial estimates. The draw-down times of pervious concrete pavement systems in various conditions are also examined to ensure appropriate longer term performance. Comments on the findings of the analysis are also provided.
5.1 Example Proposed Development
5.1.1 Development Plan
A single development is analyzed and discussed in subsequent sections. Various site conditions are considered so that comparisons of the effects of various elements are more evident. The proposed development consists of 300,000 ft2 (about 28,000 m2 or 6.9 acres) of pervious parking, onto which the runoff from 150,000 ft2 (about 14,000 m2 or 3.5 acres) of impervious roof and impervious pavement structures drain. The impervious pavement would be used in heavy traffic-load lanes, such as delivery areas, and in turning lanes. Vegetated islands, side slopes, and contiguous undeveloped land (some of which may not belong to the owner) occupying 200,000 ft2 (about 19,000 m2, or 4.6 acres) will also drain onto the pervious concrete pavement system. The islands, slopes, and undeveloped land will be landscaped with grass and some bushes. The CN of these pervious areas and the pre-development CN will depend on the soil (HSG) used in the different examples. Modifications to this adjacent, impervious area are considered later. The site use prior to development is pasture in very good condition with continuous forage available, and minimal bare areas or trails.
Pavement depth is normally controlled by anticipated traffic loading so a minimum thickness is selected prior to the hydrologic design and analysis. Pavement depth should be specified in increments of 1 in. (25 mm). Typical pervious concrete pavement characteristics are provided in Appendix D. In the example in this chapter, a pervious concrete pavement depth of 6 in. (150 mm) will be used. The design porosity of the pervious concrete in these examples is 15%. The pervious concrete pavement system is assumed to be level.
5.1.2 Site Conditions and Constraints
Hydrologic site performance of the approximately 650,000 ft2 (about 60,000 m2 or 15 acres) development is examined for a variety of site conditions or constraints, including four different soil types and the presence or absence of a base course composed of clean stone. Performance is examined in a 2-year, 24-hr storm and 10-year, 24-hour storm.
The four different soils used in the example analysis of the site are:
1. A sandy, well draining soil classified as HSG A;
2. A loamy sand with some silt, with an intermediate infiltration rate, still classified as HSG A;
3. Another silty soil with an intermediate infiltration rate, classified as HSG B; and
4. A poorly draining silty clay, classified as HSG D.
The effects of base course are examined by using 8 in. (200 mm) of clean stone. A compacted aggregate base consisting of size #57 or #67 stone has a porosity of 40%. The levels of precipitation used in this study are relatively conservative: the precipitation in the 2-year storm is given as 4 in. (100 mm) and the precipitation in the 10-year storm is given as 6 in. (150 mm).
5.2 Pre-Development Runoff and Post-Development Runoff Without Pervious Concrete Based on the soil classifications provided for pasturage in good condition at the time of analysis, estimates of the pre-development runoff can be determined. Runoff is estimated from the Curve Number and given in inches for 4 in. (100 mm) and 6 in. (150 mm) of precipitation (see Table 4).
Several comments are in order. First, although the values of runoff are reported in Table 4 to the nearest 0.01 in., such reporting precision is inappropriate given the variability in the input estimates and the uncertainty in the model itself. Second, since both Case 1 and Case 2 are in HSG A soils, the estimate of runoff is the same for both; in reality, one would expect a difference, but it is important to recognize the inherent simplifications made in this, as in all, hydrologic models. Third, the effect of the type of soil on the volume of runoff is significant and the designer should be careful to compare “apples to apples” when assessing the value of various alternatives in different areas.
Table 5A shows the pre-development CN’s and runoff values with more appropriate significant figures. Table 5B shows the post-development CN’s and runoff values anticipated from the site in the 2-year and 10-year storm without the benefit of a pervious concrete pavement system, (assuming all of the parking area has an impervious surface). Table 5B is based on composite CN’s (the area-weighted, average CN).
Table 4. Estimates of Pre-Development Runoff for Example Case
Table 5A. Pre-Development CN’s and Runoff
*Note: Metric conversions are not exact equivalents due to rounding in Table 4.
Table 5B. Post-Development CN’s & Runoff Without a Pervious Concrete Pavement
The increase in runoff associated with development is significant in the absence of some type of mitigation. Again, the effect of the underlying soil is also significant, even in the pre-development stage. As shown below, a pervious concrete pavement system appreciably improves the hydrologic performance of the site.
5.3 Preliminary Estimates for Use in the CN Method and Discussion
5.3.1 Initial Estimates of Infiltration Rate
Based on the soil classification and descriptions provided, estimates of the infiltration rates for the different soils were made based on TR-55 (SCS 1986) (see Tables 1A and 1B).
It is important to recall that these values must be estimated prior to construction and that the construction process itself will change in situ characteristics. This level of accuracy in the initial estimates of infiltration rate will provide a sufficiently robust model in most practical situations. It is best to modify marginally acceptable designs rather than depend on more accurate estimates of infiltration rates when modifying or finalizing a design, especially considering the variability inherent in the other design elements. Preliminary designs can determine feasibility and identify necessary design modifications as early as possible. Sensitivity analysis is discussed further in section 5.6.
5.3.2 Initial Estimates of the CN of Adjacent Areas
Based on the HSG of each case and the description of cover anticipated after development, the CN of the vegetated and landscaped areas on the site that will drain into the pervious concrete pavement can be estimated. Values are similar but slightly higher (more conservative) than the pre-development case to account for foot traffic, irregular watering, slope, and other reductions in the quality of the ground cover. Estimates are drawn from TR-55 (SCS 1986).
5.4 Results and Discussion of Site Analysis Including Pervious Concrete
5.4.1 Runoff and Equivalent Curve Numbers The runoff and equivalent CN’s for the proposed development, as affected by the soils considered in this study, are given in Tables 8A and 8B. Improvements in runoff (in inches) between pre- and post-development, where the site includes a pervious concrete paving system, are provided in Table 9.
Table 6. Initial Estimates of Infiltration Rate
Table 8A. Post-Development Runoff, Including Pervious Concrete
Table 8B. Equivalent Curve Number, Post-Development, Including Pervious Concrete
*Note: Calculated values of the equivalent Curve Number should not be given below about 36.
Table 9. Improvement from Pre- to Post-Development Runoff, Including Pervious Concrete
Note: A positive value indicates a positive (beneficial) improvement.
5.4.2 Discussion of Findings of Site Analysis
The pervious concrete pavement system significantly reduced post-development runoff and, in all cases where a clean stone base was used, the total runoff was actually lower than pre-development levels. This is a finding for a specific, but realistic, situa | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
