PerviousPave

Slope Considerations1

American Concrete Pavement Association Ι www.acpa.org

Introduction The hydrological design in PerviousPave assumes that the entire system is level. Cross-slopes, crowns and the traditional means of providing surface runoff for drainage are not necessary. However, when the surface of a pervious pavement is not level, the depth of the pavement and reservoir layer(s) must be designed to meet the desired runoff goals. If the slope of the system is subtle, the recommendations made in PerviousPave might be considered minimum requirements and the reservoir layer(s) slightly thickened, as necessary, to ensure that all stormwater being processed by the system is contained (stored) in the appropriate layer(s). If necessary, more complex options for handling water flow may be used.

Pervious concrete pavements have been placed successfully on slopes up to 16%. In such cases, trenches were dug across the slope, lined with 0.25 in. (6 mm) visqueen, and filled with rock (Figure 1). Pipes extending from the trenches carry water traveling down the paved slope out the adjacent hillside. Use of filter fabric also is recommended in such scenarios to prevent washout of the subgrade (Tennis, et al, 2004).

Effective Volume In general terms, the effective storage volume due to a sloped pervious concrete pavement can be calculated as:

(%) = 2 ∗ ∗

where,

d = thickness of the pervious concrete (per PerviousPave design), ft (m) s = slope, % L = length of slab, ft (m)

Figure 1. Elevation (top) and plan (bottom) views of a sloped installation (Tennis, et al, 2004).

PerviousPave

Slope Considerations2

American Concrete Pavement Association Ι www.acpa.org

This equation is only valid when the slope is greater than the ratio of the slab width to length (e.g., w/L). As an example, consider a 100 ft (30.5 m) long and 6 in. (150 mm) thick slab with a 1% slope, which will have an effective volume of 25%:

(%) = 0.5 2 ∗ 0.01 ∗ 100 = 25%

Two important features of pervious concrete pavements can be deduced from this effective volume effect:

1. Pervious concrete pavements should not be constructed with a crown because doing so will decrease the effective storage of the system.

2. Pervious concrete pavements should be constructed as level as possible to achieve the full potential storage capacity benefit of the system.

To use the effective volume as a correction factor during the design of a pervious concrete pavement, the Storm Intensity can be divided by the effective volume to artificially increase the design volume of water to be processed by the system. For example, if a 10-year storm, 24-hour precipitation of 4.0 in. (100 mm) is used with the slope, thickness (as calculated by PerviousPave) and length from the previous example, the design precipitation would be 4.0 in. ÷ 0.25 = 16 in (400 mm). Upon re-running the hydrological calculations in PerviousPave, the reservoir layer(s) would be increased as necessary.

Other Means to Accommodate Slope If the slope is excessive, increasing the reservoir layers as outlined above may not be sufficient or feasible and other means to accommodate the sloped drainage may be employed. Many methods have been proposed, including the trench and drain system shown in Figure 1. Figure 2 shows a reservoir layer that is thickened, as necessary, and a deep well used on one end of a slope to increase storage capacity of the system.

Figure 2. A recharge bed (A) or a well (B) may be necessary for sloped pervious concrete pavements (PCA EB303).

PerviousPave

Slope Considerations3

American Concrete Pavement Association Ι www.acpa.org

If a steeply-sloped pervious concrete pavement is very long in the direction of the slope, terraces or a check dam might be used (Figure 3).

See Appendix E in the Portland Cement Association’s (PCA’s) EB303, “Hydrological Design of Pervious Concrete” for more examples of calculations to accommodate slope effects in pervious concrete pavements and the ACPA’s IS334P, “Stormwater Management with Pervious Concrete Pavement” for examples on accommodating slope and poorly draining soils.

References ACPA IS334P, Stormwater Management with Pervious Concrete Pavement, American Concrete Pavement Association, Skokie, Illinois, 2009.

PCA EB303, Hydrological Design of Pervious Concrete, Portland Cement Association, Skokie, Illinois, 2007.

Tennis, P.D.; Leming, M.L.; and Akers, D.J., Pervious Concrete Pavements, Portland Cement Association, Skokie, Illinois, 2004.

Figure 3. Pervious concrete pavements that are very long in the direction of the slope might require terraces (left) or check dams (right) to accommodate the slope (PCA EB303).