I am looking for a typical C value used with the Rational Run Off Method with concrete brick pavers. I have a large driveway and large patio with pavers in gravel and sand. The underlying soil is sandy and groundwater is very deep so that infiltration is not an issue. Compacted soil under the driveway may increase the C value versus a patio C value. Any tips would be greatly appreciated.
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Rational C Value for Brick Pavers in a Sand Bed 1
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How big is the sand area between the pavers? If the sand is just "filling in the little gaps", then I would assume about C=0.95 as an estimate. If you have maybe 50% coarse sand (or gravel) space in between the pavers and a subsurface that does not limit the flow, then I might go as low as C=0.75 as an estimate. This is also highly dependent upon the slope and whether the area forms a bowl. Paver surfaces with that much "infiltration space" are sometimes refered to as "permeable pavements", but that can be misleading.
Though these have the potential to significantly reduce stormwater runoff from traffic surfaces, permeable pavements are also highly susceptible to surface sealing and pore clogging. Pore clogging, in particular, is difficult to remedy as the pavement system generally must be removed and replaced. Prevention of surface sealing and pore clogging depends on regular maintenance and prevention of damaging substance spills. Petroleum products such as oils and greases have a high potential to seal surfaces and clog pores. Permeable pavements are high maintenance systems with a high potential for hydraulic failure. That is, if the function is stormwater management (infiltration). Such surfaces can work just fine structurally and be completely clogged. Such systems may start out with very high infiltration rates, but then have significantly decreased infiltration rates through clogging and sealing. The first year might have a "C=0.20" and then decrease to "C=0.95" by the fifth year.
I do not recommend the use of permeable pavement in areas where there is a significant potential for surface sealing or pore clogging if stormwater infiltration is a desired function. This includes roadways and parking lots with regular traffic loading exceeding 10 vehicles per day, irrespective of vehicular weight, and also includes areas which are exposed to heavy loadings of fine particles (such as clay, silt, and dust). Permeable pavements are generally suitable and recommended for dedicated pedestrian pathways. If permeable pavements are employed, I recommend those which incorporate course aggregate or vegetation (eg grass or moss) between pavers over continuous asphalt or concrete material applications.
There have been several studies on these systems. One such study can be downloaded at" You could also try a W3 search for "permeable pavement pdf". Please note that the Interlocking Concrete Pavement Institute (ICPI) paints a somewhat overly positive picture of performance in my opinion (you might mull over the traffic, pollutants, and maintenance these systems experienced).
The site above was reported from another study as "XXXXX Monitoring Site: No runoff was produced at the [XXXXX] parking lot (Figure 4) during monitoring from March to December 2004." That is true. It is also true that the site failed structurally and probably no more than 25 vehicles were on that site during the entire monitoring period, those for only very brief periods, and that was immediately after construction (not several years later). The site is also located far, far away from clayey soils. The site was a demonstration for permeable pavement. This is a small parking lot with a very low traffic loading (less than 3 vehicles per week to date of photographs, generally single family type vehicles). Two failure types are evident in the first photograph. An approximately 100 square feet area has experienced excessive differential settlement relative to other areas of the parking lot. This is most likely due to improper compaction during construction or piping (pore erosion) of base/subbase materials afte construction. These are structural failures, not hydraulic failures - though these failures involved a hydraulic mechanism. Hydraulic failures are improper hydraulic functioning of the permeable paver system (primarily inadequate infiltration). Evidence of piping with subsequent settlement at the parking lot corner can be seen in the lower right corner of the first photograph.
A full structural failure is shown in the second photograph. This occurred during a single heavy precipitation event less than one week after the first photograph was taken. The piping evident at the parking lot corner in first photograph proceeded during the precipitation event, resulting in this failure. Structural failures due to piping require adequate energy gradient between the driving water surface and the outlet water surface. Given a relatively flat terrain, this is an unlikely scenario. Permeable pavement installations adjacent to harbor bulkheads, retaining walls, and relatively steep fill slopes are potential areas for such failures. About 1/3 of the site paver surface has now been dug-up and is being replaced with a standard concrete surface.
So, that's the gloom and doom stuff. These systems can work great for stormwater management (infiltration), but may require significant maintenance. Just be careful.
Anyway, C=0.95 is my suggestion.
.
tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation
How big is the sand area between the pavers? If the sand is just "filling in the little gaps", then I would assume about C=0.95 as an estimate. If you have maybe 50% coarse sand (or gravel) space in between the pavers and a subsurface that does not limit the flow, then I might go as low as C=0.75 as an estimate. This is also highly dependent upon the slope and whether the area forms a bowl. Paver surfaces with that much "infiltration space" are sometimes refered to as "permeable pavements", but that can be misleading.
Though these have the potential to significantly reduce stormwater runoff from traffic surfaces, permeable pavements are also highly susceptible to surface sealing and pore clogging. Pore clogging, in particular, is difficult to remedy as the pavement system generally must be removed and replaced. Prevention of surface sealing and pore clogging depends on regular maintenance and prevention of damaging substance spills. Petroleum products such as oils and greases have a high potential to seal surfaces and clog pores. Permeable pavements are high maintenance systems with a high potential for hydraulic failure. That is, if the function is stormwater management (infiltration). Such surfaces can work just fine structurally and be completely clogged. Such systems may start out with very high infiltration rates, but then have significantly decreased infiltration rates through clogging and sealing. The first year might have a "C=0.20" and then decrease to "C=0.95" by the fifth year.
I do not recommend the use of permeable pavement in areas where there is a significant potential for surface sealing or pore clogging if stormwater infiltration is a desired function. This includes roadways and parking lots with regular traffic loading exceeding 10 vehicles per day, irrespective of vehicular weight, and also includes areas which are exposed to heavy loadings of fine particles (such as clay, silt, and dust). Permeable pavements are generally suitable and recommended for dedicated pedestrian pathways. If permeable pavements are employed, I recommend those which incorporate course aggregate or vegetation (eg grass or moss) between pavers over continuous asphalt or concrete material applications.
There have been several studies on these systems. One such study can be downloaded at" You could also try a W3 search for "permeable pavement pdf". Please note that the Interlocking Concrete Pavement Institute (ICPI) paints a somewhat overly positive picture of performance in my opinion (you might mull over the traffic, pollutants, and maintenance these systems experienced).
The site above was reported from another study as "XXXXX Monitoring Site: No runoff was produced at the [XXXXX] parking lot (Figure 4) during monitoring from March to December 2004." That is true. It is also true that the site failed structurally and probably no more than 25 vehicles were on that site during the entire monitoring period, those for only very brief periods, and that was immediately after construction (not several years later). The site is also located far, far away from clayey soils. The site was a demonstration for permeable pavement. This is a small parking lot with a very low traffic loading (less than 3 vehicles per week to date of photographs, generally single family type vehicles). Two failure types are evident in the first photograph. An approximately 100 square feet area has experienced excessive differential settlement relative to other areas of the parking lot. This is most likely due to improper compaction during construction or piping (pore erosion) of base/subbase materials afte construction. These are structural failures, not hydraulic failures - though these failures involved a hydraulic mechanism. Hydraulic failures are improper hydraulic functioning of the permeable paver system (primarily inadequate infiltration). Evidence of piping with subsequent settlement at the parking lot corner can be seen in the lower right corner of the first photograph.
A full structural failure is shown in the second photograph. This occurred during a single heavy precipitation event less than one week after the first photograph was taken. The piping evident at the parking lot corner in first photograph proceeded during the precipitation event, resulting in this failure. Structural failures due to piping require adequate energy gradient between the driving water surface and the outlet water surface. Given a relatively flat terrain, this is an unlikely scenario. Permeable pavement installations adjacent to harbor bulkheads, retaining walls, and relatively steep fill slopes are potential areas for such failures. About 1/3 of the site paver surface has now been dug-up and is being replaced with a standard concrete surface.
So, that's the gloom and doom stuff. These systems can work great for stormwater management (infiltration), but may require significant maintenance. Just be careful.
Anyway, C=0.95 is my suggestion.
.
tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation
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