Rainwater Infiltration and Coefficient of Runoff

There are many parallels between the disciplines of Civil Engineering and Landscape Architecture. The two fields of land development expertise are in many instances dependent upon the success or failure of each industry. The relationships are most often interrelated in the grading, drainage and erosion control aspects, however many landscape architects also are educated and experienced in the horizontal and vertical road alignment.

As our population in the United States continues to grow and rural and wilderness land is consumed, one of the primary areas of overlap between civil engineering and landscape architecture is in drainage.  Material choice has become a greater factor in the design of parking lots, sidewalks, driveways, etc. especially when LEED is a consideration.  When considering different materials and drainage, the Coefficient of Runoff is a critical element.  Most people have a general idea of what coefficients of runoff are for differing materials, but there are standard equations available.  The coefficient of runoff is a percentage of rainfall that is calculated as a decimal for varying materials.

In the parking lot at the Pueblo Municipal Complex, the parking areas are paved with concrete pavers with a loose gravel joint that allows rainwater to infiltrate.

The following are the coefficients of runoff for common surfaces:

  1. Roofs, Asphalt, Brick Pavers with Grout and Concrete: 0.9 (T-2:T-6)
  2. McAdam Asphalt, Brick Pavers and/or other stone with Sand-Swept Joints (Less than 1/2 inch joint): 0.8
  3. Compacted Gravel, Breeze or Granite Fines: 0.7-0.9 (T-4:T-6)
  4. Loose Gravel and Other Inorganic Mulch: 0.5-0.7 (T-2:T-5)
  5. Bare Soil without Vegetative Growth (Range justification is due to the varying soil types): 0.4-0.7 (Varies)
  6. Loose Gravel and Organic Mulch: 0.3-0.6 (T-2:T4)
  7. Pasture Land: 0.3-0.4 (T1-T-2)
  8. Well-Maintained Turf Lawn: 0.3 (T-2: T-4)
  9. Rural Areas or Open Space: 0.1-0.3 (T-1)
  10. Parks and Golf Courses: 0.2-0.3 (T-2: T-6)
  11. Dense Woods: 0.1-0.2 (T1)
T-Zones above are based off the Rural-to-Urban Transect. Additional information on the rural-to-urban transect can be found at the Center for Applied Transect Studies.
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