Page 435 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

Low-Impact Development and Stormwater Issues 395

an upslope area of about 5.38 acres. This upslope area is mixed use and has about one-half with
a runoff coefficient of 0.80 and the other half around 0.30 (rational method). If you get a constant
rainfall of 2.9 in/h, what would you expect the steady-state (after the storage in the pervious
concrete is filled) runoff rate to be (volumetric flow) from the parking area from the entire drainage
area? Draw a rough site sketch. Note that the rate of infiltration through the pervious concrete is
much greater than the rate through the sand.

15. The parking lot in Exercise 10.14 is experiencing a lot of sand drifting and may occasionally
become fully covered with blowing sand, causing a reduction in the effective permeability of the
pervious system. It has been found by researchers that this reduces the system infiltration rate to
a fraction of the sand infiltration rate, and this fraction is the same as the fraction of the porosity in
the pervious, since this more accurately represents the flow paths to the sand. What is the worst
volumetric steady-state runoff rate that might be expected at steady state for the same conditions
in Exercise 14 if the pervious is fully clogged (covered) with this sand?
16. In Exercise 14, the engineer decides to alter the design and have this top 6-in layer of pervious
concrete of 25 percent porosity over a 6-in layer of gravel with a porosity of 45 percent. This will
be placed over the same native sand of 0.022 cm/s permeability. Assume the same upslope active
runoff conditions and the same steady rainfall of 2.9 in/h. What is the expected time to the start of
runoff from the pervious parking lot if the full upslope active runoff and the direct rainfall start at
the same time? Assume the lot to be fairly flat. (Note that there is a time delay to maximum runoff
from other areas based on the time of concentrations, but we are ignoring that here.)
17. Many engineers are worried that if placed on slopes, water will just pour out of pervious
pavement, especially if it is placed over poorly draining soils. Typically soils have the following
infiltration rate ranges:

Soil Infiltration Rate Order of Magnitudes
Gravel >1 cm/s
−2
Sand 10 cm/s
−6
Silt 10 cm/s
−8
Clay <10 cm/s
If the rainfall rate is greater than the soil percolation rate, then when the “available” storage in the
pervious and subbase layers is full, there will be runoff. In previous exercises we assumed that the
available storage was all the voids in the pavement and gravel layers, but in this case the available
storage can be reduced when built on a slope. You are proposing a 5-ft-wide sidewalk, 100 ft long
with the length sloping with the grade (width is approximately horizontal). The sidewalk is made
with a 4-in-thick top layer of pervious concrete of 25 percent porosity and a 4-in-thick subbase
of gravel of 40 percent porosity. Calculate, for a rainfall rate of 1 in/h over silt, an estimate of the
time until runoff starts if the sidewalk is on a 2 percent grade. Make the following assumptions to
simplify the analysis. Assume that friction through the pervious and gravel is negligible, so the
water fills the storage volumes right away. (Typically the flow rate through unclogged pervious
concrete is much greater than 0.3 cm/s.) Draw a rough sketch to explain your estimates. Is it
realistic to assume that the rainfall falls at this rate for the time estimated?
18. Repeat Exercise 17 if the sidewalk is on a 10 percent grade.
19. Repeat Exercise 17 if the sidewalk is built over sand.

20. Repeat Exercise 18 if the sidewalk is built over sand.