POROUS PAVEMENT MODELING
© Clear Creek Solutions, Inc., 2010
Porous pavement includes porous asphalt or concrete and grid/lattice systems (nonconcrete) and paving blocks. The use of any of these types of porous pavement requires
that certain minimum standards and requirements are met related to subgrade, geotextile
material, separation or bottom filter layer, base material, wearing layer, drainage
conveyance, acceptance testing, and surface maintenance.
WWHM4 has a new porous pavement element that incorporates both the standard basin
element and the gravel trench bed element to represent porous pavement. The basin
element input information represents the surface of the porous pavement and the gravel
trench bed element information represents the subgrade.
Porous Pavement Flow Paths
Evaporation from
pavement
Surface Runoff
Rain on
pavement
Infiltration through pavement
Infiltration to gravel subgrade
Underdrain Flow
Infiltration to
native soil
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WWHM4 Porous Pavement Modeling
Porous pavement allows for infiltration through the pavement layer into a gravel/crushed
rock subgrade with option of infiltration to the native soil and/or underdrain flow.
Surface runoff only occurs when ponding occurs on the pavement surface following
complete saturation of the subgrade and pavement layers.
The porous pavement element should only be used when the following three conditions
are met:
1. The infiltration rate of the porous pavement is greater than the peak rainfall rate.
2. The infiltration rate of the porous pavement is greater than the underlying native
soil.
3. There is subgrade layer of crushed rock/gravel between the porous pavement and
the native soil.
For the purposes of this example we will model both conventional pavement and porous
pavement side-by-side so to show the difference between the two. So let’s get started.
We will model a 5-acre parking lot adjacent to the Seattle-Tacoma (SeaTac) Airport. The
first thing that we will do is to locate our project on the project map.
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WWHM4 Porous Pavement Modeling
SeaTac Airport is located in King County, Washington. We click on the map to select
the project location. Based on our project location WWHM4 selects the appropriate
precipitation record and precipitation multiplication factor. We then have the option to
fill in the Site Information boxes.
For the Predevelopment scenario we select a standard land use basin. We have to decide
the appropriate predevelopment land use for the project site. Because this project is
located in Western Washington and has to meet Washington State Department of
Ecology regulations, we select C soil, Forest vegetation, and Flat land slope (0-5%).
The project site is 5 acres.
Because we are modeling both conventional pavement and porous pavement side-by-side
(normally you would just model one or the other), we have two basins with the same
information. POC 1 represents the conventional pavement predevelopment runoff; POC
2 represents the porous pavement predevelopment runoff. Because the predevelopment
land use is the same for both POC 1 and POC 2 the predevelopment runoff will be the
same. This will not be true for the mitigated (post-development) runoff.
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WWHM4 Porous Pavement Modeling
For the Mitigated scenario we will use the standard basin element to represent the
conventional impervious pavement and the porous pavement element to represent the
porous pavement.
The conventional pavement will still need some form of mitigation for the extra runoff
produced when the forest is removed and the 5-acre site paved. One mitigation option is
to place a stormwater vault under the parking lot to collect the runoff and then discharge
it at the predevelopment flow rate. That is what we will do in this example.
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WWHM4 Porous Pavement Modeling
First we add the basin element, rename it “Conventional Pavement”, and assign 5 acres of
Parking, Flat land use.
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WWHM4 Porous Pavement Modeling
We add a vault element (naming it “Conventional Vault”) and connect its outlet to POC
1. Now we can use Auto Vault to size the vault.
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WWHM4 Porous Pavement Modeling
We set the adjustment level (amount of optimization) to its highest level (5) and then
click “Create Vault”. Take a break while Auto Vault runs through its iterations and I will
meet you on the far side after it has finished.
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WWHM4 Porous Pavement Modeling
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WWHM4 Porous Pavement Modeling
Auto Vault is finished. It gives us a conventional vault size of 133 feet long by 133 feet
wide by 7 feet deep. Big vault.
Now we will model the same 5-acre parking lot with porous pavement.
We add the porous pavement element and designate it as POC 2. We now have to input
the element information.
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WWHM4 Porous Pavement Modeling
The first thing that we will do is to input the facility dimensions. The parking lot is 5
acres in size. We need to convert that area into an equivalent length and width. Five
acres is 217,800 square feet and that can be represented by a rectangle with a length of
500 feet and width of 435.6 feet. We will skip the effective total depth for the moment
(but we will come back to it) and enter the bottom slope (0.01 ft/ft).
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WWHM4 Porous Pavement Modeling
The next input is the layers for the porous pavement and subgrade. We can have up to
three layers. The first layer is always the porous pavement. We input the thickness of
the porous pavement layer (6 inches or 0.5 feet) and the porosity of the pavement.
Consult the manufacturer for the porosity of their particular pavement. For the purpose
of this example we will assume that the porous pavement porosity is 0.30 (30% void
space).
The next layer is the subgrade. Usually this layer consists of gravel or crushed rock. It
can be as thick or thin as we think is appropriate. For this example we will assume that
the subgrade layer consists of one foot of gravel with a porosity of 0.35 (0.35%).
We still need to decide on how we want to handle native infiltration and the underdrain
information. Let’s first talk about native infiltration.
At the project site the native soil is a C soil with poor drainage/infiltration. We need to
talk to the local municipal permitting agency about whether or not we can include
infiltration to the native soil in our porous pavement model. Most agencies are going to
be skeptical about allowing any infiltration to a C native soil. However, they may be
more agreeable to the idea if they think long-term instead of short-term.
With traditional single event modeling the focus was always on the short-term benefits of
infiltration. Only high infiltration rates are important when computing runoff from a
short-term single storm event. However, with long-term continuous simulation modeling
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WWHM4 Porous Pavement Modeling
(which is what we are doing here) even very low infiltration rates are significant. This is
because the facility (pond, tank, vault, bioretention area, porous pavement subgrade, etc.)
can drain to the native soil over weeks and months, instead of just hours and days.
For our example, let’s assume that the local municipal permitting agency allows a native
soil infiltration rate of 0.01 inches per hour (that is about 1 inch per 4 days). In a month 8
inches of water can drain into the native soil. That is 8 feet of water per year. So, an
infiltration rate of 0.01 inches per hour may be slow but it adds up over time and that is
why we want to turn on infiltration (Infiltration = YES) and include it in the drainage
calculations.
We input a measured infiltration rate of 0.01 inches/hour. A reduction factor is also
required. The reduction factor is multiplied by the measured infiltration rate to get the
model’s long-term infiltration rate (so don’t leave it at zero or there won’t be any longterm infiltration). The reduction factor is the same as 1/safety factor and accounts for the
potential of long-term clogging of infiltration facilities. For an infiltration pond the
Washington State Department of Ecology requires a safety factor of 4 (reduction factor of
¼ or 0.25). Currently there are no safety factor guidelines for porous pavement. I argue
that the porous pavement safety factor should be 1 because opportunities for sediment to
clog the subgrade/native soil interface are minimal at best. So we will use a reduction
factor of 1 for this example.
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WWHM4 Porous Pavement Modeling
There is a “Size Pavement” button below the Infiltration input that may have piqued your
curiosity. For this example project we are not going to use it, but since we are here I
might as well tell you how it works and when to use it.
We give the user the option to route runoff from another element (that other element must
be a lateral flow basin). That means that you can have some pervious area (like a lawn)
or impervious area (like an impervious road surface) drain directly to the porous
pavement element. If you have a target percent infiltrated (say, 100%) then by clicking
on the “Size Pavement” button you will let WWHM4 determine how big the porous
pavement area must be to achieve your target infiltration percentage.
It doesn’t make sense to use the “Size Pavement” option if the only area draining to the
porous pavement is the porous pavement area itself. When there is a 1 to 1 ratio of the
surface area to the infiltration area (as there is with the porous pavement element without
any lateral flow connections) then the percent runoff infiltrated is only a function of the
native infiltration rate and the storage in the pavement and subgrade layers plus whatever
surface ponding is allowed (more on that below). Size doesn’t matter.
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WWHM4 Porous Pavement Modeling
Now let’s talk about whether or not we want to include an underdrain. The underdrain
has a diameter and a height above the subgrade/native soil interface (the bottom of the
subgrade layer). You can have both an underdrain and native infiltration; in that situation
most of the water will exit via the underdrain. So, because of that, it is usually best to
have either an underdrain or native infiltration, but not both.
The advantage of an underdrain is that it keeps the subgrade from becoming completely
filled with water, especially if infiltration into the native soil is not allowed. The
disadvantage is that the discharge from the underdrain must still meet flow control
requirements. So, with an underdrain a stormwater mitigation facility (pond, tank, vault,
etc.) will be needed for the porous pavement parking lot. The stormwater mitigation
facility will be smaller than if we use conventional impervious pavement, but we are still
going to need one to control the underdrain discharge. I generally recommend against
using an underdrain unless there is absolutely no other way to handle the runoff. For this
project we are going to model the parking lot without an underdrain.
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WWHM4 Porous Pavement Modeling
The last piece of needed information is the depth of ponding allowed on the surface of the
porous pavement.
The question is: “How deep does the water get on the surface of the pavement before
runoff occurs?” Typically there is going to be some ponding. We just have to tell the
model how much. Without a curb to contain the water we are probably going to get only
about an inch of water ponding on the surface of the pavement before it starts to runoff in
one direction or another. Based on that idea we set the ponding depth to 0.1 feet. If you
are not sure about the appropriate ponding depth check first with the local municipal
permitting agency to make sure that they are okay with whatever assumption you are
using.
We have one last input to make before running the model with porous pavement.
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WWHM4 Porous Pavement Modeling
Earlier we skipped over the input of the effective total depth. Now we need to put in a
value. The effective total depth is the sum of the porous pavement and subgrade layers
plus the surface ponding depth plus any freeboard. We have not input a freeboard value,
but freeboard is just an arbitrary height above the top of the facility at which discharge is
occurring. Typically, freeboard is set to 1 foot. If we use a freeboard of 1 foot for this
porous pavement element then our effective total depth equals
1.0 ft subgrade layer +
0.5 ft porous pavement layer +
0.1 ft ponding on porous pavement surface +
1.0 ft freeboard
2.6 ft
There is nothing magical about this number. The key is to make sure that the effective
total depth is large enough so that during extreme events the simulated water level never
exceeds the effective total depth during the entire multiple-year simulation period. If, for
some reason, the effective total depth is exceeded (and you get an error message to that
effect) then simply increase the effective total depth until that doesn’t happen.
Now we are finally ready to click the “Run Scenario” button and see how our porous
pavement element does infiltrating the runoff to the native soil.
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WWHM4 Porous Pavement Modeling
The results show that we get 100% infiltration into the native soil. This might be
surprising considering the low (0.01 inch per hour) infiltration rate we used. However,
because of the large contact area between the subgrade and the native soil (5 acres in
size) there was plenty of opportunity to infiltrate when compared with the contributing
surface area (also 5 acres). This is what makes porous pavement so effective.
If we had less than 100% infiltration we could increase the subgrade layer depth (and
storage) to achieve 100% infiltration.
We don’t do any flow duration analysis when we have 100% infiltration because there
are no stormwater surface discharge flows to analyze. However, if we had an underdrain
in the project then we would have to conduct a flow duration analysis of the underdrain
flows (they are considered surface discharge flows) to find out whether or not they met
the flow control standard.
With 100% infiltration no vault is needed. And we are finished.
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WWHM4 Porous Pavement Modeling
SUMMARY (for porous pavement element only):
1. Locate project site on map.
2. Input Pre-project for each basin in the project site. Connect the
Pre-project basin to the POC 1. Run Scenario.
3. Add the porous pavement element to the Mitigated scenario.
4. Input the porous pavement element dimensions.
5. Input the subgrade, native infiltration, and surface ponding
information to the porous pavement element.
6. Decide whether or not to use underdrain.
7. Connect porous pavement element to POC 1.
8. Run Scenario. Look at Percent Infiltrated. If less than 100%
then go to Analysis screen and compare flow durations. Make
changes if necessary.
9. Finished.
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