THE INNER WORKINGS OF THE POROUS PAVEMENT HYDROLOGIC DESIGN CALCULATOR 
DESIGN CRITERIA 
Design of porous pavement requires the optimization of a two main criteria:
You will answer the two questions of hydrologic and structural design separately and choose the most conservative (thickest) pavement section. The calculator you can download here will only help you answer the hydrolgoic design question. Hire a geotechnical engineer to answer the structural question. Caveat: This page is not meant to include all the information you need to design or make decisions about using porous pavement. For more information on porous pavement design and modeling, see the fact sheet "Porous Pavements".

DOWNLOAD CALCULATOR 
Simple porous pavement Hydrologic Design Calculator (Excel 255 kB): This calculator will help you determine the depth of rock (aka subbase, base course, base rock, aggregate) needed to infiltrate your desired storm. Porous pavement hydrologic design as a substitute for detention (Excel 309 kB): This calculator expands on the simple calculator. When working in jurisdictions that require detention, to use porous pavement, a designer will have to prove that porous pavement will entirely replace (and, in fact, probably function bettter) than a detetention pond. Since porous pavement is more expensive to install than impervious pavement, savings from not building a detention pond can be applied to the porous pavement instead. In addition, residential developments that can avoid detention basins will have another lot on which to build. Elements that are common to both calculators are addressed first on this page. Elements specific to the detention argument follow at the end.

OVERVIEW & MODELING METHOD 
Let's step through the logic for the Porous Pavement Hydrologic Design Calculator by first taking a look at a screenshot.The top portion is for user inputs. Once those are entered, the worksheet will automatically generate a hydrograph for the storm and calculate a few other handy items. The hydrogaph is generated using a Santa Barbara Urban Hydrograph (SBUH) Type IA rainfall distribution, but since the Urban Hydrology for Small Watersheds TR55 model (used to size detention basins, but now is being applied sometimes for LID practices) isn't applicable for storms under 1 inch and isn't very accurate for smaller sitesized areas, this calculator uses the rational method (Q=CIA) within each 10minute time increment to calculate the peak flow for that interval. Generally, the rational method isn't recommended for volume sensitive calculations such as this, but since we're incorporating the shape of the hydrograph into the calculations, this approach will work well, and a little more conservatively to model infiltration facilities. The hydrograph continues out to 4420 minutes, which equals 72 hours.

POSTDEVELOPED USER INPUTS FOR BOTH SIMPLE & DETENTION POROUS PAVEMENT CALCULATORS 
Using an open source Excel worksheet, learn the steps to hydrologically model porous pavement when it is installed to only manage rainfall, not runoff from other areas (aka hydrologically isolated). You may be surprised to see that soils with very low infiltration rates (as low as 0.1 inch/hour) are still appropriate for porous pavement. 
24Hour Rainfall Depth [in]: Enter the size of the storm that you're required or wish to infiltrate. A good minimum rule of thumb to protect streams from scouring is the 2year 24hour design storm, but use whatever your jurisdiction requires. 
Contribution Area [sf]: For hydraulically isolated areas (ie pavements managing only their own rainfall and not runoff from other areas), this equals the pavement area. For designs where storage rock will receive runoff from other areas like a roof, add the roof area to this number. Important side note for design: Don't drain untreated runoff into the surface or subsurface of porous pavement as this can and probably will clog the pavement. 
Storage Rock Area [sf]: This equals the pavement area installed over soils that can infiltrate. In some cases, for constructability, a portion of the pavement will be installed on compacted soil (aka fill). In this case the storage rock storage area equals the infiltration area, which is the area of native soil in a wet, uncompacted condition. 
Drainage Area Runoff Coefficient: This is the C in Q=CIA of the rational method. Click here to learn more about the rational method and to see typical values if you'd like to model this for all areas, not just impervious areas, draining to the planter. This is roughly equivalent to the percentage of runoff you expect from a particular land cover. Leaving this asis will be sufficiently conservative for any situation. 
Native Soil Infiltration Rate [in/hr]: Enter the infiltration rate of the native soils. This comes from running an infiltration test (see fact sheet "Infiltration Testing") in the location of the infiltration facility at the depth where the constructed facility intersects with the native uncompacted soil. This number cannot be altered by the design. 
Depth of Storage Rock [in]: Guess a value for the depth of storage rock (equivalent to the base rock) and check the calculated values. This is an interative process. For Type IA SBUH storms such as those found in Western Oregon, 12" is a good first guess, but you could also enter the depth of rock needed for structural stability for your traffic loads on your wet uncompacted native soils if you've gotten a geotechnical report already. 
Void Ratio for Rock Trench [%]: The rock trench is made up of open graded (all almost the same size) rock. The open graded rock provides storage for the additional volume of water that must be stored in the voids between the rocks. The void ratio is ratio of solid to voids and is often 40%, but your rock supplier should be able to confirm this. Some jurisdictions require you to assume 30% void ratio to account for fines moving upwards or downwards, which will reduce storage unless you deepen the rock. In effect, this requirement becomes a safety factor applied to a physical property that can be easily assessed. Since safety factors should be applied to design elements that we're unsure of, save yourself some money and confirm the void ratio of the rock that you'll be using. 
Outflow Elevation Above Bottom of Storage Rock [in]: Enter the depth of water allowed to pond by whatever overflow control structure you employ. If there is not a large storm overflow control structure (recommended), this value equals the depth of the storage rock entered above. This will be the maximum value possible for "Maximum Ponding Depth in Storage Rock During Storm" below. If you are not using a large storm overflow, ensure that the base rock is deep enough to be structurally sound when completely inundated, since buoyancy could come into play for some very large storm. 
Is Outflow Elevation Above Bottom of Storage Rock <= Depth of Storage Rock?: This checks that you entered numbers above that don't cause the impossible situation where the overflow elevation is higher than possible ponding depth. This should always be TRUE. Is this FALSE, increase Depth of Storage Rock or decrease Outflow Elevation Above Bottom of Storage Rock. 
POSTDEVELOPED CALCULATED VALUES  PONDING 
These values will tell you whether the Depth of Storage Rock you guessed at is adequate. 
Maximum Ponding Depth in Storage Rock During Storm [in]: This is the maximum depth to which the design storm will rise in the Storage Rock. This number is calculated from the hydrograph and usually occurs in the middle of the storm when the most intense rainfall is modeled to occur. If structural design doesn't require a deeper rock section, you could set your Depth of Storage Rock to this value and build a pavement section with this depth of storage rock. 
Depth of Water Left in Storage Trench After 30 Hours [in]: In some jurisdictions, this will be required to be zero becuase with our frequency of rain storms, the rule of thumb is that all stormwater facilities should be empty in 30 hours to be ready for the next storm. Keep reading to find out why. 
Size of Following Storm That Could be Stored in Remaining Rock [in]: If the geotechnical report recommends a rock section based on structural design (which should be entered in Depth of Storage Rock) that's greater than the Maximum Ponding Depth in Storage Rock During Storm, then the value reflected hereindicates that there's additional storage available for the next storm. If this value for storm depth is equal to or greater than the required design storm, then technically, the facility does not necessarily have to be empty in 30 hours to be ready for the next storm. 
Is the Storage Trench Empty in 30 Hours?: This checks the Depth of Water Left in Storage Trench After 30 Hours to see if there's a value in it. As described above, depending on the jurisdiction, requried 24Hour Design Storm, and the Depth of Storage Rock, this value doesn't necessarily have to be TRUE to have a pavement that functions well and is ready for the next storm. 
Is the Storage Trench Empty in 72 Hours?: This checks the depth of ponded water in column (12) at 72 hours to see if there's a nonzero value. If not, it returns a TRUE. This values should always be TRUE. 72 hours is the length of time mosquitoe larvae need to hatch and while it's unlikely that mosquitoes will be able to lay eyes in the pavement, mosquitoes have been found to hatch from ponded water in rip rap in bioretention facilities, so it's a good safe practice to make sure the facility is empty by then. Whatever you do, don't leave this FALSE. Either change your design or don't use porous pavement in the proposed location. If this is FALSE, you have a few options for different design situations:

OTHER POSTDEVELOPED CALCULATED VALUES 
Peak Rainfall Intensity [in/hr]: This provides the peak rainfall intensity for the storm entered, calculated from the maximum value in column (3) of the hydrograph. 
Peak Inflow Rate [cfs]: This is the peak rate at which rain falls onto the porous pavement, calculated from the maximum value in column (4) of the hydrograph. 
Peak Outflow Rate from Infiltration [cfs]: This is the effective infiltration rate of the facility. While the infiltration rate of the native soil is fixed, the effective infiltration rate of the facility depends on the area over which a volume of water is spread. Depending on the design, this may vary. In the case of a hydraulically isolated pavement with a Contribution Area equal to the Storage Rock Area, this will always be the same number, regardless of what area you enter. In the case of a facility that's receiving runoff pretreated for sediment from elsewhere, this number will vary. 
Peak Outflow Rate from Control Structure [cfs]: This is the rate at which stormwater overflows the control structure and leaves the site. If there's a value here, but the pavement was designed with no control structure, this means water is surcharging out of the top of the pavement during your design storm. This is dangerous and could cause structural failure if the pavement section is not designed for a buoyant condition. 
Ratio of Storage Rock to Contribution Area: This is provided for agencies who would like to create a SIM form similar to that of the City of Portland. If you look on page 4 of this link to Portland's SIM Form, you'll see what's called a "Sizing Factor". This number accounts for rainfall patterns (i.e. storm type), 24hour design storm size, the infiltration rate of the native soils, etc. For hydraulically isolated pavements where the Contribution Area is the same size as the Storage Rock Area, this number is always 1.0. 
Storage Capacity of Storage Rock [cf]: This calculates the volume of storage in the voids in the rock based on the Depth of Storage Rock and Void Ratio for Rock Trench values that you entered.

POSTDEVELOPED SBUH HYDROGRAPH FOR BOTH SIMPLE & DETENTION POROUS PAVEMENT CALCULATORS 
You should never have to edit any of this. 
Column 1,Time [min]: This is the progressive hydrograph time in 10 minute intervals. 
Column 2, Rainfall Depth [in] = 24 Hour Rainfall Depth [in, user input] x a constant that represents the unit hydrograph. All the numbers in the unit hydrograph for the first 24hours while the storm is happening add up to 1. 
Column 3, Rainfall Intensity [in/hr] = Rainfall Depth [in, Column 2] x 60 [min/hr] /10 [min] time interval 
Column 4, Inflow Rate [cfs] = Drainage Area Runoff Coefficient [unitless, user input] x Rainfall Intensity [in, Column 3] x ft/12 in x hr/3600s x Drainage Area [sf, user input]. This is the equation for the rational method Q=CIA where Q = flow [cfs], C = runoff coefficient, I = rainfall intensity [ft/s], and A = drainage area [sf]. 
Column 5, Inflow Volume [cf] = Inflow rate [cfs, Column 4] x 600s/time interval. We assume that for 10 minutes the flow rate is constant and we take the time out of the equation by multiplying the inflow rate by 600 seconds because that's how many seconds are in 10 minutes. 
Column 6, Runoff Depth [in] = Inflow volumne [cf, Column 5] / Drainage area [sf] x 12 [in/ft, conversion factor]. This is the depth of runoff generated during the 10minute interval spread out over the drainage area. 
Column 7, Facility Infiltration Rate [cfs] = Storage Rock Area [sf, user input] x Native Soil Infiltration Rate [in/hr, user input]/43200 [conversion factor ft/12in x hr/3600s]. The facility infiltraton rate accounts for the fact that as the Storage Rock Area increases, so does the infiltration area. As the infiltration area increases, more runoff can be infiltrated even though the infiltration rate of the native soils stays the same. This number only varies with the Storage Rock Area since the Native Soil Infiltration Rate is a constant obtained through field testing. 
Column 8, Inflow Rate  Facility Infiltration Rate [cfs] = Inflow Rate [cfs, Column 4]  Facility Infiltration Rate [cfs, Column 7]. This number is negative as long as the storm is generating flows less than the infiltration capacity of the facility. That means all the runoff coming into the facility is being infiltrated over the 10minute interval. 
Column 9, Inflow Volume  Facility Infiltration Volume [cf] = (Inflow RateFacility Infiltration Rate) [cfs, Column 8] x 600 [conversion factor because there are 600 seconds in 10 minutes). We need this to start tracking for use in Column 10, so we know how much runoff volume is left over in the 10minute interval because runoff rates exceed the infiltration capacity of the facility. 
Column 10, Cumulative Inflow Volume to be Stored [cf] = a conditional statement that tracks cumulatively how much runoff needs to be stored. If Column 9 is negative, then no runoff volume is accumulating and the value in this cell is assigned a zero (because the storm is not exceeding the infiltration rate of the facility yet). If Column 9 is positive, then there is runoff volume and the cell is assigned a value equal to the cell above it plus the cell to the left of it in Column 9. 
Column 11, Storage Rock Ponding without Control Structure[in] = a conditional statement that performs a stage/storage analysis of the Storage Rock filling with runoff, not accounting for Overflow Elevation Above Bottom of Storage Rock. If the volume of runoff to be stored in the storage rock (Column 10) is greater than the Depth of Storage Rock, then this cell maxes out at the Depth of Storage Rock and volume starts accumulating in the storage rock voids. The depth of water ponding in the rock trench [in] = Cumulative Runoff Volume to be Stored (cf, Column 10) / Storage Rock Area [sf] x 12 [in/ft, a conversion factor] / Void Ratio for Storage Rock [unitless]. Dividing the depth of excess rainfall by the void ratio gives the true depth of the runoff as it fills the rock trench. 
Column 12, Storage Rock Ponding with Control Structure [in] = a conditional statement that looks in the value (ie. row) after itself in Column 11 and compares it against the overflow elevation (i.e. Depth Above the Bottom of the Storage Rock). If ponding depth has not exceeded the overflow elevation, then this equals the ponding depth. The value stored here equals the overflow elevation, which tells the next column that for this 10minute interval, storage in the voids has been exceeded and runoff from the porous pavement is occuring. In this case, the value stored here equals the Inflow Rate in Column 4. The maximum value in this column is reflected under OTHER CALCULATED VALUES as Peak Outflow Rate from Control Structure.

ADDITIONAL WORKSHEET FOR DETENTION POROUS PAVEMENT CALCULATOR 
When working in jurisdictions that require detention, to use porous pavement, a designer will have to prove that porous pavement will entirely replace (and, in fact, probably function bettter) than a detetention pond. Since porous pavement is more expensive to install than impervious pavement, savings from not building a detention pond can be applied to the porous pavement instead. In addition, residential developments that can avoid detention basins will have another lot on which to build.The rest of this page is dedicated to the version of the porous pavement calculator available for download above that can be used to make this argument. Here's a screenshot showing how the first half and the hydrograph are the same as above: The hydrograph continues to 72 hours. 
CALCULATING PREDEVELOPED PEAK FLOWS  USER INPUTS 
Since detention requires that you detain a certain postdeveloped storm so that peak flows from the facility do not exceed predeveloped flows, there are two worksheets in this calculator, one to model postdeveloped flows, which we've already looked at in detail above, and one that models the predeveloped condition. Comparison of the two can be found on the postdeveloped worksheet, which we'll look at later. Here's a screenshot of the predev Excel tab: 
Predeveloped 24 Hour Rainfall Depth [in]: Enter desired predeveloped 24hour storm that the postdeveloped 24hour flood storm must be attenuated back to as required by your jurisdiction. (If you're the jurisdiction, requiring that the 25year storm be attenuated to the peak flow from 1/2 of the 2year storm may be stringent enough to protect streams from scouring.) 
Disturbance Area [sf]: This is the larger of either the postdeveloped Contribution Area or the Rock Storage Area. Do nothing and this will be enter automatically. 
Contribution Area Runoff Coefficient [unitless]: Enter 0.1 for woodlands or other appropriate number for your site's predeveloped land cover. Equivalent to the C in Q=CIA.

OTHER PREDEVELOPED CALCULATED VALUES 
Peak Predeveloped Runoff Rate [cfs]: Calculated from the predeveloped hydrograph by finding the largest value in Column 4, Inflow Rate. This is summarized on the postdeveloped worksheet under CALCULATED VALUES – 25YEAR STORM ATTENUATION  COMPLETE PREDEV MODEL WORKSHEET FIRST! 
Peak Rainfall Intensity [in/hr]: Calculated from the predeveloped hydrograph by finding the largest value in Column 3, Rainfall Intensity.

PREDEVELOPED SBUH HYDROGRAPH 
Equations for Columns 1 through 6 on the predeveloped hydrograph are the same as equations on the postdeveloped hydrograph. The only difference is the predeveloped hydrograph uses the 24Hour Rainfall Depth entered on the predeveloped worksheet.

POSTDEVELOPED CALCULATED VALUES – 25YEAR STORM ATTENUATION  COMPLETE PREDEV MODEL WORKSHEET FIRST! 
Peak Predeveloped Runoff Rate [cfs]: Calculated from the predeveloped hydrograph by finding the largest value in Column 4, Inflow Rate and copied over here for visual comparison to the Peak Postdeveloped Runoff Rate [cfs]. 
Peak Postdeveloped Inflow Rate [cfs]: Calculated from postdeveloped hydrograph distribution. Represents the rate at which rainfall enters the porous pavement. 
Peak Postdeveloped Outflow Rate from Control Structure [cfs]: Calculated from postdeveloped hydrograph distribution. This is the rate of runoff actually leaving the site, which is what interests regulators. 
Is Postdeveloped 25Year Flood Storm Attenuated? A conditional statement that confirms that the Peak Postdeveloped Outflow Rate from Control Structure is less than the Peak Predeveloped Runoff Rate. Increase Outflow Elevation Above Bottom of Storage Rock until this says TRUE and make sure that other conditional statements (in purple) are acceptable. 