Prev Topic Home Up Next Topic

HydroCAD Stormwater Modeling - Since 1986


Understanding Exfiltration


"Exfiltration" refers to a loss of water from a drainage system as the result of percolation or absorption into the surrounding soil.  When performing a reach routing, a constant exfiltration (CFS) can be subtracted from the inflow hydrograph to account for these losses.  However, the more common (and recommended) technique, is to implement exfiltration within a "pond".

Three calculations options

HydroCAD provides three options for calculating pond exfiltration:

Click for complete self-training materials1) A constant flow can occur whenever a specified elevation is exceeded. For example, a drywell might exfiltrate 2.2 CFS when the level exceeds 102 feet.

2) A constant velocity can occur over an available area.  For example, an exfiltration rate of 15 mm/hour means that a water layer of 15mm will take one hour to exfiltrate.  The available area can be based on either surface area or wetted area, and may exclude areas that lie above or below specified elevations.

3) You can specify the saturated hydraulic conductivity, and have the exfiltration calculated by Darcy's Law based on the distance to groundwater.  The available area can be based on either surface area or wetted area, and may exclude areas that lie above or below specified elevations. This option was added in HydroCAD-9.0.

Setting the invert elevation

By default, exfiltration will occur if there is any water in the pond.  But if the lower areas of the pond are impervious, you can select the option to "Allow Exfiltration only above invert."   This allows an effective invert elevation to be specified, and exfiltration will occur only when the water surface exceeds this level. When using the velocity method, exfiltration will apply only to any additional area lying above the invert.

If the upper areas of the pond are impervious, you can also set a maximum exfiltration elevation by selecting "and below maximum".  This will exclude all areas at and above the maximum from the exfiltration calculation.  This can be used in conjunction with the minimum (invert) elevation to allow exfiltration only within a specific elevation range.  If you don't want to apply a minimum, make sure the invert is below the bottom of the lowest pond storage.

Click for complete self-training materialsSurface area or wetted area?

By basing exfiltration on surface area you are stating that all flow will essentially be downward. Only horizontal areas (above the invert) are available for exfiltration. All vertical areas are excluded.

If you wish to include vertical surfaces, such as the sides of a drywell, then you may want to specify wetted area. As always, it is your responsibility to ensure that this computation is applicable to your particular situation.

Gravel-filled exfiltration areas

An increasingly common means of dispersing stormwater is through a gravel-filled trench or drywell.  This can be modeled using the same techniques described above, except that an adjustment must be made for the fact that only a fraction of the overall trench volume is available for storage.  This is readily done in HydroCAD-5 (and later), by specifying the percentage of voids for the applicable storage definition.   This allows accurate modeling of the available storage, while still considering the volume of the overall trench for the purposes of determining the exfiltration area.

Click for complete self-training materialsA perforated pipe or chamber is often buried in a gravel-filled trench, serving as an inlet device, as well as providing additional storage.  This complicates the model in two ways.  First, the interior of the pipe or chamber provides 100% open storage space, in contrast to the reduced storage of the surrounding stone.  If the pipe/chamber volume is small in comparison to the overall storage, you may chose to simplify the model by neglecting the additional storage.  On the other hand, if the pipe/chamber is large and you want to allow for the full storage of the pipe  plus the gravel voids, you can use the embedded storage capability of HydroCAD-7 (and later).  For earlier versions, you will need to determine the overall stage-storage curve by hand and enter this directly into the model.

The second possible effect of a perforated inlet pipe would be as an additional flow restriction.  Modeling this situation might require that the pipe be considered as the outlet of a suitable upstream "pond", or perhaps as a compound outlet in conjunction with the exfiltration.  In most situations, these complications are readily avoided by ensuring that the perforated pipe is able to pass a greater flow that can pass from the trench by exfiltration.  This preserves the "level pool" routing assumption.

Advanced techniques

While most cases will require just a single exfiltration device, it is also possible to use several exfiltration devices on a single pond. This could be used to model multi-stage exfiltration schemes, such as a drywell that overflows into a perforated pipe.

As with all pond designs, you should view and understand the stage-discharge plot to make sure the pond is exhibiting the behavior you expect. Do not rely solely on a review of the hydrograph, since any problems in the stage-discharge relationship may not be apparent.

Design Guidelines

While some systems may have enough exfiltration capacity to dispose of stormwater runoff as it occurs, many systems will take many hours or even days before a significant fraction of a rainfall event can be discharged through exfiltration.  Under these circumstances, exfiltration is not an effective means of short-term runoff management.  These systems must have enough storage (detention) capacity to hold a large portion of the runoff volume over the longer time period that is required for complete exfiltration to occur.

A second consideration is that the infiltration capability of most sites can be expected to degrade over time.  This can occur because of inadequate sediment removal before runoff reaches the exfiltration area,  and/or lack of proper maintenance.  Because of this likelihood, some stormwater management rules may not allow credit for exfiltration as part of the runoff analysis.  This doesn't mean that stormwater systems shouldn't be designed with as much exfiltration capacity as possible - they should, if only to avoid depletion of groundwater after site development.  However, a conservative stormwater design will have sufficient capacity to handle the required events without any short-term exfiltration, allowing the detained volume to exfiltrate over an extended period time.

Groundwater Effects

Any groundwater effects must be included in the constant flow or velocity that you use with HydroCAD.  Any changes in groundwater (such as groundwater mounding) will generally occur over a longer period of time, and often have little effect on the typical peak-flow analysis.  In cases where the mounding is significant, a worst-case (minimum) exfiltration rate should be used.

Using the Discharge Multiplier

To determine the effects of reduced exfiltration capacity, you can reduce the appropriate "Discharge Multiplier" to a value less than "1".  You can even set the multiplier to zero in order to "turn off" the exfiltration entirely.  Making these changes with the appropriate report window(s) open will let you immediately see the effects of each scenario.

Note: When using multiple storage chambers, the exfiltration velocity is applied to the total storage area, which already allows for the number of chambers.  Do not increase the discharge multiplier in these cases, since this will cause a double adjustment for the exfiltration flow!  (In other words, set the storage multiplier, but leave the discharge multiplier at 1.)  To verify the actual area being used for exfiltration, see the pond summary report and the stage-storage report.

What is the Exfiltration Velocity?

The exfiltration velocity specifies the volume of water that will pass through a given area during a certain period of time.  This is also known as the flux or flux velocity.  It may also be expressed as [Volume/Time]/[Area] or [Volume]/[Area*Time], which both reduce to [Length]/[Time], which is a velocity.

For saturated media, the exfiltration velocity is related to the hydraulic conductivity by Darcy's Law:

V = Ks I   and   I = ---

Where V is the exfiltration velocity, Ks is the saturated hydraulic conductivity, and I is the hydraulic gradient.  H is the head difference across the media, and L is the media length.  Note that if the head (H) is not much greater than the distance to groundwater (L), the gradient is approximately equal to one, and the exfiltration velocity is approximately equal to the conductivity.  For other scenarios, the exfiltration velocity can be determined by multiplying the conductivity by the hydraulic gradient.

Note that Darcy's Equation applies only to saturated media.  For unsaturated conditions, other effects such as capillary action must be considered in determining the exfiltration velocity.

The term "Permeability" is sometimes used as a synonym for Conductivity. However, Permeability is a less precise term with several different meanings. The more precise term, Conductivity, is therefore used in this document.

Typical Exfiltration Rates

Exfiltration velocity can vary widely depending on soil conditions and groundwater levels. Common values may fall anywhere from 0.1" to 100" per hour (2 to 2500 mm/hour).  For designs that are critically dependent on the exfiltration rate, actual site testing is recommended.  Moisture levels, water tables, and other factors can significantly reduce the actual values.


HydroCAD 7.1 and later

HydroCAD 7.0 and earlier


0.1-100 inches/hour

0.0001-0.1 feet/minute


2-2500 mm/hour

0.03-40 mm/minute

Common Conversion Factors

bulletTo convert mm/min to mm/hr, multiply by 60.
bulletTo convert ft/min to in/hr, multiply by 720.
bulletTo convert cfs/sf to in/hr multiply by 43,200.

Using a percolation rate

A measured perc rate can be converted to an equivalent exfiltration velocity by the following equation. However, other factors must be considered to determine if this is a reasonable design value for a proposed exfiltration area. (For example, can a large pond be expected to perc at the same rate as a small test pit over a period of many hours?)

V = 60 / P   where:

    V=Exfiltration velocity [inches per hour]
    P=Perc Rate [minutes per inch]
    60=Units conversion factor

Dealing with oscillations

When setting an exfiltration velocity, oscillations can occur as the exfiltration rate increases suddenly from zero to full flow.  This can usually be corrected by setting the exfiltration phase-in depth to a small non-zero value, such as 0.01 feet.  Details here.

[Home]   [Support] 
Copyright 2022 HydroCAD Software Solutions LLC (privacy)   (legal notices)