Understanding Exfiltration
Introduction
"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:
1) 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.
Surface 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.
A 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:
H
V = Ks I and I = ---
L
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.
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.