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ACTIVITY: Underground Drainage Systems
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125
Targeted Constituents
Significant Benefit
z Sediment
Heavy Metals
Nutrients Toxic Materials
z
Partial Benefit
{ Low or Unknown Benefit
Floatable Materials
Oxygen Demanding Substances
Oil & Grease { Bacteria & Viruses { Construction Wastes
Implementation Requirements
z
High
Capital Costs
O & M Costs
Medium
z Maintenance
{ Low
{ Training
Description
This BMP covers subsurface infiltration BMPs such as drywells and vaults.
Infiltration rates in much of the state are typically poor due to clay soils and bedrock.
Such locations may not be suitable of infiltration BMPs. Infiltration systems work best
at sites having sandy loam types of soils. Areas containing karst topography and
sinkholes may initially appear to have excellent infiltration, but should be considered
as unreliable and will require very careful investigation and analysis.
Selection
Criteria
Underground drainage systems, such as drywells and vaults are suitable for
draining small impervious surfaces, such as parking lots or residential rooftops, for
which the adjacent pervious area has soils with adequate infiltration rates.
Natural sinkholes (or other evidences of karst topography and drainage) are not
considered to be infiltration systems for use in treating stormwater quality or in
providing stormwater detention. In general, stormwater drainage may continue to
flow to a natural sinkhole at a rate that is representative of natural undeveloped
conditions. No unusual or unfavorable geologic conditions shall be present near
the sinkhole that indicates subsidence, piping, increased limestone dissolution,
potential collapse or other safety concerns.
Design and
Sizing
Considerations
Infiltration can be a very desirable method of stormwater treatment for land uses which
do not heavily pollute stormwater runoff. For instance, established residential areas
typically have less pollution than industrial and commercial areas. The primary
physical conditions necessary for infiltration are: 1) permeable soils which have not
been compacted or graded, and 2) low and non-interfering groundwater tables.
Stormwater runoff from parking lots or buildings should be pretreated with a water
quality enhancing inlet, oil/water separator, grass swale or other type of stormwater
treatment BMPs. Small amounts of stormwater runoff from selected impervious areas
are given an opportunity to infiltrate. A factor of safety should be incorporated into the
design to ensure that the system still works even when partially clogged.
The recommended minimum infiltration rate is at least 0.5 inches per hour, but may
depend on type of infiltration system and the desired water quality treatment involved.
Drawdown should occur within 48 hours. An infiltration basin or trench must have at
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least 3 feet separation from seasonal high groundwater and at least 4 feet separation
from bedrock. Coarse soils are not as effective in filtering groundwater; therefore
provide at least 6 to 8 feet separation from seasonal high groundwater for sand and
gravel soils.
Unless adequate engineering documentation is submitted, an infiltration system must
be located at least 100 feet away from any drinking water well, septic tank or
drainfield. It is also recommended that an infiltration trench should not be located near
building foundations, buildings with basements or crawl spaces, major roadways,
wetlands, streams, or potentially unstable slopes and hillsides.
Overview of Infiltration Theory
The overall degree of water quality treatment achieved by infiltration is a function of
the amount of stormwater that is captured and infiltrated over time. Minimum
infiltration storage is generally required to be the first flush volume.
Typical infiltration rates are shown in Table I-03-1. The USDA soil texture
classification is based upon the triangle shown in Figure I-03-1, with the following
definitions:
Approximate size
Rough description
Gravel
Sand
Silt
Clay
> 2 mm
0.05 mm to 2 mm
0.002 mm to 0.05 mm
< 0.002 mm
> No. 8 sieve or so
> No. 200 sieve
Little plasticity or cohesion
Can be rolled and compressed
For preliminary design, infiltration rates may be estimated using a published soil
survey. However, final design must include soil gradation testing and measurement
of unsaturated vertical infiltration rates in the field by the double-ring infiltrometer
test. This test is not appropriate for clay soils or other soils which clearly appear to
be unsuitable for infiltration methods. The allowable infiltration rate is 0.5 inches
per hour, although an infiltration rate of 1 inch per hour is highly recommended.
Table I-03-1 shows that soils with a hydrologic soil group of C or D will not have
sufficient infiltration rates.
Another well-known method of categorizing soils and evaluating soil properties is
by the Unified Soil Classification System (USCS). The following soil groups are
generally acceptable as good soils for infiltration:
SW
SP
SM
Tennessee BMP Manual
Stormwater Treatment
Well-graded sands and gravelly sands, little or no fines
Poorly graded sands and gravelly sands, little or no fines
Silty sands, sand-silt mixtures
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Table I-03-1
Typical Infiltration Rates from USDA Soil Texture
Typical Water
Capacity
(inches per
inch of soil)
0.35
0.31
0.25
0.19
0.17
0.14
0.14
0.11
0.09
0.09
0.08
USDA Soil Texture
*
**
**
**
Sand
Loamy sand
Sandy loam
Loam
Silt loam
Sandy clay loam
Clay loam
Silty clay loam
Sandy clay
Silty clay
Clay
Typical
Infiltration Rate
(inches per hour)
8.27
2.41
1.02
0.52
0.27
0.17
0.09
0.06
0.05
0.04
0.02
Hydrologic
Soil Group
A
A
B
B
C
C
D
D
D
D
D
* - Suitable for infiltration with typical 6’ to 8’ separation from seasonal high groundwater
** - Suitable for infiltration with at least 3’ separation from seasonal high groundwater
Clay
Sandy
Clay
Loam
Loam
Sandy
Loam
Loamy
Sand
Figure I-03-1
USDA Soils Triangle
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Natural Depressions, Sinkholes, and Karst Topography
Much bedrock in Tennessee is composed of fractured limestone formations that are
likely to contain unusual strike angles and/or nonconformities. Karst topography is
defined as the presence of limestone or other soluble geology that is likely to form
caverns, sinkholes, or other dissolved formations. A sinkhole is a surface depression,
typically linked to an underground cavern system, which occurs primarily in limestone
regions. See Figure I-03-3 for a typical sketch of a sinkhole.
For natural depressions and sinkholes, it is generally required that the post-developed
peak flows and total stormwater runoff volume must be limited to the pre-developed
values. In addition, it may be required that no structures will be flooded from a 100year storm assuming plugged conditions (zero outflow). It is greatly desired that
runoff should be treated using one or more stormwater treatment BMPs, prior to
discharging toward a sinkhole or other natural depression.
Consideration may be given to recommendations that are based upon advanced
subsurface testing or visual inspection by experts or professional engineers with
demonstrated experience in hydrogeology. Tennessee Department of Environment and
Conservation (TDEC) requires anyone who performs a dye trace study to obtain a
TDEC registration for this activity (see TDEC website). Major sinkholes are
considered to be waters of the state; filling or otherwise altering a large sinkhole
requires an Aquatic Resources Alteration Permit from TDEC.
A drywell or dry vault can be used to infiltrate stormwater runoff from small areas of
impervious runoff, such as roofs or parking lots. The designer should be very careful
to avoid adverse impacts to foundations, basements, unstable slopes or hillsides, septic
tanks, utility lines, etc. A small pretreatment chamber with a screen is recommended
in many instances to handle leaves (roofs) or trash and sediment (parking lots).
A typical drywell adjacent to a house foundation is shown in Figure I-03-2 (without a
pretreatment chamber). A dry vault (larger than a drywell) can be constructed using
masonry blocks and a poured concrete lid to hold a larger volume of stormwater
runoff. Inspect the drywell or dry vault on a regular basis.
Construction/
Inspection
Considerations
It is very important to protect the natural infiltration rate by using light equipment
and construction procedures that minimize compaction. Stormwater must be
allowed to enter the facility until all construction in the catchment area is
completed and the work area is stabilized. If this prohibition is not feasible in
particular situations, do not excavate the facility to final grade until after all
construction is complete upstream.
Protect infiltration surface during construction.
Inspect frequently for clogging during construction.
Maintenance
Tennessee BMP Manual
Stormwater Treatment
Improperly functioning infiltration systems must be replaced by other stormwater
treatment BMPs that are capable of providing water quality treatment.
Maintenance can be difficult and costly for infiltration systems, with a potential
for high maintenance costs due to clogging. Maintenance costs and site access
should be carefully considered prior to design.
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Pretreatment of stormwater runoff may reduce maintenance costs by capturing
coarse sediments and floatable materials in a smaller structure that can be more
easily cleaned.
Inspect and observe the infiltration system several times during the first year,
particularly after heavy rainfall events. Use observation wells and cleanout
ports to monitor water levels and drawdown times. Record all observations and
measurements taken. Perform any maintenance and repairs promptly.
Inspect the infiltration system annually thereafter, and after extreme rainfall
events. If stormwater does not infiltrate within 48 hours after a storm, it is
generally time to clean, repair or replace the facility. Remove debris and
sediment at least annually to avoid high concentrations of pollutants and loss of
infiltration capacity.
The primary objective of maintenance and inspection activities is to ensure that
the infiltration facility continues to perform as designed. Regular inspection
can substantially lengthen the required time interval between major
rehabilitations.
Prevent compaction of the infiltration surfaces by physical controls such as
gates or fences. Maintain dense grass vegetation for infiltration basins. Use
rotary tillers on infiltration surfaces when needed to restore infiltration capacity
and to control weed growth.
Maintenance plans should include provisions to repair or replace this type of
structure after 5 years or so.
Maintain records of inspections and maintenance performed.
Sediment Removal
A primary function of stormwater treatment BMPs is to collect and remove
sediments. The sediment accumulation rate is dependent on a number of factors
including watershed size, facility sizing, construction upstream, nearby industrial or
commercial activities, etc. Sediments should be identified before sediment removal
and disposal is performed. Special attention or sampling should be given to
sediments accumulated from industrial or manufacturing facilities, heavy
commercial sites, fueling centers or automotive maintenance areas, parking areas,
or other areas where pollutants are suspected. Treat sediment as potentially
hazardous soil until proven otherwise.
Some sediment may contain contaminants for which TDEC requires special
disposal procedures. Consult TDEC – Division of Water Pollution Control if there
is any uncertainty about what the sediment contains or if it is known to contain
contaminants. Clean sediment may be used as fill material, hole filling, or land
spreading. It is important that this material not be placed in a way that will promote
or allow resuspension in stormwater runoff. Some demolition or sanitary landfill
operators will allow the sediment to be disposed at their facility for use as cover.
This generally requires that the sediment be tested to ensure that it is innocuous.
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Cost
Considerations
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Maintenance is difficult and costly for underground trenches.
Potential for high maintenance costs due to clogging.
Pretreatment will reduce maintenance costs by capturing gross settleable solids
and floatables in a smaller space that can be more easily cleaned.
Limitations
The four major concerns with infiltration systems are clogging, potential impact
on other structures and properties, accumulation of heavy metals, and the
potential for groundwater contamination.
Clogging and high maintenance costs are very likely to occur in fine soils that
are marginally allowable for infiltration rates. Erosion control is extremely
important to prevent clogging; infiltration systems fail if they receive high
sediment loads. Perform regular maintenance and inspections to minimize the
potential for clogging and loss of infiltration capacity. Pretreatment is highly
recommended for stormwater runoff from many land uses, prior to discharging
to an infiltration system.
Infiltration systems are not appropriate for areas with high groundwater tables,
steep slopes, lots of underground infrastructure, and nearby buildings.
Heavy metals are likely to settle in any of the stormwater treatment BMPs, but
particularly for infiltration systems (which have the lowest velocity). High
levels of heavy metals have been observed in other states where adequate
maintenance was not performed. Toxic levels are not likely to be exceeded, but
the sediments will need to be handled as hazardous waste after a few years of
neglect.
There is a higher risk of groundwater contamination in very coarse soils. It is
highly recommended that a monitoring and inspection program should be used
to verify that no contamination occurs. Infiltration systems may not be
appropriate where there is significant potential for hazardous chemical spills, or
near drinking water wells.
Additional
Information
Tennessee BMP Manual
Stormwater Treatment
Underground drainage systems are suitable only for small sites of a few acres.
Infiltration systems or wet detention should be considered where dissolved
pollutants discharging to surface waters are of concern. However, satisfactory
removal efficiencies require soils that contain loam. Coarse soils are not effective
at removing dissolved pollutants and fine particulates before the stormwater
reaches the ground water aquifer.
Problems can be expected with infiltration systems placed in finer soils. The State
of Maryland has emphasized these systems for about 10 years where they have
been installed in soils with infiltration rates as low as 0.27 inches (0.69 cm) per
hour. A recent survey (Lindsey, et al., 1991) found that a third of the facilities
examined (177) were clogged and another 18% were experiencing slow infiltration.
Dry wells that treat roof runoff had the fewest failures (4%) and porous pavement
the most (77%). Dry wells may have the lowest failure rate because they only
handle roof runoff. The primary causes of failure appear to be inadequate
pretreatment and lack of soil stabilization in the tributary watershed, as well as
poor construction practices (Shaver, personal communication).
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Based on a review of several studies of infiltration facilities in sandy and loamy
soils, it has been concluded that “monitoring . . . has not demonstrated significant
contamination . . . although highly soluble pollutants such as nitrate and chloride
have been shown to migrate to ground water” (USEPA, 1991). However, pollution
has been found in ground water where infiltration devices are in coarse gravels
(Adophson, 1989; Miller, 1987).
Clogging has not been a problem with well maintained systems discharging to
sands and coarser soils, suggesting that pretreatment for these infiltration devices
in the aforementioned soil conditions is not necessary. Pretreatment when
infiltrating to finer soils is suggested. An infiltration facility sized only for
treatment is much smaller than one sized for flood control and therefore may be
more susceptible to clogging.
For small systems treating less than a few acres of pavement, pretreatment can be
accomplished with a stormwater quality inlet, catch basin and a submerged outlet.
The diameter and depth of the sump should be at least four times the diameter of
the outlet pipe to the infiltration system (Lager, et al., 1977). Swales can also be
used although they will not likely be feasible in industrial sites that tend to be fully
utilized.
Pretreatment of the stormwater is highly recommended for drywells where access
for maintenance is difficult if not impossible. Such pretreatment may include
biofilters, sumps, stormwater quality enhancing inlets, or oil water separators.
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Roof drain or gutter
Overflow and
splash block
Cap with lock
Building
foundation
12” typical depth to drywell
Line top, sides and bottom of drywell
with geotextile filter fabric
Perforated pipe within drywell
(typically PVC)
NOT TO SCALE
Clean washed aggregate
or gravel (1” to 3” size)
Observation well (perforated PVC pipe),
anchor to bottom of trench with rebar
Figure I-03-2
Typical Drywell (With Rooftop Drainage)
Many sinkholes are located
in existing neighborhoods.
Entrance may not be
clearly visible due to
obscuring brush, trash,
rocks, or silting.
Natural sinkhole
or depression
Underground cave
with potential of
solution or collapse
Limestone rock or other
soluble formation
Increasing stormwater runoff to a natural depression may increase
sinkhole formation by further dissolving limestone. Even if amount of
stormwater runoff has not been increased, stormwater quality treatment
is necessary to prevent pollutants from entering groundwater and to
reduce potential pH changes and chemicals within stormwater runoff.
NOT TO SCALE
Figure I-03-3
Typical Schematic of Sinkholes and Karst Areas
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References
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Adolphson Associates, Storm Water Evaluation, Clover/Chambers Basin Ground
Water Management Program, for the Tacoma-Pierce County Health Department, 1989.
Adolphson Associates, Subsurface Storm Water Disposal Facilities, Interim report for
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Camp Dresser & McKee, Sevenmile Creek Basin Pilot Stormwater Quality Master
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Camp Dresser & McKee, Larry Walker Associates, Uribe & Associates, Resources
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Environmental Regulation. Stormwater Management Practices, FL, 1988.
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Management Practices, M. S. Thesis, University of Tennessee, Civil and
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Infiltration Practices, 1984.
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Urbonas, Ben and Peter Stahre. Storm Water Best Management Practices and
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