1. ?
    2. Chapter 2Making the Case for Stormwater Management
      1. _
        1. _
          1. _
        2. 2.2.3Groundwater Recharge, Stream Base Flow, and First-Order Streams
        3. 2.2.4Stream Channel Changes

 
Pennsylvania Stormwater
Best Management Practices
Manual
Chapter 2
Making the Case for Stormwater Management
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 2
Chapter 2 Making the Case for Stormwater Management
2.1
A Brief Review of Stormwater Problems in Pennsylvania……………………………………….1
2.2
The Hydrologic Cycle and the Effects of Development…………………………….…………….4
2.2.1 Rainfall, Runoff, and Flooding……………………………………………………………….6
2.2.2 The Impacts of Vegetation Loss and Soil Changes…………………………………….10
2.2.3 Groundwater Recharge, Stream Base Flow, and First-Order Streams……………...10
2.2.4 Stream Channel Changes……………………………………………………………………13
2.2.5 Water Quality…………………………………………………………………………………..14
2.3
References………………………………………………………………………………………………19
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2.1
A Brief Review of Stormwater Problems in Pennsylvania
Pennsylvania is the most flood prone state in the country. It has experienced several serious and
sometimes devastating floods during the past century, often as a result of tropical storms and
hurricanes, and heavy rainfall on an existing snow pack. To a large extent, the flooding that results
from such extreme storms and hurricanes occurs naturally and will continue to occur. Stormwater
management cannot eliminate flooding during such severe rainfall events (Figure 2-1).
Figure 2-1. Flooding impacts are devastating communities,
even with conventional stormwater management programs (F. Thorton).
In many watersheds throughout the state, flooding problems from rain events, including the smaller
storms, have increased over time due to changes in land use and ineffective stormwater
management. This additional flooding is a result of an increased volume
of stormwater runoff being
discharged throughout the watershed. This increase in stormwater volume is the direct result of more
extensive impervious surface areas (Figure 2-2), combined with substantial tracts of natural
landscape being converted to lawns on highly compacted soil or agricultural activities.
Figure 2-2. Parking lots are common impervious surfaces that
affect stormwater runoff.
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The problems are not limited to flooding. Stormwater runoff carries significant quantities of pollutants
washed from the impervious and altered land surfaces (Figure 2-3). The mix of potential pollutants
ranges from sediment to varying quantities of nutrients, organic chemicals, petroleum hydrocarbons,
and other constituents that cause water quality degradation.
Figure 2-3. Pollutant laden runoff degrades water quality.
Increased stormwater runoff volume can turn small meandering streams into highly eroded and
deeply incised stream channels (Figure 2-4). Stream meander and the resulting erosion and
sedimentation is a natural process, and all channels are in a constant process of alteration.
However, as the volume of runoff from each storm event is increased, natural stream channels
experience more frequent bank full or near bankfull conditions. As a result, streams change their
natural shape and form. Pools and riffles that support aquatic life are disrupted as channels erode to
an unnatural level, and the eroded bank material contributes to sediment in the stream and degrades
it’s health by smothering stream bottom habitat. The majority of this stream channel devastation is
intensified during the frequently occurring small-to-moderate precipitation events, not during major
flooding events.
Figure 2-4.Stormwater influenced stream bank morphology in Valley Creek.
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Rainfall is an important resource to replenish the groundwater and maintain stream flow (Figure 2
-
5). When the stormwater runoff during a storm event is allowed to drain away rather than recharge
the groundwater, it alters the hydrologic balance of the watershed. As a consequence, stream
base flow is deprived of the constant groundwater discharge and may diminish or even cease.
During a drought, reduced stream base flow may also significantly affect the water quality in a
stream.
Figure 2-5. Rainfall replenishes the groundwater, which in turn provides stream base
flow.
The groundwater discharge to a stream is at a relatively constant temperature, whereas
stormwater runoff from impervious surfaces may be very hot in the summer months and extremely
cold in the winter months. These temperature extremes can have a devastating effect on aquatic
organisms, from bacteria and fungi to larger species. Many fish, especially native trout, can be
harmed by acute temperature changes of only a few degrees.
Improperly managed stormwater causes increased flooding, water quality degradation, stream
channel erosion, reduced groundwater recharge, and loss of aquatic species. But these and other
impacts can be effectively avoided or minimized through better site design. This chapter discusses
the potential problems associated with stormwater and explains the need for better stormwater
management. The problems caused by impervious and altered surfaces can be avoided or
minimized, but only through stormwater management techniques that include runoff volume
reduction, pollutant reduction, groundwater recharge and runoff rate control for all storms.
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2.2
The Hydrologic Cycle and The Effects of Development
The movement of water from the atmosphere to the land surface and then back to the atmosphere
is a continuous process, with water constantly in motion. This balanced water cycle of
precipitation, runoff, evapotranspiration, infiltration, groundwater recharge, and stream base flow
sustains Pennsylvania’s water resources. This representation of the hydrologic cycle, while
depicting the general concept, over-simplifies the complex interactions that define the surface and
subsurface flow processes of humid regions in the United States.
Changes to the land surface, along with inappropriate stormwater management, can significantly
alter the natural hydrologic cycle. In a natural Pennsylvania woodland or meadow, very little of the
annual rainfall leaves the site as runoff. More than half of the annual amount of rainfall returns to
the atmosphere through evapotranspiration. Surface vegetation, especially trees, transpires water
to the atmosphere (with seasonal variations). Water is also stored in puddles, ponds and lakes on
the earth’s surface, where some of it will evaporate. Water that percolates through the soil either
moves vertically and eventually reaches the zone of saturation or water table, moves laterally
through the soil and often emerges as springs or seeps down gradient or is stored in the soil.
Soils are influenced and formed by vegetation, climate, parent material, topography and time. All
of these factors have some effect on how water will move through the soil. Restrictive soil horizons
may impede the vertical movement of water and cause it to move laterally. It is important to
understand these factors when designing an appropriate stormwater system at a particular
location. Under natural woodland and meadow conditions, only a small portion of the annual
rainfall becomes stormwater runoff. Although the total amount of rainfall varies in different regions
of the state, the basic average hydrologic cycle shown below holds true (Figure 2-6).
Figure 2-6. Annual hydrologic cycle for an undisturbed acre in the Pennsylvania Piedmont region.
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Changing the land surface causes varying changes to the hydrologic cycle (Figure 2-7). Altering
one component of the water cycle invariably causes changes in other elements of the cycle.
Roads, buildings, parking areas and other impervious surfaces prevent rainfall from soaking into
the soil and significantly increase the amount of runoff. As natural vegetation is removed, the
amount of evapotranspiration decreases.
Figure 2-7. Representative altered hydrologic cycle for a developed acre in the
Piedmont region.
These changes in the hydrologic cycle have a dramatic effect on streams and water resources.
Annual stormwater runoff volumes increase from inches to feet per acre, groundwater recharge
decreases, stream channels erode, and populations of fish and other aquatic species decline.
Past practices focused on detaining the peak flows for larger storms. While detention is helpful in
reducing peak flows for the immediate downstream neighbor, it does not address most of the other
problems discussed earlier.
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Figure 2-8. Average annual precipitation in Pennsylvania.
2.2.1 Rainfall, Runoff, and Flooding
In Pennsylvania, average annual precipitation ranges from 37 inches to more than 45 inches per
year (Figure 2-8), and reflects a humid pattern. Nearly all of the annual rainfall occurs in small
storm events (Figure 2-9
)
. Precipitation of an inch or less is frequent and well distributed
throughout the year. However, large storms, hurricanes, and periods of intense rainfall can occur
at any time.
Figure 2-9. Distribution of precipitation by storm magnitude for Harrisburg, PA (Original Data from
Penn State Climatological Office, 1926-2003)
3" - 4"
1%
4" +
1%
65%
0.1" - 1"
2" - 3"
6%
1" - 2"
27%
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Stormwater management has historically focused on managing flooding from the larger but less
frequent extreme event storms (Table 2-1). Traditional site design has focused on the
peak rate
of
runoff during such events; that is, how fast the stormwater runoff is leaving the site after
development. Detention facilities are built to
slow down the rate of runoff leaving a site
during large storms so that the rate of runoff
after development is not greater than the
rate before development. Regulatory criteria
is often based on controlling the “release”
rate of runoff from the 2-year through 100-
year storm events. Storm frequency is
based on the statistical probability of a storm
being exceeded in any year. That is, a 2-
year storm has a 50% probability of being
exceeded in any single year, and a 100-year
storm, a 1% probability.
2-year 5-year 10-year 50-year 100-year
Philadelphia
3.3
4.1
4.8
6.7
7.6
Pittsburgh
2.4
2.9
3.3
4.4
4.9
Scranton
2.6
3.2
3.7
5.4
6.4
State College
2.7
3.3
3.8
5.2
5.9
Williamsport
2.8
3.5
4.1
6.0
7.0
Erie
2.6
3.2
3.7
5.1
5.8
Frequency of Occurrence (Years)
Location
Table 2-1. Statistical Storm Frequency Events for locations in PA
(24 hour duration) (Source: NOAA National Weather Service
Precipitation Frequency Data Server, 2004).
Preventing increased runoff rates from large storm events is extremely important but it does not do
enough to protect streams and water quality. With a change in land surface, not only does the
peak
rate
of runoff increase, the
volume of
runoff also increases. While a stormwater detention
facility may slow the rate of runoff leaving a site, there may still be an increased volume of runoff.
This is shown graphically in Figure 2-10. Detention controls the peak runoff rate by extending the
hydrograph. So while the rate of runoff may not increase, the duration of runoff will be longer than
before development because of the increased volume.
Figure 2-10. The hydrograph is an important tool used for understanding the hydrologic
response of a given rainfall event. The area beneath the hydrograph curve represents the total
volume of runoff being discharged
.
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On a watershed basis, detention becomes ineffective downstream as the sole management
strategy for stormwater control due to the extended hydrograph and increased volume. There is
even a possibility that the peak flows may
increase
downstream flooding. The combination of
more runoff volume over a longer time period will result in downstream flow rates that are higher
than before development, as indicated in Figure 2-11.
Figure 2-11. This figure illustrates a small watershed comprised of five hypothetical Subbasin development
sites, 1 through 5, each of which undergoes development and relies on a separate peak rate
control detention basin. As the storm occurs, five different hydrographs result for each sub-
area and combine to create a resultant pre-development hydrograph for the overall
watershed. The net result of the combined hydrographs is that the watershed peak rate
increases considerably, because of the way in which these increased volumes are routed
through the watershed system and combine downstream. Flooding increases considerably
in peak and duration, even though these detention facilities have been installed at each
individual development.
The second reason that detention alone is not sufficient for stormwater management is that it does
not address the frequent small storm events in Pennsylvania. Most of the rainfall in Pennsylvania
occurs in relatively small storm events, as indicated for the Harrisburg area (Figure 2-9). In
Harrisburg, over half of the average annual rainfall occurs in storms of less than 1 inch (in 24
hours). Over 90 percent of the average annual rainfall occurs in storms of 2 inches or less, and
over 95 percent of average annual rainfall occurs in storms of 3 inches or less. This pattern is
typical of the entire state.
Detention facilities that are designed to control the peak flow rate for large storm events often allow
frequent small storm events to “pass through” the detention facility. These small frequent rainfall
events discharge from the site at a higher rate and a greater volume of runoff than before
development. There is also an increase in the
frequency
of runoff events because of the change
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in land surface. For example, little runoff will occur from most wooded sites until over an inch of
rainfall has fallen. In contrast, a paved site will generate runoff almost immediately (Figure 2-12).
After development, runoff will occur with greater frequency than before development, and runoff
may be observed with every rainfall. The design of stormwater systems that collect, convey and
concentrate runoff may further degrade conditions.
Runoff Volume from
Woodland and Impervious Surfaces
0.02
0.36
1.03
1.59
2.11
3.6
4.37
0.97
1.49
3.04
3.85
4.54
6.37
7.26
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
1 inch Rainfall
1.5 inch
Rainfall
2-yr Storm
(3.27")
5-yr Storm
(4.09")
10-yr Storm
(4.78")
50-yr Storm
(6.61")
100-yr Storm
(7.5")
Runoff (inches)
Woodland
Impervious
Runoff Values for the 1" and 1.5"
storms generated using the Small
Storm Hydrology Methodology (Pitt,
1994), and Runoff values for the
remaining storms generated using SCS
Runoff Curve Number Method (CN=98
for impervious and CN=73 for woods, C
soils, Fair Condition)
Figure 2-12. This graph generally compares the volume of runoff generated from a woodland site
with the volume of runoff generated by impervious area for different rainfall amounts.
Note that the volume increase for small storms is significant.
The combination of more runoff, more often and at higher rates will create localized flooding and
damage even in small storm events. Throughout the state, over 95 percent of the annual rainfall
volume occurs in storm events that are less than the 2-year storm event. The net effect is that
during most rainfall events, stormwater discharges are not managed or controlled, even with
numerous detention basins in place.
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2.2.2 The Impacts of Vegetation Loss and Soil Changes
On woodland and meadow areas, over half of the average annual rainfall returns to the
atmosphere through evaporation and transpiration (Figure 2-6). The vegetation itself also
intercepts and slows the rainfall, reducing its erosive energy, reducing overland flow of runoff, and
allowing infiltration to occur. The root systems of plants also provide pathways for downward water
movement into the soil mantle.
Evapotranspiration (ET) varies tremendously with season and with type of vegetative cover. Trees
can effectively evapotranspire most, if not all, of the precipitation, that falls in summer rain showers.
Evapotranspiration dramatically declines during the winter season. During these periods, more
precipitation infiltrates and moves through the root zone, and the groundwater level rises.
Removing vegetation or changing the land type from woods and meadow to residential lawnscapes
reduces evapotranspiration and increases the amount of stormwater runoff.
Soil disturbance and compaction also increases stormwater runoff. Soils contain many small
openings called “macropores” that provide a mechanism for water to move through the soil,
especially under saturated conditions. When soil is disturbed (grading, stockpiling, heavy
equipment traffic, etc.) the soil is compacted, macropores are smashed and the natural soil
structure is altered. Soil permeability characteristics are substantially reduced.
Compaction can be measured by determining the bulk density of the soil. The more compacted the
soil is, the heavier it is by volume.
Heavy construction equipment can
compact soil so significantly that the
soil bulk density of lawn soil
approaches the bulk density of
concrete (Table 2-2 Ocean County,
New Jersey Soil Conservation
District, 2001; Hanks and
Lewandowski, 2003). The result is a
surface that is functionally impervious
because the water absorbing
capacity of the soil is so altered and
reduced.
Table 2-2. Common Bulk Density Measurements
Undisturbed Lands
Forest & Woodlands
1.03 g/cc
Residential
Neighborhoods
1.69 to 1.97 g/cc
Golf Courses - Parks
Athletic Fields
1.69 to 1.97 g/cc
CONCRETE
2.2 g/cc
As discussed in Chapters 5 and 6, comprehensive stormwater management focuses on preventing
an increase in stormwater runoff volume by protecting vegetation and soils, or minimizing
stormwater impacts by restoring vegetation and soils to reduce runoff volumes and the velocity of
runoff. Vegetation and soils are a critical component of the “water balance” and are an essential
part of better stormwater management.
2.2.3 Groundwater Recharge, Stream Base Flow, and First-Order Streams
Water moves through the soil until it is evapotranspired or reaches the groundwater table and
replenishes the aquifer. The actual movement of water through the sub-surface pathways is
complex, and less permeable soils, clay layers, and rock strata are often encountered. The water
moving through the soil is generally referred to as gravitational water or drainage water. Other
types of water in soil include capillary water and hygroscopic water. Capillary water is that water
held in soil pores by surface attraction (sometimes referred to as capillary action); this is the water
that is typically available to plants for uptake. Hygroscopic water is water that is tightly held by the
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soil particles and can only be removed by physical drying. Although capillary water does play an
important role in evaporation processes, gravitational water is of primary concern from a
stormwater management prospective.
The movement of gravitational water through the soil is influenced by a soils texture, structure,
layering and the presence of preferential flow pathways (macropores). Soil textures are defined by
the percentage of sand, silt and clay present in the soil. In general, the permeability and hydraulic
conductivity of a soil will decrease with decreasing textural grain size (i.e., gravitational water
moves more easily through sands than silts and clays). Soil texture also influences the shape of
the wetting front as water moves through a soil.
It has also been observed that there is a discontinuity of soil-water movement at the interface
between soils of different textures. This layering causes percolating water to concentrate at certain
points along the layer interface and then break into the layer interface in finger-like protrusions.
The significance is that even a change in soil texture within a vertical profile will cause a disruption
in the soil-water movement. This disruption often causes water to “back up” at the interface, which
can cause water to move laterally.
Soil structure also influences the movement of water through a soil. A disruption in the movement
of soil water will occur at the interface between soil layers of differing structures. While texture and
structure are certainly important to how water moves through soils, soil layering and the presence
of dominant flow paths (macropores) play the most significant role in defining how water moves
through the subsurface.
Soils form over time in response to their landscape position, climate, presence of organisms and
parent material. Soils that have formed in place from the weathering of their parent material,
usually form a typical profile with A, B and C horizons above bedrock. However, many soils form
from a combination of the weathering of parent materials and the deposition of transported soils
creating a more complex layering effect. In general, any interface between soil layers can slow the
downward movements of water through a soil profile and promote lateral flow. This is especially
true in sloping landscapes typical of most of Pennsylvania.
Restrictive soil layers within a soil profile also disrupt the vertical movement of soil-water and
promote the lateral movement of water through the soil. Restrictive soil layers include clay lenses,
fragipans or plow pans, for example. Fragipans are layers within a soil profile that have been
compressed as a result of some external influence (glaciation for example). This compressed layer
often causes water to perch above the fragipan and promotes lateral flow. Fragipans are
commonly found in colluvial and glacial soils. In addition, many soils in agricultural regions of
Pennsylvania contain “plow-pans” which are compressed layers of soil formed by the repeated
traversing by moldboard plows.
Soil water also follows preferential flow paths through the soil. Preferential flow paths include
pathways created by plant roots, worm or rodent burrows, cracks or voids in the soil resulting from
piping action caused by the lateral movement of soil-water. Preferential flow paths also form at the
soil rock interface and within rock structures.
The groundwater level rises and falls depending on the amount of rainfall/snowmelt and the time of
year. The water cycle illustration of Figure 2-6 estimates that approximately 12 inches of the 45
inches of average annual precipitation in this natural watershed system finds its way into the
groundwater table.
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A variety of processes can occur when precipitation falls on a natural soil surface. Hillslope
hydrology processes have been identified by Chorley (1978) and are systematically illustrated in
Figure 2-12. The flow processes illustrated here are only representative examples of the complex
interactions that occur in nature. Simplified descriptions of these processes follow:
Figure 2-12 Components of hillslope hydrology (Adapted from Chorley [1978])
1. Areas marked with a “1” are areas where the infiltration capacity of the soils exceeds the
rainfall rate. All rain falling on these areas infiltrates into the ground.
2. Areas labeled with a “2” identifies an area where the rainfall rate exceeds the surface
infiltration rate, and the excess rainfall becomes surface runoff (Hortonian surface runoff).
3. Areas marked with a “3” represents areas where the soil has become saturated and cannot
hold additional moisture; all rain falling on these areas immediately becomes surface runoff.
Saturation can occur as a result of various subsurface conditions. Areas marked “3a”
illustrates where a restricting layer (fragipans, clay lenses, etc.) limits the downward
movement of soil water creating a perched water table that reaches the ground surface.
Area “3b” identifies an area where water moving through the soil (through-flow) reaches the
surface as a spring or seep (return-flow); in these cases the surface in the vicinity of the
seep or spring becomes saturated.
4. The areas marked with a “4” represent areas of through-flow. Through-flow is the lateral
movement of water through the soil. Area “4a” illustrates through-flow along preferential
flow paths in unsaturated soils; area “4b” shows shallow surface flow (a common
occurrence in PA); and area “4c” illustrates through-flow in saturated areas.
5. Areas marked with a “5” represents an area of return-flow. Return-flow is water that has
moved through unsaturated or saturated subsurface areas and re-appears as surface flow
through springs or seeps.
6. The area labeled as “6” represents an area of deep percolation or groundwater recharge.
7. Area “7” points to a location where groundwater discharges to the stream (influent streams).
For effluent streams, water moves from the stream into the ground water table in these
areas. In some streams, both processes may occur during different times of the year.
(Brown/Fennessey/Petersen)
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Most of these flow processes occur within natural watersheds in Pennsylvania. The extent to
which one or more of these processes are active within a particular area is influenced by soil
characteristics, geology and topography or landscape position.
Eventually the groundwater table intersects the
land surface and forms springs, first order
streams and wetlands (Figure 2-5). This
groundwater discharge becomes stream base
flow and occurs continuously, during both wet
and dry periods. Much of the time, all of the
natural flow in a stream is from groundwater
discharge. In this sense, groundwater discharge
can be seen as the “life” of streams, supporting
all water-dependent uses and aquatic habitat.
First-order streams are defined as “that stream
where the smallest continuous surface flow
occurs” (Horton, 1945), and are the beginning of
the aquatic food chain that evolves and
progresses downstream (Figure 2-13). As the
link between groundwater and surface water,
headwaters represent the critical intersection
between terrestrial and aquatic ecosystems.
During periods of wet weather, the water table
may rise to near the ground surface in the vicinity of the stream. This higher ground water table
coupled with through-flow, return-flow and shallow subsurface flow result in an area of saturation in
the vicinity of the stream channel. As a result, this area saturates quickly during rain events; and
the larger the rain event, the more extensive the area of saturation may be. It is understood by
researchers that a significant amount of the surface runoff observed in streams during precipitation
events is generated from the saturated areas surrounding streams (Chorley, 1978; Hewlett and
Hibbert, 1967). The runoff generated from rainfall on saturated land areas is referred to as
saturation overland flow. Hydrologists understand that the watershed runoff process is a complex
integration of saturation overland flow and infiltration excess (Hortonian) overland flow (Troendle,
1985). Areas that generate surface runoff pulsate, shrink and expand in response to rainfall. This
concept on a watershed scale is consistent with the hillslope hydrologic processes.
Figure 2-13 Leaves and organic matter are
initially broken down by bacteria and
processed into food for higher organisms
downstream.
Changes in land use cause runoff volumes to increase and groundwater recharge to decrease.
Wetlands and first order streams reflect changes in groundwater levels most profoundly, and this
reduced flow can stress or even eliminate the aquatic community. As the most hydrologically and
biologically sensitive elements of the drainage network, headwaters and first order streams warrant
special consideration and protection in stormwater management.
2.2.4 Stream Channel Changes
The shape of a stream channel, its width, depth, slope, and how it moves through the landscape, is
influenced by the amount of flow the stream channel is expected to carry. The stream channel
morphology is determined by the energy of stream flows that range from “low flow” to “bank full”.
The flow depths determine the energy in the stream channel, and this energy shapes the channel
itself. In an undeveloped watershed, bank full flow occurs with a frequency of approximately once
every 18 months. During larger flood events, the flow overtops the stream banks and flows into
the floodplain with much less impact on the shape of the stream channel itself.
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In a developing watershed, the volume and rate of stormwater runoff increase during small storm
events and the stream channel changes to accommodate the greater flows. Because the stream is
conveying greater flows more often and for longer periods of time, the stream will try to
accommodate these larger flows by eroding stream banks or cutting down the channel bottom.
Since traditional detention basins do not manage small storms, these impacts are often most
pronounced downstream of detention basins.
Numerous studies have documented the link between altered stream channels and land
development. The Center for Watershed Protection (Article 19, Technical Note 115, Watershed
Protection Techniques 3(3): 729-734) states that land development influences both the geometry
(morphology) and stability of stream channels, causing downstream channels to enlarge through
widening and stream bank erosion. These physical changes, in turn, degrade stream habitat and
produce substantial increases in sediment loads resulting from accelerated channel erosion.
As the shape of the stream channel changes to accommodate more runoff, aquatic habitat is often
lost or altered, and aquatic species decline. Studies, such as US EPA’s
Urbanization and
Streams: Studies of Hydrologic Impacts
(1997), conclude that land development is likely to be
responsible for dramatic declines in aquatic life observed in developing watersheds. These stream
channel impacts have been observed even where conventional stormwater management is
applied.
The effects occur at many levels in the aquatic community. As the gravel stream bottom is covered
in sediment, the amount and types of microorganisms that live along the stream bottom decline.
The stream receives sediment from runoff, but additional sediment is generated as the stream
banks are eroded and this material is deposited along the stream bottom. Pools and riffles
important to fish and other aquatic life are lost, and the number and types of fish and aquatic
insects diminishes. Trees and shrubs along the banks are undercut and lost, removing important
habitat and decreasing natural shading and cooling for the stream.
The runoff from impervious surfaces is usually warmer than the stream flow, and can harm the
aquatic community. When the stream flow is comprised primarily of groundwater discharge, the
constant, cool temperature of the groundwater buffers the stream temperature. As the flow of
groundwater decreases and the amount of surface runoff increases, the temperature regime of the
stream changes. Runoff from impervious surfaces in the summer months can be much hotter than
the stream temperature, and in the winter months this same runoff can be colder. These changes
in temperature dramatically affect the aquatic habitat in the stream, ranging from the fish
community that the stream can support to the microorganisms that form the foundation of the food
chain. Important fungal communities can be lost altogether. It is apparent that increasing
impervious areas can lead to significant degradation of surface water by altering the entire aquatic
ecosystem.
2.2.5 Water Quality
Impervious surfaces and maintained landscapes generate pollutants that are conveyed in runoff
and discharged to surface waters. Many studies of pollutant transport in stormwater have
documented that pollutant concentrations show a distinct increase at the beginning of a flow
hydrograph referred to as the “first flush”. In fact, the particulate associated pollutants that are
initially scoured from the land surface and suspended in the runoff are observed in a stream or
river before the runoff peak occurs. These pollutants include sediment, phosphorus that is moving
with colloids (clay particles), metals, and organic particles and litter. Dissolved pollutants, however,
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may actually decrease in concentration during heavy runoff. These include nitrate, salts and some
synthetic organic compounds applied to the land for a variety of purposes.
Managing stormwater to minimize pollutant loading includes reducing the sources of these
pollutants as well as restoring and protecting the natural systems that are able to remove
pollutants. These include stream buffers, vegetated systems, and the natural soil mantle, all of
which can be put to use to remove pollutants from stormwater runoff.
Stormwater quantity and quality are inextricably linked and need to be managed together.
Although the most obvious impact of land development is the increased rate and volume of surface
runoff, the pollutants transported with this runoff comprise an equally significant impact.
Management strategies that address quantity
will in most cases address quality.
Stormwater runoff pollutants include sediment, organic detritus, phosphorus and nitrogen
forms, metals, hydrocarbons, and synthetic organics.
The increased stormwater runoff
brought on by land development scours both impervious and pervious land surfaces. Stormwater
runoff transports suspended and dissolved pollutants that were initially deposited on the land
surface. Hot spot impervious areas such as fueling islands, trash dumpsters, industrial sites, fast
food parking lots, and heavily traveled roadways contribute heavy pollutant loads to stormwater.
Many so-called pervious surfaces, such as the chemically maintained lawns and landscaped
areas, also add significantly to the pollutant load, especially where these pervious areas drain to
impervious surfaces, gutters and storm sewers. The soil compaction process applied to many land
development sites results in a vegetated surface that is close to impervious in many instances, and
produces far more runoff than the pre-development soil did. These new lawn surfaces are often
loaded with fertilizers that result in polluted runoff that degrades all downstream ponds and lakes.
The two physical forms of stormwater pollutants are particulates
and solutes
. One very
important distinction for stormwater pollutants is the extent to which pollutants are particulate in
form, or dissolved in the runoff as solutes. The best example of this comparison is the two
common fertilizers: Total phosphorus (TP) and nitrate (NO3-N). Phosphorus typically occurs in
particulate form, usually bound to colloidal soil particles. Because of this physical form, stormwater
management practices that rely on physical filtering and/or settling out of sediment particles can be
quite successful for phosphorus removal. In stark contrast, nitrate tends to occur in highly soluble
forms, and is unaffected by many of the structural BMPs designed to eliminate suspended
pollutants. As a consequence, stormwater management BMPs for nitrate may be quite different
than those used for phosphorous removal. Non-Structural BMPs (Chapter 5) may in fact be the
best approach for nitrate reduction in runoff.
Particulates:
Stormwater pollutants that move in association with or attached to particles include
total suspended solids (TSS), total phosphorus (TP), most organic matter (as estimated by COD),
metals, and some herbicides and pesticides. Kinetic energy keeps particulates in suspension and
some do not settle out as easily. For example, an extended detention basin offers a good method
to reduce total suspended solids, but is less successful with TP, because much of the TP load is
attached to fine clay particles that may take longer to settle out.
If the concentration of particulate-associated pollutants in stormwater runoff, such as TSS and TP,
is measured in the field during a storm event, a significant increase in pollutant concentration
corresponding to but not synchronous with the surface runoff hydrograph is usually observed
(Figure 2-14). This change in pollutant concentration is referred to as a “chemograph”, and has
contributed to the concept of a “first flush” of stormwater pollutants. In fact, the actual transport
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process of stormwater pollutants is somewhat more complex than “first flush” would indicate, and
has been the subject of numerous technical papers (Cahill et al, 1974: 1975; 1976; 1980; Pitt,
1985, 2002). To accurately measure the total mass of stormwater pollution transported during a
given storm event, both volume and concentration must be measured simultaneously, and a
double integration performed to estimate the mass conveyed in a given event. To fully develop a
stormwater pollutant load for a watershed, a number of storm events must be measured over
several years. The dry weather chemistry is seldom indicative of the expected wet weather
concentrations, which can be two or three orders of magnitude greater.
Because a major fraction of particulate associated pollutants is transported with the smallest
particles, or colloids, their removal by BMPs is especially difficult. These colloids are so small that
they do not settle out in a quiescent pool or basin, and remain in suspension for days at a time,
passing through a detention basin with the outlet discharge. It is possible to add chemicals to a
detention basin to coagulate these colloids to promote settling, but this chemical use turns a
natural stream channel or pond into a treatment unit, and subsequent removal of sludge is
required. A variety of BMPs have been developed that serve as runoff filters, and are designed for
installation in storm sewer elements, such as inlets, manholes or boxes. The potential problem
with all measures that attempt to filter stormwater is that they quickly become clogged, especially
during a major event. Of course, one could argue that if the filter systems become clogged, they
are performing efficiently, and removing this particulate material from the runoff. The major
problem then with all filtering (and to some extent settling) measures is that they require substantial
maintenance. The more numerous and distributed within the built conveyance system that these
BMPs are situated, the greater the removal efficiency, but also the greater the cost for operation
and maintenance.
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Figure 2-14. Chemograph of phosphorus and suspended solids in Perkiomen Creek (Cahill, 1993).
Solutes:
Dissolved stormwater pollutants generally do not exhibit any increase during storm event
runoff, and in fact may exhibit a slight dilution over a given storm hydrograph. Dissolved
stormwater pollutants include nitrate, ammonia, salts, organic chemicals, many pesticides and
herbicides, and petroleum hydrocarbons (although portions of the hydrocarbons may bind to
particulates and be transported with TSS). Regardless, the total mass transport of soluble
pollutants is dramatically greater during runoff because of the volume increase. In some
watersheds, the stormwater transport of soluble pollutants can represent a major portion of the
total annual discharge for a given pollutant, even though the absolute concentration remains
relatively constant. For these soluble pollutants, dry weather sampling can be very useful, and
often reflects a steady concentration of soluble pollutants that will be representative of high flow
periods.
Some dissolved stormwater pollutants can be found in the initial rainfall, especially in regions with
significant emissions from fossil fuel plants. Precipitation serves as a “scrubber” for the
atmosphere, removing both fine particulates and gases (NOX and SOX). Chesapeake Bay
scientists have measured rainfall with NO
3
concentrations of 1 to 2 mg/l, which could comprise a
significant fraction of the total input to the Bay. Other rainfall studies by NOAA and USGS have
resulted in similar conclusions. Impervious pavements can transport nitrate load, reflecting a mix
of deposited sediment, vegetation, animal wastes, and human detritus of many different forms.
Pollution prevention through use of Non-Structural BMPs is very effective. A variety of Structural
BMPs, including settling, filtration, biological transformation and uptake, and chemical processes
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can also be used. Stormwater related pollution can be reduced if not eliminated through
preventive Non-Structural BMPs (Chapter 5), but not all stormwater pollution can be avoided.
Many of the Structural BMPs (Chapter 6) employ natural pollutant removal processes as essential
elements. These “natural” processes tend to be associated with and rely upon both the existing
vegetation and soil mantle. Thus preventing and minimizing disturbance of site vegetation and
soils is essential to successful stormwater management.
Settling:
Particles remain suspended in stormwater as long as the energy of the moving water is
greater than the pull of gravity. In a natural stream, the stormwater that overflows the banks slows
and is temporarily stored in the floodplain, which allows for sediment settling, and the building of
the alluvium soils that comprise this floodplain. As runoff passes through any type of man-made
structure, such as a detention basin, the same process takes place, although not as efficiently as in
a natural floodplain. Where it is possible to create micro versions of runoff ponds (rain gardens),
distributed throughout a site, the same settling effect will result. The major issue with settling
processes is that the dissolved pollutant load is not subject to gravitational settling.
Filtration:
Another natural process is physical filtration. Filtration through vegetation and soil is by
far the most efficient way to remove suspended stormwater pollutants. Suspended particles are
physically filtered from stormwater as it flows through vegetation and percolates into the soil.
Runoff that is concentrated in swales, however, can exceed the ability of the vegetation to remove
particles. Therefore, it is important to avoid concentrated flows by slowing and distributing the
runoff over a broad vegetated area.
Stormwater flow through a relatively narrow natural riparian buffer of trees and herbaceous
understory growth has been demonstrated to physically filter surprisingly large proportions of larger
particulate-form stormwater pollutants. Both filter strip and grassed swale BMPs rely very much on
this surface filtration process as discussed in Chapter 6.
Biological Transformation and Uptake/Utilization:
This category includes an array of different
processes that reflect the remarkable complexity of different surface vegetative types, their varying
root systems, and their different needs and rates of transformation and utilization of different
“pollutants,” especially nutrients. An equally vast and complex community of microorganisms
exists below the surface within the soil mantle, and though more micro in scale, the myriad of
natural processes occurring within this soil realm is just as remarkable.
Phosphorus and nitrate are essential to plant growth and therefore are taken up through the root
systems of grasses, shrubs and trees. Nitrogen transformations are quite complex, but the muck
bottom of wetlands allows the important process of denitrification to occur and convert nitrates for
release in gaseous form. Nitrates in stormwater runoff passing through wetlands is removed and
used by wetland plants to build biomass. The caution in terms of a wetland or similar surface BMP
is that if the vegetation dies at the end of a growing season and the detritus is discharged from the
wetland, the net removal of nitrate is maybe less than expected. The guidance for BMP
applications is that if biological transformation processes are considered, care must be taken to
remove and dispose of the biomass produced in the process.
Chemical Processes:
Various chemical processes occur in the soil to remove pollutants from
stormwater. These include adsorption through ion exchange and chemical precipitation. Cation
Exchange Capacity (CEC) is a rating given to soil, that relates the soil organic content to its ability
to remove pollutants as stormwater infiltrates through the soil. Adsorption will increase as the total
surface area of soil particles and/or the amount of decomposed organic material increases. Clay
soils have better pollutant reduction performance than sandy soils, and their slower permeability
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rate has a positive effect. CEC values typically range from 2 to 60 milli-equivalents (meq) per 100
grams of soil. Coarse sandy soils have low CEC values and therefore are not especially good
stormwater pollutant removers. The addition of compost will greatly increase the CEC of sandy
soils. A value of 10 meq. is often considered necessary to accomplish a reasonable degree of
pollutant removal.
Section 2.3 References
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ASCE Journal of Hydraulics
Barnes, J. H., and W. D. Sevon, 1996.
The Geologic Story of Pennsylvania
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Geological Survey, Educational Series No. 4, Harrisburg, PA
Brown, Scott A., P.E., Larry A.J.Fennessey, Ph.D.,P.E., and Gary W. Petersen,Ph.D.,
CPSS
Understanding Hydrologic Process for Better Stormwater Management
Cahill, T. H., 1989.
The Use of Porous Paving for Groundwater Recharge in Stormwater
Management Systems
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PA, Oct., 1988.
Cahill, T. H., 1993.
Stormwater Management Systems, Porous Pavement System with
Underground Recharge Systems, Engineering Design Report
, Spring, 1993.
Cahill, 1997.
Land Use Impacts on Water Quality: Stormwater Runoff
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Cahill, 1996.
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Urbanizing Areas, Cahill Associates, West Chester, PA, April 21-24, 1996.
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Cahill, 1992.
Structural and Nonstructural Best Management Practices for the
Management of Non-Point Source Pollution in Coastal Waters: A Cost-Effectiveness
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Comparison.
T.H. Cahill and W.R. Horner, The Coastal Society; Thirteenth International
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Chorley, R.J. (1978).
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CWP, 2003.
Impacts of Impervious Cover on Aquatic Systems
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Protection, Ellicott City, MD
Emerson, C., Welty, C., Traver, R., 2005,
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Stormwater Detention Basins,
ASCE Journal of Hydraulic Engineering
Fenneman, N. M., 1938. Physiography of Eastern United States
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New York, NY.
Fennessey, Lawrence A.J.,Ph.D.,P.E. and Arthur C. Miller, Ph.D.,P.E. 2001.
Hydrologic
Processes During Non-Extreme Events in Humid Regions
Fennessey, Lawrence A.J., Ph.D.,P.E. and Richard H. Hawkins, Ph.D.,P.E. 2001.
The
NRCS Curve Number, a New Look at an Old Tool
Friedman, D.B., 2001. "Impact of Soil Disturbance During Construction on Bulk Density
and Infiltration in Ocean County, New Jersey." Ocean County Soil Conservation District.
Forked River, NJ
Friedman, D.B., "Developing an Effective Soil Management Strategy." Ocean County Soil
Conservation District. Forked River, NJ.
Godfrey, M. A., 1997.
Field Guide to the Piedmont
, The University of North Carolina
Press, Chapel Hill, NC.
Hanks, D & A. Lewandowski, 2003.
Protecting Urban Soil Quality: Examples for
Landscape Codes and Specifications
, USDA-NRCS, Toms River, NJ
Hewlett, J.D., and A.R. Hibbert (1967).
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Watersheds to Precipitation in Humid Areas
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290
Konrad, C. P. & D. B. Booth, 2002.
Hydrologic Trends Associated with Urban
Development for Selected Streams in the Puget Sound Basin, Western Washington.
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Tacoma, Wash.
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Kochanov, W. E., 1987.
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Kochanov, W. W., 1999.
Sinkholes in Pennsylvania
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Maryland Department of the Environment, 2000.
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McCandless, T. L. and R. A. Everett, 2002.
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and Channel Characteristics of Streams in the Piedmont Hydrologic Region
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McCandless, T. L., 2003. Maryland Stream Survey:
Bankfull Discharge and Channel
Characteristics of Streams in the Allegheny Plateau and the Valley and Ridge Hydrologic
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, US Fish & Wildlife Service, CBFO-S01-01, Annapolis, MD
McCuen, Richard, 1979,
Downstream Effects of Stormwater Management Basins,
ASCE
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McCuen, R. and Moglen, G., 1988,
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Weather Service, Bohemia, NY
Ocean Co. SCD, 2001.
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