Pennsylvania Stormwater Best Management Practices .2005-01-12  Pennsylvania Stormwater Best Management

  • View
    212

  • Download
    0

Embed Size (px)

Text of Pennsylvania Stormwater Best Management Practices .2005-01-12  Pennsylvania Stormwater Best...

  • Section 2Stormwater and Pennsylvanias Natural Systems

    Pennsylvania StormwaterBest Management Practices

    ManualDRAFT - JANUARY 2005

  • This page intentionally left blank.

  • 2-17Draft Pennsylvania Stormwater Management Manual

    Section 2 - Stormwater and Pennsylvanias Natural Systems

    Section 2 Stormwater and Pennsylvanias Natural Systems

    2.1 The Hydrologic Cycle

    2.1.1 Precipitation2.1.2 Infiltration2.1.3 Evapotranspiration2.1.4 Ground Water Recharge and Stream Base Flow2.1.5 Surface Runoff

    2.2 Hydrologic Impacts of Development and Impervious Surfaces

    2.3 Stormwater Management

    2.4 Stormwater and Water Quality

    2.5 Major Watersheds and Stream Systems

    2.6 Important Natural Systems of Pennsylvania

    2.6.1 Geology2.6.2 Soils2.6.3 Vegetation2.6.4 Physiography

    2.7 References

  • 2-18 Draft Pennsylvania Stormwater Management Manual

    Section 2 - Stormwater and Pennsylvanias Natural Systems

    Section 2 Stormwater and Pennsylvanias Natural Systems

    Impervious surfaces greatly increase the volume and rate of stormwater produced by the land surface,which can have damaging impacts related flooding, groundwater recharge, stream baseflow, andstream channel shape. This stormwater runoff carries with it the various pollutants that haveaccumulated on the man-made landscape. The development of measures that prevent or reducethese hydrologic and chemical impacts is the primary purpose of this Manual, but before specificBMPs can be described, the processes of rainfall and surface runoff must be understood.

    2.1 The Hydrologic Cycle

    The hydrologic cycle is a dynamic system in constantmovement. The cycles dynamic quality the movement ofrainfall from atmosphere to the land surface, where vegetativesystems transpire and surface and groundwater pathwaysconvey water back to the ocean must be sustained fortruly effective stormwater management. A simplifiedschematic of the hydrologic cycle is shown in Figure 2-1. Thecycle for an average year in a typical Pennsylvania watershed,the Brandywine Creek, is shown as a flow chart in Figure 2-2.

    Impacting one part of the system means impacting other partsof the system, as the water balance is maintained. Thereare five major components to the water cycle - precipitation,infiltration, evapotranspiration, groundwater recharge, andrunoff. Altering one component of the water cycle invariablytranslates into changes in other elements of the cycle. For

    example, paving the land surface reduces the amount of infiltration and evapotranspiration whileincreasing the amount of runoff. Stormwater BMPs are intended to help maintain the natural watercycle balance.

    Figure 2-1. Natural Hydrologic Cycle for an Undisturbed Acre ofLand

    The hydrologic cycle is a dynamicsystem in constant movement.

    The hydrologic cycle is comprised offive major components: precipitation,

    infiltration, evapotransporation,

    groundwater recharge, and surface

    runoff.

    Impacting one part of this systemmeans impacting other parts of the

    system, to maintain the water

    balance.

  • 2-19Draft Pennsylvania Stormwater Management Manual

    Section 2 - Stormwater and Pennsylvanias Natural Systems

    2.1.1 Precipitation

    Precipitation in Pennsylvania reflects a relativelyhumid pattern, with average annual precipitationranging from under 34 inches to more than 50inches per year (Figure 2-3, NOAA, 2002). Stormfrequency data demonstrates some consistencyfrom one city or region to another (Table 2-1). Stormfrequency is derived based on the statisticalprobability of a storm occurring in a given year. Thatis, a 10-year, 24-hour storm has a 10 percentchance of occurring in any single year, a 50-yearstorm has a 2 percent chance, and a 100-yearstorm, a 1 percent chance. Stormwatermanagement has historically focused on managingthe flood effects from these large storms.

    Tropical hurricanes occasionally track across thestate, bringing with them heavy rainfalls and flooding. The pathways of major flood producinghurricanes that have impacted the state over the past four decades as shown in Figure 2-4. These

    Precipitation in Pennsylvania reflects a relativelyhumid pattern, with average annual precipitation

    ranging from under 34 inches to more than 50

    inches per year.

    Tropical hurricanes occasionally track across thestate, bringing with them heavy rainfalls and

    flooding.

    Precipitation varies by season, but is welldistributed throughout the year, with most of the

    annual volume of rainfall occurring in small storm

    events of modest size.

    Figure 2-2. Annual Hydrologic Cycle as Flowchart for the Brandywine Creek, (Cahill, 1995).

  • 2-20 Draft Pennsylvania Stormwater Management Manual

    Section 2 - Stormwater and Pennsylvanias Natural Systems

    coastal storms have been responsible for much of the actual flood damage experienced inPennsylvania over the years. These extreme flood events are only one component of stormwatermanagement.

    Precipitation also varies by season, but rainfall is well distributed throughout the year, occurring infrequent events of modest size. Most of the rainfall in Pennsylvania occurs in relatively small stormevents, as indicated in Figures 2-5 and 2-6. Over half of the average annual rainfall occurs in storms

    Table 2-1. Rainfall Events of 24-Hour Duration in PA (inches) (Source: NOAA National Weather ServicePrecipitation Frequency Data Server, 2004)

    2-year 5-year 10-year 50-year 100-yearPhiladelphia 3.3 4.1 4.8 6.7 7.6Pittsburgh 2.4 2.9 3.3 4.4 4.9Scranton 2.6 3.2 3.7 5.4 6.4State College 2.7 3.3 3.8 5.2 5.9Williamsport 2.8 3.5 4.1 6.0 7.0Erie 2.6 3.2 3.7 5.1 5.8

    Frequency of Occurrence (Years)Location

    Figure 2-3. Average Annual Precipitation in Pennsylvania, 1971 1990 (NOAA, 2002)

  • 2-21Draft Pennsylvania Stormwater Management Manual

    Section 2 - Stormwater and Pennsylvanias Natural Systems

    of less than 1 inch (in 24 hours). Well over 75 percent of the average annual rainfall occurs in stormsof 2 inches or less, and over 95 percent of average annual rainfall occurs in storms of 3 inches orless. A 2-year frequency rainfall is approximately 2.5 to 3 inches of precipitation. When stormwatermanagement only addresses large events (2-year storms and greater), much of the actual rainfalland runoff are overlooked (as much as 95% of the annual rainfall). Managing smaller storms thatcompromise the vast majority of the annual rainfall in Pennsylvania is also crucial.

    Figure 2-5. Number of Storm Events by Event Magnitude, (Cahill, 2004)

    8 9 71

    579

    5103

    0

    1000

    2000

    3000

    4000

    5000

    6000

    4" + 3" - 4" 2" - 3" 1" - 2" 0.1" - 1"

    Storm Magnitude (inches)

    Num

    ber o

    f Sto

    rms

    Figure 2-4. Major Flood-Producing Hurricanes through Northeastern United States

  • 2-22 Draft Pennsylvania Stormwater Management Manual

    Section 2 - Stormwater and Pennsylvanias Natural Systems

    2.1.2 Infiltration

    On an average annual basis, most rainfall soaks into the soilmantle under natural conditions. This is because there areso many small storm events that produce little if any runofffrom natural surfaces, as indicated in Figure 2-1. Thedominance of infiltration in the overall water cycle is especiallytrue when considering that most rainfall events are small. One

    inch of rain, falling in undisturbed woodlands on the moderately draining soils typical in much of thestate, produces little or no runoff. Most rainfall soaks into the soil, is stored in surface depressions, oris intercepted by vegetation as shown in Figure 2-7. Even in a larger rainfall event of 3.27 inches in24 hours, the natural system absorbs 2.24 inches according to the Runoff Curve Number Method, oralmost 70 percent of the rainfall.

    2.1.3 Evapotranspiration

    Most of the rainfall that soaks into the soil returns to theatmosphere through evaporation and transpiration throughvegetation as shown in Figure 2-2. The vegetation itself alsointercepts and slows the rainfall, reducing its erosive energy,reducing overland flow of runoff, and allowing infiltration andevaporation to occur. The root systems of plants providepathways for water movement.

    Some precipitation passes the root zone and is able to movefarther downward to the zone of saturation, or water table.Weeks or months later this precipitation emerges from springs

    or seeps as groundwater recharge or baseflow. When the natural vegetative cover is removed, all ofthese processes are altered, and the net result is a significant increase in stormwater runoff volume.

    On an average annual basis, mostrainfall soaks into the soil mantle

    under natural conditions.

    Most of the rainfall that soaks intothe soil returns to the atmosphere

    through evaporation and transpiration

    through vegetation.

    Evapotranspiration varies withseason and with type of vegetative

    cover.

    Figure 2-6. Distribution of Precipitation by Event Magnitude (PA State Climatological Office, 1926 - 2003)

    3" - 4"1%

    4" +1%

    65%

    0.1" - 1"

    2" - 3"

    6%

    1" - 2"

    27%

  • 2-23Draft Pennsylvania Stormwater Management Manual

    Section 2 - Stormwater and Pennsylvanias Natural Systems

    Evapotranspiration varies tremendously with season and with type of vegetative cover.Evapotranspiration can be a dominant force in warm weather months with good woodland cover,fully developed leaf mass, and the hot summer temperatures of July and August. Trees can effectivelyevapotranspire most, if not all, of the precipitation, which falls in summer thunderstorms and rainsho