R. Pitt

July 10, 2002

 

Regional Rainfall Conditions and Site Hydrology for Construction Site Erosion Evaluations

 

Introduction: Hydrology for the Design of Construction Erosion Controls

Factors Affecting Runoff

Alabama Rainfall Conditions

Typical Birmingham Rain Conditions

Intensity, Duration and Frequency (IDF) Information for Rains Used to Design Erosion Controls

Selection of Design Storms for Varying Risks and Project Durations

Methods of Determining Runoff

Use of the SCS (NRCS) TR-55 Method for Construction Site Hydrology Evaluations

General Description of TR-55 for Small Watersheds

Selection of the Curve Number

Time of Concentration Calculations

Sheetflow

Shallow Concentrated Flow

Channel Flow

Example Travel Time Calculation

Tabular Hydrograph Method

Example Tabular Hydrograph Calculation

Tabular Hydrograph Example for Urban Watershed

Example use of WinTR55

Program Description

Model Overview

Capabilities and Limitations

Model Input

Processes

Example WinTR-55 Setup and Operation

Example Applications to Construction Sites

Erosion Yields for Different Alabama Rain Categories

Important Internet Links

References

 

Introduction: Hydrology for the Design of Construction Erosion Controls

This module provides an overview of hydrology analysis techniques appropriate for the design of construction site erosion controls. The NRCS’s TR-55 procedure will be used in this module, as it provides most of the needed information and is generally applicable to conditions found on most construction sites.

Included as part of this module is a pdf copy of TR-55, Urban Hydrology for Small Watersheds by the US Dept. of Agric./Soil Conservation Service  (now NRCS) (1986). Recently, a Windows version of TR-55 (WinTR55) has become available (beta version) that can be used to greatly simplify these calculations. TR-55 provides a good set of tools to determine a number of hydrology parameters needed for effective design of construction site erosion controls. The following list shows typical controls and the types of hydrology information needed for complete evaluations and design (later modules will review and present examples of how this information is used in these designs):

· Mulches - water velocities and water depth

· Ditch liners - water velocities and water depth

· Slope down shoots - peak flow rates

· Diversion dikes and swales - peak flow rates

· Filter fabric fences - water velocities and hydrographs

· Sediment ponds - water volume and hydrographs

Factors Affecting Runoff

· Rainfall­

The extent of the storm, and the distribution of rainfall during the storm, are two major factors which affect the peak rate of runoff. The storm distribution can be thought of as a measure of how the rate of rainfall (intensity) varies within a given time interval. If a certain amount of precipitation was measured in a given 24-hour period, this precipitation may have occurred over the entire 24-hour period or in just one hour. The duration of the rain (and the peak intensity) directly affect the runoff rates.

The size of the storm is often described by the length of time over which precipitation occurs, the total amount of precipitation occurring and how often this same storm might be expected to occur (frequency). Thus, a 10-year, 24-hour storm can be thought of as a storm producing the amount of rain in 24 hours with a 10% chance of occurrence in any given year.

· Antecedent Moisture Content

The runoff from a given storm is affected by the existing soil moisture content resulting from the precipitation preceding the event of interest (defined as a five day period by the SCS). This has a much smaller effect in areas having mostly paved surfaces. On construction sites, this factor can be important.

· Surface Cover

The type of cover and its condition affects the runoff volume through its influence on the infiltration rate of soil. Bare soil at a construction site generates more runoff than forested or grass land for a given soil type. As a site develops, paving areas reduces the surface storage and infiltration capacity of the area and thus increases the amount of runoff.

The foliage and its litter maintain the soils infiltration potential by preventing the sealing of the soil surface from the impact of the raindrops. Some of the raindrops are retained on the surface of the foliage, increasing their chance of being evaporated back to the atmosphere. Some of the intercepted moisture is so long draining from the plant down to the soil that it is withheld from the initial period of runoff. Foliage also transpires moisture into the atmosphere thereby creating a moisture deficiency in the soil which must be replaced by rainfall before runoff occurs. Vegetation, including its ground litter, forms numerous barriers along the path of the water flowing over the surface of the land which slows the water down and reduces its peak rate of runoff.

· Soils

In general, the higher the rate of infiltration, the lower the quantity of stormwater runoff. Fine textured soils, such as clay, produce a higher rate of runoff than do coarse textured soils, such as sand. In addition, compacted soils also produce much more runoff than natural soils (Pitt, et al. 1999). Sites having clay soils are much more susceptible to compaction problems than most other soils.

· Time of Concentration

The time of concentration (Tc) is the longest time needed for runoff to originate from the complete project site. The time of concentration effects the peak and shape of the hydrograph. With land clearing and subsequent development, the drainage efficiency usually dramatically increases, with associated much greater peak runoff values that occur earlier in the storm. In addition, land development (and soil compaction) decease the infiltration capacity of the site, further increasing the runoff volume, and peak rate of runoff.

Alabama Rainfall Conditions

An assessment of typical Alabama rain conditions was conducted to determine the frequency of highly erosive rains and the relative importance of various rains in generating construction site erosion yields. Figures 4-1 through 4-3 show the general variations of rain conditions over Alabama. These figures were prepared by Pitt and Durrans (1995) as part of a research project for the Alabama Dept. of Transportation. These analyses used data from the 1976 and 1977 rain period. These two years were determined to be representative of the average conditions from 1948 through 1994 based on total rain depth and the monthly distribution of rains. These data was obtained from EarthInfo (Golden, CO) CD-ROMS which were archives of the official NOAA data.  Figure 4-1 is a contour map of the total annual rain depth, based on analyses at more than 120 rain gage stations located in Alabama and in surrounding states. There is little variability in rain conditions over most of the state (50 to 56 inches per year). The northwestern corner has less rain (down to about 46 inches), while the rain depth increases substantially moving towards the gulf coast (as high as 66 inches per year). There are usually a few more than 100 separate rain events per year in Alabama, defined using a minimum of 6 hours for the interevent period, with the smallest rains being 0.01 inches and the largest approaching 10 inches. Figure 4-2 presents the percentages of these annual rains having at least 0.25, 1.00, 2.5, and 5.00 inches. Few, if any, of the rains are likely greater than 5 inches in the central and northern portions of the state, but several rains greater than this amount likely occur each year near the coast. At least 40 to 50% of all rains are at least 0.25 inches in depth throughout the state. Figure 4-3 shows the percentages of all storm interevent periods that are at least 3 and 15 days. Most interevent periods are about 3 days throughout the state, but few last as long as 2 weeks, especially near the gulf coast.

Figure 4-1. Annual rainfall variations over Alabama (Pitt and Durrans 1995).

Figure 4-2. Probabilities of individual rain storms having various rain depths in Alabama (Pitt and Durrans 1995).

Probabilities of rains having at least 3 day

antecedent dry periods.

Probabilities of rains having at least 15 day

antecedent dry periods.

Figure 4-3. Rain storm interevent periods for Alabama (Pitt and Durrans 1995).

Typical Birmingham Rain Conditions

Monthly rain depths from 1955 to 1986 were examined to identify a single rain year that had total depths and rain distributions similar to the long‑term average conditions. The years 1975 and 1976 were found to both have similar rain conditions, that were close to these average conditions. Individual events in these years were identified using hourly rain records for descriptive statistical summaries. A rain event was defined as a series of hourly observations containing no more than six adjacent hours having no rain. This definition has been commonly used in many urban runoff studies as it produces discrete runoff hydrographs. The six hour period of no rain almost always allows urban runoff discharges to return to near baseflow conditions. Tables 4-1 and 4-2 summarize these rains.

Table 4-1.  Birmingham Rain Depth Distributions (average for 1975 and 1976)

Rain depth range (inches)

Interevent period (days)

Annual number of rains in range (out of 100 rains per year)

Total rain in range (inches)

% of annual rain in range

Accumulative % of rain in range

0 to 0.5

4

62

15.5

25

25

0.5 to 1.0

10

19

14.3

23

48

1.0 to 1.5

21

9

11.3

17

65

1.5 to 2.0

41

3

5.3

8

73

2.0 to 2.5

56

3

6.8

10

83

2.5 to 3.0

122

2

5.5

8

91

3.0 to 3.5

183

1

3.5

3

94

3.5 to 4.0

365

1

3.8

6

100

Table 4-2.  Birmingham Runoff Volume Distributions for Typical Construction Site

Rain depth range (inches)

Volumetric runoff coefficient (Rv)

Annual runoff in range (inches)

% of runoff in range

Accumulative % of runoff in range

0 to 0.5

0.27

4.2

19

19

0.5 to 1.0

0.34

4.9

22

41

1.0 to 1.5

0.36

4.1

17

58

1.5 to 2.0

0.39

2.0

9

67

2.0 to 2.5

0.41

2.8

11

78

2.5 to 3.0

0.44

2.4

10

88

3.0 to 3.5

0.45

1.5

4

92

3.5 to 4.0

0.48

1.8

8

100

Total, or weighted average:

0.36

23.7

100

 

Table 4-1 lists the expected rainfall distribution for typical Birmingham conditions. There are about 100 individual rains per year in Birmingham, ranging from 0.01 to about 4 inches in depth. Most of the rains are less than 0.5 inches in depth, but more than one‑half of the total annual rain depth is associated with rains greater than one inch. Rain interevent periods are important when determining the periods of time that bare ground may remain unprotected at construction sites. The interevent periods shown on this table are for all rains greater than the minimum rain in the range. As an example, rains greater than 2 inches occur about every 56 days, while rains greater than 0.5 inch occur about every 10 days.

Table 4-2 summarizes the runoff quantities that may be expected for each rain depth class, for a typical construction site area. More than half of the runoff from this area is associated with rains less than 1.5 inches in depth. Less than 20 percent of the runoff is associated with rains greater 2.5 inches in depth. Only rains greater than about 1.25 inches will contribute runoff quantities greater than 0.5 inches, a commonly used detention criterion contained in runoff control ordinances. The first 0.5 inch of runoff from all rains therefore includes all rains smaller than about 1.25 inches, plus portions of larger rains. The remaining runoff, after the first 0.5 inch, totals about 5.5 inches for typical construction areas using the 1975 and 1976 Birmingham rains. About 76 percent of the total runoff is therefore included in this criterion. If the storage criterion was increased to one inch of runoff, the percentage of the annual runoff affected would increase to about 93 percent. These criteria do not specify the degree of treatment, but only the fraction of the total flow to be treated.

 

A specified draining period for a detention facility is not specific enough. As an example, if most of the runoff is released in the early portion of the period, very little pollutant reduction may occur. If all of the runoff is held for the complete period, the pollutant removal would be very large. As an example, a complete 24‑hour detention period for typical urban runoff in a pond five feet deep would result in controlling all particles greater than about three microns. This would correspond to a particulate residue control of about 95 percent. Without knowing the distribution of flows from the pond, or the pond .depth, it is not possible to estimate the pond performance. If the pond was drained at a constant rate over 24‑hours, the total storm control would be only about 45 to 50 percent. In most cases, the pond would drain much faster over the first portions of the 24‑hour period, resulting in pollutant removals much less than 50 percent.

Intensity, Duration and Frequency (IDF) Information for Rains Used to Design Erosion Controls

As noted above, rains having high intensities typically contribute the greatest erosion yields. Individual rains that may occur in any month can contribute excessive erosion losses. Very rare rains, occurring at most only once every year, typically receive the most attention for flooding and drainage studies. When these rare rains do occur, great erosion yields will occur and most erosion control devices will fail. As an example, Figure 4-4 shows the peak rain intensities for short rain durations and long return periods for Birmingham, AL, while Figure 4-5 is an IDF curve for Tuscaloosa, AL. These curves are practically identical. Rains having average intensities of almost 3 inches per hour lasting for 30 minutes are expected to occur with a 50 percent probability every year. Five minute peak rain intensities of more than 6 inches per hour also occur with a probability of at least 50 percent every year. Table 4-3 lists the approximate rain depths (inches) and average rain intensities (inches per hour) associated with rain, durations from 1 to 24 hours and return frequencies of 1 to 100 years for Birmingham. Also shown on this table are three maximum probable events, associated with 6, 12, and 24 hour rain durations. It would be very difficult to design effective erosion control practices that can withstand the high runoff rates than may occur during most of these events.

 

Figure 4-4.  Intensity, duration, and frequency (IDF) curve for Birmingham, AL

Figure 4-5  Tuscaloosa, AL, IDF curve.

Table 4-3.  Rare Birmingham Rain Conditions

Duration (hours)

Probability (P, % occurrence per year)

Frequency

(1/P, years)

Rain Depth (inches)

Rain Intensity (inches per hour)

1

100

1

1.5

1.5

2

100

1

1.9

1.0

3

100

1

2.1

0.7

6

100

1

2.5

0.4

12

100

1

3.0

0.3

24

100

1

3.5

0.1

1

20

5

2.3

2.3

2

20

5

2.8

1.4

3

20

5

3.1

1.0

6

20

5

3.8

0.6

12

20

5

4.5

0.4

24

20

5

5.3

0.2

1

10

10

2.6

2.6

2

10

10

3.3

1.7

3

10

10

3.5

1.2

6

10

10

4.3

0.7

12

10

10

5.1

0.4

24

10

10

6.0

0.3

1

4

25

3.1

3.1

2

4

25

3.6

1.8

3

4

25

4.0

1.3

6

4

25

5.0

0.8

12

4

25

6.0

0.5

24

4

25

6.9

0.3

1

2

50

3.4

3.4

2

2

50

4.0

2.0

3

2

50

4.4

1.5

6

2

50

5.5

0.9

12

2

50

6.6

0.6

24

2

50

7.6

0.3

1

1

100

3.8

3.8

2

1

100

4.4

2.2

3

1

100

4.9

1.6

6

1

100

6.0

1.0

12

1

100

7.2

0.6

24

1

100

8.4

0.4

6

Maximum probable event

31

5.2

12

Maximum probable event

37

3.1

24

Maximum probable event

42

1.8

The Alabama Rainfall Atlas is available at: http://www.bama.ua.edu/~rain/. This web site, prepared by Dr. Rocky Durrans of the University of Alabama for an Alabama Dept. of Transportation project, calculates and presents IDF curves for any location in the state of Alabama. IDF equation coefficients were calculated based on long term rain records for many state locations. This web site than interpolates the coefficients for any location on the state map and presents graphical and tabular IDF information. The IDF information is presented for 2 to 500 year rains and for 5 minutes to 48 hours in duration. The web site will also produce SCS design hyetographs. Figure 4-6 is the main map that is displayed for the Atlas. The user simply clicks the mouse anywhere an IDF calculation is desired, and selects if a map or table (or both) is desired. In most cases, the “partial duration” option is probably desired in order to be more consistent with historical NOAA IDF curves (not a significant difference for the large, rare, rains, but more of an effect on the smaller events). These IDF curves are likely to vary from the “official” older NOAA IDF curves as they are obtained from more recent data (the Alabama Rainfall Atlas values seem to be slightly smaller than the NOAA values). The bottom button to accept is then clicked and the desired outputs are produced. Figure 4-7 is an example for Mobile, AL, showing both an IDF graph and a table. This is a preliminary product and the “print” options indicated are not yet functioning. However, it is possible to use a simple print screen utility to capture the calculated IDF information.

Figure 4-6.  Opening map for the Alabama Rainfall Atlas.

Figure 4-7.  IFD information produced by the Alabama Rainfall Atlas for Mobile, AL.

Figures 4-8 and 4-9 refer to the SCS rain distribution types that are commonly used in urban drainage design. The cumulative rain distribution in Figure 4-8 shows how the rain intensities vary throughout this hypothetical event. The slope of this curve, averaged over the time of concentration (described later) is the rain intensity the corresponds to the value on the IDF curve. Figure 4-9 shows which of these rain types are applicable for different southeastern US areas. Most of the US uses type II rains, but the gulf coast and eastern seaboard use type III rains. Type I and IA are used in some parts of the western states.

Figure 4-8. Cumulative distribution curves for different SCS rain types.

Figure 4-9. SCS rain distribution types for southeastern US (NRCS 2002b)

Selection of Design Storms for Varying Risks and Project Durations

The selection of appropriate control practices, as outlined in Section 4 of these notes, therefore must consider potentially high runoff flow rates. As an example, the use of filter fences is not recommended in channels that drain large areas. Filter fences are most suitable for controlling sheet flows originating from relatively small areas. More robust sediment control practices, such as wet detention ponds, are needed for treating runoff from large areas. Similarly, the use of unreinforced mulches (mulches without netting or tacking) can only be used on flat slopes with small contributing areas.

The following equation (from McGhee 1991) can be used to calculate the probability that a rain having a return period of “n” years, will occur at least once in the next “y” years:

               

Figure 4-10 is a plot (McGhee 1991) illustrating this relationship, but modified to show the probability of an event not being exceeded during the design period. A storm having a 50 year return period (T) would be the appropriate design storm frequency for a project (exposure) period (Td) of 5 years, if it was necessary, with  a 90% theoretical probability, that failure would not occur during this 5-year period.

Figure 4-10.  Probability of design storm (design return period) not being exceeded during the project life (design period) (from McGhee 1991).

Obviously, if failure could possibly lead to serious property damage or loss of life, then the probability of an event that may cause such failure not occurring during the project design life will need to be very large. Similarly, if only minor inconvenience will be associated with a failure, then the probability of that event not occurring during the design period can be much less. Table 4-4 illustrates several examples for a typical construction period of one year. The design storms could therefore vary greatly for different elements on the same project site. A filter fence failure may not be very serious if the site runoff is also being captured by a downstream sediment pond. However, the failure of the pond could cause much greater problems. Similarly, the slope along a filled embankment near a building foundation could cause structural failure if massive erosion occurred on the slope. In these cases and for a one year construction period, the filter fence may be designed using a 1.9 year design storm (acceptable failure probability of 50% in the one year period), the pond may require a 10 year design storm (acceptable failure probability of 10% in the one year period), while the slope near the building may need a 20+ year design storm (acceptable failure probability of <5% in the one year period).

Table 4-4.  Design Storm Return Periods Associated with Different Probability Levels for a 1-year Construction Period

Probability of storm not being exceeded in a one year  (Td on Fig 2.5) construction period

Design storm return period (T on Fig 2.5)

50%

1.9 year

75%

6.5

90%

10

95%

20

Methods of Determining Runoff

Many different methods of computing runoff have been developed. Some of the methods and limitations of each are summarized on Table 4-5 and summarized below (from Illinois 1989).

Table 4-5.  Selection Criteria for Runoff Calculation Methods (Illinois 1988)

Output Requirements

Drainage Area

Appropriate Method

Peak Discharge Only

Up to 20 acres

1

 

3

4

5

Up to 2,000 acres

 

2

3

4

5

Up to 5 square miles

 

2

3

 

5

Up to 20 square miles

 

2

3

 

5

Peak Discharge and Total Runoff Volume

Up to 2,000 acres

 

2

3

4

5

Up to 5 square miles

 

2

3

 

5

Up to 20 square miles

 

2

3

 

5

Runoff Hydrograph

Up to 5 square miles

 

2

3

 

5

Up to 20 square miles

 

2

3

 

5

1 Rational Method

2 SCS TR-20 Method

3 SCS TR-55 Tabular Method

4 SCS TR-55 Graphical Peak Discharge Method

5 COE HEC-1 Method

1. The Rational Method is an empirical formula used for computing peak rates of runoff that has been used in urban areas for over 100 years (Q=CiA). It is useful for estimating runoff on relatively small areas such as roof tops, parking lots, or other homogeneous areas. Use of the rational equation should be limited to drainage areas less than 20 acres that do not vary in surface character and do not have branched drainage systems. The most serious drawback of the rational method is that it gives only the peak discharge and provides no information on the time distribution of the storm runoff, disallowing routing of hydrographs through the drainage system or storage structures. Furthermore, the choice of “C” and “Tc” when choosing “i” in the rational method is more an art of judgment than a precise account of the antecedent moisture condition or an aerial distribution of rainfall intensity. Many errors have been reported in the use of the Rational Method, and it cannot be easily verified. Modifications of the rational method have similar limitations. The rational method may be applicable in small, isolated sections of construction sites.

2. The SCS-TR-20 computer program utilizes hydrologic soil and cover runoff curve numbers to determine runoff volumes and unit hydrographs to determine peak rates of discharge and combined hydrographs. Factors needed to use the method are the 24-hour rainfall amount, a given rainfall distribution, runoff curve numbers, time of concentration, travel time, and drainage area. This procedure probably should not be used for drainage areas less than 50 acres or more than 20 square miles. It is very useful for larger drainage basins, especially when there are a series of structures or several tributaries to be studied. Recently, a preliminary Windows version of TR-20 has become available, making the method easier to use.

3. The SCS TR-55 Tabular hydrograph is an approximation of the more detailed SCS TR-20 method. The Tabular Method divides the watershed into subareas, completes an outflow hydrograph for each, and then combines and routes each subarea to the outlet. It is especially useful for measuring the effects of changing land use in a part of a watershed. It can also be used to determine the effects of hydraulic structures and combinations of structures, including channel modifications, at different locations in a watershed. The Tabular Method should not be used when large changes in the curve number occur among subareas within a watershed and when runoff volumes are less than about 1.5 inches for curve numbers less than 60. For most watershed conditions, however, this procedure is adequate to determine the effects of urbanization on peak rates of discharge for subareas up to approximately 20 square miles in size. The recent preliminary Windows version of TR-55 has many improvements and is much easier to use than the older manual method or the original computer version. It is applicable for many conditions at construction sites.

4. The SCS TR-55 Graphical Method calculates peak discharge using an assumed unit hydrograph and an evaluation of the soils, slope, and surface cover characteri­stics of the watershed. The assumed unit hydrograph is based on design considerations rather than meteorological factors. Correction factors for swampy or ponding conditions can be used. This method is a component of the older TR-55 procedures and is not included in the new Windows version of TR-55. It is not a very suitable tool, as it has most of the same limitations as the rational method (specifically no hydrograph routing).

5. The COE-HEC 1 provides similar evaluation as the SCS TR-20. It is a rainfall-runoff model that can be calibrated to gauge records. Like TR-20, it can be used on both simple and complex watersheds. Several years ago, the older HEC-1 was superseded by the HEC-HMS (Hydrologic Modeling System) that is a Windows based program and much easier to use. Because of its complexity, it is not a very suitable tool for use at most construction sites. However, if complex conditions exist, like at some highway sites where relatively large streams are crossed by the construction activities, its use may be warranted.

Use of the SCS (NRCS) TR-55 Method for Construction Site Hydrology Evaluations

General Description of TR-55 for Small Watersheds

The complete User Guide for TR-55 (1986 version) can be downloaded from:

http://www.wcc.nrcs.usda.gov/water/quality/common/tr55/tr55.pdf. This pdf file is also available from this module. According to the NRCS (2002), Technical Release 55 (TR-55) Urban Hydrology for Small Watersheds was first issued in January 1975 as a simplified procedure to calculate the storm runoff volume, peak rate of discharge, hydrographs and storage volumes required for storm water management structures (SCS 1975). This initial version involved manual methods and assumed the Type II rainfall distribution for all calculations. In June 1986, major revisions were made in TR-55 by adding three rainfall distributions (Type I, IA and III) and programming the computations. Time of concentration was estimated by splitting the hydraulic flow path into separate flow phases (SCS 1986). This 1986 version is the last non-computerized version and has been widely used for drainage design in urban areas. A DOS computerized version was available, but was not necessary.

Even though the manual version of TR-55 is currently being phased out, its use may still be of interest when examining construction sites. In addition, the User Guide for TR-55 (SCS 1986) contains a more through description of the basic processes included in the model. A later discussion presents a description and example of the Windows version of the program.

Only the following site characteristics are needed to use TR-55: drainage area, curve number (CN), and time of concentration (Tc). With this information, it is possible to develop a hydrographs for a specific design storm. If in a complex drainage area, the watershed can be subdivided into subwatersheds for routing the flows through the system. The following subsections describe the elements of TR-55 that are of most interest for use on construction sites, and present examples for its use.

Selection of the Curve Number

The first part of using TR-55 is to select the curve number. The curve number is simply the single parameter that relates runoff to rainfall. This is illustrated in Figure 4-11. The following equation shows how the CN is used to calculated the runoff depth, Q in inches, from the precipitation depth, P in inches, and the curve number, CN:

                 

Figure 4-11.  Basic SCS rainfall-runoff relationship for different CN values (SCS 1986).

Tables 4-6 and 4-7 are used to select the most appropriate curve numbers for the area. For construction sites, Table 4-6 shows that newly grades areas have curve numbers ranging from 77 for A type soils to 94 for D type soils. These are relatively high compared to typical pre-development conditions (woods ranging from 30 to 77), reflecting the increase in runoff volume during the period of construction, and the associated increased runoff rate.

Table 4-6.  Typical Curve Number Values for Urban Areas (SCS 1986)

Table 4-7.  Typical Curve Number Values for Pasture, Grassland, and Woods (SCS 1986)

Time of Concentration Calculations

The time of concentration needs to be determined for each subwatershed in the study area. It is usually necessary to investigate several candidate flow paths in order to be relatively certain of the one that takes the longest time to reach the end of the subwatershed area. There are many different time of concentration formulas usually presented in hydrology textbooks, usually for different conditions and locations. The SCS/NRCS method has become relatively common recently and it is necessary to use this method when using TR-55 (and TR-20). This method separates the flow path into three segments: sheet flow, shallow concentrated flow, and channel flow. In some cases, especially for small sites, only sheet flow and possibly shallow concentrated flow may be evident. The candidate flow paths are drawn on a site topographic map, usually originate on the subwatershed boundary, and proceeding all the way to the bottom of the subwatershed. Sheetflow is usually the first element considered and normally is assumed to last for a maximum of 300ft, using a kinematic solution to Manning’s equation. Some states limit its’ use to even shorter lengths. The flow path is then assumed to occur as shallow concentrated flow, until a designated channel on the topographic map is reached (usually taken as a designated creek or stream on a USGS quadrangle map). When several candidate flow paths are evaluated, the one with the longest travel time is assumed to represent the time of concentration for the subwatershed. If a rain lasts for that time period, runoff will therefore occur from the complete area, resulting in maximum runoff rates.

The following discussions show how the travel times are calcualted for each flow path element.

Sheetflow

The following equation (a kinematic solution to the Manning’s equation) is used in the SCS procedures to calculate the travel time along the sheetflow path segment:

               

Where:


Tt = travel time (hr)
n = Manning roughness coefficient (for sheet flow)
L = flow length (ft) (maximum of 300 ft.)
P2 = 2-year, 24-hour rainfall depth (in), and
s = slope of hydraulic grade line (land slope, ft/ft)

The sheetflow Manning’s n roughness coefficient values are different from the channel lining roughness coefficients. Table 4-8 lists these sheetflow values. These are all greater than the channel lining n values for the rougher surfaces, due to the shallow nature of the flows. As an example, a common channel lining n value for grass is 0.024, while the sheetflow n value for grass is 0.24, or 10 times as high. The grass has a much greater effect on flow when the flow is shallow than when the flow is deep. However, the smooth surface sheetflow n coefficients (0.011) are very similar to the values that would be used for these surfaces in channels. This is because these smooth surfaces have a minimal effect on shallow and deeper flows due to their relative low roughness heights. An important factor for construction sites is the roughness coefficient of 0.011 for bare soils, compared to cultivated soils (with mulch covers of >20%) of 0.17, and dense grasses of 0.24. Natural woods can have n coefficients of 0.4 to 0.8, depending on the height of the underbrush. Figure 4-12 includes graphs that can be used to estimate the travel time for different sheetflow conditions.

Table 4-8. Sheetflow Manning’s Equation Roughness Coefficients (SCS 1986)

Surface Description

Sheetflow Roughness Factor, n

Smooth surfaces (concrete, asphalt, gravel, or bare soil)

0.011

Fallow (no residue)

0.05

Cultivated soils:

 

     Residue cover ≤ 20%

0.06

     Residue cover >20%

0.17

Grass:

 

     Short grass prairie

0.15

     Dense grass1

0.24

     Bermudagrass

0.41

Range (natural)

0.13

Woods2

 

     Light underbrush

0.40

     Dense underbrush

0.80

1 includes species such as weeping lovegrass, bluegrass, buffalo grass, blue gama

grass, and native grass mixtures

2 When selecting n for woods, consider cover to a height of about 0.1 ft. This is the

only part of the plant cover that will obstruct sheet flow.

Smooth surfaces (concrete, asphalt, gravel, or bare soil)

Fallow (no residue)

Cultivated soils: residue cover £ 20%

Cultivated soils: residue cover > 20%

Figure 4-12.  Sheetflow travel times.

Grass: short grass prairie

Grass, dense (weeping lovegrass, bluegrass, buffalo grass, blue gama grass, and native grass mixtures

Grass: burmudagrass

Range (natural)

Figure 4-12.  Sheetflow travel times (cont).

Woods, light underbrush (considering cover to height of about 0.1 ft)

Woods: dense underbrush (considering cover to height of about 0.1 ft)

Figure 4-12.  Sheetflow travel times (cont).

Shallow Concentrated Flow

After a maximum of 300 ft., sheetflow usually becomes shallow concentrated flow. The following equations are used to calculate the estimated velocities of this flow segment, based on the nature of the surface (paved or unpaved). Figure 4-13 contains graphical solutions for these equations.

                              (Unpaved)

                            (Paved)

Where:
V = average velocity (ft/s), and               
s = slope of hydraulic grade line (watercourse slope, ft/ft)

These two equations are based on a solution of the Manning equation with different assumptions for n (Manning roughness coefficient) and r (hydraulic radius, ft). For unpaved areas, n is 0.05 and r is 0.4; for paved areas, n is 0.025 and r is 0.2. The travel time associated with shallow concentrated flow segment is calculated using this velocity and the flow path length.

Figure 4-13.   Shallow concentrated flow velocities (SCS 1986).

Channel Flow

If the flow path includes a designated channel shown on a USGS quadrangle map, the Manning’s equation is used to calculate the velocity in the channel reach. The travel time in the reach is then calculated using this channel-full velocity and the length of the channel.

               

Where:

V = average velocity (ft/s), and
r = hydraulic radius (ft) and is equal to a/pw
a = cross sectional flow area (ft2)
pw = wetted perimeter (ft)
s = slope of hydraulic grade line (channel slope, ft/ft)
n = Manning roughness coefficient (for open channel flow)

This is the conventional Manning’s equation, and appropriate channel lining n coefficients are used.

Example Travel Time Calculation

The TR-55 User Guide (SCS 1986) includes the following example. Figure 4-14 shows a watershed in Dyer County, northwestern Tennessee. The problem is to compute Tc at the outlet of the watershed (point D). The 2-year 24-hour rainfall depth is 3.6 inches. All three types of flow occur from the hydraulically most distant point (A) to the point of interest (D). To compute Tc, first determine Tt for each segment from the following information:

                Segment AB: Sheetflow; dense grass; slope (s) = 0.01 ft/ft; and length (L) = 100 ft.

                Segment BC: Shallow concentrated flow; unpaved; s = 0.01 ft/ft; and L = 1400 ft.

                Segment CD: Channel flow; Manning’s n = 0.05; flow area (a) = 27 ft2;

                                wetted perimeter (pw) = 28.2 ft; s = 0.005 ft/ft; and L=7300ft.

Figure 4-14. Watershed for TR-55 Tt calculation example (SCS 1986).

Figure 4-15 is the SCS worksheet showing the calculations for the above problem. In this case, each flow segment is comprised of a single condition of slope and cover. In many cases, the individual flow segments may need to be broken up into subunits to represent different slopes or roughness coefficients. The travel times for each of the segments are added. For the sheetflow segment, however, the travel length must still be less than 300 ft. in total, not for each calculation interval. Worksheet 3 has two columns to facilitate two segments for each portion. Additional segments may be needed. In this example, the total travel time for this flow path from A to D is 1.53 hours, with almost 1 hour associated with the channel flow time. For small sites, including most construction sites, the sheetflow segment will likely comprise the largest portion of the total flow time.

Again, in order to determine the time of concentration for the watershed, several different candidate flow paths are usually needed to be evaluated and the one with the longest travel time is used as the time of concentration. This may not be the path with the longest travel distance, but may be a shorter path affected by shallower slopes and rougher covers.

Figure 4-15.  Calculation example for travel time problem (SCS 1986).

Tabular Hydrograph Method

The SCS TR-55 tabular hydrograph method (SCS 1986) can be used to develop a hydrograph for each subwatershed area than can then be routed through the downstream project segments. This method will also produce the total runoff volume and the peak flow rate. This method is not used in the new WinTR-55; this preliminary computerized version uses the more complete routing procedures from TR-20. However, the following is still presented as an optional method and to illustrate the sensitivity of Tc and CN selections.

Example Tabular Hydrograph Calculation

The following example is from the TR-55 manual (SCS 1986) and illustrates how the Tc, CN, and other site characteristics are used to develop and route hydrographs for a complex watershed.

This example computes the 25-year frequency peak discharge at the downstream end of subarea 7 shown in Figure 4-16. This example is for present conditions and uses the worksheets presented in SCS (1986). Calculate the present condition CN, Tc, and Tt for each subarea, using the procedures in TR-55 chapters 2 and 3. These values are entered on worksheet 5a (Figure 4-17). Then, the tabular hydrograph tables are used to determine the normalized hydrograph for downstream locations.

The hydrograph tables are presented in SCS (1986) according to rain type (there are sections of tables for types I, Ia, II, and III rain distributions). The first step is to find the table section pertaining to the rain distribution for the study area. In this case, the area has type II rains. The type II rain hydrograph tables are further grouped according to the Tc for the subarea, ranging from 0.1 to 2 hours. In the case for subarea #1, the Tc is 1.5 hours, so pg 5-37 from SCS (1986) is used (Table 4-9). Each page is further divided into three segments, corresponding to Ia/P ratios of 0.10, 0.30, and 0.50. The Ia is the initial abstractions for the area (not to be confused with rain distribution type Ia) and are a direct function of the CN value. These are given in the User Guide (SCS table 5-1). The P is the total rain depth being evaluated. The top set of values are used for Ia/P ratios of ≤ 0.2, the middle set for ratios from 0.2 to 0.4, while the bottom set is used for ratios of > 0.4 (interpolation is not used; WinTR-55 and TR-20 calculate more precise values based on actual site conditions). In this case, the #1 subarea Ia/P is 0.18, so the top set of values are used. Finally, each segment has 12 lines representing different travel times from the bottom of the subwatershed area to the location of interest. The largest values on each line start close to 12 hours for the top time, and shift to the right as the travel time increases. The shift between the largest values is equal to the differences in the travel times between each line, representing routing of the hydrographs as they travel downstream. For the #1 subarea, the Tt is 2.5 hours. Therefore, the line near the bottom of the top segment is used. The values in the table represent normalized hydrographs and are expressed as csm/in (ft3/sec of flow per mi2 of watershed area per inch of direct runoff). These values are multiplied by AmQ (the factor of the watershed area, in mi2 and the direct runoff in inches) to obtain the flow values in traditional units of ft3/sec, or cfs. These final cfs values are written on worksheet 5b (Table 4-10). As an example, the appropriate values for the peak discharge (q) for subarea 4 at 14.6 hr is:

                q =  qt(AmQ) = (274)(0.70) = 192 cfs

Once all the prerouted subarea hydrographs have been tabulated on worksheet 5b, they are summed to obtain the composite hydrograph. The resulting 25-year frequency peak discharge is 720 cfs at 14.3 hr, as shown on Table 4-10.

Figure 4-16.  Example watershed for tabular hydrograph calculations (SCS 1986).

Figure 4-17.  Worksheet 5a for showing basic watershed data (SCS 1986).

Table 4-9.  Tabular Hydrograph Table for Example Problem (SCS 1986, pg 5-37)

Table 4-10.  Worksheet 5b for Example Hydrograph Calculation (SCS 1986)

Tabular Hydrograph Example for Urban Watershed

The following example is for a typical urban watershed, having four subareas that are quite different in their development characteristics. The following lists the procedure for evaluating this area:

1) subdivide the watershed into relatively homogeneous subareas (as shown in Figure 4-18)

Figure 4-18.  Relatively homogeneous subareas in example urban watershed.

2) calculate the drainage for each subarea:

I

0.10 mi2

II

0.08

III

0.6

IV

0.32

Total:

1.12

3) calculate the time of concentration (Tc) for each subarea (TR-55 chapter 3):

I

0.2 hrs

II

0.1

III

0.3

IV

0.1

4) calculate the travel time (Tt) from each subarea discharge location to the location of interest (outlet of total watershed in this example) (TR-55 chapter 3):

I

0.1 hrs

II

0.05

III

0.05

IV

0

5) calculate the curve number (CN) for each subarea:

I

Strip commercial, all directly connected

CN = 97

II

Medium density residential area, grass swales

CN = 46

III

Medium density residential area, curbs and gutters

CN = 72

IV

Low density residential area, grass swales

CN = 40

6) rainfall distribution: Type II for all areas

7) 24-hour rainfall depth for storm: 4.1 inches

8) calculate total runoff (inches) from CN and rain depth (from SCS fig. 2-1)

I

CN = 97

P = 4.1 in.

Q = 3.8 in.

II

CN = 46

P = 4.1 in.

Q = 0.25

III

CN = 72

P = 4.1 in.

Q = 1.5

IV

CN = 40

P = 4.1 in.

Q = 0.06

9) determine Ia for each subarea (assumes Ia = 0.2 S) (SCS table 5-1):

I

CN = 97

Ia = 0.062 in.

II

CN = 46

Ia = 2.348 in.

III

CN = 72

Ia = 0.778 in.

IV

CN = 40

Ia = 3.000 in.

10) calculate the ratio of Ia to P

I

Ia/P = 0.062/4.1 = 0.015

II

Ia/P = 2.348/4.1 = 0.57

III

Ia/P = 0.778/4.1 = 0.19

IV

Ia/P = 3.000/4.1 = 0.73

11) use worksheets SCS 5a and 5b to summarize above data and to calculate the composite hydrograph. These are shown in Tables 4-11 and 4-12.

Table 4-11.  SCS Worksheet 5a for Urban Example

Table 4-12.  SCS Worksheet 5b for Urban Example

The peak flow is seen to be 910 cfs, occurring at 12.3 hours. Figure 4-19 is a plot of the 3 main components, plus the total hydrograph. Subarea III contributed most of the peak flow to the total hydrograph, while subareas II and IV contributed insignificant flows. The following module section introduces WinTR-55 and presents this same example. The main differences is that WinTR-55 requires a description of the channel as it calculates the travel times and conducts the channel routing using a more precise procedure. In addition, the hydrograph development uses TR-20, instead of the tabular hydrograph method.

Figure 4-19.  Plot of individual and composite hydrograph for urban example.

Example use of WinTR55

A WinTR-55 work group was formed in the spring of 1998 to modernize and revise TR-55 and the computer software. The current changes included: upgrading the source code to Visual Basic, changing the philosophy of data input, developed a Windows interface and output post-processor, enhanced the hydrograph-generation capability of the software and flood route hydrographs through stream reaches and reservoirs.

The availability and technical capabilities of the personal computer have significantly changed the philosophy of problem-solving for the engineer. Computer availability eliminated the need for TR-55 manual methods, thus the manual portions (graphs and tables) of the user document have been eliminated.

The WinTR-55 user manual (NRCS 2002a) covers the procedures used in and the operation of the WinTR-55 computer program. Part 630 of the Natural Resources Conservation Service (NRCS) National Engineering Handbook provides detailed information on NRCS hydrology and is the technical reference for this document.

Program Description

WinTR-55 is a single-event rainfall-runoff small watershed hydrologic model. The model generates hydrographs from both urban and agricultural areas and at selected points along the stream system. Hydrographs are routed downstream through channels and/or reservoirs. Multiple sub-areas can be modeled within the watershed.

Model Overview

A watershed is composed of sub-areas (land areas) and reaches (major flow paths in the watershed). Each sub-area has a hydrograph generated from the land area based on the land and climate characteristics provided. Reaches can be designated as either channel reaches where hydrographs are routed based on physical reach characteristics or as storage reaches where hydrographs are routed through a reservoir based on temporary storage and outlet characteristics. Hydrographs from sub-areas and reaches are combined as needed to accumulate flow as water moves from the upland areas down through the watershed reach network. The accumulation of all runoff from the watershed is represented at the watershed Outlet. Up to ten sub-areas and ten reaches may be included in the watershed.

WinTR-55 uses the TR-20 (NRCS 2002b) model for all of the hydrograph procedures: generation, channel routing, storage routing, and addition.

Figure 4-20 is a diagram showing the WinTR-55 model, its relationship to TR-20, and the files associated with the model.

 Figure 4-20. WinTR-55 system schematic (NRCS 2002a).

Capabilities and Limitations

WinTR-55 hydrology has the capacity to analyze watersheds that meet the criteria listed in Table 4-13:

Table 4-13. WinTR-55 Capabilities & Limitations (NRCS 2002a)

Variable

Limits

Minimum area

No absolute minimum is included in the software. However, carefully examine results from sub-areas less than 1 acre.

Maximum area

25 square miles (6,500 hectares)

Number of Subwatersheds

3-10

Time of concentration for any sub-area

0.1 hour < Tc < 10 hour

Number of reaches

0-10

Types of reaches

Channel or Structure

Reach Routing

Muskingum-Cunge

Structure Routing

Storage-Indication

Structure Types

Pipe or Weir

Structure Trial Sizes

3-3

Rainfall Depth1

Default or user-defined
0 – 50 inches (0-1,270 mm)

Rainfall Distributions

NRCS Type I, IA, II, III, NM60, NM65, NM70, NM75, or user-defined

Rainfall Duration

24-hour

Dimensionless Unit Hydrograph

Standard peak rate factor 484, , or user-defined (e.g. Delmarva—see Example 3)

Antecedent Moisture Condition

2 (average)

1 Although no minimum rain depth is listed by the NRCS in the above table, it must be recognized that the original SCS curve number methods, incorporated in this newer version, are not accurate for small storms. In most cases, larger storms used for drainage design are reasonably well suited to this method. Pitt (1987) and Pitt, et al. (2002) showed that rain depths less than 2 or 3 inches can have significant errors when using the CN approach.

Model Input

The various data used in the WinTR-55 procedures are user entered via a series of input windows in the model. A description of each of the input windows follows the figure. Data entry is needed only on the windows that are applicable to the watershed being evaluated.

Minimum Data Requirements. While WinTR-55 can be used for watersheds with up to ten sub-areas and up to ten reaches, the simplest run involves only a single sub-area. Data required for a single sub-area run can be entered on the TR-55 Main Window. These data include:  Identification Data-User,  -State, ‑County, -Project, and -Subtitle; Dimensionless Unit Hydrograph; Storm Data; Rainfall Distribution; and Sub-area Data. The sub-area data can be entered directly into the Sub-area Entry and Summary table: Sub-area name, sub-area description, sub-area flows to reach/outlet, area, runoff curve number (RCN), and time of concentration (Tc). Detailed information for the sub-area RCN and Tc can be entered here or on other windows; if detailed information is entered elsewhere the computational results are displayed in this window.

Watershed Sub-areas and Reaches. To properly route stream flow to the watershed outlet, the user must understand how WinTR-55 relates watershed sub-areas and stream reaches. Figure 4-21 and Table 4-14 show a typical watershed with multiple sub-areas and reaches.

Figure 4-21. Sample Watershed Schematic (NRCS 2002a)

Table 4-14. Sample Watershed Flows (NRCS 2002a)

Sub-area

Flows into

Upstream End of

 

Reach

Flows into

Area I

Reach A

 

Reach A

Reach C

Area II

Reach C

 

Reach B

Reach C

Area III

Reach C

 

Reach C

OUTLET

Area IV

Reach B

 

Reach D

OUTLET

Area V

Reach C

 

Reach E

OUTLET

Area VI

Reach E

     

Area VII

OUTLET

     

Area VIII

OUTLET

     

Area IX

Reach D

     

Area X

OUTLET

     

Reaches define flow paths through the watershed to its outlet. Each sub-area and reach contribute flow to the upstream end of a receiving reach or to the Outlet. Accumulated runoff from all sub-areas routed through the watershed reach system, by definition, is flow at the watershed outlet.

Processes

WinTR-55 relies on the TR-20 model for all hydrograph processes. These include: hydrograph generation, combining hydrographs, channel routing, and structure routing.

The program now uses a Muskingum-Cunge method of channel routing (Chow, et al. 1988; Maidment 1993; Ponce 1989). 

The storage-indication method (NRCS NEH Part 630, Chapter 17) is used to route structure hydrographs.

Example WinTR-55 Setup and Operation

An example using WinTR-55 and the previously presented urban watershed example, is shown on Figures 4-22 through 4-31. Figures 4-32 and 4-33 are other screens available in WinTR-55 that can be used to aid in the calculation of some of the site data, while Figure 4-34 is used for detention facilities.

Figure 4-22.  WinTR-55 opening screen.

Figure 4-23.  WinTR-55 small watershed basic information screen.

Figure 4-24.  WinTR-55 reach data screen.

Figure 4-25.  WinTR-55 reach flow path screen.

Figure 4-26.  WinTR-55 reach routing screen.

Figure 4-27.  WinTR-55 storm data screen (information automatically determined by location).

Figure 4-28.  WinTR-55 event selection/run screen.

Figure 4-29.  WinTR-55 calculated hydrograph summary screen.

Figure 4-30.  WinTR-55 hydrograph plot screen.

Figure 4-31.  WinTR-55 report generation screen.

Figure 4-32.  WinTR-55 land use details screen (if data not directly entered).

Figure 4-33.  WinTR-55 time of concentration details screen/calculator (if data not directly entered).

Figure 4-34.  WinTR-55 structure data screen for detention facilities.

This WinTR-55 example resulted in a peak flow for the 2-yr storm of about 730 cfs, compared to the previously calculated value of 910 cfs. This difference is due to the specific routing procedure used, plus the more precise hydrograph development procedure in the updated program version.

Example Applications to Construction Sites

As indicated previously, there are a number of situations where WinTR-55 (or TR-55) can be used to advantage when evaluation construction sites, including performing the design of erosion controls. These may include:

                · Determination of flows going away from the site affecting downstream areas. Downstream erosion controls may include filter fencing along the project perimeter, or sediment ponds, depending on flow conditions. These controls must be completed before any on-site construction is started.

                · Determination of upland flows coming towards the disturbed areas. These flows must be diverted by swales or dikes, or safely carried through the construction sites. Channel design will be based on the expected flow conditions. These controls must be completed after the downstream controls, and before any on-site controls are started.

                · Determination of on-site flows on slopes going towards filter fencing, sediment ponds, or other controls. Needed to also evaluate shear stress on channels and on slopes.

Figure 4-35 is a regional map (on a USGS quadrangle) showing a construction site, and associated upland and downslope drainages. This module show how it is possible to easily calculate the runoff characteristics affecting the site and downslope areas for different rain conditions. In addition, detailed site conditions for different project phases can also be evaluated for the design of appropriate erosion controls.

Figure 4-35.  Determination of general upslope and downslope drainage areas from construction site.

Figure 4-36 shows subdrainages for the upslope, downslope, and on-site areas for this example construction site. Table 4-15 summarizes the characteristics of these areas, along with the hydrologic information needs for each area. Most of the site will be excavated, except for the two small areas near the downslope edge. The upslope diversions will carry the upslope water to the main channel. The erosion on O1 and O2 on-site areas will be controlled by slope mulches and filter fences, before the runoff drains to the on-site main channel. A sediment pond will be constructed at the downslope property boundary before this main channel leaves the site.

 

Figure 4-36.  Subdrainage areas on and near construction site.

Table 4-15.  Upslope and On-Site Subdrainage Area Characteristics for Construction Site

Area Notation

Location

Objective

Area (acres)

Cover n

Average flow path slope

CN (all “C” soils)

Tc (min)

U1

Upslope – direct to on site stream

Hydrograph (to be combined with U2 and U3)

37.4

0.4

8%

73

29

U2

Upslope – diversion to on site stream

Peak flow rate and hydrograph  (to be combined with U1 and U3)

14.6

0.4

11.5

73

25

U3

Upslope – diversion to on site stream

Peak flow rate and hydrograph (to be combined with U1 and U2)

2.4

0.4

12.7

73

20.7

O1

On site – drainage to sediment pond and main site stream (also slope protection needed)

Peak flow rate and hydrograph

12.6

0.011

10

91

3.5

O2

On site – drainage to filter fence and main site stream (also slope protection needed)

Peak flow rate and hydrograph

7.1

0.011

10.5

91

1.6

O3

On site – towards perimeter filter fence (also slope protection needed)

Peak flow rate and hydrograph

6.1

0.011

5

91

4.1

O4

On site – towards perimeter filter fence (also slope protection needed)

Peak flow rate and hydrograph

3.1

0.011

6.7

91

3.3

O5

On site – towards perimeter filter fence (also slope protection needed)

Peak flow rate and hydrograph

1.8

0.011

11.3

91

1.5

O6

On site – nothing (will remain undisturbed)

na

1.3

0.24

6.7

na

na

O7

On site – nothing (will remain undisturbed)

na

0.3

0.24

10

na

na

All of the information needed to calculated the expected flows from these upslope and on-site areas is shown on Table 4-16, except for the design storm. The area has a SCS type III rain distribution and the construction period will be one year. The different site features will require different design storms due to the different levels of protection that are appropriate. Table 4-16 lists the features and the (assumed) acceptable failure rates during this one year period, along with the corresponding design storm frequency and associated 24 hr rain total appropriate for the area. The design storms range from 4.0 to 8.4 inches in depth and the times of concentration range from 1.5 to 30 minutes. The design rain intensities could be very large for some of these design elements.

Table 4-16.  Acceptable Levels of Protection for Different Site Activities

Site Construction Control

Acceptable Failure Rate during Site Construction Activities

Design Storm Return Period (years)

24-hr Rain Depth Associated with this Design Storm Return Period

Diversion channels

25%

6.5

5.5

Main site channel

5%

20

6.6

Site slopes

10%

10

6.0

Site filter fences

50%

1.9

4.0

Sediment pond

5% and 1%

20 and 100

6.6 and 8.4

Downslope perimeter filter fences

10%

10

6.0

In some designs (for shear stress calculations in the next module), the water depth is also needed for sheetflows. The following equation can be used to calculate the estimated water depth for sheetflow, based on the Manning’s equation (R, the hydraulic radius is equal to the flow depth for sheetflow):

               

where:    y is the flow depth (in feet),

                q is the unit width flow rate (Q/W, the total flow rate, in ft3/sec, divided by the slope width, in ft.)

                n is the sheet flow roughness coefficient, and

                s is the slope (as a fraction)

Erosion Yields for Different Alabama Rain Categories

It is possible to estimate the relative erosion contributions of different rains, as shown in Tables 4-17 through 4-21. Thronson (1973) presented the following equation to estimate the erosion potential for individual rains, when complete intensity information is not available:

               

where P is the rain depth, in inches, and dur is the rain duration, in hours. This equation was proposed for SCS original type II rains, applicable for the complete US, except for the extreme west coast. Long-term rain series data for Huntsville, Birmingham, Tuscaloosa, Montgomery, and Mobile were extracted from EarthInfo CD-ROMS (Golden, CO) and processed in SLAMM (www.winslamm.com) to combine the hourly data into individual rain records. Each rain was defined as having at least a 6 hour dry interevent period. About 50 years of data were available for each city, although some of the records were incomplete. The number of events evaluated for each city ranged from about 2500 to 5200 separate rains. The calculations were made for each of 12 rain categories and the total annual R was estimated by multiplying the partial R for each category by the number of events in each category. The calculated annual R values for these 5 cities were slightly larger (differences of 6 to 34%) than the published annual R values. The main reason for these differences is that the published annual R values are median values for many separate years, while the R values used here were averaged values, which would be larger. The calculated R values for each category were therefore adjusted to indicated the approximate portion of the total annual R associated with the different rain categories.

As indicated in Module 3, the larger rains contribute most of the erosion potential for Alabama conditions. For all of these cities, except Mobile, the rain depth associated with the median of the annual R is about 2 inches, while it is about 2.5 inches for Mobile. About 5% of the annual rains are therefore responsible for about half of the annual erosion potential. Because of the long rain record used here, these rain series include rare events, including the “50-year” event. It may be impractical to design erosion controls that can effectively withstand these very large events. Except for Mobile, rains greater than 4 inches occur less than once a year. If a “typical” rain year was examined, the effects of these very large rains would be somewhat diminished. When the 1976 rain year for Birmingham was examined (a typical year for local rains), for example, the rain depth associated with the median erosion potential was reduced to about 1.75 inches.

Table 4-17.  Erosion Potential Analysis for Huntsville Rains Occurring from 1958 through 1999

Rain range (inches)

Mid Point Rain (inches)

Duration (hours)

Average Intensity (in/hr)

#/year in range category

% of rains in category

Thronson R

% of annual R in category

Accumulative % of total R

0.01 to 0.05

0.03

3

0.01

22.5

26.0

0.1

0.0

0.0

0.06 to 0.10

0.08

7

0.01

8.1

9.4

0.2

0.1

0.1

0.11 to 0.25

0.18

8

0.02

13.3

15.4

1.7

0.6

0.6

0.26 to 0.50

0.38

10

0.04

13.9

16.0

8.1

2.7

3.3

0.51 to 0.75

0.63

12

0.05

9.3

10.8

15.2

5.1

8.4

0.76 to 1.00

0.88

14

0.06

5.7

6.6

18.0

6.0

14.4

1.01 to 1.50

1.26

16

0.08

6.6

7.6

43.0

14.3

28.7

1.51 to 2.00

1.76

18

0.10

3.2

3.8

41.9

14.0

42.7

2.01 to 2.50

2.26

20

0.11

1.6

1.9

34.2

11.4

54.1

2.51 to 3.00

2.76

24

0.12

0.8

0.9

24.9

8.3

62.4

3.01 to 4.00

3.5

30

0.12

0.8

0.9

35.2

11.7

74.2

over 4.01

5.27

36

0.15

0.7

0.9

77.5

25.8

100.0

4425 events

51.1 years

12.03 in. max rain

Totals:

86.5

100.0

300.0

100.0

 

Table 4-18.  Erosion Potential Analysis for Birmingham Rains Occurring from 1948 through 1999

Rain range (inches)

Mid Point Rain (inches)

Duration (hours)

Average Intensity (in/hr)

#/year in range category

% of rains in category

Thronson R

% of annual R in category

Accumulative % of total R

0.01 to 0.05

0.03

3

0.01

22.9

20.7

0.1

0.0

0.0

0.06 to 0.10

0.08

7

0.01

17.4

15.8

0.4

0.1

0.1

0.11 to 0.25

0.18

8

0.02

17.3

15.6

2.4

0.7

0.8

0.26 to 0.50

0.38

10

0.04

19.5

17.6

12.4

3.5

4.4

0.51 to 0.75

0.63

12

0.05

9.4

8.5

16.6

4.8

9.1

0.76 to 1.00

0.88

14

0.06

8.3

7.5

28.6

8.2

17.3

1.01 to 1.50

1.26

16

0.08

7.9

7.2

56.4

16.1

33.4

1.51 to 2.00

1.76

18

0.10

3.8

3.5

53.9

15.4

48.8

2.01 to 2.50

2.26

20

0.11

1.6

1.5

38.0

10.9

59.7

2.51 to 3.00

2.76

24

0.12

0.8

0.7

26.3

7.5

67.2

3.01 to 4.00

3.5

30

0.12

1.1

1.0

57.0

16.3

83.5

over 4.01

5.67

36

0.16

0.4

0.4

57.9

16.5

100.0

4583 events

41.5 years

13.58 in. max rain

Totals:

110.5

100.0

350.0

100.0

 

Table 4-19.  Erosion Potential Analysis for Tuscaloosa Rains Occurring from 1958 through 1999

Rain range (inches)

Mid Point Rain (inches)

Duration (hours)

Average Intensity (in/hr)

#/year in range category

% of rains in category

Thronson R

% of annual R in category

Accumulative % of total R

0.01 to 0.05

0.03

3

0.01

6.9

11.8

0.0

0.0

0.0

0.06 to 0.10

0.08

7

0.01

10.3

17.5

0.4

0.1

0.1

0.11 to 0.25

0.18

8

0.02

9.4

16.0

1.9

0.5

0.6

0.26 to 0.50

0.38

10

0.04

10.3

17.5

9.8

2.6

3.2

0.51 to 0.75

0.63

12

0.05

6.3

10.7

16.7

4.5

7.7

0.76 to 1.00

0.88

14

0.06

4.5

7.7

23.3

6.2

13.9

1.01 to 1.50

1.26

16

0.08

5.2

8.9

55.8

14.9

28.8

1.51 to 2.00

1.76

18

0.10

2.6

4.5

55.2

14.7

43.5

2.01 to 2.50

2.26

20

0.11

1.4

2.4

48.3

12.9

56.4

2.51 to 3.00

2.76

24

0.12

0.7

1.2

35.6

9.5

65.9

3.01 to 4.00

3.5

30

0.12

0.6

1.1

47.1

12.6

78.4

over 4.01

5.33

36

0.15

0.5

0.8

80.8

21.6

100.0

2535 events

43.2 years

11.76 in. max rain

Totals:

58.7

100.0

375.0

100.0

 

Table 4-20.  Erosion Potential Analysis for Montgomery Rains Occurring from 1948 through 1999

Rain range (inches)

Mid Point Rain (inches)

Duration (hours)

Average Intensity (in/hr)

#/year in range category

% of rains in category

Thronson R

% of annual R in category

Accumulative % of total R

0.01 to 0.05

0.03

3

0.01

25.1

25.2

0.1

0.0

0.0

0.06 to 0.10

0.08

7

0.01

9.6

9.7

0.2

0.1

0.1

0.11 to 0.25

0.18

8

0.02

16.9

17.0

2.2

0.6

0.7

0.26 to 0.50

0.38

10

0.04

15.8

15.9

9.6

2.7

3.4

0.51 to 0.75

0.63

12

0.05

9.5

9.6

16.2

4.5

7.9

0.76 to 1.00

0.88

14

0.06

6.2

6.2

20.4

5.7

13.6

1.01 to 1.50

1.26

16

0.08

7.8

7.9

53.6

14.9

28.5

1.51 to 2.00

1.76

18

0.10

3.7

3.7

50.4

14.0

42.6

2.01 to 2.50

2.26

20

0.11

2.0

2.0

43.7

12.2

54.7

2.51 to 3.00

2.76

24

0.12

1.0

1.0

32.7

9.1

63.8

3.01 to 4.00

3.5

30

0.12

1.0

1.0

48.7

13.6

77.4

over 4.01

5.49

36

0.15

0.7

0.7

81.1

22.6

100.0

5121 events

51.5 years

10.96 in. max rain

Totals:

99.4

100.0

359.0

100.0

 

Table 4-21.  Erosion Potential Analysis for Mobile Rains Occurring from 1948 through 1999

Rain range (inches)

Mid Point Rain (inches)

Duration (hours)

Average Intensity (in/hr)

#/year in range category

% of rains in category

Thronson R

% of annual R in category

Accumulative % of total R

0.01 to 0.05

0.03

3

0.01

30.5

26.0

0.1

0.0

0.0

0.06 to 0.10

0.08

7

0.01

12.5

10.7

0.4

0.1

0.1

0.11 to 0.25

0.18

8

0.02

19.1

16.4

3.0

0.4

0.5

0.26 to 0.50

0.38

10

0.04

17.3

14.8

12.8

1.9

2.4

0.51 to 0.75

0.63

12

0.05

10.6

9.0

21.7

3.2

5.7

0.76 to 1.00

0.88

14

0.06

6.9

5.9

27.6

4.1

9.8

1.01 to 1.50

1.26

16

0.08

8.4

7.2

69.5

10.3

20.1

1.51 to 2.00

1.76

18

0.10

4.4

3.8

72.4

10.8

30.8

2.01 to 2.50

2.26

20

0.11

2.9

2.5

78.9

11.7

42.6

2.51 to 3.00

2.76

24

0.12

1.5

1.3

58.4

8.7

51.2

3.01 to 4.00

3.5

30

0.12

1.5

1.3

86.2

12.8

64.0

over 4.01

6.03

36

0.17

1.4

1.2

242.0

36.0

100.0

5239 events

44.7 years

11.81 in. max rain

Totals:

117.0

100.0

673.0

100.0

 

Table 4-22 shows the variation of these large rains for the 1948 through 1999 rain period for Birmingham (41.5 years of data due to some missing data periods). From 1 to 8 (an average of 4.1) of these rains occur each year, but no obvious pattern is indicated. Table 4-23 examines these highly erosive rains for each month of the year, for this same Birmingham rain period. May through November appears to have fewer of these rains, however, September had the largest number of any month.


4-22. Number of Large Rains (>2 inches) per Year for Birmingham.

year

#/year

48

4

49

2

50

7

51

6

52

2

53

4

54

3

55

1

56

3

57

8

58

2

59

2

60

1

61

6

62

4

63

6

64

8

65

2

66

5

67

6

68

5

69

6

70

5

71

4

72

3

73

5

74

3

75

5

76

7

77

8

88

3

89

2

90

3

91

3

92

5

93

1

94

4

95

4

96

5

97

1

98

6

99

2

total:

172

min

1

max

8

average

4.1

st dev

2.0

COV

2.0

Table 4-23. Birmingham Rains by Month

 

2.00 to 2.50

2.51 to 3.00

3.01 to 4.00

over 4.01

total

January

7

2

4

4

17

February

7

2

4

1

14

March

9

5

5

2

21

April

5

1

5

1

12

May

7

4

4

1

16

June

6

0

5

0

11

July

5

2

2

2

11

August

4

5

1

1

11

September

9

7

5

1

22

October

0

3

5

1

9

November

8

1

1

1

11

December

6

2

6

3

17

Total for 41.5 years of record

73

34

47

18

172

Average (#/year):

1.8

0.8

1.1

0.4

4.1

Important Internet Links

Alabama Rainfall Atlas:

http://bama.ua.edu/~rain/

TR-55 computer program (new windows beta version):

http://www.wcc.nrcs.usda.gov/water/quality/common/tr55/tr55-beta.html

TR-55 1986 documentation:

http://www.wcc.nrcs.usda.gov/water/quality/common/tr55/tr55.pdf

TR-20 computer program (new windows beta version):

http://www.wcc.nrcs.usda.gov/water/quality/common/tr20/tr20-beta.html

National Engineering Handbook, Part 630 HYDROLOGY

http://www.wcc.nrcs.usda.gov/water/quality/common/neh630/4content.html

US Army Corps of Engineers, Hydrologic Management System User Guide (replacement for HEC-1):

http://www.hec.usace.army.mil/publications/pubs_distrib/hec-hms/user/user.html

US Army Corps of Engineers, River Analysis System User Guide for water surface profile calculations (replacement for HEC-2):

http://www.hec.usace.army.mil/publications/pubs_distrib/hec-ras/user/user.html

References

Chow, V. T., Maidment, D. R, and Mays, L. W., Applied Hydrology, McGraw-Hill, 586 pages. 1988.

HEC. HEC-RAS User's Manual, Version 2.0. US Army Corps of Engineers, Hydrologic Engineering Center, April 1997.

Illinois. Illinois Procedures and Standards for Urban Soil Erosion and Sedimentation Control. Association of Illinois Soil and Water Conservation Districts, Springfield, IL 62703. 1989.

Maidment, D. R. (ed.), Handbook of Hydrology, McGraw-Hill, 1422 pages. 1993.

McGee, T.J. Water Supply and Sewerage. McGraw-Hill, Inc., New York. 1991.

NRCS. National Engineering Handbook, Part 630 HYDROLOGY, downloaded June 23, 2002 at: http://www.wcc.nrcs.usda.gov/water/quality/common/neh630/4content.html

NRCS. SITES Water Resource Site Analysis Computer Program User’s Guide. United States Department of Agriculture, Natural Resources Conservation Service. 469 pp. 2001.

NRCS. WinTR-55 User Manual. US Dept. of Agriculture, Natural Resources Conservation Service. Downloaded on June 23, 2002 from: http://www.wcc.nrcs.usda.gov/water/quality/common/tr55/tr55-beta.html Version dated April 23, 2002a.

NRCS. TR-20 System: User Documentation. United States Department of Agriculture, Natural Resources Conservation Service. 105 pp. 2002b (draft).

Pitt, R. Small Storm Urban Flow and Particulate Washoff Contributions to Outfall Discharges, Ph.D. Dissertation, Civil and Environmental Engineering Department, University of Wisconsin, Madison, WI, November 1987.

Pitt, R. and S.R. Durrans. Drainage of Water from Pavement Structures. Alabama Dept. of Transportation. 253 pgs. September 1995.

Pitt, R., J. Lantrip, R. Harrison, C. Henry, and D. Hue. Infiltration through Disturbed Urban Soils and Compost-Amended Soil Effects on Runoff Quality and Quantity. U.S. Environmental Protection Agency, Water Supply and Water Resources Division, National Risk Management Research Laboratory. EPA 600/R-00/016. Cincinnati, Ohio. 231 pgs. December 1999.

Pitt, R., M. Lilburn, S. Nix, S.R. Durrans, S. Burian, J. Voorhees, and J. Martinson Guidance Manual for Integrated Wet Weather Flow (WWF) Collection and Treatment Systems for Newly Urbanized Areas (New WWF Systems). U.S. Environmental Protection Agency. 612 pgs. Expected final publication in 2002.Ponce, V.M., Engineering Hydrology, Prentice Hall, 640 pages. 1989.

SCS. Urban Hydrology for Small Watersheds. Technical Release 55, US Department of Agriculture, Soil Conservation Service. 91 pp. 1975.

SCS (now NRCS). Urban Hydrology for Small Watersheds. US Dept. of Agric., Soil Conservation Service. 156 pgs. 1986.

SCS. Time of Concentration, Hydrology Technical Note No. N4. United States Department of Agriculture, Soil Conservation Service, Northeast National Technical Center. 12 pp. 1986.

Thronson, R.E. Comparative Costs of Erosion and Sediment Control, Construction Activities. U.S. Environmental Protection Agency. EPA­430/9-73-016. Washington, D.C. 1973.

Welle, P.I., Woodward, D. E., Fox Moody, H., A Dimensionless Unit Hydrograph for the Delmarva Peninsula, Paper No. 80-2013, ASAE 1980 Summer Meeting, 18 pp. 1980.