Parking Lot Storm Drain Design
Substantial effort is spent designing a building to meet the needs of the client and effort is also expended making sure an adequate number of parking spaces are provided. Designing grades for the parking lot and handling the runoff from a rainfall event are often left to less-experienced engineers and architects. However, providing proper drainage for a parking lot can have a significant impact on the ability of the client to use the building and parking lot during a heavy rainfall event. This course will provide engineers and architects that do not have extensive training in hydrology and hydraulics with the tools necessary to properly design storm drainage for a parking lot.
Rational Formula
For simple parking lots, the peak runoff can be easily computed using the Rational Formula. While hydraulic engineers may argue that the Rational Formula is too simplistic to use for large storm drain systems, it is quite appropriate to use for a small parking lot. The Rational Formula is:
Q = CIA
Where:
Q = peak runoff rate in cubic feet per second (cfs)
I = rainfall intensity in inches per hour
A = drainage area in acres
A careful unit analysis will indicate that there should actually be a unit conversion factor included, however, the factor is almost one and therefore is ignored. It is very important to note that the units of rainfall intensity (I) are in inches per hour. This course will discuss both rainfall amounts (in inches) and rainfall intensity (in inches per hour). It is important to carefully distinguish between these two variables.
Rainfall amounts will continue to increase as the duration of the storm increases. Rainfall intensities, however, will continue to decrease as the duration of the storm increases.
This equation is based on a rainfall event only, not on a snow-melt or rain-on-snow event. Snow melt generally always occurs at much slower rates than intense rainfall events and can almost always be ignored when sizing storm drain facilities for parking lots.
Runoff Coefficients
The runoff coefficient essentially represents the percentage of the rainfall that runs off rather than infiltrates into the soil. The coefficient can vary from 0.0 to 1.0, but generally varies from 0.2 to 0.95. For a parking lot that is essentially all paved, a reasonable coefficient is 0.90. For grassed areas that are part of the landscaping, the coefficient is likely to be between 0.15 and 0.30. Numerous tables have been published for runoff coefficients. One example is shown in Table 1.
Table 1
Runoff coefficients *
Type of Drainage Area C
Downtown Business Area 0.70 – 0.95
Residential Single Family Areas 0.30 – 0.50
Streets and Parking Lots 0.70 – 0.95
* from “Urban Drainage Design Manual”, FHWA HEC-22, November 1996
Where the drainage area has two or more distinctly different conditions, a weighted average can be used. For example, if a parking lot is 80% pavement and 20% grassed area, the weighted average is computed as follows:
Cavg = 80% x 0.90 + 20% x 0.20
Cavg = 0.76
Rainfall Intensity
Data Sources
Rainfall data has been collected in the United States for over 100 years. Portions of the available data have been analyzed by many researchers, including the author. In design of storm drainage for parking lots, rainfall intensities for short durations (one hour or less) are of primary interest. While the most likely source of rainfall intensity should be the US Government, the most recent publications of the National Oceanic and Atmospheric Administration (NOAA) are not very current and do not include very recent data. For the 37 eastern states, the most current NOAA publication is Technical
Memorandum No. 35, published in June 1977. For the 11 western states, the most current publications are the NOAA Atlas 2 documents, published individually for each state in 1973. However, these documents provide maps for 6-hour and 24- hour events, with equations for one-hour events. The equations for converting these one hour values to shorter durations actually come from Technical Paper 40, published in 1961.
To summarize, while values from published US Government sources are available and widely used, more updated values may be available from local sources. In using Technical Memorandum No. 35 and the NOAA Atlas 2 documents, where a point of interest is not located exactly on one of the precipitation lines, it is acceptable to use straight line interpolation between the adjacent precipitation lines.
Technical Memorandum No. 35 is available in PDF form. It will be necessary to have a copy of this publication in order to answer some of the quiz questions at the end of this course. Click on the icon below to open the PDF file in a new window.
Many cities have done more recent studies of local rainfall. Some state transportation agencies have also done similar studies. Where this information is available, it should probably be used in lieu of the older US Government publications. In many local areas, use of the rainfall intensities published in a Storm Water Management Manual, Master Plan or similar document is required. Before using any intensity, always check with the local government regulatory authority to determine if particular values are required.
Since this course was first developed, the National Weather Service has published some additional rainfall data. In August 2003, they published the first portion of NOAA Atlas 14. This covers the arid southwest, including Arizona, Nevada, New Mexico, Utah and part of southern California. They hoped to publish another portion of NOAA Atlas 14 sometime in 2004, which will cover the Ohio River Valley. Draft values for this area are available effective May 2004. This publication can be found at: http://hdsc.nws.noaa.gov/hdsc/pfds
Using rainfall data
The Rational Formula requires that the rainfall information be in the form of rainfall intensity, with units of inches per hour. Care must be taken to distinguish between rainfall amounts (which must always increase with longer durations and can be interpolated) and rainfall intensity (which must always decrease with longer durations and cannot be interpolated). Much of the published information is in the form of amount of rain for a specified time interval. For example, information from Technical Memorandum No. 35 includes rainfall amounts in a five minute time period. It is necessary to convert this value to a rainfall intensity value. If Technical Memorandum No. 35 indicates that for the location of interest, the rainfall amount for 5 minutes is 0.6 inches, then the corresponding intensity is 7.2 inches per hour (0.6 inches per 5 minutes times 12 five- minute intervals in one-hour). This is a very important point and the cause of many computational errors.
The rainfall data in Technical Memorandum No. 35 includes rainfall amounts for 5, 15 and 60-minute intervals. In order to obtain values for 10-minute and 30-minute intervals, the publication provides the following equations:
10-minute value = 0.59 * 15-minute value + 0.41 * 5-minute value
30-minute value = 0.49 * 60-minute value + 0.51 * 15-minute value
For example, if the 5-minute value is 0.50, the 15-minute value is 1.05 and the 60- minute value is 1.90, the 10-minute value would be:
0.59 * 1.05 + 0.41 * 0.50 = 0.82 inches
and the 30-minute value would be:
0.49 * 1.90 + 0.51 * 1.05 = 1.47 inches.
One common mistake in the use of rainfall data is to take a long duration event (such as a six hour rainfall) and compute a shorter duration event by simple division. For example, if the 6-hour rainfall is 1.8 inches, it is common to divide this by 6 to get a one-hour rainfall of 0.3 inches. This process ignores the fact that rainfall throughout the United States rarely falls uniformly for long periods of time. It is therefore necessary to use appropriate methods to determine short duration rainfall amounts.
Design rainfall events are often referred to by a return period, for example, a 5-year event or a 100-year event. This course is not intended to provide a technical discussion of statistics. The technically correct description of a 5-year event is an event that has a 20% probability of occurring in any year and a 100-year event has a 1% probability of occurring in any year. For simplicity sake, a 5-year event is often referred to as a rainfall event that will occur, on average, once in 5 years and a 100-year event will occur, on average, once in 100 years. However, it is possible that a 100-year rainfall event could occur in each of two consecutive years. The return period terminology is used in an attempt to help non-statisticians understand the potential risk being accepted, but is often misused.
In the design of any storm drain system, some level of risk is taken. The risk is that there is a possibility that a rainfall event in excess of the design rainfall event will occur. If the design rainfall event is a 2-year event, there is a 50% chance that this design will be exceeded in any year. The design return period is most often specified by a regulatory authority, either at the local or the state level. There are no engineering guidelines for appropriate return periods for parking lot storm drain systems. Each system should be evaluated based on the risk involved. For example, if an event greater than the design storm would cause flooding of a low area in a park, this risk could probably be accepted on a relatively frequent basis and perhaps a 1-year or 2-year design storm would be acceptable. On the other hand, if an event greater than the design storm would cause flooding several feet deep at a hospital, this risk would be less acceptable and a longer return period, such as a 10-year or 25-year event, might be more appropriate. Therefore, there is no single correct answer to what the minimum design return period rainfall event should be for a parking lot. It could be a 1-year event in some situations and a 25- year event in others.
Many local authorities have established a design frequency (or return period) for design within the local boundaries. Always check with local officials before selecting a design return period.
Contributing Drainage Areas
For a small parking lot, calculation of the contributing drainage area should be relatively straight forward. It is, however, necessary to have a good plan view of the site, including the location(s) of the building(s). It is also necessary to have an architectural plan of the building(s) in order to determine where runoff from the roof of the building is directed. The roof drainage can have a substantial impact on the total amount of runoff from a rainfall event. From the drainage plan for the building and a site grading plan, the area that drains to each potential inlet location can be measured. For small parking lots, these areas are usually computed in square feet and converted to acres for use in the Rational Formula.
Time of Concentration
Time of concentration is a term used quite often in storm drain design and is important in the use of the Rational Formula. The time of concentration is the time that it takes a raindrop that falls on the most remote portion of the parking lot to reach the storm drain system. If the rain falls at a uniform rate for this entire time (a simplistic, but necessary assumption), the entire drainage basin contributes water to the storm drain system. Many equations and methods have been developed to determine time of concentration. However, for a simple parking lot, only the simplest of equations are necessary.
The primary reason that computation of time of concentration is necessary is to determine the appropriate rainfall duration. For example, if the time of concentration for a parking lot is 5 minutes, then the duration of the rainfall event should be 5 minutes. If the specified return period is 10 years, the 10-year 5-minute rainfall intensity should be used in the Rational Formula. When the computed time of concentration is between two values (for example, if tc = 8 minutes) it is necessary to interpolate the rainfall amounts and then convert them to rainfall intensities (interpolation rainfall intensities will yield incorrect values). When the computed time of concentration is less than 5 minutes, it is generally acceptable to use a time of concentration of 5 minutes. Some design manuals indicate that it is acceptable to use a minimum time of concentration of 10 minutes, or even longer. This may be appropriate for larger parking lots.
When rain falls on a parking lot and begins to run off, the flow is classified as sheet flow. When it reaches a gutter or other channel in the paved system, it becomes shallow concentrated flow. For small parking lots, it is unlikely that the flow reaches the condition termed open channel flow.
For sheet flow, the kinematic wave equation should be used. The kinematic wave equation is:
(1) Tt = (0.933/I 0.4)*{(n*L)/S0.5)} 0.6
Where:
Tt = sheet flow travel time, minutes
n = roughness coefficient (see Table 2 below)
L = flow length, feet
I = rainfall intensity, inches per hour
S = surface slope, ft/ft
Sheet flow is generally considered to occur only for short distances, rarely more than 300 feet. However, this could be a significant portion of the flow in a small parking lot. One of the problems with the kinematic wave equation is that it is not a direct solution. The rainfall intensity to be used should be the intensity that matches the time of concentration. However, the time of concentration is a function of the rainfall intensity. It is therefore a trial and error solution. However, for a small parking lot, a close estimate of the time of concentration can be easily made (probably 5 or 10 minutes), so the iterative solution can be completed in only several trials. When the time of concentration is less than 5 minutes, it is generally acceptable to use a rainfall intensity equal to a 5-minute event.
Table 2
Manning’s roughness coefficient (n) for overland sheet flow*
| Surface Description              n Smooth Asphalt                  0.011 Smooth Concrete                0.012 Ordinary Concrete Lining   0.013 Dense Grasses                       0.24 Bermuda Grass                      0.41  |
* from “Urban Drainage Design Manual”, FHWA HEC-22, November 1996
Table 2
Manning’s roughness coefficient (n) for overland sheet flow*
Shallow concentrated flow velocity can be computed using the following equation:
(2) V = k * Sp0.5
Where:
| V | = | velocity, feet per second |
| k | = | intercept coefficient (see Table 3 below) |
| Sp | = | slope, percent |
Table 3
Intercept coefficients for shallow concentrated flow velocity*
Land Cover k
Grassed waterway 1.5
Unpaved 1.6
Paved areas 2.0
* from “Urban Drainage Design Manual”, FHWA HEC-22, November 1996
Solution:
Next, compute the shallow concentrated flow velocity.
Using equation 2:
k = 2.0,
Sp = 1.0
V = 2.0 * (1.0)0.5
V = 2.0 feet per second
T = 100 feet/2.0 feet per second
T = 50 seconds = 0.8 minutes
Time of concentration = Total travel time = 5.3 + 0.8 = 6.1 minutes
Inlet Sizing and Location
Types of inlets
There are many types of inlets. The most commonly used are rectangular grates, curved vane grates and curb inlets. Curved vane grates are typically used where the grate is placed on a slope because they intercept more water than the same size grate with simple rectangular bars. For a parking lot, it would be more common to use either a simple rectangular grate or a curb inlet. For examples of grates types, it is recommended that the user visit the web site for Neenah Foundry at www.neenahfoundry.com. This manufacturer provides many different types of grates for many different applications.
A curb inlet is often an opening in the curb only. Sometimes a curb inlet is combined with a rectangular grate. The primary disadvantage of a curb inlet is that it is not very efficient hydraulically and requires water to pond deeper than a grate that is in the gutter. The primary advantage of a curb inlet is that it is much less susceptible to plugging by leaves and branches.
Rectangular grates are used in most municipalities. Occasionally a round grate will be used, but rectangular grates collect water flowing in a gutter much more effectively than a round inlet. The primary advantage of a rectangular grate is that they intercept flow in a gutter quite efficiently. The primary disadvantage of a rectangular grate is that they are susceptible to plugging by leaves and branches. The most effective type of grate is combination grate that includes the advantages of a rectangular grate and a curb opening.
Inlet location
Inlets obviously need to be located wherever a low point occurs. If the inlets are to connect to an existing storm drain system, it is also generally convenient to locate an inlet close to an existing manhole to facilitate connection to the existing system.
When the depth of flow at an inlet is very shallow, the inlet will operate as a weir. In this condition, weir flow controls. As the depth of flow increases, the inlet becomes submerged and operates as an orifice. Most parking lot inlets will be located in low points, where computation of the inlet capacity is straightforward. If an inlet is placed along a continuous slope, the capacity of the inlet can only be determined based on results from laboratory testing. A good description of the capacity of inlets on a continuous slope is provided in Chapter 4 of FHWA’s Urban Drainage Design Manual, Hydraulic Engineering Circular No. 22. This course will only deal with inlets located in sags.
Inlet sizing
The capacity of an inlet grate operating as a weir is:
Qi = Cw P d1.5
Where:
P = perimeter of the grate in feet, disregarding the side against the curb
Cw = 3.0
D = flow depth in feet
The capacity of an inlet grate operating as an orifice is:
Qi = Co Ag (2 g d)0.5
Where:
Co = orifice coefficient = 0.67
Ag = clear opening area of the grate, square feet
g = 32.2 feet per second per second
Note that this equation requires the clear opening of the grate, which varies depending on the grate, but generally is between 50% and 70% of the total grate opening.
When computing the capacity of an inlet grate, both the weir equation and the orifice equation should be solved. The equation that yields the lower capacity will determine the flow regime.
Example:
An inlet is rectangular, 2 feet long (parallel to the curb) and 1.5 feet wide. The open area of the grate is 60% of the total open area. The depth of water over the grate is 0.2 feet. What is the capacity of the grate?
Solving the weir equation:
Qi = Cw P d1.5
Qi = 3.0 * (2 + 1.5 +1.5) * (0.2)1.5
Qi = 1.34 cfs
Solving the orifice equation:
Qi = Co Ag (2 g d)0.5
Qi = 0.67 * (2.0 * 1.5 * 60%) * (2 * 32.2 * 0.2)0.5
Qi = 4.33
The weir equation yields a lower flow, therefore this inlet is functioning in weir flow at a flow rate of 1.34 cfs.
Example:
An inlet is rectangular, 3 feet long (parallel to the curb) and 1.2 feet wide. The open area of the grate is 50% of the total open area. The flow is 1.5 cfs. What is the depth of water over the grate?
Solving the weir equation:
Qi = Cw P d 1.5
1.5 = 3.0 * (3 + 1.2 +1.2) * (d)1.5
1.5 = 16.2 * (d)1.5
(d)1.5 = 0.0926
d = 0.20 ft.
Example:
Solving the orifice equation:
Qi = Co Ag (2 g d)0.5
1.5 = 0.67 * (3.0 * 1.2 * 50%) * (2 * 32.2 * d)0.5
1.5 = 1.206 * (2 * 32.2 * d)0.5
1.5 = 9.678 * (d)0.5
(d)0.5 = 0.155
d = 0.024 ft
The weir equation yields the higher depth. This corresponds to a lower capacity, therefore the inlet operates as a weir and the depth over the inlet is 0.20 ft.
Parking Lot Grading
While parking lot grading is mostly a function of the site and the surrounding areas, some general guidelines related to storm drainage can be included. First and foremost, the low spot in the parking lot should not be located near the entrance to the building. The ideal situation is to slope the parking lot completely away from the building entrance. When this is not possible, the low point should be placed as far from the entrance as possible, preferably against one edge of the parking lot.
The following is information that will be needed as part of the quiz.
Parking Lot #1
This parking lot is depicted in Figure 1. The parking lot is 190 feet long and 175 feet wide. The entire lot is asphalt-pavement (C=0.90). The slope of the parking lot is 2% from top to bottom and 0.5% from left to right. The bottom of the page is a curb and gutter section. Assume that sheet flow will occur from the top of the page to the bottom of the page, and that when the flow reaches the gutter at the bottom of the page, the flow will be shallow, concentrated flow. The parking lot storm drain system is to be designed for a 2-year event. The parking lot is located near the borders of Texas, Arkansas and Oklahoma. The inlet will be located in the lower right hand corner (the low spot of the parking lot).
Inlet #1
An inlet is rectangular, 1.5 feet long (parallel to the curb) and 1.2 feet wide. The open area of the grate is 70% of the total open area.
Inlet #2
An inlet is rectangular, 3 feet long (parallel to the curb) and 2 feet wide. The open area of the grate is 50% of the total open area.