RAINFALL REQUIREMENTS FOR MODELING RUNOFFRUNOFF

This chapter has presented several ways in which rainfall information may be repre-sented for stormwater runoff modeling and management. Which method to use in any system design or analysis depends on the approach used for transformation of the rainfall input into a prediction of the resulting stormwater discharge and on local reg-ulations. Chapter 5 reviews a number of different runoff prediction methods and their input requirements.

Examples of various runoff prediction methods and their corresponding rainfall requirements are as follows:

• Rational Method – This peak discharge method is commonly used for smaller projects such as inlet design and simple storm sewers. Application of the rational method requires IDF curves, either provided by a review agency or created from other sources.

2.40–2.55 1.0

2.55–2.70 0.5

2.70–2.85 0.5

2.85–3.00 1.0

Table 4.10 Incremental depth for Example 4.5 t (hr) Incr. P (mm)

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Time, hr

0.00 0.15 0.30 0.45 0.60 0.75 0.90 1.05 1.20 1.35 1.50 1.65 1.80 1.95 2.10 2.25 2.40 2.55 2.70 2.85 3.00

Incremental, in.P

• SCS Peak Discharge Method – This method produces only a peak dis-charge (no hydrograph). Its use requires the 24-hour total rainfall depths for the selected recurrence interval.

• SCS Unit Hydrograph – Hydrographs are required for modeling detention ponds and complex watershed systems. Any rainfall distribution can be used with the SCS unit hydrograph method; however, the 24-hour total rainfall depths and the 24-hour rainfall temporal distribution (Type I, IA, II, or III) applicable to the geographic location of the project is most typical.

• General Unit Hydrograph Methods – A number of methods may be applied to synthesize a unit hydrograph for a given watershed. Because unit hydrograph analyses are time-based, applications of these methods require use of a rainfall hyetograph or a temporal distribution of some type. As a general rule, no limitations exist on the methods applied to estimate the hye-tograph or temporal distribution. A number of “standardized” methods exist for design storm development, such as the SCS method, but hydrograph analyses are by no means limited to use with such methods. As a good prac-tice, it is recommended that engineers compute stormwater runoff hydro-graphs using a number of plausible rainfall distributions, as this will enlighten them as to the sensitivities of their results and the corresponding uncertainties in their designs and analyses.

4.7 CHAPTER SUMMARY

Before designing or evaluating any stormwater drainage system, the engineer must first determine the flows, and therefore the rainfall event or events, that the system must be capable of handling.

In evaluating a rainfall event, the rainfall characteristics that must be considered are the depth/volume, duration, area, and average recurrence interval of the precipitation, as well as its temporal and spatial distributions. Various types of rainfall gauges are used to collect rainfall data and develop precipitation records. Rainfall data records are available from agencies such as the NWS in the United States, and much informa-tion is available on the Internet. Often, rainfall data has been processed into a form more readily accessible to the engineer and is presented in publications such as HYDRO-35 or TP-40, or as IDF curves for the locale of interest. IDF curves can also be used to approximate the recurrence interval for an actual event, and equations found in HYDRO-35 can be used to develop IDF curves for areas where little data is available.

Temporal distributions describe the variation in rainfall intensity over the course of a storm event. Temporal distributions may be actual gauged rainfall amounts, or they may be synthetic in nature. Examples of synthetic storm distributions are the NRCS (SCS) 24-hour distributions for the United States. Temporal distributions are used to develop rainfall hyetographs, which show incremental rainfall depths or intensities for storm events.

Once the appropriate hyetograph for a design or study has been developed, this infor-mation can be used to compute the runoff flow rates that will be used in drainage sys-tem analysis and design.

REFERENCES

Chow, V. T. 1964. Handbook of Applied Hydrology. New York: McGraw-Hill.

Chow, V. T., D. R. Maidment, and L. W. Mays. 1988. Applied Hydrology. New York: McGraw-Hill.

Durans, S. R. and P. A. Brown. 2001. “Estimation and Internet-Based Dissemination of Extreme Rainfall Information.” Transporation Research Record no. 1743: 41–48.

Frederick, R. H., V. A. Myers, and E. P. Auciello. 1977. Five- to 60-Minute Precipitation Frequency for the Eastern and Central United States.NOAA Technical Memorandum NWS HYDRO-35. Silver Spring, Maryland: National Weather Service, Office of Hydrology.

Haestad Methods. 2003. StormCAD User Manual. Waterbury, Connecticut: Haestad Methods.

Hershfield, D. M. 1961. Rainfall Frequency Atlas of the United States for Durations from 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years. Technical Paper No. 40. Washington, D.C.: Weather Bureau, U.S. Dept. of Commerce.

Huff, F. A. 1967. “Time Distribution of Rainfall in Heavy Storms.” Water Resources Research 3, no. 4:

1007–1019.

Huff, F. A. and J. R. Angel. 1992. “Rainfall Frequency Atlas of the Midwest.” Illinois State Water Survey Bulletin 71: 141.

Johnson, L. E., P. Kucera, C. Lusk, and W. F. Roberts. 1998. “Usability Assessments for Hydrologic Fore-casting DSS.” Journal of the American Water Resources Association 34, no. 1: 43–56.

Keifer, C. J., and H. H. Chu. 1957. “Synthetic Storm Pattern for Drainage Design.” Journal of the Hydraulics Division, ASCE 84, no. HY4: 1–25.

Larson, L. W., and E. L. Peck. 1974. “Accuracy of Precipitation Measurements for Hydrologic Modeling.”

Water Resources Research 10, no. 4: 857–863.

Miller, J. F., R. H. Frederick, and R.J. Tracey. 1973. Precipitation-Frequency Atlas of the Coterminous Western United States. NOAA Atlas 2, 11 vols. Silver Spring, Maryland: National Weather Service.

Singh, V. P. 1992. Elementary Hydrology. Englewood Cliffs, NJ: Prentice Hall.

Soil Conservation Service (SCS). 1969. Section 4: Hydrology. In National Engineering Handbook. Wash-ington, D.C.: U.S. Dept. of Agriculture.

Soil Conservation Service (SCS). 1986. Urban Hydrology for Small Watersheds. Technical Release 55.

Washington, D.C.: U.S. Department of Agriculture.

Soil Conservation Service (SCS). 1992. TR-20 Computer Program for Project Formulation Hydrology (revised users’ manual draft). Washington, D.C.: U.S. Department of Agriculture.

Sweeney, T. L. 1992. Modernized Areal Flash Flood Guidance. NOAA Technical Memorandum NWS HYDRO 44. Silver Spring, Maryland: National Weather Service, Office of Hydrology.

U.S. Weather Bureau. 1958. Rainfall Intensity-Frequency Regime, Part 2 – Southeastern United States.

Technical Paper 29. Washington, D.C.: U.S. Department of Commerce.

PROBLEMS

4.1 The total volume of water stored in the atmosphere is estimated to be 12,900 km³. The average rate of global evaporation and transpiration is 505,000 km³/yr. What is the average amount of time (in days per year) that moisture spends in the atmosphere?

4.2 Using the methods given by HYDRO-35 and the data given in the table below, construct a set of IDF curves for return intervals of 2, 5, 10, 25, 50, and 100 years and durations of 5, 10, 15, 30, and 60 minutes.

4.3 Using Figures 4.15 through 4.20, complete the following table for Boston, Massachusetts.

T (yr)

Rainfall Intensity (in/hr) for duration (min)

5 15 60

2 5.1 3.3 2.4

100 8.4 6.6 3.4

T (yr)

Rainfall Intensity (in/hr) for duration (min)

5 15 60

2 100

4.4 Given the following data from a storm, plot the rainfall hyetograph (intensity versus time), calculate the total rainfall depth for the storm, and calculate the average rainfall intensity.

4.5 Use the appropriate 24-hour NRCS (SCS) synthetic rainfall distribution to develop a design storm hyetograph for a 10 year, 24-hour storm in Atlanta, Georgia. Use a total rainfall depth of 8.5 in.

(from TP 40).

Interval (min) Intensity (in/hr)

0–15 0.25

15–30 0.30

30–45 0.40

45–60 0.25

60–75 0.10

75–90 0.05

grassed area,