Precipitation

Everyone acknowledges that when it rains, runoff is generated. From a design point of view, we need something else. We need to know how much and how often it rains on our project location. Typically this information is generated from data collected by the U.S. National Weather Service through a system of rain gages. While the raw data is important, it is of little use to StormShed3G™.

IDF curves

Depending on the design methodology, the form of precipitation data that is relevant for design is either the Intensity Duration Frequency (IDF) curves or a Probably Maximum Precipitation. Both are tools that allow the engineer to predict what will happen in the future, instead of what happened in the past. IDF curves are typically used for the Rational Method of storm water peak runoff analysis. The most common source for runoff intensity data is U.S. Weather Bureau TP40 and HYDRO-35. Most of the reviewing agencies have developed a set of IDF curves for use within their jurisdiction.

The equation

Where

• F is the frequency
• K, b, x, and n are constants for a specific location

Given an IDF curve, the constants can be estimated using nonlinear regression analysis. Use of the equation has generally been replaced with IDF curves provided by reviewing agencies. 3G uses a variation of the above relationship

Where m and n are coefficients for a particular location.

SCS Rainfall Patterns

Probable Maximum Precipitation (PMP) is defined as the theoretically greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular location at a certain time of the year (National Research Council, 1985). Suffice to say that from a designer’s standpoint, there are a series of isopluvials that are commonly used to read PMP for a particular design event. The isopluvials are essentially maps of rainfall contour, typically in tenths of an inch. Hence if the contour over the project site is 30, it translates to 0.3 inches for the design event and duration specified on the map that is being read.

Typically, the maps are either 6 or 24 hour maps for 2, 10, 25, 50 or 100 year design events. In conjunction with the use of isopluvial maps to determine the amount of rainfall at a particular location, the SCS has developed several rainfall patterns. The patterns are defined as rainfall types that govern most of the United States .

Figure 1: Applicable region for various SCS Type curves (Soil Conservation Service, 1986).

The Type II curve is applicable to the majority of the United States and represents the most intense storm pattern. Based on the following figure, the inflection point of the curves represents the peak storm runoff for the pattern. As can be seen, the Type II pattern has an inflection point at 12 hours, whereas the Type IA storm pattern peaks at approximately 8 hours.

see Appendix for actual values.