hydrologic soil group letter A through D. The hydrologic soil group is determined by drainage class or

infiltration rate. A list of U.S. Soils and their hydrologic group is given in SCS (1986). Local soil

conservationists or soil scientists should be consulted if difficulty arises in determining soil groups.

area, the antecedent moisture condition (AMC) of the soil may be determined by summing the total

precipitation in the 5 days prior to a storm. Soil moisture will affect infiltration capacity and the resultant

runoff.

In most applications, the choice of average antecedent soil moisture condition (AMC II) is

appropriate. AMC II CNs are used for input into the loading function model. During winter months,

where soils are frozen AMC III may be appropriate for many areas. During dry conditions, for

example, mid to late summer, AMC I may be appropriate. Careful consideration should be given in

choosing an AMC for site and temporal conditions.

Curve numbers for AMC II are given in Table 1.1.1. The CNs for the other two antecedent

conditions, AMC I and AMC III, are determined by conversion equations (Hawkins 1978). Direct

runoff may be determined from Figure (1.1.2), as well as from equation (1.1.1).

The design storm may be used to drive single event loading models. Precipitation events are highly

variable infrequency, duration and intensity and are best characterized by statistical analysis of long-

term precipitation records. From historical data analysis, precipitation intensity-duration curves are

constructed. From the resulting probabilistic curves, the rainfall depth for a specified duration and

recurrence interval for the design storm may be determined.

The design storm is defined as an estimate of rainfall depth for a specified duration (10 min-24

hours) for a specified return period (2-100 years). A probability distribution derived from the historic

rainfall record is used to statistically estimate the design storm rainfall depth. The relationship between

return period and probability of design storm event occurrence is defined by the following relationship.

1

T=

(1.1.3)

P

Where:

T = return period in years

P = probability (0-1) an annual maximum event of any

year will equal or exceed some given value

1.1-5

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