The standard project storm (SPS) is

…a relationship of precipitation versus time that is intended to be reasonably characteristic of large storms that have or could occur in the locality of concern. It is developed by studying the major storm events in the region, excluding the most extreme. For areas east of 105 longitude the results of SPS studies are published in EM 1110-2-1411 as generalized regional relationships for depth, duration and area of precipitation. For areas west of 105 longitude, special studies are made to develop the appropriate SPS estimates. The standard project flood (SPF) [runoff from the SPS] is used as one convenient way to compare levels of protection between projects, calibrate watershed models, and provide a deterministic check of statistical flood frequency estimates. (USACE, 1989)

Basic Concepts and Equations

The SPS model included in the program is applicable to watersheds within the continental United States and East of 105 ^{\circ} longitude. It is limited to areas 10 to 1,000 square miles. The SPS is rarely used now because of the emergence of risk-based design techniques, the inconsistency of the method between different geographic regions, the lack of a standard SPS West of 105 ^{\circ} longitude, and no attached probability of occurrence. The 0.002 annual exceedance probability event has all but replaced the SPS for design and description purposes. However, the regulations that originally instituted the SPS have not been rescinded and it may still be necessary to compute it as part of designing a flood protection project. A detailed description of the SPS can be found in the EM 1110-2-1411 and also in HEC Training Document No. 15 (USACE, 1982). Development of the SPS begins with the specification of the index depth. The program calculates a total storm depth distributed over a 96-hour duration using:

Total depth=\sum_{i=1}^{4}\left(R_{24 H R}(i) \cdot \operatorname{SPFE}\right)

where SPFE is the standard-project-flood index-precipitation depth in inches; and R24HR(i) is the percent of the index precipitation occurring during 24-hour period i. R24HR(i) is given by:

$R_{24 H R(i)}=\left\{\begin{array}{ll}3.5 & \text { if } i=1 \\ 15.5 & \text { if } i=2 \\ 182.15-14.3537 * L O G_{\varepsilon}(T R S D A+80) & \text { if } i=3 \\ 6.0 & \text { if } i=4\end{array}\right\}$

where TRSDA = storm area, in square miles. Each 24-hour period is divided into four 6-hour periods. The ratio of the 24-hour precipitation occurring during each 6-hour period is calculated as:

$R_{6 H R(i)}=\left\{\begin{array}{ll}R_{6 H R}(4)-0.033 & \text { if } i=1 \\ 0.055 *(S P F E-6.0)^{051} & \text { if } i=2 \\ \frac{13.42}{(\operatorname{SPFE}+11.0)^{0.93}} & \text { if } i=3 \\ 0.5 *\left(1.0-R_{6 H R}(3)-R_{6 H R}(2)\right)+0.0165 & \text { if } i=4\end{array}\right\}$

where R6HR(i) is the ratio of 24-hour precipitation occurring during hour period i. The program computes the precipitation for each time interval in the jth 6-hour interval of the ith 24-hour period (except the peak 6-hour period) with:

$P R C P=0.01^{*} R_{24 H R}(i)^{*} R_{6 H R}(j)^{*} S P F E^{*} \frac{\Delta t}{6}$

where ∆t is the computation time interval, in hours. The peak 6-hour precipitation of each day is distributed according to the percentages in Table 12. When using a computation time interval less than one hour, the peak 1-hour precipitation is distributed according to the percentages in Table 13. (The selected time interval must divide evenly into one hour.) When the time interval is larger than shown in Table 12 or Table 13, the percentage for the peak time interval is the sum of the highest percentages. For example, for a 2-hour time interval, the values are (14 + 12)%, (38 + 15)%, and (11 + 10)%. The interval with the largest percentage is preceded by the second largest and followed by the third largest. The second largest percentage is preceded by the fourth largest, the third largest percentage is followed by the fifth largest, and so on.

Following the development of the distribution, the hyetograph for each subbasin is computed as the transposition factor multiplied by the distribution depth.


Table 12. Distribution of maximum 6-hour SPS in percent of 6-hour amount.

Duration (hr)

EM 1110-2-1411 Criteria (Standard)

Southwestern Division Criteria (SWD)

1

10

4

2

12

8

3

15

19

4

38

50

5

14

11

6

11

8


Table 13.Distribution of maximum 1-hour precipitation in the SPS.

Time (min)

Percent of Maximum 1-hr Precipitation in Each Time Interval

Accumulated Precent of Precipitation

5

3

3

10

4

7

15

5

12

20

6

18

25

9

27

30

17

44

35

25

69

40

11

80

45

8

88

50

5

93

55

4

97

60

3

100


Parameter Estimation

A storm area must be selected in order for the distribution to be developed. In general, the area should match the drainage area for the watershed that drains to the location where the flood protection project will be constructed. The area may be slightly larger than the drainage area at the actual proposed construction site.
The SPS index precipitation value is taken from Plate 2 in EM 1110-2-1411, as shown in Figure 20. The lowest isohyet line has a value of 9 inches and passes through central Minnesota, Northern Michigan, New York, and Maine. A high isohyet line with a value of 19 inches follows the Texas-Louisiana gulf coast and crosses to Florida. Select the best index precipitation value based on the location of the flood protection project.
Each subbasin must have a so-called transposition factor. The factors are selected by overlaying the SPS isohyetal pattern over the complete project watershed. An area-weighted average should be used to determine the factor for each subbasin. The isohyetal pattern is taken from Plate 12 in EM 1110-2-1411, as shown in Figure 21.

Figure 20.Reproduction of Plate 2 from EM 1110-2-1411.

Figure 21.Reproduction of Plate 12 in EM 1110-2-1411.