The U.S. Geological Survey 

(USGS) measures discharge in 

streams using several methods. 

However, measurement of peak 

discharges is often impossible or 

impractical due to difficult access, 

inherent danger of making measurements during flood events, and 

timing often associated with flood 

events. Thus, many peak discharge 

values often are calculated after the 

fact by use of indirect methods.

The most common indirect 

method for estimating peak discharges in streams is the slope-area 

method. This, like other indirect 

methods, requires measuring the 

flood profile through detailed surveys. Processing the survey data 

for efficient entry into computer 

streamflow models can be time 

demanding; SAM 2.1 is a program 

designed to expedite that process.

The SAM 2.1 computer program is designed to be run in the 

field on a portable computer. The 

program processes digital surveying data obtained from an electronic 

surveying instrument during slopearea measurements. After all measurements have been completed, 

the program generates files to be 

input into the SAC (Slope-Area 

Computation program; Fulford, 

1994) or HEC-RAS (Hydrologic 

Engineering Center-River Analysis 

System; Brunner, 2001) computer 

streamflow models so that an estimate of the peak discharge can be 

calculated.

Background

The purpose of the SAM 2.1 

computer program is to aid field 

personnel in the completion of a 

slope-area measurement. More 

information about estimating peak 

discharges using the slope-area 

method can be found in reports by 

Benson and Dalrymple (1967) and 

Dalrymple and others (1967).

The first version of the SAM 

program was created by Berenbrock (1996) using Quattro Pro 

Version 6 for Windows software 

(Corel Corp., 1996). The current 

version of the program, SAM 2.1, 

was created using Microsoft Excel 

2000 software (Microsoft, 2000), 

with Visual Basic Applications 

(VBA) programming (fig. 1), 

because newer versions of Quattro 

Pro would not run the original program and Excel is commonly available on USGS computer systems.

Input Data

The program reads data from a 

comma-delimited text file that usually can be exported directly from 

the surveying instrument or data 

collector with few or no modifications. The input data file contains 

the following information, in this 

order: Foresight (point) number 

(FS), Easting, Northing, Elevation, 

Note, and Comments. The data file 

must follow the specific format and 

use specific descriptor labels (Note) 

to allow the program to identify the 

type of information associated with 

each data point. The input data file 

uses the same format that was used 

in the earlier version (Berenbrock, 

1996). More information on formatting can be found in the HELP section of the program.

High-Water-Mark Data

Because the intent of the program is for use at various steps in 

the slope-area process, the program 

first should be used after the highwater-mark (HWM) data have been 

collected. Once the data file has 

been formatted and downloaded to 

the computer, the “Plan View” 

routine should be activated. The 

user will be prompted for specific 

information: site name, site number, and location of the input data 

file. The user also will be prompted 

to determine the base line for stationing. Once all the necessary 

information has been input, the program will create a plan view (fig. 2) 

of the area and an HWM profile plot 

(fig. 3). The user should use the 

profile plot to help determine where 

the slope-area cross sections should 

be located.

SAM 2.1 — A Computer Program for Plotting and Formatting 

Surveying Data for Estimating Peak Discharges by the SlopeArea Method

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2004–3028

March 2004
Cross-Section Data

After the cross-section data 

have been downloaded to the computer, the “Cross Sections” routine 

should be activated, which will 

require the user to determine the 

cross-section order and to verify the 

endpoint locations (left and right) at 

each cross section. The program 

then adjusts all data points in each 

cross section to a straight line 

between the two endpoints and 

determines cross-section stationing 

(distance across the channel) from 

the left endpoint. This adjustment 

results in a “true” distance between 

data points along the cross section 

and prevents the occurrence of 

“false” distances that can be created when data points are obtained 

in a zigzag pattern along the cross 

section. Finally, the routine calculates the average water-surface elevation for each cross section, 

generates profile plots of each cross 

section (fig. 4), and adds the locations of each cross section to the 

plan view and HWM profile plots.

Output Data Files

After all the data have been processed, the program can output the 

relevant data into two types of text 

files that can be used as input data 

files for various streamflow models. 

The text files can be used as input 

data files to SAC (Fulford, 1994), 

WSPRO (Water-Surface PROfile 

computations; Shearman, 1990), 

HEC-2 (Hydrologic Engineering 

Center-2; Hydrologic Engineering 

Center, 1991), or HEC-RAS (Brunner, 2001). Model-specific data 

such as Manning’s roughness coefficients (n-values), contraction or 

expansion coefficients, break 

points, and so forth, are not 

included in the SAM program and 

must be entered by the user prior to 

model application.

Summary

The use of the SAM 2.1 program in the field during slope-area 

measurements will greatly reduce 

the amount of time required to plot 

the data. This program also allows 

the user to quickly make informed 

decisions with regard to crosssection locations and the number of 

cross sections needed. Finally, the 

ability to process the data, estimate 

roughness coefficients and other 

site-specific information, and calculate peak discharge estimates while 

personnel are in the field and able 

to revise or add information should 

result in better quality and more 

reliable estimates.

— Jon E. Hortness

References Cited

Benson, M.A., and Dalrymple, Tate, 

1967, General field and office 

procedures for indirect discharge measurements: U.S. 

Geological Survey Techniques 

of Water-Resources Investigations, Book 3, Chap. A1, 30 p.

Berenbrock, Charles, 1996, A procedure to generate slope-area 

plots and compute estimated discharge for peak flows in the field 

using a portable computer 

[Abs.]: U.S. Geological Survey 

National Computer Technology 

Meeting, Rancho Mirage, Calif., 

May 19-24, 1996, U.S. Geological Survey Open-File Report 

96-204.

Brunner, G.W., 2001, HEC-RAS, 

River analysis system user’s 

manual: U.S. Army Corps of 

Engineers, CPD-68, 320 p.

Corel Corporation, 1996, Quattro 

Pro 6 for Windows users guide: 

Ottawa, Ontario, Canada, Corel 

Corporation.

Dalrymple, Tate, and Benson, M.A., 

1967, Measurement of peak discharge by the slope-area 

method: U.S. Geological Survey Techniques of WaterResources Investigations, Book 

3, Chap. A2, 18 p.

Fulford, J.M., 1994, User’s guide to 

SAC, a computer program for 

computing discharge by the 

slope-area method: U.S. Geological Survey Open-File Report 

94-360, 31 p.

Hydrologic Engineering Center, 

1991, HEC-2—Water surface 

profiles, user’s manual: Davis, 

Calif., U.S. Army Corps of 

Engineers.

Microsoft Corporation, 2000, 

Microsoft Excel 2000 for Windows user’s guide: Redmond, 

Wash., Microsoft Corporation.

Shearman, J.O., 1990, User’s manual for WSPRO—A computer 

model for water surface profile 

computations: Federal Highway Administration Report, 

FHWA-IP-89-027, 187 p.

For more information about the USGS and its 

products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this 

publication is for descriptive purposes only and 

does not imply endorsement by the U.S. 

Government.

Although this fact sheet is in the public domain 

permission must be secured from the individual 

copyright owners to reproduce any copyrighted 

materials contained within this report.

Hortness, J.E., 2004, SAM 2.1—A Computer 

Program for Plotting and Formatting Surveying Data 

for Estimating Peak Discharges by the Slope-Area 

Method: U.S. Geological Survey Fact Sheet 2004–

3028, 4 p.

2
AM 2.1 program in Microsoft Excel.
slope-area measurement site.
s for slope-area measurement site.
area cross section.