ASTM: D5388: Discharge, Indirect, by Step-Backwater

Summary
Analytes
Revision
Data and Sites

Official Method Name

Standard Test Method for Indirect Measurements of Discharge by Step-Backwater Method

Current Revision

Reapproved 1997. Current edition approved April 15, 1993. Published June 1993.

Media

WATER

Instrumentation

Transit-stadia survey equipment

Method Subcategory

Physical

Method Source

ASTM

Citation

Annual Book of ASTM Standards, Section 11, Water and Environmental Technology, Volume 11.01, Water (I)

Brief Method Summary

The step-backwater test method is used to indirectly determine the discharge through a reach of channel. The step-backwater test method needs only one high-water elevation and that being at the upstream most cross section. A field survey is made to define cross sections of the stream and determine distances between them. These data are used to compute selected properties of the section. The information is used along with Manning's n to compute the change in water-surface elevation between cross sections.

Scope and Application

This test method covers the computation of discharge of water in open channels or streams using representative cross-sectional characteristics, the water-surface elevation of the upstream-most cross section, and coefficients of channel roughness as input to gradually-varied flow computations. This test method produces an indirect measurement of the discharge for one flow event, usually a specific flood. The computed discharge may be used to define a point on the stage-discharge relation.

Applicable Concentration Range

None.

Interferences

The cross sections selected should be typical and representative of the reach half way to each adjacent cross section. If there are abrupt changes between adjacent cross sections, the results could be suspect. The ratio of the conveyance to the conveyance at an adjacent cross section should stay within 0.7 and 1.4.
Care must be taken in selecting the water-surface elevation for the downstream cross section. It should not be so high that it would reflect backwater at the upstream cross section or so low that it would be in super-critical flow. A good rule of thumb is to select a stage so that the conveyance of the downstream cross section is approximately equal to the conveyance of the upstream-most cross section.
The only way to be certain that the water-surface elevation is not too high or too low or that the reach is sufficiently long enough or that enough cross sections are used, or all of the above, is to use the converging profile method. In this method, several profiles are developed using a range of starting water-surface elevations. The slope of the profiles from the higher starting elevations should increase as you move in an upstream direction. The slope of the profiles from the lower starting elevations should decrease as you move in an upstream direction. At some distance upstream, the profiles will converge.
A minimum of about ten cross sections are needed to develop a smooth backwater curve.

Quality Control Requirements

Sample Handling

Maximum Holding Time

Relative Cost

Greater than $400

Sample Preparation Methods

This method has 1 analyte associated with it.

Analyte	Detection Level	Bias	Precision	Pct False Positive	Pct False Negative	Spiking Level
Discharge(N-55555) Discharge	N/A	N/A	N/A

Precision Descriptor Notes:	Determination of the precision and bias for this test method is not possible, both at the multiple and single operator level, due to the high degree of variability of open-channel flow. Both temporal and spatial variability of the boundary and flow conditions preclude the use of a consent standard for representative sampling. A minimum bias, measured under ideal conditions, is directly related to the bias of the equipment used and is listed in the following sections. A maximum precision and bias cannot be estimated due to the variability of the sources of potential errors listed in Section 11 and the temporal and spatial variability of open-channel flow. Any estimate of these errors could be very misleading to the user. NOTE: The bias in selecting roughness coefficients is very important in obtaining a good value for discharge. The error in discharge is inversely proportional to errors in roughness coefficients.
Detection Level Note:	None.

Precision Descriptor Notes:

Determination of the precision and bias for this test method is not possible, both at the multiple and single operator level, due to the high degree of variability of open-channel flow. Both temporal and spatial variability of the boundary and flow conditions preclude the use of a consent standard for representative sampling. A minimum bias, measured under ideal conditions, is directly related to the bias of the equipment used and is listed in the following sections. A maximum precision and bias cannot be estimated due to the variability of the sources of potential errors listed in Section 11 and the temporal and spatial variability of open-channel flow. Any estimate of these errors could be very misleading to the user. NOTE: The bias in selecting roughness coefficients is very important in obtaining a good value for discharge. The error in discharge is inversely proportional to errors in roughness coefficients.

Detection Level Note:

None.

Revision	PDF/Link
Reapproved 1997. Current edition approved April 15, 1993. Published June 1993.

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ASTM: D5388: Discharge, Indirect, by Step-Backwater

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