THE AERATION CAPACITY OF STREAMS

The Aeration Capacity of Streams

By W. B. Langbein and 'W. H. Durum

GEOLOGICAL

SURVEY

Washington 1967

CIRCULAR

542

United States Department of the Interior STEWART L. UDALL, Secretary

Geological Survey William T. Pecora, Director

First and second printings 1967

Free on application to the U.S. Geological Survey, Washington, D.C. 20242

CONTENTS Page

........................ ........................................... .

1

The coefficient of reaeration .......... , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

Other influences on the reaeration of streams . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . .

2

Introduction

~

Downstream variations .............. , ............................ , . . . . . . . . . . . . . . .

2

Regional variations at mean flow ..... ; ........................................... 1

3

Local variations .................... , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

Distribution of assimilative-capacity ... , . . . . . . . . . . . . • . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . .

5

Summary ........................... ; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6

Selected references .................. , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6

ILLUSTRATIONS Page FIGURES

1-4.

Graph .showing1. Relation of velocity and depth to reaeration coefficient . . . . . . . . . . . .

2

2. Decrease in the reaeration coefficient with increase in size of river, Kansas-Missou11i-Mississippi Rivers . . . . . . . . . . . . . . . . . . . . . . . . . .

3

3. Regional contrast ilil the reaeration coefficient . . . . . . . . . . . . . . . . . . . .

3

4. Schematic variation in reaeration coefficient in pools and riffies . . . . . .

4

TABLES Page TABLE

1. Hydraulic data for Kansas-Missouri-Mississippi Rivers ............... .

3

2. Hydraulic factors and total assimilative capacity of streams of different orders ............ • ........................................... .

5

iii

THE AERATION CAPACITY OF STREAMS By W. B. Lamgbein and W. H. Durum

I ntrodudion Flowing bodies of water such as streams ,have the inherent ability to assimilate organic pollution from domestic and industrial waste-water discharges, from agricultural runoff, and from VB!rious natural sources. The sanitary engineer and the hydrologist are searching for ways to define the ability of a water body to assimilate waste, the proportion of capacity being used at present, and the indices by which the capacity can be measured. One of these indices is dissolved oxygen, the fuel required for destroying organic waste. Although there are other factors-such as the amount of dissolved solids present, temperature, suspended sediment, biological organisms, and the amount of flow-the amount of dissolved oxygen is a -q.seful measure of the capacity of streams to assimilate waste. When polluted water is exposed to the air, oxygen is absorbed to replace that consumed in the. slow combustion of the organic matter. This proc~ss of reoxygenation, or reaeration, goes on at a rate that is proportional to the deficit of oxygen-that is, the difference between the amount of oxygen the stream can hold at a given temperature and the actual content.

The Coefficient of Reaeration The rate of absorption of oxygen per unit of time is often expressed by the simple equation,

where cis the concentration of oxygen (milligrams per liter), Cs is the concentration for saturation at the given temperature, t is time (days), al!ld k 2 is the coefficient of reaeration. The subscript 2

denotes this coefficient as the second coefficient in the Streeter-Phelps ( 1925) formulation for the deoxygenation ( k 1 ) and reoxygenation ( kz) of streams. The oxygenation of a stream is a function of the biologic, physical, and hydraulic properties of the stream. Oxygen may be added by such processes as photosynthesis of aquatic vegetation and by mechanical aeration of the flowing water. Oxygen may be removed by such processes as vegetal decay and plant respiration, as well as by the oxidation of pollutants. In flowing streams mechanical aeration may be the dominant factor. The ·effect of hydraulic properties of rivers on the coefficient of reaeration is usually expressed as the coefficient of reaeration, k 2 • There are available a few measurements of the coefficient of reaeration that indicate a rough sort of relation with the mean velocity, v, and depth, H. Two sets of river data and two sets of laboratory results plotted in terms ofv/H1.ss are shown in figure 1. This ratio seems to accommodate both the river and the laboratory data referenced and graphed in figure 1, which suggest that k 2 3.3vI nus.

=

The correspondence shown in figure 1 is evidence that velocity and stream depth are highly significant factors, although measurements of these factors alone are incomplete estimators of reaeration. Other hydraulic properties, such as the occurrence of pools and riffles and the degree of meandering, also affect the rate of reaeration. Other relations have been prepared but none for t"l\e set of available data in this simple form. The empirical nature of the formula (fig. 1) limits its application to the range of data on which it is based; fortunately, the range happens to be fairly large. 1

X

.,..: z

UJ

(3

u:

1.1.. UJ

8 z

0

i=