Chapter (5)
----------Stream flow
A. Introduction
·
stream flow is ultimate end product of
runoff generation
·
variations in flow affect amount &
type of geomorphic work a river can do
·
stream channel geometry
·
mechanics of flow
B. Stream Channel Geometry
1. Channel width: w
2. Channel depth: d
3. Cross sectional area: a
4. Wetted perimeter: Pw
5. Hydraulic radius: r = a/Pw
6. Channel gradient (slope): s
C. Discharge
Q=wxdxv=axv
D. Velocity
1. Driving forces
a. gravity: 32 ft/sec/sec or 9.8 m/sec/sec
b. channel gradient
2. Resisting forces
a. viscosity: resistance of a fluid to a change in shape
1) molecular viscosity: resistance due to friction between individual water molecules
as they collide and slide past one another
·
affected by temperature and suspended sediment
·
laminar flow
2) eddy viscosity: resistance due to friction along eddy lines
·
·
turbulent flow
turbulence: frequency and magnitude of changes in water velocity as
water is interchanged in eddies
3) Reynolds Number
·
provides an approximate measure of flow conditions
Re<500 = laminar flow
Re>2000 = turbulent flow
·
Re = v x r x r/m
where: v = average flow velocity
r = hydraulic radius
r = density of water
m = viscosity
b. friction with bed & banks
·
·
increased roughness causes increased resistance
roughness elements includes measures of grain size, microtopography &
gross channel shape
3. Manning equation
where v = average flow velocity
n = Manning roughness coefficient
r = hydraulic radius
s = channel slope
restricted to uniform flow: constant depth and velocity along some length of channel
of constant cross-section and slope
E. Flow Regimes: The Froude Number
1. F = v / sqrt(g x d)
where: v = average flow velocity
g = acceleration due to gravity
d = average depth
2. Interpretation
F<1 subcritical or tranquil flow
deep & slow flow
F=1 critical flow
F>1 supercritical or rapid flow
shallow and fast flow
3. Importance
·
affects velocity and depth (non-uniform and unsteady flow)
·
affects turbulence
·
affects aquatic habitat
What is stream flow and why is it important?
Stream flow, or discharge, is the volume of water that moves over a designated point
over a fixed period of time. It is often expressed as cubic feet per second (ft3/sec).
The flow of a stream is directly related to the amount of water moving off the
watershed into the stream channel. It is affected by weather, increasing during
rainstorms and decreasing during dry periods. It also changes during different
seasons of the year, decreasing during the summer months when evaporation rates
are high and shoreline vegetation is actively growing and removing water from the
ground. August and September are usually the months of lowest flow for most
streams and rivers in most of the country.
Water withdrawals for irrigation purposes can seriously deplete water flow, as can
industrial water withdrawals. Dams used for electric power generation, particularly
facilities designed to produce power during periods of peak need, often block the
flow of a stream and later release it in a surge.
Flow is a function of water volume and velocity. It is important because of its impact
on water quality and on the living organisms and habitats in the stream. Large,
swiftly flowing rivers can receive pollution discharges and be little affected, whereas
small streams have less capacity to dilute and degrade wastes.
Stream velocity, which increases as the volume of the water in the stream increases,
determines the kinds of organisms that can live in the stream (some need fastflowing areas; others need quiet pools). It also affects the amount of silt and
sediment carried by the stream. Sediment introduced to quiet, slow-flowing streams
will settle quickly to the stream bottom. Fast moving streams will keep sediment
suspended longer in the water column. Lastly, fast-moving streams generally have
higher levels of dissolved oxygen than slow streams because they are better aerated.
This section describes one method for estimating flow in a specific area or reach of a
stream. It is adapted from techniques used by several volunteer monitoring
programs and uses a float (an object such as an orange, ping-pong ball, pine cone,
etc.) to measure stream velocity. Calculating flow involves solving an equation that
examines the relationship among several variables including stream cross-sectional
area, stream length, and water velocity.
Surface Runoff
If the amount of water falling on the ground is greater than the infiltration rate of
the surface, runoff or overland flow will occur. Runoff specifically refers to the
water leaving an area of drainage and flowing across the land surface to points of
lower elevation. It is not the water flowing beneath the surface of the ground. This
type of water flow is called throughflow. Runoff involves the following events:
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Rainfall intensity exceeds the soil's infiltration rate.
A thin water layer forms that begins to move because of the influence
of slope and gravity.
Flowing water accumulates in depressions.
Depressions overflow and form small rills.
Rills merge to form larger streams and rivers.
Streams and rivers then flow into lakes or oceans.
On a global scale, runoff occurs because of the imbalance between evaporation and
precipitation over the Earth's land and ocean surfaces. Oceans make up 71% of the
Earth's surface and the solar radiation received here powers the global evaporation
process. In fact, 86% of the Earth's evaporation occurs over the oceans, while only
14% occurs over land. Of the total amount of water evaporated into the
atmosphere, precipitation returns only 79% to the oceans, and 21% to the land.
Surface runoff sends 7% of the land based precipitation back to the ocean to
balance the processes of evaporation and precipitation.
The distribution of runoff per continent shows some interesting patterns (see Table
8n-1). Areas having the most runoff are those with high rates of precipitation and
low rates of evaporation.
Table 8n-1: Continental runoff values. (Source: Lvovitch, M.L.
1972. World water balance, In: Symposium of World Water
Balance. IASH-UNESCO. Report Number 92).
Continent
Runoff Per Unit Area (mm per
yr.)
Europe
300
Asia
286
Africa
139
North and Central America
265
South America
445
Australia, New Zealand and
New Guinea
218
Antarctica and Greenland
164
Streamflow and Stream Discharge
The term streamflow describes the process of water flowing in the organized
channels of a stream or river. Stream discharge represents the volume of water
passing through a river channel during a certain period of time.
Because of streamflow's potential hazard to humans many streams are gauged by
mechanical recorders. These instruments record the stream's discharge on a
hydrograph. The graph (Figure 8n-1) below illustrates a typical hydrograph and its
measurement of discharge over time.
Figure 8n-1: Stream hydrograph.
From this graph we can observe the following things:
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A small blip caused by rain falling directly into the channel is the first
evidence that stream discharge is changing because of the rainfall.
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A significant time interval occurs between the start of rain and the
beginning of the main rise in discharge on the hydrograph. This lag
occurs because of the time required for the precipitation that falls in
the stream's basin to eventually reach the recording station. Usually,
the larger the basin the greater the the time lag.
The rapid movement of surface runoff into the stream's channels and
subsequent flow causes the discharge to rise quickly.
The falling limb of the hydrograph tends to be less steep that the rise.
This flow represents the water added from distant tributaries and
from throughflow that occurs in surface soils and sediments.
After some time the hydrograph settles at a constant level known as
base flow stage. Most of the base flow comes from groundwater flow
which moves water into the stream channel very slowly.
Not all hydrographs are the same. Actually, the shape and magnitude of the
hydrograph is controlled by two sets of factors:
Permanent Factors - slope of basin, soil structure, vegetation, channel density, etc.
Transient Factors - are those factors associated with precipitation input - size of
storm, intensity, duration of rainfall, etc.
Analog-Graphical Recorders
In general, analog or graphical recorders consist of two main elements: a clock
mechanism actuated by a spring, weight, or electric motor and a gage height
element actuated by a float, cable or tape, and counterweight. Four basic types of
recorders use these elements. Figure 6-3 shows a horizontal drum recorder, in which
the clock positions the pen along the drum axis, and the gage height element rotates
the drum. This recorder is also available with a vertical drum. Another type of
recorder also has a vertical drum, but the time and height elements have been
reversed so that the clock mechanism rotates the drum.
Figure 6-3 -- Horizontal drum water-stage recorder. The
time element records parallel to the axis of the drum.
(courtesy Leupoild and Stevens Instruments, Inc.).
References
Adopt-A-Stream Foundation. Field Guide: Watershed Inventory and Stream
Monitoring Methods, by Tom Murdoch and Martha Cheo. 1996. Everett, WA.
Mitchell, M.K., and W. Stapp. Field Manual for Water Quality Monitoring. 5th
Edition. Thompson Shore Printers.
Missouri Stream Teams. Volunteer Water Quality Monitoring. Missouri
Department of Natural Resources, P.O. Box 176, Jefferson City, MO 65102.
:For more information visit
Volunteer Stream > Assessing Water Quality Monitoring and > OWOW > EPA
Stream Flow 1.5 > Chapter 5 > Monitoring: A Methods Manual
Chapter 6 - Measuring and Recording Water - USBR Water Measurement Manual
Head, Section 5. Recording Gages Stage or
3. Stream Flow