The principal agent of erosion, and hence geomorphology, is water. It is, accordingly, only fitting that fieldworkers have at least a rudimentary knowledge of surface hydrology. Water has many properties that geographers, as well as practicioners from allied fields, find intriguing and problematic. For example, in today's growing and developing world, environmentalists are concerned with such things as the chemical composition of water, and the concentrations of pollutents in streams. From a physical (rather than a chemical) perspective, it is often important to be concerned with things such as suspended sediment load, and channel bed materials, as well as with the total amount of water and the velocity of its flow.
A number of easy to use, and relatively inexpensive test kits are available on the market today. These kits can be used to determine the presence and the amount of several things, including dissolved oxygen, hardness (concentrations of calcium and magnesium), chlorides, phosphates, iron, nitrate nitrogen, ammonia nitrogen, salinity, turbitity, and pH. They typically include (a) vials for extracting samples, (b) small trays or dishes for carrying-out the tests, (c) small bottles of test reagents in the form of liguids and powders, (d) some basic but necessary equipment such as droppers and color charts, and (e) an instruction pamphlet. Also there are electronic meters for measuring pH, dissolved oxygen, turbity, and conductivity. We will not be conducting chemical tests on water, because of the expense and disposable nature of test kits, the short life of reagents, and complexity testing procedures. Furthermore, water chemical tests are used only for some very specific circumstanaces, and, almost paradoxically, extracting the water samples involves only the dipping of a sample vial into the water and extracting it.
Suspended Sediment Load or the amount of sediment being carried by a stream can be measured with rod-mounted sampler, of which there are two types. One looks very much like a old-fashioned milk bottle mounted sideways on a long handle. The other looks like a minature submarine with a hole in the top and a very long periscope. To use either devise, fieldworkers wade into the stream and dip the collection bottle into the water. The bottle quickly fills with both water and sediment. However, it is not extracted soon thereafter. Instead, the bottle is slowly lowered to the channel bottom and slowly returned to the surface. Without taking it out of the water, the collection bottle is moved from one part of the channel to another. The idea of moving the sampler around is not unlike the taking of pinch samples for soil analyses--a little bit of water and sediment is collected from several places in the channel. Once the sample is collected, it is transferred to a separate, sealable container and carried back to the laboratory for analysis. In the lab, the total weight of the sample is determined using a balance, and then the water is evaporated in an oven. The unevaporated residue, the sediment, is then weighed and its proportion of the total sample calculated.
Discharge, or how much water is actually flowing or could flow through a channel, is an important consideration for all types of environmental analyses. Two examples should suffice. First, knowing a stream's maximum possible discharge is essential for floodplain management and planning purposes, especially in urban areas [example]. This is known as bankfull discharge and can be determined even when the channel is completely dry. A related measurement, especially in arid lands where stream flow can vary between flooding and hardly any water at all, is that of barfull discharge, that small amount of water that flows through a channel and does not exceed the height of the lowest point bar. Second, in many cases it is of fundamental importance to determine how much water is actually available if it is to be divided amongst a number of possible users.
To calculate discharge, the volume of flow per unit time (which is typically measured in cubic feet or cubic meters per second), one needs to determine the cross-sectional area of a straight stretch of the stream channel, and velocity or speed of flow. The hydraulic equation is:
Q = AV
where Q is discharge, A is the cross-sectional area of the channel, and V is velocity.
Determining the cross-sectional area of a channel, especially a relatively wide and deep channel, might seem difficult at first glance [example]. In reality, however, it is rather straightforward and involves dividing the area into discrete units based on natural breaks in the slopes of the channel banks [example], and applying simple geometry. This involves measuring the width of the channel and its depth at various distances from one bank [example]. Horizontal measurements, of course, can be made with a measuring tape, while vertical measurements can be made with a stadia rod and level. Adding together the areas of each section results in the total cross-sectional area of the channel [example].
Estimating the velocity of a stream flowing at near-flood stage might seem impossible when the channel is totally dry. However, several decades ago a prominent hydrologist derived a technique based on empirical tests. The technique which carries his name is known as the Manning Equation:
V = 1.00 / n x R to the two-thirds power x S to the one-half power
where V is velocity, R is the hydraulic radius (A/wetted
perimeter [the distance from the water line at one bank, down the side
of the channel, across the stream bottom, and up the other bank to the
water line]), S is the slope or gradient of the channel as measured upstream-downstream, and n is a
channel-bottom roughness coefficient. This last value can vary from as
low as 0.011 for plane alluvial channels with no vegetation, to 0.070
for channels that are weedy, winding, and have deep pools. Clean
winding channels with a few pools and ripples, and mountain streams
with beds of gravel or cobbles have roughness values of 0.040. For more
details about roughness coefficients, visit this website. Note,
if English measurements (feet and inches) are used in place of metric
measurements, the numerator and constant 1.00 should be replaced with
1.486.
Now, everyone today knows that everything can be found on-line somewhere. Well, that somewhere for the Manning Equation is here. Simply plug your data into this interactive website, and voila, you have Q.
The discussion thus far has dealt with determining bankfull discharge. Determining barfull discharge is no different, except for scale. Measurements are made for a cross-sectional area where water flows at its minimum [example]. In many cases, however, knowing the maximum or the minium discharge is not as important as knowing the current discharge. This is the amount of water that is actually flowing through a stream channel at the present time. Calculating the current discharge can be done by having one person wade into the stream with a stadia rod while holding the tail of a measuring tape. A second person holds the head of the tape at the water's edge. The person in the stream measures depth at various "breaks" in the channel bottom and conveys this information to the person on dry land who records it and the distance from the bank in a field notebook. The process is repeated at various places until the person in the water reaches the other bank. Once the process is completed, the cross-sectional area of the stream can bedetermined using simple geometry as illustrated above.
Velocity can be determined in one of two ways. Streamflow meters look amazingly like sticks with a propeller at one end and a meter at the other because that is exactly what they are [example]. To measure velocity with such a device, one fieldworker wades into the stream and inserts the propeller to a depth of 15 centimeters below the surface, at the approximate center of each segment of the cross-sectional diagram [example]. It is held there for 60 seconds before a reading from the meter is taken and recorded. Applying simple math: Q = A1V1 + A2V2 + A3V3 + A4V4 + A5V5. A second, and cruder, method of measuring total stream velocity involves the use of floating object such as a cork or even a piece of bark, and measuring the time it takes to float ten feet (ft/sec) or ten meters (m/sec) [example]. However, given that water flows faster at the surface than along the channel bottom, the rate should be multiplied by 0.85 to get an average velocity.
Created by William E. Doolittle. Revised 19 February 2018