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 and Velocity are related. Indeed, discharge or volume of water per unit time (e.g., cubic feet per second, or cubic meters per second) cannot be determined without first determining velocity. Q = AV where Q is discharge, A is the cross-sectional area of the wetted channel, and V is velocity.
To determine the cross section of the stream one must measure the width of the stream and its depth at various distances from one bank. This can be done by having one person wade into the stream with a stadia rod or a range pole 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 data are converted into a diagram and using simple geometry the cross-sectional area of the stream is determined.[example] Stated another way: Q = AV = A1V1 + A2V2 + A3V3 + A4V4 + A5V5.
Velocity can be determined in one of three 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. To measure velocity with such a devise, 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. A second, and cruder, method involves any floating object and measuring the time it takes to float ten feet (ft/sec) or ten meters (m/sec). 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. The third method involves 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, 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.
Bankfull Discharge is the maximum amount of water a specific
channel can carry before the stream exceeds its banks and floods. Being
able to calculate a stream's bankfull discharge is critical for
floodplain management and planning purposes, especially in urban areas.
One of the beauties of bankfull discharge is that it can be determined
even at times when the channel is completely dry. To calculate bankfull
discharge, simply determine the cross-sectional area of a straight
stretch of the stream channel, and employ the Manning Equation. Barfull discharge is simply that
small amount of water that flows through a channel and does not exceed
the height of the lowest point bar.
Created by William E. Doolittle. Last revised 12 November 2007, wed
![[example]](http://uts.cc.utexas.edu/%7Ewd/courses/373F/gif/hyd01.gif){kind=link}
![[example]](http://uts.cc.utexas.edu/%7Ewd/courses/373F/gif/hyd02.gif){kind=link}