Choosing a cross section of the stream to study:  Why would it be important to select a site that has a measurable flow rate (flow rates > 0.05 m/s)?  Why should you avoid areas that have rocks above the surface of the water?  Why should you avoid areas that have a rocky bed?  Why might it be better to select a cross section that has a uniform depth rather than a site that may have a large drop off? 

PART I.  WATER DISCHARGE 

Discharge (Q) = width (w) x depth (d) x velocity (v); or: 

                            Q= w x d x v 

To use this equation to find the amount of discharge (m³/s), a single cross-section across the channel is chosen.  The area of this section can be found by multiplying the width (m) by the average depth (m) and velocity (m/s), respectively, in that cross-section.  Several depth and velocity estimates must be taken in order to determine these averages. 

Measure the width and depth of your cross-section, using the tape measure and a pole (any old broom handle will do or a meter stick).  Use a flow meter to determine the average velocity in your cross-section or calculate the flow by using a small floating object and a stop watch (m/s).  Finally, calculate the discharge from these data in the space below.  Note that discharge is measure in m³/sec. 

Water discharge _________________________ 

Part II.  PARTICULATE AND DISSOLVED LOADS 

While at the creek sites, collect at least a 500 mL water sample, taking care to avoid material from the banks or bed.  (Where is the best place to collect a sample? In the center of the stream about 2/10th below the surface of the water)  Back in the lab, obtain a filter and weigh it in the analytical balance; record the weight below.  Filter your water sample through the weighed filter paper (it will take as much as ten minutes to filter all 500 ml).  Dry the filter paper in the drying oven (60^oC), and re-weigh the paper.  The difference between these two weights is the mass of suspended sediment in your 500 ml sample.  Convert this figure to grams per liter. 

Mass of filter paper (g) ______________________ 

Mass of filter paper + sediment (g)______________ 

Mass of suspended sediment (g)________________ 

Suspended sediment concentration (g/L)____________ 

The suspended load is only part of the load a stream carries.  There is dissolved (chemical) load and bed load (which moves along the stream bottom) which we must consider as well.  In the field, use the conductivity meter to measure the electrical conductivity of the water.  Conductivity increases with increased dissolved load, and the conductivity meter gives the result both in electrical conductivity units and in units dissolved solids (in milligrams per liter). 

Total dissolved solids carried in milligrams/liter? _________________ 

Convert this to grams/liter__________________________________ 

The concentration of dissolved substances is commonly estimated by assuming that it is equal to 30% of the suspended sediment concentration.  How accurate an estimate would this have been for the creeks? 

We are now ready to calculate the dissolved and suspended loads form the dissolved and suspended concentrations calculated above.  To do so, we simply multiply the concentrations by the water discharge calculated earlier.  Watch your units! 

Suspended load_____________________ 

Dissolved load______________________ 

The bed load is much more difficult to measure.  We will follow common practice and estimate it by assuming that the bed load is 10% of the suspended load. 

Bed load________________________________ 

Total particulate load (bed load plus suspended load)_________________ 

Total particulate plus dissolved load______________________________ 

PART III.  EROSION RATE 

We are now in a position to use the total particulate plus dissolved load measured today to calculate an "erosion rate".  Note that the values you calculated above represent a snapshot in time; the figure we'd really like to have is the average sediment and dissolved loads over the course of the year.  Nevertheless, we can calculate an apparent erosion rate from our data if we assume that they do represent average values; and if we assume that sediment moves steadily (that is, that sediment doesn't accumulate in storage temporarily on it's way downstream). 

Assuming that today's particulate and dissolved loads represents their annual averages, we will then "spread" this sediment evenly over the area of the drainage basin.  We usually express the result in mm lowering per year. 

Here are some data that you will need to know: 

1. Average annual particulate plus dissolved load:  We'll use your values for loads calculated above, which are in units of grams per second.  You will need to convert to the annual load by multiplying by 3.1536 x 10⁷; the approximate number of seconds in a year.  Results are in grams per year. 
2. Area of drainage basin.  Determine the area of the drainage basin using the topographic map.  Be careful to outline the area on the topographic map that will drain into the sample site (Keep in mind the direction of flow of the stream and the contour lines on the map).  Note that for the erosion rate calculation this value must be convert of mm². 
3. Density of material eroded. This information has been estimated for the soils in the Princeton area. 

The calculations: 
If particulate plus dissolved load = mass removed from landscape (annual), write: 

Annual particulate + dissolved load = (Mass removed/yr) 
                                                      = (Volume removed/yr) x (density of material) 
                                                      = (Erosion rate) x (Area) x (density of material) 
           or  
 
     S = EAp 

S is in units of g/yr; so for E to be in unit of mm/yr; A must be in units of mm² and p in units of g/mm³. 

Given: 

p (for soil) = 1.2-2.4 g/cm³ 
         Let's use 1.8 g/cm³ 
         convert to g/mm³ by dividing by 1000 
         = 0.0018 g/mm³ 

From your data calculate an erosion rate given the data above. 

Erosion rate (mm/yr)___________________________ 

Did you see any evidence that might lead you to believe that even this figure will not accurately predict the actual erosion rate?  Do you expect the actual erosion rate to be higher or lower than the figure you calculated? Why?