Exercise 4

Stream Flow/Runoff

Objectives:

            1)         To learn how to calculate the discharge of a stream

            2)         To make comparisons between storm hydrographs of two different drainage basin environments.

 

1)         Solve for the missing values:

Horizontal         Depth (d)        Average          Width (W)        Area (A)          Mean (V)     Discharge

  distance          at interval        depth (D)        of section         (W * D)           Velocity       Q=A * V

     (ft)                   (ft)                (ft)                      (ft)                (ft²)                  (ft/sec)         (ft³/sec)

     10                     0                -edge of water

      15                    5                ________               5              __________            3.0       ________

      20                    3                ________               5              __________            4.5       ________

      25                    15              ________               5              __________            5.0       ________

     30                     17              ________               5              __________            6.2       ________

      35                    18              ________               5              __________            7.0       ________

     40                     12              ________               5              __________            3.75     ________

      45                    9                ________               5              __________            3.85     ________

     50                     3                ________               5              __________            2.75     ________

      55                    1                ________               5              __________            1.66     ________

      60                    0                ________               5              __________            0.96     ________

 

2)         a)         What is the total area for all the sections?

 

 

            b)         What is the average velocity?

 

 

            c)         What is the total discharge?  Figure this two ways:  First using the values calculated in a and b, and then by adding the discharge numbers from the table.

 

 

 

 

3)         Draw the stream bed profile (cross section) by plotting depth at interval vs. horizontal distance.  Also draw a circle in the area of greatest velocity.

4)         What is the general relationship between stream depth and velocity?   Explain this relationship and why it occurs.

 

 

A hydrograph is a graph of the change in stream discharge over time.  The effect that precipitation has on a stream's discharge can be seen on a hydrograph.  The following hydrographs are for streams A and B.  The streams have equal size drainage basins which received one inch of rainfall at time zero.  The hydrographs follow stream discharge for 3.5 days after the rainfall.  Notice that both streams have the same initial flow, and stream B has a higher peak flow and shorter lag time.

5)         a)         Which hydrograph might you expect in

                        a vegetated environment            ____________

                        a desert environment                 ____________

            b)         What effects do plants have on runoff?

 

            c)         Disregarding all other factors, where would the most erosion occur (A or B)? Why?

 

            d)         Which hydrograph would you expect for a stream flowing through an urban area?  (A ro B) Why?

 

            e)         What could be done to improve the conditions in urban areas so that less erosion occurs?

 

            f)          If stream A's drainage basin were to experience a fire that burns most of its area,  would you expect its hydrograph to change?  Why and how?

 

 

            g)         Using this knowledge, what do you suppose is happening in the rain forests of the  world?

 

2 points. Due thursday, 5 August