This assessment brief contains a lot of information. It is important that you read through the assessment brief carefully before you attend the field work.
Learning File
Sub Task 3 Geomorphological assessment and cross-section measurements of depth and velocity in the River Sheaf and Burbage Brook (35%)
Preparation for this assessment
• Make sure you have accessed the assessment subtask 3 folder in the assessment section of the module site
• Make sure you have suitable clothing for the river work
• Make sure you have completed a medical consent form and given it to Beth
• Read the River Surveys guidance in your module handbook (p28-29)
• Read the risk assessments for the river work in your module handbook (p30-35)
• Read pages 1–12 of the River Surveys Handbook
There are two activities that you will be undertaking for this assessment:
1. Geomorphological Assessment
You are to carry out a geomorphological assessment of a stretch of the River Sheaf and a stretch of Burbage Brook, these should include:
• Completed sheets A, B, C and D
• A fully annotated photographic record for the section being surveyed this should be taken on the survey day
• Any maps and aerial photographs for the section being surveyed
2. Cross-section Measurements
In addition to the geomorphological assessment you will also carry out cross-section measurements of the river Sheaf and Burbage Brook these should include:
• The measurement of two different cross-sections where width, velocity and a suitable number of vertical depth measurements have been taken. The locations of these cross-sections will be shown to you on site.
Make sure you bring with you a camera and a calculator.
You are expected to work as a team under your own initiative. This means that you need to allocate tasks within the group. When recording your data remember that you will need to share the results with other members of your group and with other groups so they need to be presented in a neat and legible format that can be easily interpreted.
Method for cross-section measurements
• On the sheet provided make a note of the date, time, location, weather conditions and group members before you start the survey.
• Indicate the cross-section being measured on a location map.
• Make sure you have also photographed the locations being surveyed.
• Measure the width of the river channel using a 30m survey tape.
• Stretch the survey tape horizontally across the channel and at right angles to the channel. Secure it in place using the pegs provided.
• Divide the cross-section into a suitable number of slices (see figure 1.0 below).
• For each slice measure and record the width of the slice and the depth from the channel bed to the water surface using the metre rule.
• Having determined the depth of the slice measure the velocity of the slice using the current meter and the ultrasonic velocity probe and record them on the sheet provided (make sure you have recorded which meter you are using and in the case of the current meter which propeller).
o Note: The velocity should be measured at 0.6d from the water surface (where d is the depth of the water)
Calculations for cross-section measurements
You need to calculate:
• Mean depth, d (m) of each slice
• Mean velocity, v (m/s) of each slice
o You will have two different values of velocity for each slice since you have used two different meters to record this data. The ultrasonic meter provides a direct reading of velocity in m/s. To calculate the velocity for the current (propeller) meter you need to calculate n which is the number of revolutions of the propeller each second. n (revs/sec) is found by dividing your reading by 60 you then use one of the two equations provided on the flow velocity table below to calculate the velocity. Which equation you use will depend on the value of n (revs/sec)
NB: make sure that you use the right flow velocity table – each propeller has its own flow velocity table and calibration equations
• Cross-sectional Area, A (m2) of each slice
A = b d
where A = Area (m2)
b = Width of slice (m)
d = Mean depth of slice (m)
• Discharge, Q (m3/s) of each slice
Q = v A
where Q = Discharge of slice (m3/s)
A = Area of slice (m2)
v = Mean velocity of slice (m/s)
• Total Discharge (Qtotal, m3/s) of the cross-section
Qtotal = the sum of Q from each slice in your cross-section – in other words, add up the discharge values for each slice to obtain the total discharge for the cross-section
NB: you will have two total discharge values for each cross-section since you have used two different velocity meters.
Cross-section plots
• Plot each surveyed cross-section by hand on 1mm graph paper (depth against width).
(Note: You will need to use an exaggerated vertical scale for the depth measurements)
Figure 1.0 How to divide up the cross-section into slices
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