Assignment: Case study
This assignment is to be completed and submitted individually but collaborative work is encouraged.
You should work on the assignment progressively through the module. There will be discussion
around each topic at the relevant workshop.
There are three sources of information for this case study (all available on Canvas)
1. Case study briefing paper
2. Case study photos
3. Case study data
4. Case study rubric
Before you commence the assignment, you should read the briefing paper and peruse the photo
This assignment is to be submitted through a submission link on Canvas (Submission deadline: 11:59
pm, Sunday 6th September 2020). Overall, the assignment should be equivalent to 2,000 words
(including calculations where appropriate) and it should take no longer than approximately 20 hours.
This assignment consists of three parts. Answer questions chronologically and succinctly, showing
simple calculations in text and extended calculations in an appendix. Although each section is marked
independently, later questions require reflection on your responses to earlier ones.
Assignment Task for Topic 1. Quantify the impact of flow regulation on flow depths and velocities
in the Old Thomson River and Rainbow Creek.
Flow regulation has reduced flows in the Thomson River. Further, flow in the Thomson River is split
between the Old Thomson River and Rainbow Creek. The split is controlled by a weir across
Rainbow Creek (the Cowwarr Weir). The effect of flow regulation on the flow regime has been
analysed for the Old Thomson River and Rainbow Creek, as illustrated by the difference between
“current” and “natural” flow duration curves in the case study data. Median flows are also
Typical cross sections for Rainbow Creek and the Old Thomson River are shown in the case study
data in graphical and tabular form.
1) Assume uniform flow and calculate the impact of flow regulation on water levels and velocities
for the median flow at the typical sections in the Old Thomson River and Rainbow Creek.
Present your results in a tabular format including a brief comment on the impact of flow
regulation. Show all working. Clearly state assumptions and sources of information. You may
choose to make appropriate simplifications to the cross section to make your calculations
Assignment Task for Topic 2. Calculate how to set the Cowwarr Weir to allow Environmental Flow
in the Old Thomson River
The Cowwarr Weir (shown at left) controls
the split of flow between the Old
Thomson River and Rainbow Creek. It is a
concrete gravity structure with three steel
vertical lift gates each 2 m wide. The bed
level below the gates is 93.0 m AHD.
Cowwarr Weir Layout. Arrows indicate the direction of flow.
The flow down the Old Thomson River depends on the level of the weir pool, as indicated by the
stage discharge curve in the case study data.
It has been determined that for environmental flow purposes, when the flow in the Thomson River
upstream of Cowwarr is 5.0 m3
s‐1, 2.0 m3
s‐1 must flow down the Old Thomson River as an
a) If all gates are to be open in the Cowwarr Weir, and the height of opening gate is 25cm
(above the bed), it has a free outflow, which creates a hydraulic jump downstream. If the
depth immediately before the hydraulic jump is observed to be 18 cm, estimate:
i) the distance downstream of the gate where the jump occurs.
ii) the flow velocity after the jump.
b) Flow measurements subsequently indicate that with the gates set as determined in (a), the
flow in Rainbow Creek is actually 10% less than calculated above. Why might this be the
c) With this reduced flow, what will happen:
i) To water levels in the weir pool?
ii) To flow in the Old Thomson River?
Assignment Task for Topic 3. Interpret water surface profiles from HEC‐RAS
For the flow conditions specified above, the software HEC‐RAS has been used to calculate the water
surface profile through the Old Thomson River and Rainbow Creek.
The results are shown in the case study data. Note that these are only a selection of the output
formats available from HEC‐RAS. Examine the results in graphical and tabular form and become
familiar with their meaning.
The “typical sections” that you analysed in Question 1 above were:
Cross section number 5 for Old Thompson at river station 202.89
Cross section number 6 for Rainbow Creek at river station 245.12
a) For these sections:
i) Recalculate the normal depth for the Old Thompson River “typical section” for a flow of
ii) Recalculate the normal depth for the Rainbow Creek “typical section” for a flow of
iii) Compare these values for normal depth with the water levels predicted by HEC‐RAS for
these same flows at these sections. Explain any differences.
b) For each section, determine whether the flow at normal depth is subcritical or supercritical.
c) We know that in subcritical flow, water levels are calculated from downstream to upstream.
Looking at the HEC‐RAS results for Old Thomson,
i) How do you think the water level was set at the downstream end in HEC‐RAS?
ii) Under what conditions would this not be a suitable method?
iii) What alternatives are available?
d) Looking at the HEC‐RAS results for Rainbow Creek, where is the flow closest to critical?
Explain why the flow is closest to critical at this location.
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