1. Overview
This assessment is designed to test your achievement of selected learning objectives of Modules 2 to 5. In particular, the assignment involves flood frequency analyses and working with historical streamflow and rainfall data.
The assignment is intended to reinforce and extend your knowledge on hydrological methods and the (substantial) uncertainty associated with design flood estimation.
The purpose of the assignment is to estimate design flood discharges for the Emu Creek at Emu Vale streamgauge site (Queensland gauge 422313B) for a range of average recurrence intervals (2, 5, 10, 20, 50 and 100 year ARIs). The flood frequency analysis (FFA) will be based on consideration of the outputs of three techniques: Annual Series, Partial Series and the new Regional Flood Frequency Estimation (RFFE) developed as part of the AR&R update (Rahman et al, 2015).
The assignment is based on the following main tasks:
- Obtaining and reviewing the streamflow data
- Annual Series FFA based on fitting a log Pearson 3 (LP3) distribution to annual discharge maxima
- Partial Series FFA based on monthly discharge maxima
- Design discharge estimation using the RFFE method. A catchment plan will be also be prepared.
- Compilation of the estimates and selection of a set of design flood discharges, based on consideration of the above outputs
- Assessment of the rainfall frequency associated with the January 2011 flood event
- Preparation of an assignment report
2. Assignment Tasks
Streamflow Data
BASIC STREAMGAUGE DATA
Streamflows and relevant gauging information for station 422313B can be downloaded from the Queensland DNRM Water Monitoring Portal https://water-monitoring.information.qld.gov.au/
Access the Portal and find the streamgauge data. Tabulate the following basic gauge information: location (latitude and longitude), the catchment area, when streamgauging started, when rainfall measurements started, the maximum observed instantaneous discharge and when this occurred, and the maximum observed daily total rainfall and when this occurred.
The hydraulic control and associated rating curve are also of interest. Include in your report a photograph of the control weir and an image of the creek cross section at the streamgauge.
Provide an image of the rating curve and the flood gaugings that have been undertaken at the site.
How many gaugings have been done to the end of 2013? What is the highest gauging (level and discharge) and how does this compare with the highest recorded flood? Make a comment of the adequacy of the rating curve based on this information, especially the reliability of the discharges associated with the largest floods.
MISSING DATA
Download the monthly discharge data from the start of record to the end of the 2014/2015 water year (October-September). Generate a timeseries plot of monthly peak discharges. What percentage of the data is missing (code 255)?
Annual Series FFA
The Annual Series FFA is to be first undertaken on the following basis:
- Include data for all water years from the start of record to the end of 2014/2015.
- Exclude from the analysis the water years that have 4 or more months of missing flow data. AR&R recommends several approaches to handle missing data, but this simple exclusion method will be used. This may require an adjustment to the number of years of available data N.
- Fit a LP3 distribution to the Annual Series, estimate 5% and 95% confidence limits and prepare a flood frequency chart. Extract the design discharge estimates for the specified ARIs.
Once you complete the above analysis, you will find that the Annual Series has a negative high skew. This is due to the presence of a few very small discharges. Including these floods may result in a poor LP3 fit to the much higher major floods, and thus influence the design discharge estimates in this range (> 10 year ARI). AR&R 1987 provides recommendations on removing low outliers or the possible deletion of the lower portion of the Annual Series to improve the LP3 fit. The following approach is suggested:
- Exclude the lowest 9 floods from the Annual Series and repeat the LP3 analysis. You will need to reduce N and also make adjustments to the probability estimates. ARR&R 1987 provides technical guidance on low flows and an extract from this document is included on Studydesk as a resource (File ExtractAR&R1987.pdf)
Partial Series FFA
The Partial Series FFA is to be undertaken on the following basis:
- Use the monthly discharge maxima to extract the Partial Series. Monthly discharge maxima are acceptable for small catchments as typically the peaks between months are sufficiently separated by time to be hydrologically independent.
- Use the monthly discharge data from the start of record to the end of the 2014/2015 water year.
- For months with missing data, assume that no floods occurred during these periods. This assumption differs to that made in the Annual Series, so may give a different N to use in the analysis. The value of N can be non-integer.
- Current Australian Rainfall and Runoff (AR&R 1987) recommendations suggest the Partial Series threshold should be selected such that K (the number of independent flood peaks above the threshold) is between N and 3N. (N is the number of years of streamflow data). For consistency, use K equal to N in defining the Partial Series threshold.
- Plot the Partial Series and, from fitting a suitable regression line, provide design discharge estimates for the specified ARIs. It is appropriate to exclude large floods from the regression analysis.
- Prepare a Partial Series flood frequency chart
RFFE design discharge estimates
The Regional Flood Frequency Estimation (RFFE) approach to estimate design peak discharges up to 1% AEP for Australian small to medium sized rural catchments has been developed as part of the AR&R update. The RFFE method supersedes the probabilistic Rational Method recommended in AR&R 1987. A draft online tool has been developed to generate RFFE estimates that depend on catchment location and this website can be found at https://rffe.arr.org.au/.
Details of the RFFE can be found in Rahman et al (2015). Generate a set of design discharge estimates for the Emu Creek catchment using the RFFE method. Also provide a map showing the catchment boundary and the location of the catchment centroid. A base map, obtained from the QTopo online mapping system, can be downloaded from StudyDesk (File QTopo_A3)
Assessment of January 2011 flood rainfalls
The largest recorded floods have an impact on the fitting of a LP3 probability distribution to the Annual Series, more so than the frequent minor floods. It is thus worthwhile investigating the top ranked floods in some detail.
BOM have prepared detailed reports on notable floods in Australia and Queensland reports are available from https://www.bom.gov.au/qld/flood/fld_reports/reports.shtml. There are no flood reports for the relatively small Emu Creek. However, the February 2009 flood of the Burdekin River provides a typical example of the contents of a flood event report (this report is referred to in the Study Book Module 2).
Download the Burdekin River February 2009 flood report and familiarise yourself with how the hydrological data is analysed and presented. In this part of the assignment, you will be undertaking a frequency analysis of the observed rainfall intensities (but not to the level of detail covered in this example report).
Available pluviograph data is limited for the Emu Creek catchment. Rainfall measurements are recorded at the streamgauge and the more recent January 2011 flood was selected for analysis. This is the second ranked flood on record. The objective of the analysis is to determine the frequency of the observed rainfall that fell during the January 2011 flood event.
The rainfall analysis involves the following tasks:
- Access the Water Monitoring Portal, find and plot the recorded flood hydrograph for the January 2011 flood. Include in the plot at least 2 days of flow record after the flood peak and at least 3 days of record prior to the peak. Generate a plot of rainfall data for the same period of time. These plots can be generated online using the Water Monitoring Portal.
- From the plots, you will be able to identify the period of rainfall that was associated with the flood. Download 15 minute rainfalls for this period of time.
- Access the BOM IFD site and download the 2013 AR&R IFD data for the gauge location. It is appropriate to use this data as it represents the latest estimates of rainfall frequency.
- Estimate the time of concentration of the Emu Creek catchment (refer to Section 5.3.2 of the Study book).
- Estimate the approximate AEP of the observed rainfall depth for the duration corresponding to the time of concentration (round off to the nearest hour). Estimate the corresponding AEP of the 2011 flood discharge (use the Annual Series plotting position estimate).
The probabilistic Rational Method assumes that a flood discharge of a certain frequency is caused by design rainfall of the same frequency (corresponding to the catchment time of concentration). The above analysis demonstrates how this may not be the case for ‘real' floods.
Compile and select design discharge estimates
Compile the design discharge estimates for the Annual Series (both scenarios), the Partial Series and RFFE analyses as a table. Plot the discharge estimates on the same graph against ARI. It is recommended that an undistorted linear scale (both horizontal and vertical) be used so that the differences in the design discharge estimates at moderate-to-large floods (>10 year ARI) can be clearly seen.
Select design discharge estimates for each of the required ARI. It may not be appropriate to choose design discharges from the same method across the full range of ARIs. Provide statements that justify the choice of design discharge for each ARI.
Prepare assignment report
For reasons of consistency, a report template (Report Template.docx) must be downloaded from Study Desk and used to report your work. Complete each section of the report.
https://www.dropbox.com/s/pewqshz7oylv26w/Assignment.rar?dl=0
Attachment:- ReportTemplate.docx