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: ï‚· Obtaining and reviewing the streamflow data ï‚· Annual Series FFA based on fitting a log Pearson 3 (LP3) distribution to annual discharge maxima ï‚· Partial Series FFA based on monthly discharge maxima ï‚· Design discharge estimation using the RFFE method. A catchment plan will be also be prepared. ï‚· Compilation of the estimates and selection of a set of design flood discharges, based on consideration of the above outputs ï‚· Assessment of the rainfall frequency associated with the January 2011 flood event ï‚· Preparation of an assignment report 2. Assignment Tasks 2.1. 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.
ENV3105 Hydrology 2016
2 | P a g e
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)?
2.2. Annual Series FFA The Annual Series FFA is to be first undertaken on the following basis: ï‚· Include data for all water years from the start of record to the end of 2014/2015. ï‚· 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. ï‚· 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: ï‚· 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)
2.3 Partial Series FFA The Partial Series FFA is to be undertaken on the following basis: ï‚· 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. ï‚· Use the monthly discharge data from the start of record to the end of the 2014/2015 water year. ï‚· 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. ï‚· 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. ï‚· 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. ï‚· Prepare a Partial Series flood frequency chart