The use of hydrologic models such as RORB, for regional and urban flood modelling, greatly increases required model complexity beyond that envisioned when these models were developed. This post outlines the key differences between traditional hydrologic modelling and the complex problems that these models are now being used to address. Traditional RORB modelling RORB dates from the 1970s and was developed to provide design flood hydrographs usually at a single location at the lower end of a catchment. An example of catchment delineation for a RORB model is show in Figure 1 (left), where a smal ..read more

I’ve written before about issues with a small number of areal temporal patterns available from the data hub, where there appear to be gross errors.  An example is areal pattern 5644,  Murray-Darling Basin, 36 hour, 500 km2 area, 18 x 2 hour increments (Figure 1). Figure 1: Areal temporal pattern 5644. Murray-Darling Basin, 500 km2, 36 hour Most rainfall (63.6 %), occurs in the 6th increment (between 10 and 12 hours) and 33.9 % occurs in the final increment (between 34 and 36 hours).  There is 24 hours between the two large rainfalls which suggests an error in ..read more

An article on 17 Jan 2022 on the ABC mentioned that 20 stream gauges would be installed in response to flooding from the Darling River. https://www.abc.net.au/news/2023-01-17/nsw-government-installing-water-gauges-menindee-flooding/101860120 I was interested in the cost. There are quotes in the article: “A typical gauge might cost in the vicinity of $40,000 to $60,000 to install”. “For a river flow site, it will cost on average $15,000 a year to operate”. This seems a reasonably small amount of money compared to the cost of flood damage. If information from the new gauges could result in a red ..read more

There is now a “Jurisdictional Specifics” page for Victorian on the ARR data hub which is based on a project undertaken by HARC which I was involved in. Our work looked at whether adopting losses from the ARR Data Hub would result in “good” flood estimates from hydrologic modelling. We defined good estimates as when the design flood peaks from modelling are close to the equivalent peaks from flood frequency analysis of gauged data. In short, the Data Hub losses tend to be too large, which means modelled flood estimates are biassed low. We recommend this is addressed by increasing the preburst ..read more

Paper presented to the Hydrology and Water Resources Symposium 2021 Abstract The recommendation in Australian Rainfall and Runoff (ARR) is that an ensemble of 10 rainfall temporal patterns is used for modelling. Patterns depend on region, catchment area and duration and are provided on the ARR Data Hub.  Most available patterns are consistent and appropriate but there are a few which are physically unrealistic and appear to be erroneous.  A method was developed to identify problem patterns and all available areal and point patterns across Australia were checked.   ..read more

In 1961, Minnesota agreed to ceed land to North Dakota because of a change in course of the Red River of the North as a result of flood control works. Two parcels of land had become detached from Minnesota and attached to North Dakota because of a project undertaken by the US Army Corps of Engineers. The State legislatures agreed that State boundaries should be redraw and sought and were granted approval from the US House and Senate. The Minnesota-North Dakota Boundary Line Compact was approved by the US House and Senate on August 25, 1961 as Public Law 87-162. Details are available here. Both ..read more

The usual assumption in regression is that that all the error is in the y values; the dependent variable; the x values (the independent variable) are known without error. For a recent project, I had to undertake some analysis where this assumption wasn’t reasonable. There were errors in both variables. In this case, we were looking at the relationship between turbidity and clarity. Turbidity was measured by a meter; clarity was measured as the minimum depth of water that just obscured a target. As example of the issues, let’s consider a simple case; 10 made up values. Figure 1: 10 values with ..read more

Preburst rainfall is the rain that falls before a ‘burst’.  Burst rainfall is what you get from IFD information on the Bureau of Meteorology website.  In hydrologic modelling, it is often important to add preburst to burst rainfall to create a ‘design’ storm. You can look up preburst rainfall on the ARR data hub.   An example is shown below which is for the data hub default location in central Sydney (Longitude = 151.206E, Latitude = -33.87N). These are median preburst values. Data is also available for the 10th, 25th, 75th, and 90th percentiles. Each cell p ..read more

A sampling distribution is the probability distribution of a statistic calculated from a sample. In this case, the statistic I’m interested in is the number of floods that may occur in a sample of a certain number of years. In engineering hydrology, and land use planning, we are often interested in the 1% (1 in 100) AEP flood. If the period of interest is 100 years, say the design life of a house, how many 1% floods should I expect? We can calculate this using the binomial probability distribution. The general formula is: Where, n is the number of years (the sample size), p is the probability ..read more

As discussed in the previous post, the areal reduction factors, developed for Australia must be interpolated for durations between 12 and 24 hours. The long-duration equations apply for 24 hours or more. The short duration equations apply for 12 hours or less. In between, the recommendation is to interpolate, linearly (Figure 1) (Podger et al., 2015). This post looks at an alternative to linear interpolation. Figure 1: Areal reduction factor as a function of duration. ARFs are linearly interpolated between 12 and 24 hours One approach would be to interpolate in such a way that that there is a ..read more