Rainfall analysis

The process of planning a water storage facility requires that several issues be addressed. Typically, this will require the design to account for the specific conditions at the chosen site. Issues to consider include the rainfall statistics for the catchment; the geography, geology and topography of the area where the dam is planned; any impacts that the proposed facility will have on the existing flora and fauna; and finally, should the facility fail – any impacts that this will have on the environment and any communities downstream.

This article focuses on the use of rainfall statistics to design water storage facilities in the Queensland context. Background information is provided on the relevant methods and criteria for assessment. An example of the statistical rainfall analyses is provided for Mt. Larcom, a locality about 30 km west of Gladstone.

Background

The Department of Environment and Heritage Protection’s (EHP) Manual for Assessing Hazard Consequence and Hydraulic Performance of Structures (version 4, EHP, 2013 https://www.ehp.qld.gov.au/land/mining/guidelines.html) “the Manual” sets out the requirements of the administering authority, for consequence category assessment and certification of the design of ‘regulated structures’, constructed as part of environmentally relevant activities (ERAs) under the Environmental Protection Act 1994 (EP Act).

The term regulated structures includes land-based containment structures, levees, bunds and voids, but not a tank or container designed and constructed to an Australian Standard that deals with strength and structural integrity.

The Manual is a comprehensive document, but for the purposes of this article, only the salient points will be discussed. These include the dam category and statistics (and methods of calculating these) for specific rainfall events in the dam’s catchment.

Rainfall events

The Manual defines the wet season as the seven month period between November and May of the following year, inclusive. The three day storm event is almost self-explanatory – it is the period of three consecutive days during the wet season when the highest rainfall was recorded. The critical wet period is a period of a number of consecutive days during the wet season when the highest rainfall was recorded. The length of the critical wet period ranges from 60 to 120 consecutive days, depending on where in Queensland the proposed dam is to be constructed. The Manual contains a map showing the length of the critical wet period for various regions in the state of Queensland.

As far as dam management requirements go, the dam should have sufficient capacity at the start of the wet season (1 November) to accommodate a specific critical wet period. This is called the Design Storage Allowance (DSA). Also, the dam should, at all times, ensure that a three day storm event would not cause a failure. This is known as Extreme Storm Storage (ESS).

Each dam should be designed so that at the start of the wet season there is sufficient capacity to accommodate two types of severe weather (i.e. rain) events. These are a short (72 hour or 3 day) duration storm event and a long period (60 to 120 day) wet season event – designated as the “critical wet period”.

Dams fall into one of three categories and each category has its own set of guidelines for accommodating the rainfall characteristics of the catchment.

Dam categories and failure scenarios

The parts of the Manual that are relevant to this article concern the design of the dam such that it can safely withstand rain events described above. The document also presents a scheme for classifying the type of dam according to the amount of damage to society and the environment downstream that would result should the dam fail in any way.

There are three dam categories. In order of priority, these are High Consequence dams, Significant Consequence dams and Low Consequence dams. The dam category is essentially a classification of the dam in terms of the cost incurred to the community and environment in the event of failure, with High Consequence dams incurring the highest cost.

Below is a brief summary of only some of the criteria pertaining to the High Consequence dams:

• Persons routinely present in failure path and likely to result in more than 10 fatalities
• Contamination of drinking water results in the health of 20 persons being compromised
• Results in adverse environmental effects including chronic and acute effects on living organisms that result in severe symptoms
• Environmental remedial costs exceed $50 million or remediation likely to take more than 3 years or permanent change to ecosystem or area impacted likely to be greater than 5 km²
• Rehabilitation or compensation or repair/rectification costs to third party assets (eg. farms) expected to exceed $10 million

Two dam failure scenarios are described in the Manual. These are the “overtopping scenario” and the “dam break scenario”. The overtopping scenario occurs when water flows over the dam wall. In this situation, either the DSA or ESS requirements (or both) were not met. The dam break scenario does not, as the name suggests, imply that the dam has broken, but that water is flowing over the spillway. The criteria for the various dam categories for overtopping are as follows:

Dam category	Wet season containment (DSA)	Storm event containment (ESS)
High consequence	1 in 100 year critical wet season	1 in 100 year storm
Significant consequence	1 in 20 year critical wet season	1 in 10 year storm
Low consequence	Not specified	Not specified

 

Further information regarding dam categories and associated criteria and rain event guidelines can be found in the Manual.

Method of Analysis

One recommended method described in the Manual is the Method of Deciles to determine the return period when using rainfall data from the Bureau of Meteorology. The return period is the expected interval between occurrence of rainfall events of a given intensity, thus the 1 in 20 year storm has a return period of 20 years. Some points to note if using this method are:

• At least 50 years of useable records are required
• Any particular year must include the wet season and (obviously) the rain gauge be located in the same catchment
• The length of the critical wet period is determined from the map supplied in the Manual
• Statistical analyses are carried out to determine the rainfall event intensity for specific return periods. These include selecting a position plotting formula
• Position plotting formulae are used to assign a probability to a list of values sorted in ascending or descending order. Position plotting formulae are distribution specific – i.e. the position plotting formula for the Normal distribution differs from the Weibull position plotting formula and so on. The Weibull distribution is commonly used for rainfall analyses.
• Since the results are derived from annual data, the position plotting formula yields the annual probability that the associated rainfall value will be exceeded and its reciprocal yields the return period in years

The Bureau of Meteorology Method of Deciles is a simple process for ranking events such as annual rainfall into categories as follows:

Decile	Ranking
10th – Highest 10% of values in the sorted list	Very much above average
9th – Second highest 10% of values in the sorted list	Much above average
8th – Third highest 10% of values in the sorted list	Above average
7th	Average
6th	Average
5th	Average
4th	Average
3rd – Third lowest 10% of values in the sorted list	Below average
2nd – Second lowest 10% of values in the sorted list	Much below average
1st – Lowest 10% of values in the sorted list	Very much below average

 

From the above table we see that four deciles (40%) of the values are classified as “average”.

Worked example

In this example the rainfall analyses for Mt. Larcom, a locality about 30 km west of Gladstone, are carried out. Rainfall data for Mt. Larcom are available on the Bureau of Meteorology web site and consist of daily rainfall records since 1912 – about 104 years of data.

The length of the critical wet period is obtained from the map in the Manual. However, the line on the map separating the 90 and 120 day regions is relatively thick in relation to the overall map and it passes in the vicinity of Mt. Larcom, making the task of selecting the critical wet period length difficult. In this case, analyses would be carried out for both the 90 and 120 day critical wet periods. Critical wet periods for each year are determined by sliding a 90 and 120 day window along the time axis and recording for that year the highest rainfall for that 90 and 120 day period. The result is 104 critical period rainfall values for each of the 90 and 120 day analyses.

The critical wet period rainfalls are arranged in descending order. Binning the data using the position plotting formula yields the deciles. For Mt.Larcom, the results for the 90 day critical wet period are as follows:

Ranking	90 day critical wet period rainfall (mm)
Very much above average	964 – 1257 (inclusive)
Much above average	824 – 956 (inclusive)
Above average	709 – 809 (inclusive)
Average	646 – 697 (inclusive)
Average	584 – 634 (inclusive)
Average	514 – 582 (inclusive)
Average	461 – 510 (inclusive)
Below average	430 – 455 (inclusive)
Much below average	353 – 425 (inclusive)
Very much below average	195 – 340 (inclusive)

So, if the critical wet period rainfall for a year were 900 mm, the critical wet period for that year would be classified as “Much above average”.

Next, a graph of the return period (the reciprocal of the value determined using the position plotting formula) for each rainfall event is constructed. The 90 day critical wet period graph for Mt. Larcom is shown below. From this graph it is a relatively simple process to obtain the required rainfall intensity corresponding to a 20 year return period – in this instance 1145 mm. Thus this dam would be designed to a specification that accommodated a rainfall event of 1145 mm (or greater) in the catchment without failure.

 

Similar analyses would be carried out for the 120 day and three day storm statistics.

Conclusion

This article presents an overview of the rainfall analyses that would be undertaken when designing water storage facilities. Areas that were covered were characteristics of dams and the relationship to the rainfall characteristics of the catchment. The Method of Deciles for rainfall analysis was discussed and illustrated with an example. This process consists of the application of rather basic statistical analyses. Other methods exist, however, for example modelling rainfall. These are beyond the scope of this article and are not presented here, suffice to say that the method chosen must be rigorously validated and justified. Due to space restrictions, some of the more detailed statistical issues are not expanded.

A number of aspects of dam design, for example soil porosity and conductivity, systems consisting of multiple storages, are outside the scope of this article. Interested readers are referred to the Manual for further information.