Design rainfall information is used as a risk assessment tool in water management


Design rainfall calculated using precipitation observations is generally used as a tool in the planning of long-term structures and functions. Design rainfall can be used to calculate the volume of water to be handled by, for example, a storm water runoff system or the flow rate that a dam must be able to withstand. Increasing precipitation caused by climate change can be prepared for by dimensioning correctly.

Climate change will intensify heavy precipitation events and increase flooding

Precipitation is estimated to increase in Finland due to climate change: by approximately 7–8 per cent by the year 2040 and 12–20 per cent by the end of the century, as compared to the mean precipitation for 1971–2000. Precipitation will increase and intensify throughout the year, but in relative terms, it will increase the most in winter. [1] Heavy precipitation events are also estimated to intensify [2], and most of them will occur in summer and early autumn [3]. In summer, the heaviest precipitation events may increase by 10–25 per cent [4], [5]. However, there is significant uncertainty in estimating changes in heavy precipitation events which can cause saturation of the soil, strong flow rates (discharges) and flooding.

Floods are estimated to increase in Finland as a consequence of climate change [6], [7]. They are caused not only by heavy precipitation events, but also snowmelt, slush forming or ice damming in rivers, a rise in sea level, or combinations of these. Floods can compromise the load-bearing capacity of soil, cause damage or problems for agricultural lands, buildings, roads and other structures, and pose a health threat. For example, if drains and flood routes cannot channel storm water runoff quickly enough, more intensive summer heavy precipitation events will increase storm water floods (flash floods, urban floods) [6], [7].

Information on risks is needed in dimensioning water management systems

Future flood and climate risks should be taken into consideration when planning storm water runoff and flood management systems, such as rainwater drainpipes and dams, because they are built and dimensioned to withstand decades of use. Thus, the impacts of climate change should be prepared for (adapted to) in advance.

In a planning process, risks are identified, analysed and their extent is estimated by evaluating the probability and impact of each incident [8]. Then decision-makers decide on how great or small of a risk can be taken and what the costs of risk management and potential damages are.

Design rainfall is used to prepare for a certain risk level

Design rainfall is used, for example, as a tool in planning the dimensions for storm water runoff channel systems and dam structures. Design rainfall indicates the probability of precipitation exceeding certain amount and is used to prepare for a risk of a certain level. Precipitation that exceeds the design rainfall will cause flooding in the system being dimensioned. [3] In land use plans, specific attention must be given to where and how flood water will be channelled.

Design rainfall is determined based on the catchment area, the duration, probability (return period) and intensity of the rainfall as well as the amount of precipitation (table 1) [3]. Return period does not literally describe the interval between recurrence of a certain phenomenon, but rather its probability. If a certain amount of precipitation occurs on average once in a 50-year period, its occurrence probability is 1/50, or 2 per cent. This means that, even if the phenomenon were to occur this year, there is still a 2 per cent chance of it recurring next year. The heaviest rains are rarely measured. This is why the inaccuracy of design rainfall is the greater the more seldom that rainfall level occurs.

Table 1. Design rainfall and flood risk management terms. Drafted based on Hulevesiopas (the National Storm Water Runoff Guide) [3].
Term Definition
Storm water runoff Rain or meltwater to be channelled away from the ground surface, off of building roofs or other similar surfaces.
Design rainfall Design rainfall is determined based on the catchment area, the duration, probability (return period) and intensity of the design rainfall, and precipitation. Precipitation that exceeds the design rainfall will cause flooding in the system being dimensioned.
Rainfall intensity The average amount of rainfall during a certain time period (for example 10 minutes).
Relative frequency In relative frequency distribution, the incidents in each category are set in proportion to the total number of incidents.
Return period The time interval during which a certain phenomenon recurs. Assessing return period is based on long-term observations and the statistical probabilities derived from them.
Probability (= statistical probability) A statistical probability is a figure that the relative frequency approaches. For example, the relative frequency of precipitation is calculated based on long-term observations.
Flood risk Combination of flood probability and potential flood damage (risk = flood probability x potential damage)

The return period and probabilities of weather and climate phenomena can change rapidly due to climate change [9], which is why probabilities only based on observations are always somewhat outdated. Particularly in plans extending decades into the future, climate change impacts on the probability of phenomena should be taken into account. In practise the dimensioning of water systems and applicable guidelines, greater attention should be given to design rainfall intended especially for that purpose and any changes to occur.

Design rainfall is based on observations made using rain gauges or weather radar [10]. Particularly in previous decades, the precipitation measurement network has been sparser in Lapland than in other parts of Finland. As a result, there is greater inaccuracy in design rainfall values in the northernmost parts of Finland than elsewhere.

Storm water runoff management systems are designed according to short-duration design rainfall

A short-duration (less than one hour) design rainfall can be used to determine the volume of storm water runoff, for which a residential drainage system, building roof drainage system or culvert is being dimensioned as a direct drainage channel. For example, storm water runoff drains are not dimensioned according to the heaviest rainfalls - occasional flooding is allowed. The drain dimensioning is a compromise between increasing building costs as the size of drain increases, and the damage caused by drain flooding [3].

Information on the rainfall intensity at a chosen probability level (or return period) and the duration of rainfall is needed for dimensioning a storm water runoff drain. In many cases, the probability or return period is chosen and the duration of rainfall needed for dimensioning is determined based on the characteristics of the runoff area (surface area, flow rate, absorption). The interactive visualization of "Intensity and frequency of short-duration rainfall in Finland" can be used to assess the probability of heavy rains occurring in the present climate in a single location (a link to the visualization is below the article).

The return period of rainfall used in dimensioning may be based on guidelines or can be determined in a case-specific risk analysis. For example, the return periods used in the dimensioning of storm water runoff drains typically vary from rainfall occurring once every two or once every ten years depending on the operator of the system and the site. The corresponding statistical probability of this kind of rainfall occurring during a given year is 50 per cent (once every 2 years) and 10 per cent (once every 10 years). Hulevesiopas (the National Storm Water Runoff Guide) contains detailed information on the management of storm water runoff and taking climate change into consideration. [3]

Dams are designed to be safe by taking long-duration rainfall into consideration

Heavy rainfall lasting over 24 hours and longer may cause extensive damage. Long-duration design rainfall is used in designing dams and their safety. Dams should be designed so that they will not fail (collapse), but also without their building costs rising to unreasonable levels. A dam failure is a hazard to human life, health, the environment, and property. The safety requirements are higher for dams with a significant hazard potential than those with a low hazard potential [11].

Design flood, which is determined by hydrological model calculations, is used in dimensioning dams. Input data needed include design rainfall, thus the total rainfall accumulating over the catchment area and return period in question [12]. Different classes are used in the dimensioning of dams. The return period for the most demanding class is 1000 years, which means that the dam is designed to endure floods whose occurrence of probability is 0.1 per cent.

Apart from rescue operations, official supervision of dam safety is the responsibility of Centres for Economic Development, Transport and the Environment (ELY Centres). Dam safety guides provide guidelines for long-term planning, in which climate change is also taken into consideration.

See also