2.2.3. Type Plan—Flash Flood

Flash floods are most frequently caused by sudden surface flow caused by significant and concentrated rainfall, which quickly concentrates in the river network due to broken terrain.

Flash floods are characterized by high rainfall over a shorter duration, i.e., a high intensity and limited areal extent. In general, potentially dangerous precipitation is usually connected to the transition of frontal disorders or the influx of warm and humid air, which supports the development of convective clouds. The probability of the occurrence of heavy precipitation increases in mountain ranges, which create conditions for the occurrence of extreme rainfall on windward sides thanks to the increased flow of air. The resulting flood flows are, in addition to the intensity and duration of the causal rain and size of the afflicted area, also dependent on the physical/geographical characteristics of the afflicted territory. The main factors here represent the size and shape of the river basin, the inclination ratios of the terrain, soil permeability and saturation of the river basin prior to the rainfall. In some cases of flood situations of a regional nature, which are caused by long-lasting, less heavy precipitation, there is a considerable worsening of the course of the flood in the afflicted territory by locally limited heavy rainfall. Flash floods can be expected more in rural areas.

Damage caused by excessive rainfall in rural areas including the erosion of agricultural areas and subsequent transport of sediments into built-up areas are, for the purposes of preparing type plans, considered to be a part of flash floods. The course of a flood is characterized by the type of the flood itself, the value of the culmination flow, the shape and volume of the flood wave and the season of its occurrence.

Summer flash floods caused by short but intense rains usually affect a small area. They may affect any smaller river, but have catastrophic consequences especially when affecting sloping fan-like rivers. They exhibit a very rapid rise in water levels, followed by a very quick decrease. In addition to high precipitation intensity, an important role is also played by the ability of the soil to hold/soak rainfall water, which is affected by vegetation and anti-erosion measures as well as the current saturation of the soil due to previous precipitation. The most dynamic onset and largest death tolls are usually caused by rivers that are approximately 1–20 km<sup>2</sup> large, with a concentration period of less than 1 h.

The risk factors for flash floods are:


• configuration of the terrain with steep slopes, narrow river valleys.

The possibility of predicting flash floods is very limited due to the sharp dynamic of the development of convective clouds from which heavy precipitation comes. Even if meteorological conditions for the occurrence of heavy precipitation can be forecast relatively successfully, the precise localization of the occurrence, the duration and intensity of the heavy precipitation, and thus the specific threat to a locality can essentially not be predicted.

The societal impacts may include:


### 2.2.4. Type Plan—Excessive Precipitation (Rainfall)

For the purposes of crisis planning, excessive precipitation and its consequences refer to: the occurrence of intense precipitation in rural areas, leading to exceeding the capacity of the outflow network, flooding of lower floors of buildings and technical infrastructure by outflowing water. Damage caused by sudden excessive precipitation in rural areas including the erosion of agricultural areas and the subsequent transport of sediments into built-up areas is considered to be part of the flash flood or flood.

In unfavorable conditions, rainfall may lead to a quick build-up of flowing water, especially on reinforced, less permeable or saturated surfaces, and to the flooding of lowerplaced areas, buildings or underground areas, or to a rise of the water level in rivers and subsequent flooding. Excessive precipitation connected with storms is a fairly common phenomenon in the summer, but in most cases lasts only a short time (up to 30 min). However, in some cases storm cells may remain exceptionally active and emit an extreme amount of precipitation over a very short period of time. Moreover, storm cells may agglomerate into a group of cells which repeatedly pass over the affected area. In addition to flash floods, storms are also usually accompanied with sudden gusts of wind, electric discharges, and possibly hailstorms.

The occurrence of excessive precipitation is very random, and so it is very difficult to forecast the specific affected areas. They may also cause other unfavorable phenomena, notably soil erosion and landslides, which may then also damage traffic infrastructure, clog sewage systems, reduce flow capacities of river beds and the retention area of water recipients.

2.2.5. Type Plan—Disruptions of Dams of Significant Waterworks Associated with a Special Flood

This category covers floods caused by artificial factors, which are situations that may occur on certain kinds of water works. The owners (users) or managers of water works are obliged to ensure that these are supervised by specialized technical and safety officers with the aim of continuously tracking the technical condition of the water works in view of its stability, safety and possible disruptions along with the proposal of suitable corrective measures.

A special flood needs to be distinguished from natural floods, notably due to the different characteristics of its occurrence, course and the different flood measures that are to be used before and during a special flood. The handling of a special flood requires the careful preparation of bodies involved in the managemen<sup>t</sup> of floods and crises and the preparation of a basic planning document (plan for protecting the landscape below a selected water facility against a special flood) aimed at handling arising critical situations. This critical situation can only be forecasted in the case of an unmanageable waterworks defect, gradually increasing overflow or water inflow into the waterworks following after long-lasting and/or sudden precipitation.

Disruption of a water work (dam) and adjacent buildings due to accumulated surface water associated with the occurrence of a flood wave via a special flood will, due to its extensive destructive effects, be a reason for the announcement of a critical situation and subsequent declaration of an emergency/crisis. The area where the emergency will be announced directly depends not only on the amount of held water, the landscape configuration below the waterworks and the characteristics of the flood wave, but also on the size of the cities and towns (in terms of residential zones and production facilities) that may be affected by the special flood. There will be immense loss of human life, health, property and infrastructure, but the precise scope of these will, to a significant extent, depend on how well prepared the local bodies and citizens are for this critical situation.

Table 1 contains an overview of the plan characteristics, the application scale and the societal impacts


**Table 1.** Overview of the plan characteristics.

> Source: own research.
