1.1 Problem definition

**Figure 1.1:** Two-step approach for translation of weather station data to driving rain data on an building envelope.
$\begin{figure}\begin{center}% \fbox{% \begin{diagram}[height=3ex] \text{\fbox{w... ...& \text{undisturbed rain} & & \\ \end{diagram}} \end{center} \end{figure}$

The modelling of driving rain will be presented in detail in section 2.3, but here we will simply state that the driving rain amount on a particular position on the envelope of a particular building is determined by the wind flow around the building, and the rainfall. Estimations of driving rain amounts thus rely on data of wind and rain. Unfortunately, such data, especially driving rain data for the building in question, is almost always lacking. If a building is designed (and not yet built), such data can simply not be acquired. Therefore one tries to estimate driving rain with wind and rain data available from weather stations. These data are then `translated' to driving rain data. Two translation steps are often taken: (1) from the weather station to an intermediate reference, and (2) from the intermediate reference to the building. This two-step approach follows --so to speak-- changes in wind due to the topography: the wind blows from the countryside (where the weather station often resides), over the town to the site and eventually to the building. ``Site'' is here understood as the vicinity of the building in question. The intermediate reference wind speed is obtained at the site where the wind is not disturbed (influenced) by the building. If the site is densely built, the exact position where the intermediate reference has to be measured, is not determinable because the wind flow is influenced by both the building and its surroundings. In such cases the intermediate reference is hypothetical.

Figure 1.1 illustrates the two-step approach. Not always one will clearly find these steps in literature; sometimes even the first step or the second step is left out. For example in the British standard on the estimation of driving rain quantities [BSI 1992] one can find features of the two-step approach, as each step can be identified by the one or several coefficients in the formulae. Figure 1.1 introduces the term ``free driving rain'', which is defined as rain through the vertical.

To elaborate on the problem one has to (1) select and define the considered quantities and the kind of relationships, (2) select topographies and building types and (3) gather in situ measurements. As we will see in the literature survey (section 1.3), much knowledge is lacking or insufficient, namely:

The present study is intended to contribute to these items, although one particular building and topography will be considered.