Introduction. 2. The BS method. 2.1 Annual totals. The total amount of driving rain over a year on a wall in open airfield site is given by

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1 Comparison of the British tandard and French methods for estimating driving rain impacts on walls IA Annex 41 Glasgow meeting Chris anders Glasgow Caledonian University Introduction Driving rain is an increasingly important component of the moisture input to buildings, and a considerable amount of work has gone into the analysis and prediction of the likely amounts on different building facades. A major programme of measurements was undertaken in the UK in the 195 s and 6 s to collect the data that formed the basis of the British tandard Code of Practice for assessing the exposure of walls to wind driven rain, that was eventually published in 1992 (1). This standard used a methodology developed by Lacey and other to estimate the amounts of rain passing through a vertical area on an exposed airfield site in a year and in continuous spells of driving rain. This gave the airfield annual and spell indices. Correction factors were then used to estimate the rain that might hit a real building in an urban environment. These allowed for the roughness of the surrounding terrain, the local topography, obstructions from other buildings or trees in front of the wall and the distribution of rain on a wall. In recent years much effort has been put into refining these factors, especially the distribution on wall surfaces, using CFD analysis of wind flows (2, 3). Also a measurement programme has investigated the performance of different types of driving rain gauge (4). The methodology that formed the basis of B814 was used to develop a draft C standard (5), which has been proceeding very slowly through the C system for a number of years. This methodology requires the use of hourly values of rainfall, wind speed and wind direction, all measured at the same location. In their comments on the draft the French pointed out that they would not be able to use it because they had no meteorological stations that measure wind and rain at the same location. They have therefore developed an alternative method based on wetting and drying half days which are derived from wind speed, humidity and the observers assessment of whether or not it was raining (6, 7). This method is conceptually different from the B method because it assesses the time that the wall is wet rather than the amount of water hitting the wall. o that the can make progress, a comparison has been made between the results of the two methods using twenty years of meteorological data from three UK stations. 2. The B method 2.1 Annual totals The total amount of driving rain over a year on a wall in open airfield site is given by

2 2 8 A = v r cos(d Θ) l/m 2 /yr (1) 9 I 9 here v is the wind speed in m/s r is the rain fall in mm/hr D is the wind direction in degrees from north Θ is the orientation of the a normal to the wall in degrees from north The summation is taken over all the hours when cos(d- Θ) is positive, i.e. the wind is blowing against the wall. 2.2 pell totals A spell is considered to be a period of driving rain during which the risk of penetration through masonry increases i.e. a period in which the input of water due to driving rain exceeds the loss due to evaporation. Generally, spells are periods of 1 to 2 hours during a shower or 8 to12 hours during the passage of a depression. There are, however, occasionally long spells when successive depressions cause repeated periods of rain with little or no net evaporation in between. There can be periods of as long as 96 consecutive hours with no driving rain within the spell before evaporative loss exceeds gain from the rain. The spell totals are calculated from equation I = v r 9 cos ( D Θ) l/m 2 /spell (2) 9 where the symbols are as before and the summation is taken over all hours in the spell for which cos(d - Θ) is positive, i.e. the wind is blowing against the wall. 3. stimation of the effect of driving rain from average wind and present weather code for rain An alternative method, that is used in France (2,3), does not require simultaneous hourly measurements of wind and rain, but makes an estimate of the periods of when the wall is likely to moisten or dry due to rain. The available data are divided into twelve hour periods (7-18 and 19-6) called half days. A half day is defined as moistening if the following conditions apply: a) There is more than 4mm of precipitation on a horizontal surface in the half day b) The present weather code reports some precipitation for at least three of the five three hourly observations during the half day (i.e. at 6, 9, 12, 15 and 18 and at 18, 21,, 3 and 6). c) The average wind speed during the half day is greater than 2 m/s.

3 d) The average wind direction during the half day is within ± 6 of the perpendicular to the wall i.e. D Θ > 6 o Under these conditions, it is assumed that a wall surface will be wetted by driving rain, with subsequent water migration into the wall by capillarity. The present weather code is an internationally standardised number between 1 and 99 which meteorological observers use to characterise the type of weather on each hour; codes equal to or above 5 imply some form of precipitation was occurring. A half day is defined as drying if the wind and atmospheric humidity allow the evaporation of water at the wall surface: a) The average atmospheric relative humidity during the half day is lower than 7 % b) The average wind speed during the half day is greater than 2 m/s. c) The average wind direction during the half day is within ± 6 of the perpendicular to the wall i.e. D Θ > 6 o All other atmospheric conditions are considered as neutral. A moistening half day is given the value "+1", a drying half day "-1" and a neutral half day "". A cumulative time series is built for a given meteorological station, with the assumption that, the wall does not dry below some base level. As an example three years results from a south facing wall are shown on Figure 1. A maximum value, equal to the length in half days of the longest moistening spell in the year, is found for each year and each wall orientation, 1 5 Cumulative sum (half days) Figure 1 Accumulated wetting half days over three years on a south facing wall, with and without the assumption that walls do not dry below a zero base

4 Results e have twenty years ( ) of hourly values of rain in mm/hr, wind speed in m/s and wind direction in degrees from orth, relative humidity and present weather code from London (Heathrow airport), Manchester (Ringway airport) and dinburgh (Turnhouse airport). The results from the three locations for walls facing north, east, south and west are shown in Table 1, together with the overall mean wind speed, wind direction and total rainfall. Also shown are the 9 percentile values of the driving rain totals, i.e. the value that might be exceeded once every 1 years. Table 2 shows the percentiles of the size the spells of driving rain on the four walls in the three locations. In the B the spell index is based on the maximum value of I s likely to occur once every three years This is given by the Y 3 percentile, where Y 3 = 1 - y /3 for y years of data, so for 2 years of data Y 3 = 93.3, or 93 to the nearest whole number. Table 3 shows the percentiles of a) the maximum spell of accumulated wetting half days in each year, and b) total number of accumulated wetting half days in each year, from the three location and four wall orientations. Figure 2 shows the results from the two methods for individual years compared for a south facing wall at Heathrow. o direct relationship between the two methods is obviously present Longest Moistening period Total moistening period B Annual Total Half days l/m2/yr Figure 2 : Year by year comparison of the annual total driving rain from the B method and the longest moistening period and total moistening periods from the French method (south facing wall at Heathrow).

5 However if the extreme values predicted by the two methods are compared, better agreement results. This is shown in Figure 3, which shows the 9 percentile value of the annual driving rain total calculated with the B method, against the 9 percentile value of the longest annual moistening period from the French method. 6 9 percentile value of annual driving rain (BI) London Manchester dinburgh percentile value of annual longest moistening period half days (France) Figure 3 Comparison of the 9 percentile values from the BI annual total and the French annual longest moistening period. 93 percentile of annula spell totals (B) London Manchester dinburgh percentile value of annual longest moistening period half days (France) Figure 4 Comparison of the 9 percentile values from the BI annual maximum spell and the French annual longest moistening period. Figure 4 shows the same comparison for the 9 percentile value of the BI annual maximum spell.

6 Both these figures show that the two methods are in reasonable, but not perfect, agreement. As they are conceptually different the B method considers amounts of rain while the French method considers time of wetness very close agreement would not be expected. It is also difficult to conceive how the correction parameters for topography, sheltering etc. could be included in the French methodology. It would be useful to compare the results with actual measurements of moisture impacts on walls, however it is difficult conceive an experiment that would make this possible. The draft is being revised to allow the French method as an alternative option, and will be processed through the C system over the next year. References 1. British tandards, B 814 Code of practice for assessing exposure of walls to wind driven rain. 2. Blocken, B. ind driven rain on buildings, measurements, numerical modeling and applications. PhD thesis, University of Leuven, May Karagiozis, A. et al, ind driven rain distributions on two buildings, Journal of ind ngineering and Industrial Aerodynamics, 67 & 68, , Hogeberg, A. Microclimate load: Transformed weather Observations for use in design of durable buildings. PhD Thesis, Chalmers University of Technology, ovember C Hygrothermal performance of buildings Climatic data Part 3 : Calculation of a driving rain index for vertical surfaces from hourly wind and rain data. Draft pr acré, C., Concomitance de la pluie et du vent en France, approche statisque, Cahiers du CTB no 1792, eptember Beguin, D., tude pour la France du risque de mouillage par la pluie des parois verticales des constructions. Cahiers du CTB no 216 October 1986.

7 Table 1 : Annual driving rain totals from three sites on walls in four different orientations London - Heathrow Rainfall ind ind Annual driving rain index l/m 2 /yr Year Mm Direction peed m/s orth ast est outh Mean percentile Manchester - Ringway Year Rainfall ind ind Annual driving rain index l/m 2 /yr Mm direction peed m/s orth ast est outh Mean percentile

8 dinburgh - Turnhouse Year Rainfall ind ind Annual driving rain index l/m 2 /yr Mm direction peed m/s orth ast est outh Mean percentile Table 2 Percentiles of driving rain spells in l/m 2 /spell London - Heathrow Manchester - Ringway dinburgh - Turnhouse %tile orth ast outh est orth ast outh est orth ast outh est

9 Table 3 Percentiles of a) maximum spell of accumulated wetting half days in each year, and b) total number of accumulated wetting half days in each year London - Heathrow orth ast outh est %tile Maximum Total Maximum Total Maximum Total Maximum Total Manchester - Ringway orth ast outh est %tile Maximum Total Maximum Total Maximum Total Maximum Total dinburgh - Turnhouse orth ast outh est %tile Maximum Total Maximum Total Maximum Total Maximum Total