Roed, Jørn

Abstract. In order to find the run-off from roof material a roof has been constructed with two different slopes (30° and 45°). Beryllium-7 and caesium-137 has been used as tracers. Considering new roof material the pollution removed by runoff processes has been shown to be very different for various roof materials. The pollution is much more easily removed from silicon-treated material than from porous red-tile roof material. Caesium is removed more easily than beryllium. The content of caesium in old roof materials is greater in red-tile than in other less-porous roof materials. However, the measured removal from new material does not correspond to the amount accumulated in the old. This could be explained by weatherinq and by saturation effects. This last effect is probably the more important. The measurements on old material indicates a removal of 44-86% of the caesium pollution by run-off, whereas the measurement on new showed a removal of only 31-50%. It has been demonstrated that the pollution concentration in the run-off water could be very different from that in rainwater. The work was part of the EEC Radiation Protection Programme and done under a subcontract with Association Euratom-C.E.A. No. SC-014-BIO-F-423-DK(SD) under contract No. BIO-F-423-81-F

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Downloaded from orbit.dtu.dk on: Dec 20, 2017Run-off from roofsRoed, JørnPublication date:1985Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Roed, J. (1985). Run-off from roofs.  (Risø-M; No. 2471).RISØ-M-2471 RUN-OFF FROM ROOFS Jørn Roed Abstract. In order to find the run-off from roof material a roof has been constructed with two different slopes (30° and 45°). Beryllium-7 and caesium-137 has been used as tracers. Considering new roof material the pollution removed by runoff processes has been shown to be very different for various roof materials. The pollution is much more easily removed from silicon-treated material than from porous red-tile roof material. Caesium is removed more easily than beryllium. The content of caesium in old roof materials is greater in red-tile than in other less-porous roof materials. (Continued on next page) INIS Descruptors: BERYLLIUM 7; BUILDING MATERIALS; BUILDINGS; CESIUM 137; DECONTMINATION; PRECIPITATION SCAVENGING; RAIN WATER; REVIEWS; RUN-OFF; TRACER TECHNIQUES; WEATHERING. January 1985 Risø National Laboratory, DK-4000 Roskilde, Denmark However, the measured removal from new material does not correspond to the amount accumulated in the old. This could be explained by weatherinq and by saturation effects. This last effect is probably the more important. The measurements on old material indicates a removal of 44-86% of the caesium pollution by run-off, whereas the measurement on new showed a removal of only 31-50%. It has been demonstrated that the pollution concentration in the run-off water could be very different from that in rain-water. The work was part of the EEC Radiation Protection Programme and done under a subcontract with Association Euratom-C.E.A. No. SC-014-BIO-F-423-DK(SD) under contract No. BIO-F-423-81-F. ISBN 87-550-1070-9 ISSN 0106-6435 - 3 -CONTENTS Page 1. INTRODUCTION 4 2. RUN-OFF IN URBAN AREAS : 4 3. RUN-OFF ON ROOF MATERIAL 7 3.1. Measurements 9 4. CONCLUSION 16 REFERENCES 17 - 4 -1. INTRODUCTION Run-off is that excess of deposited rainwater that is not re-tained on the area receiving the rainfall. This phenomenon has been extensively studied in the context of hydrology (Chow 1964). As run-off water can carry away a part of the deposited radioactive material from roofs and roads through sewers it clearly could have an important impact on consequence calcul-ation, in that material deposited in urban areas may not all be retained there, thereby reducing the dose to the urban population. 2. RUN-OFF IN URBAN AREAS The total run-off can consist of surface run-off and infil-tration, i.e. the flow of water through the soil surface. Infiltration is dependent upon the type of surface^ Artificial materials comprising most surfaces in urban areas, are suf-ficiently impervious to prevent infiltration. For these surfaces the following equation will be valid Q=P-Ia 2.1. where Q is the actual direct run-off in mm, P the total rain-fall in mm, and Ia the initial accumulated rainfall in mm until run-off occurs. - 5 -Ritchie et al. (1976) assumed for an urban area that run-off from artificial surfaces would be virtually 100% for all rain-fall above an initially accumulated 3 mm. If there has been rain within the previous hour the run-off will occur sooner. In order to allow for patches of grass, etc. in the urban areas the amount of run-off was reduced to between 70 and 95%. In their model Ritchie et al. made the assumption that the concentration of radioactive material in the run-off water is equal to the concentration in the rainwater. The relation between rainfall and maximum run-off has been represented by many empirical and semiempirical formulas (Chow, 1962). The so-called rational formula is one of the most commonly used formulas for estimating maximum rate of surface run-off: Q=C«p-A 2.2. Where C is the run-off coefficient, p the rainfall intensity, and A the area considered. The rational method assumes that the rainfall intensity is uniform over the area during the duration of the storm. The reasoning of the method states that after some time the run-off rate pr. unit area reaches a maximum Q — = O p 2.3 h C then represents the part of the rainfall that runs off. Prom the Handbook of Principles of Hydrology the following table gives run-off coefficents for urban areas: - 6 -Description of A m Flat, residential, with about 30% of area impervious Moderately steep, residential, with about 50% of area impervious Moderately steep, built up, with about 70% of area impervious Runoff Coefficient 0.40 0.6S 0.80 Table. 2.1. From Horner and Flynt (1936). Relation between rainfall and run-off frc iirban areas. In the Handbook of Applied Hydrology C values are given as reported by a joint committee of the American Society of Civil Engineers and the Water Pollution Control Federations. These values are applicable for heavy storms that occur only once every 5 to 10 years. Vain« of Runoff Coefficient C Type »f drauMøc aras Rvmf cotfidmt, C Lawns: Bandy »oil, flat. 2% 0 06-0 10 Bandy awl. svamp. 2-7% 0.10-0.15 Sandy soil: stoop. 7% 0.16-0.20 Haavy soil. flat. 2% 0.13-0.17 Haavy toil, STaraft, 2-?% 0.18-0.22 Boavy soil, stosp, 7% 0.26-0.36 Downtown anas 0.70-0 85 Naifbborbood ana* 0.60-0.70 BaBdaatial: Binfta-family afas* 0 80-0 60 Mnhiunits, datacnad 0.40-0.80 Mutti mut*, attached 0.60-0.76 Suburban 0.36-0.40 Apartment dwelling areas 0.60-0.70 Industrial: Li«bt areas 0.60-0.80 Haavy »raw 0.60-0.SO Parks, cemeteries 0 10-0 25 Playaronndt 0.20-0.36 Railroad yard araat 0.20-0.40 TJaianrorad areas 0.10-0.SO Striata: AtpbaHic 0.70-0.06 Concrete 0 80-0.96 Brick 0 70-0 86 Drives and walk* 0 76-0.86 Roofs 0.76-0.M Table 2.2. Some values of the run-off coefficient C reported by a joint commitee of the American Society of Civil Enqineers and the Water Pollution Control Federation, (ASCE and WPCF, 1960). - 7 -3. RUN-OFF ON ROOF MATERIAL Run-off is normally defined as the amount of water that is transported away from an area exposed to precipitation. Evaporation is not included in the run-off mechanism. To find the run-off from a roof, a construction has been built that allows measurement of the amount of water per m^ horizontal projected area that runs off. This is done by collecting the run-off water from the roof construction in a vessel as shown in Fig. 3. The amount of rainwater per m2 projected area is found by direct sampling of rainwater in a vessel. Some authors (Ritchie 1976) assume that the concentration of pollution in the run-off water is equal to the concentration in rainwater, so that the amount of pollution removed by the run-off processes can be found from knowledge of the amount of run-off water and the concentration of the pollution in rain-water. In order to investigate the validity of this hypothesis and determine the amount of material removed by run-off processes we have measured the concentration in rainwater as well as in run-off water. To do this the run-off water is passed through an ion-exchanger in which the pollution is trapped. The amount of beryllium-7 and caesium-137 is then measured by gamma-ray spectrometry by means of a Ge(Li)-detector. - 8 -Fig. 3 B A: roof material B: buffer container C: ion exchanger D: container - 9 -In the same way the collected rainwater is passed through an ion-exchanger to find its content of ^Be and 137cs. The roof construction used for the measurements has two dif-ferent slopes and is divided in 6 sections, 2 with a 30° slope and 4 with 45° slope. 3.1. Measurements The roof was covered with new roof material in the following way: section 1 cement tile 45° slope section 2 red tile 45° slope section 3 corrugated eternite 45° slope section 4 silicon-treated eternite 45° slope section 5 corrugated eternite 30° slope section 6 silicon-treated eternite 30° slope Just after the roof material was mounted the total construction was covered with tarpaulins; the first measurement (Table 3.1.1.) was made when the construction was covered. In this measurement the run-off water can easily pass from one section to another because of the tarpaulins so that the results should be treated as one for each slope. From the measurement of run-off from tarpaulins we found that nearly all the rainwater runs off. However a substantial part of the contamination is retained by tarpaulins resulting in a lower concentration in the run-off water than in the rain water. We found a strong dependence on the slope of the tarpaulin, so that about 50% of the pollution of caesium-137 and beryllium-7 are removed by run-off when the slope is 45°, but only about 10% is removed with a slope of 30%. - 10 -Table 3.1.1. Run-off from tarpaulin March 1984 Section No. 1 2 3 4 5 6 1 2 3 4 5 6 Concentration Bqm_J ?Be 2174.20 2030.87 1015.33 2366.37 314.94 296.15 137Cs 1.715 1.582 0.666 2.287 0.364 0.319 on roof in run-off water in % of concentration in rainwater 68.8 64.2 32.1 74.6 10.0 9.4 60.0 55.3 23.3 80.0 13.4 11.2 Run-off water m3 per horizontally projected m* 0.0128 0.0170 0.0226 0.0090 0.0178 0.0152 0.0128 0.0170 0.0226 0.0090 0.0178 0.0152 % of water that runs off 78.0 103.7 137.8 54.9 108.5 92.7 78.0 103.7 137.8 54.9 108.5 92.7 % material removed with run-off 54 67 44 41 11 9 47 57 32 44 15 10 Concentration of 7Be in rainwater: 3162 Bq nr3 Concentration of *37Cs in rainwater: 2.86 Bq m~3 Mount of rainwater: 0.0164 m3 nr2 - 11 -The first measurement of real roof material was made in April 1984; the results are shown in Table 3.1.2. The main result is again that the concentration in run-off water is very much lower than the concentrations in rainwater, and that sil icon-treated material showed the highest values for removed material. For beryllium the removal was 20% for a slope of 45° and 10% for the 30° slope. For the more porous material the removal of the pollution was smaller. From cement tile about 7% was removed and from red tiles and eternite the removal was only a few percent. The measurements made in May 1984 (see Table 3.1.3.) showed the same overall picture as those made in April. From silicon-treated eternite the removal was 26% for beryllium and 50% for caesium independent of the slope. From cement tile the removal was 6% for beryllium and 9% for caesium. From red tile and eternite the removal of beryllium was 1-2% and of caesium 7-9%. Beside the measurements on new roof material the amount of caesium trapped on old roof material was also measured. The accumulated fallout of caesium in the trapped material was compared with caesium in the rain water during the period of exposure. All the figures are decay corrected to the time of the sampling. - 12 -Table 3.1.2. Run-off from roof material April 1984 Section No. 1 2 3 4 5 6 1 2 3 4 5 6 Concentration Bqm -•* 7Be 219.94 132.95 76.56 684.18 60.16 441.49 137CS 1.933 3.366 0.949 8.910 0.618 11.089 in run-off water in % of concentration in rainwater 7.24 4.37 2.52 22.51 1.98 14.52 8.06 14.03 3.96 37.14 2.58 46.22 Run-off water m3 per horizontally projected m2 0.0112 0.0031 0.0111 0.0113 0.0096 0.0085 0.0112 0.0031 0.0111 0.0113 0.0096 0.0085 % of water that runs off 91.1 25.2 90.2 91.9 78.0 69.1 91.1 25.2 90.2 91.9 78.0 69.1 % material removed with run-off 6.6 1.1 2.3 20.7 1.5 10.0 7.3 3.5 3.6 34.1 2.0 31.9 Concentration of 7Be in rainwater: Concentration of 137Cs in rainwater: Amount of rainwater: 3039.52 BqnT3 23.993 Bq m"3 0.0123 m3 ttT2 Section No. 1: 2: 3: 4: 5: Cement tile slope 45° Tile slope 45° Corrugated eternite slope 45° Silicon-treated eternite slope 45° Corrugated eternite slope 30° 6: Silicon-treated eternite slope 30° - 13 -Table 3.1.3. Run-off on roof material Nay 1984 Section Concentration in run-off water Bqm"=3 No. 1 2 3 4 5 6 1 2 3 4 5 6 7ee 282.85 162.39 121.71 764.30 83.20 894.35 137CS 0.460 0.779 0.575 1.480 0.414 1.775 in % of concentration in rainwater 13.97 8.02 6.01 37.76 4.11 44.18 21.77 36.87 27.21 70.04 19.59 84.00 Run-off % of water water m3 per that horizontally runs off projected m^ Concentration of ?Be in rainwater: Concentration of 137cs in rainwater: Amount of rainwater: 0.0172 0.0082 0.0133 0.0285 0.0212 0.0240 0.0172 0.0082 0.0133 0.0285 0.0212 0.0240 2024.19 2.113 0.0404 Bq mJ 42.6 20.3 32.9 70.5 52.5 59.4 42.6 20.3 32.9 70.5 52.5 59.4 m-3 rt-3 nr2 % material removed with run-off 6.0 1.6 2.0 26.6 2.2 26.2 9.3 7.4 9.0 49.3 10.0 49.8 Section No. 1: Cement tile 2: Tile slope 45° slope 45° 3: Corrugated eternite slope 45° 4: Silicon-treated eternite slope 45° 5: Corrugated eternite slope 30° 6: Silicon-treated eternite slope 30° - 14 -From the figures given in Table 3.1.4. we find that the 137Cs material removed by run-off is 44-86% for red-tiles and from 84-99% for other materials. This discrepancy can be explained in at least three ways: First, the weathering cannot be neglected because of its efficiency during the more than 20 years that some of the roof material has been exposed to this effect. Yet it must be noted that in the case of eternite, as shown in Table 3.1.4. the trapped material in percent of the corresponding accumu-lated fallout seems not to be strongly dependent on the time of exposure; if so, the percent should be higher for a shorter exposure. This gives us the reason for believing that the weathering effect is not essential for explaining the discrepancy. The second possible explanation is that the new material traps the pollution more efficiently than the old. This could be due to a saturation effect that appears after a certain time has passed. This effect could be different for each type of roof material. The third possibility is that the new material is so different from the old that the two can't be compared. This could cer-tainly be true for most of the material, but not for red-tile which has been fabricated approximately the same way for de-cades. It must be emphasized that red-tile is the material that shows the closest agreement between the measurement on the new and the old material. Sample No. RED TAIL 38 15 34 36 14 23 1 GLAZED TAIL 30 7 Slope of the roof degree 45 20 45 50 45 40 45 45 45 Built year 1900 1900 1914 1914 1952 1952 1956 1918 before 1950 - 15 -Table 3.1. Run-off from old Area of sample m"2 0.097 0.121 0.051 0.108 0.083 0.096 0.048 0.097 0.070 4. roofs 137 Cs-deposition on roof Bq Kl-2 383 * 23 346 * 29 1037 i 25 217 i 13 430 ± 46 267 * 21 726 * 58 219 i 17 179 i 21 Percent *' of Cs that runs off from roofs 78 85 44 86 76 85 57 87 90 31 32 11 SLATE 8 35 45 45 45 45 45 1910 1935 1943 1906 1918 0,166 0.116 0.106 0,251 0,277 212 t 21 182 t 38 131 t 10 112 t 14 161 i 6 84 90 93 94 91 CORP0GATFD ETKRMITE 13 5 6 2 3 ETERflJTE 33 "SLATE" 15 5 45 45 45 20 before before 1954 1950 1950 1960 1971 1947 0,120 0,119 0,104 0,101 0,099 0,189 99 * 281 i 112 * 47 ± 10 * 16 » 7 11 9 7 7 2 95 87 94 97 97 99 calculated from the a- ount of Cs on the roof 137 and the corresponding content of C» in rainwater - 16 -4. CONCLUSION For new roof material the degree to which pollution is removed together with run-off is very different for various types of material. The pollution is much more easily removed from silicon-treated material than from porous roof material that is in the form of red-tile. Caesium is removed more easily than beryllium. The content of caesium in old roof materials is greater in red-tile than in other less-porous roof materials. However, the measured removal on new material does not cor-respond to the amount accumulated in the old material. This could be explained by a weathering effect, or by a saturation effect. The last effect is probably the most important in explaining the discrepancy. The measurement on old material indicates a removal of 44-86% of the caesium pollution by run-off, whereas the measurement on the new material showed a removal of only 31-50%. Purther, it has been demonstrated that the pollution concen-tration in the run-off water could be very different from the concentration in rainwater. - 17 -REFERENCES ASCE and WPCF (1960). Design and Construction of Sanitary and Storm Sewers. ASCE Nan. Eng. Practice No. 37 and WPCF Man. Practice No. 9. BECK, H.L. (1966). Environmental Gamma Radiation from Deposited Fission Products, 1960-1964. Health Physics Y2, 313-322. CHOW, V.T. (ed.) Handbook of Applied Hydrology. Mc GRAW-HILL. HORNER, W.W. and FLYNT, F.L (1936). Relation between Rainfall and Run-off from Urban Areas. Trans. Amer. Soc. Civil Engr. 101, 140-206. RITCHIE, L.T. (1976). Effects of Rainstorms and Runoff on Consequences of Nuclear Reactor Accidents. SAND76-0429. Risø National Laboratory R?so-M-QlzD T i t l e and author(s) RUN-OFF FROM ROOFS Jørn Roed Group's own r e g i s t r a t i o n number(s) pages + tab le s + i l l u s t r a t i o n s Date J a n u a r y 1985 Department or group H e a l t h P h y s i c s Abstract Abstract. In order to find the run-off from roof material a roof construction with two different sloDes (30° and 45°) has been built. Beryllium-7 and Caesium-13 7 has been used as tracers. Considering new roof material the pollution removed by run-off processes has shown to be very different for various roof material. The pollution is much more easily removed from silicon-treated material than from por-ous red-tile roof material. Caesium is re-moved more easily than beryllium. The content of caesium in old roof materials is greater in red-tail than in other less-porous roof materials. (Continued next page) copies to Available on request from Risø Library« Risø National Laboratory (Risø Bibliotek), Forsøgsanlag Risø), DK-4000 Roskilde, Denmark Telephone: (03) 37 12 12, ext. 2262. Telex: 43116 However, the measured removal from new material does not correspond to the amount accumulated in the old. This could be explained by weathering and by saturation effects. This last effect is probably the more important. The measurements on old material indicates a removal of 44-86% of the caesium pollution by run-off, whereas the measurement on new showed a removal of only 31-50%. It has been demonstrated that the pollution concentration in the run-off water could be very different from that in rain-water. The work was part of the EEC Radiation Protection Programme and done under a subcontract with Association Euratom-C.E.A. No. SC-014-BIO-F-423-DK(SD) under contract No. BIO-F-423-ol-F. ISBN 87-550-1070-9 ISSN 0106-6435 

Run-off from roofs

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Run-off from roofs

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