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(1) Systems and Mechanics Research Group, Department of Civil and Offshore Engineering, Heriot-WattUniversity, Edinburgh, Scotland, UK, EH14-4AS ([email protected]).

(2) Drainage Research Group, Department of Building Engineering & Surveying, Heriot-Watt University,Edinburgh, Scotland, UK, EH14-4AS ([email protected]).

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Siphonic roof drainage systems have been in existence for approximately 30 years. In that time,the construction industry has been gradually persuaded by the benefits which these systems offer whencompared to the traditional approach. A great deal of these benefits arise from the fact that systems canbecome de-pressurised. However, this condition only arises at the design condition – typically a stormwith a return period in excess of 30 years. When the funding application was being made for the workreported herein, it was known that the majority of data published relating to siphonic roof drainage systemperformance related to “ideal” laboratory conditions. Additionally, it was recognised that theoverwhelming majority of rainfall events any siphonic system would have to drain would be well belowthe design condition. This, coupled with reports of siphonic system failures, convinced the authors thatthis was an area worthy of further research.

The work reported herein documents the instrumentation of a large, high profile building withinEdinburgh (Scotland), the aim of which is to investigate how a real siphonic system performs under realrainfall conditions in a Northern European climate. Details are given of the instrumentation used, and datacollection protocols established. The data generated at the site are discussed in detail. The ability of thesystem to drain rainfall events of known intensity and duration is considered. Conclusions are drawnregarding the ability of the system to drain the monitored storms, and those outside the envelope of thedata collected. Plans for future work are outlined.

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Siphonic roof drainage, design, data collection, numerical model.

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Over the past 30 years, an ever increasing amount of industrial and commercialroof space has been drained using siphonic roof drainage – currently it is estimated thatover 30,000 systems exist in the UK alone. This continuing increase in usage is largelydue to the many advantages the systems have over “conventional” systems forequivalent sized roof areas. However, notwithstanding the increasing use of thesesystems, there are still uncertainties regarding just how these systems operate –

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particularly during priming. This lack of understanding of system operation means thatif a system fails, it is often difficult to appreciate why the failure has occurred.Furthermore, as siphonic system design represents a higher level of expertise than maybe required for conventional systems, the performance of these systems can be morereactive to small inaccuracies or erroneous assumptions. There has also been growingconcern about the use of these systems in developments where it is often necessary tominimise the rate at which flows enter drainage systems. To resolve this, over the past 5years there has been an ever increasing amount of independent research undertakenattempting to understand how siphonic systems actually perform(1-7).

For any given application, siphonic roof drainage systems are normally designed#

to cope with the steady state pressures associated with a selected ‘design storm’, whichis normally specified in terms of a steady rainfall intensity (in the UK this is inaccordance with BS 6367(8)). Selection of a rainfall intensity at the design stage is basedupon the geographical location, and by balancing the risk of failure against the cost ofallowing for additional roof drainage capacity. However, it can be seen that thisapproach will lead to one of two post installation eventualities each time a storm occurs:

1. A storm occurs which exceeds the design rainfall intensityPractically, no matter what design rainfall intensity is selected, this will always

eventually occur, and will result in flooding to some extent. Well designed systemsmake allowance to ensure that any overspill is directed to areas where it can bemanaged, or any damage caused is limited.

2. A storm occurs which is less than the design rainfall intensity For any well specified system, the vast majority of the storms encountered will

fall into this category. Where rainfall events of low intensity are encountered, thesystem will perform as a ‘conventional’ roof drainage system. However, as increasingrainfall intensities are considered partial unsteady de-pressurisation of the system willoccur. Laboratory testing(ref) has shown that this de-pressurisation results in air beingdrawn into the system, this can exceed the volume of water entering the system in somecircumstances.. The unsteady nature of the flow regime, which has been observed to becyclic in nature, leads to varying amounts of noise generation, and structural vibrationwithin the system.

As the first of these eventualities is relatively easy to model, and is currently

accounted for in well designed systems (although the priming of the system to reach thateventuality is less well understood), it is the latter of these cases which is of the mostinterest. Previous work(2) at Heriot-Watt University has investigated how siphonicsystems operate at rates of inflow below that of the design. This has lead to a goodunderstanding of the range of flow conditions which can be expected, these areillustrated in Figure 1. As the figure illustrates, the system reacts quite differently todiffering rates of constant inflow. At quite low rates of constant gutter inflow it can beseen that free surface flow dominates the mode of operation (D). As the rate of inflowbegins to increase the pressures plotted indicate that plug flow is the mode of operation(E & F). Subsequently, as the rate of inflow begins to near the system capacity

# Currently siphonic roof drainage systems are designed to accommodate a specified storm which fills, and primes, the wholesystem rapidly with 100% water. This assumption means that the system may be designed easily using elementary steady statehydraulic relationships. The steady flow energy equation is used almost universally(3) as the backbone of the design procedure forsiphonic roof drainage systems. The pressure drop between any two points X and Y can be determined using the energy equation.

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(a) Inflow =16% of capacity

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(d) Inflow =64% of capacity

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Figure 1: Pressure history data for differing rates of constant gutter inflow.

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observations and the pressure data indicate that the phase boundaries between theplugs begin to blur (G), bubble flow predominates (H). As the rate of inflow approachesthe design capacity, the proportion of air within the flow steadily diminishes until thesystem is primed (I). Although it is relatively easy to study these flow conditions byapplying rates of steady inflow in a laboratory, it should be noted that in an actualsystem these flow conditions will be transitory and may well appear concurrently –Figure 1a illustrates how the system accelerates from free surface flow to full bore flowin the space of less than 15 seconds.

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Based on the considerations outlined above, it was therefore decided to collectdata from an installed siphonic roof drainage system in order to understand how thesesystems drain the vast majority of rainfall events – i.e. those which are below the designcapacity of the system. Buildings adjacent to the university campus were surveyed withthe aim of finding a suitably sized property which the occupier would allow access to.The building which was selected is the National Archives of Scotland documentrepository - Thomas Thomson House, which is owned and operated by the ScottishExecutive (National Government). The main building has a total roof area ofapproximately 3000 m2, which is divided into three principle areas. It is the larger ofthese areas (1988 m2) which is forming the main data collection programme. Thebuilding is illustrated in Figure 2.

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The section of roof studied measures 26.8m x 74.2m, and is served by 6 separateroof siphonic roof drainage systems, within which each outlet drains an individualsection of roof gutter (a single gutter on each side of the roof which is sub-divided).Only two of the 6 separate roof siphonic roof drainage systems have been instrumented,these are highlighted in the roof plan schematic (Figure 3) and in dimensions are givenin Figure 4 . The construction of the roof itself is polished aluminium in a curved form

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(see Figure 2). The pipework is wholly 50mm internal diameter stainless steel. Allbends within the system are 90° smooth radius bends. The siphonic roof drainagesystem installed at Thomas Thomson house is known to have a high maintenance cost,this is principally due to the roof being used by a large number of birds as an eveningroost. The birds then foul the roof surface with their droppings, feathers andoccasionally their own expired bodies. This debris then fills the gutters and blocks thesiphonic roof outlets. To ensure that the systems operate efficiently, the propertymanager currently has the roof gutter cleaned at approximately bi-monthly intervals.These maintenance issues should also allow the project the opportunity to evaluate howmaintenance effects system performance.

System 1

System 2

(607 m2)

(311 m2)

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= Siphonic Roof Outlet = Transducer Location

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The main aims of the data collection exercise are as follows:1. Obtain an accurate survey of the systems under consideration so that their

capacity can be accurately estimated.Subsequently, during rainfall events, the following data are also required:

2. Rainfall intensity variation with time.3. Wind direction and velocity variation with time.4. Gutter flow depth variation with time5. System pressure variation with time.

In Scotland, rainfall is regular – but is predictably unpredictable. Therefore whenestablishing the test site it was decided to design the instrumentation so that it wasautonomous – i.e. automatically controled data collection. This approach reduces therisk that data relating to an important rainfall event would not be lost, and it also meansthat data does have to be collected continually. To meet these aims the instrumentationwas design so that is reacts to real-time rainfall data. Data collection software wascoded which continually monitored data from a 0.1mm resolution raingauge. Oncerainfall intensity exceeds a pre-set threshold (5mm/h), the software initiates overall datacollection from the roof drainage system. Once data collection is underway, therainguage input is monitored and the rate of data collection is varied based on rainfallintensity – higher intensity rainfall events are afforded a higher sampling rate. Thirtyminutes after the rainfall has ceased, data collection is terminated. Once operating thesoftware will continue to collect data almost indefinitely. Figure 5 illustrates theprotocols used by the software for the collection of data.

Data collected relating to the wind conditions is collected separately using a weatherstation 2 km from Thomas Thomson House.

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At the time of writing, monitoring at the site has been underway for 40 days. In thattime data has been collected which with relates to a total of 76.2mm of rainfall. Twelveof the rainfall events which have occurred had peak intensity in excess of 5mm/h.

As expected at the outset, the majority of the storms monitored did not result in anysustained period of depressurisation. However, of the twelve storms monitored, threewere found to be of sufficient intensity to result in depressurisation. Each of these threeevents are illustrated on the cumulative rainfall plot (Figure 6). The following three sub-sections deals with each of these events in turn:

��6WRUP��This event occurred just a few hours after the instrumentation was installed. Rainfalldata relating to the event are illustrated in Table 1. The response of the system indraining this rainfall is illustrated in Figure 7. The data illustrate that although therainfall event was relatively small, it was sufficient to result in a depressurization of

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Monitor @ 100Hz

Tip?

Yes

No

Record Tip &Start Timer (T1)

T1>60s ?Yes

Reset &Stop Timer

No

Yes

Stop T1, Start T2 &Start Logging:

T1 = 60 - 30s : 1HzT1 = 30 - 15s : 2HzT1 = 15 - 10s : 5Hz

T1 = 10 - 0s : 10Hz

Tip?

No

T2>1800s? No

Yes

Save Data to:mmhhddyy.prn

Restart

YesRecord TipT2 > T1?

YesReset T2

Change Collection RateT2 = 60 - 30s : 1HzT2 = 30 - 15s : 2HzT2 = 15 - 10s : 5HzT2 = 10 - 0s : 10Hz

Reset T2

No

START

Tip?

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No

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both System 1 and System 2. The data also indicate that there is a 1.25 minute delaybetween the rainfall peak intensity and the peak in-pipe depressurisation.

3DUDPHWHU 9DOXHEvent Duration 14.4 minutesPeak intensity 21.1 mm/hTotal rainfall depth 1.2 mmAverage rainfall intensity 5.0 mm/hTotal rainfall volume (over 1988 m2) 2.39 m3

Average run-off rate 2.76 l/s7DEOH��'DWD�UHODWLQJ�WR�VWRUP�RI��

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��6WRUP��Data relating to this event are illustrated in Table 2. The response of the system indraining this rainfall is illustrated in Figure 7. Again this data illustrates that althoughthe rainfall event was relatively small, it was sufficient to result in a depressurization ofboth System 1 and System 2. This data set also indicated that there is a 1.5 minutedelay between the rainfall peak intensity and the peak in-pipe depressurisation.

3DUDPHWHU 9DOXHEvent duration 16.9 minutesPeak intensity 19.5 mm/hTotal rainfall depth 1.5 mmAverage rainfall intensity 5.35 mm/hTotal rainfall volume (over 1988 m2) 2.98 m3


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3DUDPHWHU 9DOXHEvent duration 5.5 HoursPeak intensity 8.47 mm/hTotal rainfall depth 10.9 mmAverage rainfall intensity 1.98 mm/hTotal rainfall volume (over 1988 m2) 21.7 m3


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��6WRUP��Data relating to this event are illustrated in Table 3. The response of the system indraining this rainfall is illustrated in Figure 9. As the data illustrate, the peak rainfallintensity of this event was only 8.47 mm/h. Figure 9 also illustrates that the 5 mm/hthreshold set for data collection was actually too low, as it appears that some of thedepressurisation data was not collected (i.e. that prior to t =0). It is not yet clear howrepresentative this particular data set is, as this rainfall event lasted for 36 hours(starting 24 hours before the recorded depressurisation) – the full rainfall data set forthis event is illustrated in Figure 10. It is therefore possible that the system respondeddifferently than it would have if the rainfall plotted in Figure 9 occurred in isolation.

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Based on a preliminary survey of the property, it was estimated that the roof drainagesystem, as a whole, was designed based on a design rainfall intensity of 100 mm/h.Applying the laboratory findings illustrated in Figure 1 to the system installed atThomas Thomson house indicated that there would be a degree of measurabledepressurisation when the rainfall intensity exceeded 25mm/h (25% of the systemcapacity). However, the data collected to date indicates that the systems is capable ofbecome partially depressurised at rainfall intensities less than 10% of the estimateddesign capacity. If this result is transferable to other installed systems it means that theymay have better than expected performance when draining rainfall events of lowerreturn periods. Additionally, the ability of the system to become depressurised at quitelow rates of inflow means that the system will have an increased propensity to be selfcleansing.

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Regarding the work reported herein, the following conclusions may be drawn:• Laboratory data has been presented which represents the range of flow

conditions which lead to the system working at its design capacity.• To complement the laboratory based work underway at Heriot-Watt University,

a field data collection site has been established based on both novel andestablished data collection techniques.

• Data has been presented which indicates the siphonic rainwater drainage systemshave the ability to become depressurised at lower then expected rainfallintensities.

• The ability of the system to become depressurised when draining low intensitystorms indicates that the self cleansing velocity may be achieved on a frequentbasis.

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It is anticipated that data collection will continue for at least one year. It is hoped that inthat time a data base of storm events will be established which will demonstrated theability, or otherwise, of the installed system to operate under varying conditions. Thedata collected will then be used to validate the numerical model being developed atHeriot-Watt University to represent the flow conditions within siphonic roof rainwatersystems.

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1. Arthur, S, & Swaffield, J.A., (1999a), 1XPHULFDO� 0RGHOOLQJ� RI� D� 6LSKRQLF5DLQZDWHU� 'UDLQDJH� 6\VWHP� Proc. Water Supply & Drainage for Buildings :CIB W62 1999, Edinburgh.

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2. Arthur, S. & Swaffield, J.A. (1999b�� 1XPHULFDO� 0RGHOOLQJ� RI� 6LSKRQLF5DLQZDWHU�'UDLQDJH�6\VWHPV�±�7KH�,PSRUWDQFH�RI�$LU, Arthur, S., & Swaffield,J.A., (1999c), 1XPHULFDO� 0RGHOOLQJ� RI� WKH� 3ULPLQJ� RI� D� 6LSKRQLF� 5DLQZDWHU'UDLQDJH� 6\VWHP��The Proceedings of CIBSE : Building Services EngineeringResearch and Technology, Vol 20, No. 2.

3. Bowler, R. & Arthur, S, (1999), 6LSKRQLF�5RRI�5DLQZDWHU�'UDLQDJH� ±�'HVLJQ&RQVLGHUDWLRQV, Proc. Water Supply & Drainage for Buildings : CIB W62 1999,Edinburgh.

4. Sommerhein, P., (1999), 'HVLJQ� SDUDPHWHUV� IRU� URRI� GUDLQDJH� V\VWHPV� ProcWater Supply & Drainage for Buildings : CIB W62 1999, Edinburgh.

5. Bramhall, M & Saul, (1999a),� +\GUDXOLF� SHUIRUPDQFH� RI� VLSKRQLF� UDLQZDWHURXWOHWV�� Proceedings of the 8th International Conference on Urban StormDrainage, Sydney, Australia.Bramhall, M & Saul, (1999b),� 7KH� K\GUDXOLFSHUIRUPDQFH� RI� VLSKRQLF� UDLQZDWHU� RXWOHWV� UHODWLYH� WR� WKHLU� ORFDWLRQ� ZLWKLQ� DJXWWHU� Proc Water Supply & Drainage for Buildings : CIB W62 1999,Edinburgh.

6. Slater, J.A,. Cockerham G. and Williams, P.D., (1999)� /RVV�IDFWRUV�LQ�VLSKRQLFURRI�GUDLQDJH��Proc Water Supply & Drainage for Buildings : CIB W62 1999,Edinburgh.

7. May, RWP & Escarameia, M, (1996), 3HUIRUPDQFH� RI� VLSKRQLF� GUDLQDJHV\VWHPV�IRU�URRI�JXWWHUV, Report No SR 463, HR Wallingford.

8. British Standards BS 6367 : 1983, &RGH�RI�SUDFWLFH� IRU�GUDLQDJH�RI� URRIV�DQGSDYHG�DUHDV, BSI.

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The researchers at Heriot-Watt University remain grateful for the assistance given inprogressing this project by the following companies; Dales Fabrications Ltd, DallmerLtd, EPSRC, Fullflow Ltd, Geberit AB, HR Wallingford Ltd, Pick Everard, RoyalAcademy of Engineering, Scottish Record Office, Simona UK Ltd, Sommerhein ABand The Scottish Office – Administrative Services, Royal Mail Property Holdings andAshby Developments Limited.

216,7( (9$/8$7,21 2) $1 ,167$//(’ 6,3+21,&5$,1:$7(5’5$,1 ... · roof siphonic roof drainage...

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