HYDRAULIC & DRAINAGE SYSTEMS DESIGN

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1) Flow Rates and Slotdrain SizingElkington Gatic has been closely involved with designing surface water drainage systems in partnership with consultingengineers for many years. It has become increasingly apparent throughout this period of time that due to the numberof variables applicable to each individual project, the information contained in published capacity charts provides only avery limited amount of useful data. Any combination of the following variables will have a direct and significant impactupon the surface water drainage design:

geographic location rainfall intensity and duration natural gradients on the site length of each channel run and outlet position effects of adjoining runs or rainwater inlets permeability of surface material specific local environmental requirements Stormwater Attenuation

In view of these factors, Elkington Gatic, in conjunction with a leading drainage hydraulics consultancy, has developedthe Slotdrain Interactive Design Software programme, a unique and extremely versatile surface water drainage designsoftware tool. The design software enables all of the above criteria to be considered and provides comprehensive datasheets and CAD/pdf drawings for each individual run plus links to standard details for all channels and systemcomponents.

To arrange a demonstration and free installation of the software please call +44(0)1304 203545.

2) Benefits of Step Fall DesignsGatic Slotdrain is uniquely available in 7 channel sizes ranging from a small and compact 100mm wide channel throughto an extremely high capacity 600mm wide channel. Whilst single size channel runs may at times prove to be moresuitable and appropriate, there are significant improvements in hydraulic performance, invert depths and total installedcost when Gatic Slotdrain designs incorporate a Stepped Fall channel layout.

Designing a step fall system: Creates an 'installed gradient', causing the channel invert to be steeper than the ground slope Optimises the use of smaller channels running much closer to capacity Provides greater self-cleansing velocities Increases flow capacities, often resulting in a reduction in channel sizes Achieves the shallowest invert depth at the outlet point Reduces the amount of excavation and size of concrete surround required for channels by selecting the shallowest

and most economic channel layout.

Hydraulic & Drainage Systems Design

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3) Procedure for the manual design of Gatic Slotdrain systemsIf you do not have access to the Slotdrain Interactive Design Software, or would prefer to calculate or check Slotdrainsizes manually, please refer to the detailed notes on page 32

4) Slotdrain Inlet CapacityThe inlet capacity of each type of Gatic Slotdrain channel is greater than would be required with any realistic level of rainfall in any location across the world, despite the fact that slot widths are 10mm and 30mm respectively. The table below shows the inlet capacity for channels withboth the 10mm and 30mm slot width. Extensive testing of Gatic Slotdrain regarding channel intake capacity has been carried out by an independent drainage andhydraulics consultancy company. More comprehensiveresults are available on request.

* Flow rates determined independently by CRM Rainwater Drainage Consultancy Ltd

To conclude, even with the smallest intake capacity figure shown above, this demonstrates that both the 10mm and30mm slot width of Gatic Slotdrain channels will cope easily with any realistic level of rainfall in any region of the world. Therefore, in all cases, capacity of the Slotdrain channel will be determined by the flow carrying capability of thehexagonal channel, not the inlet capacity.

Width of Slot

p y p

Inlet Capacity (l/s per linear metre)*

10mm 8.5

30mm Treadsafe 14

30mm Standard 14

Hydraulic & Drainage Systems Design Continued

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5) Spillage Interception from Forecourt and Fuelling AreasRegulation 6(4) of DSEAR states 'it is the duty of the site operator to put in place measures to collect and safely retainany spillages of dangerous substances'.

Gatic Slotdrain has been empirically tested at an operational filling-station forecourt by a leading Fire Service petroleumofficer. The results demonstrated that Gatic UltraSlot with the 30mm throat orifice (standard or treadsafe) is eminentlysuitable for use in all forecourt and fuelling areas, as it provides a continuous interception slot that prevents wash-overof surface liquids. Whilst the test demonstrated that a single channel coped with almost all of the spillage, a strongrecommendation was given to incorporate a twin, parallel run of drainage channels to ensure full compliance with theabove regulation even in the event of a partial blockage or build up of silt in the channels.

Copies of the full Fire Service Test Report are available upon request.

6) RoughnessGatic Slotdrain is manufactured from 3m lengths of galvanised steel, which leads to lower frictional losses from boththe surface and joints, giving improved flow performance over some rougher concrete based products, or productssupplied in shorter lengths requiring a greater number of joints.

7) Self-Cleansing Properties of Gatic SlotdrainThe hexagonal profile of Gatic Slotdrain ensures that self-cleansing is improved even with the smallest amount ofrainfall. The v-shaped channel invert ensures that at low flows, water is channelled into a narrower cross section,increasing the flow depth and thus effective hydraulic gradient. Additionally, the flow is always running in the samewell defined course, reducing the likelihood of siltation.

8) WatertightnessGatic Slotdrain has been independently tested and certified by BSI to comply with the watertightness requirements ofBS EN 1433:2002 - Drainage Channels for Vehicular and Pedestrian Areas. A test certificate is available upon request.

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Hydraulic & Drainage Systems Design - Stormwater Attenuation

Stormwater Attenuation and SuDS

AttenuationWith the ever increasing development of greenfield sites, pressure on the storm drainage system has reached crisispoint. Over the past few years, the Environment Agency, SEPA and their partners have pushed forward the concept ofmaking drainage sustainable, by replicating natural processes (SuDS). The EA, SEPA and planning authorities now lookfor a drainage solution on most sites that will:

Reduce the quantity of water flowing from site Improve the quality of water flowing from site Provide an amenity on site for people and wildlife

On most heavily developed sites (such as retail or distribution) it is only realistically possible to provide solutions to thefirst two of the key objectives, but this still provides enormous environmental benefits over a traditional approach.

Designing for SuDSThere are many and varied ways of incorporating stormwater attenuation requirements into a drainage scheme.Unfortunately, many of the options available are:

Complex for the engineer to design and specify accurately Complicated and time consuming for the contractor to install correctly Often difficult or practically impossible to access, maintain and remove silt build-up

Gatic Slotdrain offers an excellent, proven solution to all of these issues, providing all parties involved with a simple,effective and highly efficient method of storage and flow regulation.

Gatic Slotdrain can be utilised to provide an efficient and effective component of the SuDS strategy for any hardsurfaced site. In most cases, a discharge restriction will be placed on site, which can limit flow to nearly seventy timesless than the amount the site would have discharged with a traditional system. This reduced flow means that a largeamount of the surface water on the site must be attenuated (stored for a period and slowly released). By utilising largerGatic Slotdrains, surplus capacity can be generated, which can be used to provide attenuation very close to the sourceof run-off. A Gatic Slotdrain system can be designed to cope quite easily with the attenuation requirements for mostsmall to medium sites. Whilst it may not be large enough to provide the full site attenuation requirements for very largedevelopments, it can provide a valuable component of the storage network, particularly when positioned close to thesource, where it can be used to store the peak short duration run-off, enabling pipe sizes across the site to be reduced.

Flow RegulationIn order for water to be attenuated on site, a flow control must be introduced to slow the passage of water. There are anumber of ways of controlling flow but the most suitable for a channel drain system is an orifice plate. The orifice plateis simply a sharp edged hole in a plate of metal, but its hydraulics are very well proven and well understood. By carefully designing the size of the hole, very accurate maximum flow levels can be established.

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Calculating the Outlet Orifice SizeThe size of the outlet orifice is precisely calculated using the Slotdrain Interactive Design Software (available to engineers upon request) to exactly match the permissible discharge for each site. Furthermore, the softwarecontains a unique 'Optimisation' tool that enables the designer to maximise utilisation of each controlled run, making itpossible to reduce discharge rates further, which in turn may facilitate reductions in outlet pipe sizes and possibly thesize of any fuel interceptors required.

Performance and Maintenance of the Flow RegulatorThe Gatic Slotdrain flow regulator utilises a semi-circular sharp edged orifice, located at the invert position of theSlotdrain to ensure that in normal flow conditions there are no obstacles or restrictions to the flow. In the unlikely event of a blockage, an overflow weir, proportional to the outlet orifice is positioned at the top of the flow regulator.The regulator can be installed at any point where the Slotdrain discharges into an access, outlet or silt box, and thusflow can be regulated at any point on site. This is particularly useful on sloping sites, where water can be retained at alllevels of site, rather than just at the bottom.

The Flow Regulator is in all cases neatly positioned on the inside wall of the appropriate outlet unit, enabling easy visualinspection and maintenance access (see pages 49-52).

Storm ceases, controlled outflow continues and water level in theSlotdrain system falls.

Water level in the Slotdrain system rises as storm intensity increases.Outflow reaches designed regulated flow rate without flooding the

surrounding area.

Rainfall starts, the Slotdrain begins to fill with water and dischargesvia the flow regulator.

Dry weather conditions.

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Hydraulic & Drainage Systems Design - Channel Sizing

Procedure for the design of Gatic Slotdrain1) Calculate the area of the catchment in m2.

2) Apply a permeability factor to get an impermeable area (Ap) in m2.

Suitable figures for area multipliers are shown in the table below:

3) Decide upon a design rainfall rate (r) in l/s/m2.

4) Calculate required flow from impermeable area x rainfall rate (Ap . r) in l/s.

5) If the Slotdrain is to be used for conveyance of flow from other sources, such as roof drainage downpipes, then addthe sum of the point inflows to the required flow.

6) Determine the required length of channel (L) in m.

7) Determine the ground slope along the length of the channel (S) as a %. The maximum value for S should not exceed3.3% and S should not be adverse to the direction of flow along the channel.

8) Decide upon the design loading for the channel. If F900 loading is used, decide upon the slot type (Standard orTreadsafe). Hence select a channel range that is to be used for the design.

9) Decide if a stepped channel design can be incorporated; if so, proceed to step 16 in the list.

Single Size Channel Design10) Calculate the slope factor b using the following rules:

for S __0.5% b = 0.132S - 0.00022

for 0.5% < S _ 3.33% b = 0.00044

11) Select a suitable channel from the range. When full, the water in the channel will be just below the supports in theslot. Determine the maximum depth (h) in m and the channel cross sectional area (A) in m2 for the chosen channel.

12) If L/h exceeds 1000 then select a larger channel and repeat from step 11, or reduce the channel length and repeatfrom step 6.

13) Calculate the channel capacity from:

14) If the required flow calculated at step 5 is larger than the channel capacity calculated at step 13, select a largerchannel and repeat from step 11.

Material

Concrete 1.0Roofing materials 0.95Tarmac 0.90Asphalt 0.85Block/stone paving - cement joints 0.80Block/stone paving - open joints 0.60High permeability materials (gravel etc)

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15) If the required flow calculated at step 5 is smaller than 40% of the channel capacity calculated at step 13, it may bepossible to use a smaller channel. To check, select a smaller channel and repeat from step 11.

Stepped Channel Design16) First time through, use the smallest channel in the range and perform the calculations in steps 10-13.

17) If the required flow calculated at step 5 is less than the channel capacity calculated at step 16, proceed to step 20,otherwise select the next channel in the range.

18) Calculate the imposed slope given by the sum of the steps in the invert divided by the channel length (-step.100 /L) as a %.

19) Add the channel slope calculated at step 7 to the imposed slope calculated at step 18 to give S and repeat thecalculations from step 16.

20) Calculate the flow per unit length of the channel from the flow divided by the channel length (Ap.r /L) in l/s/m.

21) First time through, use a 3m length of channel (Lx =3m) or any other length that the designer considers may beclose to the finished design length.

22) Calculate the flow in the channel from Lx times the flow per unit length calculated at step 20. Add any point inflowsthat occur in channel length Lx from the upstream end to get the design flow.

23) Using the slope calculated at step 19 and hence the last calculated value of b, and the values of A and h for the sizeof channel being considered, calculate the flow in the channel from:

24) If the required flow calculated at step 22 is less than the channel capacity calculated at step 23, then increase Lx by3m (or any other length that the designer considers may be close to the finished design length) and repeat fromstep 22.

25) Determine the length of channel at the current size by subtracting the previous length of channel from Lx.

26) Use the next size unit in the design until all sizes set at step 17 have been allocated. Increase Lx by 3m (or anyother length that the designer considers may be close to the finished design length) and repeat from step 22.

Key

AP impermeable arear rainfall rate (l/s/m2)L length of channel runS ground slope along length of channelb coefficienth depth from invert to bottom of Slotdrain throatA cross sectional area of channel (excluding throat)Q flow capacity of channelLx part length

CONTENTSUnique FeaturesRange of Channel Types

Product RangeGatic PaveSlotGatic UniSlotGatic UltraSlotAccess, Outlet & Silt BoxesAccessoriesCatch Pit and Drop ConnectorsFlow Regulators

Hydraulic & Drainage System DesignFlow Rates and Slotdrain SizingBenefits of Step Fall DesignsDesigning a step fall systemProceedure for the manual design of Gatic Slotdrain systemsSlotdrain Inlet capacitySpillage Interception from Forecourt and Fuelling AreasRoughnessSelf-Cleansing Properties of Gatic SlotdrainWatertightnessStormwater Attenuation

Structural Design & Application DetailsSingle Block Concrete Base and SurroundUltraSlot - Protective Surface LipUltraSlot - Concrete Surround Anchor PointCoefficient of ExpansionExpansion Joints & Crack Inducers in Concrete SurfacesApplication DetailsTypical PaveSlot Channel in block pavingTypical UniSlot Channel in an asphalt surface150mm PaveSlot Section Plan and Details150mm UniSlot Section Plan and DetailsTypical UniSlot Channel in concreteTypical UltraSlot Channel in concrete600mm UniSlot Section Plan and Details225mm UltraSlot Section Plan and DetailsSmall Access box at the head of runSmall Access Box at a junction or at any point along a runLarge Access Box c/w double triangular gratingSmall Outlet Box positioned at the end of a runLarge Outlet Box positioned at the end of a runSmall Outlet Box with rainwater pipe connctionSmall Silt Box c/w 300mm deep removeable silt bucketLarge Silt Box c/w 300mm deep removeable silt bucketSilt Box with trapped outletEnd Cap positioned at the head of a short channel runEnd Cap outlet c/w spigot to accept standard pipe couplingsDrop ConnectorSmall Outlet Box with 225-300mm Drop ConnectorPre-formed CornersChannel Pipe500mm Short LengthsSlotdrain Width/Radius chartUltraSlot F900 Access PointCatchpit ConnectorsUltraSlot F900 Access Point Section Plan & DetailsStandard Catchpit Connector DetailDetail showing connection of UniSlot or PaveSlot channelsFlow RegulatorFlow Regulator DetailsFlow Regulator fitted into a constructed catchpit or manholeFlow Regulator fitted into a constructed catchpit or manhole DetailsSpecification Clause

Handling, Installation & MaintenanceHandling, Transit and UnloadingSitework and InstallationChannel Throat Protection StripsInstallation of Silt, Outlet and Access BoxesExcavation RequirementsUnit PreparationSetting the unit in placeConnecting Slotdrains to BoxesConnecting Outlet Pipes or Rainwater Inlet Pipes to BoxesPlacing the frame and cover/gratingBack-filling with concreteInstallation of Flow Regulators

Installation of Slotdrain ChannelsPAVESLOTSetting and Bedding the channelConcrete PourSetting Adjacent Paving MaterialsLaying Consecutive Slotdrains

UNISLOTSetting and Bedding the ChannelIn full concrete surroundIn Asphalt SurfacesSurface FinishingIn Paved SurfacesLaying Consecutive Slotdrains

ULTRASLOTMethod 1 - Laying on a concrete bed. Setting and bedding the ChannelOptional use of Anchor pointsConcrete PourLaying Consecutive SlotdrainsMethod 2 - SuspendedPreparing and positioning the channelOptional use of Anchor PointsFirst Concrete pour Second Concrete PourExpansion Joints and Crack inducersCutting Slotdrain ChannelsForming a RadiusInspection and Maintenance of Slotdrain Systems

Index