1) Flow Rates and Slotdrain Sizing
Elkington Gatic has been closely involved with designing surface water drainage systems in partnership with consulting engineers for many years. It has become increasingly apparent throughout this period of time that due to the number of variables applicable to each individual project, the information contained in published capacity charts provides only a very limited amount of useful data. Any combination of the following variables will have a direct and significant impact upon 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 developed the Slotdrain Interactive Design Software programme, a unique and extremely versatile surface water drainage design software tool. The design software enables all of the above criteria to be considered and provides comprehensive data sheets and CAD/pdf drawings for each individual run plus links to standard details for all channels and system and components.
2) Methods of Calculating the Hydraulic Capacity of Drainage Channels
The established and universally accepted methodology for calculating the hydraulic performance of linear drainage channels is the HR Wallingford Report SR581 'Hydraulic Capacity of Drainage Channels with Lateral Inflows'. The Gatic Slotdrain Interactive Drainage Design Software has successfully incorporated this methodology since its launch, during which time the results produced have been routinely verified to be accurate by many external engineers.
Whilst the HR Wallingford method continues to be the tried, trusted and preferred methodology used by Elkington Gatic to calculate the hydraulic capacity for Gatic Slotdrain, evidence has recently emerged that an alternative method is now being used to provide drainage designs for a particular channel drain system.
This lesser-known alternative methodology uses the Saint-Venant Equation, which was originally used to calculate flows in rivers and canals, but when used in the context of drainage channels, produces hydraulic capacities much greater than those produced by HR Wallingford. In practical terms, this means that when the Saint-Venant Equation is used to calculate channel drain sizes, for any given hydraulic inflow conditions, the result will often indicate that a much smaller size channel (average 30% smaller) will be suitable.
In the interests of choice, and to enable you to make a more informed decision in respect of which methodology you would prefer to incorporate into your design, we have upgraded our design software to offer both methodologies.
If you routinely utilize other general drainage design software programmes for designing the remainder of the drainage network, to avoid any issues of inconsistency between the hydraulic capacities generated by the two methodologies in the Slotdrain software programme and your general design software, we would recommend that you run a simple check. For example, a 225mm Slotdrain laid to a 1/100 gradient should give a similar result to a typical 225mm pipe laid to the same gradient.
3) Benefits of Step Fall Designs
Gatic Slotdrain is uniquely available in 7 channel sizes ranging from a small and compact 100mm wide channel through to an extremely high capacity 600mm wide channel. Whilst single size channel runs may at times prove to be more suitable, there are significant improvements in hydraulic performance, invert depths and total installed cost where Slotdrain designs incorporate step falls.
Designing a step fall system:
* Generates 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.* Minimi ses invert depths at the outlet point.* Reduces the amount of excavation and concrete surround required, by selecting the shallowest and most economical channel layout.
4) Spillage Interception from Forecourt and Fuelling Areas
Regulation 6(4) of DSEAR states 'it is the duty of the site operator to put in place measures to collect and safely retain any spillages of dangerous substances'. Gatic Slotdrain has been empirically tested at an operational filling-station forecourt by leading Fire Service petroleum officer. The results demonstrated that Gatic UltraSlot with the 30mm throat orifice (standard or treadsafe) is eminently suitable for use in all forecourt and fuelling areas, as it provides a continuous interception slot that prevents wash-over of surface liquids. Whilst the test demonstrated that a single channel coped with almost all of the spillage, a strong recommendation was given to incorporate a twin, parallel run of drainage channels to ensure full compliance with the above regulation even in the event of a partial blockage or build up of silt in the channels.
5) Roughness
Gatic Slotdrain is manufactured from 3m lengths of galvanised steel, which leads to lower frictional losses from both the surface and joints, giving improved flow performance over some rougher concrete based products, or products supplied in shorter lengths requiring a greater number of joints.
6) Self-Cleansing Properties of Gatic Slotdrain
The hexagonal profile of Gatic Slotdrain ensures that self-cleansing is improved even with the smallest amount of rainfall. 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 same well defined course, reducing the likelihood of siltation.
7) Watertightness
Gatic Slotdrain has been independently tested and certified by BSI to comply with the watertightness requirements of BS EN 1433:2002 - Drainage Channels for Vehicular and Pedestrian Areas. A test certificate is available upon request.
8) Procedure for the manual design of Gatic Slotdrain systems
If you do not have access to the Slotdrain Interactive Design Software, or would prefer to calculate or check Slotdrain sizes manually, please contact our technical department for a copy of our manual design procedure.
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