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9.1 |
Drain and Culvert |
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The schematic details of U-shaped and trapezoidal drains and box culvert are shown in  Drawing No. 1. Drains and culverts shall be designed to be hydraulically adequate, structurally sound and geotechnically stable in accordance with the current codes, specifications and requirements. |
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9.2 |
Roadside Drain |
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All roadside drains shall be constructed in accordance with the standard Land Transport Authority's specifications shown in Drawing No 2 or such other drawings to be issued by Land Transport Authority or Board. The roadside drains shall be provided with false bottom as may be required by Board. |
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9.3 |
Transition |
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A transition is required where there is a change of drain cross-section. The purpose of a transition is to change the shape of flow and surface profile in such a manner that minimum energy losses occur and cross waves and other turbulence are reduced. This may be achieved using tapering walls with no sudden changes of cross-section. The minimum length of a transition shall be 1.5 times the width of the wider drain section. |
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9.4 |
Curve and Bend |
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9.4.1 |
The presence of curves or bends in drain alignment is sometimes unavoidable. Difficulties in design often arise because of the complexity of the flow around a curved path. A drain curve will increase frictional loss and lead to the danger of serious local erosion due to spiral flow. Hence, the radius of any horizontal curve shall be as large as possible, consistent with the general terrain, in order to reduce the super elevation of the water surface and preserve the freeboard. A horizontal curve shall have a minimum radius of 3 times the width of the drain channel. |
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9.4.2 |
The benching of the drain at the bend shall be configured to minimise sedimentation at the inner side of the bend. For this purpose, the dry weather flow channel at the bend shall be aligned towards the outer side of the bend, with the centre of the channel spaced at a quarter of the drain width from the outer cope of the drain. |
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9.5 |
Sump for Drain Intersections |
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A sump of sufficient size shall be provided where drains converge. The minimum internal width of the sump shall not be less than 1.5 times the width of the drain leading away from the sump. Drains shall enter the sump at angles less than a right angle and at different levels wherever possible. The invert level of the downstream drain shall be lower than the invert level of the sump so that no stagnant water will collect in the sump. |
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9.6 |
Maintenance Access for Drainage Reserve |
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9.6.1 |
The maintenance access within a Drainage Reserve shall be level and continuous. The requirements of the levelled maintenance access are shown in Drawing No. 3. If there is an approved structure overcrossing a Drainage Reserve, ramps (not steeper than 12%) shall be provided at both sides of the structure for the continuity of maintenance access. |
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9.6.2 |
Access from adjacent road to Drainage Reserve shall be provided. At the entrance of the access, detachable bollards or posts and chains shall be provided as shown in Drawing No. 4. |
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9.7 |
Markers along Edge of Drainage Reserve |
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If a Drainage Reserve is not fenced-out from the adjoining premises, concrete markers shown in Drawing No 4 shall be installed at maximum 50m spacings along the edge(s) of the Drainage Reserve to demarcate the Drainage Reserve line(s). Closer spacing may be necessary at curves/bends. |
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9.8 |
Drop-inlet Chamber and Slot-outlet |
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9.8.1 |
Runoff from road carriageways and carparks shall be effectively drained away to prevent water stagnation and to ensure road safety. Drop-inlet chambers (shown in Drawing No. 5 shall be provided at maximum 6m spacings along all road carriageways and concealed scupper drainage shall be provided in accordance with the Land Transport Authority's standard specifications. |
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9.8.2 |
At all T-junctions of roads with one or more of the connecting side roads sloping down towards the junction, drop-inlet chambers shall be provided at half the normal spacings or alternatively, slot-outlets with gratings shall be provided as shown in Drawing No. 5. Slot-outlet may also be used where an existing roadside drain is less than 500 mm deep or at areas outside the road carriageways such as carparks. |
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9.9 |
Safety Railings |
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Standard safety railings shall be provided for all open drains more than 1.0m deep. The railings shall be galvanised, painted and erected as specified in Drawing No. 6. |
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9.10 |
Special Railings for Park Connectors |
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If approved by the Board, special railings may be installed in conjunction with the development of park connectors or waterbodies. The railings shall be easy to maintain, safe and compatible with the surroundings. Special railings which have been approved for use are given in Drawing Nos. 7A & 7B. |
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9.11 |
Rungs in Drains |
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Non skid aluminium rungs shall be provided at 60m apart for open drains with internal widths equal to or less than 2m and at every opening/grating for closed drains with internal widths equal to or less than 2m. Rungs shall be embedded in the drain wall as shown in Drawing No. 2. |
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9.12 |
Grating over Closed Drain/Culvert |
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9.12.1 |
All gratings provided over closed drains/culverts shall be hinged to fixed frames securely embedded into the drain structures. Mild steel heavy duty gratings shall be used for closed drains subjected to vehicular loadings, whereas light duty gratings shall only be used for pedestrian loadings. Chequered plates shall be fixed on the pedestrian gratings for those closed drains narrower than 2m (internal width). The gratings, frames and chequered plates shall be galvanised. The details of the gratings and chequered plates are shown in Drawing No. 8, 9, 10, 11, 12. |
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9.12.2 |
Where a culvert runs across the road, no sump/grating shall be sited on the road carriageway. Where necessary, sumps with galvanised mild steel grating shall be provided at the two side-tables of the road. In the case of a dual carriageway, a sump with galvanised heavy duty mild steel grating shall be provided at the centre divider. |
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9.12.3 |
The size and spacing of gratings required shall be based on the internal width of the closed drain, as follows :- |