Surface Water Drainage

The width of a drainage reserve is based on the required drain size as shown in Appendix 1.

The drainage reserve shall be delineated as a separate plot in Urban Redevelopment Authority’s (URA) Written Permission to be subsequently vested or surrendered to State.

Land set aside as drainage reserve shall be kept free of all encumbrances

A drainage reserve may be realigned subject to the following conditions:

1. The proposed drain diversion is technically and physically feasible and acceptable to the Board; and

2. The developer/owner shall construct the proposed drain to the size as determined by the Board.

Under special circumstances, the developer/owner may be allowed the use of the drainage reserve subject to the following conditions:

1. The developer/owner shall allow the Board or the Board’s agent access to carry out drainage works within the drainage reserve as and when necessary;

2. The developer/owner shall not construct any structures or facilities on, over or under the drainage reserve;

3. The developer/owner shall be responsible for the physical maintenance of the land and storm water drainage system within the drainage reserve;

4. The developer/owner shall be responsible for the structural maintenance of the storm water drainage system within the drainage reserve;

5. The developer/owner shall be responsible for public safety within the drainage reserve;

6. The developer/owner shall at all times indemnify and keep indemnified the Government fully and completely against all liabilities of whatsoever nature and description which may be incurred or suffered by the Government in connection with or arising out of the use of the drainage reserve by the developer/owner and against all actions, proceedings, claims, costs and expenses therefrom;

7. The Board reserves the right to rescind the approval for the use of drainage reserve as and when required by the Board for drainage works and without the need for reinstatement of the affected facilities;

8. The developer/owner shall be responsible for safeguarding the structural integrity of the affected storm water drainage system within the drainage reserve when carrying out the work; and

9. The developer/owner shall reinstate or rectify any damage to the storm water drainage structures caused by their work at their own cost and to the Board’s satisfaction upon the completion of the work.

10. Any additional conditions deemed necessary by the Board.

The minimum platform level of a development site as specified by the Board is the required minimum ground level of that proposed development. The platform levels shall be indicated on the layout plans.

The minimum platform level required for a proposed development depends on its locality and the type of development, as categorised below:

1. General Developments

   The minimum platform level shall not be lower than:

   1. 4.0m above Singapore Height Datum for developments along the southern coast; and 4.5m above Singapore Height Datum for developments along the northern coast; or

   2. 300 mm above the adjacent road/ground level; or

   3. Any other level as may be specified by the Board; whichever is the highest.

   whichever is the highest.

2. Industrial/ Institutional/Commercial/Multi-Unit Residential Developments

   The minimum platform level shall not be lower than:

   1. 4.0m above Singapore Height Datum for developments along the southern coast; and 4.5m above Singapore Height Datum for developments along the northern coast; or

   2. 600 mm above the adjacent road/ground level; or

   3. 300mm above modelled flood levels (as advised by the Board) if the development is a CI or KI; or

   4. Any other level as may be specified by the Board;

   whichever is the highest.

3. Special Facilities and Developments with Direct or Indirect Linkages to Underground Special Facilities

   The minimum platform level shall not be lower than:

   1. 4.0m above Singapore Height Datum for developments along the southern coast; and 4.5m above Singapore Height Datum for developments along the northern coast; or

   2. 1 m above the adjacent road/ground level; or

   3. 300mm above modelled flood levels (as advised by the Board) if the development is a CI or KI; or

   4. Any other level as may be specified by the Board;

   whichever is the highest.

If, subject to approval of the Board, the minimum platform levels as specified in Clause 2.1.1 cannot be met, the following conditions (a) and (b) must be met:

1. The platform level of the building structure must be at least:

   1. 3.5 m above Singapore Height Datum for developments along the southern coast; and 4.0 m above Singapore Height Datum for developments along the northern coast; or

   whichever is higher

   and;

2. Flood protection measures must be included in the design to provide at least the same level of flood protection that the minimum platform and/or crest levels would provide for the building. Details of proposed flood protection measures should be submitted to the Board for approval. Implementation of the flood protection measures shall comply with the requirements stipulated in Clause 2.4. Notwithstanding the above, the Qualified Person shall advise the developer/owner to raise the platform level and/or crest level to the highest possible levels before considering the deployment of flood protection measures. In addition, the developer/owner shall be advised that the development site may be subjected to flood risks despite the implementation of flood protection measures.

1. All new development proposals including reconstruction proposals shall comply with the requirements for minimum platform levels except those listed in Clause 2.1.3(b).

2. The following proposals need not comply with the required minimum platform levels:

   1. Addition & alteration (A&A) works to an existing building where there are no direct and/or indirect linkages to underground special facilities;

   2. Partial reconstruction works to an existing building involving only the building of additional floors without reconstruction of the first storey and where there are no direct and/or indirect linkages to underground special facilities;

   3. Conservation projects;

   4. Temporary development works intended for use for a temporary period;

   5. Any other works as may be specified by the Board.

For these development proposals, the Qualified Person shall advise the developer/ owner that the minimum platform levels are still recommended as a protection measure to reduce flood risks. If lower platform levels are adopted, the developer/ owner shall be advised that the development site may be subjected to higher flood risks. The developer/owner should consider installing flood protection measures where possible (See Clause 2.4 and Clause 13).

1. Where the minimum platform levels for developments in low-lying areas are significantly higher than the adjacent road/ground levels, the Qualified Person may apply in writing to Board for approval to construct the ancillary areas at below the minimum platform levels so as to tie-in with the adjacent low-lying road/ground levels or to satisfy other planning and environmental considerations. Notwithstanding the above, the Qualified Person shall advice the developer/owner that the platform level adopted for the ancillary areas should be as reasonably high as possible.

2. Ancillary areas include:

   1. Entrance driveways;

   2. Bin centres;

   3. Turfed compound areas;

   4. Car porch/porches for single unit development(s); and

   5. Other areas as may be approved by the Board.

Crest level is required for all entrances, exits or openings to basement(s) or underground structures (e.g. tunnels, underground facilities). The minimum crest level required depends on the type of development, as categorised below:

1. Basements of General Developments

   The minimum crest level shall be at least 150 mm above the minimum platform level as specified in Clause 2.1.1(a).

2. Basements of Industrial/Institutional/Commercial/Multi-Unit Residential Developments

   The minimum crest level shall be at least 300 mm above the minimum platform level as specified in Clause 2.1.1 (b).

3. Special Facilities and Developments with Direct or Indirect Linkages to Special Facilities,

   The minimum crest level shall be at least 300 mm above the minimum platform level as specified in Clause 2.1.1 (c).

For land reclamation works, the minimum reclamation level shall not be lower than:

1. 4.0 m above Singapore Height Datum for developments along the southern coast; and 4.5 m above Singapore Height Datum for developments along the northern coast; or

2. Any other levels as may be specified by the Board.

The platform levels required for any development subsequently carried out on the reclaimed land shall be higher than the reclamation level and shall be separately determined by the developer/owner’s Qualified Person to ensure effective internal drainage of the development site.

Flood protection measures as stipulated under Clauses 2.1.2(b), 2.1.3(b) and 2.2 shall comply with the following requirements, including those stipulated in Clause 13 and Appendix 2:

1. General Development

   Flood protection measures shall be provided at all points of entry, exit and all openings (including windows, ventilation and services openings) for the building premises (at ground/platform level).

2. Industrial/Institutional/Commercial/Multi-Unit Residential Developments, Special Facilities and Developments with Direct or Indirect Linkages to Underground Special Facilities

   Flood protection measures shall be provided at:

   1. At all points of entry, exit and all openings (including windows, ventilation and services openings) for the building premises (at ground/platform level); or

   2. All points of entry and exit for the entire development area.

3. The developer/owner shall obtain the necessary clearance from relevant authorities to install the flood protection measures during the Development Control (DC) stage for approval.

4. Details of the flood protection measures and Standard Operating Procedure (SOP) should be submitted to the Board during Detailed Plan (DP) stage for record.

The QP shall advise the developer/owner on the need to identify power, communications, and other critical M&E equipment prior to the design of the development site.

Critical M&E equipment shall be located /installed at the minimum platform level or higher.

For critical M&E equipment located at the basement, the structural crest level for all entrances, exits and openings leading to the basement must comply with crest level requirements as stipulated in Clause 2.2.

A higher level of protection against flood risks is required for the underground Rapid Transit Systems, vehicular underpasses, road tunnels and their ancillary buildings. The principles and standards of protection are outlined in Appendix 3.

All developments with direct/indirect linkages to the underground Rapid Transit Systems, vehicular underpasses, road tunnels and their ancillary buildings shall have the same protection requirements as stated in Clause 2.1.1(c) and Clause 2.2(c). All development proposals (including addition & alteration (A&A) and reconstruction proposals) having such linkages shall be submitted to the Board for approval.

No drain shall be interfered with or altered without the approval of the Board.

All runoff from the development site shall be discharged into a roadside drain or an outlet drain and not into the adjacent premises. A minimum 600 mm high solid wall shall be erected along the boundary of the development site abutting adjacent premises to prevent surface runoff from overflowing into adjacent premises.

Where the runoff from the development site is discharged through an existing or proposed drainage outlet to the adjacent premises, the downstream drain at the adjacent premises must be able to cope with the runoff from the development site. The developer/owner may be required to upgrade the downstream drain to the required size as determined by the Board. The Qualified Person shall liaise with the downstream owner(s) before implementing the drainage proposal.

Where there is existing runoff flowing from adjacent premises or roads into or through the development site, the runoff shall continue to be allowed to discharge through the development site.

If a development site is sloping towards the adjacent premises, a drainage system shall be provided along the site boundary to prevent the runoff from discharging into adjacent premises.

The internal width and clear depth of a roadside drain (including entrance culvert) shall be at least 600 mm.

The provision of gratings, openings and chequered plates for closed drains shall comply with Clause 9.11.

Culverts Across Roads

Only box culverts shall be provided. The internal width and clear depth of a box culvert shall be at least 1.0 m. For culverts across expressways and semi-expressways, the minimum size shall be 2.0 m (width) x 1.5 m (depth).

Internal drains shall be designed with sufficient capacity and openings to intercept and discharge all runoff from the development site.

Internal drains shall be designed for safety and ease of maintenance.

Prior to discharging any storm water within the development site to the roadside/outlet drain, vertical grating(s) shall be installed at the outlet discharge point(s) of the internal drain(s) located within the development site. The height for the vertical grating shall not encroach into the drain freeboard.

Internal Conduit Drain

Where the open backyard area of each intermediate terraced development is not linked to the front yard by a corridor at ground level, a conduit drain for conveying surface runoff may run beneath the building itself. The conduit drain shall be at least 300 mm in diameter. The conduit drain shall be laid straight without any bend or sag in its alignment. At both the inlet and outlet ends of the conduit drain, maintenance sumps shall be provided with invert level drops of at least 150 mm deep. The conduit drain shall be fully supported along its entire length by (and not attached to) the ground beams of the building and shall form as part of the permanent structure of the building to ensure against settlement.

Drainage for Air Wells/ Courtyards

Conduit drains serving the air wells or courtyards shall be designed to cater for the runoff and shall be at least 150 mm in diameter. Maintenance sump(s) should also be provided.

The internal width of a common drain shall be at least 450 mm.

Where the existing common drain is located outside the existing boundary wall or fencing of the development site, the location of the existing or proposed boundary wall or fencing shall not be altered or realigned unless otherwise approved by the Board.

The size (width and depth) and the alignment of the proposed drain shall be determined and approved by the Board.

Pursuant to a development proposal, the developer/owner may be required to improve the existing drain such as the outlet drain flowing through the development site, the entrance culvert and the roadside drain.

No structure shall be constructed over the roadside drain or outlet drain without the approval of the Board. If approved, the drain under the structure shall be upgraded to the design requirements as determined by the Board.

Hydraulic Calculations

For developments of area exceeding 0.4 hectare, the hydraulic calculations for internal drainage system (including the catchment plan), shall be endorsed by a Professional Engineer and submitted to the Board for record.

Structural Calculations

The structural calculations and plans for the following categories of drains shall be endorsed by a Professional Engineer and submitted to the Board for record:

1. Drains within the drainage reserve; and

2. Closed roadside drains which do not follow the standard designs used by Land Transport Authority.

Basements, tunnels and other underground facilities shall as far as possible be designed to prevent any ingress of storm water. Where ingress of storm water cannot be completely prevented, the catchment contributing to the ingress shall be kept to the minimum and an adequate pumped drainage system shall be provided. The minimum design and operation criteria for the pumped drainage system shall be as follows:

1. A pumped drainage system of sufficient size and capacity, as described in Clause 10, shall be provided.

2. The catchment of underground facility shall be totally segregated from the surface water catchment. There shall be no direct connection from the internal drain(s)/pumped drainage system within the underground facility to the roadside drain(s)/outlet drain(s). This can be achieved by adopting, but not limited to the following design considerations:

   1. Adequate crest levels, as stipulated in Clause 2.2, at the entrances/ access points to the underground facility shall be provided before discharging any surface runoff collected within the development to the roadside drain(s)/outlet drain(s).

   2. Channel runoff from the roofs, rainwater downpipes and all premises at and above ground level into surface gravity drains, but not into the underground structures.

   3. Surface runoff collected within the underground facility shall be discharged to the internal drainage system at the ground level via a swan neck connection which complies with the minimum crest level requirement as stated in Clause 2.2. A typical section of the swan neck connection is shown in Drawing 1.

   4. Provide cut-off drains across the ramps or access ways to prevent surface water from entering underground facilities. Cut-off drains serving the underground structure catchment must not be linked to the surface water drainage system.

1. Proper drainage system, either temporary or permanent, shall be provided at all stages of reclamation works. All proposals of drainage works in connection with reclamation shall be submitted to the Board for approval.

2. Before the commencement of any reclamation works, drains affected by the reclamation shall be upgraded and extended to channel off the overland flows from the existing land upstream of the reclamation site.

3. All temporary drainage systems provided during the construction stage shall be cleansed and maintained regularly by the reclamation developer/owner to remove all obstructions, silt and rubbish until such time when the reclaimed land is developed and provided with a permanent drainage system.

4. The developer/owner shall maintain and manage the completed reclaimed land at all times to ensure that runoff is effectively drained away without causing drainage problems within the land or to the adjacent lots. The finished levels of the completed reclaimed land shall be formed in such a manner to ensure free surface water drainage and no localised ponding. All subsidence, low-lying areas and depressions shall be filled-up and graded to effect proper surface water drainage.

All outfall structures shall be extended under and beyond the sand bar area in an encased form and to discharge flow at below the low tide level. Where there is no sand bar formation along the shore, the outfall structure may be in an open or encased form subject to the approval of the Board. In all cases, the outfall shall extend adequately beyond the shoreline such that the sea-bed level is at least 500 mm below the outfall invert at the point of discharge. All outfall structures shall be protected against undermining and scouring effects.

No structure shall be constructed within a drain or drainage reserve.

Drain connection within a drainage reserve shall be closed to maintain the continuity of access along the drainage reserve. The closed drain or the slab and its supporting structure/foundation shall be designed for heavy vehicular loading in accordance with Land Transport Authority’s standards.

All embankments and structures shall be set outside the drainage reserve. They shall be stable and independent of the drainage structures.

The design of embankments and structures near a drain or drainage reserve must not take into consideration the passive resistance offered by the existing or proposed drainage structures. The embankments and structures shall be stable when excavations up to 1.0 m below the invert or proposed invert of the adjacent drain are carried out. In addition, the Qualified Person shall ensure that the embankments and structures can be built without affecting the drainage structures.

The structural and stability calculations for an embankment or a structure within 2 m from the edge of the drainage reserve shall be endorsed by a Professional Engineer and submitted to Board for record. The submission of such calculations shall not in any way absolve the Professional Engineer from his/her responsibility.

Drainage for earth retaining structures shall not be discharged to the adjacent premises.

1. Developments at higher ground

   1. For new developments, if an earth retaining structure is erected to raise the ground level of the development site above adjacent premises, runoff from the development site, including the sub-soil flow from weepholes of the earth retaining structure (where applicable), shall be drained away by cutoff drains situated along the earth retaining structure or other drainage system as proposed by the Qualified Person within the development site without overflowing into the adjacent premises.

   2. In addition, a cut-off drain shall also be provided along the top of the retaining wall to capture the surface runoff from the development and maintenance access shall be provided to facilitate access to the cut-off drains.

2. Developments at lower ground

   1. If the earth retaining structure is erected at a development site where the ground level is lower than adjacent ground, the earth retaining structure shall not in any way obstruct existing sub-soil and overland flows that are discharging through the development site from the adjacent premises.

   2. The flows shall be drained off by cut-off drains located along the top and bottom of the retaining wall within the development site or other drainage system as proposed by the Qualified Person. These cut-off drains shall be properly connected to an existing roadside/outlet drain and maintenance access shall be provided to facilitate access to the cut-off drains.

Where a structure is allowed to be located within a drainage reserve or drain by the Board, the following general technical requirements shall be complied with:

1. Drainage Capacity

   The affected drain must be hydraulically adequate as determined by the Board, otherwise it must be upgraded to the required size in conjunction with the proposal.

   No structure shall be built within the flow channel. Where intermediate support is necessary to be provided within the channel for technical reasons, the reduced discharge capacity must be compensated by suitable enlargement of the channel. Such support, where approved, shall not be located at the centre of the channel or within the dry weather flow channel but at a suitable location where it will cause least obstruction to the flow.

2. Structural Integrity

   The Board may require the drain affected by the structures to be upgraded in conjunction with the proposal. The proposed structures to be erected within the drain or drainage reserve shall be designed and constructed to be independently supported.

   All foundation structures/piers, in the vicinity of or within the drain or drainage reserve shall also be designed and constructed as independent structures from the drainage structures. These foundation structures/piers must be stable when excavations up to 1.0 m below the invert or proposed invert of the affected drain are carried out.

   For a proposed structure to be constructed over a drain or drainage reserve, the vertical clearance between the base of the proposed structure and the cope of the existing or proposed drain shall be at least 100 mm.

   The independent supports of the proposed structure must be located at minimum distance of 300 mm from the drainage structure(s) or at any other distance as specified by the Board.

3. Maintenance

   The proposed structure to be erected within the drain or drainage reserve shall not inhibit drainage maintenance work. The following provisions for maintenance shall be incorporated:

   1. For elevated structures (such as Rapid Transit Systems or road viaducts, pedestrian overhead bridges, buildings, etc.) over the drain or drainage reserve:

      Sufficient headroom, viz minimum 5.4 m vertical clearance between the base of the proposed elevated structure and the vehicular maintenance access within the drainage reserve, shall be provided. Any deviation shall be approved by the Board.

   2. For at-grade structures (such as road decks, pedestrian malls, linkways etc.) over the drain or drainage reserve:

      In addition to the provision of gratings and drains as specified in Clause 9.11 and Clause 9.13.4, access openings (4 m by 2 m) with 6 m wide lay-bys shall be provided within the development site but outside the road carriageway or mall. Requirements on the provision for maintenance openings and lay-bys shall be determined and approved by the Board.

4. Effective Drainage for Proposed Structure

   The proposed structure shall be provided with an effective drainage system to intercept and discharge the surface water runoff.

   Where a slab is built over a roadside drain to provide access to premises, runoff from the slab shall be effectively drained away without causing drainage problem on top of the slab and the adjacent areas.

No services, manholes or other services structures shall be laid within a drain or drainage reserve.

Where there is no drainage reserve, services, service manholes or other services structures shall be located at a minimum distance of 300 mm from the drainage structures.

Where services are approved to be laid within a drain or drainage reserve, the services shall be laid to undercross the drain/drainage reserve throughout the full width of the drain/drainage reserve with the following minimum clearance below the invert of the existing or proposed drain:

1. 1.0 m for lined drain;

2. 1.5 m for earth drain or

3. Other minimum clearance as may be specified by the Board.

Under exceptional circumstances, minor services may be laid to overcross the drain subject to the approval of the Board. The Qualified Person may apply in writing to the Board and substantiate the proposal with technical justification. The Board may require the affected drain to be reconstructed to the size as determined by the Board. A typical plan for minor services to overcross the drain is shown in Drawing No. 9.

Services located within a drainage reserve, if approved, shall be designed to withstand heavy vehicular loading in accordance with Land Transport Authority’s standards and with their own independent supports.

Where new drains are to be constructed or existing drains are to be reconstructed, all existing services within or across the drains and drainage reserves shall be re-laid at the cost of the developer/owner to undercross the new or reconstructed drains.

Services ducts may be incorporated in the top slab of the culverts, subject to the approval of the Board.

The execution of any work shall not change, disrupt, fill, block, divert or disturb the existing overland flow or the existing system of drains unless an alternative drainage system has been approved by the Board.

The runoff within, upstream of and adjacent to the worksite shall be effectively drained away without causing drainage problems within the worksite or in areas outside the worksite.

All earth slopes shall be set outside the drainage reserve.

No obstruction of any kind shall be placed, laid or erected within drainage reserves and drains unless authorised by the Board. If so, the drainage reserves and drains shall remain accessible at all times to workers and plant/equipment of, or authorised by the Board.

No temporary structures such as cofferdams, crossings, stagings etc. shall be constructed in or over an existing drain without the approval of the Board.

Any temporary structure if approved within a drain or drainage reserve shall be removed immediately on completion of the construction works or as directed.

Drainage inlets such as drop-inlet chambers and scupper drains shall not be blocked by any material or debris.

Drains shall not be covered without the approval of the Board.

Bunds of stockpiled materials such as earth from trench work shall not be longer than 10 m and gaps of at least 1 m width shall be provided between the bunds to allow the free flow of surface runoff.

Material from any stockpile or construction activities shall not be allowed to fall or be washed into the drain. Adequate preventive measures, including the provision of proper and stable barricades or screens where necessary, shall be provided.

Any damage to existing drain/drainage facilities arising out of any work shall be immediately reported to the Board. Any obstruction to flow arising from the damage shall be immediately and completely cleared. The damaged drain shall be repaired or reinstated to the requirements of the Board.

Any opening from the site that is not in use shall be properly sealed to prevent the escape of silty water out of the site area.

No services shall be laid or left (in conjunction with development works) within a drainage reserve. Where the services have to cross a drainage reserve, approval of the Board shall be obtained.

The discharge from any construction / earthwork sites into storm water drainage system shall not contain Total Suspended Solids (TSS) in concentrations greater than, the prescribed limits under Regulation 4 (1) of the Sewerage and Drainage (Surface Water Drainage) Regulations.

Developers / owners and Qualified Persons should include the earth control measures (ECM) specifications and schematic or conceptual ECM plans in the tender documents and allow for the cost of ECM in the tenders.

The site operator/contractor shall obtain approval from the Board and implement adequate ECM before the commencement of any earthwork.

The site operator/owner shall submit the detailed ECM proposal, endorsed by a QECP, to the Board to apply for an approval. The ECM proposals (with calculations) shall include a plan (a typical plan is as shown in Drawing No. 8).

The ECM shall be designed to cope with at least a storm of return period of 5 years.

The ECM shall be installed by the site operator/contractor according to the endorsed plans and the completed ECM at site shall be approved by the QECP before commencement of earthworks and construction works.

An effective ECM requires 2 components which shall include, but is not limited to, the following minimum measures in order to meet the legal requirements cited under Regulation 4(1) of the Sewerage and Drainage (Surface Water Drainage) Regulations:

(1) Erosion Control Measures

The erosion control measures shall minimise the extent and duration of any exposed/bare/erodible surfaces by:

1. Proper Work Sequencing - Proper construction staging and work sequencing shall be adopted to ensure that no large bare/ erodible surfaces are exposed for long duration.

2. Covering up of all bare / erodible surfaces - Bare surfaces (including earth stockpiles) shall be covered by concrete-lining, concrete-paving, erosion control blankets, close turfing or other suitable materials. Access roads within the site and at exit/entrance as well as the surfaces around the site facilities (such as office, fabrication and storage yards) shall be covered or paved. Work areas shall be covered with erosion control blankets or other suitable materials during rain or before work stops every day.

   Minimising of the bare erodible surfaces as mentioned above will reduce the volume of silty water to be contained and treated. For areas which have been paved/ covered, the clean surface runoff could be channelled directly into the drains.

3. Progressive and timely revegetation and stabilisation – All bare surfaces shall be restored immediately upon completion of work at every stage.

If a construction site has very little bare/erodible surfaces, the operator/contractor will have less difficulty in containing and treating the silty discharges as described in Clause 6.3.7 (2) below. For those areas within a construction site which have been paved/covered and will not cause silty discharge, it is possible to drain these areas directly into the storm water drainage system without treatment.

(2) Sediment Control Measures

The sediment control measures shall trap, contain and treat the silty discharges from within a construction/earthworks site (including rain, runoff, water from washbay, underground water at basement, etc.) by providing:

1. Perimeter Cut-off Drain – Perimeter cut-off drains shall be concrete-lined and adequate to capture and channel all runoff from the site to the holding pond/sump. For sites located above slope, a boundary wall of at least 600 mm high shall be provided along the entire perimeter of the site to prevent overflow onto adjacent properties.

2. Perimeter Silt Fence – Silt fences shall be erected along the perimeter cut-off drain (between the construction site and perimeter cut-off drain). The silt fence is to be embedded firmly into the ground and made from an approved geotextile filter fabric or equivalents to capture the sediment from stormwater runoff. The sediment built-up behind the silt fence must be regularly removed.

3. Intermediate Silt Trap – Intermediate silt traps of suitable size shall be installed at regular intervals along the perimeter lined cut-off drain. Within the intermediate silt traps, suitable geotextile filter fabric or equivalents shall be installed across the full depth and width and /or coagulation-assistance materials shall be placed. Silt traps relying primarily on hardcore, granite chips or sands for filtration, are not acceptable.

4. Holding pond/sump – All silty runoff shall be collected and channelled to ground holding pond/sump for treatment to the required water quality standard before discharging the runoff into the drain. All silty water shall be treated and discharged within 10 hours after the rainstorm so as to prepare the pond/sump for the next rainfall event.

   1. No processed/used water, such as slurry water and sullage water shall be discharged to or kept in the holding pond/sump.

   2. The holding pond/sump shall be located near the drainage outlet.

   3. Water level markings shall be provided within the holding pond/sump to indicate the level of water in the holding pond/sump and the depth of the holding pond/sump.

   4. The holding pond/sump shall have a storage capacity to cope with a storm of return period of 5 years.

   5. Cut-off drains could be used as part of the holding pond/sump. The capacity of the cut-off drains available to hold the silty runoff should be clearly tabulated, taking into consideration the site terrain. The wash bay sump shall not be included in the calculation for storage.

   6. For above ground holding tank design proposal, contractor shall provide justification that the system is able to function at all times, in particular, during heavy rain in the middle of the night. The justification shall include detailed calculations of number of pumps 23 and pump size, provision of redundancy to cater for maintenance and breakdown, positive suction head at the sump pit, provision of power supply for the automated system throughout the entire operation and the standby manpower as necessary.

   7. The above information, schematics and technical drawings shall be clearly enclosed in the ECM plan for the contractor to strictly adhere to. Contractor shall engage a QP (mechanical) to design and endorse the pumping system and monitor the performance and revise the ECM design accordingly.

5. Treatment System – Adequately-sized treatment system shall be installed to treat all silty surface runoff before it is discharged into the drains. The treatment system shall be sized to treat and empty the silty runoff water in the holding pond/sump within ten (10) hours after the rainstorm so as to prepare the pond/sump for the next rainfall event. Any other water such as used water, sullage water and slurry shall be handled separately and shall not be channelled to the holding pond/sump for treatment. The treatment system shall be installed, calibrated, commissioned, operated and maintained regularly according to the manufacturer’s specifications and maintenance manual. The treatment system shall be free from any material/substance that will affect the quality of treated water.

   1. The treatment plant treats the silty water at a prescribed flow rate. Treatment plants do not have holding capacity and shall not be included as part of the computation of total holding pond/sump capacity.

   2. The quality of discharge into public drain shall be continuously monitored by CCTV linked to the Silt Imagery Detection System (SIDS) for sites 0.2 hectares and above.

   3. It is strongly encouraged for the treatment system to be equipped with an automated intervention feature to prevent silty water exceeding legal limits from being discharged into public drain.

      Examples of automated intervention features include but are not limited to:

      • Automatic shutting down of treatment system; and

      • Automatic diversion of silty water to holding pond with a motorised valve.

6. Turbidity Curtains - For works that are within or adjacent to the water bodies, such as canals, rivers, sea or reclamation works, turbidity curtain(s) shall be installed along all the exposed slopes/riverbanks. The silt within the turbidity curtain(s) shall be removed regularly and disposed off accordingly to the relevant regulations.

7. Wheel wash – Used water from wheel wash shall not be discharged directly to the drain. The silty water within the wash bay could be channelled into the holding pond/ sump.

8. Minimal or No Discharge - Recycling of water should be practised at construction/earthwork sites. The recycled water could be used for non-potable purposes in order to minimize discharge into the drain.

9. Site Hoarding – Gap at the base of site hoarding shall be properly sealed to prevent the outflow of silty water from the site.

The site operator/contractor shall inspect the conditions of the ECM regularly and during/after every rain event.

The site operator/contractor shall arrange for his QECP to carry out regular audit/review for every stage of the earthworks and construction works, and also revise/amend the ECM at site according to the advice of the QECP. All inspection/audit/review reports shall be kept on site and made available to the Board upon request.

The site operator/contractor shall inspect the conditions of the ECM regularly and during/after every rain event.

The site operator/contractor shall arrange for his QECP to carry out regular audit/review for every stage of the earthworks and construction works, and also revise/amend the ECM at site according to the advice of the QECP. All inspection/audit/review reports shall be kept on site and made available to the Board upon request.

The ECM implemented on site shall be checked and maintained regularly to ensure that the ECM remains effective throughout the whole duration of works. This shall include:

1. Replacing of silt fences and erosion control blankets

2. Re-paving of worn-out concrete surfaces

3. Replacing of membrane modules

4. Removal of silt accumulated in drain and the holding pond/sump

5. Removal of silt accumulated at the silt fence and beside the boundary wall

6. Removal of sludge accumulated in the ECM treatment plant

The site operator/contractor shall monitor the discharge water quality as cited in Clause 6.3.1 before it enters the roadside/ outlet drain(s).

The site operator/contractor shall for this purpose, provide a monitoring instrument and CCTV system upon request by the Board. The CCTV system if installed, shall be positioned at the drain so that it is able to view the treated water flow from the discharge outlet(s) along with the upstream of the drain clearly. The CCTV image quality shall be able to distinguish the clear water and the silty water clearly.

The site operator/contractor shall keep the CCTV in operations and link it to the Silt Imagery Detection System (SIDS) at all times upon request by the Board.

The site operator/contractor shall promptly respond to the SIDS alerts and take immediate remedial actions for abnormalities captured by the alerts.

The site operator/contractor shall submit regular reports (including photographic and monitoring records) of the site ECM as well as those for discharge quality upon requested by the Board.

All ECM shall not be removed before the completion of work. The site operator/ owner shall inform the Board prior to removal of the ECM on completion of the project.

1. The approval of the Board shall be sought for all earthworks affecting the existing drainage system or the flow pattern of surface runoff. In the event that earthworks are to be carried out adjacent to major drains and canals, the developer/owner shall engage a Qualified Person to carry out soil investigation works and slope stability analysis to ensure that the earthworks will not cause any damage to the drainage structures.

2. The developer/ owner shall take all necessary steps and measures, such as implementing suitable soil stabilisation methods and temporary protection works to ensure the stability and structural integrity of the existing drainage structures.

3. Earth control measures shall be provided in accordance with the requirements as stipulated in Clause 6.3.

A temporary drainage system shall be provided for all roadworks to prevent flooding or water ponding on the road. Where the road is being raised and the new road drainage outlets are at a higher level than the existing ones, the existing outlets shall not be blocked unless temporary alternative drainage system has been provided. The developer/owner and the Qualified Person shall be responsible for implementing all necessary measures to ensure that the site is kept flood free at all times. Where necessary, the Board may require the submission of hydraulic calculations, programme of works, temporary drainage arrangements at various stages of the roadworks, etc. by the Qualified Person.

All reclamation works shall be carried out in accordance with Clause 2.3 and 4.10.

During reclamation, a temporary drainage system including perimeter cut-off drains, silttraps, silt fences and outlet connections shall be provided. The temporary drainage system shall include erosion protection linings, such as concrete, geotextile mattress and other suitable materials to be approved by the Board. The proposals of the temporary drainage system shall be submitted to the Board for approval.

The temporary drainage system shall be cleansed and maintained regularly by the reclamation agency to remove all obstructions, silt and rubbish during the reclamation period.

The Rational Formula shall be used to compute the peak runoff:

where Qr = peak runoff at the point of design (m3 /s)

C = runoff coefficient

I = average rainfall intensity (mm/hr)

A = catchment area (hectares)

The runoff coefficient (C) depends on the degree and type of development within the catchment. Catchments are classified according to the expected general characteristics when fully developed. The C values are as follows:

For a storm of return period (T) years, the rainfall intensity (I) is the average rate of rainfall from such a storm having a duration equal to the time of concentration (tc). The rainfall intensity (I) can be obtained from the Intensity-Duration-Frequency (IDF) curves by estimating the duration of rainfall (equals to the time of concentration, tc) and selecting the required return period of (T) years. The Intensity-Duration-Frequency (IDF) curves for Singapore Island are shown in Appendix 4.

The return periods (T) adopted for the design of drainage systems in Singapore shall be as follows and may be subjected to any other specific requirement imposed by the Board:

The peak runoff (Qr) occurs at the point of design when all parts of the catchment receiving a steady, uniform rainfall intensity are contributing to the outflow at this point. This condition is met when the duration of rainfall equals the time of concentration (tc). The time of concentration (tc) consists of the overland flow time (to) plus the drain flow time from the most remote drainage inlet to the point of design (td), viz. tc = to + td.

The overland flow time (to) varies from 5 minutes to 15 minutes, depending on the overland travel distance, land topography and characteristics. The drain flow time (td) shall be estimated from the hydraulic properties of the drainage channel.

The following types of developments are required to control the peak runoff discharged from the affected development sites:

1. New erection and reconstruction works to all developments greater than or equal to 0.2 hectares in size; and

2. Additions & Alterations (A&A) works to all existing developments where affected area is greater than or equal to 0.2 hectares in size such as:

   1. Addition of new building(s); or

   2. Extension to existing building(s); or

   3. Partial reconstruction of existing building(s);

   4. Any combination of the above

The maximum allowable peak runoff to be discharged to the drains will be calculated based on a runoff coefficient of 0.55, and for design storms with a return period of 10 years and for various storm durations of up to 4 hours (inclusive). Peak runoff reduction can be achieved through the implementation of ABC Waters design features and/or structural detention and retention features, such as:

1. Detention tanks/drains;

2. Retention/sedimentation ponds;

3. Wetlands;

4. Bioretention swales;

5. Bioretention basins or rain gardens;

6. Porous pavements, etc.

Details (calculations and/or hydraulic model results) showing how the proposed system meets the required peak runoff rates shall be submitted and endorsed by PE (Civil). For systems that include ABC Waters design features to meet the required peak runoff rates, the details must be endorsed by an ABC Waters Professional, who is also a PE (Civil).

The general requirements for detention tank systems are outlined in Appendix 6.

For design guidance on detention tank systems, QPs can refer to the Technical Guide for Onsite Stormwater Detention Tank Systems, available on the PUB website.

For design guidance on the ABC Waters design features, QPs can refer to the ABC Waters Guidelines and relevant chapters in the Engineering Procedures, available on the PUB website. Due consideration shall be given to meeting ABC Waters storm water quality objectives, which will often require treatment of storm water runoff using ABC Waters design features.

Drains are designed for steady uniform flow conditions and one-dimensional method of analysis is used.

Drains shall be designed to have discharge capacities (Qc) adequate to cope with the estimated peak runoffs (Qr). The size, geometry and the bed gradient of a drain determine its discharge capacity (Qc). With the required discharge capacity (Qc) determined [which must be equal to or larger than the peak runoff (Qr)], the size of the drain is computed from the Manning's Formula:

where Qc = discharge capacity of drain (m3 /s)

n = roughness coefficient A = flow area (m2 )

P = wetted perimeter (m)

R = A/P = hydraulic radius (m)

S = bed gradient

The value of the roughness coefficient (n) depends on the drain's flow surface and is given below:

The velocity of flow in a drain shall not be lower than 1.0 m/s for self-cleansing action to take place. However, the flow rate during dry weather may fall to a low level where this minimum velocity cannot be achieved. The problem can be solved by introducing a small channel in the drain to confine the dry weather flow to a smaller flow section. The dimensions of such a dry weather flow channel is given below or unless otherwise stated by the Board. The design of Type C3 channel is as shown in Drawing No. 1.

The velocity of flow in a drain shall not be too great to cause excessive scouring or hydraulic jumps. Hence the velocity of flow in a concrete-lined drain shall be limited to a maximum of 3.0 m/s or below the critical velocity, whichever is lower. For an earth stream, the maximum velocity shall be limited to 1.5 m/s. Further limitation of the maximum velocity shall be complied with when specified by the Board.

Drains are designed to carry sub-critical flows. Critical state of flow exists when the Froude Number is equal to one. An open channel flow at or near the critical state shall be avoided as under such a condition the water surface is unstable and wavy. In order to secure greater flow efficiency, channel flow shall be designed so that the Froude Number shall fall within the range from 0.8 decreasing to such minimum value as to achieve a practical flow depth and permissible flow velocity.

Freeboard refers to the depth from the top of the drain (cope/bank) to the top of the water surface in the drain at design flow condition. Sufficient freeboard shall be provided to prevent waves or fluctuation of the water surface from overflowing the cope/bank. Generally, a depth of freeboard equivalent to 15% of the depth of the drain is required.

As part of the Active, Beautiful, Clean Waters (ABC Waters) Programme, integration of adjacent watercourses with proposed developments is encouraged, whilst satisfying the engineering requirements for drainage functions without posing any public safety, maintenance or environmental problems. These include, but are not limited to:

1. Enhancing the accessibility or connectivity to waterbodies/waterways, eg. creating community spaces for adjacent to watercourses and providing boardwalks to bring people closer to water, etc.;

2. Providing aesthetic treatment to waterways, eg. use of a more natural finish such as block pitching or imprints on canal walls, planting along the canal walls, etc.;

3. Installing ABC Waters Design features to detain and treat storm water runoff closer to the source (see Clause 8.2 for further details).

The designs of the aesthetic waterbodies, waterways and safety barriers need not be constrained by the standard features as shown in this Code of Practice as long as these facilities fulfill the drainage requirements and do not give rise to any public safety, maintenance or environmental problems.

The approval of the competent planning authorities shall also be sought for all such proposals. Where appropriate, the aesthetic waterbodies and watercourses shall be integrated into the development proposals and be maintained by the developers/owners of these developments.

Qualified Persons together with the developer/owner are strongly encouraged to enhance the development by adopting appropriate ABC Waters design features as described in the latest version of “ABC Waters Design Guidelines” and “Engineering Procedures for ABC Waters design features” published by the Board (available at https://www.pub.gov.sg/abcwaters/designguidelines). These guidelines were developed based on the following principles:

1. Reduction of runoff and peak flow from the development site by implementing local ABC Waters design features that provide detention measures and minimise impervious areas;

2. Improvement of quality of water draining from the development into receiving environment. For example, through effective filtration and retention measures via ABC Waters design features, runoff from the site can be treated to remove pollutants and silt, thereby protecting the water quality in waterways downstream;

3. Integration of stormwater treatment into the landscape by incorporating multipleuse ABC Waters design features that also maximise the aesthetics and recreational amenities of developments; and

4. Protection and enhancement of natural water systems within the development site.

The developer/ owner shall engage an ABC Waters (Active, Beautiful, Clean Waters) Professional to plan, design, oversee the construction of, and develop a maintenance plan for the ABC Waters design features.

For developments of area exceeding 0.4 hectares, DC submissions for ABC Waters design features, comprising the following items, should be submitted to the Board as part of DC submission:

1. Hydraulic calculations for the ABC Waters design features, endorsed by an ABC Waters Professional who is a PE (Civil). The calculation shall be accompanied by completed relevant design templates in Excel format, available on the PUB website.

2. Catchment/location plan of ABC Waters design features endorsed by an ABC Waters Professional who is an Accredited Landscape Architect or a registered Professional Architect.

3. Design drawings and maintenance checklists endorsed by an ABC Waters Professional of any profession.

If there is any change to the catchment or design of the ABC Waters design features subsequent to DC approval, QP shall submit a DC amendment with the full set of updated items (a), (b) and (c) above with respective endorsement and a summary table for all the amendments..

For ABC Waters design features that are designed to be part of the measures to comply with Clause 7.1.5, the developer/owner shall maintain the ABC Waters design features regularly to ensure that these features are well maintained and remain effective in serving the functions as stipulated in Clause 7.1.5.

All U-shaped drains and box culverts shall be designed to be hydraulically adequate, structural sound and geotechnically stable in accordance with the current codes, specifications and requirements.

All roadside drains shall be constructed in accordance with the standard Land Transport Authority's specifications shown in Drawing No. 1 or such other drawings to be issued by Land Transport Authority or the Board. The roadside drains shall be provided with false bottom as may be required by the Board.

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.

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.

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.

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 connecting into the sump shall not form an acute angle with the outgoing drain flow and shall connect 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.

Drain connection shall not join an existing drain at an angle that is against its flow. Invert level of the drain connection shall be as high as hydraulically possible and must not be lower than the benching level of the drain receiving the flow. It is a good design practice to consider a minimum vertical distance of 300mm between the invert of the drain connection to the benching level of the drain receiving the flow.

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.

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.

If a drainage reserve is not fenced-out from the adjoining premises, concrete markers shown in Drawing No. 4 shall be installed at maximum 50 m spacings along the edge(s) of the drainage reserve to demarcate the drainage reserve line(s). Closer spacing may be necessary at curves/bends.

Runoff from road carriageways (including viaducts) and carparks shall be effectively drained away to prevent water stagnation and to ensure road safety. Drop-inlet chambers and concealed scupper drainage shall be designed in accordance with the Land Transport Authority's latest Standard Details of Road Elements and shall be provided at maximum 6 m spacings along all road carriageways. Intermediate sump shall be provided if the length of the concealed scupper drainage exceeds 6m. For curves and bends, inlet openings shall be provided at every 3m interval. For flood prone areas, hotspots or any other areas as specified by the Board, enhanced dropinlet chambers (shown in Drawing No. 5) shall be provided.

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 every 3m or alternatively, slotoutlets as specified in Land Transport Authority’s latest Standard Details of Road Elements shall be provided. Slot-outlets 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.

The size of openings and discharge pipe into the storm water drainage system has to be at least 250 mm diameter. The discharge pipe that is constructed has to avoid 90 degree bend for discharge towards downstream.

Standard safety railings shall be provided for all open drains more than 1.0 m deep. “Type A” railings as specified in Land Transport Authority’s latest Standard Details of Road Elements shall be installed for new open drains. Otherwise, railings as specified in Drawing No.6, to match the existing railings on site, shall be installed when requested by the Board.

Non skid aluminium rungs shall be provided at every opening/grating for closed drains with internal depth equal to or greater than 0.9m. Rungs shall be embedded in the drain wall as shown in Drawing No. 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. The gratings, frames and chequered plates shall be galvanized. The details of the gratings and chequered plates shall be designed in accordance with the Land Transport Authority’s latest Standard Details of Road Elements.

For drains with internal width greater than 4m, access opening (4m x 2m) shall be provided as shown in Drawing No. 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 gratingsshall be provided at the two sidetables 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.

The size and spacing of gratings required shall be based on the internal width of the closed drain, as follows:

Where a closed drain exceeds 3 m deep, access shaft (2 m by 1.5 m) may be required by the Board. If the access shaft is deeper than 4 m, intermediate platform shall be provided as shown in Drawing No. 3.

Where an entrance culvert/crossing is proposed at a stretch of closed drain, gratings/ openings shall be provided at the closed drain sections upstream and downstream of the proposed entrance culvert/ crossing. The gratings shall be designed in accordance with the Land Transport Authority's latest Standard Details of Road Elements.

Under the exceptional circumstance where the entrance culvert/crossing is shallower than 600 mm or when requested by the Board, one number of hinged open gratings as per Drawing no. 7C shall be installed in the middle of the entrance culvert/crossing.

In the design of drains, stability of the slope and upheaval shall be considered. A nominal live load surcharge of 10 kN/m2 shall be taken into consideration in the design of drains except as qualified by Clause 9.13.2 and Clause 9.13.4.

For drains that are adjacent to roads and are affected by vehicular loading, a live load surcharge of 20 kN/m2 shall be taken into consideration in the design of drains.

Culverts carrying vehicular loading shall be designed to withstand bridge loading in accordance with Land Transport Authority’s standards.

For drains that are required to install 4 m x 2 m grating as stipulated in Clause 5.4.cii, the drains shall be designed to withstand bridge loading in accordance with Land Transport Authority’s standards.

The minimum design and operation criteria for the pumped drainage system shall be as follows:

1. The pumping capacity shall be adequate to cater for immediate discharge of the storm water ingress of not less than 150 millimetres per hour from the entire source catchment area; i.e.:

   

   where P = pumping capacity (m3 /s)

   I = rainfall intensity (mm/hr)

   A = catchment area contributing to ingress of storm water (m2 )

2. The minimum standby pumping requirement depends on the type of development, as categorised below:

   1. General Developments

      The minimum standby pumping requirement including standby pumps and pumping mains shall be at least N duty + 1 standby. “N” refers to the number of pumps to achieve the design pumping capacity.

   2. All Type of Developments except for General Developments

      The minimum standby pumping requirement including standby pumps and pumping mains shall be at least N duty + N standby.

      The pumped drainage system shall be supported by a generator should the main power supply fail.

3. All pumping installation shall be designed with an automated device to start the pumping operation at times of storm water ingress, with operational option for manual control to override the automated device whenever desired.

4. Adequate pump sump shall be provided with sufficient storage capacity to cater for the total quantum of inflow from the entire source catchment area over a duration of at least 3 hours or such longer period as may be deemed necessary by the Qualified Person or as required by the Board for the re-activation of the pumping installation in the event of emergency breakdown/repairs or power failure, based on the maximum recorded rainfall given below:

   Duration (hrs)	3	4	5	6	12	24
   Total Rainfall (mm)	196.9	210.6	253.4	281.9	376.7	533.2

5. The base of the pump sump shall be designed with a gradient of 1:40 or steeper, and which shall be graded towards the pumps. The pumps shall be located within a small sump pit which should be deeper than the pump sump so that there will be no stagnant water in the pump sump at all times.

The criteria specified above are minimum requirements which shall be complied with. Nevertheless, the Qualified Person shall be fully responsible for the complete design of the pumped drainage system, incorporating such additional features or requirements as the Qualified Person may deem necessary to achieve reliable protection of the basements, tunnels or underground facilities against flood.

The civil and structural components of the pumped drainage system shall be designed and endorsed by a Professional Engineer (Civil) while the mechanical and electrical components shall be designed and endorsed by a Professional Engineer (Mechanical/Electrical). Design computations duly endorsed by the Professional Engineers shall be submitted to the Board for record, including the operation sequence and monitoring measures of the pumped drainage system and other relevant information.

The Qualified Person shall formulate a well-regulated procedure for the maintenance, operation and monitoring of the pumped drainage system. The developer/owner shall be responsible for the maintenance, operation and monitoring of the pumped drainage system. The Qualified Persons shall liaise with the developer/owner to ensure that a wellestablished management set-up is operational to undertake this function before applying for the issue of Temporary Occupation Permit and Certificate of Statutory Completion. Upon obtaining Temporary Occupation Permit (TOP), the Developer/Owner/Managing Agent/MCST/Town Council shall make annual declarations and submissions that the inspections to storm water drainage system including flood protection measures as stipulated in Clause 13.2 have been carried out.

The Board may in special cases introduce amendments or additional requirements for the design and operation of the pumped drainage systems.

QPs are required to declare that their platform, crest levels and maximum allowable peak runoff from the development are in compliance with the Code of Practice on Surface Water Drainage and constructed in accordance with approved plans when applying for Temporary Occupation Permit (TOP) clearance.

The declaration shall consist of the application for TOP clearance and be supported by as-constructed survey plans indicating:

1. the crest levels, platform levels (based on the approved flood protection measures);

2. common drain;

3. pump drainage system for basement;

4. structural detention and retention features and/or ABC Waters design features, if they are used to satisfy the detention requirement as stipulated in Clause 7.1.5; and

5. any other relevant information as required by the Board prepared and endorsed by a Registered Surveyor.

Submission requirements for “As- Constructed” drawings for structural detention and retention features and/or ABC Waters design features are shown in Appendix 5.

The Board will only issue TOP clearance to the developer/ owner when the declaration and all necessary supporting documents are submitted and assessed to be in compliance with the requirements of approved plans and the Code of Practice on Surface Water Drainage.

Drainage works shall be constructed in accordance with the approved drainage plans, the Sewerage and Drainage Act (Chapter 294), the Sewerage and Drainage (Surface Water Drainage) Regulations and this Code of Practice on Surface Water Drainage.

1. The Qualified Person shall comply with the relevant sections of the Sewerage and Drainage Act (Chapter 294).

2. The Qualified Person shall be fully responsible for the supervision and construction of drainage works based on the approved plan.

3. The Qualified Person shall notify the Board of any proposed deviation of works from the approved plan and obtain approval for the deviation by submitting drawings showing the amendments before proceeding with the works.

4. The Qualified Person shall, if necessary, notify/apply directly to all the relevant authorities such as the Land Transport Authority (LTA), National Parks Board (NParks) and National Environmental Agency (NEA) before carrying out drainage works and shall abide by the conditions stipulated by the relevant authorities.

On completion of drainage works/facilities, the Qualified Person shall certify and submit to the Board a Certificate of Inspection for Drainage Works.

On completion of drainage works/facilities, the Qualified Person shall submit the “As-constructed” drawings and piling records to the Board for record. “As-constructed” drawings for the following categories of drainage works/facilities need to be submitted:

1. Drains and approved structures within drainage reserve

2. Roadside drains

Submission requirements for “As- Constructed” drainage drawings are shown in Appendix 5.

A survey plan shall be submitted if a development or structure is constructed within less than 300 mm from a drainage reserve. The survey plan shall consist of a site plan and cross-sections showing the development or structure and lot boundary in relation to the drainage reserve. The survey plan shall be prepared and endorsed by a Registered Surveyor.

Drainage works to be handed-over to the Board shall have a one year Defects Liability Period. The Defects Liability Period shall commence from the date the Board gives no objection to the issue of the CSC. The developer/owner shall pay for the cost of rectification of defects in the drainage works during the Defects Liability Period.

Upon the expiry of the Defects Liability Period, the Qualified Person shall arrange for the handingover of the drainage works to the Board after all the necessary or required rectification works are satisfactorily completed.

The Developer/Owner/Managing Agent/MCST/Town Council shall be responsible for the maintenance, operation and monitoring of the storm water drainage systems within their premises. These include, but are not limited to, regular adequacy checks on the flood protection measures such as minimum crest levels, detention/retention pond systems and pumped drainage system.

Upon obtaining the Temporary Occupation Permit (TOP), the Developer/ Owner/ Managing Agent/ MCST/ Town Council shall make annual declarations and submissions for the following storm water drainage systems including flood protection measures via PUB website- Business & Professional (B&P) Portal:

Any change in original design or specifications for flood protection measures, the Developer/ Owner /Managing Agent /MCST /Town Council shall engage a PE (Civil and/or Mechanical) to endorse the design and ensure they are able to meet the flood protection requirements stipulated in the Code of Practice on Surface Water Drainage, and the Board should be informed of the change.

1. During Development Control (DC) stage

   For all developments with flood protection measures, the following documents, endorsed by a Qualified Person, shall be submitted:

   1. Proposed plans indicating the crest levels and/or platform level with and without flood barrier at various entrance and exit points;

2. During Detailed Plan (DP) stage

   For developments where flood barriers are automated and/or with more than 2 points of entry/ exit, or when requested by the Board, the following shall be submitted:

   1. Proposed size, type and design of the flood barrier.

   2. Details on the standard operating procedure (SOP) including the proposed maintenance plan of the flood barrier to be endorsed by a PE (Civil and/or Mechanical), and the developer/ owner.

3. During Temporary Occupation Period (TOP) stage

   For all developments with flood protection measures, the following documents, endorsed by a Registered Surveyor, shall be submitted together with the declaration that their platform and crest levels are in compliance with the Code of Practice on Surface Water Drainage and constructed according to approved plans:

   1. As-constructed survey plans indicating the crest levels, platform levels and the flood protection levels (based on the approved flood protection measures).

4. Certificate of Statutory Completion (CSC) stage

   For developments where automated flood barriers are implemented, the following documents, endorsed by a PE (Civil and/or Mechanical), shall be submitted:

   1. Leak Test Certification on the installed flood barrier, in compliance with relevant international standards or any requirements specified by the Board.

5. Maintaining the integrity of Storm Water Drainage System

   Upon obtaining Temporary Occupation Permit (TOP), the Developer/Owner/Managing Agent/MCST/Town Council shall make annual declarations and submissions that the inspections to storm water drainage system as stipulated in Clause 13.2 have been carried out.

Underground Rapid Transit Systems, vehicular underpasses, road tunnels and their ancillary buildings must be stringently protected against flood risks. The salient principles and standards of protection to be complied with are outlined below:

1. STRINGENT CATCHMENT SEGREGATION

   [i.e. segregating the Underground Rapid Transit Systems, vehicular underpasses, road tunnels and their ancillary buildings from surface catchments and runoffs]

   • Absolutely minimise the storm catchments of the Underground Rapid Transit Systems, vehicular underpasses, road tunnels and their ancillary buildings (such as by means of profile designs and roofing), and channel all at-grade and above-grade runoffs (eg. that from the roofs) into surface drains segregated from the underground facilities.

   • Segregate the Underground Rapid Transit stations, tunnels, vehicular underpasses, road tunnels and their ancillary buildings by means of integrated water-tight barriers of at least 1.3 m above flood and ground levels.

   • All ingress and egresses to the Underground Rapid Transit Systems, vehicular underpasses, road tunnels and their ancillary buildings (including pedestrian/traffic linkages and ventilation/services openings) are to be built with a segregation threshold of at least 1.3 m above road and ground levels.

2. RELIABLE PUMPED DRAINAGE SYSTEM

   [for removal of any water ingress that cannot be cut-off owing to constraints inherent in the Underground Rapid Transit System's, vehicular underpasses, road tunnels and their ancillary buildings interface with the surface tracks and linkages]

   • Install an active pumping capacity equivalent to the 100-year return storm intensity corresponding to the time of concentration of the storm catchment (excluding the internal infiltration route of the tunnel surface) as determined from the Code of Practice on Surface Water Drainage, capped at 280mm/hour for a typical 10-minute concentration duration.

   • Provide a bypass opening directly between Underground Rapid Transit Systems, vehicular underpasses, road tunnels and their ancillary buildings (including pedestrian/traffic linkages and ventilation/services openings) and storage tank below without any intermediate gratings

   • Provide duplicated standby pumping equipment, including back-up pumps and pumping mains.

   • Install standby fuel generators for uninterrupted operation of the pumping facilities.

   • Construct pump sumps with an active operational storage capacity of 280mm (for 6-hour inflow) where sumps are accessible during train operations or 530mm (for 24-hour inflow) where sumps are inaccessible during train operations.

   • Institute a well-regulated maintenance and operation procedure for the pumped drainage facilities.

     The Pumped Drainage System (PDS) shall be fully automated with a supervisory monitoring system in place to allow operators to remotely monitor the water level in the pump sump and critical operation status of the pumps such as “start”, “stop” and “tripped”. An independent water level sensor, dedicated to detect the extreme high sump water level shall be in place. This extreme high sump water level shall be monitored by the supervisory monitoring system and a siren shall be triggered upon detection.

3. DEVELOPMENT CONTROL OF ALL CONNECTED DEVELOPMENTS

   [such that the same principles and standards of flood protection in (A) & (B) above are applied to all developments/facilities with existing/proposed linkages to the Underground Rapid Transit System, vehicular underpasses, road tunnels and their ancillary buildings (including pedestrian/traffic linkages and ventilation/services openings)]

   • Carry out advanced, comprehensive planning of all proposed linkages to the Underground Rapid Transit System.

   • Implement the same flood protection safeguards mentioned above for all such developments.

4. OPERATIONAL AND MANAGEMENT CONTROL

   [to ensure continued functioning and safeguarding of the protection measures implemented under (A), (B) & (C)]

   • Have in place a comprehensive map/database of the overall underground networks with linkages to the Underground Rapid Transit System vehicular underpasses, road tunnels and their ancillary buildings (including pedestrian/traffic linkages and ventilation/services openings.

   • Subject all building and development planning (including Addition & Alteration and reconstruction) of such linked developments/facilities to the vetting and control of the Land Transport Authority.

   • Institute stringent operational and management measures to be undertaken by the owners of the linked developments/facilities under the supervision of Land Transport Authority.

1. The “As-Constructed” drawings shall be submitted in CAD and PDF format. The “As-Constructed” drawings shall be endorsed by a Professional Engineer and, where applicable, also by a Registered Surveyor.

2. The “As-Constructed” drawings shall be prepared in SVY21 coordinates. Note of survey such as date of survey, datum, coordinate system accuracies and tolerances, as stipulated in

   • Standard and Specifications for 3D Topographic Surveying (Mapping) in Singapore

   • Standard and Specifications for Utility Survey in Singapore

3. The “As-Constructed” drawings shall show the following details:

   1. Alignment and width of the drainage reserve.

   2. Size and type of the constructed drain within the drainage reserve.

   3. Longitudinal section showing the gradient, invert and cope level of the constructed drain.

   4. Cross-section of the constructed drain in relation to the drainage reserve/lot boundary/retaining wall/boundary fence.

   5. Locations, sizes and types of all drain connections.

   6. Piling records for drains and structures within the drainage reserve and roadside drains including entrance culverts. The piling records shall include the extent of piling, sizes, types, lengths and spacing of the piles. Reasons shall be stated if no piling is carried out.

   7. Details of structures within drainage reserve such as bridge, crossing over drain, pedestrian crossing including the geographic co-ordinates.

   8. Geographic co-ordinates (X and Y co-ordinates) of drainage appurtenances (e.g. gratings, drop inlet chambers, scupper mouth, scupper drain, railings etc.)

   9. Structures (including piling records) beside drainage reserve, as and when required by the Board.

4. The “As-Constructed” drawings for structural detention and retention features (including drains that are used fully/partially for detention) shall show the following details:

   1. Site plan indicating the type, dimension and volume of the constructed structural detention and retention features in the development site

   2. Longitudinal section showing the gradient, invert and cope level of the constructed structural detention and retention features

   3. Cross-section of the constructed detention tank(s)/drains showing the pumping facilities & swan neck pipe (if applicable), the effective depth of the structural detention and retention features including the connection points to the internal and public drainage system

5. The “As-Constructed” drawings for ABC Waters design features should include the following details:

   1. Site plan for ABC Waters design features with surrounding ground levels, invert level of the features, top levels, dimensions and invert levels for overflow sumps etc.

   2. Cross-section/profiles of the features with levels and dimensions

   3. Locations, levels and dimensions of overflow sumps, check dam, flushing pipes and other ancillary (as applicable).

   4. Longitudinal profiles (for swales) showing the gradient, invert and cope levels

   5. Geographic co-ordinates (X and Y co-ordinates) of all ABC features and accompanying appurtenances (e.g. railings, decks, seating, gabion walls, lightings, play equipment etc.)

6. Records of piles installed shall be submitted in Excel sheet format in accordance with the forms stipulated in BCA, Building Control & Management, Record Structural Plan and Test Records.

(A) DESIGN REQUIREMENTS

1. All access/openings into the detention tank shall be located at the ground level.

2. All types of discharge system shall be designed to empty the tank within 4 hours after a storm event to ensure that the detention volume is available for the next storm event.

3. An overflow structure shall be incorporated in the design of detention tank system to allow drainage of the site in the event that the detention tank system malfunctions (e.g. the orifice clogs or a power outage disables the pumps) or is completely full. The overflow structure shall be sized for a maximum allowable peak discharge based on a runoff coefficient of 0.55.

4. For detention tank with pumped discharge system, the minimum design and operation criteria shall be as follows:

   1. The maximum operating pumping capacity shall be less than the maximum allowable peak discharge.

   2. All pumped discharge systems shall be designed for automated operation of the pumping system, with an option for manual control to override the automated system when required.

   3. The pumped discharge system for detention tank shall not be combined with the pumped drainage system required for the drainage of underground facilities/ basements as stipulated in Clause 4.9.

   4. There shall be minimally one standby pumping equipment, including standby pumps i.e., (N duty + 1 Standby)¹.

   5. The pumped discharge system shall be supported by a generator should the main power supply fail.

   6. The pumped discharge system shall discharge storm water from the detention tank into the internal drainage system at the ground level via a swan neck connection which complies with the minimum crest level requirement as stated in Clause 2.2. Direct pumping discharge into the roadside/outlet drain is not permissible.

   7. The pumps shall be located within a small sump pit which should be deeper than the pump sump so that there will be no stagnant water in the pump/ discharge sump at all times.

   ¹For pumped drainage system designed for underground facilities, the minimum standby pumping requirement is stipulated in Clause 10.1 (b) and dependent on the type of development.

   8. The QP shall formulate a well-regulated procedure for the maintenance, operation and monitoring of the pumped discharge system.

   9. The criteria specified above are minimum requirements which shall be complied with. Nevertheless, the QP shall be fully responsible for the complete design of the pumped discharge system, incorporating such additional features or requirements as the QP may deem necessary to ensure the detention tank system is able to perform as required during a storm event.

5. The Developer/Owner shall be responsible for the maintenance, operation and monitoring of the detention tank system. The QPs shall liaise with the Developer/Owner to ensure that a well-established management set-up is operational to undertake this function before applying for the issue of Temporary Occupation Permit and Certificate of Statutory Completion.

6. For more information such as design, operation and maintenance considerations for detention tank systems, QPs can refer to the Technical Guide for On-Site Stormwater Detention Tank Systems, available on the PUB website.

(B) SUBMISSION REQUIREMENTS

1. During DC submission stage, the following documents, endorsed by a QP, shall be submitted:

   1. Proposed drainage plans indicating catchment and sub-catchment boundaries. If more than one detention tank is required, the plans should indicate clearly the specific sub-catchment(s) of each tank including the outlet discharge point of the internal drainage system to the public drain.

   2. Proposed site plan with clear indication of the following details:

      1. Runoff coefficients and area of development with varying characteristics of catchment/subcatchment;

      2. Proposed structural detention and retention features and the catchment/sub-catchment area of each feature, to attenuate stormwater runoff to comply with COP requirements;

      3. Proposed location and footprint of the detention tank(s), pumping facilities (if applicable), the effective depth of the detention tank(s) and connection point to the internal drainage system. For a detention tank that is located in the basement and is operated with a pumped discharge system, the plan should indicate clearly the location(s) of the swan neck connection (showing the crest level) of the pumped discharge system.

2. During DP submission stage, the following documents, endorsed by a QP, shall be submitted:

   1. Detailed drawing plans and sections of the detention tank system, clearly indicating the inlet and outlet configuration and levels, connections to upstream drainage network and downstream internal and external drains.

   2. Design calculations or modelling results.

   3. Details of the Standard Operating Procedure (SOP) on the operation and maintenance of the detention system (including pumped discharge system, if applicable).

   4. Details of the proposed pumped discharge system (pump capacity, crest level of the swan neck, power requirements), if applicable.

   The civil and structural components of the pumped discharge system shall be designed and endorsed by a Professional Engineer (Civil) while the mechanical and electrical components shall be designed and endorsed by a Professional Engineer (Mechanical/Electrical). Design computations duly endorsed by the Professional Engineers shall be submitted to the Board, including the operation sequence and monitoring measures of the pumped discharge system and other relevant information.

3. During TOP/CSC stage, the following documents, endorsed by a QP, shall be submitted:

   1. Certificate of inspection on the detention tank and pump discharge system (if applicable).

   2. Written declaration by the QP that the maximum stormwater discharge from the development is in compliance with the maximum allowable peak runoff stipulated in the COP and constructed according to approved plans.

   3. Written confirmation by the QP that he has liaised with the Developer/Owner to ensure that a Maintenance/Managing Agent has been established to undertake the SOP of the maintenance, operation and monitoring of the detention tank system.

   4. As-built drawing plans of the detention tank systems, as-built survey plan and final design calculations indicating the detention systems were constructed in accordance with the approved plans. The as-built survey plan shall be prepared and endorsed by a registered surveyor.

4. Upon obtaining Temporary Occupation Permit (TOP), the Developer/Owner/Managing Agent/MCST/Town Council shall make annual declarations and submissions that the inspections to storm water drainage system including flood protection measures as stipulated in Clause 13.2 have been carried out.