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Stormwater Drainage: A Runoff Conveyance System Design

posted Sep 10, 2021, 3:50 AM by jeffery jim   [ updated Sep 10, 2021, 4:51 AM ]

Another inquiry was made today in regards to stormwater drainage design and its conveyance system. In the previous short note on drainage, I spoke briefly about stormwater drainage and its function. Let us recap on the previous short note where the drainage system that we usually have are meant to mitigate risk coming from flash flood and hence, design usually considers a period of 180 minutes.

2. To many people, drainage system looks like the easiest part of civil engineering and most of the time this particular branch of engineering is taken for granted. In actual fact, drainage and its conveyance system is not as easy as most think especially when it involves large development or in town planning for a proposed large township. Similar goes to drainage involving agricultural canal and development at elevated or undulating areas. Further explanation of differences shall be based on experience involving in all of these kinds of projects.

3. Unlike structural design which require only a single software for design, drainage design need more than just stormwater design software. The most cumbersome and strenuous part of stormwater design and hydrological engineering is in deriving and generating unique data which changes from one region to another or worst, its temporal and spatial variability within a large site. The design starts based on LiDAR and remote sensing in determining the topographical and geological features based on proformas before placing entry into GIS software for geostatistical analyses in order to see water spatial movement and flow intensity map. Subsequently waterbodies, water shedding areas, aquifers and subsurface water drainage are considered for hydrological cycle. These values are then checked to gauged water level for validation process and tested during hydrological modelling. Once model is tested for its accuracy in hydraulic modelling software, it shall be the base for modelling and simulation as calibrated model. Only with a calibrated model, a proper peak discharge can be considered based on probable maximum precipitation.

4. Probable maximum precipitation may need a lot of statistical analyses by using Gumbel’s Distribution or L-Moment Ratio Diagram in order to generate an accurate rainfall frequency model for ungauged site. Besides this, one can refer to IDF parameters from guideline from nearest weather station when designing for rainstorm from IDF relationship in order to keep life easier unless requested by the client. From these, we can generate both Intensity Depth Frequency or IDF and Depth Duration Frequency (DDF). There are software out there with pre-build database for several weather station however the accuracy of the region precipitation and micro climate may not suit the actual site condition. Therefore, customization is required by overwriting or reprogramming the duration for longer hour precipitation which requires the change of hydrograph block duration. The peak storm should also include the consideration for areal reduction factor for larger catchment area (which is more than 100km2) and climate change factor (based on projected value provided in the guideline or climatology analyses). After all parameters were derived, Q value can be generated based on rainfall duration and Peak Q value can be established for pre-construction. The model is then modified based on post construction surface condition and the introduction of drainage conveyance system in order to generate Peak Q value for post construction. When both values are compared based on maximum of 50 years or 100 years ARI, the value of post construction should be smaller than pre-construction.

5. Else, remodelling is required by making changes in town planning level or by introducing more stormwater structural mitigation facilities or even reroute before reaching the confluence point or the discharge point in order to avoid sudden surge of torrent at estuary or at floodplain which are common areas with dense population. Toward the end of this modelling process, the best outcome from drainage size and length as well as on-site detention facilities will be adopted for drainage design. It is quite tedious as the whole large area is divided to smaller parcels or catchment areas based on specific development condition and criteria.

6. For smaller areas when it is less critical, it is sufficient to use Rational Method instead of SWMM or SCS method which need a lot of consideration involving many parameters and considerations for time of consideration. Peak Q value is generated by multiplying coefficient, intensity and area of development.

7. For agricultural drainage or conveyance canal, the most important criteria are similar to the processes I have mentioned earlier with exception where net water demand and gross demand with certain level efficiency shall be provided depending on the irrigation stage; pre-saturation or normal stage. The only additional structural facility available in this kind of project is the water gate in order to control the water on main season and off-season.

8. There are various types of drain design which can be use depending on the site condition and the required volume of conveyance with at least one foot of freeboard. Nevertheless, the introduction of interceptor drain may need specific consideration if it involves overflowing runoff from adjacent land especially from high hills. It is sufficient to allow for high intensity rainfall based on Rational Method for interceptor drains but it is best to use Hydrograph as this method will include critical parameters especially on overland slope gradient and flow length. Other drain design which require due care and consideration would be the horizontal drain and subsoil drainage. These should allow for subsurface drainage and geological mapping to ensure these types of drain is free from aquifer or is not part of waterbodies which may take time to discharge and desaturate. Instant desaturation can trigger loss of mass and sudden settlement and changes in slope equilibrium.

9. Culvert construction is another design which needs a lot of geotechnical considerations and have to be constructed on stabilized soil and preferably have a very low settlement when tested with plate bearing test. This is essential as small differential settlement can trigger tension crack on subgrade on top of the culvert and destroy the integrity of the subgrade after water infiltration This will cause the road to undergo degradation and leads to rutting or sudden depression on machine direction. It would be best to have the (concrete surround) culvert sitting on treated soil via geotextile. The crux to this practice is to ensure uniformity of level when soil starts to consolidate over time under heavy traffic loading.

10. I hope this short note will give you some insights on stormwater drainage design and the complexity of modelling which involves GIS, geology, geotechnic, geohydrology, hydrology and other soil pedology in deriving an accurate calibrated model. As a comparison, structural engineering is less strenuous with one design software, probably hundreds of hours in drafting and months of headache when integrating MEP into structural drawings. That may not be the case in a turnkey project when all in-house consultants are on BIM (AutoCAD suite) and sharing information across the server.