Project Drainage System and RC Pipe Design: Components, Pipe Sizes and Standards (2026)

Flooding within a project is a problem owners face every rainy season, and fixing it afterwards costs many times more than designing it right from the start, because the whole drainage system is buried under the roads — any fix means tearing up the road. This article is written for project owners, contractors and engineers laying out housing estates, factories or project sites that need a completely new drainage system.

What Does a Project Drainage System Consist Of?

A project stormwater drainage system has 4 main components: collection pipes or channels, manholes, main collector routes and an outfall to a public waterway. All of them work together by gravity, so levels must be designed to flow in one direction across the whole project.

• Drain pipes / drainage channels: collect rainwater from road surfaces and house roofs through roadside inlets. Typical projects use circular RC pipes buried under the road shoulder or footpath, while open channels/U-channels suit areas that need easy cleaning.
• Manholes: points where pipes join and change alignment, trap sediment and provide access for maintenance. They are typically placed about every 10–15 m, as required by the local authority, and at every point where the pipe bends or changes size.
• Main collector routes: pipes grow larger as the accumulated catchment area increases, collecting water from the side streets to the outfall. Some projects include a detention pond to attenuate flow before discharge, as required by the relevant authorities.
• Outfall: the point where water is discharged into a canal or public waterway. The water level of the receiving waterway must be checked; if it is higher, a pump sump and pumps are needed, and a permit to connect to the public drainage must be obtained from the relevant authority.

← Scroll to see the full image →

How Do You Choose RC Pipe Size and Strength Class?

Selecting RC pipe involves 2 separate decisions: the diameter (from the runoff calculation) and the strength class per TIS 128 (from the load on top of the pipe). The current standard, TIS 128-2560, divides pipes into 4 strength classes (Class 1–4), where a lower class number carries more load. A large pipe with too low a class will crack under the road, while a high-class pipe that is too small cannot drain the water fast enough.

← Scroll to see the full table →

Typical applications of RC pipe by strength class (TIS 128-2560)

Strength class	Load capacity	Suitable application
Class 1	Max	Under roads, traffic lanes, truck areas, or very deep burial
Class 2	Medium	Moderate-load areas, as specified by the engineer based on cover depth
Class 3	Lower	Areas with no vehicle traffic, e.g. under footpaths or green areas
Class 4	Lowest	Lightest-load applications, as specified by the engineer (the standard defines this class only for pipes ⌀0.80 m and larger)

As for size, branch pipes in housing estates typically start at a diameter of about 0.40–0.60 m and grow with the accumulated catchment area to about 0.80–1.50 m on the main collector routes. Each province's land-allocation requirements often also set a minimum pipe size within the project, so always check them before designing.

How Do You Calculate Runoff and Pipe Size?

The most widely used method for project sites is the Rational Method, which estimates the peak stormwater flow rate from three variables: the surface type, the rainfall intensity and the catchment area.

Once the runoff is known, the engineer selects the pipe size and gradient so that the pipe's discharge capacity exceeds the calculated flow. Areas with a lot of hard surface (roofs, roads, concrete yards) have a high C value and generate many times more runoff than natural ground — which is why filling land for a project always calls for a new drainage design rather than relying on natural infiltration as before.

Why Do Pipe Gradient and Levels Matter?

Water in the pipe flows by gravity, so the pipe gradient governs both the flow velocity and the depth of the pipe along its entire length. There are 3 key principles:

• Minimum velocity: design the flow velocity in the pipe to be no less than about 0.6 m/s (self-cleansing velocity) so that the water carries sediment along instead of letting it settle and block the pipe — pipes laid too flat are the main cause of chronic blockages.
• Continuous levels throughout the system: the downstream end must always be deeper than the upstream end. The longer the route, the deeper the downstream pipe, so alignments should be as short and straight as possible, and the outfall level must be checked to be above the water level of the receiving waterway.
• Level data from an actual survey: the design must use levels from a topographic survey of the actual site, not estimates from general maps, because a level error of just a few tens of centimeters can make an entire pipe line flow backwards. See the scope of SPN's topographic survey services

Which Standards Apply to Drainage System Design?

Project drainage work involves product standards, land-allocation requirements and local-authority requirements at the same time. The main groups to check are:

• TIS 128-2560 (reinforced concrete pipes): the product standard for RC pipes, defining strength classes 1–4 by load capacity; used as the basis for specifying pipe class in construction drawings and for material acceptance.
• Land-allocation requirements (under the Land Allocation Act B.E. 2543 (2000)): housing estates must provide a drainage system in line with the requirements of each province's Land Allocation Committee, which often specify minimum pipe sizes, manhole spacing and treatment/detention before discharge.
• Local-authority requirements: connecting pipes to a public drain or canal requires a permit from the relevant local authority, which may set additional conditions on levels, location and discharge.
• Engineering design criteria: the design storm return period, runoff coefficients and minimum–maximum pipe velocities, chosen by the design engineer according to hydraulic engineering principles to suit the site.

Drainage is part of a project's overall civil design and must be coordinated in level with the roads and the master plan. Read the overview in our guide to RC structural and civil engineering design, the article on RC road design for projects, or see the scope of SPN's civil engineering design services