Detention drain may look like an overly-designed standard drain, but the two differ in their objectives of managing the runoff- conveyance vs. storage.

While the standard drain acts as the ‘transporter’ for the runoffs, conveying the water from the highest to the lowest point of a land as fast as it can, the detention drain is designed as an ‘interceptor’, storing the water from the upstream temporarily to control the downstream discharge.

Therefore, the larger dimensions of the detention drain are intentional; it is actually a requirement to ensure the development area is not flooded during intense storms, especially at the downstream.

In this article, we will compare the maximum flow and drain size between pre-development, post-development without water management, and post-development with water management to further see the effect of the drain’s physical size on the flow discharge.

Site Characteristics and Catchments Overview #

The physical size of a drain, putting aside the purposes it serves, depends on the characteristics of the development site- Different sites will have their own unique characteristics that will affect the hydraulic requirements. For this particular reason, it is important to know the site’s topography, land use, and catchment areas.

Site Details:

1. Catchment Area: 0.367 ha

2. Pre-development Land Use: Grass Cover (Coefficient: 0.5), Commercial and Business Centres (Coefficient: 0.95)

3. Post-development Land Use: Commercial and Business Centres (Coefficient: 0.95), Grass Cover (Coefficient: 0.5), Park (Coefficient: 0.4)

4. Storm Design ARI: 5 Yrs, 50 Yrs, 100 Yrs (For the comparison, we will focus on the 50 Yrs ARI as an example)

Given the compact site area of only 0.367 ha, designing an On-Site Detention (OSD) or Detention Pond will be an impractical option as they require side slopes, which is equivalent to excessive land takes. By opting for a detention drain system, we can maximize the land use while maintaining effective water management- the drains act as primary conduits and reservoirs simultaneously.

Comparison of the drain size and maximum flow #

Post-development (Without water management- as conveyance): Sample Project File here

Post-development (With water management- as storage): Sample Project File here

A development in Kota Kinabalu, Sabah, which applied detention drain as the water management design, has been made as a reference to observe the physical comparisons.

The network above is analysed in two different scenarios- Scenario 1: Analyse the network using the Design Method, whereby the drain sizes are to be iterated by the software, and Scenario 2: Analyse using the Analysis Method, in which the drain sizes are fixed to a certain size.

In Scenario 1, where the computation engine is similar to MiTS 2, the software determines the drain sizes purely based on the most efficient size for transporting runoff. In the analysis, the software finds that the box drain with a width of 600mm and a height of 300mm is already optimal for water transport; the drain successfully fulfills its conveyance purpose by moving water from a higher point to a lower point, with a discharge rate of 0.097 m^{3}/s.

In Scenario 2, the Analysis Method in the Storm Sync Module requires pre-determined sizes, and in this case, the box drains have been fixed to larger dimensions- especially for the drain before the discharge point, with a width of 1200mm and height of 300mm. With larger dimensions, we can “restrict” the downstream discharge to a relatively lower rate, such as 0.055 m^{3}/s, as the drain not only conveys the water, but is able to store water temporarily.

So, why detention drain size?

Discussion: #

Why Drain Size Matters? #

As the site has been developed, the natural pervious surface will be replaced with the impervious concrete, roofing, and pavements- this will turn the previous slow-soak runoffs into a fast surge. With high-intensity storms, the downstream areas will be prone to flooding, something that the authorities want to avoid at all costs.

In preventing the flood catastrophe, engineers must design the drain while strictly adhering to the key fundamentals in water management:

Post-development discharge (Q_{post}) \leq Pre-development discharge Q_{pre}

The fundamental is not something easily achievable with no storage in the system- the drain will be similar to a hose, only able to transport the water from point A to point B. With a detention drain, it acts similarly to a bucket with a small hole at the bottom to control downstream discharges.

During massive storms, a higher volume of runoff will flow into the drains (buckets), and yet the discharge rate is controlled to a relatively lower rate; this creates a condition where the rate of water leaving the system is less than the rate of water coming into the system, causing the water to be “stacked up” inside and filling up the drains.

If the provided storage is small, the water will fill up quickly and overflow almost immediately during the runoff. To prevent the overflow, a larger storage must be provided to give extra space for the water to reside while it waits for its turn to be discharged.

How do we determine the “right” orifice size for our detention drain? #

When it comes to deciding the best orifice size, there is no right or wrong- it is more of iterations of the diameter that will give the right balance between two requirements: (1) (Q_{post}) \leq Q_{pre} and (2) the water depth to drain depth ratio, y/D < 1.0.

While it is our main goal to always keep the post-development discharge small, it is also important to take into account the y/D ratio; One of the safety measures in preventing flooding, whereby it is always preferable to have the ratio less than 1.0, rather than equal to 1.0 (We don’t want the water to be filled up to the brim and easily overflow the drains).

To illustrate the above, a table of comparison has been created following the different orifice sizes.

For a network governing an area of 0.367 ha, the 250mm orifice is considered the most effective in controlling the discharge at the downstream. Not only did the size result in a discharge of 0.055 m^{3}/s, which is less than the post-development discharge of 0.082 m^{3}/s, but it also maintains a y/D ratio of 0.89– with maximum flow, only 89% of the drain will be filled, providing the required freeboard.

While the 150mm and 175mm orifices provide a similar y/D ratio of 0.89, they are considered conservative options due to the lower discharge rate. The water is released more slowly at the rates of 0.022 m^{3}/s and 0.029 m^{3}/s, which creates the possibility of flooding during frequent storms (one intense storm followed by another in a short duration).

As for the 300mm orifice, it is already out of the windows; Not only is the resulting discharge rate of 0.086 m^{3}/s, more than the pre-development discharge, but to accommodate the larger opening, it requires the drain height to be increased to 450mm- with 300mm orifice, the drain height is not possible to be maintained at 300mm due to no clearance available at the top or bottom of the orifice to secure the plate. For this particular size, it will definitely increase the construction cost.

Conclusion #

The significantly large dimensions of the drain are definitely not due to over-design, but it is a practical necessity in managing runoffs within a limited development area. While the typical 600mm drain width is designed solely for conveyance, the 1200mm drain width is designed as a linear reservoir for both conveyance and storage.

With detention drain, it allows engineers to design a proper water infrastructure within a compact area, which not only follows the authority regulations but also satisfies the hydraulic requirements- thus providing peace of mind to the surrounding communities, especially during intense storms.