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MSMA Pond: Basin Outflow Limit ARI Feature

What is an outlet for a pond? #

Outlets for ponds – both detention and retention – are multi-stages hydraulic structures that regulate the discharge of runoff for the post-development stage. These structures are crucial in maintaining a lower peak flow rate compared to the pre-development stage, hence reducing the risk of downstream flooding and erosion. For a pond, the gradual release of runoff occurs at the mid-section as orifice flows, and at the top of the control structure through weir flows during high water flow conditions. 

Schematic figure of a pond with primary outlet and secondary outlet (Source: Optimal Design of Stormwater Detention Basin on Researchgate)

Orifice Flows #

Orifice flows of a pond, also known as the primary flows, should be designed to decrease peak flows after development for both minor and major return periods (ARI). This requires a two-stage outlet design, one to handle minor storm discharge and the other for major storm discharge. Both the minor and major outlet(s) can be constructed in rectangular or circular shape, depending on the pond’s structure.

Weir Flows #

Weir flows is the excess of the major design storm ARI that may flow from a pond in the event of a high water flow situation. A secondary outlet, or better known as the emergency spillway should be designed for this type of flow to prevent overtopping while maintaining the stability of the pond.

Basin Limit Outflow ARI #

The Basin Limit Outflow ARI is a feature of the MiTS Pond Module that helps engineers create a primary outlet for the pond, taking into account both major and minor design storms. It gives users the flexibility to select their desired return periods (ARI)for the pre-development runoff outflows.

With only a single outlet for the pond, the discharge comparison is no longer between major pre-development to major post-development or minor pre-development to minor post-development. Instead, the discharge for post development minor and major will be compared with the pre-development outflow limit, in which the post-development flows should be lesser. Hence, this feature enables users to easily do the comparison, making sure that the outlet size is adequate for runoff management.

Where can we find the feature? #

The feature is available under the Minor Design Storm Primary Outlet only when the Major Design Storm Primary Outlet is not included in our design.

Go to Outlet Design tab > Scroll down to the Major Design Storm Primary Outlet > Untick the box > Scroll up to the Minor Design Storm Primary Outlet > The Basin Outflow Limit ARI will be available under the Number of Orifice section

Outlet Design for Major Design Storm Primary Outlet

Basin Outflow Limit ARI for Minor Design Storm Primary Outlet

How does this feature work? #

Referring to the sample project file here;

Location of development = Sungai Buloh, Selangor

Catchment Area = 5.2 ha

Pond design ARI = 50 yrs ARI

Pre-development Tc = 27.42min

Post-development Tc = 20.3min

Maximum allowable pond depth = 2m

Primary outlets in the riser to control 10 and 50-yrs ARI

Spillway design to cater for 100-yrs event

For minor storm design, 10-yrs ARI #

Orifice type = Circular; Diameter = 400mm; No. Of orifice = 1

With the pre-development design flow limit set to 0.574m3/s for 10 yrs-ARI, the discharge routing for major design will definitely be higher. This is because, for major design storm routing, the rainfall intensity considered is based on a higher return period (ARI).

Hence, with this single routing, designers cannot simply say that the sizing for the outlet is not suitable and the discharge should also be compared with the pre-development flow limit of major design storms.

For Major Storm Design, 50-yrs ARI #

Orifice type = Circular; Diameter = 400mm; No. Of orifice = 1

From the image above, the post-development discharge is lesser compared to the pre-development flow limit, which reflects the main purpose of the pond as the part of the stormwater management. From here, users will need to ensure not only the discharge, but also the freeboard is following the requirements stated in MSMA 2nd Edition Guidelines.

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