Multiple Series of Detention Ponds

Civil engineers may require to design multiple detention ponds due to reasons below:

1. The catchment area is too big and spread out, making it impractical to direct all runoff into a single pond downstream. 
2. Not enough space to build a large single storage facility.
3. Local regulatory authorities may set the allowable discharge limit of the area. So splitting into multiple ponds makes it easier to meet design storm requirements without creating excessively deep or large detention structures.
4. Adopting multiple ponds provides localized peak flow attenuation before discharging the runoff into the downstream system. 

Incorporating Storage Unit in MiTS 2 #

In MiTS 2, detention ponds (storage units) were not directly incorporated inside the drainage conveyance network. Instead, the process requires a workaround approach.

1. Design detention ponds in MSMA Module
2. In the Drainage Module, set the discharge of the drain connected to the pond (assuming node is the pond) equal/close to the peak outflow from the first pond, according to rational method in MSMA module
3. By using formula Q=CiA/360, adjust the parameter value so that the calculated discharge value approximates to the maximum post-development discharge (Qpost) from the MSMA module.

What is the drawback of this MiTS 2 approach? #

By this method, the assumption is made that the drain connected to the second pond will carry the maximum discharge from the first pond outlet. This assumption may lead to an overdesign of the outlet drain, as a larger dimension would be required to accommodate the peak flow discharge. Moreover, since the drain is analysed in a steady state flow calculation, the dynamic storage behaviour of the pond (such as water level fluctuation and temporary storage) is not realistically represented.

Let’s delve further on the case study.

Project files can be downloaded below

1. MiTS 2 Manual Method
2. MSMA Calculation 
3. MiTS 3 Dynamic Routing Analysis

MiTS 2 Methodology  #

Total catchment area: 10 ha

Design ARI: 50 years

MSMA Module #

From the Qpost calculated from MSMA guideline, these peak flows will be used at the respective pond outlet drain in Drainage Module. To achieve this, the discharge parameter (C.i.A) should be adjusted, so that the computed peak flow closely matches with the MSMA orifice discharge from the first and second pond.

MiTS 2 Drainage Module #

When adjusting the peak flow manually to approximate the Qpost obtained in MSMA calculation, the drain capacity of 600mm x 450mm is not sufficient to cater for the Qpost. The drain fails if size of 600mm x 450mm is selected.

N/B: The gradient of drain M2 and M3 are fixed to 1/200

From the comparison above, a larger outlet drain size (900mm × 900mm) is required to ensure that drain M2 can adequately convey the maximum peak flow from the upstream pond to downstream. This can result in increased construction costs and inefficient material usage, as the drain may be larger than what is hydraulically necessary.

This is where Analysis Mode in MiTS3 is useful, as storage units can be defined directly in the drainage network, allowing direct hydrograph routing and dynamic interaction between pond and conveyance. It allows us to observe the actual discharge rate and determine the most appropriate drain dimension based on dynamic simulation results. This can be further explored as follows: 

MiTS 3 Analysis Mode (Dynamic Routing) #

Discussion/ Results #

Outflow discharge of 2 outlet drains #

Drain M2 – Outlet drain from upstream pond (Pond 1) connecting to downstream pond (Pond 2)

N/B: Negative flow depicted above indicates the water is flowing back inside the pond. Dynamic Wave Routing can capture complex hydraulic behaviour such as backwater effects, flow reversal, pressurized flow and etc. Refer to article here

Drain M3 – Outlet drain from downstream pond (Pond 2) discharge to another water body

Storage Time Series for Ponds (Inflow/Outflow Hydrograph) #

Pond 1 (Upstream Pond) #

MSMA Module

MiTS 3 Dynamic Routing Analysis

Alternative pond size (Pond 1) is tested to see the inflow and outflow hydrograph. Pond top area is assigned 600m2.

Pond 2 (Downstream Pond) #

MSMA Module

MiTS 3 Dynamic Routing Analysis

Alternative pond size (Pond 2) is tested to see the inflow and outflow hydrograph. Pond top area is assigned 300m2.

Table Comparison #

Pond 1

Design ARI: 50 years

Selected Storm Duration: 80min

Pond 2

Design ARI: 50 years

Selected Storm Duration: 100min

Under MSMA methodology, each detention pond is analysed independently, assuming no hydraulic interaction or awareness of other ponds within the same system. It is as if each pond has no clue the others exist, as they’re analysed as a standalone system. Since MSMA calculation is on the conservative side, the inflow/outflow and water depth of the pond is higher than in MiTS 3 Analysis Mode. 

But with MiTS 3, the hydraulic behaviour of the ponds is simulated dynamically, where upstream and downstream ponds interact with each other through hydrograph routing. This provides a more realistic view of how flow and storage behave across the entire drainage network.

In Analysis Mode, observe that a smaller drain (600mm × 300mm) is sufficient to connect between Pond 1 and Pond 2. Likewise, the outlet drain at Pond 2 can be smaller since the upstream pond has already reduced the peak flow. And also, even after reducing the pond size, the water level remains safely below the maximum pond depth, indicating sufficient storage performance. That’s a major win for cost efficiency because now we can optimize for drain and pond size!

Why Dynamic routing is more cost effective #

Unlike steady-state analysis, which assumes that peak flow conditions occur continuously throughout the system, dynamic routing simulates the time-dependent hydraulic behavior of drains and detention ponds. In steady-state analysis, concurrent inflows are treated as if their peak discharges occur simultaneously, resulting in a conservative (and often overestimated) total flow equal to the sum of individual peak flows. 

In contrast, dynamic routing accounts for the temporal variation of inflows, allowing for the attenuation and phase difference of hydrographs. As a result, the combined flow at any given time may be lower than the arithmetic sum of peak discharges.

This more realistic representation of system hydraulics typically leads to more cost-effective designs, enabling smaller storage or conduit sizes without compromising hydraulic performance or safety.

Conclusion #

When it comes to multiple series of ponds and drains, using routing for analysis is a must because it takes into account of the actual hydrographs, and will give us a more optimized pond and drain sizes, resulted in more economical and realistic design, without sacrificing safety or performance.