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A case study between OSD Rational Method and Pond Rational Method has been done, to compare the difference between these two stormwater managements. 

Project references:

Project File for Pond Rational Method

Project File for OSD Rational Method

 

Similarity between two rational methods:

Rational method is comparing between the discharge of Pre development and Post Development, in order to determine the dimension of the control (OSD/Pond) and the size of outlets. 

 

The applicability of the two rational methods:

OSD is suitable to be used for a development area of less than 5 ha, while Pond is used for an area larger than 5 ha. However, MSMA 2nd Edition has come out with a shorter, simplified method for OSD called “Simplified Method” and is often encouraged by the authority to use this new method. (refer blog Comparison of OSD results between software calculation and excel spreadsheet using ‘Simplified Method)

 

The difference in methodology: A summary

Same concept is applied for both OSD and Pond, however there are some differences in terms of the calculation, which results to different output. Below are the examples:

  Pond Rational Method OSD Rational Method
Qpre Use Rational Equation, Q=CiA Use Rational Equation, Q=CiA
Qpost

Use Rational Hydrograph Method (RHM)

Routing calculation for Peak Qpost

Use Rational Equation, Q=CiA
Pond Storage

Based on Preliminary Pond Volume

Based on SSR

tcs is considered in calculation

Outlet Design

Has three design ARI (minor, major and secondary)

Involves hydrograph routing to calculate Water Level

Has two design ARI (primary and secondary)

Water Level is not calculated

Catchment Area

> 5 ha

< 5 ha

i) PRE DEVELOPMENT

Given the same situation of pre development;

Location: Kuala Lumpur – Puchong Drop, K Lumpur

Time of concentration:

L (m) = 315 

S (m/km) =

Area (ha) =

Tc (min) = 21.6

The ARI used for comparison is 50 years

For Pre Development Discharge (Qpre), both are calculated by using Rational Equation Q=CiA

Therefore, Qpre = 1.123 m3/s

The difference between OSD and Pond as you can see in the calculation is that Pond considers the calculation of three ARI which consists of Minor, Major and Secondary. In this case, the comparison is made by using Major design value.

Calculation of Qpre of Pond 

 

ii) POST DEVELOPMENT 

Given the same details for the Post Development;

Overland Flow Time, td

L (m) = 10

S (%) = 2

N = 0.015

To(min) = 3.01 

 

Drain Flow Time, tc

L (m) = 350

V (m/s) = 1

Td(min) = 5.83

Therefore, tc = 8.8 min

Catchment Runoff Coefficient

Total Area = 5 ha (impervious)

C Impervious = 0.95

The calculation of Post Development Discharge (Qpost) is different between OSD and Pond. For OSD, Rational Equation Q=CiA is used while for Pond, Rational Hydrograph Method (RHM) is used to determine the Qpost. 

RHM is used to route the Preliminary Pond Volume and is used to determine the Qpost which is based on the highest pond volume. Due to the difference, the end output of the Qpost also will be different.

Pond: Preliminary pond volume is calculated based on RHM and the peak Qpost is determined based on the highest preliminary pond volume

The Pond Qpost is 1.72 m3/s which is based on the highest preliminary pond volume, 2775 m3.

OSD: The Qpost is calculated by using Rational Equation

The OSD Qpost is 3.976 m3/s.

 

iii) POND STORAGE

The estimation of pond sizing is based on the calculated Preliminary Pond Volume. Based on this project, the volume is 2775 m3 at 1.72 m3/s discharge. 

OSD dimension estimation is based on the Site Storage Requirement (SSR), which also consider the value of tcs (refer to MSMA 1st Edition Chapter 19, Figure 19.2)

tcs is calculated in OSD calculation 

Based on the result, the SSR is 2257.92 m3 with an additional 20% to account for inaccuracies in construction and further loss of storage due to build-up of the lawn surface.

OSD SSR = 2257.92 x 1.2 = 2709.505 m3

 

iv) POND SIZING

The preliminary pond volume and the SSR are referred to estimate the pond volume. To determine the end dimension, the calculation of the dimension has also to be compared to the outlet design (based on trial and error)

POND Sizing with control 3328.416 m3
OSD sizing with control 2774.539 m3

 

v) OUTLET DESIGN 

Pond outlet design is accounting for the minor, major and secondary design (emergency spillway). The adjustment of the orifice sizing and the number are adjusted to comply with the requirement of MSMA Qpost < Qpre.

Pond: The Qpost with control is calculated and compared to Qpre

Inflow hydrograph routing also is involved in order to calculate maximum water level. 

Pond: Routing is involved in Pond design

OSD shows the result of the outlet discharge to be compared to the discharge without control. Water level is not calculated in OSD. 

OSD: Example of calculation OSD Primary Outlet

 

CONCLUSION 

  Pond Rational Method OSD Rational Method
Qpre

Rational Method

1.123 m3/s

Rational Method

1.123 m3/s

Qpost

RHM

1.72 m3/s

Rational Method

3.976 m3/s

Pond Storage (estimated)

2775 m3

SSR 2257.92 m3

SSR (additional 20%) 2709.505  m3

Pond Sizing

3328.416 m3

2774.539 m3

Based on the summary above, OSD gives you a smaller size of detention due to different methods of equations.

However, the choice of the control structure is still dependent on your requirement. If the area is small or less than 5 ha, it is a common practise to use OSD and much more convenient to use the OSD Simplified Method. Other outputs such as water level with control, can only be obtained by using Pond routing. The ARI consideration for minor and major for flood protection of several years also can only be done in Pond as the routing iteration is available in the Pond. The conclusion is that the best option to use between Pond and OSD is subjective and relies on your requirements (sizing, ARI, water level etc.).