A case study between OSD Rational Method and Pond Rational Method has been done, to compare the difference between these two stormwater managements.
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|
Use Rational Hydrograph Method (RHM)
Routing calculation for Peak Qpost
|Use Rational Equation, Q=CiA|
Based on Preliminary Pond Volume
Based on SSR
tcs is considered in calculation
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
> 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) = 2
Area (ha) = 5
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.
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.
The Pond Qpost is 1.72 m3/s which is based on the highest preliminary pond volume, 2775 m3.
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)
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)
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.
Inflow hydrograph routing also is involved in order to calculate maximum water level.
OSD shows the result of the outlet discharge to be compared to the discharge without control. Water level is not calculated in OSD.
|Pond Rational Method||OSD Rational Method|
|Pond Storage (estimated)||
SSR 2257.92 m3
SSR (additional 20%) 2709.505 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.).