Sunday, November 23, 2008

Sustainable Flash Flood Mitigation Solution

A new Urban Storm Water Management Manual for Malaysia was completed in 2000 to replace the 1975 Urban Drainage Design Manual. In January 2001 the Cabinet approved its application for all types of development projects in the country.

The new manual correctly pointed out that widening and channeling river and drain to cater for increased discharges of storm water runoff as the urban area develops is inherently defective from the environmental point of view. It also recognise the need to apply structural Best Management Practices (BMP) to control storm water from the aspect of quantity and quality runoff to archieve zero development impact contribution. It is hope that this new strategy will be a sustainable solution to mitigate the existing floods problems but it also to prevent the occurence of such problem in the new area developed.

I am really curious why this mandated approach enforced since 2001 is totally absent from the Penang Flood Mitigation Plans. There is even a video on Workshop on Urban Storm water Management Manual posted on YouTube:

Using structural Best Management Practices to manage storm water runoff is a very cost effective and efficient solution to mitigate flash floods problem.

For example, in 1999 one maintenance station in Keary Mesa, California was selected for retrofit pilot with the installation of The Storm-Filter™, a proprietary water quality treatment device that uses cartridges filled with different types of media to filter storm water runoff before channel into storm water well.

Siting Criteria:

SiteLand UseWatershed Area (Hectare)Impervious Cover %
Kearny Mesa Maintenance Station0.6 100

Design Characteristics of the Storm-Filter™

SiteDesign Storm (mm )Design Storm Discharge
(L/s )
WQVa (m3)Number of CanistersNumber of Chambers
Kearny Mesa3676194863

Construction Costs for Storm-Filter™ (1999 dollars)

Storm-Filter™Construction Cost, $Cost/WQV ($/m3)
One location305,3561,572

The cost per unit water quality volume treated is $1572/m3. Total construction cost is US$305,356. This is the solution deployed to manage storm water runoff for a watershed of 0.6 ha. with tributary area of less than 8 ha.

This is just one example of structural BMP that can be deployed to Georgetown areas which have land space constrains. There are other cheaper options that can be considered for use to manage storm water on ultra urban areas.
(Source California Department of Transportation)

Consider the cost involved and other side benefit like harvesting storm water for non potable need. With average rainfall of 2000mm per annual, the volume of storm water harvested will be significant.

For a factory in the Free Trade Zone (FTZ) in Penang, the water yield is calculated as:
Total Rainfall (Water) Yield = Roof Area X Drainage Coefficient (0.8) X Annual Rainfall
= 1000 m2 X 0.8 X 2505 mm per year
= 1000 m2 X 0.8 X 2.505 m
= 2004.0 m3
= 2,004,000 litres per year
Based on normal usage of a 500 workers:
= 500 persons X 100 litres (Only Flushing toilet)
= 50,000 litres per day
= 18,250,000 litres per year
The rainfall harvested can fulfill about 11.0 % of total water needs of this factory. In monetary terms, based on the average rate of 78 sen per 1,000 litres (or 0.078 sen/litre), the factory will save RM156,312.00 (US$42,246.49) per year.

(Source: Penang water Watch)

The state government can use such kind of solution to solve flash floods problems at the same time harvest rain water for sell in order to recover some of the cost spent.

Schematic of a Storm-Filter™

Surface View at Kearny Mesa

Internal View at Kearny Mesa

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