ABSTRACT This paper examines the contribution that Constructed Wetlands can make to increase stromwater storage and reduce the volume of runoff during rain events to the drainage system, streams and rivers in Depok, Indonesia. A review of Constructed Wetlands (CW) projects are discussed and the examples are taken from Southeast Asia and USA. Review of examples from Southeast Asia are discussed in details as they are closely related to Indonesia’s geography and climate. Based on the reviews, the author analyzed the prospects of a CW in Depok to reduce flooding especially during rain events. A brief description of flood protection mechanisms in Jakarta Metro Area, Indonesia is discussed with focus on the approaches to mitigation. This paper traced the causes of floods over the years and that how different ecofriendly approaches such as CW can be implemented to mitigate/reduce flooding in the future. INTRODUCTION City of Jakarta also known as Jakarta Metropolitan Area (JMA), Indonesia has been suffering from seasonal flooding 2 to 5 years intervals (Pawitan, 2012) for a long time and has adopted different mechanisms, both short and long terms to reduce flooding and damage to life and property. During the last flood of January 15 – 21, 2013 resulted in 47 fatalities and economic losses up to US$ 695 million (IRIN News, 2013). Unfortunately, Indonesia and JMA are likely to face more devastating and frequent floods in the future, especially under the threats posed by projected climate change and sea level rise global warming of 1.4 to 5.8 C, on average, and sea level rise of 9 to 88 cm, on average, by 2001 are in prospect according to IPCC Assessment Report (IPCC, 2011), which would increase flood risks, among other consequences, in temperate and topical Asia. Local, national and international agencies have spent millions of dollars on flood control projects; however, the results are mixed. There's consensus among the local and international planners that majority of the mega projects such as waterways and pumping stations have helped to mitigate flooding, however; due to rapid growth in population and urban development of JMA, effectiveness of structural measures have not produced long term solutions to flooding ( Putri and Rahmanti 2010). Area of Jakarta The population of Greater Jakarta Metropolitan Area, comprised of DKI Jakarta, Bogor, Depok, Tangerang and Bekas i (Jabodetabek), reached 27.9 million according to the 2010 national census, with a growth rate of
Transcript
ABSTRACT
This paper examines the contribution that Constructed Wetlands can make to increase stromwater storage and
reduce the volume of runoff during rain events to the drainage system, streams and rivers in Depok, Indonesia.
A review of Constructed Wetlands (CW) projects are discussed and the examples are taken from Southeast
Asia and USA. Review of examples from Southeast Asia are discussed in details as they are closely related to
Indonesia’s geography and climate. Based on the reviews, the author analyzed the prospects of a CW in
Depok to reduce flooding especially during rain events.
A brief description of flood protection mechanisms in Jakarta Metro Area, Indonesia is discussed with focus
on the approaches to mitigation. This paper traced the causes of floods over the years and that how different
ecofriendly approaches such as CW can be implemented to mitigate/reduce flooding in the future.
INTRODUCTION
City of Jakarta also known as Jakarta Metropolitan Area (JMA), Indonesia has been suffering from seasonal
flooding 2 to 5 years intervals (Pawitan, 2012) for a long time and has adopted different mechanisms, both
short and long terms to reduce flooding and damage to life and property. During the last flood of January 15 –
21, 2013 resulted in 47 fatalities and economic losses up to US$ 695 million (IRIN News, 2013).
Unfortunately, Indonesia and JMA are likely to face more devastating and frequent floods in the future,
especially under the threats posed by projected climate change and sea level rise global warming of 1.4 to 5.8
C, on average, and sea level rise of 9 to 88 cm, on average, by 2001 are in prospect according to IPCC
Assessment Report (IPCC, 2011), which would increase flood risks, among other consequences, in temperate
and topical Asia.
Local, national and international agencies have spent millions of dollars on flood control projects; however, the
results are mixed. There's consensus among the local and international planners that majority of the mega
projects such as waterways and pumping stations have helped to mitigate flooding, however; due to rapid
growth in population and urban development of JMA, effectiveness of structural measures have not
produced long term solutions to flooding ( Putri and Rahmanti 2010).
Area of Jakarta
The population of Greater Jakarta Metropolitan Area, comprised of DKI Jakarta, Bogor, Depok, Tangerang
and Bekasi (Jabodetabek), reached 27.9 million according to the 2010 national census, with a growth rate of
3.6 percent per annum over the period 20002010( Census , GOI). The land and sea areas of Jakarta (JMA)
are 661.52 km^2 and 6977.7 km^2 respectively. Out of that approximately 40 % of land is low laying, with
an elevation of 0.8 m + 1.2m. Mostly the northern part of the JMA is below the sea level (Pawanti, 2002).
Sources of Flooding in Jakarta
Low lying areas: Approximately 1653.8 km 2 land is low laying and heavily populated both by tenured and
illegal settlements, especially along the bank of the Ciliwung river.
Land subsidence: The rate of land subsidence measured by Rochman (2004) using GPS monitoring sites
shown the average deformation variations from 7.5 cm to 32.8 cm within four years period. The major cause
of land subsidence is groundwater extraction especially for industrial water use.
Inadequate Garbage /solid waste disposal: Every day the urban population of Jabotabek (Jakarta, Bogor,
Tangerang and Bekasi) generates over 35,000 cubic meters of garbage (enough to cover a soccer pitch to
depth of over 5 m), of which 26,000 cubic meters is from the city of Jakarta. Approximately 83% of the
garbage produced in Jakarta is collected by the local community, scavengers, and the private sector. The
remaining 17% is thrown into the rivers. Outside the city of Jakarta, only about 50% of the waste generated is
collected, and 20% of that does not find its way to a disposal site – which in any case may only be a
smoldering open dump (UNESCO, Indonesia, 1997)
Flood Control Measures over the decades
Jakarta flood control plans have a long history dating back to early 1970s, for example East Flood Canal
(EFC) which was initially started in 1973 along the Ciliwung River. Most of the flood control measures are
structural (engineering). According to government census of 2010, JMA has a population density of 14,464
people /km^2 (GOI, census, 2010). Due to growing population and physical growth of JMA, government
can't keep up with flood control infrastructure. In such scenario planners have advocated that government can't
simply solve the problem with physical infrastructure and need imagination to reduce the flooding 25 years in
the future ( Putri and Rahmanti ,2010). The severity of January,2013 flooding was a result of collapse of an
embankment and a levy resulted in sudden burst of flow on the impervious surface that caused “flashy” runoff,
and resulted in overflow of drainage systems in western and central parts of Jakarta. Flood control systems are
expensive to build and maintain for example the Western Floodway was built in 1992, which intercepted the
Ciliwung, Cideng and Krukut rivers costing US$100 million, mainly financed by Japan’s Overseas
Development Agency (ODA). (JanJaap, 2012)
OBJECTIVE
Indonesia is a developing country, according to World Bank the GNI per capita $3420 in 2012. Out of a
population of 234 million, more than 32 million Indonesians currently live below the poverty line and
approximately half of all households remain clustered around the national poverty line set at per month of US
$22 (World Bank, 2013). As the national economic priorities are focused on the development of human
resources such as education and poverty alleviation, government of Indonesia cannot afford multi billion
dollars projects. The governments of Indonesia and Jakarta need to think outside the box regarding flood
control and management and adopt *Ecohydrology concepts of water resource management at varying
scales. Currently flood mitigation plans are at city wide scale and these projects generally tend to ignore
communities that are part of the problem for example the inhabitants of the slumps along the river banks.
Under such scenarios the government needs to find solutions for the communities that are living in the flood
zones. There's a need for nonstructural measures such as Constructed Wetlands that are incorporated with
the flood control planning.
Examples of Constructed wetlands for flood prevention
Singapore
Government of Singapore constructed wetlands for stormwater management and wastewater reuse. The
wetland project known as Sungai Buloh wetland preserve located at latitude 1,26 47.85N , longitude
103,0430 48.29 E spread over 1000 square meters. This wetland is composed of 6000 plants of 5 species
and has a nutrient removal performance of 64% of total nitrogen and 24.4% of total phosphorus (Lee, 2009).
Below is the list of the species that were planted at Sungai Buloh wetland preserve.
Common reed Phragmites Karka
Tube sedge Lepironia articulate
Bulrush Scirpus mucronakus
Cattail Typha angustifolia
Spike rush Eleocharis palustris
Jakarta, Indonesia
A pilotscale Constructed wetland was built in Kelurhan Srengseng Sawah South Jakarta to improve the
wastewater treatment. This project showed lot of promise as it reached efficiency between 5890 % in
breaking down Biochemical Oxygen Demand (BOD5).This project showed that Constructed wetland is a
viable option particularly in response to the problem of limited availability land in urban areas (Hendrawan,
2013).
Portland, Oregon USA
The City of Portland Environmental Services constructed a wetland named Glencoe Rain Garden in 1990 to
reduce runoff and to protect the homes from sewer backup. After 10 years in 2000, the plants and soils in the
wetland are soaking up 85% of the annual volume of runoff, and have reduced the annual peak flow by 80 %
(Portland Environmental Services, 2013)
Chicago City, Chicago USA
City of Chicago built a rain garden in October, 2013 at Haines Elementary School. This project replaced
1,000 square feet of asphalt's pavement with native plantings and vegetation. It's expected that this project
would increase the stormwater collection and increase the flow waiting time (residence time) and help to
reduce flash flooding in the neighborhood.
METHODOLGY
Location of the proposed CW site
Source: Google Earth, 2014
The location of the site is in Depok, West Java, and 15 miles south of Jakarta. This location is frequently
effected by seasonal flooding. The site is 2,204 square meters. The site is next to the Ciliwung River. The
average slope of the site is 2 percent. The site is next to highly dense residential area and the storm water flows
west toward the river. The geographic location of the site is 6, 23 33, 77 S and 106 49’ 54.76E. The soil type
of this area is classified as Andisols as it was developed on volcanic materials. The subgroup of Andisols is
Hydric Andisols has high water retention due to low bulk density due to high organic and mineral contents. The
upper layer of Depok soil is silt and clay (Liong, 2010).
Watershed Area of Jakarta Metro Area
Source: ESRI, 2013
Stormwater runoff of Jakarta watershed during rainy seasons is about 1.020 billion centimeters per year
(cmy), the runoff coefficient of the watersheds is increasing due to land use and improper agricultural
management, (Naik, 2012). Jakarta is extensively urbanized; it needs about one percent area allocated for
water storage system (open or underground) with capacity at least 50% of total stormwater during floods
(Pawitan, 2011)
Flood Analysis
Rainfall analysis showed that average annual rainfall of the sub watershed of Depok from 1990 2006 is 55
mm. The maximum rainfall range from 204mm to 76 mm. There are seven years (1994, 1995, 1998, 2001,
2002, 2004, and 2007) where the rainfall spiked and passed 80 mm of rainfall and witnessed flooding.
Source: Office of Public Works Water Resources Management Sector, Depok and Jakarta BMG, 2008
Ciliwung River Discharge Analysis (Depok gauging station)
Source: Office of Public Works Water Resources Management Sector, Depok and Jakarta BMG,
2008
Stream discharge analysis showed that yearly discharge (19902006) of Ciliwung River (Depok Station)
ranged from 30.2 to 80.1 Q (m^3/s). The yearly average discharge is 50.8 Q (m^3/s) showing seasonal
variation of dry summer and monsoon rainfall.
There’s a strong correlation between high rainfall and the discharge of Ciliwung River. Heavy rainfall causes
bankfull and small floods when the maximum rainfall ranges from 80 mm 120 mm for example in the month of
May,2001, Jakarta received 118 mm of rainfall ( Depok Station) and it resulted in heavy flooding.
Flood Frequency and Magnitude Analysis
According to the flood frequency data analysis Ciliwung River discharge (Depok Station) at bankfull level
between 44.7 – 52.7 Q (m^3/s) and the probability of 33 and 50 percentages for 1.5 to 2 years respectively.
Small floods with high probability of 80 to 90 percentages can occur between 70.9 to 78.2 Q (m^2/s) at 510
years return interval. 20 and 100 years flood predication indicate that discharge level of 82.2 to 85.5 can
return to Ciliwung River with 95 percentage of probability.
Bankfull
RI Prob ln (Q) Q (m3/s)
1.5 2 yr. RI 1.5 0.33 3.80 44.7
2 0.50 3.96 52.7
Small Floods
5 10 yr. RI 5 0.80 4.26 70.9
10 0.90 4.36 78.2
Large Floods
> 10 yr. RI 20 0.95 4.41 82.2
100 0.99 4.45 85.5
Rainfall and discharge analysis showed that Ciliwung River at Depok Station can flood above 45 Q (m^3/s).
Rainfall analysis of Depok station that’s located in the middle reach of Ciliwung River from the year 1991 to
2007 shows average daily rainfall of 55mm for the year and maximum daily rainfall for the year 204 mm. Total
annual rainfall from 19912007 is 95356 mm.
Stream discharge analysis showed that yearly discharge of Ciliwung River is ranged from 80.1 Q (m^3/s) to
32.3 Q (m^3/s) with monthly variation of 16 Q (m^3/s) in dry seasons to 44 Q (m^3/s) in the rainy seasons.
The total yearly discharge from 1990 to 2006 is 931.7 Q (m^3/s).
Peak Flow Calculations
In order to attenuate peak runoff through the construction of wetland; the primary method to determine peak
flow is the Rational Method. The Rational equation is given as
Q = C*I*A
Q = Estimated Design Discharge (cfs)
C = Composite runoff coefficient (0.54, estimated by Sinukaban, 2012)
I = Rainfall Intensity (mm/hr), Depok rainfall intensity during monsoon season is 100 mm per hour.
Pawitan, Haydith (personal communication, April 18, 2014)
A = Depok rain effected area 68 ha (0.68) Sq. Km ^2
Q = 0.54 X 100mm/year X 0.68 square km ^2
Peak Q discharge = 36.72 Q (m^3/s)
Retention Time of storm water in a Constructed Wetland
Retention Time: Area (acres) X Depth (feet) / Flow per hour (acres foot)
168 acres x 6.674 feet / 107.16 acres foot/hr
Retention time: For 10.46 hours equals to 101.16 acft/hr (1.686 m^3/min) or 2,428 m^3/day of
storm water storage
Design and water Retention of the Constructed Wetland
Length: 23.59m
Width: 7.86m
Total Depth: 2m
Depth of water: 0.8m
Surface Area: 185.42m^2
Criteria of Constructed Wetland
It is recommend that the size of wetland should be at least one percentage of the watershed area. In our case
study Depok, west Java is considered as a sub watershed of Ciliwung River. We divided Depok flooded
area (68 ha) along the Ciliwung River by proposed wetland surface area of 0.01854 (ha). The result size of the
constructed wetland is 2.7 Percent of the frequently flooded acreage. It’s recommended that a buffer of 5 to
15 meters should be created between the proposed wetland and the stream or river (outlet).
Results
Depok subwatershed had a history of high flood. According to news report and city officials estimated that
flood of 2004, inundated 68 (ha) mostly the residential areas (The Jakarta Post, 2004). However, the Depok
City covers about 94 Km^2 and dominated by the slope steepness of <15% (Sinukaban, 2010). In such
scenarios the size of the wetland should be lot bigger than the proposed CW but due to lack of open space the
size of the CW was modeled to the bankfull flooded acreage of 68 (ha). The scaling down of sub watershed
helps to analyze flood protection plans; as the hydrology of flood waters fluctuates due to varying land use and
topography. The bankfull flood at Depok occurs at 44.7 Q (m^3/s) and this CW could increase the retention
of stormwater by 0.0281 m^3/s.
Conclusion
Jakarta Metro Area rivers hydrology, floods and urbanization are interlinked and the problem of flooding can’t
be solve in vacuum. The magnitude of population growth, housing needs and water demand need to be
address with engineering and ecofriendly projects like CW. There are no easy answers to reduce flooding in
Jakarta Metro Area but ecofriendly CW all over the Jakarta watershed would help to reduce the intensity of
the flooding. More research is needed to determine the impact of wetlands and Constructed Wetlands in
reducing flooding in Jakarta Metro Area.
Footnote
Ecohydrology is a concept that study the interaction between the water body and the ecosystem. Such
interaction helps to understand the impact of water on the terrestrial ecosystem.
References
1. Brinkman, JanJaap and Hartman, Marco, 2012, Jakarta Flood Hazard Mapping Framework
2. Chung Lee, 2009, Nitrogen removal in constructed wetland systems. Engineering Life Science, Volume 9.
