Retarding Basin

“Retarding Basin” dan Banjir Jakarta

oleh Dr.-Ing. Ir. Agus Maryono

Jakarta diterjang banjir bandang lagi. Kali ini lebih luas dan menyedihkan, setelah banjir besar 2002 dan banjir kecil dan menengah tahun 2003, 2004, 2005, dan 2006.

Adakah metode efektif yang ramah lingkungan untuk mengatasi banjir sekaligus bisa dimanfaatkan untuk mengatasi kekeringan kota?

Oleh banyak negara, masalah serupa diselesaikan dengan metode retarding basin ramah lingkungan. Filosofi metode ini adalah mencegat air yang mengalir dari hulu dengan membuat kolam-kolam retensi (retarding basin) sebelum masuk ke hilir. Retarding basin dibuat di bagian tengah dan hulu kanan-kiri alur sungai-sungai yang masuk kawasan yang akan diselamatkan.

Contoh implementasi metode retarding basin adalah penyelesaian banjir di wilayah hilir Sungai Rhine di Eropa. Untuk mengurangi banjir yang menerjang kota-kota di wilayah Jerman dan Belanda bagian hilir, dimulailah (integriertes Rheisprogram) dengan membuat retarding basin-retarding basin di sepanjang Sungai Rhine di bagian tengah dan hulu, mulai dari kota Karslruhe (di perbatasan Perancis dan Jerman) sampai ke kota Bassel di perbatasan Jerman, Swiss, dan Austria.

Retarding basin ini dibangun untuk memotong debit puncak banjir Sungai Rhine yang akan menyusur menuju hilir masuk kota-kota penting, seperti Koeln, Dusseldorf, dan akhirnya Rotterdam. Volume air bah pada puncak banjir akan disimpan di retarding basin selama banjir berlangsung dan akan dikeluarkan setelah banjir reda. Retarding basin ini terbukti efektif menurunkan banjir yang terjadi di sepanjang Sungai Rhine di bagian hilir.

Program pembangunan retarding basin besar-besaran ini terus dikerjakan mengingat keberhasilannya cukup signifikan dan efeknya bagi perbaikan kualitas lingkungan serta konservasi air di daerah tengah dan hulu tinggi.
Penyimpanan Air

Fungsi retarding basin selain untuk memangkas puncak banjir, juga sebagai penyimpan air untuk dilepaskan pada saat musim kemarau dan meningkatkan konservasi air tanah karena selama air tertahan peresapan air terjadi. Dengan adanya cadangan di retarding basin, pada musim kemarau air dapat dipakai untuk penggelontoran saluran drainase dan sungai-sungai di daerah hilir.

Retarding basin harus didesain ramah lingkungan, artinya bangunannya cukup dibuat dengan mengeruk dan melebarkan bantaran sungai, memanfaatkan sungai mati atau sungai purba yang ada, memanfaatkan cekungan-cekungan, situ, dan rawa-rawa yang masih ada di sepanjang sungai, dan dengan pengerukan areal di tepi sungai untuk dijadikan kolam retarding basin.

Disarankan, dinding retarding basin tidak diperkuat dengan pasangan batu atau beton karena selain harganya amat mahal, juga tidak ramah lingkungan dan kontraproduktif dengan ekohidraulik bantaran sungai. Tebing-tebing itu cukup diperkuat dengan aneka tanaman sehingga secara berkelanjutan akan meningkatkan kualitas ekologi dan konservasi air.

Untuk penanganan banjir di Jakarta, retarding basin dapat dibuat di bagian tengah dan hulu dari 13 sungai yang mengalir ke jantung kota Jakarta, seperti Sungai Ciliwung, Cisadane, Mookervart, Pesanggrahan, Grogol, Krukut, Kali Baru Barat, Cipinang, Sunter, dan Cakung.

Pembuatan retarding basin ramah lingkungan dapat diawali dengan inventarisasi lokasi sepanjang alur sungai dengan prioritas dari bagian tengah hingga hulu. Inventarisasi ini dimaksudkan untuk menemukan lokasi-lokasi kanan-kiri sungai yang bisa dijadikan lokasi retarding basin. Setelah lokasi-lokasi yang cocok ditemukan, dapat dilakukan pembebasan tanah dan dimulai pembuatan retarding basin secara bertahap. Pembebasan tanah di pinggir sungai di daerah tengah dan hulu, yaitu di daerah Bekasi ke arah hulu, kiranya tidak memakan biaya mahal seperti pembebasan tanah di Jakarta Pusat.

Pembuatan retarding basin ini jauh lebih murah jika dibandingkan dengan pembuatan banjir kanal-banjir kanal. Karena selain lokasinya di luar daerah pusat perekonomian, konstruksinya juga ramah lingkungan dan tidak diperlukan konstruksi-konstruksi tambahan lain, seperti jembatan pelintasan, tanggul, dan perlindungan tebing.
Masih ada lokasi

Menurut studi makro peta Jakarta, penulis berkesimpulan, ke-13 sungai di Jakarta hampir semua masih mempunyai areal pinggir sungai yang bisa dimanfaatkan sebagai kolam retarding basin, terutama di daerah Jakarta Selatan, Depok, dan masuk Kabupaten Bogor. Untuk daerah Jakarta Selatan sampai perbatasan dengan Depok, misalnya, di Sungai Ciliwung kolam retarding basin bisa dibangun di sepanjang pinggir sungai dari Kompleks TNI-Cilandak hingga daerah MT Haryono, pada Sungai Pesanggrahan di daerah Cirendeu, Kompleks Lebak Bulus, dan Kebayoran Lama; pada Sungai Krukut di daerah Ksatriaan Marinir Cilandak, Cilandak Timur, daerah sekitar Kemang dan Karet, pada sungai Sunter, daerah Cipinang dan Kelapa Gading Barat. Juga untuk sungai-sungai lain masih banyak daerah dapat digunakan areal retarding basin pinggir sungai.

Dengan dibangunnya retarding basin-retarding basin yang ramah lingkungan dengan jumlah cukup, diyakini banjir Jakarta dapat diredam. Air dari bagian tengah dan hulu dapat direm sementara masuk retarding basin dan akan keluar jika gelombang banjir mulai menyurut. Jumlah retarding basin yang harus dibangun sesuai hitungan volume banjir yang akan direduksi. Semakin banyak retarding basin, tinggi dan volume genangan yang dapat diatasi kian besar. Penanganan banjir di suatu lokasi tertentu dapat diprioritaskan dengan cara membuat retarding basin di bagian hulu dari sungai yang menuju lokasi itu. Jadi untuk mengatasi banjir di sepanjang Ciliwung hilir dan Istana Negara, misalnya, dapat dibuat retarding basin dalam jumlah cukup banyak di sebelah hulu aliran sungai tersebut.

Berdasarkan telaah itu, Pemerintah DKI sebaiknya memprogramkan pembuatan retartding basin secara simultan terus-menerus sehingga banjir Jakarta dengan keyakinan penuh dapat diatasi sekaligus konservasi air pada musim kemarau terjaga. Namun, perlu diingat, penanggulangan banjir dengan metode ekohidraulik ramah lingkungan lain, seperti memanen hujan, ekodrainase, sumur peresapan, areal resapan, penghijauan, penghutanan kembali, penghentian penebangan hutan, revitalisasi sungai rawa dan situ, peninggian jembatan rendah, serta menghidupkan kembali transportasi sungai di Jakarta harus dilakukan secara serius dan terintegrasi.

Agus Maryono Peneliti Sungai, Banjir, dan Ekohidraulik; Dosen Fakultas Teknik, MST FT UGM

Sumber: Kompas, Kamis, 08 Februari 2007, Rubrik Opini

■ Tsurumi River Multipurpose Retarding Basin

Whay Doce ‘multipurpose’ Mean?

The Tsurumi River and the Toriyama River intersect in the Kozukue/Toriyama area of Kohoku-ku, Yokohama.

A retarding basin has been built here as a flood control measure. The area features a wide open space which is used for recreation fields, sports facilities, and venues for competitive events. In addition to protecting people from flooding, the basin can thus serve multiple purposes as a sports venue, a recreation area, and a lush natural park for local residents.

This multipurpose retarding basin, located near Shin-Yokohama station, offers the city’s residents a sense of security as well as a valuable recreation area.
With the International Stadium Yokohama as its centerpiece, the basin is used as a sports park and is managed by the City of Yokohama. Here local residents can enjoy a wide array of sporting and recreational activities.

Sports

Featuring International Stadium Yokohama and Kozukue Field, this sports and recreation facility allows local residents to enjoy the thrill of both participating in and attending lots of exciting sporting events.

International Stadium Yokohama
This is the largest sports stadium in Japan, boasting a capacity suitable for international games of 70,000 spectators. It can be used for a wide variety of events, including track and field, soccer, rugby, and other field sports.

Parks

It’s also a relaxing space where visitors can take advantage of the lush green environment near the water.
Take a stroll or enjoy some recreation time.
This is a place you can come just to relax for a while.

Athletic Field and Recreational Area
There’s an inviting recreational field in the middle of the athletic complex where anyone is welcome to take a walk or enjoy other recreational activities.
There’s also a water recreation area where people can have fun in the water and the outdoors.
It offers all the outdoor fun you can imagine!

Nature

Left: Grey heron / Middle: Cormorant / Right: Little egret Photos by: Yoshiyuki Usui

This verdant area along the nearby Tsurumi River lets visitors fully enjoy the river and its rich natural surroundings.

Water Recreation Area

The area’s ponds and wetlands are inhabited by a rich array of wildlife. Various species of wild birds can be seen here throughout the year, making it a great outdoor learning environment.

Helath Care

The retarding basin is also home to such medical and community welfare facilities as the Yokohama Comprehensive Care Continuum (YCCC), the Yokohama Rehabilitation Center, and the Sports and Cultural Center for the Disabled (Yokohama Rapport).

But what’s the real purpose behind this “multipurpose” area?

This is what makes the basin a “multipurpose” retarding basin. It affords space for recreation and relaxation and is designed to serve a variety of purposes. This pamphlet explains the invaluable role it plays for residents of the Tsurumi River basin.

Protecting People From Flooding

The Tsurumi River has long been known as a raging river prone to repetitive flooding, and the Kozukue/Toriyama region where the multipurpose retarding basin is located has always served as a natural retarding basin for the river.

By upgrading its natural functions with some human engineering, we were able to provide protection against the dangers of flooding to the surrounding communities as well as the communities further down the river.

The multipurpose retarding basin upgrade is being promoted as a joint project with the City of Yokohama.
Effectively utilizing the limited land available in this densely populated city, the basin improvements enable the site to serve flood control functions in times of flooding and to be used as an athletic park in normal conditions.

The Tsurumi River Flood Control Plan sets as its target the safe discharge of the rains of a flood of a magnitude seen only once in 150 years, but as shown in the flow rate distribution chart below, the current improvements to the river basin aim to allow it to safely discharge the amount of rainfall that occurred in the largest postwar flood (from the Kanogawa Typhoon of 1958) at a rate of 200m3/sec.

A retarding basin regulates the amount of water that is allowed to flow down a river during a flood.

The retarding basin that regulates flooding on the Tsurumi River is encompassed by a surrounding embankment, and by excavating its interior, engineers have been able to ensure that it can hold a certain volume of floodwater.

This basin can hold a maximum of 3.9 million cubic meters.

1 When it looks like a flood is going to cause the river to overflow, water from the river can flow over the overflow type levee into the retarding basin.

Water flows in over the overflow type levee built along the Tsurumi River.

Because the overflow type levee along the Tsurumi River are built about 3 m lower than the separating levees, they allow floodwaters to flow into the retarding basin. By temporarily storing the water, the basin prevents the water from causing massive flooding in the downstream communities of the Tsurumi River basin.

2 floodwater is temporarily stored in the retarding basin.

When the flood is over, the stored water is gradually discharged back into the Tsurumi River.

When the flood has passed, the water is allowed to flow through the spillway back into the Tsurumi River.

3 When the water level of the river drops, the sewage gate is used to allow water to gradually flow back into the river.

If the same amount of rain that fell during the Kanogawa Typhoon in September 1958 were to fall again today, it would likely flood an area of 664 ha and cause flood damage for about 90,000 people.
The estimated value of the damage would be around 400 billion.
Once the Comprehensive Flood Control Measures, which include the Tsurumi River Multipurpose Retarding Basin, are completed, damage from the inundation of river waters will become preventable.

• This simulation predicts what will happen when the river basin measures and improvements to the  sewerage system are complete.
• The results will differ depending on these conditions.

Sumber dan untuk selengkapnya di: http://www.ktr.mlit.go.jp/keihin/english/tsurumi/oasis_01.htm