STUDI PERENCANAAN KOLAM RETENSI (RETARDING BASIN) TIPE SAMPING PADA SUNGAI BABON SEBAGAI UPAYA PENGENDALIAN BANJIR DI KOTA SEMARANG

Flooding is the phenomenon where rivers are unable to contain the water discharge, resulting in overflow. Broadly speaking, floods in Semarang City are caused by local rainfall and tidal floods. The overflow from the Babon River impacts Trimulyo Village covering an area of 19.70 hectares and Bangetayu Wetan Village covering 1.04 hectares. Additionally, coastal areas near the mouth of the Babon River, such as Terboyo, experience tidal flooding.
Based on these factors, there is a need for the planning of effective flood control structures to mitigate flooding. One such structural measure is a retention pond. In this planning study, a retention pond is proposed downstream of the Babon River with a storage capacity of 477,064.581 m3 over an area of 159,021.527 m2 and a total depth of 4 m (including a 1-meter freeboard and 1-meter dead storage). The effectiveness of the retention pond in reducing floods reaches 85.15%.
The outlet of the retention pond utilizes a pump with an effective discharge of
11.5 m3/s. The inlet of the retention pond is equipped with a side spillway featuring an OGEE I crest. The stability of the spillway crest under normal and flood conditions meets safety factors for earthquake and non-earthquake scenarios. Reinforcement of the dam foundation employs CCSP W-450 B 1000 with a total depth of 9 meters, and a capping beam of 750 × 750 mm reinforced with 9D16 mm bars is planned. The channel directing water from the spillway to the retention pond is made of concrete, 10 meters wide and 0.85 meters high, including freeboard. A reinforced concrete embankment, 5 meters deep, is used for the retaining wall of the retention pond belt structure. The structural stability under normal and extreme conditions, including earthquake and non-earthquake scenarios, meets safety factors.