Mathematical Modelling on the Operation of Water Control Structures in a Secondary Block Case Study: Delta Saleh, South Sumatra

Abstract

The project Land and Water Management of Tidal Lowlands (LWMTL) was set forth in Indonesia to reclaim vast areas of waterlogged tidal land for agricultural exploitation. The objective of this study was to simulate water management systems near the farmers field by controlling flap gate culverts in secondary canals (SDU) and stop logs in tertiary canals under different scenarios of irrigation, drainage and flushing. Based on data (hydraulic and hydrometric) collected during a field survey in August 2005, a 1D mathematical model (DUFLOW) for the canal system was developed and calibrated. For boundary conditions of the model, tidal water level fluctuations in the primary canals were used. Different rainfall intensities (10 mm/day, 30 mm/day and 80 mm/day) and evaporation of 3 mm/day were modelled in order to evaluate the hydraulic response of the system. Water normally flows from the secondary canals to the tertiary canals. Water level is kept and maintained as high as possible. The model shows that water level in the tertiary canals increases in time; the level also depends on tidal water level outside and rainfall intensity. In the case of 80 mm/day rainfall, water level in tertiary canals will reach around 2.00 m above median sea level (+MSL, more or less the ground surface elevation) after one day. At this level, water can be utilized for agricultural production. Flushing can be done by operating the culverts and water can flow out from SDU to the primary canal. Water in the tertiary canal can be set at low elevation +1.30 m +MSL for flushing to the SDU and primary canal. Based on the hydraulic performance of the mathematical model, water level can be controlled to improve the hydro-topographical conditions of the area which can change from C/D to A/B category. A one way flow system can be realized where the flow will start from the SDU to tertiary canal and to the field. During low water, drainage can also be regulated from field to the tertiary canal and to the SDU. Finally, related to the water management system in this secondary block some suggestions can be presented. In the future this temporary dam in the SDU should be replaced by a permanent control structure and culverts should be completed with a flap gate. Based on the water level in the SDU (behind the dam), operational rules of the tertiary water control structures have to be derived and used by the farmers.