Analyzing the hydraulic performance of water gates due to the impact of extreme rainfall on tidal lowlands: a case study in Katingan rice fields area, Central Kalimantan

Rice fields in the Katingan tidal lowlands are inundated due to spring tides and extreme rainfall. Excessive inundation in the rice fields causes crop failure. Therefore, the study was conducted to analyze the use of water gates in the secondary canals for controlling the inundation. The unsteady flow simulation boundary conditions are the upstream discharge with a return period of 5 and 25 years and measured tidal. The water flow is controlled by closing the sluice gates at the water level elevation of +3.50 m, and flap gates close during high tide and rainfall. This research shows that controlling the water flow by gates decreases the water level and inundation area. Under rainfall with a return period of 5 years, the inundation area decreased by 79.03 %, and the maximum water level decreased by 0.14 m. In the rainfall with a return period of 25 years, the inundation area decreased by 75.31%, and the maximum water level decreased by 0.11 m. The water gates’ operation incompletely decreases the inundation and water level beneath rice crops’ threshold inundation depth and duration. Therefore, there is a need for additional water control to optimize the decrease of inundation in the rice fields.


Introduction
Lowlands are naturally inundated due to the low topography.Inundation can be caused by tidal influences, overflowing river water, or rainfall, where this source of inundation differentiates between tidal and lebak lowlands.The characteristics of tidal lowlands are located near the coast, while the lebak lowlands are far from the coast [1].Indonesian government developed the lowlands for various commodities such as agriculture, fisheries, and ponds.However, the marginal nature of the lowlands resulted in several problems, such as flooding, drought, and low water quality for agriculture [2].
Katingan tidal lowlands are located near the coast of Central Kalimantan Province.Currently, there are several issues concerning water quantity.During the rainy season, there is excess water in the rice fields, but in the dry season, the water is deficient [3].This problem is caused by high tides during the rainy season, while in the dry season, the tides are low and cannot flow into the rice fields.Sedimentation in canals [4] and the malfunctioning water structure resulted in uncontrollable high tides and rain discharge.The characteristics of Katingan tidal lowlands also affect inundation conditions in rice fields, 1313 (2024) 012005 IOP Publishing doi:10.1088/1755-1315/1313/1/012005 2 where the area is located in river zonation IIa [5] with hydro-topography A and B [6], resulting in the influence of high tides, and rice fields continuously inundated [7].
The impact of climate change will worsen inundation conditions.In agricultural land, climate change caused decreased food crop production [8].In Indonesia, the impact of climate change has increased temperature, sea level, and rainfall intensity [9][10][11][12].The low-lying characteristics, decrease of the water system, and the impact of extreme rainfall cause an increase in the depth and area of inundation in the Katingan rice fields [13].Inundation with a depth of more than 0.32 m will damage rice crops [14], and the duration of inundation over three days results in crop failure [15].Therefore, controlling inundation in rice fields is crucial to maintain rice production.
There are several studies related to water management systems in lowlands.In Rotterdam, the Netherlands, water level control in the lowlands uses polders and pumps so that flood discharge does not flow into land or settlements [16].In the Dadahup lowlands, Indonesia, the water gates decrease the water level in the land during the rainy season [17], and in the Anjir Serapat lowlands, combining gates and canal normalization decreases the area of inundation due to rainfall with a return period of 5 years [18].Rice fields in the Katingan tidal lowlands have an essential role in maintaining rice sufficiency in Central Kalimantan.The lack of water management systems, low-lying characteristics, and the impact of extreme rainfall will decrease rice production.However, the research on the impact and controlling the inundation due to extreme rainfall in tidal lowlands is limited, especially in Katingan.Therefore, it is necessary to simulate and analyze the hydraulic performance of water gates to control the inundation in rice fields.

Study Area
Katingan tidal lowlands are in the Katingan watershed and located in an estuary directly adjacent to the Java Sea, as shown in Figure 1.The area of the Katingan tidal lowland reaches 8,083 ha with dominant rice commodities.In this research, the study area is a productive rice field with an area of 1,871 ha, located between secondary canal six left and secondary canal six right (white area in Figure 1).For rice crop water needs, local farmers use water sources from the Katingan River, which is influenced by tides and rainfall.The planting period reaches two times a year, with rice production reaching 2.75-3.5 tons ha -1 .The topography of the land is flat and low [3], at an elevation of +3.20-3.80m, with the highest tidal water level at +4.52 m in spring tide.

Impact of Rainfall on Katingan Rice Fields
Climate change increases rainfall intensity [11,12], and extreme rainfall impedes agricultural sustainability [19].In tidal lowlands, increased rainfall intensity will raise the flow discharge from upstream, impacting the lowland area vulnerable to inundation.The inundation area in the Katingan tidal lowland is only 486.66 ha under spring tide.Meanwhile, under rainfall with a return period of 5 and 25 years, the inundation area increases to 504.84 ha and 512.71 ha.The impact of extreme rainfall in Katingan tidal lowlands also increases the depth and duration of inundation [13].The previous study [13] presented the depth and inundation area due to extreme rainfall with a return period of 5 and 25 years.However, this study conducted a re-simulation to re-calculate the inundation conditions, which caused several improvements in geometry and unsteady flow data.

Hydraulic Analysis
Hydraulic analysis using HEC-RAS.The method used one-dimensional flow in the canals and twodimensional flow in the rice fields.The flow connection between the canal and the rice fields uses a lateral structure along the secondary canal, as seen in Figure 2. Inundation control on rice fields using water gates in the secondary canals.The sluice gates are used on the left secondary one and right secondary one canals, and flap gates on the other eighteen secondary canals.The dimensions of sluice gates are 2x1.5 m and 1.5x1.5 m for flap gates, as shown in Figure 3. Unsteady flow data uses the measurement of tides in the Katingan River and primary canals for six days from February 17-22, 2023.Upstream discharge is used from rainfall transformation with a return period of 5 and 25 years.The rainfall data is the maximum of six daily rainfall for nine years from 2014-2022.Rainfall data was taken from a rain gauging station in Katingan, located in the tidal lowlands of Katingan.The calculation method of return period rainfall uses the frequency distribution analysis, while the transformation of rainfall into flow discharge uses the drainage modulus formula.The unsteady flow simulation period was conducted for six days at the highest tide conditions from January to March 2023, where the highest tide was on February 17-22, 2023.

Hydrodynamics of the Canals
The operation of sluice and flap gates decreases the discharge from the downstream secondary canal (Sta.0); instead, the discharge from upstream (Sta.2000) increases due to rainfall, as seen in Figure 4 and Figure 5.The sluice gate operating system causes tides water higher than +3.50 m to retain the downstream of secondary canals, thus decreasing the discharge into the secondary canals.The sluice gates open when the water level decreases to +3.00 m; thus, the water in the secondary canals can drain out to the primary canals.The operation system of flap gates causes the discharge from high tides to be lower than without water gates, as presented in Figure 5.Meanwhile, when the water level upstream of the gates increases, the flap gates will open, and the water in the secondary canals drains to the primary canals.
The high flexibility of the flap gate operations allows the gates to be adjusted under certain hydrology conditions.During high tides and rainfall, the gate is positioned downstream of the secondary canals; thus, the water cannot flow into the canals.Otherwise, when the rice fields need water, the flap gates are positioned upstream of secondary canals; thus, the water flows into the canals.Reduction flow discharge into the secondary canals decreases the maximum water level.In Figure 6, under rainfall with a return period of 5 years, the maximum water level elevation in the canal is at +3.92 m, while after the water gates become +3.83 m.In the rainfall with a return period of 25 years, the maximum water level elevation in the canal decreases from +3.93 m to +3.84 m.

Dynamics of Inundation in Rice Crop
The water gates operation decreases water level elevation and inundation areas in rice fields.In Figure 7, it is shown that during spring tide, the maximum water level elevation in the rice fields decreased from +3.92 m to +3.65 m or decreased by 0.27 m.However, under rainfall with a return period of 5 and 25 years, the decrease in water level elevation was insignificant, from +3.92 m to +3.78 m or by 0.14 m and from +3.92 m to +3.81 m or decreased by 0.11 m.   Figure 7e and 7f show that the controlled water flow under rainfall with a return period of 5 and 25 years, the water level in the rice fields is higher than the controlled water flow at spring tide.This occurs in rice fields located in low areas so that the excessive water cannot be drained to primary canals and stagnates.
Water gates decrease the discharge into secondary canals during high tides and rainfall, as presented in Figure 4 and Figure 5.This condition decreases the overflow into rice fields so that the inundation area is lower than without gates, as shown in Table 1.In the rainfall with a 5-year return period, the inundation area in rice fields reached 504.84 ha, while with the water gates, the inundation area decreased to 105.88 ha or 79.03%.Likewise, under rainfall with a 25-year return period, the inundation area decreased from 512.71 ha to 126.57ha or 75.31%.Operation of the water gates decreased the inundation area, but under rainfall 5 and 25 years return period, 105.88 ha and 126.57ha of rice fields were still inundated by more than 0.32 m. Figure 8 shows that the inundation depth is decreased after controlling the water flow in the canals.However, the inundation depth under extreme rainfall is still above the inundation threshold for rice crops.
Under a 5-year return period rainfall, the duration of inundation with a depth of more than 0.32 m is over two days.Likewise, under a 25-year return period rainfall, the duration of inundation with a depth of more than 0.32 m is over three days.Inundation durations over three days at critical phases of rice crops result in crop failure [15], so it is obvious to have an additional water control to decrease the inundation duration below three days.Figure 9 compares water levels in rice fields before and after control with water gates under rainfall with a 25-year return period.The water level in rice fields can be lowered but is still above the threshold of inundation depth for rice crops.Therefore, additional water control is needed to decrease the inundation in the rice field beneath 0.32 m; thus, the inundation condition is no longer threatening and sustains the production of rice crops.

Conclusions
The research showed that controlling the water flow in the secondary canals by closing the sluice gates when the high tides at +3.50 and flap gates closing during high tides and rainfall results in the water being retained downstream of the gates, and the flow discharge becomes lower than without gates.Controlling the flow discharge in secondary canals, resulting in decreased water overflow into rice fields; thereby, inundation can be decreased.Under rainfall with a return period of 5 years, the inundation area decreased by 79.03 %, and the maximum water level elevation decreased by 0.14 m.In the rainfall with a return period of 25 years, the inundation area decreased by 75.31%, and the maximum water level elevation decreased by 0.11 m.
The water gates can decrease inundation conditions in the rice fields.However, under rainfall with a return period of 5 and 25 years, 105.88 ha and 126.57ha of rice fields were still inundated, and the depth and duration of inundation were still above the threshold inundation for rice crops.Therefore, further research is needed concerning additional water control to decrease the inundation depth in the rice field beneath 0.32 m and inundation duration below three days; thus, the inundation is no longer threatening and sustains the production of rice crops.

Figure 1 .
Figure 1.Location of Katingan tidal lowland where yellow shows the total area while white is the study area (Base map from Google Earth Pro)

Figure 2 .Figure 3 .
Figure 2. Location of lateral structure, sluice, and flap gates on secondary canals

Figure 4 .
Figure 4. Comparison of discharge in secondary canals with and without sluice gates

5 Figure 5 .
Figure 5.Comparison of discharge in secondary canals with and without flap gates

Figure 6 .
Figure 6.Comparison of water level elevation in the secondary canal without and with water gates

Figure 7 .
Figure 7.Comparison of water level elevation in rice fields without water gates under (a) spring tide, (b) 5-year, and (c) 25-year return period rainfall and with water gates under (d) spring tide, (e) 5-year, and (f) 25-year return period rainfall.

Figure 8 .
Figure 8.Comparison of water level elevation in rice fields without and with water gates.

Figure 9 .
Figure 9.Comparison of water level elevation in canals and rice fields under 25-year return period rainfall without and with gates

Table 1 .
Comparison of inundation area and water level elevation in rice fields with and without water gates