ECODRAINAGE SYSTEM EVALUATION AND CONTROL OF FLOODS IN JOMBANG DISTRICT, EAST JAVA

Jombang sub-district is the most densely populated sub-district because it is located in the middle of Jombang district and is also the center of government. Because it is a densely populated area, resulting in many changes in land use, green land for absorption has turned into a watertight area. The results of the analysis from this aspect indicate that several channels in Jombang District are not able to accommodate the design flood discharge. The calculation of the height of rain design in this study used the Log Person type III method with a return period of 5 years, which resulted in the design rainfall of 157 mm. The results of the analysis show that there are 8 channels out of 74 that are unable to accommodate the design flood discharge. After planning the infiltration wells, a different number was obtained for each road. For Jalan Sentot Prawirodirjo, there are 8 infiltration wells with a reduction power of 51.62%.


Introduction
Flood is a form of natural phenomenon that occurs due to high rainfall intensity where there is excess water that is not accommodated by an area's drainage network. This condition has an impact on the emergence of inundation in the area which can harm the community [1].
Drainage is one of the basic facilities designed as a system to meet community needs and is an important component in urban planning (especially infrastructure planning). The urban drainage system is a component of urban infrastructure that is closely related to spatial planning. Flood disasters that often hit most regions and cities in Indonesia are caused by chaotic spatial planning [2].

Material and Methods
Planning was carried out in Jombang District, Jombang Regency. Starting from January 2019 to March 2019. The data used in this study are rainfall data for the last 10 years, research location data, drainage network maps, and existing drainage data.

Description ot the Research Location
Jombang District is one of the sub districts located in Jombang Regency. The area of Jombang sub district is around 36,40 km 2 , divided into several parts of land including industry, factories, settlements and rice fields.

Data Analysis
Hidrological data analysis Calculating the average rainfall using the Polygon Thiessen method and calculating the value of rain intensity using the Mononobe equation and performing frequency analysis by calculating statistical parameters and then using the selected distribution based on the Chi Square and Smirnov-Kolmogorov test. Design flood discharge analysis Analize the design flood discharge using the Rational method, the Haspers method, the Nakayasu Hydrograph Synthesis Unit (HSS) and the HSS Gama I. The four methods are the Rational method that will be used in this study.

Infiltration well analysis
Analysis of infiltration wells in this study using the method according to Sunjoto. According to Sunjoto (1998), the volume and efficiency of infiltration wells can be calculated based on the balance of water that enters the well and water that seeps into the ground [3].

Result and Discussion
Hydrological analysis is used to determine the design rainfall and the results of processing rainfall data using frequency analysis obtained by Log Person III as follows: The results of the discharge recapitulation using the rational method can be seen in the table below: One of the objectives of applying the infiltration stream is to reduce surface runoff so as to avoid excessive surface runoff that can cause flooding. The amount of surface runoff that can be reduced through infiltration wells depends on the volume and number of infiltration wells. = 23,12 meter The water depth of the well is very high. If the groundwater level is at a depth of 23.12 meters from the ground level, then the infiltration well needs to be turned into a series system where each well with H = 3.Then the number of wells required for the series system is: n = 23,12/3 = 7,70 ~ 8 buah This means that with 8 infiltration wells on Jalan R.E Martadinata, it can store groundwater reserves and reduce flood discharge by 0.00236 m3 / second, or it can reduce flood discharge by around 97.17%. Efficiency = Qsumur / Qeks x 100% Efficiency = 0.08104 / 0.0834 x 100% = 97.17% From the results of calculations using the Sunjoto method (1998) above, it can be concluded that the number of infiltration wells of 8 units can reduce flooding by 97.17%. The discharge reduction is able to reduce the surface runoff load to infiltrate the soil layer as groundwater reserves.

Conclusion
Based on the results of data processing and data analysis, the following conclusions can be drawn: 1. The largest flood discharge in Jombang District is located in the S.LBK.M channel, which is equal to 0.29572 m3 / second. 2. The results of calculations using the Sunjoto method (1998) show that there are 8 infiltration wells on Jalan R.E Martadinata. 3. The existence of 8 infiltration wells can reduce the volume of flooding by 97.17%. 4. Infiltration wells were chosen because they are considered more effective and efficient, because they are also widely used in ecodrainage solutions in Indonesia.