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Box culverts are utilized in situations where natural stream flow intersects with roads and railway lines. This research utilizes a digital elevation model and the water Modeling System software to assess the catchment area of the primary valley and identify the factors contributing to flooding in Qoshtapa City. The study involves an analysis of the existing culvert and generated the necessary data for the design of a new culvert. Despite the presence of the existing culvert, floodwater levels rose to over 1 m above the roadway elevation of Erbil-Kirkuk during the last flood event in 2021–2022. The research collected hydrological and climatic data for the study area, conducted soil type analysis using the Harmonized World Soil Database software, and performed hydraulic calculations to estimate the maximum flood discharge of the valley using the Hydrological Engineering Center-Hydrological Modeling System software for flood return periods of 50, 100, and 200 years, for design, to select the best economic alternative. The new culvert design was executed using Bentley Culvert Master software to ensure that floodwaters can flow through the culvert without rising to street level. The results indicated that the new culvert design surpasses the capacity of the existing one. The results show that the best economic alternative hydraulic design is the first alternative capacity of 201 m³/s of a 100-year return period; the new design cross-section area of the culvert is 52.5 m². [ABSTRACT FROM AUTHOR]

This research uses a new engineering approach in the management of rainfall storms in the Koya basin to delineate the inundation boundaries for the floods that occur in the study area. This covers some agricultural lands and residential areas in the countryside. According to this approach, it is possible to determine the best sites for building small dams and water collection basins for water harvesting. The above-mentioned approach consists of a meteorological and morphological study, soil classification, flood hydrograph modeling, and demarcation of inundation boundaries, to find the depth and velocity of flood flow. The average runoff depth was based on curve number (CN) values. These values were determined based on soil cover types and soil class according to the Harmonized World Soil Database program. Accordingly, the soil class in the study area is Group B alluvial silt and the CN value is determined to be 71. The results showed that the runoff potential varied with land use and soil characteristics. Hydrological modeling is performed by HECHMS software (Army Corps of Engineers Hydrologic Engineering Center - Hydrologic Modeling System), which simulates runoff precipitation process using a SCS Soil Conservation service CN model. A flood hydrograph generates at the catchment area outlet, and the delineation of the floodplains is verified by HEC-RAS software (US Army Corps of Engineers Hydrologic Engineering Center - River Analysis System). Furthermore, the results showed that the flood return period of 100 years could reach sensitive areas such as urban, agricultural, and residential areas. According to the results of this study, there were suitable sites in the catchment area for the establishment of small dams and ponds for water harvesting. [ABSTRACT FROM AUTHOR]