Many diversion head-works for irrigation have been designed and constructed in the past few years. Some of them are performing successfully, but some of the schemes have failed. So, it is important to deal with the assessment of the causes of failure diversion weirs and identify the main design parameters implemented for irrigation. In this work, the existing design practices, failure causes, and failure mechanisms in the four Community-Managed Small-Scale Irrigation (SSI) diversion weirs in the Arib Watershed, North Gojjam Zone, Amhara Region, Ethiopia, have been assessed. To identify and characterize the existing irrigation schemes, field observation and measurements, group discussions, interviews, and questionnaires were used, and the data was analyzed by SPSS statistical software. Arc-GIS 10.7 was applied to delineate the watershed of the study area; peak discharge was estimated by the Soil conservation system (SCS); and the HEC-HMS was used for building the hydrologic models. Peak rainfall from Bahir Dar metrological station analyzed for feature 50 years return by using Gumbel distribution method was 148.46mm. The analysis resulted the river peak design flood was 217.7m3/s and 202.9m3/s computed by HEC-HMS and SCS-CN method respectively. The main constraint parameters that contributed to the failure of the irrigation schemes, including planning, institutional, social, operational, and economic, have been assessed. The main problems observed in the diversion schemes are sedimentation at the head-works and canals, damage to intakes and sluice gates, clogging of intakes, and damage to distribution systems. Some of the problems recognized at the institutional, social, and operational levels are due to a lack of adequate community participation, the absence of Water Users Associations (WUA), proper handing over, a lack of training and maintenance, and evaluation and monitoring issues. The study suggests rehabilitation measures for the failed SSI schemes. The results of the study will help the SSI schemes perform better and more efficiently to increase agricultural productivity in the study region, as well as other small-scale irrigation schemes in our country. Key words: Weir failure, HEC-HMS, design parameter, Arib Watershed, SCS, Hydrologic, Hydraulic