Flood risk management are considerably influenced by several factors, such as all sources of flooding, social circum-stances, policy and even the potential for local economic growth. To encourage government, business, community and oth-er parties to continue investing in flood risk management projects, it is necessary to give understanding that the projects can also provide economic benefits through systematic predictions and assessments of costs, benefits and social values, espe-cially on flood-affected communities. This study aims: (1) to develop knowledge and understanding on small-scale flood risk management project in Malang City, Indonesia, and; (2) to assess the economic efficiency of the project investment considering all benefits, both monetary and non-monetary. The research method is a mixed method combining quantitative questionnaires (N = 53 from 162 families) with qualitative in-depth interviews (N = 10) and field observations. The runoff discharge and the inundation depth were calculated using hydrology and hydraulic analysis, while the economic efficiency was analysed using cost benefit analysis (CBA). The results show that the community-based flood risk management system can reduce the flood risk up to 30% compared to before the implementation of that system. This system also provides direct financial benefits through the use of drainage channels for fish and vegetables farming. It causes the increase of the net so-cial benefit about 70–90% and the net present value (NPV) greater than zero (NPV > 0). Therefore, the project investment is recommended to be proceeded.

Lake Limboto, situated in Gorontalo, Indonesia, confronts severe threats jeopardizing its sustainability. Widespread deforestation in the watershed area has led to excessive sedimentation within the lake, consequently diminishing its water storage capacity and compromising its flood control function. This degradation has manifested itself in downstream droughts during the dry season and flooding during the rainy season. Historical data reveals a stark reduction in Lake Limboto’s size, plummeting from approximately 3,644.5 ha in 1991 to around 2,693.9 ha in 2017. This study aims to provide comprehensive examination of the interplay between the socioeconomic status of the local community and their understanding of the lake ecosystem. Furthermore, it delves into how these factors produce synergies that shape and impact community involvement in sustainable lake management initiatives. Hypothesis testing yielded significant results, affirming the existence of a positive correlation between socioeconomic status, knowledge of the lake ecosystem, and active community participation in sustainable lake management efforts. The findings underscore the critical importance of socioeconomic factors that need to be considered when designing strategies for the preservation and sustainable management of Lake Limboto. Integrating the community into conservation initiatives is necessary, given their intrinsic relationship with the lake. By acknowledging and leveraging the nexus between socioeconomic status, ecological knowledge, and active participation, stakeholders can formulate more effective and inclusive strategies for safeguarding Lake Limboto’s ecological integrity. This study contributes valuable insights for policymakers, environmentalists, and local communities alike, emphasizing the necessity of collaborative efforts to ensure the long-term resilience and vitality of Lake Limboto.