I don’t know the specifics described. But if flood records are not kept up to date, and the data used to estimate floods are not updated, then it is possible to have a recent observed flood to exceed the estimate. Flood estimates are just that, estimates. Even with 100 years of flood data, which is unusual in most areas, floods can occur that would be outside of this record, especially in this period of climate change, documented situations which are considered atmospheric rivers, funneling extreme amounts of water to local areas for many days. With flood estimates, one should look at the confidence limits and how far the estimate curve was extended beyond the existing data. Besides the climate considerations, one needs to review the land use, vegetation cover, any periods of drought or severe fire that may have affected data, vegetation, condition of the soil, etc. Knowledge of dams, hydrologic modifications such as stream straightening, gullying, topographic features such as mountainous influences relative to weather patterns may also be considered. Unless the dam is designed to be overtopped during extreme flows, be sure to include bypass channel or waterway to help prevent failure. concerning liability, it is always best to have a licensed engineer review and sign off plans. I would make sure the extreme observed value is included in the hydrologic flood plotting of the annual data or partial duration series. It would be unusual with all the data plotted including the extreme observed to have an extreme observed value to be much different from the estimated flood, but if the extreme observed value has been evaluated to be the 1000 year storm/flood, and your design management preference was the 100 year storm/flood, you would probably stick with the 100 year design and include management practices to lower dam before severe forecast and bypass channel help prevent overflow. If your area is subject to tropical cyclones, hurricanes or similar severe massive thunderstorms or atmospheric rivers, one should consider these factors along with life and property damage potential, and bring these forward into the decision. The licensed and insured engineer is trained to consider these varied factors before approving the project. If the dam is likely to overtop and fail during a 100 year storm, that is a 1% chance of failure in any year, so if the dams life expectancy is 100 years, there is a good chance 100 x 1% of failure. In some instances, the active lowering of the reservoir pool and bypass channel can prevent or mitigate failure damage.

As a hydrologist, you are likely aware that the estimation of flood values involves a degree of uncertainty due to the complexity and variability of the natural systems involved. There are several potential sources of error in flood estimation, including errors in the rainfall data used to drive hydrological models, errors in the model structure and parameterization, and uncertainties in the historical flood record used to calibrate and validate the model.

To address the issue of observed flood values exceeding estimated values, there are several potential steps that could be taken:

As for authentic references, I recommend reviewing the following resources:

• "Flood Estimation Handbook" by the UK Environment Agency: This is a comprehensive guide to flood estimation that provides detailed guidance on the various methods and sources of uncertainty in flood estimation.
• "Hydrology and Floodplain Analysis" by Philip B. Bedient, Wayne C. Huber, and Baxter E. Vieux: This is a textbook that provides an in-depth introduction to hydrology and floodplain analysis, including methods for flood estimation and sources of uncertainty.
• "Handbook of Applied Hydrology" by Ven Te Chow, David R. Maidment, and Larry W. Mays: This is a comprehensive reference book on hydrology that covers a wide range of topics, including flood estimation, hydrological modeling, and sources of uncertainty.

We do not need to build a new dam in Iran.

A elaboração de um projecto seja ele qual gor a especialidade ( ponte, barragem, hidrologia, porto), face 'a sua importância o tempo de vida util e o periodo de retorno de um determinado evento. Ou seja, qual a probabilidade de ocorrência de um determinado evento ser excedido para um determinado período de tempo.

para o efeito necessita-se de informação confiável (base de dados) sobre os registos historicos de um evento ser excedido. Por exemplo: Se estiveres a projectar uma pequena barragem com um período de vida util de 20 anos, teras que considerar uma cheia com um periodo de retorno não inferior a 20 anos, ou seja em que a probabilidade de ocorrencia vir a ser excedido sera 1/T ou seja 1/20. Se for mais importante ( período de vida útil > 200 anos), terás que considerar como cheia de projecto, uma cheia que tenha um período de retorno superior a 200 anos, ou seja a probabilidade que essa situação se verifique podera' vir a verificar-se uma vez a cada 200 anos. Como facilmente poderás concluir que os caudais serão muito diferentes, cada vez maiores quanto a probabilidade de ocorrência será mais rara.

Em conclusão, deveras ter em atencao como caudal ( mediante boa ponderaçã/reflexão), os seguintes aspectos:

1- Período de vida útil da infraestura ou estrutura

2- Período de retorno de um evento (cheia/sismo/vento), ou seja a probabilidade de ocorrência que esse evento se pide verificar.

3- Analise de risco, calculo da probabilidade que essa (cheia/sismo, etc), pode vir a exceder a cheia de projecto . Terás que avaliar até que ponto, no caso dessa probabilidade ser excedida ( vir a ocorrer) se o risco é admissível (perda de vidas, custo de danos, ou só custo de reparacao. Enfim, principalmente independente da análise técnica o mais importante é o bom senso e sensibilidade técnica-human do engenheiro.

Espero tercsido util. Abraço

ernesto carneiro

Be sure that the estimated flood data were missing or instrument error, unless our observed data can be taken a minimum of two years flood data recording and adjust the previous data uncertainty errors

Observed data