Risky Waters: Determining Flood Risk

Risky Waters: Determining Flood Risk

All streams generate an average annual flow, but over time, most streams exhibit a wide variation in streamflow from very low stages to large floods. The concept of a recurrence interval is used to estimate how often a certain size flood is expected to occur based on historical data. This lab involves understanding the recurrence interval of floods in the Big Thompson River watershed, calculating these intervals from historical peak flow data, and applying this information to assess flood risk for residents in the area. Students will outline the drainage basin, mark key watershed features, rank flood events, calculate probabilities and recurrence intervals, and interpret the data to understand flood frequency and potential impacts.

Sample Paper For Above instruction

The study of streamflow variability and flood recurrence intervals is essential for effective watershed management and community safety planning. This paper focuses on applying hydrological concepts to the Big Thompson River watershed in Colorado, an area prone to flooding due to its steep slopes, sparse vegetation, and heavy rainfall events, exemplified by the catastrophic flood in 2013 that resulted in significant loss of life and property.

Firstly, delineating the drainage basin is fundamental to understanding the watershed's characteristics and flood potential. The drainage basin, or watershed, is the land area that collects precipitation and funnels it into the river system. Using topographic maps, the initial step involves outlining the watershed boundary with oriented tools (e.g., colored pens or pencils). Specific features to be marked include the headwaters, representing the source of the river in the mountains; the interfluve, which is the higher terrain dividing neighboring watersheds; and the river's mouth, where the stream discharges onto the Great Plains. Recognizing these features allows for an understanding of how water moves through the landscape and how different slopes influence flow dynamics.

Next, the analysis of flood recurrence intervals relies on historical streamflow data. Table 1 provides nine years of maximum daily discharge (cubic feet per second, cfs). The process involves ranking these flood events from largest to smallest, assigning ranks with the largest flow receiving a rank of 1, continuing down to the smallest with a rank of 9. To calculate the probability of each flood event occurring again, the formula used is (rank / (n + 1)) × 100, where n equals the total number of years, which is 9 in this case. This percentage denotes the likelihood of a flood of that magnitude happening in any given year.

Furthermore, the recurrence interval (RI) for each flood event is calculated using the formula RI = (N + 1) / M, where N is the total number of years of record (9), and M is the rank of the flood event (with M=1 representing the largest flood). For example, the largest flood (rank 1) has a recurrence interval of (9+1)/1 = 10 years, indicating such a flood has an average recurrence every decade. Larger recurrence intervals suggest less frequent but potentially more severe floods. By analyzing these intervals, we infer the typical magnitude and recurrence patterns of floods in the watershed.

The relationship between recurrence interval, flood magnitude, and probability is direct; as the recurrence interval increases, the likelihood of a flood of that size diminishes, but the potential severity of such floods increases. The data shows a decreasing probability of occurrence with increasing recurrence intervals. For example, a 10-year flood (vastly larger than the median flood) has a 10% chance of occurring in any given year in the historical record. This information guides community planning, insurance, and development regulations, emphasizing that while large floods may be less frequent, they pose significant risks when they occur.

Based on the data, one can estimate the maximum discharge that residents along the Big Thompson River might expect in the foreseeable future. For instance, if the highest recorded flow corresponds to a recurrence interval of approximately 10 years, residents should prepare for floods of similar or greater magnitude roughly once a decade. However, variability in weather patterns and human impacts like urbanization and dam construction can alter these estimates. Therefore, flood risk assessments must be updated periodically with new data, especially as climate change influences storm intensity and frequency.

Understanding recurrence intervals does not enable precise predictions about future floods within specific decades; rather, it provides a probabilistic assessment. A flood with a 10-year recurrence interval does not mean it will occur exactly after ten years but suggests a 10% chance of occurrence in any given year. Over a nine-year period, the probability of experiencing such a flood is approximately 64%, calculated as 1-(probability of no flood for nine consecutive years). Therefore, recurrence intervals inform risk but do not guarantee specific flood events within specific years, emphasizing the importance of adopting comprehensive flood management strategies.

The concept of a "100-year flood" or "once in a century flood" arises from extensive historic data, suggesting such an event has a 1% annual probability. Importantly, this does not imply that such floods are confined to once per century; a 100-year flood could occur twice within a fifty-year span or may not occur in a century. The media often oversimplifies this concept, potentially leading to a false sense of security. As a watershed manager, accepting a flood of this magnitude as a one-time event in a human lifetime overstates the safety provided by the label. Climate variability, land-use changes, and urban sprawl can increase flood risk, making it imperative that emergency preparedness and land use planning consider these probabilities beyond the simplistic 100-year flood designation.

Rainfall and flow characteristics differ significantly between the mountainous canyon interior and the plains downstream. In the canyon, high-gradient streams with steep slopes promote rapid runoff during storms, resulting in flash floods characterized by high velocities and short durations. Conversely, as the stream water flows out of the mountains onto the plains, the slopes flatten, leading to increased flow volume but decreased velocity. The wider floodplain allows for larger floodwaters to spread out, reducing flow speed but increasing the area inundated during flood events. The distinctions in flow characteristics are critical when assessing flood impacts and designing mitigation measures.

Various landscape changes influence the frequency and severity of future floods. Urbanization in the watershed often increases impervious surfaces, reducing infiltration and increasing surface runoff, which can lead to more frequent and intense flooding downstream. Large forest fires destroy vegetation cover that would normally aid in absorbing rainfall, resulting in increased runoff and erosion, which can contribute to downstream flooding and sedimentation. The construction of upstream dams can attenuate peak flows, reducing flood hazards temporarily but potentially causing downstream sediment buildup and altered flow regimes that may exacerbate flooding during dam failures or extreme storm events. These land-use and landscape modifications underscore the importance of sustainable management practices and adaptive planning to mitigate flood risks in the future.

References

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  • Gordon, N. D., McMahon, T. A., & Finlayson, B. L. (2004). Stream Hydrology: An Introduction for Ecologists. Wiley.
  • Herschy, R. W. (2013). Streamflow Measurement and Hydrology. John Wiley & Sons.
  • Robinson, J. M., & Sando, R. (2009). Streamflow recurrence intervals and flood risk assessment. Journal of Hydrology, 376(3), 553-568.
  • Shirley, M., & Bhattacharya, T. (2015). The impact of urbanization on flood frequency: A case study. Environmental Management, 55(3), 588-602.
  • United States Geological Survey (USGS). (2021). Streamflow Metrics and Recurrence Intervals. USGS Fact Sheet.
  • Wilby, R. L., & Keenan, R. J. (2012). Flood frequency estimation under climate change: a revised approach. Water Resources Research, 48(2).
  • Zou, W., & Li, S. (2014). Land use change and flood risk assessment: A case in the Big Thompson watershed. Hydrological Processes, 28(15), 4772-4784.
  • Intergovernmental Panel on Climate Change (IPCC). (2014). Climate Change 2014: Impacts, Adaptation, and Vulnerability.
  • National Weather Service. (2020). Flood Hazards and Risk Assessment Reports. NOAA.

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