ESCI409 Floods

  • Floods are the most widely experienced catastrophic geologic hazard 

  • Flooding in NH is the most common and most expensive geologic hazard 

  • Flood = river stage that is “too” high 

  • When the watershed delivers more water than the river can hold, the excess water spills out of the channel and onto the floodplain 

  • River stage (aka gauge height) = how high the water is at a specific location  

  • Bankfull stage 

  • How high the water needs to be to fill the river to its banks 

  • Flood occurs when stage exceeds bankfull 

  • Damage worsens as stage increases 

  • Horizontal flood extent increases as stage rises 

  • Bankfull discharge 

  • How much discharge Is needed to fill a river to its banks 

  • Most rivers are big enough to hold their typical flor 

  • If too small, will erode bed + banks and increase cross-sectional area 

  • If too big, then sediment will deposit within the channel and decrease cross-sectional area 

  • Flood stage categories 

  • Below flood – normal levels 

  • Action –  

  • Minor 

  • Moderate 

  • Major 

  • River flooding factors 

  • Excessive rainfall 

  • Snowmelt 

  • Severe storms 

  • Hazardous blocking of stream channel (increases stage without change in discharge) 

  • Trees 

  • Rock avalanches 

  • Stream discharge fluctuates over time 

  • Extreme event frequency 

  • Magnitude of extreme events is inversely proportional to how often they occur ( higher magnitude = lower frequency) 

  • Magnitude = 1/frequency 

  • Typically assume that historical record (frequency of past events) is useful in predicting the future (probability of future events) 

  • Flood recurrence interval: R=(N+1)/M 

  • N = number of years, M – ranking 

  • Probability of occurrence = P=1/R 

  • Human impacts on flooding 

  • Climate change 

  • Watershed development 

  • River corridor development 

  • Annual peak flows are changing for many rivers – higher peaks are becoming much more common 

  • Human impacts change floodplains by changing land use and climate 

  • Buildings and fill replace water volume, raise stream height 

  • Infrastructure increases costs/financial impact when floods occur 

  • Storm drains: built to rapidly deliver storm water to streams, which increases stream height 

  • Urbanization causes floods by routing water more quickly to streams 

  • Flood control: Levees 

  • Raised banks along a stream channel 

  • Causes increased flow further down the stream and therefore worsened floods 

  • Flood control: Dams 

  • Store river water, delaying arrival downstream 

  • Also trap sediment – can create scour downstream 

  • Flood control: retention ponds 

  • Trap surface water and sediments before they get to water 

  • Delays arrival of water 

  • Water may also be lost to infiltration and evaporation 

  • Netherlands “Room for the River” project – began in 2007 after severe river flood damage in the 1990s, approx 2.3bil euros cost 

  • Fun Fact – Weeks Act in 1911 permits federal govt to purchase private land to protect headlands of rivers in the eastern US