FLOODS AND FLOOD CONTROL   

A.  INTRODUCTION

        There are various types of floods, including flash floods,
        tidal (or storm-surge) floods, and dam-break floods, but 
        our focus here is on riverine (or regional) floods. 

        Floods are more meteorological than geological events,  
        although geology plays an important role in the severity
        of flooding.  Of all the extreme events we shall consider, 
        floods are the closest to being truly random.

B.  IMPORTANT CONCEPTS

        1.  THE HYDROLOGIC CYCLE

             A picture of how water continually cycles through the
             physical environment.  Surface water runoff  (or runoff),
             the part of this cycle that causes floods, is composed of 
             the portion of rainfall that does not soak into the ground
             or evapotranspirate back into the atmosphere.
                     

        2.  DRAINAGE BASINS

             The total land area that contributes runoff to a given
             stream.  A drainage basin (or watershed) is bounded
             by high points in the terrain that form drainage divides.
                            

        3.  FLOODPLAINS

             Flat-lying areas adjacent to streams that are subject to
             flooding when streams overflow their banks.  Floodplains
             form naturally by the process of stream meandering and
             flooding (which creates natural levees).

C.  RAINFALL AND RUNOFF

         1.  STORM HYDROGRAPHS

              A hydrograph is a plot of surface runoff versus time 

              a.  "Flashy" (Efficient) Drainage Basins

              b.  Inefficient Drainage Basins

         2.  INFLUENCE OF GEOLOGY

              a.  Drainage Pattern:  influence of geology

              b.  Topography

              c.  Permeability

        3.  INFLUENCE OF HUMANS

             Urbanization and development lead to the removal of
             forests and wetlands, which serve to naturally retard
             the runoff of surface water.  In contrast, the pavement
             and storm sewers that accompany urbanization inhibit
             infiltration and send runoff to the nearest stream more 
             quickly (thus decreasing lag time and increasing peak
             flows).  Infiltration basins are intended to counteract
             these effects of urbanization. 

         4.  FLOOD FREQUENCY

              Any given stream discharge has a long-term (average)
              recurrence interval.  Natural streams in humid regions
              overflow their banks once every 2 to 3 years, but larger
              floods occur less frequently.  The probability of a given
              magnitude flood depends on its recurrence interval.

D.  LIVING IN FLOODPLAINS

         1.  REGULATORY FLOODPLAINS

              In "a perfect" world, people would live outside of the
              100-Year Floodplain, which would reduce the risk of
              flooding in any given year to 1 percent or less.  

              a.  Floodway:  

              b.  Floodway Fringe:

         2.  FLOODPLAIN MANAGEMENT

              However, because many cities have "grown up" within
              100-year floodplains, approaches are needed to manage
              floods that threaten people living in these communities.  

              a.  Levees, Floodwalls, and Dikes 

                   The situation in New Orleans 

              b.  Channelization (aerial view)

               c.  Flood Control Reservoirs

               d.  River Diversions (picture)

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