FLOOD CONTROL 
  

A.  INTRODUCTION

        In the U.S., floods rank second only to earthquakes in
        terms of the damages caused by geologic processes.     
        This reflects two facts:  (1) floods are widespread, and  
        (2) many people live within floodplains. Consequently,
        considerable time and money have been invested in
        efforts to control floods.
 

B.  BACKGROUND

        1.  RUNOFF

             Surface runoff occurs when rainfall intensity exceeds
             the rate at which water can soak into the ground.  As
             runoff from a drainage basin reaches a stream, it will 
             cause a rise in stream flow (or discharge).
                     

        2.  FLOODS

             Floods occur when streams overflow their banks and
             spill out onto their adjacent floodplains.  This process  
             allows streams to deposit some of their sediment load,
             which can create natural levees.

         3.  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; and larger
              floods occur less frequently.  Because floods are truly
              random, it is impossible to predict when they will occur.

C.  RAINFALL AND RUNOFF

         1.  STORM HYDROGRAPHS

              a.  "Flashy" (Efficient) Drainage Basins

              b.  Inefficient Drainage Basins

         2.  INFLUENCE OF GEOLOGY

              a.  Drainage Pattern

              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. 

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.  

              However, because many communities have grown up
              within 100-year floodplains, the approach has been to
              build engineering structures that protect people living
              in these communities. 

         2.  FLOODPLAIN MANAGEMENT

              Attempts to control rivers through engineering are not
              without their consequences, as rivers tend to adjust to
              engineering structures in order to maintain a "graded"
              condition.

              a.  Levees, Floodwalls (picture)

                   The situation in New Orleans:  rebuilding plan

              b.  Channelization

              c.  Flood Control Reservoirs

              d.  River Diversions

              The Lower Mississippi River is an example of a river
              where all of these approaches have been used.