FLOODS AND DROUGHTS

 

 

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    Floods and droughts are both direct responses to rainfall, or the lack of it.  Both also involve soil moisture.  However, the two are  operative on very different time scales, and usually on different space scales.

    A flood is primarily the result of a single meteorological event which is rather short-lived.  Only a well defined, often small, area is involved.  Frequently the major atmospheric concern is with forecasting the timing, location, and amount of rainfall so that timely watches or warnings can be issued.   

    Drought, in contrast, is a more widespread and long-lived feature.  It develops slowly and usually vanishes almost as slowly.  Most of the time a drought will affect a third or more of the state, straining resources for a wide area.  Long-range forecasts are helping to chart the course of drought.

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Photo 5.3.1: Drought conditions at Concord Lake in Cabarrus County 

Photo 5.3.2:  The Tar River in flood in 1999 at Louisburg, Franklin County

 

Types and causes of floods

 

    In this section we are concerned with the river-based floods associated with the surface run-off discussed in section 3.5.  It is convenient to divide these into two main types likely in North Carolina:

- flash floods: associated with a short period of intense rainfall, usually falling onto a small river and causing a rapid increase in stream flow; 

- widespread river floods: greater area involved than in flash floods, usually taking longer to develop, involving a major area and larger rivers

 

    The third possible type, the storm surges associated with hurricanes, were considered in section 5.1 and are not mentioned further here

 

Flash Floods

 

    A flash flood is possible whenever there is an intense downpour onto a local area. However, a downpour alone is not likely to cause a flood unless other factors are favorable (Fig 5.3.?).  These factors can be summarized as the things which impede percolation of rain water into the soil and encourage surface run-off.  

 

    Many, such as steep slopes, may be characteristic of a particular area.  We do associate flash floods more with the mountains than with the Coastal Plain. Some factors, such as thin soils or sparse vegetation, may be directly associated with the slopes.  They may also be a result of changes wrought on the surface when humans develop the area.  Some factors, such as the soil moisture content prior to the storm, are more meteorologically oriented.  Finally, the diagram indicates that there should be something, or somebody, in the way.  Without it the event will probably not be recorded.

 

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Fig 5.3.?: Factors important in the development of a flash flood

 

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    Probably the best known cause of flash floods are convective storms. Convective storms are ideal rainfall sources, but even then it is common to need a saturated soil before a flood can be generated.  Hence spring is the common time of occurrence - after the onset of convective storms, but before high evaporation rates are established.

 

Photo 5.3.?: Results of a flood in Chapel Hill from a summer thunderstorm giving over 5" of rain in a few hours, July 2000

 

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In the mountains flash floods in winter or early spring, when the water falls on frozen, rather than saturated, soil  are possible.

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Fig 5.3.?: flash floods in western NC as the result of a winter storm 

Fig 5.3.?: Frequency of occurrence of flash floods for each month of the year.

River floods

 

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Rainfall from Floyd & Dennis

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Fig 5.3.?: Denis Rainfall

Fig 5.3.?: Floyd rainfall

 

 

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Fig 5.3.?: Total precipitation, July 1916

Another bridge wash-out, this time from a flood in 1916 - see below - and involving a railroad (photo courtesy of Southern RR??)

Fig 5.3.?: Precipitation. July 15-16, 1916

 

 

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A gauging station after a flood - sandbags on Little Fishing Creek near Enfield after the Dennis/Floyd 1999 floods Dennis/Floyd 1999 Floods at Tarboro - USGS view

 

 

Floods-NC_seasonal_regimes.gif (11206 bytes)     Taking all kinds of floods associated with rivers into account, the following three regimes have been identified.  Note that in all cases floods are most likely in late winter or realy fall but outside the mountains there is a second peak in fall.
Fig. 5.3.?:  The three basic flood regimes likely to occur in North Carolina (from Lecce, 2000)

 

 

Measuring and monitoring drought

 

Drought definition difficult:

        normal (precipitation & evaporation)

        common variability

 

    length of time involved

 

    area of impact:

          agriculture

          hydrology (water supply)

 

Originally, in USA work by Palmer led to the Palmer Drought Severity Index (PDSI) and the Palmer Hydrologic Drought Index (PHDI).  These used for many years - some legal applications - now various alternative methods used.

 

 

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Fig: 5.3.?: The current approach to drought monitoring - a composite  map using several different approaches to defining and monitoring drought

                    

 

 

Did North Carolina have a Dust Bowl?

 

 

 

 

 

Above diagrams suggest drought has been fairly frequent, and usually affected much of the state at the same time.  Seems to have been concentrated in 1920's and 1930's, but no period free. (Pattern of wet spells looks similar, but more concentrated into latter part of century).

    The diagram at right ephasizes the decadal variability in one of the Climate Divisions.

 

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Fig 5.3.?: Number of months per decade with severe drought (PDSI <-3) for the NC Northern Piedmont.

   

    Over the years, there has been a changing impact of drought -

(1) - impacts decreased through government policies, social infrastructure, drought monitoring (NC and US efforts)

(2) - impacts increased through increasing water use (streams and deep wells)

                   scale of effects may increase (drought unlikely to be for 'just' NC) - major disruptions possible -  'global conflict'?

 

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