1 - Introduction

4- Types of Floods

5 - Occurrence of widespread floods

     The focus of this section is on the variability of water availability.  The day-to-day variability of precipitation was considered much earlier, while the seasonal variability of soil water - its nature depending on the climate regime - was discussed more recently.  Within these, there are minor fluctuations from one period to the next, fluctuations that a water supply system has to deal with.  A reservoir, for example, must be built sufficiently large to provide water during a long dry spell.

    Most of this section, however, emphasizes extreme events - events that test any water system, and sometimes the whole social system itself.

At one extreme are droughts - usually lasting at least a few months, and covering at least a portion of a state.

At the other extreme are floods, lasting a much shorter time, and usually covering a much smaller area

Drought is obviously a time of water shortage - vegetation wilts, rivers are low or dry up completely.  But in fact drought is very difficult to define - a drought in moist North Carolina probably seems like a flood in arid Arizona - we see and respond to a drought in our summer, when water is needed for vegetation growth (or lawn watering), in a different way to a similar water shortage in winter - there may be a shortage of soil water, and crops wilt or die, at the same time as there is a shortage of water for domestic use - or it may be the opposite, bumper crops in the field while cities have water restrictions.

More formally, the definition of drought:

- will vary from place to place, depending primarily on the normal climatic conditions and the common variability of the climate, and 

- will depend on the areas of human activity which are likely to be influenced by the water shortage 

Consequently there are many possible definitions of drought.  The most common is the Palmer Drought Severity Index.  This has a scale from -4 to +4, with drought getting increasingly severe as the number becomes more negative (positive numbers indicate 'wet spells', although this index is not used to suggest the likelihood of floods).  The maps below employ a modification of the Palmer scheme. The Palmer index itself has two main versions, to deal with the two major types of drought, dependent on the aspect of the environment of concern:

Agricultural drought: this primarily concerns the status of the soil water, and is geared towards agricultural interests.  In North Carolina these are usually relatively short-term events, often starting a month of so into the growing season and lasting throughout the rest of the season. The rains, and low evaporation rates of winter usually refill the soil, so in our state an agricultural drought rarely extends from one summer to the next.

Hydrologic drought: is concerned primarily with the status of the ground water.  The amount of ground water changes rather slowly with time, and droughts may take months to build-up, and they may equally take months to be overcome.  The water in the soil on top of an area with drought may be more than adequate, so the two types of drought are not completely connected.

    The situation in North Carolina towards the end of summer 2000 was a mild drought in the west and adequate moisture elsewhere.  (This map is based largely on the Palmer indices) 

    Since this August map, the situation got considerably worse, with no rain falling in the North Carolina mountains from map-time until early November.  Forest fires (fortunately involving mainly the burning of underbrush and consequently not too severe) began to occur.    The fires were extinguished, mainly by human fire-fighter action. 

     Then a period of rain set in, ensuring that the fires were out.  The drought has by no means ended - indeed, the November 14 map shows that it is considerable worse.

http://drought.unl.edu/dm/

Is the source of both maps.:

There is an automated sequence of maps there, showing the evolution of the current drought in the west - well worth a visit)

Another way of looking at drought is through stream flow.  In the map at right each colored dot represents a gauging station with at least 30 years of record.  The bluer the dot, the more the gauge is at the 'high flow' end of the record, the green indicates reasonably close to normal, while the red shows low flow.  Many of the western rivers clearly show a low flow - a drought condition.

The French Broad River looking downstream from the Asheville gauging station

The Asheville hydrograph for the time when the picture at left was taken (September 2, 2000). The same gauge is used for the hydrograph of the flood of 1916 below.

Drought is not, historically, confined to the mountains of North Carolina.  The dates and locations of a few major previous events are shown at left.

Coastal/Storm Surge Floods: associated with water 'pushed' ashore as waves which have been driven inland by strong winds off shore.  Many of these are associated with hurricanes or nor'easters.  However, they can occur at any time. (The map at right indicates the areas likely o be affected)

Flash floods: these are associated mainly with local (thunder) storms, giving intense rainfall for a short period over a rater small area.  They give a rapid rise in the hydrograph for the basin affected, and often an almost equally rapid fall.  They tend to be difficult to forecast far enough in advance to give adequate warning.  Most occur in the mountains.

Wide-area floods:  commonly associated with situations where a large amount of rain falls - not necessarily highly intensely - over a large area onto already saturated soils.

The best known, and probably the biggest wide-area flood is that associated with Dennis and Floyd in 1999.  Dennis gave a large amount of rainfall which caused some, but not too severe flooding.  More importantly, it ensured that the soil was saturated, so that the rains from Floyd had nowhere to go but overland.

Most major floods in our state seem to be associated with two weather events - usually two hurricanes one behind the other.  The diagrams here indicate the synoptic weather events leading to some of the major floods of the twentieth century.

Returning to almost the same spot in the mountains as for the current drought, we can also look at one of the major floods of the last century (which also emphasizes that floods are by no means confined to the Coastal Plain).. The hydrograph for July 1916 at Asheville (at the gauge shown at right), makes the flood obvious.

Hydrograph diagram source:

US Geological Survey, North Carolina District Office

 http://nc.water.usgs.gov

    This great 1916 flood completely covered the walls of Biltmore Railroad station (at right), and reached some way up the roof.  Downtown Asheville (seen in the left distance) was sufficiently high to be above flood level.

   The masonry railroad bridge across the French Broad just upstream of teh gauge acted as something of a dam - increasing the level of flood at Biltmore, but preventing the gauge showing an even bigger record stream flow.