1887

oa Civil Engineer in South Africa - Hydrological myths

Volume 25, Issue 7
  • ISSN : 0009-7845

 

Abstract

Hydrological techniques for the estimation of floods have not advanced much since the development of unit hydrograph methods in the 1930s. The technique of discounting losses using a coefficient has been readily adopted without a great deal of thought. The linear hydrological methods have all sprung from the same reasoning. It is invariably assumed that the design storm for any catchment is that with a duration corresponding to the travel time down the catchment. The flood due to 2 mm of effective rainfall is assumed to be twice that due to 1 mm of rainfall.


Hydrographs for different storm durations are obtained by subtraction of cumulative hydrographs. Risk is accounted for by assuming the recurrence interval of the storm is that for the flood. The intensity of the storm is generally assumed to be constant. The method of isolating storms of specific duration is often not understood by the drainage engineer.
Attempts have been made to rationalize soil losses, for example the Soil Conservation Service in the United States has proposed a method for evaluating losses based on soil type.
In an attempt to rationalize and establish a more theoretical basis for estimating floods, research is now being undertaken in the field of overland flow routing and analysis of drainage systems. Numerical methods render the use of linear methods unnecessary and it is recommended that engineers adopt the kinematic method where possible. The mathematical technique referred to as kinematic theory has been developed to some extent but has not been applied extensively yet.

Die hidrologiese metodes vir die berekening van vloede het nie veel gevorder sedert die eenheid-hidrograaf in die 1930's ontwikkel is nie. Die metode van afvloeiberekening met gebruik van 'n koëffisient is te geredelik aanvaar. Die lineêre metodes in die hidrologie berus almal op die selfde beredenering. Dit is aanvaar dat die ontwerpstorm vir enige opvanggebied dié is met 'n duurte wat ooreenstem met die reistyd met die opvanggebied af. 'n Vloed veroorsaak deur 2 mm van effektiewe reënval is aanvaar as twee keer die wat veroorsaak is deur 1 mm reënval.


Hidrograwe vir verskillende stormlengtes is bereken deur bloot van totale hidrograwe af te trek. Risiko is bereken deur te aanvaar dat die tyd tussen storms dieselfde is vir die vloed as vir die reënval. Dit is aanvaar dat die storm deurgaans dieselfde sterkte het. Die dreineringsingenieur het dikwels nie die metode verstaan waarvolgens storms van 'n spesifieke tydsduur geïsoleer is nie.
Pogings is aangewend om grondverliese te rasionaliseer - die VSA se grondbewaringsdiens het byvoorbeeld 'n metode voorgestel waarvolgens verliese volgens grondtipe bepaal word. Navorsing word tans gedoen op die gebied van roetebepaling van oorlandse vloei en ontleding van dreineringstelsels. Numeriese metodes maak die gebruik van lineêre metodes onnodig en dit word aanbeveel dat ingenieurs die kinematiese metode waar moontlik volg. Die wiskundige tegniek bekend as die kinematiese metode is reeds redelik ver gevorderd, maar die aanwending daarvan was tot dusver nog maar beperk.

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/content/civeng1/25/7/EJC24916
1983-07-01
2016-12-10

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