La corrélation entre les pluies torrentielles et l'intensité de l'érosion (d'après les observations faites dans un périmètre torrentiel) - Avant-propos

SUMMARY. THE CORRELATIONBETWEEN TORRENTIAL RAINFALLAND THE INTENSITY OF SOIL EROSION. Rain erosion in generai is of importance and considerable interest; the part played by torrential rainfall however deserves specialattention.All rainfall does not cause soi) erosion, but ther is a correlationbetween heavy rainfall and such erosion. The point at wich raiacauses noticable erosion to start is more or less specific for a givenarea and depend up on different factors such as the slope the vegetation,the physical and chemical properties of the soi), the geologica)substrata and so on.Apart from these factors wich have a more or less auxiliary cha-racter in the phenomenon of erosion, it is intensity rainfalwich plays the most important and decisive role. This erosive characterof torrential rain manifests itself partly through the directe actionof rain drops with consequent separation of soi) particles and partlyby transport of materials through run-of f and their removal in thebed of water courses.There is a very definate correlation between these two aspects i. e.between erosion of the soi) and the transport of materials on solidson the ,hand the intensity of torrental rainfall on the other hand.Torrential rainfall is considered as rain with a minimum preci-pitation of 3o m in one day/usualy limited to the sumer seasonwhen rainfall is more concentrated/, or alternatively any rainfall ofless than one hours duration but with a minimum intensity equivalentto 0,3 mm per minute.After many observations on a mountanous area subject to torrentialrain and consisting of 31 bassins draming finto torrents, the followingsfacts were noted :I) That rain erosion becomes most marked and transport of materialbecomes most noticable when rainfall reaches an intensity of 0,3to 0,5 mm per minute.2) That the quantity of solids removed rises very steeply withincrease in intensity of rain above this limit.3) That the total quantity of material removed f rom an area as a result of one exceptionally heavy rain/ in this specific case some2 inni per minute / is often greater than the average amount of materialsmoved per year. The intensity of torrential raia being morenoteworthy f rom the viewpoint of erosion than total precipitation inan area.To determine the carrying capacity of a torrential water coursein terms of quantity of solids it is very convenient to apply Stiny'sformula modified by Hercheulidze and completed whit a coefficienttorrential « m », signifying the degre of torrentiality of the bassin.This formula however does not take finto account the intensity oftorrential rainfall wich is meritorious for soil erosion in the bassin,nor the f requency of such rainfall in a region.The author proposes a new formula to correct this and to take themessing factors finto account so that one can obtain a mor accuratirepresentation of relative degree of torrentiality and erosion in agivon area.To determine the degree of erosion / including the formation andtransport of materials by rainfall /, it is necessary bef ore all to determinethe rains in an area, wich have an erosive character as well astheir frequency during a given period - i. e. their average precipitaarea.-tion per year and nothe total precipitation of al rainfal in theThe paper also records some actual data regarding quantity materialsand solids transported. Il one wishes to apply or compare thesefacts and the results trought out in the text, it is necessary to takeall relevent factors which have been considered by the paper whitaccount, such as :a) An average rainfall intensity of 2 mm per minute.b) An mean duration about 20 minutes.c) A mean frequency of such rains six times in Ioo years.d) A slope of 18 to 22%.e) The geological strata - crystalline schists in the upper reechesof the bassins covering about 50 % of the surface and neogene sedi-ments in the lowereches also covering about, 50 % of the totalsurface.f) Forest cover very modest - average about 10 % over the wholearea.g) Climate-continental with slight Mediterranean influence in theregion.h) Latitude of the area — 40° North.i) Annual precipitation averages 65o mm