عنوان مقاله [English]
The runoff response from arid and semi-arid catchments is a problem that is difficult to approach and is not satisfactorily nswered yet. One of the reasons is that runoff events in dry regions are rare, and difficult to observe and to monitor. Another reason is the fact that overland flow, which plays a crucial role in dry watersheds, is difficult and complicated to quantify Channel incision is part of denudation, drainge -network development and landscape evolution. The causes of river channel are numerous. It is generally agreed that runoff generation in drylands is caused by infiltration excess or Hortonian overland flow. Partial area contribution and variably contributing areas are a phenomenon in drylands as well The focuses on time factors is very important in practical study and its bring in modeling is fundamental. Slope erosion by runoff can be investigating in related to time scale . Disturbance of slope surface by runoff can be explanation in during time. When flow water on slope is faster and time concentration is short ,surface potential for erosion is very high. In this conditions is shaping liner erosion forms on slopes, such as , Ravin , Gully and Rill. In these slopes also is occurred flash flood . Ojhan Chay catchament located in the eastern slopes of Sahand mountain .Load sediments is very high in this catchement .The purpose of this paper therefore is measurement of runoff concentration time and is estimated flow velocity on slopes.
Ojan Chay catchament is one sub-basin that located on eastern slopes of Shand mountain .This basin is located at 37° 54' N to 37° 44 ' and from 46° 50' to 46° 31'.Ochan River is one reach of Achy Chay that it at western site adjacent to Saaied Abad Sub-basin, and at eastern site is adjacent to other sub-basin. Ojan basin is experience intensive liner erosion and all forms of liner erosion is created on foot and sharp slope .Geology and lithology of this basin is various .Sediment and Igneous rocks formed base of this basin. Thickness of these rocks is high .Diorit ,Volcano sediments ,Ash ,Igenberits are sample of these rocks in Ojan basin. Miocence sediment rocks are contained of Marl-Gyps and Sandstone. Quaternery sediments as trase and alluvial fans is in northern site of basin. This formation is made of lay ,clay and sand .
Material and Methods
In order to estimation of runoff time concentration on slope surface in Ojan Chay basin ,is divided to 140 parts and all accounts is made in these parts . Many data is obtained from these sub-basin properties .Some data is obtained from digit maps and others data is obtained from field studies.In this paper, we focused on time concentration estimation by used of Tc1,Tti and Tc. Tci ,Tti and Tc is defined in terms of channel Lengths ,storm and slope .Where l is channel length ,S is slope ,p is storm and n is Maning Index. We used in Tl1 , maning index table . Velocity of flow is estimated by used of V1, V,Tt,T1,... V1,V are defined in term of slope ,channel length,storm ,k (is constant that related to flow type ).The results are show that on maps .
Results and Discussion
Surface flow that on slope is concentrating and is flowing in single channel ,have very high power for erosion .When surface material are suitable to erosion ,is created liner erosion forms ,such as ,Rill,Gully ,...,on slope by flow . River Ojan have very high bar load in water .All parts of these sediments are due to slope erosion by rill and gully erosion on slope and footslopes. Ojan Basin is a mountainous basin ,for this reason ,slope factor play main role in erosion .Central and northwestern site of Ojan slope is high because erosion in these sites is intensive . In study area time concentration is related to many factors .slope ,length of channel ,storm are affects factors on slope erosion. In this study these factors is considered .The results of investigation on time concentration show that rate of time concentration is high in central and northwestern site of Ojan slope.In this site surface of slope is disturbance by rills and gullies .In these areas lengths of channel is short ,such as , rills. Water flow due to shower is moving quick. Basins these single channel are not big and these shapes are round that water is flow into channel that is located at foot slope. Investigation on velocity of runoff in Ojan (by V1) show that flow velocity is under 10 second in all parts of basin .In this method ,channel length play very important role in velocity of flow on slope ,therefore short rills is made on slope . Velocity of flow is quick on slope that rills is created .
Runoff on slope is creating initial channels and then is disturbance surface and delivered sediment to foot slope channels. Sensitiveness of surfaces for intensive erosion is high when flow velocity is high and time concentration is low. In these slope we can seen vary forms of liner erosion ,such as ,rills, gullies,....Also in these slope is experience flash floods. Some parts of rain water is not influences into soils ,because many parts of water is reduced to runoff. Investigation is suggested that surface materials and topographical factors are suitable to liner erosion ,short channels are made by concentrated water . Channels have high depth in foot slope in Ojan Chay. At upslope area of basin is not vast. Length of channel have short in these site. Due to , time concentration is short .When area at upslope is vast , rill is not created ,but gully is created ,in foot slopes. In these condition ,slopes is gentle and velocity of flow is slow. Bcause area is vast and therefore amount of water is high ,gullies big and deep.