◙ Objectives of the lecture: ●Understanding the effects of lithology, structure, climate, topography and permeability (infiltration capacity) on the drainage texture. ●Understanding the effect of lateral erosion in the development of stream meandering. ●Understanding the effect of dip angle of rock strata in the formation of different topographic features. ●Knowledge or identification of the characteristics of the fluvial cycles. ◙ Lecture contents: ●Drainage texture and its implications: -Drainage density. -Stream frequency. ●Influencing factors on the drainage texture: -Climate. –Infiltration capacity. –Amount of initial relief. -Rock structure. –Erosion cycle. –Sheet wash and creep. ●Stream meandering and lateral erosion. ●Influence of homoclinal structure upon topography. ●Topographic types on homoclinal structure: -Cuesta. –Hog back. –Homoclinal ridge. –Mesa. –Butte. ●An idealized fluvial cycle: -Young stage. -Mature stage. -Old stage.
Drainage texture *Drainage texture is the relative spacing of drainage lines and it includes: A- Drainage density: It is the total stream length divided by the total area of the basin. Drainage texture = (Total length of the streams in a given basin / Total area of the basin) Unit: km/km2 → normal basin, m/m2 → miniature badlands B- Stream frequency: It is the total number of stream segments divided by the total area of the basin. Stream frequency =1/km2
*Drainage texture does not refer to the steepness of slopes, amount of relief and stages in the geomorphic cycle. *Drainage texture is expressed by relative terms (fine, medium & coarse) texture. *The factors which influence on the drainage texture are: 1- Climate: It affects on drainage texture both directly and indirectly, the amount of precipitation influence directly the quantity and character of runoff, for example: In areas of equal factor except in rainfall percentage, those have larger rainfall percentage have more drainage line due to high surface runoff, and indirectly climate affects drainage texture by its control upon the amount and type of vegetation cover through their influence upon the amount and rate of surface runoff.
2- Infiltration capacity: i)It is referred also to the permeability of the mantle rocks and bed rock. ii)It is the most important single factor. iii)Drainage lines are enormous over impermeable materials (for example clay, shale)) than over permeable materials ( for example sand and gravel). iiii)It is influenced by a large number of factors which are: ⋆ Soil texture (size of individual particles). ⋆ Soli structure (arrangement of individual particles). ⋆ Amount and type of vegetable cover, biological structure in the soil (root performation, animals borrows). ⋆ Temperature of the soil (frozen or not). 3- Amount of initial relief: drainage lines develop in large number upon irregular surface than upon one which lacks conspicuous relief. 4- Rock structure: includes massiveness, abundance of joints and faults.
5- Erosion cycle: it appears that drainage texture is finest during maturity stage when drainage lines are enormous. 6- Sheet wash and creep: drainage lines decrease in areas of dominant sheetwash and creep. So: ⋆ Bad land topography → fine drainage texture. ⋆ Impermeable clays & shales, high vegetation&high rainfall→ extremely fine drainage texture. ⋆ Sand & gravel, outwash plain & glaciated areas→ coarse drainage texture. ⋆ Gravel plain fewer surface drainage than till plain underlain by impermeable clay till.
Stream meandering and lateral erosion 1) When a river has cut- down nearly to base level, it flows more slowly with a reduced gradient and begins to swing from side to side of its valley. The energy imparted to the load of sediment which it carries is expended in the widening of the valley by lateral erosion, and the course of the river develops big loops called meanders.
2) The length of the loop (meander) when fully formed is about 16 times the width of the stream. 3) On the concave side of a curve (bend) the bank is undercut and eroded, while detritus is deposited on the convex side, with the formation of banks of sand and gravel. 4) The flow of a river at a bend moves from one side of its channel to the other side. 5) The scouring effect of the bed current often results in a deepening of the channel on the concave side of the bend. 6) As the meander progress, a flood plain (alluvial flat) is eventually developed, and the river flows its own deposits in the widened valley. 7) When the meanders reach their full size, they tend to shift bodily downstream. 8) If a river in the meandering stage breaks through the neck of land at the end of the loops (below figure –d-), it will leave a crescent-shaped lake called an (ox-bow) or (mort-lake).
Influence of homoclinal structure upon topography Sedimentary rocks of uniformly gentle to steep dips are called homoclinal structure. Due to varying resistance of rocks to the weathering and erosion in the homoclinal structure, the following topographic features are formed whose major differences are related to the steepness of dip. ⋆ Cuesta: - General expression of gently or moderately dipping strata, dip angle is less than 25o. - Slope differed at both sides. - May shift many miles. a cuesta is a hill or ridge with a gentle slope (backslope) on one side, and a steep slope (frontslope) on the other.hillridge
In geology and geomorphology, cuesta refers specifically to an asymmetric ridge with a long and gentle backslope called a dip slope that conforms with the dip of a resistant stratum or strata, called caprock.dip slopedipstratumstrata caprock Because the slope of a cuesta dips in the same direction as the sedimentary strata, the dip angle of this bedding (&theta) can be calculated by (v / h) = tan(&theta) where v is equal to the vertical distance and h is equal to the horizontal distance perpendicular to the strike of the beds.
⋆ Hog back: Sharp crested ridges. Dip angle more than 45o. No too much difference in the slope of both sides (approximately equal). Remain fixed in position. hogback In geology and geomorphology, a hogback is a long narrow ridge or series of hills with a narrow crest and steep slopes of nearly equal inclination on both flanks.ridge Typically, this term is restricted to a ridge created by the differential erosion of outcropping, steeply dipping (greater than 30° - 40°),erosionoutcropping
A homoclinal ridge or strike ridge is a hill or ridge with a moderate, generally between 10° to 30°, sloping backslope.hillridge ⋆ Homoclinal ridge: Homoclinal ridges are the expression of regional outcrops of moderately dipping strata, typically sedimentary strata, that consist of alternating beds of hard, well-lithified strata, i.e. sandstone and limestone and weak or loosely cemented strata, i.e. shale, mudstone, and marl.stratasedimentarysandstonelimestoneshalemudstonemarl Its backslope is a dip slope, that conforms with the dip of a resistant stratum or strata, called caprock.dip slopedipstratumstratacaprock Caprock or cap rock is a harder or more resistant rock type overlying a weaker or less resistant rock typerock
⋆ Homoclinal ridge: - Develop in areas of moderately dipping strata. - Difference in the slope of both sides is observable. - Intermediate form between cuesta and hog back. ⋆ Mesa: 1- isolated tableland capped with a protective covering. 2-horizontal in altitude. is the American English term for tableland, It is a characteristic landform of arid environments, particularly the Western and Southwestern United States in badlands and mountainous regionslandformWesternSouthwestern United Statesbadlands ⋆ Mesa:
⋆ Butte: similar to mesa but with more limited summits. A butte is an isolated hill with steep, often vertical sides and a small, relatively flat top. Buttes are smaller than mesas.
An idealized fluvial cycle Stages of fluvial cycle: Young stage. Mature stage. Old stage. Young stage: -there will be a few large tributaries, numerous short tributaries will be extending themselves by headward erosion. -valleys have V-shape cross section and will be deep valleys. -There will be general lack of floodplain development. -waterfalls and rapids may exist where stream courses cut (cross) beds of resistant rocks. - stream divides will be broad and poorly defined. -stream meandering may exist, but the meanders are those on a flat surface.
Mature stage: -valley have extended themselves, so the region now has a well integrated drainage system. -adjustment of streams to lithologic variations, such as existence of some longitudinal tributaries along belts of weak rock. -stream divides will be sharp. -any waterfalls that existed in young stage have been elminated. -floodplains are more existent. -meanders are more conspicuous but in contrast to those of young stage, they are free to shift their position over the floodplains.
Old stage: -tributaries to main (trunk) stream are fewer in number than in mature stage. -valleys are extremely broad, gently sloping laterally and low sloping longitudinally. -There is marked development of floodplain. -height of interstream areas have been reduced and stream divides are not sharp. -lacks, swamps and marshes are present but they are on the floodplains and not in interstream as in young stage. -Extensive areas are at or near the base level of erosion.
◙ References: 1-Thornbury, W. D., Principles of geomorphology (2 nd ed.). John Wiley & Sons, Inc., New York, 594p. 2-Bloom, A. L., Geomorphology- A systematic analysis of Late Cenozoic landforms (3 rd ed.). Prentice-Hall of India Private Limited, New Delhi, 482p. 3-Ritter, D. F., Process geomorphology (2 nd ed.). Wm. C. Brown Publishers, lowa, USA, 579p. 4-Spark, B. W., Geomorphology (2 nd ed.). Longman Group Ltd, London, 530p. 5- www. Wikipedia.org