Drainage Basin

Part IMorphological Systems

ContentsDrainage Basin CycleStream Characteristics

Stream channelStream flowHydrograph

Fluvial MorphologyBasic form elements of a fluvial erosion landscape.MeasurementsDrainage patterns

Drainage Basin Cycle

Stream Characteristics

Stream ChannelStream flowHydrograph

Stream ChannelChannels may be thought of as a long, narrow trough, shaped by the forces of moving water.Depth:

It is measured at any specified point in the stream as the vertical distance from the surface to bed.

Width: It is the distance across the stream from one water edge to the other. It may be so narrow that a person can jump across them, or as wide as 1.5km (eg. Mississippi, Changjaing)

Stream ChannelCross sectional area (A):

It is the are in square metres or square feet.Wetted perimeter (P):

It is the length of the line of contact between the water and the channel.

Channel Slope (S): It is the angle between the water surface and the horizontal plane. Eg. 50cm/km: it means the stream surface undergoes a vertical drop 50cm for each 1km of horizontal distance downstream. Eg. 0.03 means the stream drops 3 m for ever 100m of horizontal distance.

Stream FlowVelocity:

Gravitational force causes stream flow.Water close to the bed and banks moves slowly that in the deepest and most centrally located zone flows faster.Water will flow faster in a channel of steep gradient than one of low gradient.Mean velocity (V) in streams is commonly equal to about 60% of the maximum velocity.

Discharge (Q):It is the volume of water passing through a given cross section of the stream in a given unit of time. (m3/s)Q = VA

Stream FlowThe optimum channel would be semi-circular shape in cross section.For Q=VA

As velocity ‘V’ increases, cross-sectional area ‘A’ must also decrease.

Q = VA (waterfall, rapids,……)

As cross-section area ‘A’ increase, velocity ‘V’ must increase.

Q = VA (lake, delta, estuary…..)

HydrographIt shows the variation of discharge with time at a certain location of a river.

Components of hydrographDischarge:

It is the amount of water flow in the river. It varies over times and place to place.

Base flow:It is much more constant in amount and the water level is nearly the same.It is discharged from delayed ground water flow in soil during ordinary time.

Rainfall:Its intensity is partly responsible for the magnitude of the peak.

Components of hydrographThe peak:

It shows the maximum flood level.

Rising limb:Its steepness determined by the rate of surface runoff.The higher the peak, shorter the basin lag with the steeper rising limb.

Basin lag:It is the time between the heaviest rain and the peak.The lag is determined by the nature of soil moisture, the structure, relief of the basin and the intensity of the rainfall.

Recession limb:The rate of passing flood.

HydrographIt is great use to engineers and hydrologists in planning of irrigation, drainage system, power (HEP) development, water supply and flood forecasting.In many parts of the world, spring is a time of floods for spring rain and snow-melt.Although some rivers flood quite regularly, the recurrence interval varies from basin to basin and varies with the magnitude of the flood.

Shape of hydrographsThe variation of the hydrograph shape from basin to basin, which shows the dependence of the discharge on geological and morphological characteristics of a drainage basin.A flash stream has a hydrograph with sharp peak, for high immediate surface runoff, with little absorption and storage of water in the basin.

Fluvial MorphologyMeasurement of geometrical properties of the land surface of a fluvial erosion system.Basic form of a fluvial erosion landscape.

Linear Properties of Stream Channel systemAreal Properties of Drainage BasinRelife Properties of the Fluvial System.

MeasurementStream OrdersStream NumbersStream LengthStream SlopesBasin AreasDrainage Density Density and Texture of Topography

Drainage patterns

Basic form elementsLinear Properties of the stream channel.

They are the numbers, lengths and arrangements of sets of line segments.

Areal Properties of Drainage BasinIt is a two-dimensional properties, includes size and shape.

Relief Properties of the Fluvial system.Relative heights of points on surfaces and lines with respect to the horizontal base of reference.It is a vertical dimension, includes gradient of ground surface and stream channel, rate of drop of the runoff and measures of the intensity of the processes of erosion and transportation.

Stream orders

Stream ordersEach fingertip channel is a segment of the first order stream.Two first order streams merge to form second order streams and so forth.The trunk stream of any watershed bears the highest order number of the entire system.First and second order streams usually carry flowing only in wet weather.

Stream orders – Bifurcation Ratio

The order of a stream segment is designated by ‘u’.The bifurcation ratio (Rb) is defined as

Stream orders – Bifurcation Ratio

If the region of uniform climate, rock type, and stage of development, the bifurcation ratio tends to remain constant from one order to the next (3 to 5)

Stream Numbers

R.E. Horton: “The numbers of stream segments of successively lower orders in a given basin tend to form a geometric series, beginning with a single segment of the highest order and increasing according to a constant bifurcation ratio.”Eg. Bifurcation ratio is 3, the trunk segment is the sixth order, the number of segments will be 1, 3, 9, 27, 81 and 243.

Stream NumbersThe formula should be

The total numbers of stream segments of the entire drainage basin should be

Stream Length

The 1st order stream segments have the shortest length, and the segments become longer as order increases.The mean length of stream segments increases by a length ratio (R1) with each increase in stream order.

Stream LengthR.E. Horton: “The cumulative lengths of stream segments of successive orders tend to form a geometric series beginning with mean length of the 1st order segments and increasing according to a constant length ratio.”

Stream SlopesSlope is stated as a ratio.0.01 is a ration of 1:100. (a drop of 1 metre vertically in 100 metres horizontally).

Profile a and profile b have the same slope

Basin Areas

Drainage DensityThis is found by measuring the total length of all of the streams within the basin. (km/km2)

Drainage density are controlled by several factors, but the most import control factor is infiltration capacity.

Factors affecting infiltration capacityGeology (Rock or soil type)

Hard, resistant rocks (granitic rock): low densityWeak rocks (shale and clay): high densityPermeable rocks or soil (sand): low densityImpermeable rocks or soil (clay): high density

TopographySteep slope: high densityGentle slope: low density

VegetationForest: low densityFarmland: medium densityBadland /desert: high density

ClimateHigh rainfall: high densityLight rainfall: low density

Drainage patternsDrainage patterns are largely controlled by two factors

Slopes of the drainage areaLocal differences in the resistance of rocks or geological structures (faults….)

http://www.il-st-acad-sci.org/kingdom/geo1005.html

Drainage Patterns

Dendritic PatternRectangular PatternTrellised PatternRadial PatternCentripetal

Dendritic Pattern

Tree-like, random branching pattern developed in a region of uniform or homogeneous rock.

Rectangular PatternIt is a rectilinear or grid-like pattern developed in a region with strong marked joint systems or faults intersecting at approximately right angles.This pattern is largely controlled by structural weakness.

Trellised PatternIt is a rectilinear stream pattern developed in region of alternate layers of resistant and less resistant rocks which dip in the same direction.The streams join one another at right angles

Radial Pattern

A radial pattern of drainage is a stream pattern developed on a structure dome, volcanic cone, uplifted fault block or a conical hill.

Centripetal Pattern

It consists of streams drainage from different directions towards the centre of a depression or basin.It is very common in inland drainage system, where streams terminate ina lake at the centre.Streams in intermontane basin or desert basins often develop this pattern.