Practical Morphometric Analysis_watershed.pdf

Practical Morphometric Analysis_watershed

1. Morphometric Analysis of
Watershed

2. Morphometric Analysis of Watershed
Morphometricparameters are categorized into three
aspects:linear, areal, and relief.
Linear parameters:Stream order, stream number,
bifurcation ratio, stream length, and mean stream length
Areal or basin parameters:relating to, of, or involving an
area, such as a region of land or a space.
Circularity ratio, elongation ratio, drainage density, and
drainage frequency
Relief parameters:Dissection index and ruggedness index

3. Morphometric Analysis of Watershed
1.Delineate watershed and sub-watersheds in given
topographical map, measure watershed perimeter
2.Identify stream order/ number them, Draw stream
path measure lengths of stream segments
3.Find out watershed and sub-watershed relief
Difference of elevation between highest and lowest
elevation
4.Compute drainage density
Total length of all stream divided by area of the
watershed

4. Area: Calculate the area by counting the number of boxes traced in
tracing paper.
Perimeter: The perimeter of watershed can be calculated by aligning
thread along the ridge line and measuring it through scale.
Calculate the real perimeter by using the scale of the toposheet.
Translating map to real ground value.
Watershed relief: It can be calculated by identifying the highest and
lowest elevation which was taken by studying contour lines.
Watershed relief = Highest elevation in WS–Lowest elevation in WS
Watershed length: Align the thread along the longest streams and all
streams for measuring the watershed length and total length of the all
streams respectively.
Translate the map value to real ground value.
(Longest stream length) = ?? cm (map) = ??*25000 = ?? (Ground) Km
Measure the total length of all streams on the map and translate it to
real ground value.

5. Drainage density (Dd)
•Drainage density is the total length of streams of all orders (km)
per drainage area (km
2
). The Ddmostly are in km/km
2.
•There are five classes of drainage density with the following
value ranges (km/km
2
), i.e., very coarse (<2), coarse (2-4),
moderate (4-6), fine (6-8), and very fine (>8)
•Ddclasses can be highlighted that there are two main classes:
low/coarseand high/fine class.
•Low class of Ddshows a poorly drained basin with a slow
hydrologic response. Surface runoff is not rapidly removed from
the watershed making it highly susceptible to flooding, gully
erosion, etc
•High class of Ddshows a quick hydrological response to rainfall
events. Besides, high class of Ddhas an impermeable subsoil
material, sparse vegetation and high/mountainous relief.

8. Morphometric Analysis of Watershed
5. Calculate stream Bifurcation Ratioof the watershed
A ratio of the number of streams of a low order to the
number of streams in the next higher order.
Rb= Number of So-1/Number of So
Average of Rbbetween two subsequent river orders is
the Rbof watershed
(RbSo1/So2+ RbSo2/So3 +RbSo3/ So4)/ 3
The lower Rbvalues are the characteristics of structurally
less disturbed watersheds without any distortion in
drainage pattern.

9. Bifurcation ratio
Calculation
So#Rb
112
12/4 = 3
24
4/2 = 2
32
2/1 = 1
41
Average Rb= (3 + 2 + 1)/3
=2

11. Morphometric Analysis of Watershed
6. Elongation Ratio (Re)
Ratio of the diameter of a circle of the same area as the
basin to the maximum basin length
Re = D/L,
Where D is the diameter of the circle with same area as
watershed,
L is watershed length (Longest stream length)
A (calculated) = Area of circle = πr
2
r
2
= A/π, D = 2r
Values of Re generally vary between 0.6 and 1.0 over a wide
range of climatic and geological environments.
Values close to 1.0 are characteristic of areas with very low
relief, whereas values in the range of 0.6 -0.8 are
representative of catchments described with high relief and
steep slopes.

13. Morphometric Analysis of Watershed
7. Circularity Ratio (Rc), ratio between the area of watershed and area
of the circle having equal perimeter
Rc= Watershed area/ Area of circle with same watershed
perimeter
Watershed Area = A (calculated)
Watershed Perimeter = P (measured)
Watershed perimeter (P) = Circle perimeter, P = 2πr
Area of circle (A) = πr
2
= π(P/2π)
2
= P
2
/4π
Rc= A/ (P
2
/4π) = 4πA/P
2
π(constant) = 22/7 or 3.1416
A lower Rcvalue indicates elongation with relatively slow discharge
and less risk of erosion and flood hazard. Higher Rcvalues indicate
potentially faster runoff.

14. circulatory ratio
•The circulatory ratio of a watershed isthe ratio of the
watershed's area to the area of a circle with the
same perimeter as the watershed.
•A lower Rcvalue indicates elongation with relatively
slow discharge and less risk of erosion and flood
hazard. Higher Rcvalues indicate potentially faster
runoff.
•The value of the ratio can range from 0 (in line) to 1
(in a circle), but most watersheds have values
between 0.2 and 0.8.

16. Interpretation of parameter
•Watershed is classified according to its shape, i.e., elongated
and circular. The elongated watershed has high relief and
young geomorphic stage. Due to its high relief, it leads to
more vulnerable to erosion.
•Circular watershed has low relief and mature geomorphic
stage. It leads to more vulnerable to flooding.
•Elongated watershed is identified by a high value of
drainage density (Dd), texture ratio (Dt), stream frequency
(Fs), relief ratio (Rh), and low value of elongation ratio (Re),
circularity ratio (Rc), form factor (Rf), length of overland
flow (Lg), and bifurcation ratio (Rb).
•The opposite of those elongated parameter values are the
characteristics of a circular watershed.