This document discusses watersheds and concepts related to watershed hydrology. It begins by defining a watershed as a drainage area that contributes runoff to an outlet point. It then discusses key characteristics of watersheds including size, shape, slope, soils and land use. The document also covers watershed delineation, functions of watersheds, types of watersheds, and hydrologic analysis parameters such as outfall and watershed boundary. Finally, it discusses runoff estimation methods including the Rational Method and provides examples of applying the Rational Method to calculate peak runoff rates.

1. CONCEPT AND
MEASUREMENT OF
WATERSHED
KMJ26403 HYDROLOGY AND WATER RESOURCES ENGINEERING
MRS SITI KAMARIAH MD SA’AT
FKTM
UNIMAP

2. Watershed
 A basin, drainage or catchment area that is the
land area that contributes runoff to an outlet
point
Outlet point
Watershed
boundary

3. Watersheds
 Area of land draining into a stream at a given location. In US called watershed.
 Also known as Catchment, Catchment area, Catchment basin, Drainage area, River
basin, Water basin
 Rainfall that falls in a watershed will generate runoff to that watershed outlet.
 Topographic elevation is used to define a Watershed boundary (land survey or
LIDAR)
 Scale is a big issue for analysis
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4. We all live in a watershed!
Area of land from which all
water drains, running downhill,
to a shared destination - a river,
pond, stream, lake, or estuary

5. Functions
 Captures precipitation – its characteristics influence how much
is captured
 Stores water once it infiltrates into soil (important to plants)
 Slowly releases water into streams, rivers, oceans

6. Why are watersheds important?
 Activities within a watershed impact runoff and water quality of water
leaving the watershed
 Must manage at a watershed level rather than other boundaries to attain
goals related to runoff and water quality

7. Types of watershed
 Forested watershed
 Urban watershed
 Agricultural watershed
 Rural watershed
 Coastal/swamp/desert watershed
 Combination of above
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8. Hydrologic analysis
 Two important hydraulic parameters in hydrologic analysis
 – Outfall/drainage outlet,
 i.e. the common point of discharge
 – Watershed boundary
 – any rain that falls within the boundary will be directed towards point of discharge
 – Because of various nature of river system, a watershed may have any number
of sub-watershed within it
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10. Watershed Characteristics
 Size
 Slope
 Shape
 Soil type/Land use
 Storage capacity
Reservoir
Divide
Natural
stream
Urban
Concrete
channel
1 mile

11. Watershed size
 Watershed area (km2, ha)
 Runoff generation on these watersheds can be considered in two phases: i)
land phase and ii) channel phase
 Small watersheds (< 250 km2)
 They have dominant land phase and overland flow, have relatively less conspicuous
channel phase.
 They are highly sensitive to high-intensity, short-duration rainfalls.
 Large watersheds (>250 km2)
 They have well-developed channel networks and channel phase, and, thus, channel
storage is dominant.
 They are less sensitive to high-intensity rainfalls of short duration.
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12. Watershed slopes
 The slope govern how fast water will drain to the channel
 • steep slopes - peaked hydrograph
 • gentle slopes - flat hydrograph
 • Consider the average gradient of hill slopes
 (slope: vertical/horizontal distance)
 • Formula for land slope, S:
 Where:
 L = total length of contours (m),
 CI= contour interval (m)
 A= watershed area (m2)
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13. • Important hydrologic
characteristic
• Elongated Shape
• Concentrated Shape
• Affects Timing and
Peak Flow
• Determined by geo -
morphology of stream
Watershed shapes

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Sub-watershed

16. Sub-watershed
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17. Watershed delineation
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18. Fig. 1. The selected 13
watersheds and 12 coastal
regions of Peninsular Malaysia.
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19. Langat river basin
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20. RUNOFF WHAT IS RUNOFF?
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21. What is runoff?
 Runoff = draining or flowing off of precipitation from a catchment area through a surface channel enters
into stream channel.
 Output from catchment in a given unit of time.
 Based on the time delay between the precipitation and the runoff, the runoff is classified into two
categories; as
 (a) Direct runoff : runoff which enters the stream immediately after the rainfall. It includes surface
runoff, prompt interflow and rainfall on the surface of the stream. Direct storm runoff and storm runoff
are also used to designate direct runoff.
 (b) Base flow : The delayed flow that reaches a stream essentially as groundwater flow is called base
flow.
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22. Components of runoff
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23. Factors affecting catchment runoff
 a) Precipitation characteristics
 b) Shape and size of catchment
 c) Topography
 d) Geologic characteristics
 e) Meteorological characteristics
 f) Storage characteristics of a catchment
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24. Runoff estimation RATIONAL METHOD

25. Runoff estimation method
 Rational method
 Infiltration Approach
 Hydrograph method
 NRCS Curve Number (Formerly SCS) Method
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26. Rational Method
 Used for determination of peak flow rate
 Assumption: A constant intensity of rain uniformly distributed over an area
 maximum runoff: occur when the rainfall duration equals the time of
concentration
 For small size (<50 km2) catchments
 This not cover what is MSMA (Manual Saliran Mesra Alam Malaysia)/Urban
Stormwater Management Manual.

27. Rational Method
 The maximum possible flow generated by rainfall event of a watershed
• Knowledge of Qp is required for drainage design to avoid/minimize project from flooding
• Rational method: is a prediction method and based on characteristics of watershed and
rainfall
 Assumptions:
 • Waters is small (<200 acres)
 • Peak flow occurs when the entire area is contributing
 • Rainfall intensity is uniform over a time of concentration
 • A rational coefficient represent rainfall:runoff ratio
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28. Rational Method
 Standard Rational Method
 Qp = C i A
 Where
 Qp=peak discharge
 C=coefficient of runoff
 i = mean intensity of precipitation for
duration equal to tc
 A=drainage area
 To compute Qp, requires tc,i and C
Rational runoff Coefficient, C
– Defined as the rate of rainfall over a
watershed
to rate of runoff from that watershed
– The value is highly dependent on
land use and
slope (Table)

29. Rational method
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30. Rational Method

31. Runoff Coefficient
 Coefficient that represents the fraction of runoff to rainfall
 Depends on type of surface
 When a drainage area has distinct parts with different
coefficients…
 Use weighted average
A
A
C
..
A
C
A
C
C
i
n
n
2
2
1
1







32. Runoff
Coefficient

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35. Time of Concentration (tc)
 Time for water to flow from hydraulically most
distant point on the watershed to the point of
interest
 Assumes peak runoff occurs when i lasts as long
or longer than tc

36. Time of concentration, tc
 For other catchment, use Kirpich Equation (1940)
tc=0.01947 L0.77S-0.385
where
 tc in min
 L= maximum length of travel time in m
 S= slope catchment = ∆H/L
 ∆H = difference of elevation between the most remote point on the catchment
and the outlet

37. Time of concentration, tc
 Sometimes its written as
 tc= 0.01947 K1
0.77
 Where K1=
 L = max length of travel (m)
 ∆H = different of elevation
H)
/
(L3


38. Time of concentration, tc
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39. Time of Concentration (tc)
 Depends on:
 Size and shape of drainage area
 Type of surface
 Slope of drainage area
 Rainfall intensity
 Whether flow is entirely overland or whether some is
channelized

40. Rainfall Intensity, i
 Corresponding to a duration tc and the desired
probability of exceedence P
 Return period, T=1/P
 Found from rainfall intensity—duration-frequency (IDF)
curve

41. Intensity-Duration-
Frequency(IDF) Curve
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42. Rainfall Intensity, i
 Average intensity for a selected frequency and duration
 Based on “design” event (i.e. 50-year storm)
 Overdesign is costly (what else?)
 Underdesign may be inadequate

43. Rainfall Intensity, i
 Based on values of tc and T
tc = time of concentration
T = recurrence interval or design frequency
 As a minimum equal to the time of concentration,
tc, (mm/hr)

44. Recurrence Interval (Design Event)
 2-year interval -- Design of intakes and spread of water
on pavement for primary highways and city streets
 10-year interval -- Design of intakes and spread of water
on pavement for freeways and interstate highways
 50 - year -- Design of subways (underpasses) and sag
vertical curves where storm sewer pipe is the only outlet
 100 – year interval -- Major storm check on all projects
ARI = Average recurrent interval
- Average length of time between rain events
that exceed the same magnitude, volume
and duration

45. Design according to
MSMA V2
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46. Example 1:
Rational Method
 An urban catchment has an area of 85 ha. The slope of the catchment is 0.006 and
the maximum length of travel of water is 950m. The maximum depth of rainfall
with a 25-year return period is as below:
Duration
(min)
5 10 20 30 40 60
Depth of
rainfall
(mm)
17 26 40 50 57 62
 If a culvert for drainage at the outlet of this area is to be designed for a
return period of 25years, estimate the required peak-flow rate, by assuming
runoff coefficient is 0.3

47. Solution
tc using Kirpich formula:
tc=0.01947 L0.77S-0.385
= 0.01947 x (950)0.77 x (0.006)-0.385
= 27.4 minutes
 By interpolation, i = (50-40)/10 x 7.4 + 40 = 47.4 mm
 Average intensity, i tc,p = 47.4/27.4 x60 = 103.8 mm/hr
 Q = 0.3 x 103.8 x 0.85/3.6 = 7.35 m3/s
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48. Example 2:
Rational Method
 If the urban area of example 1, the land use of the area and the
corresponding runoff coefficients are as given below, calculate the
equivalent runoff coefficient.
Land Use Area (ha) Runoff coefficient
Roads 8 0.70
Lawn 17 0.10
Residential Area 50 0.30
Industrial Area 10 0.80

49. Solution
C =
C1A1 + C2A2 + ….. + CnAn
ΣAi
=
0.7 (8)+ 0.1(17)+0.3(50)+0.8 (10)
(8+17+50+10)
= 0.36
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50. EXAMPLE 3
 A catchment area of 120 ha has a time of concentration of 30 min and
runoff coefficient of 0.3. If a storm of duration 45 min results in 3.0 cm of
rainfall over the catchment, estimate the resulting peak flow rate.
 i = 30 mm/(45 min/hr x 60) = 40 mm/hr
 C = 0.3
 A = 120 ha
 Q = 0.3(40)(120)/360 = 4 m3/s
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51. Worked example using MSMA
 Determine the design peak for flow generated from residential area of 10 hectares
in Kuala Lumpur for design return period 50 years. Assume 80 m of overland flow
followed by 400 m of flow in open drain. Catchment area average slope is 0.5%.
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1. Determine tc = to + td = 8 + 7 = 15 min
From overland flow chart: to = 8 minutes
td = L/v (Manning equation), Assume v = 1 m/s, then td = 400 s = 6.7 min.
Use 7 min
2. Refer IDF curve to obtain i. i = 200mm/hr
3. Choose rational coefficient, C (Table) = 0.7
4. Compute Qp = C I A

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53. IDF curve Kuala Lumpur
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54. Try: Problem
 Information on the 50-year storm is given below:
 A culvert has to drain 200 ha of land with maximum length of
travel 1.25 km. The general slope of the catchment is 0.001 and its
runoff coefficient is 0.20. Estimate the peak flow by rational
method for designing the culvert for 50-year flood.
 Ans: 10 m3/s
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Duration
(min)
15 30 45 60 180
Rainfall
(mm)
40 60 75 100 120

55. Thank you
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Student password
3is1jd