Runoff

1
Runoff

• Hydrographs and the SCS Curve Number Method

2
Runoff vs. Streamflow

• Streamflow is baseflow runoff that makes it to
  the stream channel.

• Runoff is the result of a precipitation event

3
What is a watershed?

• A watershed is entire drainage area of a stream
  or river, from which the surface and groundwater
  leave at a single point.
• Usually defined by elevation.
• Large rivers with several tributaries are
  typically called river basins
• Watershed is used for tributaries / small streams

4
Watershed
5
Watershed Factors that Affect Surface Runoff

• Topography
• Surface runoff occurs in the direction of land
  slope
• Perpendicular to the contour lines
• Land slope has a significant effect on water
  velocity
• Overland flow
• Channel flow

6
Elevation Contours with Surface Runoff
7
Watershed Factors that Affect Surface Runoff

• Watershed shape
• Circular watersheds have higher rates of runoff
• Runoff from different points reach outlet at same
  time
• Elongated watersheds have lower rates than
  circular
• Downstream tributaries reach outlet first
• Soil
• Controls the infiltration process

8
Watershed Shapes
9
Watershed Factors that Affect Surface Runoff

• Land use
• Vegetative cover reduces runoff
• Improves soil structure
• Depletes soil moisture creating a dry antecedent
  moisture
• Bare soil results in increased runoff
• Poor structure
• Less permeable and can have surface sealing
• Fine soil particles washed into larger soil pores

10
Flow measurement
11
How is flow measured?
12
Streamflow Components

• Baseflow Continuous flow supported by seepage
  and groundwater flow.
• Runoff-AKA Storm water runoff. Excess rainfall
  from storm events.
• What is a runoff hydrograph?
• Continuous record of streamflow over time.

13
Runoff Hydrographs

• Complete Runoff Hydrograph Contains
• Runoff Volume-Area under the hydrograph
• Peak Flow Rates-maximum flow or peak of the
  runoff hydrograph.
• A complete time history of the flow.
• For small watersheds we ignore baseflow so that
• Runoffeffective rainfallprecipitation -
  abstractions

14
Conceptual Model
t3
t4
Dt
t2
t5
t1
a5
area
a3
a1
a2
a3
a4
a5
a4
t1
time
a1
a2
Rainfall with uniform intensity of r1 and
duration of Dt falling uniformly on the watershed.
15
Conceptual Model for Runoff

• Conceptual model is very steeply rising and
  falling with a base time only slightly longer
  than the duration of the rainfall excess.
• An actual hydrograph would have a lower peak and
  slower recession, due to tremendous capacity for
  storage.

16
Stream Hydrograph
Time (T)
17
Storm Hydrograph
(Surface runoff only / base flow removed)
Time (T)
18
Comparison of Urban andForested Hydrographs
19
Comparison of Upstream and Downstream Hydrographs
20
Runoff

• Typically determined in one of two ways
• Mass balance approach
• Effective rainfall

21
Mass Balance Approach

• Continuity Equation
• Inputs Outputs Change in Storage
• Input is typically precipitation.
• Outputs can be infiltration, ET, etc. depending
  on scale.
• Storage can be interception, soil water storage,
  etc. again depending on the scale and scope.

22
Effective Rainfall

• The SCS Curve Number Approach

23
Effective Rainfall

• Rainfall that becomes runoff.
• Effective Rainfall Precipitation Initial
  Abstractions
• Also called Rainfall Excess.
• Effective RainfallRainfall ExcessRunoff

24
SCS Curve Number Approach

• By far the most popular method.
• Combines initial abstractions and infiltration
  losses and estimates rainfall excess as

25
Curve Number

• A parameter that combines soil type and land use
  to estimate runoff potential.
• Based on the Hydrologic Soil Group (HSG), land
  use and condition.
• Range between 0 and 100. The greater the curve
  number, the greater the potential for RO.
• Impervious areas and water surfaces are assigned
  curve numbers of 98-100.

26
Hydrologic Soil Groups and Land Use

• SCS classified more than 4000 soils into four
  general HSG (A, B, C, and D)
• Based on soils minimum infiltration rate when the
  soil is bare and after prolonged wetting.
• In general A have the highest infiltration
  capacity and lowest runoff potential (sandy
  soils) and D have lowest infiltration rates and
  highest runoff potential (clay soils)
• Curve numbers for various land uses ranging from
  cultivated land to industrial and residential
  districts.

27
Curve Numbers
28
Antecedent Moisture Conditions

• Curve numbers in tables are for CN(II) or normal
  antecedent soil moisture conditions.
• If conditions are dryer than normal than a CN(I)
  should be used.
• If conditions are wetter than normal than a
  CN(III) should be used.
• Table 5.2 gives adjustment factors for CN(II) to
  obtain CN(I) or CN(III).

29
Mixed Land Uses and HSGs

• An area weighted CN is used when the area
  considered is for mixed land uses and HSGs.

30
Example Problem

• Given
• Precipitation (P) 4.04 in.
• A watershed that has
• 35 cultivated with a D soil group
• 30 meadow with a B soil group
• 35 thin forest with a C soil group
• Required
• Calculate the surface runoff (excess rainfall)

31
Watershed with Land Use and HSGs Listed
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Example Problem

• Find the curve numbers
• Use HSG CN
• Cultivated D 35 91
• Meadow B 30 58
• Thin Forest C 35 77
• Table 5.1 text (reference is important)
• Calculate a weighted CN
• Weights based on area
• CNavg 0.35(91) 0.30(58) 0.35(77)
• CN avg 76.2 76

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Example Problem

• Calculate the S term
• S 1000 / CN 10 (1000 / 76) 10
• S 3.16 in.
• Check to see if P gt 0.2S
• 0.2S 0.2(3.16) 0.63 in. ? P gt 0.2S?
• Calculate surface runoff (Q)
• Q (P - 0.2S)2 / (P 0.8S)
• Q (4.04 0.2(3.16)2 / 4.04 ((0.8)3.16)
• Q 1.77 in.
• For a rainfall event 4.04 in. on the given
  watershed with average soil moisture conditions

34
AGSM 335

• Homework 4

35
Rainfall Hyetograph
36
Effective Rainfall Hyetograph
37
Creating an Effective Rainfall Hyetograph

• Calculate the accumulated P for each time step
  from a rainfall hyetograph.
• Calculate the appropriate weighted CN.
• Calculate S using Equation (5.3).
• Find 0.2S.
• For each time step where the accumulated P gt 0.2
  S calculate the accumulated Q using Equation
  (5.4).
• Find the incremental Q at each time step.
• Plot the incremental Q vs. time.

38
Effective Runoff Hyetograph Calculations
Development of the Rainfall and Effective Rainfall Hyetographs Development of the Rainfall and Effective Rainfall Hyetographs Development of the Rainfall and Effective Rainfall Hyetographs Development of the Rainfall and Effective Rainfall Hyetographs Development of the Rainfall and Effective Rainfall Hyetographs
P24 5 S 2.19
0.2 S 0.438
A B C D E F G
Time Ordinate Depth Increment Depth Accumulated Accum. Effective Incremental Effective
(hrs) (Pt/P24) (in) (in) Rainfall (in) Rainfall (Q) (in) Rainfall
8 0.11 0.57 0.00 0.00 0.00 0.00
9 0.15 0.73 0.16 0.16 0.00 0.00
10 0.19 0.95 0.22 0.38 0.00 0.00
11 0.25 1.25 0.31 0.68 0.02 0.02
12 0.50 2.50 1.25 1.93 0.60 0.58
13 0.75 3.75 1.25 3.18 1.52 0.92
14 0.81 4.06 0.31 3.49 1.77 0.25
15 0.85 4.27 0.21 3.70 1.95 0.18
16 0.89 4.43 0.16 3.86 2.09 0.13
      3.86     2.09
  (Table 3.4) BP24 Differencing of C Addition of D Q (P - 0.2S)2/(P0.8S) Differencing of F