The Drainage Basin “your friend, and all of its secrets”

The Drainage Basin

“your friend, and all of its secrets”

I. Introduction--two general relationships realized concerning drainage basins


* streams form their valleys in which they flow


* streams form their valleys in which they flow* every river consists of a major stream that is fed by a number of mutually adjusted branches that diminish in size away from the main stem.

I. IntroductionII. Slope Hydrology and Runoff Generation

I. IntroductionII. Slope Hydrology and Runoff GenerationA. Basin Hydro. Cycle and Budget

Input = Output + change in storage

I. IntroductionII. Slope Hydrology and Runoff GenerationA. Basin Hydro. Cycle and Budget

Input = Output + change in storage

Overland flowInterflowBaseflow

I. IntroductionII. Slope Hydrology and Runoff GenerationA. Basin Hydro. Cycle and BudgetB. The Storm Hydrograph

B. The Storm Hydrograph

Unit hydrograph: ‘where the runoff volume is adjusted tothe same unit value (e.g., one inch of rainfall spread evenly over a basin in one day”

Overland flow

Baseflow

Direct ppt

Interflow

Basin Area

Discharge

III. Initiation of Channels and the Drainage NetworkA. The concept


Shear stress…….


Shear stress…….

Effects of Vegetation….

τ = ρgds


Rills develop, and they begin to “bifurcate”

“repeated divisions of single channel segments”





III. Initiation of Channels and the Drainage NetworkB. Basin Morphometry

refers to the geometry of the basin

B. Basin Morphometryrefers to the geometry of the basin

Drainage Compositionrefers to the distinct fabric of the drainage basin

1. Drainage Compositionrefers to the distinct fabric of the

drainage basin

Strahler (1952)

Shreve (1967)

2. Linear Morphometric Relationships

Certain linear parameters of a basin are proportionallyrelated to stream order.

a. Bifurcation ratio (Rb)streams of one orderstreams of the next highest order



a. Bifurcation ratio (Rb)streams of one orderstreams of the next highest order

Ex: 1 6th order stream3 5th order9 4th order 27 = 327 3rd order 981 2nd order

Ratio value is nearly constant between adjacent ordersAND….where geology is homogeneous, Rb = 3.0 – 5.0



b. Length ratio ave length of streams of one order ave length streams of the next highest order

Can be used to determine the average length of streams inan unmeasured order.

3. Aerial Morphometric Relationships

a. Drainage density (Dd) total length of streams in basin basin area

Reflects the interaction between geology and climate

In general….

4. Relief Morphometric Relationships

a. Relief Highest elevation – lowest elevation

Reflects the vertical dimensions of drainage basin

Includes factors of gradient and elevation

IV. Basin Morphology and Flood Hydrograph

IV. Basin Morphology and Flood Hydrograph

V. Basin Evolution and Denudation

V. Basin Evolution and DenudationA. Factors affecting sediment yield* climate

V. Basin Evolution and DenudationA. Factors affecting sediment yield* climate

V. Basin Evolution and DenudationA. Factors affecting sediment yield* basin size

V. Basin Evolution and DenudationA. Factors affecting sediment yield* elevation and relief

V. Basin Evolution and DenudationA. Factors affecting sediment yield* rock type

V. Basin Evolution and DenudationA. Factors affecting sediment yield* Human Factor

V. Basin Evolution and DenudationA. Factors affecting sediment yield* Human Factor

V. Basin Evolution and DenudationA. Factors affecting sediment yieldB. How do basins erode (denute?)

Sediment Transport and Denudation

0.398 km2

Before After

? ?

?

13, 364 m3

0.398 km2

Volume = 13, 364 m3

Area 0.398 km2 or 398,000 m2 = 0.034 m

= 3.4 cm

Summary of Basin Denudation of Blue Ridge Systems

Event

BasinBasin Area (km2)

Volume ofSediment Eroded

(m3)

Mean Basin

Denuda-tion (cm)

HurricaneCamille

Willis Cove

4.08 173,488 4.25

HurricaneCamille

Ginseng Hollow

1.75 88,727 5.07

HurricaneCamille

Polly Wright Cove

2.47 87,707 3.55

Rapidan Flood

Jenkins Hollow

0.398 13,364 3.36

Rapidan Flood

Teal Hollow 0.123 2,492 2.03

Long Term and Episodic Denudation

0

1

2

3

4

5

6

7

8

9

10

Nelson County Madison County

Den

ud

atio

n (

cm) Long Term

Episodic

3,500 Years 1 Day

2,500Years 1 Day

HurricaneCamille

RapidanStorm

B. How do basins erode (denute?)

High MagnitudeLow Frequency

Low MagnitudeHigh Frequency

VI. Basin HydrologyA. Subsurface Water

Hydrogeology (GEOL 460)M W 10:10-11, Friday Lab

VI. Basin HydrologyA. Subsurface Water

Hydrogeology (GEOL 460)M W 10:10-11, Th lab

VI. Basin HydrologyB. Surface Water 1. TermsDischargeQ = V * A

VI. Basin HydrologyB. Surface Water 1. TermsDischargeQ = V * AVelocity is highly variable; depends where measured.

Velocity is highly variable; depends where measured.

VI. Basin HydrologyB. Surface Water2. Reporting discharge Daily vs annual discharge

mean vs median discharge

3. Flood Frequency

y = 1708.5Ln(x) + 1705.1

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

1 10 100

RI (yrs)

Dis

char

ge

(cfs

)

What does this mean???

y = 1708.5Ln(x) + 1705.1

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

1 10 100

RI (yrs)

Dis

char

ge

(cfs

)


the curve estimates the magnitude of a flood that can be expected within a specified period of time


the curve estimates the magnitude of a flood that can be expected within a specified period of time The probability that a flow of a given magnitude will occur during any year is P = 1/RI.


the curve estimates the magnitude of a flood that can be expected within a specified period of time The probability that a flow of a given magnitude will occur during anyyear is P = 1/RI. EX: a 50 year flood has a 1/50, or a 2 percent chance of occurring

in any given year

For multiple years: q = 1- ( 1-1/T)n

where q = probability of flood with RI of T with a specified number of years n

For multiple years: q = 1- ( 1-1/T)n

where q = probability of flood with RI of T with a specified number of years n

EX: a 50 year flood has an 86% chance of occurring over 100 years

CAUTION!!! ANALYSIS GOOD FOR 1.5x length of data set

The Mechanics of Determining RI

Flood recurrence interval (R.I.)

use Weibull Method - calculates the R.I. by taking the average time between 2 floods of equal or greater magnitude.

RI = (n + 1) / m where n is number of years on record, m is magnitude of given flood

VI. Basin HydrologyC. Paleoflood Hydrology Slackwater deposits……

VI. Basin HydrologyC. Paleoflood Hydrology Which one is right????

“carbon is your friend”

The Drainage Basin “your friend, and all of its secrets”

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