Unit Hydrograph

Coverage

Flood pictorial views

Hydrograph - Review

Unit Hydrograph

Unit Hydrograph. Why ?

Assumptions For UH

Terminology for UH

Creating Unit Hydrograph

Applications of Unit H

Text in

here Text in

here

FLOOD PICTORIAL VIEWS

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Prediction

The only hydrograph that predicts the behavior of a flood from a storm of any duration

Why Construct & Analyse Hydrographs ?

To find out discharge patterns of a particular drainage basin

Help predict flooding events, therefore influence implementation of flood prevention measures

14

Hydrograph Theory

Hydrograph

Graphical representation of time (hours) versus

discharge (cfs or cms) at a particular point on a

stream or channel which drains the watershed area

Peak Discharge

?

Text in

here

-------------------------YES ? Then we will be able to

1. Manage the Storm water.

2. Identify the Flood Plans on downstream side.

3. To place the Hydraulic structures at safe level.

4. Efficient Urban Storm water management plan.

5. Design the Different types of Hydraulic structures.

6. Minimize the effects of Floods.

Text in

here

Micro Hydro Projects

Unit Hydrograph

A conceptual direct runoff hydrograph

resulting from a rainfall excess of unit depth

and constant intensity for a particular

watershed is called a unit hydrograph

Text in

here

The unit hydrograph method is employed to

calculate the direct runoff hydrograph at the

watershed outlet given the rainfall excess produced

by a storm event.

This method is categorized as a lumped model in

which the hydrologic characteristics of the entire

watershed are combined and typified by one or more

parameters, simple mathematical expressions, or

graphs.

Text in

here

The Unit hydrograph is a useful tool in the process of

predicting the impact of precipitation on stream flow.

The Unit depth is 1cm in the SI unit system and 1inch

in the U.S. system.

It is usually abbreviate as a Uhc.

The subscript “c” indicate the Duration of the rainfall

excess.

Text in

here

For instance, the direct runoff hydrograph

produced by a rainfall excess that has a

duration of 3 hr and constant intensity of 1/3

in/hr is denoted by UH3 and depth of the

rainfall excess is (1/3 in./hr)(3 hr) = 1 in

Text in

here

We can develop a unit hydrograph for a gaged

watershed by analyzing the simultaneous

rainfall and runoff records.

Unfortunately, most small, urban/rural

watersheds are ungaged. However, there are

several synthetic unit hydrograph methods

available to develop a unit hydrograph for

ungaged watersheds e.g. Espey Ten-Minute Unit

Hydrograph.

Text in

here Text in

here

UNIT HYDROGRAPH—WHY ?

Simplifying our task / work / procedures.

Gives us a base line for a specified watershed.

Standardize the hydrograph for different watersheds.

Gives us information that how the flow of a stream

will be affected over time by the addition of one unit

of runoff.

Text in

here Text in

here

The role of Unit Hydrograph theory in the flood

prediction process is to provide an estimate of stream

flow given and amount precipitation.

Once we know how much rainfall or snowmelt has

occurred, or is likely to occur, and we have an idea of

how much of this will turn into runoff, we still need to

know how the flow of a stream will be affected over time

by that runoff. The unit hydrograph provide us with a

way to estimate this, and is an integral part of many

hydrological modeling systems.

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Text in

here Text in

here

ASSUMPTIONS

The primary assumption of unit hydrograph theory is

that the rainfall has uniform distribution, both in

space-with minimal variations across the basin-and in

time; in other words, the rainfall rate did not vary

much during the event.

But In reality,

precipitation events are rarely uniform in space and time.

Often, one portion of the basin experiences higher

intensity precipitation than another portion.

Text in

here Text in

here

The base duration of direct runoff hydrograph due to different

effective rainfall of unit duration is constant.

The ordinates of DRH are directly proportional to the total amount

of DRH of each hydrograph (principles of linearity, superposition,

and proportionality)

For a given basin, the runoff hydrograph due to a given period of

rainfall reflects all the combined physical characteristics of basin

(time-invariant)

Text in

here Text in

here

BASIN-AVERAGED RAINFALL

In typical non-snow situations, we begin the hydrologic process with

rainfall. In particular, we start with a basin-averaged rainfall. This

simply tells us how much rain fell, or is forecast to fall, on a given

basin and typically takes the form of a rainfall depth per time. In unit

hydrograph theory, we assume that this rainfall has fallen uniformly

across the basin

Text in

here Text in

here

BASIN-AVERAGED EXCESS RAINFALL

From averaged rainfall, we need to know how much of the basin-

averaged rainfall will become runoff. In unit hydrograph theory, runoff

is often referred to as “excess precipitation” or “excess rainfall.”

Rainfall runoff models will typically provide an estimate of what

becomes excess rainfall.

So, for example, if 25% of our 4.00 cm basin-averaged rainfall

becomes excess rainfall, then we have a basin averaged excess rainfall

of 1.00 cm

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Direct Runoff

The unit hydrograph represents the excess

precipitation or quick – response runoff

Text in

here Text in

here

TERMINOLOGY - UH

Duration

Rising Limb

Recession Limb (falling

limb)

Peak Flow

Time to Peak (rise time)

Time of Concentration

Recession Curve

Base flow Separation line

Base flow

Quick Response Run off

Point of inflection

UH Components / Terminology

Text in

here Text in

here

CREATING U.HYDROGRAPH

From Stream flow Data

Synthetically

Espey Ten-Minute Unit Hydrograph

Snyder

SCS Unit Hydrograph

Time-Area Unit Hydrograph(Clark, 1945)

Gamma Function Unit Hydrograph

“Fitted” Distributions

Geomorphologic

Contents

- Click to add Text

- Click to add Text

- Click to add Text

A unit hydrograph can be derived from a total stream

flow hydrograph at a given stream gauge location

along with the following information:

• The Basin Area

• The Basin-averaged rainfall depth

• The duration over which the excess precipitation

occurred.

STEPS FOR DERIVING THE UNIT HYDROGRAPH

Text in

here Text in

here

Text in

here When deriving a unit hydrograph it is important to

start with an archived hydrograph in which the

quick-response runoff portion is from one single

storm event. In addition, that storm should have

produced its excess precipitation with nearly

uniform coverage in space and time over the basin

Select Appropriate Precipitation Event Step-1

Text in

here

Remove Base flow Contribution Step-2

Text in

here The total volume of water from the quick-response runoff needs

to be calculated. This is done by summing the areas under the

QRR Hydrograph for each time step, in this case, hourly.

Calculate Quick – Response Volume Step-3

Text in

here

Determine Excess PPT Depth from Basin Step-4

Text in

here

Determine Excess PPT Depth from Basin Step-4

For example, assume we have a basin area of 100 square km,

which is 100,000,000 sq.m and calculated volume of quick-

response to be 2,000,000 cum-then the depth will be

Text in

here

Adjust the Quick-Response Hydrograph Step-5

The excess ppt depth probably won`t be exactly one unit as unit

hydrograph requires. So, we have to adjust the QRR

hydrograph to show what the response from one unit would be.

Text in

here

Adjust the Quick-Response Hydrograph Step-5

Once we multiply each point on the hydrograph by our

adjustment factor of 0.5, our resulting unit hydrograph is for

exactly 1 cm of excess precipitation

Text in

here

Determine Duration of UH Step-6

The duration of a unit hydrograph refers to a continuous

time period during which one unit of excess ppt occurred.

If it took 6 hours for the one unit of excess to occur, we

have a 6-hr unit hydrograph. Remember, the unit

hydrograph duration does not refer to the duration of the

stream flow response.

Text in

here

The difficult part of determining the duration of a unit

hydrograph is estimating which portion of the entire

precipitation event actually contributes to excess ppt.

Recall that the water that infiltrates & percolates into

deeper storage and base flow is not part of excess ppt.

We can estimate this portion of the ppt. by applying a

constant loss function to the rainfall.

Recall that we have already calculated the depth of the

excess ppt to be 2.0 cm. Now, we need to know how

long it took for that excess to occur.

Text in

here

So we move this loss function line such that the

amount of ppt. above the line is equal to the depth

of excess ppt. that we already calculated for the

basin.

Below that line the ppt. goes to long-term storage.

Above the line is the excess ppt.

Text in

here

Text in

here

Text in

here

Now we have an excess precipitation bar graph of 6-hr.

Notice that the amounts from hour to hour on this

graph are not truly uniform. This is typical.

For purposes of calculating a unit hydrograph

duration, however, we assume that all excess ppt

occurred uniformly in time.

Text in

here

Final Unit Hydrograph

At the end of these steps, we have a 6-hr unit hydrograph.

It show the stream flow response to 6 hrs of excess ppt

that produced one unit of depth.

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Example:

Two storm each of 6-hr duration and having

rainfall excess values of 3.0cm and 2.0 cm

respectively occur successively. The 2-cm ER rain

follows the 3-cm rain. The 6-hr UH for the

catchment is the same as given in previous

example. Calculate the resulting DRH.

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Unit Hydrograph of Different Durations

Under condition where lack of adequate data in

developement of unit hydrograph

D-hour unit hydrograph is used to develop unit

hydrographs of differing durations nD

Two method available:

1. Method of superposition

2. The S-Curve

If a D-h unit hydrograph is available, and its desired

to develop unit hydrograph of nD, its is easily

accomplished by superposing n unit hydrographs with

each graph separated from the previous on by D-h.

Method of Superpositions

D = 2-Hr Unit Hydrograph

Adjusted Net Rainfall

one inch over basin

Qp

Change UH Duration

Consider 1 hr UH

Add and Lag two UH

by one hour

Sum and divide by 2

Results in 2 hr UH

Example

Given the ordinates of a 4-hr unit hydrograph as

below derive the ordinates of a 12-hr unit

hydrograph for the same catchment

Time (hr) 0 4 8 12 16 20 24 28 32 36 40 44

Ordinates of

4-hr UH

0 20 80 130 150 130 90 52 27 15 5 0

Contents

- Click to add Text

- Click to add Text

- Click to add Text

Text in

here Text in

here

S-Curve

Also known as S-hydrograph

Hydrograph produced by continous effective

rainfall at a constant rate for infinite period.

Curve obtained by summation of an infinite

series of D-h UH spaced D-h apart.

Contents

- Click to add Text

- Click to add Text

- Click to add Text

S-Curves for UH

S-Curves

• Convert 2 hr UH to 3-hr

• Lag each 2-hr UH by

Duration D

• Add to produce S-curve

S-curve

Example

Solve previous example with S-curve method:

Given the ordinates of a 4-hr unit hydrograph as below

derive the ordinates of a 12-hr unit hydrograph for the

same catchment

Time (hr) 0 4 8 12 16 20 24 28 32 36 40 44

Ordinates of

4-hr UH

0 20 80 130 150 130 90 52 27 15 5 0

Contents