The Methods Of Modelling Catchment Runoff Environmental Sciences Essay

In the modeling of catchment overflow, catchment type and informations handiness may find the pick of method to utilize. Besides the pick of method can sometimes interact with the catchment type and the intent for which the inundation estimation is required. Thus consideration and judgement is required for the choice of a suited method i??Reedi??1999. Table 1 below lineation a figure the advantages and disadvantages of the theoretical accounts antecedently described in this study.

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In decision to the features of the selected theoretical accounts discussed in subdivisions X-y and summarised in Table 1 it was found that in line with the findings of the World Meteorological Organisation comparing survey discussed in subdivision ( SWM in history spot ) . The Stanford Watershed Model carried more complexness, standardization and package issues and restrictions than the other theoretical accounts described. And as “ it could non be proven that complex theoretical accounts give better consequences than simpler 1s ” i??Remote Feeling Applications to Hydrology: Future Impacti??1996 it was decided to utilize a hybridized theoretical account of the combination of the staying theoretical accounts

Using a intercrossed attack will better the calculating potency of the theoretical account. Through hybridisation the restrictions of single patterning techniques can be overcome

As the FEH provides two processs for the appraisal of inundation frequence ; the statistical and rainfall overflow method as mentioned in subdivision ( FEH/FSR ) and is recommended for usage in the UK it shall be used.

As mentioned in subdivision ( FEH/FSR ) the Flood Estimation Handbook is by and large considered as the criterion for rainfall overflow analysis in the UK. Furthermore the Environmental Agency has officially endorsed the FEH as the method that should be used for rainfall overflow appraisal i??Balmforthi??2006.

The catchment in inquiry, the Heriot Watt Edinburgh campus and the environing Murry Burn Tributaries catchment, has a plan country of 6.7km2 ( 670ha ) ( see Mistake: Reference beginning non found ) and therefore the utilizing the modified FSR rainfall overflow method from the Flood Estimation Handbook would be acceptable.

3 Description of the method

Method/results

FEH modified version of the FSR rainfall overflow method

1 Appraisal of Tp ( 0 ) and unit hydrograph

In state of affairss where no records exist the unit hydrograph clip to top out should be estimated by deducing a unit hydrograph from records of rainfall overflow.

An option

combining weight. ( 0 )

where:

DPSBAR = Index of catchment Steepness ( m/km )

DPLBAR = Index describes catchment size & A ; drainage way ( kilometer )

PROPWET= Index of proportion of clip that dirts are wet

URBEXT = FEH index of fractional urban extent for 1990

To happen DPSBAR and DPLBAR, catchment landform forms were generated by reproducing the Institute of Hydrology Digital Terrain Model ( IHDTM ) techniques ( see Mistake: Reference beginning non found and Error: Reference beginning non found )

The catchment country was separated utilizing a 500m grid and the height of each grid node was assigned utilizing a 3D contours theoretical account obtained from EDINA multi map. Drainage way lines were assigned to each grid node based on the steepest path to one of its eight neighboring nodes.

The intermodal incline was calculated for each grid and DPSBAR was found to be 29.3m/km by taking the norm over the catchment as shown in Table 2 below.

DPLBAR was found to be 2.35km by taking the mean drainage length of the combined drainage way lines throughout the catchment The longest drainage way from beginning to outlet ( LDP ) was found to be 5.1km as shown in Table 3 below

Table 2. Derivation of DPSBAR ( to be read in concurrence with Mistake: Reference beginning non found )

Table 3.Derivation of DPLBAR ( to be read in concurrence with Mistake: Reference beginning non found )

PROPWET values are defined as the proportion of clip that dirts are wet or dry harmonizing to what is known as the 6mm threshold. This states that a series of n back-to-back yearss with a dirt wet defect greater than 6mm is defined as a dry enchantment and conversely wet enchantments are defined as dirts with a wet defect of less than 6mm. The PROPWET values for gauged catchments around the UK was calculated between 1961 and 1990 and are shown in Mistake: Reference beginning non found. The PROPWET value was interoperated to be 0.5.

URBEXT was found from Fig. 1 below and taken as 0.05.

Fig. 1. FEH index of fractional urban extent for 1990 i??Houghton-Carri??1999

The reasonable clip measure interval is chosen to give a well defined triangular unit hydrograph profile. A practical clip measure interval should be used so non to hold an inordinate and unneeded figure of subdivision when happening the hydrograph response. “ In pattern, a information interval of 10-20 % of the value of Tp ( 0 ) is normally suited ” i??Houghton-Carri??1999

Fig. 2. FSR triangular unit hydrograph i??Houghton-Carri??1999

The triangular unit hydrograph can be drawn up utilizing the parametric quantities from the undermentioned equations:

combining weight. ( 0 )

Once has been adjusted utilizing the chosen clip measure interval it is reffered to simplt as

combining weight. ( 0 )

where:

AREA = The program country of the catchment ( See Mistake: Reference beginning non found )

combining weight. ( 0 )

combining weight. ( 0 )

2 Calculation of design storm continuance D

The design storm continuance is based on a expression which approximates the continuance giving the largest inundation magnitude and so is a map of the unit hydrograph clip to top out.

combining weight. ( 0 )

where:

SAAR = Standard mean one-year rainfall ( see Mistake: Reference beginning non found )

The deliberate value of storm continuance is rounded up or down to the nearest uneven whole number of the storm continuance to let a remarkable cardinal extremum rain event period to be formed in the entire rainfall hyetograph ( see Fig. 5 )

3 Calculation of design storm deepness P

The design storm deepness is distributed within the design storm continuance D during an appropriate design storm profile harmonizing to the degree of urbanization. For reasonably rural catchments with such as the one considered inundations usually occur in winter seasons.

The rainfall return period TR is obtained from Fig. 3 utilizing a flood return period TF.

Fig. 3 Recommended return period to give flood extremum of needed return period i??Houghton-Carri??1999

The design storm deepness P referred to as the catchment MT – D rainfall is calculated by scaling the MT – D rainfall by a decrease factor which is a ratio of the rainfall deepness as shown in the followers:

combining weight. ( 0 )

where:

ARF = Areal Reduction Factor

The FEH usage rainfall continuance frequence informations presented on a CD-ROM to cipher the MT – D rainfall i??Houghton-Carri??1999. It is nevertheless possible to cipher this rainfall utilizing the Modified Rational Method, Volume 4 of the Wallingford Procedure, utilizing the recommended storm return period TR found from Fig. 3 and ramp continuance calculated in eq.20.

This method provides the computation of rainfall strengths of continuances between 5 proceedingss and 48 hours, and the return periods between one and 100 old ages. The value of the 5 twelvemonth – 60 minute rainfall deepness ( M5 – 60min ) and the ratio of the ( M5 – 60min ) are determined utilizing Mistake: Reference beginning non found and Error: Reference beginning non found severally.

The ( M5 – Calciferol ) for the storm continuance calculated in eq.20 is determined utilizing the followers:

combining weight. ( 0 )

where:

Z1 = unitless factor read from Mistake: Reference beginning non found

The Z1 factor is read from either one of a figure of graphs depending on the values of the ratio R between of 0.12 and 0.45.

Finally the needed MT – with the coveted storm continuance D and rainfall return period TR is found by multiplying combining weight. 22 with a 2nd ratio Z2 interpolated from table Table 4.

Table 4. Z2 ratio for Scotland and Northern Ireland i??Wallingford Procedure

The ARF is so merely read from Fig. 4 utilizing the values of D and AREA

Fig. 4. Areal Reduction Factor ( ARF ) % , related to country of catchment AREA and ramp continuance D i??Houghton-Carri??1999

4 Derivation of design storm profile

The undermentioned equation is used to happen the rainfall block or interval % which is used in Table 5 to plan the rainfall hyetograph

combining weight. ( 0 )

For the D-hour storm each rainfall block has a continuance equivalent to the fraction of the entire storm continuance. The extremum rainfall even is centred due to the uneven whole number multiple of the storm continuance. From Table 5 it can be seen that for each 0.5 hr rainfall block of the 5.5 hr storm will hold a continuance equivalent to 1/11 or 9.1 % of the storm continuance. The % of storm deepness % P is interpolated from the winter storm profile shown in Fig. 5 utilizing the % D. It can be seen that the cardinal block proportional to 9.1 % of the storm continuance represents 22 % of the overall storm deepness, the cardinal 3blocks relative to 27.3 % of the storm continuance represents 55 % of the storm deepness and so on.

Table 5 Derivation of the rainfall hyetograph

The % difference in block interval sets is converted into millimeter by multiplying by the storm deepness P and the rainfall hyetograph is so plotted.

Fig. 5 Derivation of the winter design storm profile i??Houghton-Carri??1999

5 Derivation of Design Antecedent Catchment Wetness CWI

The antecedent catchment wetness refers to the province of catchment prior to the storm event and is found through the catchment wetness index CWI. The CWI relates to the catchment SAAR values and is an of import factor in act uponing per centum funoff giving indicants on dry conditions flow conditions such as baseflows.

Recommended CWI values are found from Fig. 6 below.

Fig. 6 Recommended design values for catchment wetness index CWI i??Houghton-Carri??1999

6 Calculation of per centum overflow

The per centum overflow PR is dependent on the province of the catchments soil wetness conditions prior to a storm event, storm magnitude and particularly the degree of urbanisation URBEXT. The dynamic per centum overflow DPR is made up of a figure of component properties such as catchment rainfall per centum and the catchment wetness index.

The catchment per centum overflow is found from the followers:

combining weight. ( 0 )

combining weight. ( 0 )

combining weight. ( 0 )

The standard per centum overflow SPR gives a representation of the catchments normal capacity to bring forth overflow and is found from Mistake: Reference beginning non found with mention to Error: Reference beginning non found

Table 6 SPR and BFI values for HOST categories i??Baylissi??1999

combining weight. ( 0 )

7 Derivation of Net Event Hyetograph

The net event hyetograph is derived by using the per centum overflow PR each block of the entire rainfall hyetograph.

8 Derivation of rapid response overflow hydrograph

The rapid response hydrograph is derived as set out below. The values from the triangular unit hydrograph are set out across the top of the tabular array in the heading row

Fig. 7. Derivation of the rapid response overflow hydrograph

9 Calculation of Baseflow

( 2.19 ) combining weight. ( 0 )

10 Derivation of entire overflow hydrograph

Differences between theoretical accounts and methods

In pattern the consequences of different theoretical accounts frequently show big differences. This is due to “ the assorted computational methods, the changing discretization of the delineated systems every bit good as dissensions in the applied precipitation informations and the parametric quantity set of the theoretical accounts ” . ( book 1_page15_Background )

Note: so travel on to explicate why have used a generic theoretical account utilizing the approved! ? ? Methods form the FEH and FSR! ? Because ‘unoficialy Superseeded the fsr

Reasons for taking FEH

“ it could non be proven that complex theoretical accounts give better consequences than simpler 1s ” i??Remote Feeling Applications to Hydrology: Future Impacti??1996. (

4 Consequences

My Model

Discuss etc… .

Comparing theoretical accounts

See CIRIA C635 tabular array 6.3

Comparison of the short-cut method with the rational method ( FEH_4 )

“ A comparing of extremum flows obtained from the two methods concluded that,

topic to an assumed usage of indistinguishable overflow coefficients for little lowland

catchments, the rational method output flood extremums typically twice every bit big as those

from the FSR rainfall-runoff method ”

5 Discussion of consequences

1.1 The demand for catchment forms

Flood extremum informations are available at a big figure of estimating Stationss throughout

the UK, but for many of the sites where inundation appraisal is required there are no

such informations. It is utile to quantify the physical and climatological belongingss of a

catchment so that inundation top out informations may be transferred and applied to hydrologically

similar catchments and so that ungauged sites can be allocated to an appropriate

pooling-group. Most significantly, relationships between cardinal variables ( such as

the average one-year inundation QMED ) and catchment forms provide a technique

for deducing a inundation estimation at an ungauged site. Estimates produced in this manner

are ( in about all instances ) far less dependable than those obtained by utilizing inundation extremum

informations, but they can however be considered for usage in minor inundation design

strategies, and they are utile in supplying a probationary appraisal when more

major plants are being proposed.

Short Cut Method

6 Decisions

ie split catchment countries up for each feeder and theoretical account a UH for each, so add together ( hypertext transfer protocol: //books.google.com/books? id=Rtjtazovs9AC & A ; pg=PA354 & A ; sig=d0C-o0lLoOH0LvQWUlL97KDsNxo & A ; hl=en # v=onepage & A ; q=dawdy % 20and % 20o’donnell & A ; f=false )

Decisions Made

A7.12 Advantages and disadvantages of the Flood appraisal

enchiridion techniques CIRIA ( C635 )

The losingss mentioned in subdivision 2.3… .then see below

This appraisal of the rainfall loss is considered by many to be the most complex, and perchance least studied, constituent in the rainfall-runoff procedure. McPherson ( 1978 ) and Aron ( 1982 ) have considered rainfall losingss to be the weakest nexus in the proper appraisal of overflow ; and others, such as Yen ( 1982 ) and Pilgrim ( 1986 ) , have stressed that the survey of rainfall losingss should be given well more attending… … … … … … … ..

..Together, interception and depression storage are important factors in the overall H2O budget and, harmonizing to Goldman et Al. ( 1990 ) , could account for the major losingss in up to 80 % of ascertained storm events. However, infiltration is often the lone procedure of the three to be simulated in rainfall-runoff theoretical accounts, particularly with event theoretical accounts ( A REVIEW OF RRM page 46 )

Use GIS

FEH1

5.5.2 Type of catchment

aˆ? The statistical attack should by and large be used for big catchments,

e.g. when the drainage country exceeds 1000 km2. The construct of a

catchment-wide design storm is less realistic for big catchments,

doing the rainfall-runoff attack less appropriate.

Chapter 5 Which method to utilize 17

5.1 Introduction 17

5.2 Preliminary considerations 17

5.3 Choice of method within the statistical attack 18

5.4 Choice of method within the rainfall-runoff attack 19

5.5 Factors act uponing the pick between statistical and

rainfall-runoff attacks 20

5.6 Accommodating estimations from the statistical and

rainfall-runoff attacks 21

5.7 Choice of method when the catchment is urbanized 21

5.8 Extra possibilities

Excel spreadsheet could be huffy compatable with GIS as variables such as catchment country and incline etc can be merely inputed.

Validation of FEH

Further Research

It is just to indicate out before doing farther remark on the FEH attack that

it had river catchment hydrology in head and non urban drainage analysis. If it

delivered all that urban drainage applied scientists required it would hold been a

fillip but it was non the aim ( unluckily ) .

One consistent characteristic of these consequences is that there is range for bettering the

FEH theoretical account for continuances below 1 hr. In some instances in Scotland it

performed better than expected but the current advice on the FEH CD non to

usage it below 30 proceedingss is reasonable. The FSR theoretical account performed really good and

demonstrates that widening the FEH theoretical account to include continuances every bit abruptly as 5

proceedingss is possible and would be a really worthwhile undertaking. The FEH theoretical account

Conceptual

1.1 General

Water resources are indispensable renewable resources that are the footing for being and development of a society. Proper use of these resources requires assessment and direction of the measure and quality of the H2O resources both spatially and temporally.

Water crises caused by deficits, inundations and decreasing H2O quality, among others, are increasing in all parts of the universe. The growing of population demands for increased domestic H2O supplies and, at the same clip, consequences with a higher ingestion of H2O due to enlargement in agribusiness and industry. Mismanagement and deficiency of cognition about bing H2O resources and the altering climatic conditions have effects of an instability of supply and demand of H2O. The job is pronounced in semi-arid and waterless countries where the resources are limited.

Establishing a rainfall-runoff relationship is the cardinal focal point of hydrological modeling from its simple signifier of unit hydrograph to instead complex theoretical accounts based on to the full dynamic flow equations. As the computer science capablenesss are increasing, the usage of these theoretical accounts to imitate a catchment became a criterion. Models are by and large used as public-service corporation in assorted countries of H2O resource development, in measuring the available resources, in analyzing the impact of human intervention in an country such as land usage alteration, deforestation and other hydraulic construction such as dikes and reservoirs.

Establishing a rainfall-runoff relationship is the cardinal focal point of hydrological modeling from its simple signifier of unit hydrograph to instead complex theoretical accounts based on to the full dynamic flow equations. As the computer science capablenesss are increasing, the usage of these theoretical accounts to imitate a catchment became a criterion. Models are by and large used as public-service corporation in assorted countries of H2O resource development, in measuring the available resources, in analyzing the impact of human intervention in an country such as land usage alteration, deforestation and other hydraulic construction such as dikes and reservoirs.

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