HR Wallingford

Hydrological Conditions for Design inUrbanising Areas and using Detention Storage

Report SR 302March 1992

31 lvlarch 1992

Address: Hydraulics Research Ltd, Wallingford, Oxfordshire OX10 88A, United Kingdom.Tclcphone: 0491 35381 lntemational +44491 35381 Telex: 848552 HRSWALC.Facsimile: A49l 32233Intemational + 44 491 32233 Registered in England No. 1622174

A:\TSRREP.WP

This repor t was prepared und.er contract No. PECD 7/6/193, funded by theDepartment of the Environment, for the research into Hydrological -

condi t ions for Design. in urbanis ing Areas and using oelent io i i to t .g" . Thework was carried out in the Resear6h oepartment. of HR warlingtora.

HR nominated project officeri Dr W R whiteSect,ion leader; M p Osborne

The DoE nominated officer is Mr Peter woodhead. This report is publishedon behalf of the.DoE but any opinions expressed are those of the authorsand not necessarily t,hose oi tne Department.

@ Crown copyright 1992

Published by permission of the Controller of Her Majesry's Srarionery office,and on behalf of the Deparunent of the Environment

A: \TSRREP.WP 31 March 1992

ABSTR,ACT

Most sewerage design :Ln the IIK uses slmthetic rainsto:ms derived fromrainfall statistics. These were originalJ-y developed to represent peakflow rates at average sucqEr conditioag. They may therefore not beapplicable to winter conditions, or !o systenrs where totat volune of runoffis i-nportant,' the design of detention storaete taaks, and predicting spiJ.J-from overflows.

One alternative method of design is to use J.ong timeseries of real rainfal.J.records. Ihis has the disadvantage of long corputation tines, anddifficuJ.ties i.rr obtal-ning the dati. This project aet out to develop amethod to rePtesent a 1ong tineseries with just a few slmthetic ator:m.s.

Anrrual rainfall ti-meseries representing a tlpical- year's rainfal-J- for threelocations in the IIK rere analysed to detemine the stonm characteristics.lhe stor:ms rere classified by depth of rain, catcbment rretness, peakednessand skew. lhis led to a new definition of peake&ress which gave higherpeak iatensitLes in 1ong durations.stores. Tbis work al-so gave an insightinto hor well the tLmeser:i.es represented the raLnfatJ. patterns in a t1p5.calyear.

Slmthetic stotms were generated using the averagie sto:m characteristics andwere found to give a good representation of the timeseries. Sensitivitystudies trere carried out to deternine hor rel'I the stor.ms would perfor:n inless than optinal conditions. Final testing of the slmthetic atolsts rrascarried out on uodels of real catchments incJ.uding detention Lanks andoverflows.

A : \ T S R R E P . W P 31. March 1992

CONTENTS

3 .

4 .

1 .

2 .

5 .

6 .

7 .

TNTRODUCTION

ANALYSIS OF RAINFALL CHARACTERISTICS

2.L Int roduct ion2.2 The lrlRc Annual Timeseries2.3 Analys is of the annual t imeser ies

2 . 3 . L S t o r m D e p t h2.3.2 Catchment weLness2 .3 .3 Peakedness2 . 3 . 4 S k e w

SYNTHETIC RAINFALL SERIES

3.1 Choice of storms for synthetic rainfall series3.2 Storage Prograrn3.3 Sensi t iv i ty test ing of skew3.4 Sensi t iv i ty test ing of s torm durat ion3.5 Conrparison with the annual t imeseries

TESTING ON REAL SYSTEI'{S

4. L Choice of model-s4 . 2 R e s u l t s

4 . 2 . 1 M o d e l 1 -4 . 2 . 2 M o d e l 2

4.3 Effect of sampling the annual t imeseries

CONCLUSIONS

FUTURE WORK

ACKNOWLEDGEMENTS

REF'ERENCES

Page

1

2

223

457

Ll_

12

t21 313l 41 5

1 6

1 61 8

1 81 8

l-9

1 9

2 L

2 2

2 3

A: \TSRREP.WP 31 March 1992

FIGURES

1 . Depth ratio/Return perioda South East , Durat ions L5 - 48 minutesb South East , Durat ions 60 - 1920 minutesc Yorkshi re, Durat ions 15 - 480 minutesd Yorkshi re, Durat ions 60 - L9Z0e South West , Durat ions L5 - 480 minutesf South west , Durat ions 60 - 1920 minutes

Not included in this report

Freguency/SkewSouth EastSouth East 20 largest stormsYorkshireYorkshire 20 largest stormsSouth WestSouth West 20 largest storms

4 . Frequency/UCwt ratioSouth East - annualYorkshire - annualSouth glest - annualSouth West - seasonaLYorkshire - seasonalSouth East - seasonal

2 .

3 .abcdef

abcdef

5 .6 .7 .8 .9 .l - u .

1 1 .L 2 .1 ?

L 4 .1 5 .l - o -

t 7 .' l q

Comparison with annual t imeseriesSouth East - 30? perv iousSouth East - 60? perv iousSouth East - 70? perv iousSouth West - 30t perv iousSouth West - 60? perv iousSouth West - 70% perv iousYorkshi re - 30? perv iousYorkshire - 60t perviousYorksh.ire - 702 perviousLayout of model 1Layout of model 2ModeL l Storage,/Return periodModel 1 Overflow,/Return periodModel 2 Overfl-ow Return period

Appendix A Skew Storms

A : \ T S R R E P . W P 31 March 1992

1. II{TRODUCTION

one of the longest runningr arguments in the use of

urban drainage models concerns whether the

behaviour of the system j.s best represented using

synthet ic s torms der ived f rom staList ics, or us ing

a t imeser ies of real s torms.

The argument in favour us using storms is that they

are readily available for aII locations in the UK,

and that they are easy to use and require only a

few storms. However the normal synthetic storms

represent only one set of rainfall conditions,

average surruner conditions, and have only been

proved to be correct in predicting peak flow rates'

not storage volumes. The argiument in favour of

timeseries is that they include a wider range of

conditions, and therefore are l ikely to contain the

condi t ions which are cr i t ica l on each catchment .

There are trdo crucial areas in which synthetic

storms may not adequately represent the response of

a catchment. The first of these is in the design

of detention storage tanks, where modest storms

fall ing on a very wet catchment may give a larger

volume of runoff than a larger storm with averag:e

catchment wetness. The second area is in

predicting spil l from overflows in small frequent

storms for pollution impact studies. The first

area is the main area of in terest of th is pro ject ,

although the second has been covered as well.

A : \ T S R R E P . W P 31 March 1992

2 . t

2 . 2

AI.IALYSIS OF

RAINFAjJIJ CITAR,ACTERI ST ICS

In t roduc t ion

The I{Rc Annual

Timeseries

The first slage of this study was to investigate

the variation that existed in long rainfall records

of real storms to see whether this variation could

be represented by a few synthetic storms. To do

this a representative timeseries !{as required. The

$lRc annual ,Timeseries were used for this. These

were chosen, because although they did not include

the more extreme storma which were also of interest

to this project they were a convenient source of

representative rainfalt data. This work is

reported more fulty elsewhere IHR wal]-ingford

1 9 9 1 1 .

Timeser ies ra infa l l is a sequence of h is tor ic

ra infa l l eventa stat is t ica l ly representat ive of the

precip i ta t ion pat terns for a g iven locat ion. The

hlRc annual t imeseries are series of storm events

representing typical years for each of three

regions of the country. They have a duration of

one year and are sequences of f ine resolution rain

data suitable for input to WASSP-SIM. The storms

are available in three forms, a chronological

ser ies of a l l s igni f icant events in a t lp ica l year ,

the same eventa ranked in order of severity and a

pair of similarly ranked series for summer and

srinter periods. Annual t imeseries are available

for the South East , South West and Yorkshi re. Each

ser ies consist of approximately 99 storms.

A: \TSRREP. WP 31 March 1992

2 . 3 Anal-ysis of the

annual t imeseries

In deriving the timeseries a prelj-minary seltection

of month ly ra infa l l to ta ls was made. The tota ls

were screened and any months with values within the

range of the mean plus or minus one standard

deviation were accepted. Approximately one third

of alt the monthly rainfalls were outside this

rang'e, representing very wet or vary dry months;

these were re jected. f rom the analys is . This

selection of representative months is not thought

to be significant, ' however t,he analysis of the fuII

rainfall record may provide better results. The

annual t, imeseries are currently ranked by severity;

this involved using the timeseries rainfall for

simulation in a number of models in order that the

Iargest discharge volumes could be identif ied.

This project attempts to rank the series by

compar ison wi th s tat is t ica l ra infa l l data.

The storms in the annual t imeseries r.tere analysed

for four parameters: storm depth, catchment wetness

(UCWI), and two parameters describing the storm

shape, peakedness and skew. For each storm these

parameters were compared to the standard values

used for synthetic storms for the locations from

which the timeseries data was derived.

The data assumed for the t imeser ies locat ions is :

A: \TSRREP.TIP 31 March 1992

South

East

6 0 3

Yorkshi re

L 2 2

Average annual rainfal l mm

M5 60 mm

Standard UCWI winter tnm

Standard UCWI surruner nm

Location lndex

5 1

2 . 3 . t Storm Depth

Each storm in the series was analyzed to find the

maximum depth in each of a series of durations.

The durat ions used where: 1-5, 30 ' 60, L20, 240r

480, 950, 1920 minutes. The depth rat io was

defined as the maximum depth divided by the depth

in a storm of one year return period of the sarne

durat ion. The cr i t icat durat ion was def ined as the

duration with the largesL depth ratio.

The depth ratios were then ranked in ascending

order for each duration, and the rank converted to

a return period, 1/ (number of storms - rank) and a

graph of depth ratio against return period was

p l o r r e d ( F i g L a - 1 f ) .

The results for each duration are similar' although

for all of the regions the long storms with

durations of 240 minutes and above show a decrease

of depth rat io wi th increasing durat j -on. This is

particularly noticeable for the South West region

i f the out l ier s torms ax !2 and L20 minutes

duration are iqnored. This indicates that long

duration storms are under-represented in the

t imeser ies. For durat ions up Lo 244 minutes the

depth ratio is independent of duration.

A: \TSRREP.WP 31 March 1992

The variation of depth ratio with return period for

the South East and Yorkshire regions are similar in

that the depth ratio steadily increases up to a

return per iod of about 0.5 then r iees more s lowly.

The graph for the South West region rises much more

rapid ly to a value of 0.8 then more s lowly to a

maximum of approximately 1 (Fig 1e). The

difference in these graphs is indicative of two

dist inct ra infa l l pat terns. South East and

Yorkshire regions representing the rainfall

patterns in Eastern EngJ-and and the South West

representing Western England.

The curves of depth ratio were redrann so as to

give a smooth curve which passed through a depth

rat io of l - .0 at a return per iod of 1 year . ( fh is

is requi red by the def in i t ion of depth rat io . ) The

averalJe of the values for durations of 60 minutes

to 240 minutes are tabulated bel-ow.

2.3.2 Catchment hretness

Catchment wetness is defined in the Wallingford

Procedure by the Urban Catchment Wetness Index

(UCWI). Standard values of UCWI are g iven for a1l

locations in the UK for summer and winter

ReturnPer iod yrs

South East South West Yorkshi re

1 : 1 L . 0 0 0 1 . 0 0 0 I n n n

2 : t 0 . 8 1 3 0 . 7 7 5 0 . 8 5 5

4 . 1 0 . 6 3 5 0 . 6 5 0 0 . 7 1 0

8 : 1 0 . 4 0 3 0 . 5 6 3 0 . 4 8 8

1 6 : 1 0 . 3 0 0 0 . 4 1 6 0 . 3 0 2

3 2 z L 0 . 1 9 6 0 . 2 9 3 0 . 1 8 5

6 4 . 1 0 . 1_05 0 . L 7 7 0 . L 1 0

A : \ T S R R E P . W P 31 March 1992

conditions. Surwner is taken as April to September

and winter as October to March. These standard

values nrere derived by taking the average of the

values calculated for the last day of each month in

the season over a per iod of many years.

An UCWI ratio was calculated for each storm as the

UCWr for the storm divided by the standard UCWI for

the appropriate season. (Not.e the original

timeseries was derived for version 6 of WASSP which

does not allow UCWI values greater than 170. The

latest version of the timeseries which has

corrected values was therefore used.)

No correlation was found between UCWI ratio and

ret,urn period.

The results are surprising. There is a very large

spread of values but most results are above the

sLandard values. The results are shown in

Figures 4A, 48 , 4C and the mean and standard

deviation are tabulated belo$t.

South East South West Yorkshi r

Mean 1 . 5 7 9

Standard Deviat ion 0 . 4 5 2 0 . 3 3 2 0 . 6 3 7

If summer and winter.storms are considered

separately (Figrs 4D-4F) a different pattern

emerges. In winter there is a smaller spread of

values, and the average is close to the standard

value. Thj -s is l ike1y to be because in winter

A: \TSRREP . WP 31 March 1992

catchments may remain consistently wet.. fn sunmer

there is a large spread of values including some

very much higher than the standard. ?he result has

been noted previously by other researchers using

other rainfall series. As the standard suruler UCWI

is the basis of most urban drainage design' and

these higher values wil l increase the amount of

runof f f rom a stormr. th is is of great concern. A

hlpothesis for this variation can be put forward.

The pattern of rainfall in summer is generally to

have of dry weather followed by a series of sununer

st,orms. There is therefore a greater chance that a

storm has been recently preceded by another storm

than if the stoxms !{ere uniformly distributed

throughout the suruner. The average UCwf within

these periods of rainy weather wil l- be greater than

the overall suruner average which is given by

averaging the last day of each month.

This phenomena is worthy of further investigation,

but these results show that the higher, ninter'

standard. values of UCWI are in fact appropriate for

both sununer and winter.

2 . 3 . 3 P e a k e d n e s s

The rainfalJ- anal-ysis in the flood studies report,

did not explicitJ-y investigate the variation of

peakedness with durations greater than l- hour. The

f i rs t analys is of the t imeser ies a lso looked at

peakedness independent of duration. Peakedness was

or ig inal ly def ined as:

cL/c2

where C2 depth of rain in the crit, ical duration

of the storm

A: \TSRREP.WP 31 March 1992

C1 maximum depth of rain in half the

cr i t ica l durat ion of the storm.

This showed similar values of peakedness to

synthet ic s torms generated ;us ing the Flood Studies

Report.s standard 50 percentile summer or 75

percentile winter profi les. The standard storms do

not give a constant .value of peakedness using this

definit ion, but have values varying by about 20

percent .

No correlation was found between peakedness and

return period of storm.

A better definit ion of peakedness was then used'

which allowed for the variation of peadkedness wit,h

durat ion. This was def ined as:

r I / 1 2

where r1 the maximum depth in a given duration

in the storm

12 the maximum depth in half that duration

in the storm.

The resul ts f rom th is analys is are tabulated below.

They show that peakedness increases with storm

durat ion.

A: \TSRREP.WP 3 L M a r c h 1 9 9 2

Average Peakedness

Using this definit ion of peakedness t.here are

physical l imit.s that Lhe value must l ie between 0-5

and 1.0. However there is another bray in which the

peakedness at J-ong durations can be calculated.

This is by us ing the rat io of f ive year return

period 60 minute and 2 day storms which are mapped

for the whole of the UK.

The 2 day depth vras assumed to represent the depth

in 48 hours. (There is in pract ice a smal l

d i f ference between the two but th is was ignored.)

The peakedness factor must be applied for each

doubl ing of the durat ion. From one hour to 48

hours there are n doublinqs where:

4 8 : 2 "

n - 5 . 5 8

Durat ion minutes South East South 9{est Yorkshi re

3 0 0 . 6 9 3 0 . 6 7 7 0 . 6 6 9

6 0 0 . 7 5 2 0 . ? 0 9 0 . 7 1 8

L20 0 . 8 1 5 0 . 7 5 7 0 . 7 7 8

240 0 . 9 0 4 0 . 8 2 3 0 . 8 4 2

4 8 0 0 . 9 8 8 0 . 9 8 5 1 . 0 0 0

9 6 0 0 . 9 9 ? 1 . 0 0 0 1 . 0 0 0

L920 1 . 0 0 0 r . . 000 1 . 0 0 0

A: \TSRREP . WP 31 March 1992

Therefore assuming an average peakedness P:

P r . t , = r

where r = M5_60

M5_60

/ r45-2DAt

t hour depth at 5 year return

period

2 day depth at 5 ;year return

per iod

M5 zDAY

These value were taken as maximum limits for the

peakedness at durations of four hours and above.

For durations less than L hour the peadkedness was

taken as the min imum observed value of 0.67. A

Iinear variation was used betri leen one hour and for

hou rs .

The resulting storm profi le was compared with the

standard 50? sunurner profi les which are generally

used for urban drainagre design. At short

durations, less than one hour, the new profi le has

much lower peaks than the 50t summer storm.

However as the duration increases the new profi le

shows almost no decrease in peak intensity, whereas

the 50t suruner profi le shows a rapid reduction in

peak intensi ty . At long durat ions, greater than 4

hours, the new profi le therefore has a higher peak

intensi ty

Yorkshire

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1 031 March 1992

2 . 3 - 4 S k e w

Skew is a measure of the position of the peak

rainfa l l in tensi ty wi th in the storm. Skew was

expected to have a significant effect on the

response of the system to ra infa l l . The usual

synthetic stonns are syrrtrnetrical (ie central skew)

Several different definit ions

but the following was adopted

usefu l

skew were tried,

being the most

o f

as

The crit ical duration (D) of the storm and the

start and end of this duration were identif ied.

This period was then taken as representing the

storm, and rainfall before and after this was

ignored. The time for half of the rainfall depth

within this duration (T50) !,ras then caLculated.

Skew is def ined as:

S k e w = T 5 0 / D

A skew of 0 would represent a storm with most of

the ra infa l l a t the star t o f the storm, a skew of

0.5 would have most of the ra infa l l in the centre

of the strom, and a skew of 1 would have most of

the ra infa l l a t the end of the storm.

The resul - ts (F ig 3a - 3c) show that there is a

variation of skew, \dith most storms having fairly

centa l skew. The average values of skew are: 0.5

in the South East , s l ight ly above 0.5 in the South

West and 0.5 in Yorkshi re. To invest igate the

relationship between skew and storm size the skews

of the twenty largest storms for each series were

p lo t t ed (F ig 3d - 3 f ) . The resu l t s a re s im i l a r .

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1131 March 1992

3 .

3 . L

STIIXHETTC R,AINFAIII

SERIES

Choice of storms

for synthet.ic

ra infa l l ser ies

From th is analys is i t is reasonabLe t .o use storms

wi th a centra l skew, but the s igni f icance of skew

was investigate further in the sensitivity testing

carr ied out in the next phase of the pro ject .

The storms for the synthetic rainfall series were

chosen to have the average characteristics of the

t imeser ies stonns. Separate character is t ics were

used for each region. In sumfirary these

characteristics ri lere :

The depth of rain was calculated using the depth

rat ios g iven in sect ion 2.

The catchment wetness was taken as the standard

winter va lue.

The peakedness was varied with duration.

The skew was centra l .

Sensi t iv i ty test ing of these storms and in i t ia l

comparisons of the synthetic storms and the

timeseries were carried out using a simple storage

simulation program. This work is reported more

fu l ly e lsewhere [Rainey & Osborne L991] .

A: \TSRREP,WP

L23 l - March 1992

3.2 Storag,e Program

3 . 3 Sensi t iv i ty

test ing of skew

A simple storage simutation program was developed.

This represented a catchment as a lumped hydrologic

runoff model, and the sewerage system as a single

storage tank hrith a l imit on the continuation flow.

The results of the model showed the maximum volume

of water stored in t.he tank during a storn.

The hydrologic model uses the runoff model from the

WALLRUS program where the rrunoff is defined by the

proportion of the area which is covered by

impermeable surfaces, a soil index and the

catchment rdetness (UCWI) .

The storage tank i-s of infinite size and has an

outflow limit on the continuation flow. Any flow

above this l imit wil l be stored unti l such time

that the inflow drops below the outflow limit. Any

stored ri later can then flow from storagte to bring'

the tota l out f low up to the l imi t . The out f low

limits can be varied to represent different

catchment character is t ics. The storage model was

validated by comparison with a simple WALLRUS

model-.

From our analys is of the t imeser ies, no real

tendency for skew was found. Hordever the effect of

skew was investigated using the storage model and

checked using a 2 pipe test system on WALLRUS.

Various catchment characteristics were used in the

model . a cr i t ica l skew of 0.52 was most common

with l i t t le var iance. The ef fect of th is smal l -

skew compared to a centra l skew of 0.5 was

A : \ T S R R E P . n P

1 33 1 M a r c h 1 9 9 2

3 . 4 Sensi t iv i ty

test ing of

storm duration

negl ig ib le. Even using largrer skew values of 0.6

and 0.7 a maximum increase in s torage of 10t was

found.

The crit, ical duration of storm will carry with

catchment characteristics, in part.icular the

catchment. size and the amount of attenuation

storage in the catchment. The crit ical duration

may also vary with the storm size. The sensitivity

of the calculated storage vol-umes to the ueing

durations other than the crit ical duration was

therefore invest igated.

Synthet ic s torms of f requency of 2:1 year and

durat ions of 1, 2, 4 8 and 16 hours were s imulated

using the storage program to find the crit ical

durat ion for a set of catchment character is t ics.

Storm durations in excess of 16 hours were not

normally considered, as for most systems the

cr i t ica l durat ion was found to be less than th is .

The storm duration with the largest storage volume

was then taken as the cr i t ica l durat ion. Using the

cr i t ica l durat ion the storage volume for a ser ies

o f s to rms o f r e tu rn pe r i ods o f l - : 8 yea rs t o 64 :1

year were calculated. The var iat ion of cr i t ica l

duration wLth return period was investigat.ed by

simulat ing the fu l l set of return per iods (1:8

years to 64:1- year) for durat ions of t hour to

1 6 h o u r s .

A: \TSRREP.WP

1 431 March 1992

3 , 5

The crit ical durations were found to vary with

return period. However, the error introduced by

choosing the cr i - t ica l durat ion f rom the 2:1 year

storm was less than 10t . This procedure wi l l

t .herefore normal ly be sat is facLory.

Comparison with

the annual t. imeseries

The effect of the synthetic rainfall series was

compared srith t,hat of the annual t imeseries using

the storage program. The synthetic series was

using 10 storms to repreaent the complete range of

return periods, whereas the timeseries uses 99.

The comparisons were done for each of the three

t imeser ies.

Three different sets of catchment characteristics

were used. These had perv ious areas of 60*, 30t

and ?0t. Some tests were done with a range of soil

ind ices, but most test used a value of 0.4. For

each set of catchment character is t ics two d i f ferent

outflow limits were used. These represented

catchments with small amounts of attenuati-on

storage, and vrith very large amounta of attenuation

s to rage .

For the synthet ic ra infa l l - ser ies the cr i t ica l

duration was first identif ied, and then storms of

1 : 8 , L : 4 , L z 2 , 1 : 1 , 2 z L , 4 z L r 8 : 1 - , 1 5 : 1 , 3 2 : 1 a n d

64:1 year were analysed. Al t 99 storms of the

appropriate timeseries were analysed. The two sets

of storage volume results were plotted together for

compar ison.

The comparisons are generally good (Figs 5-13)

The South East synthet ic ra infa l l ser ies

A: \TSRREP . WP

1531 l ' tarch 1992

4 .

A 1

TESTING ON

REAI. SYSTEMS

Choice of models

consistently indicate a slight under-prediction of

storage compared to the timeseries. The Yorkshire

synthet ic ra infa l l ser ies g ives a good compar ison

for all of the catchments at the low outflow limit

with some over-prediction on the more pervious

catchments at high outflow limits.

The South west synthetj-c rainfall series did not

perform as well with a LO-20? under-prediction of

storage at low outflow limits and a slight over-

predict ion,at h igh out f low l imi ts .

These results sho!i led that the accuracy of the

synthetic rainfall- series was generally insensitive

to catchment character is t ics.

The f ina l s tage of th is s tudy was to test the

synthetic series against the annual t imeseries

using WALLRUS models of real- caLchments.

The Water Service Companies suppl-ied a number of

models. Two suitably representative models were

selected as there was insuf f ic ient t ime to test the

synthet ic ra infa l l ser ies on a l l the models.

However the test ing of synthet ic ra infa l l ser ies

using the storage model had indicated that the

results were not verv sensitive to catchment

cha rac te r i s t i cs .

The models were selected to have typical values of

the fo lJ .owing parameters: s ize, perv ious area,

tpaved, t roof and soi l index. The deta i l -s of the

A: \TSRREP . WP

1 631 March 1992

models are given below, and the details of the

storage tanks and over f lows are g iven in F igures L4

a n d 1 5 .

Model 1 Model 2

Permeable area 4 5 4 . 6 2 L . 9

Paved area 4 7 . 0 5 . 9

Roof area 0 . 0 6 . 1

Soi l index 0 . 3 0 0 . 4 5

Pipes 8 5 1 0 9

Tanks L 2

Volumes 3 3 0 0 . 3

0 . 3

Locat ion D11 Yorks

Cr i t i ca l

duration

1 5 1 5

A timeserJ-es appropriate to the region of the

catchment was then selected. A synthet ic ser ies of

s t . o rms o f r e tu rn pe r i od re tu rn pe r i od 1 :8 , 124 ,

L z 2 , L : 1 , 2 : 1 - , 4 : ! , 8 : 1 , 1 - 6 : 1 , 3 2 : L . 6 4 : 1 - y e a r w a s

generated using the standard data for the

t imeser ies locat ion. Al though durat ions of 1, 2,

4, 8 and 16 hours were s imulated, the resul ts

presented here are a l l for durat ions of 16 hours as

the error in making this simpLification was found

to be small. The synthetic series was compared

$rit,h the annual t imeseries by simulation using the

WATLRUS program. The results of storage and

overf low, against return per iod for the synthet ic

ser ies and the annual t imeser ies were compared.

A: \TSRREP.WP

L731 March 1992

4 . 2 R e s u l t s

4 . 2 . 1 M o d e l 1

The graph of storage against return period (Fig 16)

shows significant storage for the return periods

considered. a good comparison between the

synthetic rainfall series and the annual t imeseries

is obtained for storms which occurred less

frequently than 5:1 year. For storms which

occurred more frequentJ-y than 6:1 year the

synthetic rainfall series under predicted the

atorage by up to 20t. The effect of storm duration

on this was tested, and a storm of 32 hours

duration and return period 32:1 year provided a

storage value of 183.6m2 which reduced the error at

th is return per iod by 5-6t .

The overflow from the storage tank spil led in many

of the storms and the comparison of spil l volume is

shown in Figure L7. The predictions of the annual

t imeser ies and the synthet ic ra infa l l ser ies are

s im i l a r .

4 . 2 . 2 M o d e l 2

The overflow at tank 1 did not operate in any of

the storms which were used.

The graph of overflow spil l against return period

for tank 2 is given in Figure 18. Good agreement

is shown between the annual t imeseries and the

synthet ic ra infa l l ser ies.

A: \TSRREP.WP

1 831 March 1992

4 . 3 Effect of sampling

the annual t imeser ies

5 . coNcrusroNs

The annuat timeseries contains 99 stonns and it is

therefore not usual to analyse all of these because

of the long computing times which this would

enta i l . Instead a sample of the storms is

analysed, and the results for the whole series

ext,rapolated from this. The most usual sampling is

to use the first f ive large storms, and then every

fifth storrn for the rest of the series. The effect

of this on the predictions for the two models was

checked. In both cases the synthetic rainfall

series rrras a better representation of the full

t imeseries than the sampLe. It should be

remembered that the sampled timeseries st,i l l

consis ts of 23 storms compared to 10 for the

synthet ic ser ies.

The annual t imeseries does not contain enough longr

duration storms to correctly represent the true

ra infa l l pat tern.

The variation of storm depth with return period for

return periods less than one year appears to be

independent of storm duration, but does vary

regional ly .

The published average sununer values of UCWI appear

to be too Iow, as many suruner storms have much

higher values

If storm peakedness is defined to vary with

duration then it gives much higher intensities for

A: \TSRREP.wP

1 931 March 1992

long duration storms, although lower peak

intensi t ies for shor t durat ion storms.

For models using the constant runoff coefficient

model of the Wallingrford Procedure, the skew of the

storm profi le has l itt le effect on storage volume

or over f low spi I I , and centra l skew can be used.

Design of detention storage in urban drainage

systems should use the standard ldint.er value of

UCWI and the new storm profi le with peakedness

varying with duration. A range of durations

including very long durations should be analysed.

To predict overflow spil1, the use of the

ra infa l l ser ies, us ing the new storm

characLer is t ics, g ives resul ts as good as

annual t imeseries and usually better than

sampled timeseries, and uses fewer storns

e i t he r o f t hese .

synthetic

the

the

than

A: \TSRREP.WP

2 031 March 1992

5. E"UII'RE WORK

The work carried out i-n this study has made several

imporLant advances, but has raj-sed more questions

than iL has answered. Some of these should be the

subject of fu ture research work.

The work has indicated that the annual t imeseries

is not a good representat ion of ra infa l l pat terns.

The derivation of the synthetic rainfall series

should therefore be repeated using longer rainfall

records which wil l be more representative.

The proposed changes to UCWI and storrn profi le

should apply to the design of pipe systems as weLl

as detention storage. As pipe systems are normally

designed for short duration storms the tero changes

are l ike ly to cancel out . However the ef fect

should be tested-

The analysis carried out here has not looked at all

of the differences between sunmer and winter

condi t ions. As many types of s tudies, for example

overf low spi l l , requi re separate considerat ion of

summer and winter conditions, future analyses

should be done separate ly for the two seasons.

A modified runoff model is being introduced for the

Wallingford Procedure which uses a different

measure of catchment wetness. The analysis should

be updated to provide results for this model. This

model wil l change the catchment $retness and runoff

during a storm. This may therefore make skewed

storms more s igni f icant .

The use of the synthet ic ra infaLl ser ies should be

analysed for water qual i ty modelJ- ing. I t is again

A: \TSRREP.WP

2 L31 March 1992

7. ACKNO}]II.EDGEMENIIS

l ikely that storm skew will be more significant for

t h i s app l i ca t i on .

The analysis has ahosrn significant regional

var iat ions in ra infa l l character is t ics, and future

work should look at more regions.

The definit ions of skew and peakedness are sti l l-

clumsy and consideration should be given to

developing better definit ions.

The authors are grateful to glRc for providing the

timeseries rainfalL on which the eynthetic rainfall

series have been based. To Thames wat.er Util i t ies'

Yorkshire water, Norfolk District Council and

Hertsmere Drainage for supplying real models which

were used for test ing.

A: \TSRREP.WP

2231 March 1992

8. REFERENCES

HR gfa l l ingford. The c lass i f icat ion of s torms in

the annua l t imese r i es . Ex 2429 . Oc tobe r 1991 - .

Rainey C M L t Osborne M P. Design storms.

HR Wal l ingford Report No SR 291. November 1991.

A: \TSRREP.wP

2 331 March 1992

Figures

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Example I

TANK

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SOFFITLEVEL AD AREAha

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84.975

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Example 2a

PIPE DIAMETER

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SOFFIT LEVEL AD AREA ha

LENGfi{ UPSTREAM DOWNSTREAM PAVED

Example 2b

TANK IBedlevel 25l.MOOverflow crest level 251.390

Plan area 0.866m2

ROOF PERVIOUS

| 225

2 750

3 750

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95 215.052 210.811

r0 2r5.45r 215.580

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19 21836t 215.055

5.332 4.784 16.837

5.332 4.784 16.837

5.312 4i64 16.777

0.020 0.020 0.060

Example 2b

SOFFITLEVELAD AREAhA

PIPE DIAMETER LENGTH IJPSTREAM DOWNSTREAM PAVED ROOF PERVIOUS

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2 300 13 252.44 25r.390 0.787 0.755 2.879

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APPENDIX A

A: \TSRREP.T'P

2 5

31 uarch 1992

Annual Series Rainfall Data for SOUTH EAST U.K. **'t'k IUNKED IEAII ****

315FF3 .0

ucltll=9o ** I ** EVENT

0

1 4 : 4 1 4 / B / 7 LI

1

67 .000 66 .000 65 .000 64 .000 63 .000 62 .00057 .000 56 .000 55 .000 54 .000 53 .000 52 .00047 .000 46 .000 45 .000 44 .000 43 .000 42 .ooo37 .000 36 .000 35 .000 34 .000 33 .000 32 .00027 .000 26 .000 25 .000 24 .000 23 .000 22 .o0017 .000 16 .000 15 .000 14 .000 13 .000 12 .0007 .000 6 .000 5 .000 4 .000 3 .000 2 .0000 .000 0 .000 0 .000 0 .000 0 .000 0 .0000 .000 0 .000 0 .000 0 .000 0 .000 0 .000

08:01 25/9/67 610 vAtUES

3 .000 4 .000 5 .000 6 .000 7 .000 8 .00013 .000 14 .000 15 .000 16 .000 17 .000 18 .00023 .000 24 .000 25 .000 26 .000 27 .000 28 .00033 .000 34 .000 35 .000 36 .000 37 .000 38 .00043 .000 44 .000 45 .000 46 .000 47 .000 48 .00053 .000 54 .000 55 .000 56 .000 57 .000 58 .00063 .000 64 .000 65 .000 66 .000 67 .000 68 .00067 .000 66 .000 65 .000 64 .000 63 .000 62 .00057 .000 56 .000 55 .000 54 .000 53 .000 52 .00047 .000 46 .000 45 .000 44 .000 43 .000 42 .00037 .000 36 .000 35 .000 34 .000 33 .000 32 .00027 .000 26 .000 25 .000 24 .ooo 23 .000 22 .00017 .000 16 .000 15 .000 14 .000 13 .000 12 .0007 .000 6 .000 5 .000 4 .000 3 .000 2 .0000 .000 0 .000 0 .000 0 .000 0 .000 0 .0000 .000 0 .000 0 .000 0 .000 0 .000 0 .000

F

4 (AUG)

90320 VATUES

0 .3090 60 350 600

9 .000 0 .000 1 .000 2 .000 3 .000 4 .000 5 .000 6 . o0o 7 . oo0 8 .00019 .000 10 .000 11 .000 12 .000 13 .000 14 . o0o l s .000 16 .000 17 .000 18 .00029 .000 20 .000 21 .000 22 ,ooo 23 .ooo 24 .OOO 25 .000 26 .000 27 .OOO 28 .00039 .000 30 .000 31 .000 32 .000 33 .000 34 . OOO 35 .000 36 .OOO 37 .000 38 . OOO49.000 40 .000 41 .000 42 .Ooo 43 .000 44 .000 45 .ooo 46 .ooo 47 .ooo 48 .00059 .000 50 .000 51 .000 52 .000 53 .ooo 54 .000 55 .000 56 .000 57 ,000 58 .00069 .000 60 .000 61 .000 62 .000 63 .000 64 .000 65 .000 66 .000 67 .000 68 .0000 .000 0 .000 0 .000 0 .000 0 .000 0 .000 0 .000 0 .000 o .o0o 0 .0000 .000 0 .000 0 .000 0 .000 0 .000 0 .000 0 .000 0 .000 0 .000 0 .000

ucwr=146 ** 2 ** EVENT 3 (APR) 01:07 5/4/sB 380 vALUEs0 .30 146

90 60 360 60061 .000 70 .000 69 .000 68 .00051 .000 60 .000 59 .000 58 .00041 .000 50 .000 49 .000 48 .00031 .000 40 .000 39 .000 38 .00021 .000 30 .000 29 .000 28 .00011 .000 20 .000 19"000 18 .0001 .000 10 .000 9 .000 8 .0000.ooo o.o0o o.ooo o.ooo0 .000 0 .000 0 .000 0 .000

ucl^il=65 ** 3 ** EVENT 15 (SEP)0 .30 65 I

160 60 480 6009 .000 0 .000 1 .000 2 .000

19 .000 10 .000 11 .000 12 .00029 .000 20 .000 21 .000 22 .00039 .000 30 .000 31 .000 32 .00049 .000 40 .000 41 .000 42 .000s9 .000 50 .000 51 .000 52 .00069 .000 60 .000 61 .000 62 .00061 .000 70 .000 69 .000 68 .000s1 .000 60 .000 59 .000 58 .00041 .000 50 .000 49 .000 48 .00031 .000 40 .000 39 .000 38 .0002 r .000 30 .000 29 .000 28 .00011 .000 20 .000 19 .000 18 .0001 .000 i 0 .000 9 .000 8 .0000 .000 0 .000 0 .000 0 .0000 .000 0 .000 0 .000 0 .000