The effect of urbanization on storm runoff from two catchment areas in
North London
M.J. Hall
Abstract.
The development of urban areas within a drainage basin may have a marked effect
upon its hydrological regime. Owing to the dearth of hydrometric records from catchments
undergoing urban development, the changes in flow regime which take place are often difficult to
quantify. One possible approach to overcoming this problem involves firstly, the derivation of
unit hydrographs from storms recorded on several catchment areas at different stages of
urbanization; and secondly, the correlation of variables describing the shape of the unit hydrograph
with catchment characteristics, which include descriptions of the urban area and its growth. A
study of rainfall and river flow records from two adjacent catchment areas located in the
northern suburbs of London has shown that simple measures of urban development, such as the
proportion of impervious area, are insufficient to describe the variations in catchment response
between ostensibly similar drainage areas. Greater attention should be given to both channel
conditions and their modification, and the distribution of urban area within the catchment.
L'effet de l'urbanisation sur les écoulements d'orage de deux bassins au nord de Londre
Résumé.
Le développement d'une zone urbaine dans un bassin versant peut changer radicalement un régime hydrologique. Par suite du manque de données hydrométriques, il est souvent
difficile de quantifier les modifications du régime qu'on peut attendre de l'aménagement. Une
approche possible pour surmonter ces difficultés consiste à établir en premier lieu les hydrogrammes unitaires afférents à des averses observées pour plusieurs bassins correspondant à différents
degrés d'urbanisation; dans un second temps, on cherche à établir des corrélations entre des paramètres représentant la forme de l'hydrographe et des caractéristiques du bassin tenant compte de
l'état d'avancement de l'urbanisation. L'étude des précipitations et l'observation des débits dans
deux bassins adjacents de la banlieue nord de Londres ont montré que des mesures simples concernant le développement urbain, telles que celles du pourcentage des surfaces imperméables, ne
suffisent pas à rendre compte des variations de la réponse du bassin dans des zones qui pourtant
présentent des conditions de drainage manifestement semblables. Il faudrait accorder davantage
d'attention à l'état du chenal et à son évolution, ainsi qu'à la distribution des surfaces urbanisées
dans le bassin.
INTRODUCTION
The changes in flow regime that can be expected as a catchment area undergoes
urbanization have been described by Savini and Kammerer (1961), Leopold (1968),
Hall (1973) and Cordery (1976) among others. Owing to the increased proportion of
impervious area, greater volumes of surface water are discharged from an urbanizing
catchment than from the same catchment in a rural state when subjected to the same
magnitude and frequency of storm conditions. Furthermore, owing to the alterations
to the pattern of surface water drainage which result from storm water sewerage
supplementing or even completely replacing the natural channel system, the velocities
of flow from an urbanizing catchment area are higher, and time characteristics of the
runoff hydrograph, such as the lag time and the time base, are shortened. Since
greater volumes of runoff are discharged within shorter time intervals, peak discharges
are inevitably increased. Moreover, since sou moisture recharge decreases as the volume
of runoff increases, low flows between storm events are generally reduced.
144
The effect of urbanization on storm runoff
145
These changes in flow regime are of fundamental importance when considering the
incidence of flooding downstream of an urbanized catchment area. Unfortunately,
although the effects of urbanization on both the flood frequency distribution and the
shape of the flood hydrograph have been appreciated in qualitative terms, little
quantitative information has become available concerning the magnitude of the
changes to be expected at different stages in urban growth. A number of investigators,
including Espey et al. (1965, 1969), Rao et al. (1972) and Hall (1974), have approached
the problem of quantifying the effects of urbanization by employing the finite period
unit hydrograph (TUH) as an index of catchment response. Rainfall and river flow
records from catchment areas at different stages of urban development within the
same hydrologically homogeneous region have been used to derive TUHs of a
predetermined duration. Parameters describing the shape of the TUH have then been
correlated with catchment characteristics and descriptors of the extent of urban
development. The relationships so obtained may subsequently be applied to derive
TUHs for both gauged and ungauged catchment areas at different stages of
urbanization.
Where hydrometric data from urbanized catchment areas are limited, a method of
synthesizing the TUH which employs as few hydrograph parameters as possible is an
obvious advantage. Using records from a group of urbanized catchment areas in the
southeast of England, Hall (1974) was able to show that their response could be
described successfully by a one-parameter dimensionless one-hour unit hydrograph.
The parameter, lag time, defined as the same time interval between the time origin and
the centroid of the TUH minus 0.5 h, was expressed as a function of the length and
slope of the main channel and the percentage impervious area of the catchment.
However, the results obtained tended to indicate that changes to the drainage system
and perhaps the distribution of impervious area within the catchment should also be
considered when determining an appropriate value of lag time. This paper describes a
subsequent study which was carried out on two adjacent catchment areas within the
northern suburbs of London.
THE CATCHMENT AREAS
The Silk Stream and the Dollis Brook are two adjacent catchment areas in North
London which drain into the Brent Reservoir. The geology of both catchments is
predominantly London Clay, with outcrops of the Claygate beds and the Pebble beds
occurring on the higher ground, particularly to the north. Minor deposits of glacial
gravel overlain by boulder clay occur to the south and southeast of the Dollis Brook
catchment. The average annual rainfall of the region (1941—1970) is approximately
700 mm.
The general extent of urban development within the two catchment areas is indicated
in Fig.l. Both drainage basins were predominantly rural until the early 1920s.
Between 1920 and 1940, extensive urbanization took place within both catchment
areas. From 1955 until about 1970, development was concentrated mainly in small
pockets of rural area within the existing urbanization, which raised the proportion of
impervious area to approximately 21 per cent in the Dollis Brook drainage basin and
to about 25 per cent in the Silk Stream catchment.
Information on the proportion of impervious area within each catchment was
abstracted by Packman (1974) from different series of Ordnance Survey maps to a
scale of 1:10 560, supplemented by field surveys. Between 1916 and 1973, four series
of such maps were issued, the revision dates being 1916-1920, 1935—1938, 1954—1955
and 1961 — 1969. More recently, a number of the new edition of Ordnance Survey
maps to a scale of 1:10 000 have been produced for the region. In addition, a limited
number of maps were issued at various times between the major revision dates.
146
M. J. Hall
K£Y
GAUGING STATION
AUTOGRAPHIC
s{
RAINGAUGE
WATERSHED
BUILT-UP AREA
»
, - '
_,..<
BRENT
FIGURE 1.
TABLE 1.
River
Dollis Brook
Silk Stream
TABLE 2.
RESERVOfi
Map of North London catchment areas.
Details of gauging stations
Station
Area [km 2 1
Records from
Hendon Lane
Colindeep Lane
23.99
31.25
1952-1969
1929-1944
Details of autographic raingauges
Catchment
Station
Records from
Silk Stream
Silk Stream
Dollis Brook
Dollis Brook
Stanmore
Edgeware
Barnet
Mill Hill
1 9 2 9 - 1 9 3 6 ; 1942-date
1931-date
1954-1967
1960-date
In order to obtain the variations in the proportion of impervious area with time
for both drainage areas, the development which took place between successive revision
dates was marked on to a master set of the most recent Ordnance Survey maps.
Smooth curves drawn through a plot of the percentage of paved area within each
catchment against time allowed the estimation of the amount of development at
intermediate dates.
Details of the two gauging stations from which flow records were used are presented
in Table 1, and their locations are indicated on Fig.l.
Table 2 shows the details of the autographic raingauges from which records were
available during the period of discharge measurements. For the Silk Stream catchment,
the data from Stanmore were used for storms prior to 1931 and the readings from
The effect of urbanization on storm runoff
147
Edgeware for the remainder of the streamflow record. For the Dollis Brook catchment,
the data from Barnet were used during the period 1954—1967, and the records from
Mill Hill for 1967—1969. Since neither of the latter gauges was operating during the
first 2 years of record at Hendon Lane, the Edgeware raingauge was assumed to be
representative for the period 1952—1954.
ANALYSIS OF DATA
Selection of storm events
The first step in the analysis involved the inspection of the weekly charts from the two
gauging stations and the preparation of a short-list of suitable storm events. In general,
attention was confined to isolated rainfall episodes which produced hydrographs having
both a well-defined peak and a smooth rising limb and uninterrupted recession. However,
minor events of this type which yielded less than 1 —2 mm of runoff over the catchment
area were ignored. The rainfall hyetographs corresponding to each of the chosen storm
events were then constructed from the available daily rainfall charts. This selection
procedure provided 40 events for the Silk Stream and 29 events for the Dollis Brook
catchment. Ordinates of both the runoff hydrographs and the corresponding rainfall
hyetographs were then abstracted at 30-min intervals using a chart recorder whose
output was punched directly on standard 80-column punched cards.
Baseflow separation
The separation of baseflow from the total runoff hydrograph and the reduction of the
recorded rainfall hyetograph to a distribution of rainfall excess are prerequisites to any
procedure for the derivation of unit hydrographs.
One of the most widely-used methods of baseflow separation involves the plotting
of the recession limb of the recorded runoff hydrograph with the discharge ordinates on
a logarithmic scale and the corresponding times on an arithmetic scale. In general, the
lower portion of the recession on such a plot is found to follow a linear relationship,
and the point at which the curve departs from this relationship is assumed to mark the
time at which direct runoff ceases. The variation of baseflow with time during the
storm may then be represented by a straight line joining this point on the recession to
the beginning of the rising limb of the recorded hydrograph. The adopted method of
baseflow separation was essentially a computer-based version of this approach.
Rainfall loss determination
Once the point at which direct runoff ended has been determined as described above,
the direct runoff hydrograph can be obtained by deducting the baseflow ordinates
from those of the recorded runoff hydrograph. The volume of rainfall excess is, by
definition, equal to the total volume of direct runoff, which can be estimated by
integrating the area under the direct runoff hydrograph using (say) Simpson's rule.
The variation of rainfall excess with time is then obtained by distributing the rainfall
losses, i.e. the difference between the volumes of recorded rainfall and rainfall excess,
over the duration of the storm.
In the present study, the 0-index method was adopted for determining the
distribution of rainfall excess. In this approach, the rainfall losses are averaged over
the number of time intervals with nonzero rainfall to give a uniform loss rate. This
procedure is readily implemented on a digital computer. A trial distribution of rainfall
excess is first derived by subtracting the average loss per time interval from the recorded
volumes of rainfall, any negative differences being set to zero. If the difference between
the known and the trial volumes of rainfall excess exceeds a predetermined threshold
value, that difference is again spread uniformly over the time intervals with nonzero
148
M.J.Hall
rainfall. Iteration proceeds until the difference falls below the threshold value, which
was taken as 0.02 mm in this analysis.
Derivation of unit hydrographs
Having separated baseflow from the recorded runoff hydrograph and determined the
distribution of rainfall excess using the procedures described above, 30-min unit
hydrographs were derived for all storm events on both catchment areas using a modified
version of the harmonic method. When applied in its original form as described by
O'Donnell (1966), the harmonic method, in common with other well-known techniques
of inversion, is liable to produce TUHs that are distorted by high-frequency oscillations
of varying amplitude. A particular advantage of the harmonic method in this respect
is the direct connection between the high-frequency oscillations in the derived TÛH
and the magnitude of the high-frequency terms in the harmonic series representation
of that TUH. Truncation of the harmonic series representation of the TUH therefore
offers a means of controlling such oscillations. The computational scheme for
determining the amount of trunction that can be tolerated with a given set of direct
runoff and rainfall excess data, which was applied to the selected events from the
North London catchments, has been fully described elsewhere by Hall (1977). This
technique is naturally less effective in dealing with low-frequency oscillations. Where
storm events produced TUHs which were distorted by low-frequency movements, these
data were discarded, leaving a sample of 19 TUHs for the Silk Stream and 14 for the
Dollis Brook.
Derivation of dimensionless unit hydrographs
The changes in response of the two catchment areas that had occurred as urbanization
had progressed were examined by dividing each sample of TUHs into groups that were
both consistent in date order and similar in hydrograph shape. In order to determine
the dates marking the limits of each grouping, plots of the individual TUHs were
superimposed using a common starting time. This procedure produced three readilyidentifiable groups of TUHs for the Silk Stream catchment covering the period
1929-1930(4 events), 1932-early 1942(9) and late 1942-1944(6). However, with
the Dollis Brook catchment, the changes in shape of the TUHs that were evident
appeared to occur at random throughout the period of record and not in the consistent
manner exhibited by the Silk Stream data. The whole sample of 14 storms from the
Dollis Brook catchment was therefore assumed to be representative of a similar flow
regime.
The derivation of a representative response for each grouping of TUHs was
approached in three stages. Firstly, each of the 30-min unit hydrographs was converted
into a one-hour TUH using the principles of superposition and proportionality. Each
one-hour TUH was then made dimensionless by multiplying all ordinates and dividing
the corresponding times by the lag time of the hydrograph as defined in the
Introduction. Finally, the ordinates of the dimensionless TUHs within each grouping
were combined, and polynomial functions ranging in order from 3 to 12 were fitted
to the pooled data set using the method of least squares. In choosing the most
appropriate representation of the dimensionless TUH, a compromise was sought
between minimizing the root mean square residual and selecting a function with a
minimum number of coefficients. For the three groupings of Silk Stream data,
polynomials of order 7, 10 and 9 were chosen respectively. For the Dollis Brook data,
a ninth-order polynomial was found to provide the most satisfactory fit. Each of the
fitted functions enclosed an area close to unity up to approximately 3.5 times the lag
time. Figure 2, which shows the dimensionless one-hour TUH for the Silk Stream
catchment derived from storm events occurring in 1929—1930, is typical of the results
obtained using this approach.
The effect of urbanization on storm runoff
149
KEY
2 0.1
16 NOVEMBER 1929
»
27 NOVEMBER 1929
o
6 DECEMBER 1929
+
11 DECEMBER 1930
•
7TH-ORDER POLYNOMIAL
FIGURE 2.
Dimensionless one-hour unit hydrographs for Silk Stream at Colindeep
Lane, 1929-1930.
DISCUSSION OF RESULTS
The effects of urbanization
As noted above, the data from the Dollis Brook catchment did not exhibit any obvious
systematic changes in the shape of the TUH between 1954 and 1969. During this
period, the percentage impervious area of the catchment increased from 17.6 to 20.2
per cent, mainly through the development of isolated rural pockets within the
existing built-up area. The 14 storm events which were analysed gave a one-hour TUH
with an average lag time of 8.0 h. The standard deviation of the lag times of 1.48 h
reflects the variability in the shape of the response functions which were obtained.
Although there were insufficient data to confirm the tendency, a number of the later
storm events were found to yield longer lag times than storms which occurred earlier
in the record.
In contrast, the Silk Stream catchment showed a marked change in its response to
storm rainfall between 1929 and 1944. In order to illustrate this variability, the
polynomial functions representing the dimensionless one-hour TUHs for each of the
three groupings of unit hydrographs were used to construct dimensional TUHs by
dividing all ordinates and multiplying the corresponding abscissae by the appropriate
average lag time. The results obtained are presented in Fig. 3. Of the three polynomial
functions, only that for 1942—1944 exhibited any departure from a smoothly varying
hydrograph form. However, examination of the plots of the individual TUHs within
this grouping showed that the small secondary peak on the recession limb could be
attributed to minor oscillations in the recession ordinates of the derived unit
hydrographs which the modified harmonic method did not suceed in removing. Figure
3 shows that the Silk Stream displayed all the features expected from an urbanizing
catchment: a reduction in the lag time and time base, and an increase in the peak of
the TUH as the urban area developed. The percentage impervious area of the catchment
increased from approximately 15 per cent in 1929—1930 to an average of 18.6 per cent
between 1932 and 1942 and 20.6 per cent from 1942 to 1944. During this period, the
lag time of the one-hour TUH decreased to 40 per cent of its original value.
Comparison with previous work
Following a study of five catchments in West Sussex that had undergone some
urbanization, Hall (1974) suggested tentative relationships for the prediction of lag
150
M. J. Hall
0
5
10
15
20
TIME, H
FIGURE 3.
One-hour unit hydrographs for Silk Stream at Colindeep Lane.
time for both rural and urban drainage areas. The equation for predominantly rural
areas (less than 4 per cent impervious cover) was
r=3.61Z°- 4 2
(1)
where T is the lag time of the one-hour TUH [h], and Z is the basin ratio of the
catchment, which is defined as the quotient of main channel length and the square
root of main channel slope. For this purpose, main channel slope is defined by the
difference in altitude between points on the main channel located 10 and 85 per cent
of the distance upstream from the gauging station divided by 75 per cent of the main
channel length. In computing the basin ratio, main channel length is measured in km
and main channel slope in m/km. [The units of the basin ratio on the abscissa of Fig.3
of Hall (1974) are erroneously labelled as km.] For a predominantly urban area (with
approximately 25 per cent impervious cover and some channel improvements and
sewerage), the suggested equation was
r=i.ioz 0 - 5
(2)
For the Silk Stream catchment, equations (1) and (2) yield estimates of lag time of
6.34 and 2.15 h respectively. These values compare with observed figures of 6.66 h at
approximately 15 per cent impervious area and 2.65 h at approximately 21 per cent
impervious area. For the Dollis Brook catchment, equations (1) and (2) provide
estimated lag times of 6.85 and 2.36 h respectively. These figures compare with an
observed lag time of 8.0 h at approximately 19 per cent impervious area.
Concluding remarks
The inadequacy of equations ( 1) and (2) when applied to the two North London
catchments clearly indicates that the three independent variables, main channel
length and slope and percentage impervious area, are insufficient to explain the
The effect of urbanization on storm runoff
151
variations in lag time that have occurred as the Silk Stream and the Dollis Brook have
undergone urban development. The largest discrepancy is observed with the Dollis
Brook catchment, which exhibits a lag time over three times the predicted value for
its state of development. This anomaly may be partly attributable to the nature of the
development during the period of record, which consisted of infilling within the
existing built-up area. This type of urbanization, in which new property would no
doubt be connected to an existing pipe network, contrasts with that which took place
in the Silk Stream catchment, where the development of large tracts of previously
rural land would involve the construction of additional sewerage. However, even with
the latter catchment, observed lag times exceeded the values computed from equations
(l)and(2).
This consistent underestimation of lag time by equations (1) and (2) indicates that
the amount of storage provided by both North London catchments has also been
underestimated. Unlike the catchment areas in West Sussex studied by Hall (1974),
neither drainage basin contains any large lakes or detention ponds, which tends to
to suggest that the source of the retarding action may be the sewerage systems. The
construction of sewerage is generally assumed to increase flow velocities and to
accelerate runoff. Where the pipe network discharges directly to the local water course
at the nearest convenient point, a faster response to rainfall is to be expected.
However, if the sewerage system is laid out so that the length of the flow path is
increased, the effect of the longer length may more than offset that of the increased
flow velocities, resulting in a longer rather than a shorter lag time. The age of the
sewerage system must also be borne in mind, since many of the sewer design methods
employed in the United Kingdom during the 1920s and 1930s tended to underestimate
the storage effect within the pipe system (Watkins, 1963). Moreover, the ageing of a
sewer, which is often accompanied by a reduction in carrying capacity, may also
contribute to a lengthening of the lag time.
In summary, the results obtained from the two North London catchments tend to
indicate that perhaps more attention should be paid to the characteristics of the manmade drainage systems which evolve as urban development proceeds. Unfortunately,
the collection of information on individual sewerage systems and their behaviour can
be both laborious and time-consuming. Despite the statutory obligation laid upon local
authorities in the United Kingdom to maintain sewer records, there are often
insufficient data immediately available for a detailed study. In these circumstances,
surrogate parameters describing the distribution of impervious area within the
catchment, such as the characteristic impervious length factor proposed by Evelyn et al.
(1970), or empirical factors describing the condition of the channel system, as
employed by Espey et al. (1965, 1969), are worthy of further consideration.
Acknowledgement.
The assistance of the Greater London Council, Non-Tidal Rivers
Department, in providing the hydrometric data upon which the present study was based is
gratefully acknowledged.
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