The effect of the construction of Harlow, Essex on the hydrology
of the Canon's Brook is investigated using pre-recorded rainfall and
runoff records. Similar hydrological data covering a per od of urbanisation
was found for 32 other English catchments. The 8.25 sq. mile clay
basin of the Canon's Brook had an average rainfall of 23.9 inches per year
during the study period 1950-68; a three year rural period was followed by
urban expansion resulting in 16.6% of the catchment having impervious
surfaces by 1968. Water yield was increased by urbanisation; a digital simulation model of the rural catchment revealed increases in/water yield
of between 0.3 and 4.9 inches with about 16% of the basin paved. The
increase in yield was smallest in wet years and greatest in dry years,
confirming the results of a synthesis of data from published papers. Low
flows increased with modal flow rising from 2 to 4 or 5 cusecs during the
study period. The mean maximum monthly floods increased 220% because of
the urbanisation and the frequency of summer floods, particularly those in
the range 40-100 cusecs, increased markedly whilst the frequency of winter
floods did not alter. The mean unit hydrograph for the 16% paved basin had
a peak 4.6 times greater than its rural counterpart and the time of rise and
width of the unit hydrograph at 50% of peak flow were 44% and 20% of the
rural values respectively. However, large floods of over 150 cusecs, with a
return period of perhaps 20 years, were largely unaffected by urbanisation,
thus confirming the results of a synthesis of published data which showed
that the effect of urbanisation on floods is inversely related to the
recurrence interval of the floods. The apparent enlargement of the channel
because of the changed flood conditions was not statistically significant.
Sediment accumulation in a regulating reservoir gave a rate of erosion of
0.088 inches per century for a period of construction activity which when
compared with published figures supports the view that construction
activity increases erosion and sediment yields
