Hydrological regime modifications

Studies in the UK have failed to relate annual and seasonal rainfall differences to urban development (Tabony, 1980). In the London area, rainfall differences are influenced mainly by altitudinal differences. On a short timescale, the proportionally greater incidence of severe thunderstorms in built-up areas compared with rural areas is well noted and can be ascribed to greater concentrations of condensation nuclei in the air, increased turbulence and urban overheating. Information on such extreme events is naturally more readily available (and perhaps more likely to be recorded) in centres of population, and the hydrological consequences are usually of immediate concern. Storm magnitudes and their frequency of occurrence are of greater importance than annual rainfall totals in urban hydrology.

Urbanisation can have a dramatic effect on all aspects of the terrestrial hydrological cycle. Urban development changes the relative proportions of water that infiltrates the ground, evaporates, travels as subsurface drainage or is carried as overland flow and surface drainage. Fig. 18.1 shows generalised estimates of the partition of water into different flow pathways given by Endreny (2005), based on assumed typical land cover characteristics. Of course, in any particular catchment, the actual impacts of these types of changes may vary greatly depending on the actual proportions of pervious and impervious surfaces, their spatial arrangement, the design of the drainage system and the connectivity of flow pathways.

Fig. 18.1 Generalised typical partition of the water balance between evaporation, surface runoff, shallow subsurface infiltration and deeper infiltration for four different land cover classes defined in terms of the percentage impervious surface area. Reprinted from The Encyclopedia of Hydrological Sciences, Chapter 117, Endreny, T. A., Land Use and Land Cover Effects on Runoff Processes: Urban and Suburban Development, Pages 1775-1804, Copyright (2005) with permission from John Wiley & Sons.

Fig. 18.1 Generalised typical partition of the water balance between evaporation, surface runoff, shallow subsurface infiltration and deeper infiltration for four different land cover classes defined in terms of the percentage impervious surface area. Reprinted from The Encyclopedia of Hydrological Sciences, Chapter 117, Endreny, T. A., Land Use and Land Cover Effects on Runoff Processes: Urban and Suburban Development, Pages 1775-1804, Copyright (2005) with permission from John Wiley & Sons.

The covering of the land surface by a large proportion of impervious materials means that a much larger proportion of any rainfall forms immediate runoff. In addition to extensive ground coverage by the buildings in a city, the paved streets and car parks contribute large areas to the impervious surfaces. Any slope of the land also greatly enhances the runoff response of a paved area. In a defined catchment area, the effect on the stream discharge is dependent on the extent of the impervious area. Contributions to groundwater are limited to rainfall on the remaining pervious surfaces, where normal infiltration into the soil and percolation into the underlying strata can take place.

After major urban developments in a catchment, the following differences in the river flow from that of an equivalent rural catchment can be identified:

(1) for a specific rainfall event, the response of the catchment is accelerated, with a steeper rising limb of the flow hydrograph; the lag time and time to peak (see Chapter 13) are reduced;

(2) flood peak magnitudes are increased;

(3) in times of low flows, discharges are decreased since there is a reduced contribution from the groundwater storage that has received less replenishment;

(4) water quality in streams and rivers draining urban areas is degraded by effluent discharges, increased water temperature and danger from other forms of pollution.

Many of these modifications are promoted by structural changes made to drainage channels. Surface water drainage systems have usually been designed to remove rain water quickly from developed areas, although measures may also be taken to delay and attenuate the runoff response, particularly in modern sustainable drainage design (see sustainable urban drainage systems; Section 18.7). In many old established settlements, storm water runs into the domestic waste water sewers, but in some countries, e.g. Australia, the cities have separate storm water and sewerage systems. When an area is newly developed, it is sometimes expedient to modify the natural stream channels; re-alignment of the water courses, lining and regrading of the channels are improvements made to facilitate drainage.

The long-term partition between the stream flow draining an urban catchment and other components of the water balance may not necessarily be very different to that of a less developed catchment. For example, in catchments with a scale of the order of 100 km2 in the area around Atlanta in the USA, Rose and Peters (2001) studied stream flow and precipitation data between 1958 and 1996, and found that annual runoff coefficients did not differ significantly when comparing catchments with different percentage urban areas. However, the same study showed that in the more urban catchments peak discharges were 30-100 per cent greater for the 25 largest storm events with 25-35 per cent less baseflow and higher flows during the recession limb of the storm hydrographs. Brun and Band (2000) found a similar pattern of decreased baseflow but relatively constant overall runoff coefficients in an 18-year study of suburban growth in a 170 km2 watershed in Maryland, USA.

Similar patterns are seen in the response of The Cut at Binfield, a catchment in the Bracknell area west of London (Fig. 18.2). This catchment has rural headwaters and around 30 per cent urban area in total. Flows have been monitored in the channel upstream and downstream of outfalls from the urban drainage system. Whilst the

Fig. 18.2 Storm hydrographs for The Cut at Binfield showing rapid urban runoff response. Reprinted from The hydrology of the UK: a study of change, Chapter 2, Robinson, M., Boardman, J., Evans, R., Heppell, K., Packman, J. and Leeks, G., Land Use Change, Pages 30-54, Copyright (2000) with permission from Routledge.

Fig. 18.2 Storm hydrographs for The Cut at Binfield showing rapid urban runoff response. Reprinted from The hydrology of the UK: a study of change, Chapter 2, Robinson, M., Boardman, J., Evans, R., Heppell, K., Packman, J. and Leeks, G., Land Use Change, Pages 30-54, Copyright (2000) with permission from Routledge.

total runoff scales approximately with drained area, the influence of the urban system is clearly seen in the hydrographs with much higher peak flow rates and more rapid responses.

The interaction of the artificial nature of urban catchments and the need to accommodate the changed hydrological characteristics is complex. The solving of one drainage problem may easily exacerbate another feature of the catchment runoff, e.g. rain events on the planned surface drainage of a new housing estate could produce higher peaks downstream than formerly, and these might cause flooding at previously safe points along the channel.

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