Introduction

As cities grow and develop, the urban landscape undergoes a high degree of change.  Most often the natural water courses and waterways maintained in the past suffer encroachment both planned and unplanned – legally and illegally.  As a result the flow cross section is rapidly reduced resulting in frequent flooding in wet weather.  Secondly, as cities develop and expand, the once fringe areas with lots of open land is converted to concreted surfaces or built up area.  As a result of this the level of infiltration of rain water is greatly reduced and the volume of run-off greatly increased.  The narrowing of natural water courses as well as increased run-off contributes to urban flooding.  In addition, in many developing country cities such as India, flooding in metropolitan cities has become a common problem largely due to reduction of passage for natural drains due to illegal or poorly planned developmental efforts (Gupta and Nair, 2011; Gupta and Nair, 2010; Ramachandra and Mujumdar, 2006; ENVIS Report, 2005).  The causes of such urban flooding are therefore significantly different from other types of flooding like river flooding or coastal flooding that occur as one time events.  The consequences of such urban flooding can be classified into three categories: a) direct consequences (material damages), b) indirect consequences (traffic detours) and c) social and economic consequences (Konig et al., 2002).  

In many of urban areas in India after a short length of flow of sewage underground (200mm to 1200mm dia pipes), the sewage pipes are no longer able to take the large flow.  The combined sewage from these large pipes generally flows in open sewers – sewers that were originally storm water courses.  These now carry sewage continuously and occasionally storm water from intense weather events.  When these are choked or encroached to a point of being too narrow, the water flow spills over its banks into nearby settlements.  This occurs generally following high intensity rainfall wherein the sewer /open storm drain can no longer take the large combined water flow.  Among the many reasons which cause urban flooding in such water courses / open sewers are a) encroachment of drain and reduction of flow area, b) improper maintenance of drainage /sewer system, c) blockages of drainage channels by USW dumped along its flow path upstream of these sensitive points.  The most common reasons found for such blockages are a) accumulation of silt or dumped debris or b) disposal of solid waste in the channel.

Poor and unplanned solid waste management and insensitive attitude among the upstream residents results in USW being dumped into the open sewers (Figure 4).  Such dumped USW cannot be transported through these narrow streams and results in sewer blockages (Kolsky and Butler, 2000).  Solid wastes which enter the drain comprise mainly of fermentables, cloth, plastic and paper.  Fermentable organic wastes of the dumped USW (e.g. food and garden wastes) as well as paper are rapidly degraded under such wet conditions leaving behind a predominantly non biodegradable complex of wastes that occasionally flows in the sewers or remains stuck at vulnerable points along the water course.  These non biodegradable wastes flow along with sewage in these drainage channels and gradually accumulate at shallow regions in the path where the flow rates are very low or the wastes encounter physical obstruction due to a shallow nature – especially when the only sewage is flowing in these streams.  The extent of such mass accumulating at specific points along the flow gradually increases with increasing quantities of USW being discharged into the streams.  Following high intensity rains that lead to a large runoff, the water level in these drains rise and releasing all the obstructed USW components especially accumulated plastics and cloth that do not undergo rapid degradation.  The sudden influx of water as runoff carrying with it a large volume of non-degradable materiaIs (cloths and plastics) results in the narrow sections of the sewers /flow channel becoming choked with these wastes and concomitant rise is water levels to create a local flood.  As such flows are ephemeral the occurrence of flooding is also short lived.  Therefore it is not easy to delineate areas where floods had occurred after the event.  As most of such events occur very late in the evening – cloudbursts, the floods are also generally later in the evening or at night and are difficult to record.  Only traces of the flood events are recordable from visible traces of waste accretion.  Water mark on house walls, compound walls, fences or electricity poles remain the sole indicators of flooding and the water level reached.  Pieces of wastes hanging on fences or adhering to drainage walls, bridge railings or water and drainage pipes crossing the water stream or found hanging on nearby vegetation indicate that clogging of the flow by solid wastes could be one of the reasons for the flood.  Typically it is found that the flow path under the bridges built over these drains also carry water supply pipes, communication network pipes, electricity cables, etc. all of which also significantly obstruct the flow when the water levels are high.  These also act as filters for a large quantity of cloth and plastics that can potentially block the flow when water levels are high and result in flooding.  In this paper we examine the instances of such cases and record the vulnerability of the drainage system to such events of choking and flooding.

Bangalore city generates around 3600 tons of urban waste daily (Chanakya et al., 2009). There are three waste treatment and disposal facilities; Mandur, Mavallipura and KCDC (Karnataka Compost Development Corporation, now not working).  Mavallipura and Mandur have capacity to treat 600 tpd and 1000 tpd of waste (BBMP report, 2010).  City has both type of collection facilities; door-to-door as well as community bin collection.  The door to door collection system is either incomplete or not easily accessible to many in slums depending upon the layout and placement of common bins.  Generally a large number of slums have come up in these low lying areas surrounding these storm water drains.  In all such cases solid wastes are dumped near or into the water channels – and sometimes on land but very close to  the eater channel.  Sometimes the collection facility is not regular. In all such cases solid wastes are dumped into the drainage network – even if there is a high fence to prevent such events.  USW dumped in various parts of the storm channel /open sewers is gradually carried to locations of shallow and slow flow where they tend to accumulate till a storm event carries this large mass away.  Such large floating mass easily chokes the narrow path below road bridges and results in flooding.  In this paper we examine patterns of such choking in order to determine if there exist alternatives that could avoid such choking and flooding.  However, controlling such flooding may not be very straightforward as USW deposition occurs all along the course while these could cause flooding only in specific locations.  Marking of such vulnerable points, anticipation of peak rainfall events and preventive activities could greatly arrest or avoid flooding in areas where solid wastes is the major cause.  This article focuses on flood prone locations of the city and develops indicators to identify the flooding region due to poor solid waste management.