Floods in an urbanised landscape refer to the partial or complete inundation from the rapid accumulation or run-off resulting in the damage to property and loss of biotic elements (including humans). Urban flooding is a consequence of increased impermeable catchments resulting in higher catchment yield in a shorter duration and flood peaks sometimes reach up to three times.  Thus, flooding occurs quickly due to faster flow times (in a matter of minutes). Causal factors include combinations of loss of pervious area in urbanising landscapes, inadequate drainage systems, blockade due to indiscriminate disposal of solid waste and building debris, encroachment of storm water drains, loss of inter connectivity among lakes, housing in floodplains and natural drainage and loss of natural flood-storages sites. Flood mitigation in urban landscape entails integrated ecological approaches combining the watershed land-use planning with the regional development planning. This includes engineering measures and flood preparedness with the understanding of ecological and hydrological functions of the landscape.

Bangalore is experiencing unprecedented urbanisation and sprawl in recent times due to concentrated developmental activities with impetus on industrialisation for the economic development of the region. This concentrated growth has resulted in the increase in population and consequent pressure on infrastructure, natural resources and ultimately giving rise to a plethora of serious challenges such as climate change, enhanced green-house gases emissions, lack of appropriate infrastructure, traffic congestion, and lack of basic amenities (electricity, water, and sanitation) in many localities, etc. This study shows that there has been a growth of 1028% in urban areas of Greater Bangalore across 45 years (1973 to 2017). Urban heat island phenomenon is evident from large number of localities with higher local temperatures. The study unravels the pattern of growth in Greater Bangalore and its implication on local climate (an increase of ~2 to 2.5 ºC during the last decade) and also on the natural resources (88% decline in vegetation cover and 79% decline in water bodies), necessitating appropriate strategies for the sustainable management.

CAUSAL FACTORS OF RECURRING FLOODS

Violation that persist in Bellandur – Agara wetland (Figure 1) are:

1. Abuse of wetlands (with dumps, building materials, etc.) -Non-removal of building debris (used for filling the drains – highlighted in Figure 2)
2. Narrowing Rajakaluves;
3. Concretised storm water drains;
4. Construction of compound wall altering the hydrologic regime – leading to escalation of flooding impact;
5. Buildings in the buffer zone of Bellandur lake (and valley zone of Agara-Bellandur);
6. Existence of labour sheds and pollution due to open defecation (lack of appropriate sanitation for labourers);
7. Dumping of solid waste – plastic bottles, polythene covers, etc.;
8. Unauthorised occupation of wetlands in the valley zone;
9. Not clearing encroached lake area (dumped debris continue to exist);
10. Inefficient custodians of wetlands (BDA and revenue department);
11. Unplanned activities – proposing SEZ in the city (that would enhance traffic density, leading to further traffic chaos; decline in groundwater table; pollution of wetlands) by KIADB, which only highlight the inefficient and incompetent planning system in the city.
12. Lack of accountability – decision makers and hence inefficient and incompetent management of natural resources.

Belladur Agara wetland is the pristine ecosystem supporting dependent biota and was aiding as a zone for the inundation of floodwater, percolation of precipitation, sustained recharge of ground water resources, while reducing overland flow. The wetlands are part of flood plains that follows the terrain. Consequences of continued abuse of wetlands are (i) increased instances of flooding, (ii) enhanced velocities of the discharge (iii) reduced recharge of ground water, (iv) loss of life and damages to the property.
Bellandur-Agara wetland’s integrity is altered due to  (i) filling of low lying area with construction debris, (ii) removal of interconnecting drains (visible in remote sensing data of 2000, Figure 2) , (iii) narrowing and concretizing drains to aid the encroachment. Figure 1 indicates the Revenue map of Agara village indicating Wetland under study. Cyan line in the revenue map indicates channels connecting Agara and Bellandur lakes along the wetland

Figure 1: Agara Bellandur Wetland

Source: www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Mannings_n_Tables.htm

DESIGN – ILLUSRATION OF NATURAL DRAINS SUPREMACY
Highest rainfall in a day was observed to be 109 mm in the year 2013 (as recorded between 31st May 2013   8:30 AM and 1st June 2013 8:30 AM), w.r.t which an yield of 1022 kilo.cum of water is generated. Considering that the rainfall has occurred within 6 hours, Discharge would be about 47.31 cum/s. Open channels along the valley zones should be able to carry the discharge of 47.31 cum/s, without clogging or surpassing the flood plain.
In order to quantify the discharge along the channel, various scenarios were observed, i.e., current width of which is about 7 to 8 meters in width, and width as per cadastral maps along with various bed characteristics such as concrete bed, soil bed and bed with vegetation., details are as presented in table 1.2.
The analysis of Channels shows that in the current scenario, where the channel width is about 7 – 8 meters in width, has a capacity to carry discharge of 51.47 cum/s at minimum roughness coefficient of 0.40 for channels with presence of weeds/reeds and stones along the beds indicating that the channels would easily cater to the exiting discharge of 47.31 cum/s.

Velocity estimates indicate that presence of vegetation along with other bed materials (Soil, stones) would help in reducing velocity of flow. In the existing scenario, flow velocity was estimated to be 1.87 to 2.57 m/s, compared to soil bed or concrete bed which has lower friction where the velocity ranges were between 3.43 to 6.86 m/s.
If the drainages were reclaimed as per the cadastral maps, the width of the channel would be around 15 to 18 meters, considering maximum flow depth as 2 meters, channel with natural vegetation (reeds and weeds) would have low flow velocities between 1.29 to 2.07 m/s which is much lower as against the current scenario, and with discharges upto 61.96cum/s, indicating that the valley in its natural condition had a higher sewage carrying potential even with lower depth of flow.
Studies showing relation between Bed material and Surface flows: Studies carried by Nepf in 2012, Miyab et al 2014, Miyab et al 2015, Morri et al 2016, Jarvella, 2002, Green 2005, highlights the role of vegetation on the river channels:

• Vegetation forms an integral part of the ecosystem providing goods and services (biological, geomorphological, landscape ecology, chemistry), ecological trapping.
• Vegetation uptakes nutrients (nitrogen and phosphorous), increase oxygen levels and helps in betterment of water quality.  
• Vegetation is known to increase bank stability, reduce erosion and turbidity, provide habitat for aquatic and terrestrial wildlife, attenuate floods, present aesthetic properties, and filter pollutants.
• Vegetation increases resistance to flow, influence water depth, alters mean velocity.
• Presence of vegetation alters flow velocities across several scales, ranging from branches to blades of a single plant to cluster of plants.
• Since time immemorial (before the birth of current breed of senseless consultants and civil engineers with fragmented knowledge of the subject) aquatic vegetation was considered for flow retardation, local treatment of water, groundwater recharge – without any contamination).
• Surface flow velocities was maximum on sections with bare banks, whereas maximum
  flow velocity occurs under the water surface in both vegetative and open channels.
• Vegetation cover changes uniform flow to non-uniform flow, showing a nonlinear distribution of Reynolds stress
• Aquatic plants are flexible, they can be pushed over by currents, resulting in change in morphology called reconfiguration
• Reconfiguration also reduces flow resistance
• Reduces the frontal area of the vegetation
• Reconfigured shape tends to be more streamlined
• Drag on a plant increases more slowly with velocity.
• Vegetation reduced the variation of drag
• Vegetation growing in waterways and rivers increase turbulence
• Within a cluster/Canopy, flow is forced to move around each branch or blade, so that the velocity field is spatially heterogeneous.
• Flow resistance due to vegetation is determined primarily by the blockage factor, A strong correlation exists between blockage factor and Mannings roughness coefficient.
• Vegetation is flexible, an increase in velocity is associated with a decrease in vegetation height
• Vegetation drag increases with increasing depth ratio if plants area emergent, and if plants are submerged hydraulic resistance decreases.
• Baffling of flow reduces bed stresses and vegetation creates region of sediment retention, enhances retention of organic matter, nutrients and heavy metals.
• Because of the positive impacts vegetation provides for water quality, habitat and channel stability, researchers now advocate replanting and maintenance of vegetation in rivers.
• Presence of vegetation in the beds mitigate bed souring, reducing bed impact.
• In many vegetated flows, the near bed turbulent stress is zero, or close to zero.
• In the recent years, vegetation has become a major component of erosion control and stream restoration

Solutions:

• Vegetation in the drain takes the load during peak monsoon, there is no need to concretise the channel (and waste public money)
• Vegetation allows groundwater recharge while treating the water (bioremediation);
• Drains with vegetation without any bottlenecks (hindrances) would be the best option to mitigate floods.
• Narrowing channel and concretizing would only increase the quantum of water and velocity, which would be disastrous.
• Objective should be towards mitigation of floods and not to generate high overland flows (with increased quantum and flow velocity)
• Experts should think sensibly with holistic knowledge (considering all subject knowledge) than fragmented narrow sectorial knowledge. Advice by pseudo experts would be detrimental as the society would be deprived of ground water, frequent floods and unnecessary livelihood threats.

Reference
Nepf, Heidi M., 2012, Hydrodynamics of Vegetated Channels. Journal of Hydraulic Research 50(3) pp: 262-279.
Miyab, N. M., Afzalimehr, H., Singh, V. P., 2015, Experimental Investigation of Influence of Vegetation on Flow Turbulence, International Journal of Hydraulic Engineering, 4(3), pp: 54 – 69, DOI: 10.5923/j.ijhe.20150403.02
Miyab, N. M., Afzalimehr, H., Singh, V. P., Ghorbani, B., 2014, On Flow Resistance Due to Vegetation in a Gravel-Bed River, International Journal of Hydraulic Engineering, 3(3), pp: 54 – 69, DOI: 10.5923/j.ijhe.20140303.02
Morrie, M., Soulamia, A., Belleudy, P., 2016, Mean Velocity Predictions in Vegetated Flows, Journal of Applied Fluid Mechanics, 9(3), pp: 1273-1283
Marjoribanks, T. I., Hardy, R. J., Lane, S. N., Tancock, M. J., 2017, Patch-scale representation of vegetation within hydraulic models, Earth Surface Processes and Landforms, 42, pp: 699 – 710, DOI: 10.1002/esp.4015
Jarvela, J., 2002., Flow resistance of flexible and stiff vegetation: a flume, study with natural plants. Journal of Hydrology, 269: 44–54.
Green, J. C. 2005. Modelling flow resistance in vegetated streams: review and development of new theory. Hydrological Processes 19: 1245–1259

CAUSAL FACTOR 5: LOSS OF INTERCONNECTIVITY AMONG LAKES

The undulating terrain (varying from about 700 m to about 962 m AMSL) in the region aided in the formation of interconnected lakes in the region (Figure 2.2).  Bangalore City was once aptly known as ‘city of lakes’ due to the presence of large number of lake (about 285 lakes in an area of 161 sq km, Spatial extent of Bangalore in 1980’s). These lakes were all interconnected with canals / drains (kaluveys’s) to enable transferring excess water to the next lake. These lakes catered the basic needs such as maintaining and recharging ground water, drinking water to the surrounding people, habitat for fishes and other aquatic ecosystems, sustaining food (fish, etc.) and agricultural activities, etc.
The drainage network in Bangalore carries water to the River Cauvery through its tributaries Arkavathi, Pinakini or Pennar and Shimsha. The central, northern and eastern portion is undulating with the upland tracts occupied by scrubs, while the low lands occupied by series of tanks formed by embanking the streams along the valley. These valleys consists of varying size water bodies from small ponds to large lakes. The southern portion of the land consists of hills that are close together and are surrounded by thick jungles.
Bangalore being located on the ridge, forms three watersheds as precipitation flows as runoff in three directions along  the valleys (Figure  2.2) - Koramangala Challaghatta Valley (K&C Valley), Hebbal Valley (H Valley) and the Vrishabhavati Valley (V Valley). Under the administrative boundary of Bruhat Bengaluru, K&C valley is the largest encompassing an area of 255 square kilometers, followed by Hebbal valley with an area of 207 square kilometers and Vrishabhavati valley with an area of 165 square kilometers. Both K&C valley and Hebbal valley joins at Nagondanahalli village (BBMP Ward 94 – Hagadur) which further flow to Dakshina Pinakini River, where as Vrishabhavati valley joins Arkavathi river which is a tributary of river Cauvery.
During 1800, there were 1452 water bodies with the storage capacity of 35 TMC (in the current spatial extent of 741 sq.km.). The number of lakes in Bangalore has reduced from nearly 285 (spatial extent of Bangalore: 161 sq.km. in early seventies) to 194 (spatial extent of Bangalore: 741 sq.km. in 2006). Unplanned rapid urbanisation during late nineties, witnessed large-scale unrealistic, uncontrolled developmental activities in the neighborhood of lakes, which  led to

1. encroachment of lakes and storm water drains resulting in decline in ground water table, while increasing the instances of flooding;
2. dumping of solid waste (MSW), Construction debris, etc. in storm water drains, lake catchment and in lakes.;
3. sustained inflow of partially or untreated sewage, polluting existing surface and subsurface water resources;
4. reduced water holding capacity due to accumulation of silt; construction debris, etc.;
5. topography alterations in the lake catchment; and
6. sustained inflow of untreated industrial effluents.

During the last four decades there has been 79% reduction in water bodies and the number of lakes in Bangalore is given in Figure 12. Loss of interconnectivity is evident in Figure 13 due to encroachments.

Figure 12: Status of Lake in Bangalore (Red colour indicate lost lakes between 1970’s to 2016)	Figure 13: Loss of interconnectivity among lakes