A Study on Flood Hazard Mitigation of Guwahati City

Transcription

1 A Study on Flood Hazard Mitigation of Guwahati City P. J. Barman, B. Sarma and A. K. Sarma 1 & 3 2 Department of Civil Engineering, Centre for Environment, Indian Intitute of Technology Guwahati, Guwahati , Aam, India. pjb@iitg.ernet.in,.banari@iitg.ernet.in, ak@iitg.ernet.in (Received on 11 September 2011 and accepted on 16 March 2012) Abtract - The problem of urban flood of Guwahati, a city located in the northeat part of India, i one of the burning problem at preent. The topography of the city i uch that rainwater cannot drain out eaily from the heart of the city. With rapid growth of reidential area in the urrounding hill, ediment yield from the immediate upper catchment i increaing and depoition of the ame in drain i adding another dimenion to thi problem. Therefore innovative planning and efficient deign of the drainage network i neceary. An effort ha been made to tudy finer detail of the iltation proce by invetigating a mall part of the drainage ytem. Theoretical analyi ha revealed that for the exiting cro ection and bed lope, flow velocity in the drain hould have exceeded the required couring velocity to make the channel a elf cleaning one. Thu, edimentation occur due to other factor. Poible factor reponible for ediment depoition have been analyzed and ome remedial meaure have been uggeted. Scope of retricting ediment and water yield from the hilly waterhed by implementing ecological management practice (EMP) ha alo been invetigated and a concept of adopting optimal EMP ha been uggeted. Keyword: Urban Flood, Sediment Control, Ecological Management Practice 1. INTRODUCTION Improper deign of the ewer ytem lead to urban flooding in many countrie. Proper deign to prevent thi flooding ha emerged a a challenge for the engineering community. Variou tudie were carried out all over the world for modeling and prevention of thi phenomenon. Geere carried out a tudy to ae the performance of urban drainage during dry period and during torm event by developing performance [1] index. Niemezynowicz dicued preent and future trend in urban hydrology [2]. Mannima preented the reult of a tudy in which the uncertainty level aociated with a detailed and implified ewer ediment modeling approach have been compared [3]. Cro dicued the vulnerability of megacitie toward hazard and howed that in many repect reident of mall citie and rural communitie are more vulnerable to diater [4]. Mark preented a one dimenional model to imulate urban flooding [5]. Beven preented event baed uncertainty aement in urban drainage modeling. They applied the Generalized Likelihood Uncertainty Etimation (GLUE) methodology [6]. Burton reviewed catchment and receiving water modeling in relation to tormwater. They claified the catchment model primarily according to the complexity ranging from imple method (baed on export coefficient or event-mean concentration multiplied by runoff volume) to complex model that are typically patially ditributed and proceed baed [7]. Thoma preented a detailed dual drainage imulation model baed upon hydraulic flow routing procedure for urface flow and pipe flow. Special conideration i given to the interaction between urface and ewer flow in order to compute accurately water level above ground a a bai for further aement of poible damage cot [8]. Swan explore the relationhip between increaed urbanization and ubequent capacity problem within drainage infratructure [9]. Moder developed a code in matlab for the deign of urban drain [10]. Delectic carried out an extenive laboratory tudy on artificial gra in order to examine the procee of ediment tranport in runoff over gra. The tudy wa focued on gra utilization in control of urban runoff, and therefore low to moderate load of fine ilt were ued a inflow into the gra [11]. The focu of thi paper i to preent 1) a detailed experimental invetigation on particle ize characteritic in variou part of a elected drain to make further analyi for couring velocity, 2) an approach for determining optimal ecological management practice for controlling ediment and water yield from an urbanized hilly waterhed. The drainage ytem of Guwahati city i not atifactory a it cannot drain out floodwater during the monoon period. Topography of the city i uch that it i difficult to have required gradient in many part of the drainage ytem. The outlet of the drainage ytem i connected to river Bharalu which i again connected to river Brahmaputra. Another problem of the drainage ytem i that at the time of peak flow the water level of river Bharalu rie to uch a level that the water of the drainage ytem form a backwater profile and the flow velocity drop almot to zero. Thi contribute toward edimentation and aggravate the flood problem in the city. A luice gate wa contructed at the confluence point of the river Bharalu and Brahmaputra to prevent the backflow flow from river Brahmaputra to Bharalu and then to the city drain. It wa propoed to pump out the tagnant water during the flood time. However inufficient pumping ytem could not olve thi problem in the city. A high ediment generation from the hill and their ubequent depoition in the drain located in the plain are the primary caue of urban flooding, a mall part of city drain, which wa renovated in 1994 to have a ufficiently large ection to carry the monoon flow, wa elected to tudy variou iue of edimentation and flow tagnation. On the other hand, to retrict ediment yield from the hill at minimum poible cot, an optimization model ha been developed to decide optimal EMP combination that can retrict the ediment and water yield within permiible limit. 9

2 P.J. Barman, B. Sarma and A.K. Sarma II. THEORY OF SEDIMENTATION The ettling velocity of ediment particle i governed by the ize and the pecific gravity of the ediment and alo the vicoity of the ewage. In thi tudy ettling velocity wa computed by uing the approximate equation a given below. Stoke' law: 2 V =g/18(s -1) d /µ (1) Where, V = Settling velocity in m/ec. g = Acceleration due to gravity. S =Specific gravity of the grit particle. d=size of the particle in metre. 2 µ=kinematic vicoity m /ec Stoke' law hold good for Reynold' number below 1. III. THEORY OF EROSION AND SCOURING The oil particle which are detached from the current location and moved with the flow of water are referred to a ediment. When the hear tre between the water and the ediment exceed the frictional reitance, the ediment tart to move in the direction of flow. When the hear tre i almot equal to the frictional reitance, the particle jut tart to move and thi condition i called incipient motion condition. But the original bed of the channel under tudy i made of concrete and the bed level rie due to accumulation of ediment carried by the flowing water. So, the channel bed, having ediment depoited in it, behave like a mobile boundary channel. Water flowing over a channel bed ha a gradient in the vertical direction. Due to thi velocity gradient, water exert a hear tre on the particle lying on the tream bed. Thi hear tre i known a boundary hear tre. The channel carrie a large amount of upended matter which remain floating or upended due to the flow of the ewage. Therefore while deigning the channel the velocity of flow hould be uch that no olid get depoited and that minimum velocity i called elf cleaning velocity. The formula for elf cleaning velocity i given a, V= 8K/f(S-1)gd (2) Where V=Velocity of flow K=characteritic of olid uually which i taken a.04 to.06 f =Darcy' coefficient of friction which i uually taken a 0.03 S=Specific gravity of olid which i uually taken a 2.65 d =Effective grain ize IV. FIELD AND LABORATORY INVESTIGATION To tudy the problem of artificial flood a part of the whole drainage ytem wa elected and ilt ample were collected at regular interval from the channel bed. A ampler wa fabricated for ample collection. Collected ample were brought to the laboratory and a part of the whole ample wa kept for determining the decompoable matter in it. The remaining part of the ample wa dried for a week in unlight. The ample wa then oven dried for 24 hour at a temperature 0 of 105 C. The oven dried ample wa then ieved in the ieve haker of ieve ize IS4.75mm, 2.00mm,1.18mm, 1.00mm, 0.6mm, 0.3mm 0.15mm and 0.075mm.The experimental data obtained from thi analyi wa plotted to obtain the uniformity coefficient, coefficient of curvature and effective grain ize of each ample. The debri contained in the each ample were alo eparated during ieve analyi and weighed eparately. The percentage of debri in each ample wa then calculated. A. Fabrication of Sediment Sampler The ampler conit of a round hollow cylinder of diameter 4.5cm. One end of the cylinder having a cutting edge i inerted in to the bed for ample collection. The ampler can be opened and cloed at the bottom by operating a valve from the top. The valve i baically a thin circular plate, hinged at one circumferential point of the cylinder and can be operated by a mall rod connected to the plate at angle 120 degree with the plane of the plate. B. Experimental Finding Sample were collected from following ten point along the drainage line tarting from uptream to downtream and reult are tabulated in Table I. Cae1: When The Channel i in Nearly Full Flow Condition (Figure 2) Cro ection of the channel =1/2( )( ) = q.m Perimeter of the channel=4.3+(2 x 3.22)=10.74m So hydraulic mean depth R=8.225/10.74=0.76m Here bed lope= Uing Manning' formula, velocity =3.982m/ec Scouring velocity = m/ec 10

3 A Study on Flood Hazard Mitigation of Guwahati City Obervation Location TABLE I FIELD DATA OF VARIOUS POINTS Uniformity Coefficient Coefficient of Curvature Decompoable Organic matter (%) Zoo Road Tiniali R.G.Barua road In front of AIDC petrol pump In front of Uday Path In front of AIDC bu toppage In front of Rupali Path In front of Akahi Path At the point where the Bharalu and the drain meet In front of La Mare intitute of IT In front of office of the director of border area Fig. 1 Plot of particle ize V percent finer of ilt particle Different Condition of Flow Fig. 2 Channel nearly full Fig. 3 Channel flowing half full 11

4 P.J. Barman, B. Sarma and A.K. Sarma The analyi of ediment characteritic ha hown that becaue of poor management of olid wate, the ediment getting mixed with garbage behave in a much different way o far it property toward couring under water current i concerned. Exitence of narrow bottle neck in many place caue afflux at uptream which reduce the flow velocity and promote ediment depoition. With the increae in the depoition of ilt, tarting from the confluence point of the drain, and the river Bharalu, the bed lope of the channel alo reduce progreively and ilt get depoited quickly. Fig. 4 Channel during lean period Uniform flow velocity and couring velocity i calculated in three different ection a hown in the figure2, figure3 and figure4 Cae 2: When The Water Level i Half of the Full Flow Condition (Figure 3) In thi cae depth of flow=1.75/2=0.875m So cro ectional area=( )(0.875)(0.5)=3.94 q m Perimeter of the ection=4.3+ (0.8056) x 2 =5.91m So hydraulic mean depth R=3.94/5.91=0.67m So uing Manning' formula velocity=3.64m/ec which i much greater than the couring velocity Cae 3: Flow During Lean Period (Figure 4) Flow depth (1-0.65) = 0.35m So cro ectional area = ( )(1/2)(0.35) = q.m Perimeter of the channel = 4.3+ (0.129) x 2 = 4.56m So hydraulic mean depth R = 1.533/4.56 = 0.336m So velocity = 2.299m/ec From the above calculation it ha been oberved that couring of the particle in the channel bed hould have been taken place. Thu, the baic caue of edimentation i not jut the inadequate ize but ome other factor are reponible for iltation in the drain. V. POSSIBLE CAUSES OF SILTATION Due to rie in the water level in the river Bharalu during the rainy eaon, the water level in the channel alo increae. But the water level in the channel remain low with repect to the river Bharalu due to which a back water profile i formed at river Bharalu.Thi reult in decreae in the flow velocity toward the outlet of the channel. Thu couring action decreae at that point and ilt get depoited lowly. VI. REMEDIAL MEASURES After analyzing the entire problem we have found that effort hould be diverted more toward controlling ediment yield from the upper catchment. For controlling ediment yield and water yield from the hilly urban area it i neceary to apply ecologically utainable and economically viable management practice that can retrict ediment and water yield within a permiible limit. Such practice can be named a Ecological management Practice (EMP). A. Ecological Management Practice: So far, there i no well etablihed EMP recognized a uitable for urban reidential development. However, variou traditional land management practice are available for controlling the ediment yield and runoff volume for agricultural area. Some of uch practice are contour terracing, mulching, and gra land development, creation of buffer zone with gra and tree, agro foretry, vegetated waterway and gully control meaure. Thee practice, if uitably modified conidering the neceity of an urban area, can alo be ued for land management of urban reidential area. Some of poible EMP for urban reidential area are uggeted below: 1) Gra Land: Gra reduce the velocity of urface runoff, minimize the impact of rain on oil and it root ytem help in increaing infiltration. In urban area, gra land can alo erve a open land that i needed according to the municipal rule. 2) Foret Land: Tree canopy reduce the direct impact of rain on oil. Beide, foret land, covered with falling leave, alo reduce the urface runoff velocity and increae infiltration. Falling leave and decaying brache act a mulche and thu tree cover can control ediment yield and runoff volume. 12

5 A Study on Flood Hazard Mitigation of Guwahati City 3) Covering Rain Impacted Area With Pebble, Vegetation or Wood Chip: Eroive power of rain drop depend on ize of rain drop and it falling height. Size of rain drop that fall from inclined roof of a houe become quite large and thu trike the ground with very high eroive power. Thu, the portion of ground lying below the line of the roof edge i prone to more eroion a the accumulated rain over the roof fall with a high velocity. Such drop line area of water around the houe can be covered with pebble or wood chip or eroion reiting vegetation can be allowed to grow, which protect the oil from the direct impact of rain drop and alo allow more infiltration. 4) Detention Drain And Retention Pond: To capture the exce urface runoff, detention drain can be contructed acro the lope and retention pond can be contructed in a uitable location. Thi can minimize downtream eroion and flooding. 5) Vegetated Waterway: If the path (or channel) of accumulated urface runoff are covered with vegetation, then the vegetation provide an obtruction to the flowing water. It reduce velocity and hence the eroive power of the flowing water. Thi reduce eroion of channel bed and bank and prevent gully formation. Root ytem of a vegetated waterway not only increae bondage of oil and make it reitant to water eroion, but alo promote infiltration. Depending on the tatu of degradation of the waterway, different type of vegetation can be uggeted. 6) Rainwater Harveting Sytem: Rainwater harveting i a technique of collection and torage of rain water in urface (torage tank) or ub urface aquifer before it i lot a urface runoff. Rainwater harveting ytem help in reducing the peak runoff and alo recharge ground water. The collected rainwater through the rainwater harveting ytem can alo be ued during the period of water hortage. VII. CONCEPT OF OPTIMAL EMPS EMP differ in their implementation and maintenance cot form ite to ite and time to time. Scope of implementing an EMP in a particular ite will have to be aeed by taking into conideration variou ite condition uch a lope, oil characteritic, land ownerhip, land availability, preent land cover and logitic of future maintenance. Beide, pollutant removal efficiencie of EMP are alo ite-pecific. Baed on thi, planning and deign of EMP hould be done in uch a way that they are efficient to capture and treat the pollutant loaded water and are ecologically and economically feaible. A Eliminating ediment and debri from drainage ytem. Even after adopting EMP, ome amount of ediment and debri will reach the drain. Similarly the water yield may alo increae beyond channel capacity if rainfall exceed the deign torm. For uch ituation following meaure can be adopted. 1) Debri Control: Flow of debri in to the channel can be controlled by placing a uitable creen at inlet of the channel. Thi will prevent the flow of debri and large ized object into the channel which may obtruct the channel and can promote depoition of ilt. The creen hould be cleaned at regular interval of time to increae it efficiency. 2) Proviion of R.C.C. Cover Slab on the Top of the Channel: The obtruction caued by throwing large ized object like cartoon, bamboo piece etc may be topped by covering the whole channel with R.C.C. lab throughout it length. 3) Silting Bain at the Inlet Point: At the confluence of the primary and econdary drain a ilting bain can be provided. The velocity of flow at the bain will get reduced and depoition of ilt will occur at the bain. A a reult, water containing leer amount of ilt will enter the channel main channel. 4) Silting Bain Throughout the Length of the Channel at Regular Interval: Suitable number of ilting bain can be provided throughout the length of the channel to collect the ilt carried by the flow. The bain hould be cleaned after regular time interval. 5) By Pumping of Sewage During Peak Flow: If backflow occur at the junction point of river Bharalu and the drain, pumping of ewage can be done by contructing a luice gate. The pump hould be uch that it can pump out both water and ilt very efficiently. 6) Proper Maintenance: To prevent further depoition of ilt the bed of the channel hould be cleaned at regular interval of time becaue depoition of ilt in the channel bed reduce the cro ection a well a longitudinal ection of the bed. 13

6 P.J. Barman, B. Sarma and A.K. Sarma VIII. CONCLUSION A tudy conducted on drainage problem of Guwahati, a city located in the northeatern part of India ha revealed that ediment yield from the urrounding hilly area i a major factor that lead to flooding of the city area located in the plain. Drain deigned on the bai of couring velocity capable of removing ediment depoited in the bed, though apparently look like efficient, cannot perform in real ituation due to variou other factor. While EMP can be implemented in the hilly urban area for controlling ediment and water yield from the upper catchment, due emphai will alo have to be given toward controlling ediment and debri from entering into the drainage ytem by variou traditional method. Pumping in uitable location i alo required from increaing efficiency of the drainage network. REFERENCES [1] U.M.J. Geere and H.A. Lobbrecht, Aeing the performance of urban drainage ytem: 'general approach' applied to the city of Rotterdam, Urban Water, Vol. 4, pp ,2002. [2] J. Niemczynowicz, Urban hydrology and water management: preent and future challenge, Urban Water, Vol. 1, pp.1-14,1999. [3] G. Mannina, A.N.A.Schellart, S. Tait and G. Viviani, Uncertainty in ewer ediment depoit modeling: Detailed v implified modeling approache, Phyic and Chemitry of the Earth, Vol. 30,2011. [4] A.J.Cro, Megacitie and mall town: different perpective on hazard Vulnerability, Environmental Hazard, Vol. 3, pp , [5] O. Mark, S. Weeakula, C. Apirumanekula and B.S.Aroonneta, Potential and limitation of 1D modeling of urban flooding, Journal of Hydrology, pp , [6] S. Thorndahl J.K. Beven, B.J. Jenen and S.K.Jenen, Event baed uncertainty aement in urban drainage modeling, applying the GLUE methodology, Journal of Hydrology, pp , [7] G.A. Burton and R.E. Pitt, Planning and urban deign tandard, 2006 [8] T.G. Schmitt, M. Thoma and N. Ettrich, Analyi and modeling of flooding in urban drainage ytem, Journal of Hydrology, pp , [9] A. Swan, How increaed urbanization ha induced flooding problem in the UK: A leon for African citie?, Phyic and Chemitry of the Earth, Vol. 35, pp , [10] S.Achleitner, M. Moderl, and W. Rauch, CITY DRAIN :An open ource approach for imulation of integrated urban drainage ytem, Environmental Modeling & Softwar, Vol. 22, pp , [11] A.Delectic. Sediment tranport in urban runoff over graed area, Journal of Hydrology, pp ,