Flood Modelling in Waidina Tributary, Fiji Islands

rd

3 International Symposium on Advances in Civil and Environmental Engineering Practices for Sustainable Development (ACEPS – 2015)

Flood Modelling in Waidina Tributary, Fiji Islands U.S. Rathnayake1 and S.M.A. Arachchi2 1

Department of Civil Engineering, Department of Mechanical Engineering, Faculty of Engineering, Sri Lanka Institute of Information Technology, Malabe, SRI LANKA 2

E-mail: [email protected] and [email protected]

Abstract: Rewa River is the widest river and the second longest river in Fiji Islands. It is in the islands of Viti Levu. The river is 145 km in length and starts from the highest peak of the island, Tomanivi. Rewa River experiences frequent floods. However, a detailed flood model for Rewa River is still to be tabled. This paper briefs the flood modeling of part of Waidina tributary of the Rewa River. US Army Corps Engineers HEC-RAS hydraulic model is being successfully applied to the Rewa River and initial results are drafted for the Waidina tributary. These results are promising; however, a completed flood model should be developed for sound conclusions. Inundation top widths due to a random flood were presented. These inundations can effectively be used to inform the residents in the vulnerable areas in a flood event. In addition, the final expected results can be used for the flood protection structural measures. Keywords: Flood inundation, inundation top widths, HEC-RAS hydraulic model, Rewa River, Waidina tributary 1. INTRODUCTION River discharges and river water heights are usually used to warn the general public about the floods. However, the general public would not worry about the discharges, but they highly concern about the potential inundation areas. Flood analyses are usually carried out to find out the inundation areas. Not only these analyses help the general public, but also the engineered results will be highly used in urban planning. Rapid urbanization has increased the direct runoff. This is due to the increased impervious pavements, not only in cities, but also in rural areas (Ku et al, 1992). In addition to the rapid urbanization, climate change has some adverse effects on the runoff (Arnell, 2004). For some countries, unusual rainfall patterns can be distinguished. These unusual rainfall patterns have increased the number of floods per annum. However, authorities including civil engineers have to identify the potential solutions to overcome the disastrous damage from the floods. These potential solutions can either be structural or non-structural. Construction of levees, revetments, flood control reservoirs are among the structural solutions. In addition, an efficient drainage network can be used to convey the additional flows. People’s awareness, proper land-use, improving the infiltration characteristics in upper catchments and avoiding marshy lands for development activities are among the non-structural measures for floods. However, it is always better to carry out a detailed flood analysis before the implementation of structural or non-structural measures. With advancements in computing and numerical analysis, hydraulics of flood modeling is not any more a difficult engineering problem to be solved. There are many commercial and non-commercial numerical solvers to compute the flood modeling. River roughness is one of the most important parameters to be determined in flood modeling (Timbadiya et al, 2011). False roughness can lead to inaccurate solutions and therefore, urban planning can be affected. Among many other commercial and non-commercial numerical solvers, Hydrological Engineering Centers River Analysis System (HEC RAS) by the United States Army Corps of Engineers is a widely used numerical solver for flood modeling. HEC RAS has successfully applied to many rivers around the world (Prata et al, 2011;

307

rd


Timbadiya et al, 2011; Yuan and Qaiser, 2011; Rathnayake et al, 2007; Goodell and Warren, 2006; Tate, 1999). HEC-RAS is a powerful hydraulic model and it is capable of analyzing one-dimensional steady, unsteady flow hydraulics, sediment transport computations and water temperature modeling. It has a graphical user interface so that the users can get the benefit of graphical modeling (Brunner, 2010). This paper briefs the flood modeling of part of Waidina tributary in Fiji Islands. Waidina tributary is a branch of the Rewa River in Fiji Islands and experienced severe floods in February 1965, October 1972, April 1980 and January 1993 (McGree et al., 2010). HEC-RAS model is being successfully applied to the Rewa River and initial results are drafted for the Waidina tributary.

2. HYDRAULIC MODELING OF RIVERS Flow continuity and the conservation of momentum are the two basic physical laws, which govern the flow in an open channel. River itself is an open channel; however, it usually does not have uniform (lined) banks and uniform bed profiles. However, these governing equations are still valid for the river flows.

Figure 1 Schematic view of the river flow. Figure 1 shows the schematic diagram of a river flow. Considering the control volume presented in Figure 1, the following equations (Equations 1 & 2) are derived. Equation 1 gives the unsteady continuity of the flow. ∂A ∂t

+

∂Q ∂x

− q1 = 0

(1)

where A, Q, q1, t and x are the total flow cross sectional area, volume flow rate, the lateral inflow per unit length, time and longitudinal direction coordinate, respectively. Equation 2 presents the unsteady momentum equation for the river flow. ∂Q ∂t

+

∂QV ∂x

 ∂z + S  = 0 f   ∂x 

+ gA 

(2)

where V, ∂z/∂x and Sf are the river flow velocity, water surface profile variation and the friction slope, respectively. Sf is expressed using Manning’s equation as shown in the following equation. Sf =

QQn

2

(3)

4

2.208 R

3

A

2

where n and R are the Manning’s friction coefficient and the hydraulic radius. However, the governing equations have to enhance when there is a flood. During a flood, the water flows not only along the channel direction, but also to the lateral direction.

308

rd


Figure 2 Schematic view of the flood plain. Figure 2 illustrates the two dimensional characteristics of riverbanks and a flood plain. Water moves laterally away from the channel in an event of a flood. The excessive flood flow inundates the flood plain and temporally stores the water. The following equations govern the total flow in a flood event. Equation 4 expresses the continuity of a flood flow. ∂A ∂t

+

∂f Q ∂xC

+

∂ [(1 − f ) Q ] ∂x f

= 0

(4)

where ɸ is the conveyance ratio and given in the following equation (Equation 5).

f=

KC

(5)

KC + K f

where KC and Kf are the conveyance in the channel and the flood plain. The conservation of momentum of a flood flow can be expressed in the following equation (Equation 6).

∂Q ∂t

(

f Q ∂ +

2

2

∂xC

AC

)

 (1 − f 2 ) Q 2

∂ +

 ∂x f

 Af   + gA  ∂z + S  + gA C  fC  f  ∂xC 

 ∂z + S ff   ∂x f

 0 = 

(6)

where AC, Af, SfC and Sff are the flow cross sectional area in channel and flood plain and friction slopes in channel and flood plain. These governing equations (Equations 4 & 6) can be approximated using implicit finite differences and solved numerically for flood flows using Newton-Raphson iteration method. Sections, j and j+1are in Figure 2, are two example sections used for finite difference scheme.

3. GEOMETRIC MODELING OF RIVERS Topography information is a must for the flood modeling in rivers. Hydraulic models incorporate Geographic Information System (GIS) maps are often used to model the river flows or floods. HECGeoRAS is a GIS based freely available hydraulic model. If the GIS maps are readily available for the location or the river that is interested, then, the hydraulic model can be easily applied and the flood flows can then be analyzed. However, if these GIS maps are not readily available for the interested area, then, one has to go to the basic version of the hydraulic model, HEC-RAS. HEC-RAS requires the cross-sectional profiles for the river, including the surroundings. A better river profile can be achieved, if the distance between two cross-sections can be minimized. However, these crosssections have to be fed manually.

309

rd


4. WAIDINA TRIBUTARY FLOOD MODELING Rewa River is the widest river and the second longest river in Fiji Islands. It is in the islands of Viti Levu. The river is 145 km in length and starts from the highest peak of island, Tomanivi. Rewa River drains approximately one thirds of island to Luacala bay. Wainibuka and Wainimala are the two major tributaries in Rewa River; however, there are many other tributaries, including Waimanu and Waidina rivers. Few research work was carried out for Waidina tributary (Brodie and Morrison, 1984, Greenbaum, 1979, Terry et al., 2002); however, a flood model was never discussed. Fiji Islands has a good spatial and temporal variation of precipitation through rainfall. Cyclones are very frequent for Fiji Islands; however, most of them are tropical. Tropical cyclone "Kina" tracked in December 1992 and intense rainfalls were found in Waidina tributary. Not only Kina, but also many other cyclones were happened throughout the recent history. Therefore, Waidina tributary is experiencing some variable rainfalls but they are intense. The authors could not find the GIS maps of Rewa River. Even though the digitized maps are available at Fiji department of Lands and Survey, these maps could not use in this research work. This is due to the lack of support from the respective authorities. Therefore, the manual method was used to obtain the topographical view of the Waidina tributary. 1:50,000 topographical maps were obtained from the Fiji Lands and Survey department. Cross-sections, along the Waidina tributary, were taken at an interval of 500 m. Figure 3 shows the Waidina tributary and its connection to the Rewa River. This research is an ongoing research and the highlighted section of the Waidina tributary is completed for its geometrical modeling.

Figure 3 Plan view of Waidina tributary.

Figure 4 Longitudinal section of Waidina tributary for 30 km.

310

rd


Figure 4 shows the longitudinal section of the section shows in Figure 3. Tributary has a very steep slope in its first few kilometers; however, it changes to a mild slope in downstream. About 200 m elevation drop can be seen along the 30 km length of the tributary (Figure 4). As it was stated above, the cross sections were obtained for this section of the tributary. The cross-sections were manually taken along the highlighted route of the tributary. On average eight to ten kilometers in width were considered. About 30 km along the tributary is being completed. These cross-sections were manually fed to the hydraulic model, HEC-RAS for the flood analysis. Tributary is a mountainous stream, and assumed with no vegetation in channel. Banks were assumed to be steep with considerable vegetation. Bottom of the stream has cobbles with large boulders. Therefore, a slightly high Manning’s roughness coefficient (0.05) was used for the flow calculations. In addition, the banks and the floodplain were assumed to be dense with vegetation. Therefore, 0.07 Manning’s roughness coefficient was used for the flood calculations. Several random flood flows were used to visualize the flood inundation; however, the results are presented herein for a flood flow of 100 3 m /s.

5. RESULTS AND DISCUSSION The simulation has taken 0.44 seconds of simulation time on an Intel® Core™ i3 desktop personal computer with a 3.40 GHz processor and 4 GB of RAM. The following figures (Figures 5a and 5b) 3 show the inundation maps for the 100 m /s of flood. Figure 5a presents the plan view of the inundation whereas Figure 5b shows a 3-D view. Two red lines are the banks of the tributary. Perpendicular lines to the banks (Figure 5a) are the fed cross-sections. Flood inundations can be seen from 8 to 25 km. th th However, a considerable flood inundation can be observed from the 10 to 16 kilometer of the tributary. This result is further illustrated in Figure 6. Figure 6 shows the comparison of the top tributary width against the top inundation width.

a) Plan view

b) 3-D view 3

Figure 5 Inundation maps of the tributary for 100 m /s flood Tributary top widths are around 50 m. However, floods even have laterally spread to 2 km from the 3 riverbanks in an event of 100 m /s of flood. This is because of the shallowness of the tributary. These inundation widths can be projected to the real map of the tributary area and the real inundation locations can be identified. Those identifications can effectively be used to inform the residents in the vulnerable areas. In addition, these results can be used for the flood protection structural measures.

311

rd


3

Figure 6 Inundation width for 100 m /s flood th

Figure 7 shows the cross-section at the 14 km of the tributary. Circled section gives the existing Waidina tributary. However, a flood water profile (rectangular section) can be seen to the right hand side of the existing tributary. Even though, the tributary water cannot flow directly from the circled section to rectangular section, the lower land (to the right hand side) shows the flood inundation. This is due to the existing lower lands close to the tributary in near by cross-sections (cross-sections at 13, 13.5, 14.5 km). This can further be identified by analyzing the near by 3-D topography.

th

Figure 7 Cross-section at 14 km

6. CONCLUSIONS AND RECOMMENDATIONS This is an ongoing research work to develop a flood model for the Rewa River in Fiji Islands. Initial flood modeling is being completed for a section of (30 km) of the Waidina tributary of the Rewa River. US Army Corps Engineers HEC-RAS hydraulic model was successfully applied to that section and initial results were obtained. They are promising; however, a completed flood model should be developed for sound conclusions. 3

3

3

The developed flood model has run for several random floods (including 100 m /s, 200 m /s, 500 m /s 3 3 and 1000 m /s) and presented for a 100 m /s of flow. However, the flood model is still to be validated for a real world flood flow. Developing a detailed flood model, including cross structures (bridges, culverts and so on) is being carried out. Furthermore, usage of GIS maps is highly encouraged. The support from the authorities is highly required to compile a holistic flood model to the Rewa River.

312

rd


7. ACKNOWLEDGMENTS Authors would like to extend their appreciation to Mr. Mausio R. Anise and Mr. Faijal Ali (lecturers in School of Building and Civil Engineering, Fiji National University) for their support in obtaining the scanned maps for this research.

8. REFERENCES Arnell N.W. (2004). Climate change and global water resources: SRES emissions and socio-economic scenarios. Global Environmental Change. 14(1). 31–52. Brodie J. E. and Morrison R. J. (1984). Coastal and inland water quality In the South Pacific; A review of existing information, monitoring programmes, Monitoring facilities and legislation. Topic review No. 16. South Pacific Commission, Noumea, New Caledonia. Brunner G.W. (2010). Hydraulic reference manual – HEC-RAS River analysis system. US Army Corps Engineers, Davis, CA, U.S.A. Goodell C. and Warren C. (2006). Flood inundation mapping using HEC-RAS. Obras y Proyectors, 2, 18-23. Greenbaum D. (1979). Water-quality studies in the Waidina and Rewa rivers. Progress Report No. 1. Mineral Resources Division, Ministry of Lands and Mineral Resources, Suva. Ku H.F.H., Hagelin N.W. and Buxton H.T. (1992). Effects of urban storm-runoff control on groundwater recharge in Nassau County, New York, Ground Water. 30(4). 507-514. McGree S., Yeo S.W. and Devi S. (2010). Flooding in the Fiji islands between 1840 and 2009. Risk Frontiers. Fiji Islands. Prata D., Marins M., Sobral B., Conceição A. and Vissirini F. (2011). Flooding analysis using HECth RAS modeling for Taquaraçu river in the Ibiraçu city, Espírito Santo, Brazil. 12 International Conference on Urban Drainage, Porto Alegre, Brazil, September 11 – 15, 2011. 8. Rathnayake U., Weerakoon S.B., Nandalal K.D.W. and Rathnayake U. (2007). Flood modeling in the Mahaweli River reach from Kothmale to Polgolla. International conference on Mitigation of the risk of natural hazards, Peradeniya, Sri Lanka, March 27 – 28, 2007. 8. Tate E.C. (1999). Floodplain mapping using HEC-RAS and ArcView GIS. Thesis, The University of Texas at Austin, United States of America. Terry J.P., Ollier C.D. and Pain C.F. (2002). Geomorpological evolution of the Nauva River, Fiji. Physical Geography. 23(5). 418-426. Timbadiya P.V., Patel P.L. and Porey P.D. (2011). Calibration of HEC-RAS Model on Prediction of Flood for Lower Tapi River, India. Journal of Water Resource and Protection. 3. 805-811. Yuan Y. and Qaiser K. (2011). Flood modeling in the Kansas River basin using Hydrologic Engineering Center (HEC) models – impacts of urbanization and wetlands for mitigation. Report EPA/600/R-11/116 Environmental Protecion Agency, Washington, U.S.A.

313