INTERACTION BETWEEN SEDIMENT TRANSPORT AND FLOOD FLOW: THE CASE OF KOMPSATOS RIVER BASIN, GREECE

Transcription

1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 Setember 013 INTERACTION BETWEEN SEDIMENT TRANSPORT AND FLOOD FLOW: THE CASE OF KOMPSATOS RIVER BASIN, GREECE S. KOUTSOYIANNIS 1, P. ANGELIDIS 1 and V. HRISSANTHOU 1 1 Democritus University of Thrace, Deartment of Civil Engineering, Xanthi, Greece vhrissan@civil.duth.gr EXTENDED ABSTRACT For the flood event of November 1996 (7-30 Nov., total rainfall deth 64.5 mm), the hydrograh and the corresonding sediment grah at the outlet of the mountainous art of Komsatos River basin (Thrace, northeastern Greece) were comuted. The mountainous art of Komsatos River basin has an area of about 567 km, consisting mainly of forest and bush. For more recise calculations, the basin was divided into 18 natural sub-basins. Komsatos River has a length of about 65 km, from which 9 km flow through the flat art of the basin, and discharges its water into Vistonis Lake. The comutation of the hydrograh was enabled by means of the following models included in the well-known hydrologic software HEC-HMS: (a) Soil Conservation Service (SCS) Curve Number, (b) synthetic dimensionless unit hydrograh of SCS, (c) hydrograh routing model Muskingum-Cunge. The comutation of the sediment grah was enabled by combination of the above comosite hydrologic model with the soil erosion model of Schmidt (199) and the stream sediment transort model of Yang and Stall (1976). The flood hydrograh and sediment grah comuted for the outlet of the mountainous art of Komsatos River basin were routed in the flat art of the basin by means of the well-known hydraulic software HEC-RAS, for quasi-unsteady flow. The comutational rocess is based on the Manning equation for the comutation of energy sloe, on the energy conservation equation for the comutation of the flood deth, and on the sediment continuity equation, in combination with Yang equation, for the comutation of the geomorhological bed evolution. From the comutational results of HEC-RAS alication, the water surface rofile, the stream bed rofile and the bed variation in the cross-sections of the flat art of Komsatos River during the flood event can be visualised. Keywords: basin, flood, hydrograh, HEC-HMS, soil erosion, sediment transort, sediment grah, HEC-RAS, Komsatos River. 1. INTRODUCTION In the ast, hydrologic models included in the well-known software HEC-HMS (Technical Reference Manual, 000) were alied to basins located near the city of Xanthi (Thrace, northeastern Greece). For examle, a comosite hydrologic model consisting of three models (Soil Conservation Service (SCS) model for the comutation of rainfall excess, kinematic wave model for the transformation of rainfall excess to direct runoff hydrograh, Muskingum-Cunge model for the routing of the hydrograh through the main stream of

2 the basin) was alied to Kimmeria Torrent basin, with an area of about 35 km, for a rainfall event in June 004 (Ziogas et al., 006). A similar comosite hydrologic model consisting of three models (SCS model for the comutation of rainfall excess, SCS model for the transformation of rainfall excess to direct runoff hydrograh, Muskingum-Cunge model for the routing of the hydrograhs from the outlets of the sub-basins to the outlet of the whole basin) was alied to Kosynthos River basin, with an area of about 37 km, for three rainfall events in June 006 (Charalabidis et al., 009). The comarison result between comuted and measured stream discharges was very satisfactory for both basins. The hydrologic model described in Charalabidis et al. (009) was also alied to Kosynthos River basin in order to calculate the flood hydrograh of November 1996 at the basin outlet. In this case, a direct comarison between comuted and measured flood discharges was imossible because of lack of flood discharge measurements (Hrissanthou and Theodorakooulos, 005). The hydrologic model described in Charalabidis et al. (009) was combined with the soil erosion model of Poesen (1985) and the stream sediment transort model of Yang and Stall (1976) in order to calculate the sediment grah corresonding to the flood hydrograh of November 1996, at the outlet of Kosynthos River basin (Hrissanthou and Theodorakooulos, 005). In the resent case of Komsatos River basin with an area of about 567 km, located between the cities of Xanthi and Komotini, the hydrologic model mentioned above is combined with the soil erosion model of Schmidt (199) and the stream sediment transort model of Yang and Stall (1976) in order to calculate the hydrograh and the sediment grah, due to the flood event of November 1996, at the outlet of the mountainous art of the basin. Additionally, in contrast to the studies given above, the water surface rofile and the geomorhological stream bed evolution during the flood event are comuted for the flat art of Komsatos River by means of the well-known software HEC-RAS (Hydraulic Reference Manual, 010).. HYDROLOGIC MODEL.1. Rainfall excess model According to Soil Conservation Service (SCS, 197) method, the rainfall excess is comuted by: h R h R ( hr 0.S) (.1) h 0.8S (mm) is the rainfall excess, (mm) is the rainfall deth and S maximum hydrologic losses (mm), which are comuted by the equation: h r r (mm) are the 5400 S 54 (.) CN CN is the curve number which can be estimated as a function of land use, hydrologic soil tye and antecedent moisture conditions, by using tables ublished by the SCS... Model for the transformation of rainfall excess to runoff hydrograh The hydrograh of direct runoff is comuted on the basis of the unit hydrograh theory. In the dimensionless synthetic unit hydrograh of SCS, discharges are exressed as a

3 ortion of the eak discharge hydrograh, T. q and time stes as a ortion of the rise time of the unit The values of q and T the recession time is equal to 1.67 are estimated by a simlified triangular unit hydrograh, T. The area which is surrounded by the curve of the unit hydrograh must be equal to the rainfall excess of 1 cm. The eak discharge s -1 ) is C =.08, A hydrograh, which is given by q CA T (km ) is the sub-basin area and T q (m 3 (.3) (hr) is the rise time of unit T t R t (.4) t R (hr) is the duration of the rainfall excess and which is aroximately equal to 0.60 t C, t C t (hr) is the lag time of the basin, (hr) is the concentration time..3. Routing model of Muskingum-Cunge The routing of the direct runoff hydrograh from the outlet of a sub-basin to the outlet of the whole basin is enabled by means of Muskingum-Cunge model. The basic equation of the model is given below: k1 k1 k k C Q C Q C Q (.5) Q Qi 1 o i 1 i i1 (m 3 s -1 ) is the direct runoff discharge, designates the time ste C o c x (1 x) c C t 1. The coefficients c x (1 x) c C o C i, designates the sace ste C 1 and C (1 x) c (1 x) c x are defined as follows: and k t (.6) x The roduct c c( t / x) C is called the Courant number and is equal to the ratio of the celerity of small waves c to the grid celerity x / t. The arameter x is obtained by the relationshi: 1 qo x (1 ) (.7) S cx f S f is the energy sloe and inlet hydrograh). q o is the reference discharge er unit width (from the 3. SOIL EROSION MODEL According to Schmidt (199), the erosive imact of drolets and overland flow is roortional to the momentum flux contained in the drolets and the flow, resectively. The momentum flux exerted by the falling drolets, r (kg m s - ), is given by the relationshi (Schmidt, 199; Hrissanthou, 006):

4 C is the soil cover factor, r r CrAu r sin a (3.1) (m s -1 ) is the rainfall intensity, (kg m -3 ) is the water u r density, A (m ) is the basin area, (m s -1 ) is the mean fall velocity of the drolets and a ( o ) is the mean sloe gradient of the sub-basin area. The momentum flux exerted by the overland flow, f (kg m s - ), is given by qbu (3.) f q (m 3 s -1 m -1 ) is the runoff rate er unit width, b (m) is the width of the sub-basin area and u (m s -1 ) is the mean flow velocity. The sediment suly to the main stream of a sub-basin is estimated by means of a comarison between the available sediment in the corresonding sub-basin area and the sediment transort caacity by overland flow (Hrissanthou, 006). The sediment transort caacity by overland flow, (kg s -1 m -1 ), is comuted by (Schmidt, 199): s c max q t q t c q (3.3) max s (m 3 m -3 ) is the concentration of susended articles at transort caacity and (kg m -3 ) is the sediment density. 4. STREAM SEDIMENT TRANSPORT MODEL The sediment yield at the outlet of the main stream of a sub-basin can be estimated by means of a comarison between the available sediment in the main stream and the sediment transort caacity by streamflow (Hrissanthou, 006). For the comutation of the sediment transort caacity by streamflow, the following relationshis are used (Yang and Stall, 1976): wd50 u* log c t log log w wd50 u* us ucr s ( log 0.314log )log( ) (4.1) w w w u.5 cr 0.66 w log( u D / ) 0.06, if 1. u / 70 * D50 (4.) c t * 50 u cr.05 w, if u / 70 * D50 (4.3) (m) is the total sediment concentration by weight, w (m s -1 ) is the terminal fall velocity of susended articles, D 50 (m) is the median article diameter, (m s -1 ) is the kinematic viscosity of the water, u * (m s -1 ) is the shear velocity, u (m s -1 ) is the mean flow velocity, u cr (m s -1 ) is the critical mean flow velocity and s is the energy sloe. 5. FLOOD AND SEDIMENT ROUTING MODEL The flood hydrograh and the corresonding sediment grah at the outlet of the mountainous art of a basin, comuted by the models described above, can be routed through the flat art of the basin with the aid of the energy conservation equation and the sediment continuity equation, which is exressed as follows (HEC-RAS, Hydraulic Reference Manual, 010):

5 Qs ( 1 ) b (5.1) t x is the active layer orosity, b (m) is the channel width, (m) is the channel bed elevation, Q s (m 3 s -1 ) is the transorted sediment load, x (m) is the distance and t (s) is the time. For the comutation of Q s, Equations (4.1), (4.) and (4.3) are used. 6. APPLICATION TO KOMPSATOS RIVER BASIN 6.1. The case study area The mountainous art of Komsatos River basin has an area of about 567 km consisting of forest (37.3%), bush (36.1%), cultivated land (6.1%) and urban area (0.5%). The highest altitude of the basin is about 100 m. The length of the main stream of the basin is about 65 km, from which 9 km flow through the flat art of the basin to Vistonis Lake. For more recise calculations, the mountainous art of the basin was divided into 18 natural sub-basins (area: between 13 and 50 km ) (Figure 1; Hrissanthou et al., 010). Figure 1. Komsatos River basin Sub-basins with main streams. 6.. Analysis of the results The extreme rainfall in the time eriod from 7 November 1996 to 30 November 1996 has a total deth of 64.5 mm, registered at the meteorological station of Genisea (Koutsoyiannis, 01). However, the daily rainfall deth from the above time eriod is distributed over the day considered. Figure shows the flood hydrograh and the corresonding sediment grah at the outlet of the mountainous art of Komsatos River basin, comuted by means of the hydrological model described in Section for the extreme rainfall of November 1996, and the soil erosion model and the stream sediment transort model described in Sections 3 and 4, resectively.

6 Stream Discharge (m 3 /s) Sediment Discharge (kg/s) Time (hr) Stream Discharge Sediment Discharge Figure. Flood hydrograh and corresonding sediment grah at the basin outlet. It is clear from Figure that the eak of the flood hydrograh follows temorally the eak of the corresonding sediment grah, which was also observed in other hydrometric stations (Maniak, 1988). From Figure, by comuting the area between the sediment grah curve and the x-axis, results that the sediment transorted during the flood considered amounts to about t. It seems to be a reasonable arithmetic result, if it is taken into account that the mean annual sediment yield, for the years , equals about t (Hrissanthou et al., 010). Figure 3 shows a three-dimensional grahical reresentation of the flat art of Komsatos River, the road bridge (ustream) and the railway bridge (downstream) can be seen Figure 3. Three-dimensional grahical reresentation of the flat art of Komsatos River. The routing of the flood hydrograh and the corresonding sediment grah (Figure ) through the flat art of Komsatos River was enabled by alying the energy conservation equation in combination with the sediment continuity equation (5.1) for quasi-unsteady flow (HEC-RAS, Hydraulic Reference Manual, 010). Figure 4 illustrates the maximum water level along the flat art of Komsatos River during the flood event, ten hours after the rainfall start, on 30 November 1996, as well as the

7 corresonding river bed rofile. In this figure, the site of the road bridge (ustream) and the railway bridge (downstream) can be seen. Additionally, the local scour downstream of the railway bridge can be seen. The bridge iers cause a reduction of the wetted area of the river cross-section, which again imlies a local velocity increase and the scouring. According to Figure 4, the flow deth is higher in the cross-sections ustream of the road bridge, near the basin outlet, than in the downstream cross-sections near the Vistonis Lake, which is due to the fact that the downstream cross-sections are wider than the ustream cross-sections. Additionally, a rise of water surface rofile ustream of the railway bridge is observed due to the damming effect of the railway bridge iers. Figure 5 illustrates the bed variation (erosion) in the cross-section 6 (Figure 4), downstream of the railway bridge, at the beginning and the end of the simulation rocess. The bed erosion deth is not considerable because the cross-section considered lies near Vistonis Lake, in the flattest art of Komsatos River, the deosition of sediment dominates under usual rainfall conditions Komsatos River Legend WS 7Nov Ground 8 Elevation (m) Main Channel Distance (m) Figure 4. Maximum water level of Komsatos River during the flood event. 8 6 Legend 7Nov Nov Elevation (m) Station (m) Figure 5. Bed variation in cross-section 6, downstream of the road bridge,during the flood event.

8 7. SUMMARY AND CONCLUSIONS In the case study of Komsatos River basin (Thrace, northeastern Greece), the hydrologic software HEC-HMS was combined with the hydraulic software HEC-RAS in order to comute the flood hydrograh, due to the extreme rainfall event of November 1996 (7-30 Nov., total rainfall deth 64.5 mm), at the outlet of the mountainous art of the basin considered, and to route the flood hydrograh through the flat art of Komsatos River. The combination of the hydrologic model (HEC-HMS) with a soil erosion model (Schmidt) and a stream sediment transort model (Yang and Stall) enables the transition of the hydrograh, resulting from the extreme rainfall, to the corresonding sediment grah at the outlet of the mountainous art of the basin. The routing of the hydrograh and the corresonding sediment grah through the flat art of the main stream of the basin is enabled simultaneously (HEC-RAS), so that the flood deth and the geomorhological bed evolution along the main stream can be comuted. The ractical merit of the hydrologic and hydraulic comutations, esecially for flood events, is the ossibility of lanning flood rotection measures both in the mountainous art and the flat art of a river. REFERENCES 1. Charalabidis G., Ziogas K. and Hrissanthou V. (009) Measurements of water discharge, sediment transort and water quality arameters in Kosynthos River basin, Proceedings 11 th International Conference on Environmental Science and Technology (CEST009), Chania, Crete, Greece, Vol. A, (CD-ROM).. HEC-HMS (000) Technical Reference Manual, US Army Cors of Engineers, Hydrologic Engineering Center. 3. HEC-RAS (010) Hydraulic Reference Manual, Version 4.1, US Army Cors of Engineers, Hydrologic Engineering Center. 4. Hrissanthou V. (006) Comarative alication of two mathematical models to redict sedimentation in Yermasoyia Reservoir, Cyrus, Hydrological Processes, 0(18), Hrissanthou V. and Theodorakooulos E. (005) Comutation of sediment grah for a flood event, Proceedings 6 th International Conference of the Euroean Water Resources Association (EWRA), Menton, France, 13. (CD-ROM). 6. Hrissanthou V., Delimani P. and Xeidakis G. (010) Estimate of sediment inflow into Vistonis Lake, Greece, International Journal of Sediment Research, 5(), Koutsoyiannis S. (01) Alication of the hydrologic software HEC-HMS, the hydraulic software HEC-RAS and sediment transort models to Komsatos River basin for the flood event of November 1996, Postgraduate Studies Thesis, Deartment of Civil Engineering, Democritus University of Thrace, Xanthi, Greece (in Greek). 8. Maniak U. (1988) Hydrologie und Wasserwirtschaft, Sringer-Verlag, Berlin, Heidelberg, New York, 576 S. 9. Poesen J. (1985) An imroved slash transort model, Zeitschrift für Geomorhologie, 9(), Schmidt J. (199) Predicting the sediment yield from agricultural land using a new soil erosion model, Proceedings 5 th International Symosium on River Sedimentation, Karlsruhe, Germany, SCS (Soil Conservation Service) (197) National Engineering Handbook, Section of Hydrology, Washington DC, USA. 1. Yang C.T. and Stall J.B. (1976) Alicability of unit stream ower equation, Journal of the Hydraulics Division, ASCE, 10(5), Ziogas K., Voudouri A. and Hrissanthou V. (006) Comarative alication of surface runoff models to Kimmeria Torrent basin (Xanthi), Proceedings 10 th Panhellenic Scientific Conference of the Greek Hydrotechnical Union, Xanthi, Greece, Vol. A, (in Greek).