ESTIMATE OF SEDIMENTATION IN KALAVASOS RESERVOIR, CYPRUS TENTH INTERNATIONAL SYMPOSIUM ON RIVER SEDIMENTATION MOSCOW, RUSSIA, AUGUST 1-4, 2007 ESTIMATE OF SEDIMENTATION IN KALAVASOS RESERVOIR, CYPRUS V. HRISSANTHOU AND M. ANDREDAKI DEPARTMENT OF CIVIL ENGINEERING DEMOCRITUS UNIVERSITY OF THRACE 67100 XANTHI, GREECE

INTRODUCTION Computation of sedimentation in Kalavasos Reservoir in terms of soil erosion in the corresponding basin Kalavasos Reservoir is located northeast of the town of Limassol, Cyprus Storage capacity of the reservoir: 17 100 000 m³ Water surface area of the reservoir: 875 000 m² Basin area of the reservoir: 96 km² Main stream of the basin: Vasilikos River A mathematical model is used for the computation of the mean annual reservoir sedimentation

MATHEMATICAL MODEL Three submodels: Hydrological submodel Soil erosion submodel (Poesen, 1985) Stream sediment transport submodel (Yang and Stall, 1976) The calculations are performed on a monthly time basis

HYDROLOGICAL SUBMODEL Simplified water balance model for the root zone of the soil: Sn´ = Sn-1 + Nn - Ern S: available soil moisture [mm] N: rainfall amount [mm] Er: actual evapotranspiration [mm] n: index for the time step

HYDROLOGICAL SUBMODEL If Sn´<0, then Sn=0, hon=0, INn=0 If 0≤Sn´≤Smax, then Sn=Sn´, hon=0, INn=0 If Sn´>Smax, then Sn=Smax, hon=K(Sn´-Smax), INn=K´(Sn´-Smax) where K´=1-K ho: direct runoff [mm] IN: deep percolation [mm] Smax: maximum available soil moisture [mm] K, K´: proportionality coefficients

SOIL EROSION SUBMODEL (Poesen, 1985) qrs = C(KE)rs-1cosa qr = qrs[0.301sina+0.019D50-0.22(1-e-2.42sina)] qrs: mass of detached particles per unit area [kg/m2] C: soil cover factor KE: rainfall kinetic energy [J/m2] rs: soil resistance to drop detachment [J/kg] a: slope gradient [o] qr: downslope splash transport per unit width [kg/m] D50: median particle diameter [m]

SOIL EROSION SUBMODEL qf = rqt (Nielsen et al., 1986) qf: sediment transport by runoff per unit width [m3/(s m)] r: entrainment ratio (r=1 for noncohesive soils, r<1 for cohesive soils) qt: sediment transport capacity by overland flow per unit width [m3/(s m)]

qt = [0.04(2g/f)1/6q5/3s5/3] / [(ρs/ρ-1)2g1/2D50] SOIL EROSION SUBMODEL qt = [0.04(2g/f)1/6q5/3s5/3] / [(ρs/ρ-1)2g1/2D50] (Engelund and Hansen, 1967) g: gravity acceleration [m/s2] f: friction factor q: runoff rate per unit width [m3/(s m)] s: energy slope ρs: sediment density [kg/m3] ρ: water density [kg/m3]

If qrf > qt, then ES = qt If qrf < qt, then ES = qrf SOIL EROSION SUBMODEL Available sediment on the soil surface of a sub-basin (qrf) = downslope splash transport (qr) + sediment transport by runoff (qf) Estimation of sediment ES reaching the main stream from the respective sub-basin area If qrf > qt, then ES = qt If qrf < qt, then ES = qrf qt: sediment transport capacity by overland flow

STREAM SEDIMENT TRANSPORT SUBMODEL Estimation of sediment load FLO at the outlet of the main stream of a sub-basin If ESI > qts, then FLO = qts If ESI < qts, then FLO = ESI ESI: available sediment load in the main stream considered qts: sediment transport capacity by streamflow

STREAM SEDIMENT TRANSPORT SUBMODEL logct=5.435-0.286log(wD50/ν)-0.457log(u*/w)+ +[1.799-0.409log(wD50/ν)-0.314log(u*/w)]log(us/w-ucrs/w) (Yang and Stall, 1976) ct: total sediment concentration by weight [ppm] w: terminal fall velocity of suspended particles [m/s] D50: median particle diameter of bed material [m] ν: kinematic viscosity of the water [m2/s] u: mean flow velocity [m/s] ucr: critical mean flow velocity [m/s] u*: shear velocity [m/s] s: energy slope

APPLICATION OF THE MATHEMATICAL MODEL Basin of Kalavasos Reservoir: Area: 96 km2 Main stream length: 18 km Soil cover: forest (87.7%), bush (2.7%), urban area (0.9%), cultivated land (8.3%), no significant vegetation (0.4%) Permeable soils: 23.4%, semi-permeable soils: 76.6% Highest altitude: 1300 m

Division of Kalavasos Reservoir basin into 5 natural sub-basins

AVAILABLE DATA Monthly rainfall data for 16 years (1986 – 2001) from 7 rainfall stations (mean annual rainfall amount 572 mm) Mean daily values of air temperature, relative air humidity, sunlight hours and wind velocity for every month of the 16 years from 1 meteorological station Symbols: YD: annual erosion amount in the basin [t] YA: annual sediment yield at the basin outlet [t] DR: sediment delivery ratio (YA/YD)

COMPUTATIONAL RESULTS Year YA [t] YD [t] DR [%] 1986 4200 4300 98 1987 74 000 194 000 38 1988 72 000 125 000 58 1989 44 000 65 000 68 1990 40 000 51 000 78 1991 118 000 365 000 32

COMPUTATIONAL RESULTS Year YA [t] YD [t] DR [%] 1992 50 000 60 000 83 1993 25 000 30 000 1994 183 000 346 000 53 1996 41 000 52 000 79 1999 1400 100 2000 20 000 2001 65 000 149 000 44

COMPUTATIONAL RESULTS Mean value of YA: 57 000 t Mean value of YD: 112 500 t Mean value of DR: 70% Mean annual rate of soil erosion: 0.44 mm (computation) 0.50 mm (measurement) (Water Development Department, Nicosia, Cyprus) Trap efficiency of Kalavasos Reservoir: 100% Mean annual deposition volume in the reservoir: 30 700 m³ Useful life of the reservoir: 556 years

SENSITIVITY ANALYSIS

SENSITIVITY ANALYSIS

CONCLUSIONS The soil erosion submodel underestimates slightly the measured mean annual rate of soil erosion for the whole basin. The input data of the mathematical model, that influence the annual erosion amount of the whole basin and the annual sediment yield at the basin outlet more strongly than the other input data, are the rainfall amount, the curve number, the sub-basin areas and the soil slope gradient of the sub-basins. There is not an immediate and sharp danger of reservoir filling with sediments.