Sediment Accumulation in River Deltas of VistonisEcosystemM.V. Malioka, MSc Civil EngineerVM & Associates Consulting
IntroductionSediment yield in river basins Natural process that involves:1. Fluvial erosion2. Sediment deposition in water bodiesPotential threat if becomes excessive due to anthropogenic interventions (diversions of rivers, disruption of land use patterns, agriculture, etc.)Sediment management is required for :• the protection of water bodies when the anthropogenic activities affect the quality and quantity of the sediments• rational conservation of sediment load which is a natural constituent of the water bodies
Handling excessive sediment loadsExcessive sediment loads may:• affect the ecological and chemical status of the water bodies• cause flooding occurrences• impact the storage capacity of reservoirs• require high capital improvements in drinking water treatment facilitiesHowever, sediments can be utilized as a valuable resource in:• fertilizing farmlands with sediments rich in organic manure and fertilizers• providing construction material for flood control (earth embankments, dikes, etc.) at low cost, short transportation and convenience
Case Study – Aquatic System ofLake VistonisCharacteristics of Lake Vistonis:• shallow coastal lake in Northern Greece which is fed by a fluvial system that comprises 3 major rivers (Kosynthos, Kompsatos and Travos) – Area: 45Km2.• protected wetland (Ramsar, Natura 2000)• wildlife refuge, special protected area and important bird area
Scope of the paperThe research of the sediment problems of the aquatic system ofLake Vistonis covered many different aspects of the problems.This presentation focuses on the following:•calculation of the sediment yield into the lake•estimation of the aggradations in the 3 river deltas•estimation of the penetration of the 3 river deltas into the lake•proposal of sediment management strategies
MethodologyAssessment of the sediment accumulation of the three main riverdeltas of Lake Vistonis ecosystem, based on:• Mathematical models for the calculation of the annual sediment yields.• Longitudinal estimation of the development of delta aggradations, based on historical data.
Mathematical ModelCalculation of the sediment yield, for the 3 major river basins(Kosynthos, Kompsatos and Travos), using scientifically acceptedmethods :• The sediment yield for river basins, Y(m3/yr), was calculatedfrom the Walling equation (Walling, 1983): Y = SDR x Wwhere:•SDR (Sediment Delivery Ratio), which is the ratio of the sedimentproduction and the sediment yield, calculated from the Boyceequation (Boyce, 1975) – SDR = 62.1/A0.1419•W(m3/yr), which is the average sediment production (byerosion), for a river basin was calculated from the Gavrilovicequation (1972) - W= TxhxπxΖ1.5xA,
Data for the Case Study Area of Catchment for Lake Vistonis Mountainous Plain Region Region Total Sub-catchment (km2) (km2) (km2) Kosynthos 370.4 102.2 472.6 Kompsatos 565.0 27.2 592.2 Kopteros-Iasmos 46.5 31.3 77.8 Travos 43.3 98.7 142.0 Sydini-Selinos 1.0 38.3 39.3 Total 1,026.2 297,7 1,323.9 Mountainous and plain region of the catchment is 71% and 29%
Longitudinal estimation of thedevelopment of delta aggradationsExamination of the development of the delta aggradations in the 3river deltas and Lake Vistonis using:•historical data for a time period of 175 years•maps for the years 1833 (British Army Headquarters map), 1921,1945, 1949, 1970, 1977 (Greek Military Geographic Service andGreek Ministry of Exterior maps) and 2005-2008 (Google Earthmap)•bathymetric map with isodepths (from bathymetric measurementsfor a total lake area of 600 hectares)
ResultsThe results include:1. Calculation of sediment yields in the three larger sub-catchments of the rivers Kosynthos, Kompsatos and Travos.2. Historical development of aggradations in River Deltas of Kosynthos, Kompsatos and Travos3. Historical development of aggradations in Lake Vistonis4. Sediment management strategies
Sediment Yield in Sub-catchment ofKosynthosMountainous catchment: 370.4 km2Plain catchment: 102.2 km2• average annual sediment yield from mathematical models: 203,206 m3/yr• average annual aggradations from maps (1920-2005): 139,400 m3/yr• bed load which contributes to aggradations: 139,400 / 203,206 = 68%• suspended load which enters the lake: 203,206 - 139,400 = 63,806 m3/yr
Sediment Yield in Sub-catchment ofKompsatosMountainous catchment: 565.0 km2Plain catchment: 27.2 km2• average annual sediment yield from mathematical models: 220,899 m3/yr• average annual aggradations from maps (1920-2005): 127,000 m3/yr• bed load which contributes to aggradations: 127,000 / 220,899 = 57%• suspended load which enters the lake: 220,899 - 127,000 = 93,899 m3/yr
Sediment Yield in Sub-catchment ofTravosMountainous catchment: 142.0 km2Plain catchment: 99.4 km2• average annual sediment yield from mathematical models: 38,141 m3/yr• average annual aggradations from maps (1920-2005): 13,000 m3/yr• bed load which contributes to aggradations: 38,141 / 13,000 = 34%• suspended load which enters the lake: 38,141 - 13,000 = 25,141 m3/yr
Historical Data on River Kosynthos•3,000 years ago River Kosynthos was flowing through farmlands into the sea.• The inflow of Kosynthos into Lake Vistonis occurred around 100 AD.•Before the diversion of the river in 1952, the river delta had been developed at the west part of the lake, with a broad front of about 6 km.•The diversion of the river – river channelization - was necessary in order to receive the flows of smaller torrents in the area and prevent flooding of the farmlands.•Today the river delta has been developed at the north part of the lake.
Historical Development ofAggradations in Delta of Kosynthos•The sediment load of torrents is now transferred into River Kosynthos resulting in higher sediment load into Lake Vistonis.•Prior to the diversion, the sediment load from the torrents was precipitating in the broader plain region. Average Annual Increase of Delta of Kosynthos (1920-2005) Surface Area Cumulative Time Volume Length Length Period (hectares/yr) (103 m3/yr) (m/yr) (m) 1920-1945 4.0 120.0 5.8 145 1945-1970 7.2 217.2 49.2 1,230 1970-2005 3.3 97.7 22.9 800 1920-2005 4.7 139.4 25.6 2,175
Historical Development of Aggradations in Delta of Kompsatos•Before 1950, it was flowing into the Lake through two river beds, which formed a river delta at the east part of the lake, with a broad front of 6 km.•In 1952 the construction of a series of embankments forced the river to flow through only one river bed .•The expanded river bed (maximum width of 1,585 m and length of 2,365 m) resulted in the accumulation of large amounts of sediments at the river delta. Average Annual Increase of Delta of Kompsatos (1920-2005) Surface Area Cumulative Time Volume Length Length Period (hectares/yr) (103 m3/yr) (m/yr) (m) 1920-1945 2.0 60 18.0 450 1945-1970 6.4 193 60.0 1,500 1970-2005 2.2 67 11.4 400
Historical Development ofAggradations in Delta of Travos Average Annual Increase of Delta of Travos (1920-2005) Surface Area Cumulative Time Volume Length Length Period (hectares/yr) (103 m3/yr) (m/yr) (m) 1920-1945 0.16 5 8.0 200 1945-1970 0.43 14 8.0 200 1970-2005 0.63 19 12.9 450 1920-2005 0.45 13 10.0 850
Conclusions – Threat by sediments•Constant occurrence of sediment deposition within the lake.•The average annual increase of the volume of aggradations in the 3 riverdeltas is approximately 255,000 m3/yr, for the period 1920-2005.•The cumulative average increase of the delta length of Rivers Kompsatos,Kosynthos and Travos, for the period 1920-2005, is 2,175 m, 2,350, and 850m respectively.•Reduction of the water volume and alteration of the morphologicalcharacteristics of the lake.
Conclusions – Reasons of the problemThe main reasons for the sediment increase in Lake Vistonis are:•Channelization of the fluvial system of the lake, that increases therate of sediment yield and deposition in the interior of the lake.•Deforestation near the river deltas, that results in the destabilizationof the soil and the retreat of the coastline of the lake.
Conclusions – ActionsSediment management strategies for the protection of the lake mayinclude:•Gabions’ constructions in specific locations in the rivers to stabilize theriver bed and contain the sediment load.•Smoothing of the river beds to ensure safe flow conditions in the rivers forparticle deposition and flooding control.•Localized woodland planting to reduce the rate of river channel migration.