Geomorphic Assessment of in-Channel Sand Excavation on the River Meriç, Turkey
Hüseyin Turoğlu, Musa Uludağ, Tunahan AykutIn-channel sand excavation has been implemented on the River Meriç flowing through Turkey for the last 15 years to prevent or mitigate floods and to generate income. Sand removal was carried out by digging in-riverbed sand deposits in the 12 km section of the river running alongside the city of Edirne. This study aimed to investigate the geomorphological effects of in-channel sand excavation on the River Meriç in the Edirne province and to evaluate the results. The 12 km section of the River Meriç in the Edirne province was examined between 2008 and 2019. The braided channel pattern of this part of the river was mapped for a one-year period. Annual variation characteristics were determined using GIS and RS. Flow observations and change characteristics in the river channel morphology were then correlated. Changing the natural river channel form and channel cross-sectional characteristics of the river has led to changes in river erosion and deposition. It is predicted that these changes will play a triggering role in causing unexpected fluvial geomorphology problems. We suggest that the expected economic input for river sand mining should be re-evaluated from the perspective of cost-benefit analysis.
Meriç Nehri Yatak İçi Kum Alımının Jeomorfolojik Değerlendirmesi
Hüseyin Turoğlu, Musa Uludağ, Tunahan AykutHem taşkın önleme ve/veya zarar azaltma ve hem de ekonomik girdi sağlanması amacıyla, son 15 yıldır Meriç nehrinin Edirne bölümünde, yatak içi kum alımları yapılmaktadır. Kum alımları; Meriç Nehri’nin Edirne ilindeki 12 kilometrelik bölümde yer alan yatak içi kum birikimlerinin kazılması ile gerçekleştirilmektedir. Bu çalışmada, Meriç Nehri’nin Edirne ilindeki yatak içi kum alımlarının jeomorfolojik etkilerinin araştırılması ve sonuçlarının değerlendirmesi amaçlanmıştır. Meriç Nehrinin Edirne ilindeki 12 km’lik bölümü, 2009-2019 yılları için mercek altına alınmıştır. Meriç Nehrinin bu bölümdeki örgülü mecra patterni her yıl için haritalanmıştır. Yatak içi kum birikiminin yıllık değişimleri, akarsu yatak genişliği Coğrafi Bilgi Sistemleri ve Uzaktan Algılama teknolojileri kullanılarak belirlenmiştir. Akım rasatları, akarsu yatak morfolojisi değişim özellikleri birlikte ilişkilendirilmiştir. Akarsu kum madenciliğinin; Meriç nehrinin bu kısmında, hidrolojik ve jeomorfolojik doğal yapısının değişmesinde etkili bir rol oynadığı anlaşılmaktadır. Meriç Nehrinin doğal yatak formunun ve yatak kesit özelliklerinin değiştirilmesi; akarsu aşındırma ve biriktirme faaliyetlerinin değişmesine neden olmuştur. Değişikliklerin; beklenmedik jeomorfoloji problemleri için tetikleyici rol oynamasına neden olacağı öngörülmektedir. Akarsu kum madenciliği açısından beklenen ekonomik girdinin; fayda maliyet analizi perspektifinde yeniden değerlendirilmesi önerilmektedir. Tespit ve öngörüler, devam etmekte olan araştırmanın ilk sonuçlarını temsil etmektedir
Since 2009, work for the prevention and mitigation of flash floods and flooding within and close to Edirne have been carried out by enlarging the channel of the River Meriç and performing in-channel sand excavation. Initially, in-channel sand excavation was begun both for commercial sand mining and to strengthen embankments along the riverbanks to manage flooding. Later, the in-channel sand excavation was largely transformed into river sand mining. The aim of this research was to investigate the effects of in-channel sand excavation on the hydrographic and geomorphological properties of the river’s fluvial environment in the Edirne section.
The research was carried out on the 12 km section of the River Meriç within the Edirne province. For this section of the river, the annual change in size, shape, and spatial distribution of the sand islands was mapped using satellite images from 2009 to 2019 by means of a Geographic Information System and Remote Sensing. Regression analysis was applied to the trend of sand island change between 2009 and 2019. In addition, regression analysis of flow data observed at Kirişhane Flow Observation Station (General Directorate of State Hydraulic Works - “DSI” in Turkish) during the years 2009-2019 was performed.
River sand mining is carried out in the northern section close to Mehmet Muezzinoğlu Bridge. The sand islands’ areal change in this section of the river was also compared with measurements of the southern section. Inferences were made by associating annual flow statistics and changes in the sand island area and location with current statistics and other data.
The first results of our research on the effects of in-channel sand excavation on the hydrographic and geomorphological properties of River Meriç are as follows.
In the 12 km section of the River Meriç within the borders of Edirne province, it was calculated that there are approximately 100 million tons of sand reserves. Morphologic changes in the sand islands of the river channel show that the intake of sand from the river channel has continued consistently throughout the last 10 years. River sand mining is being carried out in the river channel north of the Mehmet Müezzinoğlu Bridge with an extraction rate of sand at around 1 million m3 annually. Removal of sand from the river bed caused the size, shape and location of the sand islands in this part of the river to change constantly
Due to the sand mining, the channel geometry, hydrodynamic, and fluvial geomorphology characteristics of the river were altered in this part of the Meriç. These changes have arisen from erosion and deposition in the Meriç because of the removal of sand in the river bed.
River bed cleaning and the construction of embankments along the riverbanks were made within the scope of a project initiated by the DSI between 15/12/2008 and 26/11/2010. This project caused an increase in the flood flow limits from 1000 m3 /sec to 1400 m3 /sec in the Edirne section of the River Meriç.
In principle, the sand islands in the stream occupy space within the bed, reducing the river bed cross-section of water flow and encouraging floods. It can be accepted that sand mining plays a preventive role against floods by reducing the accumulation of sand in the river bed. However, two issues are important here. One is that the balance between sand removal from the river bed and sand accumulation in the bed must be taken into consideration. River sand mining should be carried out with this balance in mind. The other issue is that care must be taken not to affect the river’s hydrodynamic characteristics and groundwater circulation by excavating too deeply into the river bed.
The average flow values in humid and dry periods, calculated from observed data between 1986-2018 at Kirişhane Flow Observation Station (26°34′20” East, 41°38′50” North) (Figure 1) were analyzed. Linear regression analysis for the whole 32-year period shows that the flow trend in general is increasing.
The linear regression analysis (y) equation on the annual change of the sand islands shows a negative in sand accumulation in the sand mining part of the River Meriç for the 2009-2019 period. However, in the section after the Mehmet Muezzinoğlu Bridge, the regression equation is positive. In addition to the removal of sand from the river in-channel, the reason for this difference in the behaviour of the river is the important role played by the rock embankment built perpendicular to the flow direction in the river bed. The rock embankment made in the River Meriç channel has negatively affected the fluvial system and changed the erosion and deposition of the river.
