A Study on the Causes of Sediment Accumulation in the Drainage Systems ()
1. Introduction
Humans have long used open-air canals for land drainage in Mesopotamia around 9000 BP [1] as well as Egyptians and Greeks around 2400 BP [2] . The long March canal in 1789 was the first recorded canal project in Maryland. By the early 20th century, land drainage was a large-scale endeavor involving state and federal partnerships which focused on the removal of surface water and groundwater [3] . Agricultural drainage canals are essential for the removal of surface and groundwater in order to allow for crop production in poorly drained agricultural landscapes. Canals also mediate the flow of pollutants from agro-ecosystems to downstream water bodies. Stream flow velocities and sediment transport regime in the drainage canals cause accumulation of fine sediments and silts along the canal network. Deposition of sediment in the drainage canal network is an undesirable, yet inevitable occurrence in the course of the use and operation of any canal network system. Apart from the sediment deposited in the bottom and consequently reducing the designed, basic purpose and functional performance of the canal network as well as hydraulic works constructed on them, the physical, chemical and biological properties of these sediments deposits are becoming issues of more immediate concern.
This study was initiated in Nazilli irrigation scheme D-25 drainage canals group of Büyük Menderes basin in Turkey, to determine the effects and causes of silt accumulation and mechanical silt cleaning which are expected to alleviate the problem on the water table, and to evaluate the current situation. This problem needs to be properly addressed considering the total length of the canal network, that is, the total volume of sediment to be removed by dredging if the functional performance of the drainage canal network is to be restored and maintained.
2. Materials and Methods
2.1. Material
Nazilli irrigation D-25 drainage canal group, which forms the work area for this study, is located in Turkey’s west, in Buyuk Menderes basin. Work area is located at 61˚40' - 65˚00' east longitude, 41˚84' - 42˚02' north latitude (Figure 1). Total service area of studied D-25 drainage group is 1165 ha. Slope is in north-south, east-west direction and varies between 0.02 - 0.006. Rainfall shows an irregular distribution by months and seasons. The average annual rainfall is 608 mm. Downstream conditions of drainage system is limited by water surface elevation of Büyük Menderes. When the water surface elevation of Büyük Menderes is high, downstream conditions of drainage canals are not appropriate [4] . Effects which cause the need for drainage in the research area include rainfall, hillside waters, side streams, floods, irrigation and water leaks. Flow direction of water table complies in general with topographic slope. While soil in Nazilli irrigation area usually consists of alluvial soils; light textured soils form the majority. Because of this feature, permeability of the soil is high [5] . The main crops are cotton, corn, vegetables, alfalfa, wheat, citrus and fig.
Total length of the canals located in D-25 drainage group which has been chosen as study area is 29 + 342 km (24 km + 342 m). Properties of built drainage canals are given in Table 1 [6] . Drainage canal length per unit area is 25.1 m/ha. On the drainage channels, 3 bridges, 1 walkway, 1 gallery, 47 culverts have been built.
2.2. Method
In the study area, methods used for calculating sediment amount accumulated in investigated drainage canals and for determining the factors effecting sediment accumulation and drainage problem dimensions are given in Figure 2.
2.2.1. Drainage Systems in the Study Area
In order to determine the drainage systems in the study area, Land Classification Reports, Drainage Planning or Detailed Drainage Reports and Drainage System Projects of the areas chosen are initially provided. By examining drainage reports, canals and qualifications located in recommended drainage system are recorded. As a result of the investigation of drainage projects, which have been prepared based upon the drainage report, canal characteristics, its artistic structure and properties of anticipated drainage system are collected. By comparing drainage systems which have been formed as a result of the application and the ones recommended in the drainage report and drainage project; the differences between them are determined.
2.2.2. Identifying the Dimension of Sediment Problem
In order to determine the dimension of sediment problem in the drainage system, elevation controls were performed before or after mechanical cleaning in the canals. To determine the black elevations of the canal, levelers were built from the canals or different drainage groups with the interval of minimum 30 m up to 60 m. By com-
Figure 1. Nazilli irrigation D-25 drainage canal group.