DRAINAGE OF IRRIGATED LANDS

Transcription

1 CVE 471 WATER RESOURCES ENGINEERING DRAINAGE OF IRRIGATED LANDS Assist. Prof. Dr. Bertuğ Akıntuğ Civil Engineering Program Middle East Technical University Northern Cyprus Campus CVE 471 Water Resources Engineering 1/19

2 Overview Introduction Sources of Excess Water Surface Drainage Mc Math s Method Rational Method Subsurface Drainage Steady State Subsurface Drainage Unsteady State Subsurface Drainage CVE 471 Water Resources Engineering 2/19

3 INTRODUCTION Through the ages of civilization, people have tried to improve the filed conditions when ever there were problems related to drainage. Recently, drainage facilities were proposed for Highways Low-lying areas Parking lots Airports Urban areas Storm water systems. This chapter deals only with the drainage of irrigated lands. The removal of excess water which exists either on the surface of the soil or in the root zone. CVE 471 Water Resources Engineering 3/19

4 Overview Introduction Sources of Excess Water Surface Drainage Mc Math s Method Rational Method Subsurface Drainage Steady State Subsurface Drainage Unsteady State Subsurface Drainage CVE 471 Water Resources Engineering 4/19

5 Sources of Excess Water The main source of excess water: the seepage losses from reservoirs or canals, operational wastes in irrigation systems, surface runoff losses deep percolation losses etc. Drainage is a necessary operation irrespective of how the irrigation water is perfectly applied in the filed since excess storm water should also be removed from the area. CVE 471 Water Resources Engineering 5/19

6 Overview Introduction Sources of Excess Water Surface Drainage Mc Math s Method Rational Method Subsurface Drainage Steady State Subsurface Drainage Unsteady State Subsurface Drainage CVE 471 Water Resources Engineering 6/19

7 Surface Drainage Mc Math s Method The peak drainage discharge, Q p Q p = C i S 1/5 A 4/5 where C: runoff coefficient (C = C 1 + C 2 +C 3 ) i : the rainfall intensity (mm/hr) S : 1000 x S 0 (where S 0 is the bed slope of drainage channel) A : area to be drained. CVE 471 Water Resources Engineering 7/19

8 Surface Drainage Mc Math s Method The rainfall intensity can be determined from the rainfall-intensityduration frequency curves of a neighboring station with the known time of concentration, t c. For small agricultural watersheds having areas less than 50 ha, Kirpich proposed an equation for t c as: t c 3 L = H where t c : the time of concentration (min). L: the length of the drainage canal (m). H : the elevation difference between upstream and downstream ends of the canal (m) CVE 471 Water Resources Engineering 8/19

9 Surface Drainage CVE 471 Water Resources Engineering 9/19

10 Surface Drainage Rational Method This method is applicable for small basins having rolling terrain A < 8 km 2 Duration of P eff > t c The peak flow rate: Q p = 1/(3.6) x CiA C from Table 11.2 i from the rainfall intensity-duration-frequency curve for known t c. CVE 471 Water Resources Engineering 10/19

11 Overview Introduction Sources of Excess Water Surface Drainage Mc Math s Method Rational Method Subsurface Drainage Steady State Subsurface Drainage Unsteady State Subsurface Drainage CVE 471 Water Resources Engineering 11/19

12 Subsurface Drainage The preliminary study for the solution of any drainage problem is to obtain information concerning the elevation of groundwater table (GWT) relative to soil surface, the characteristics and texture of soil topography, etc If GWT is close to the root zone, it restricts the air entrainment through the pores and ventilation is not allowed. The elevation of GWT can be decrease by pumping out the water from closely spaced observation wells. However, drilling and operating wells is expensive. Similarly, the construction of open ditches is not recommended because the net effective area for cultivation is reduced and farming instrument cannot be operated effectively. CVE 471 Water Resources Engineering 12/19

13 Subsurface Drainage Instead subsurface drains are utilized. Perforated plastic drains are effectively used. Selection of suitable depth and spacing is very important. Giving suitable slopes permit sufficient self cleansing. Drains are connected to an interceptor. CVE 471 Water Resources Engineering 13/19

14 Subsurface Drainage In the selection of proper depth and spacing for subsurface drains the followings must be considered jointly. the soil permeability the type of crop irrigation method loads exerted by soil, agricultural instruments, and animals CVE 471 Water Resources Engineering 14/19

15 Subsurface Drainage Steady State Subsurface Drainage L can be obtain under the following assumptions: The soil is homogeneous with a constant hydraulic conductivity, K. The drains are evenly spaced a distance L apart. The hydraulic gradient at any point is equal to the slope of the water table above the point. Darcy s Law is valid for flow of water through soils An impermeable layer underlies the drain at a depth D. Rain falls or irrigation water is applied at a constant rate q. The origin of coordinates is taken on the impermeable boundary below the center of one of the drains. CVE 471 Water Resources Engineering 15/19

16 Subsurface Drainage Steady State Subsurface Drainage Donnan s Equation q x = K A i where q x : flow contribution to the drain in x-direction K : the hydraulic conductivity of the soil. A : the flow area i : the hydraulic gradient (dy/dx) If a linear variation is assumed: CVE 471 Water Resources Engineering 16/19

17 Subsurface Drainage Steady State Subsurface Drainage Donnan s Equation CVE 471 Water Resources Engineering 17/19

18 Subsurface Drainage Steady State Subsurface Drainage Hooghoudt s Equation Assumed that the flow is radial and it contributes to the drain within a certain depth, equivalent depth, d e. CVE 471 Water Resources Engineering 18/19

19 Subsurface Drainage Steady State Subsurface Drainage Hooghoudt s Equation CVE 471 Water Resources Engineering 19/19

20 Subsurface Drainage Steady State Subsurface Drainage Hooghoudt s Equation CVE 471 Water Resources Engineering 20/19