Lab report on Subsurface Drainage Design

Introduction of Subsurface Drainage Design

The subsurface drainage design is used to remove the excess ground water that is located below the ground surface. The system depends on the spacing and depth of the drain tubes with the lateral hydraulic conductivity of the soil layers. The system is based on the size of pipes and rate of water removal from the field. The size of the drainage pipes is based on the land and slope.

Objectives of Subsurface Drainage Design

Calculate drain spacing for pattern-drained fields using steady-state and transient flow methods.

Calculate required pipe sizes.

Procedure of Subsurface Drainage Design

Calculate the drain spacing required for a 32-ha field (400 m by 800 m). The soil is a silty clay loam with an impermeable layer at a depth of 2.5 m. The drains should be installed 1.2 m below the soil surface. The drainage coefficient should be 13 mm/day. Use 114 mm pipe for the laterals. The highest point of the water table should be 0.3 m below the soil surface. The soil hydraulic conductivity is 22 mm/hr. 

Analysis of Subsurface Drainage Design

area of drain spacing =  32 field 

soil type=  silty clay loam 

depth =d  =  2.5 m 

drain installation depth=m =  1.2 m 

drainage coefficient =13 mm/day  =  0.3 m/d

pipe size for laterals =   114 mm 

highest point =0.3 m 

hydraulic conductivity of soil=K_e   = 22  ( mm)/( hr)  =  0.5 m/

Soil profile of Subsurface Drainage Design

0 - 0.20 m below soil surface K = 10.0 m/d

0.20 - 1.20 m below soil surface K = 0.35 m/d

1.20 - 6.20 m below soil surface K = 0.35 m/d

Below 6.20 m, the soil is considered impervious K = 0.70 m/

In the present condition, the highest point of water table is 0.3 m and drains installation is 1.2 m therefore K = 0.35.

drain spacing of pipe =    S_d  =[(4  K_( e)  m  (2 d_( e)  +   m))/DDR]^( 1/2)

On the basis of iteration, we calculated the required drain spacing and equivalent depth, de, by using the Hooghoudt equation. The number of iterations and solutions (S and de) are mentioned in table 2 below, 

Depth of pipe S_d  =[(4  K_( e)  m  (2 d_( e)  +   m))/DDR]^( 1/2)

0.2 3.577709

0.3 3.794733

0.4 4

0.5 4.195235

0.6 4.38178

0.7 4.560702

0.8 4.732864

0.9 4.898979

1 5.059644

1.1 5.215362

1.2 5.366563

Then sketch the system and calculate the required spacing using the van Schilfgaarde transient flow equation. Assume that you will be growing valuable crops and wish to lower the water table from the soil surface to a depth of 0.4 m in 24 hours. The drainable porosity of the soil is 0.05.

van Schilfgaarde transient flow= S =[(9 K t d_e)/(f^'  [ln⁡〖m_0 〗   (2〖 d〗_( e)  + m)  - ln⁡〖m  ( 2 d_e  +m_0 )〗  ] )] 

hydraulic conductivity of soil=K_e   = 22  ( mm)/( hr)  =  0.5 m/d

t  =  24 hours =  1 day 

equivalent depth  =  d_e  = 0.2 -  1.2 

initial depth =  m_( 0 )  =   0.4 m 

m =height of water table after initial feet =  0.2

f^'  =   drainable porosity  =   0.05 

de S =[(9 K t d_e)/(f^'  [ln⁡〖m_0 〗   (2〖 d〗_( e)  + m)  - ln⁡〖m  ( 2 d_e  +m_0 )〗  ] )]

0.2 24.39765

0.3 30.80766

0.4 35.46677

0.5 39.00615

0.6 41.78617

0.7 44.02752

0.8 45.87295

0.9 47.41884

1 48.73264

1.1 49.86298

1.2 50.84578Size of the pipe in the scenario is measured as 

d =  (π L/8)/ln⁡〖 (L/(π〖 r〗_0 )  +  F(x))〗

Additional problem:

Length of the pipe =  970 m 

number of wet spots  =3 

line drain of soil  =15 m 

drainage coefficient  =  13 mm/hr     =  0.3 m/d

de =  15 m 

de  =[d/(1 +  d/S_d    [8/π  〖ln 〗⁡〖(( d)/〖 r〗_( e) )   -  3.4 〗 ]  )]

de  =[(15  m)/(1 +  970/(0.3 )   [8/π  〖ln 〗⁡〖(( 15)/15)   -  3.4 〗 ]  )]  =  0.0013