7.6 Water Logging and Drainage

7.6 Water Logging and Drainage

Introduction to Water Logging and Drainage

Water logging is a condition where the soil profile in the root zone becomes saturated with water, either temporarily or permanently, due to a rise in the groundwater table or inadequate drainage of surface water. This condition is detrimental to agriculture, construction, and the environment. Effective drainage systems are engineered solutions to remove excess water from land surfaces (surface drainage) or from within the soil profile (subsurface drainage), thereby reclaiming waterlogged lands and preventing future occurrences. Understanding the causes, effects, and design of drainage systems is crucial for sustainable land and water management.

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1. Water Logging: Causes, Effects and Preventive Measures

1.1 Definition and Criteria

• Water Logging: A condition where the water table rises to such an extent that the soil pores in the crop root zone (typically top 1-2 meters) become saturated, severely inhibiting soil aeration.

• Critical Depth: The depth of the water table below the ground surface at which water logging is said to occur. This varies with soil type and crop, but generally, a water table within 1.0 to 1.5 meters of the surface is considered harmful for most field crops.

1.2 Causes of Water Logging

1. Over-Irrigation and Inefficient Water Management:

   • Applying water in excess of crop requirements.

   • Poor on-farm water management leading to deep percolation.

2. Inadequate Natural Drainage:

   • Flat topography with very low gradients preventing surface runoff.

   • Presence of an impermeable layer (hard pan, clay layer) close to the surface hindering vertical percolation.

3. Seepage from Water Bodies:

   • Seepage from canals, reservoirs, and rivers (especially if unlined) raises the adjacent groundwater table.

4. Obstruction of Natural Drainage Lines:

   • Construction of roads, railways, or embankments without adequate cross-drainage structures (culverts, bridges) blocks natural surface flow.

5. High Rainfall and Flooding:

   • Excessive rainfall in poorly drained areas.

   • Flooding from rivers, leading to surface ponding and infiltration.

6. Poor Soil Conditions:

   • Soils with low permeability (heavy clays) have very slow infiltration and internal drainage rates.

1.3 Effects of Water Logging

1. On Agriculture:

   • Reduced Soil Aeration: Oxygen deficiency in root zone inhibits root respiration and nutrient uptake.

   • Accumulation of Harmful Salts and Toxins: Anaerobic conditions produce compounds like ferrous oxide, hydrogen sulphide, and methane, which are toxic to plants.

   • Difficulty in Field Operations: Implements cannot be used on soggy land. Delays sowing and harvesting.

   • Reduced Crop Yield: Stunted growth, yellowing of leaves, and ultimately crop failure.

2. On Engineering Structures:

   • Reduced Bearing Capacity: Saturated soils lose shear strength, leading to foundation failures.

   • Increased Lateral Pressure: On retaining walls, basement walls.

   • Deterioration of Building Materials: Dampness leads to efflorescence and corrosion.

3. On Public Health:

   • Creates breeding grounds for mosquitoes and other disease vectors.

   • Contamination of drinking water sources.

1.4 Preventive and Reclamation Measures

1. Preventive Measures:

   • Efficient Irrigation Management: Adopt drip/sprinkler irrigation, use warabandi (rotational water supply), avoid over-irrigation.

   • Canal Lining: Line irrigation canals to reduce seepage losses.

   • Proper Land Leveling: Ensures uniform water application and prevents ponding.

   • Watershed Management: Increase infiltration and reduce surface runoff in catchment areas.

2. Reclamation Measures (Curative):

   • Installation of Drainage Systems: Both surface and subsurface drains to evacuate excess water.

   • Bio-Drainage: Planting deep-rooted, high water-consuming plants (e.g., Eucalyptus) to lower water table through transpiration.

   • Pumping: Direct pumping from wells to lower the groundwater table.

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2. Design of Surface and Sub-Surface Drainage Systems

2.1 Surface Drainage System

• Purpose: To remove excess water ponding on the land surface after heavy rainfall or irrigation, primarily through overland flow.

• Applicability: Most effective in areas with low soil permeability (clay soils) where subsurface drainage is difficult or where the primary problem is surface ponding.

2.1.1 Components and Design

1. Field Drains (Shallow, Small Channels):

   • Collect water from the field surface.

   • Gradient: 0.1% to 0.5%.

   • Cross-section: Small trapezoidal or V-shaped.

2. Lateral Drains:

   • Receive water from field drains.

   • Larger cross-section than field drains.

3. Main Drain (or Outfall Drain):

   • Collects water from all laterals and conveys it to a river, lake, or sea.

   • Design is based on peak surface runoff.

4. Design Discharge (Q) - Rational Method: $Q = \frac{C \cdot I \cdot A}{360}$ Where:

   • $Q$ = Peak runoff rate (m³/s).

   • $C$ = Runoff coefficient (dimensionless, depends on land use and soil).

   • $I$ = Rainfall intensity (mm/hr) for a design storm duration equal to the time of concentration.

   • $A$ = Catchment area contributing to the drain (hectares).

5. Hydraulic Design:

   • Use Manning's equation to size the drain cross-section (Area A, Hydraulic Radius R) for the design discharge Q and a chosen bed slope S. $Q = \frac{1}{n} A R^{2/3} S^{1/2}$

   • Ensure non-scouring velocity (typically 0.6 - 1.0 m/s for earthen drains).

2.2 Subsurface Drainage System

• Purpose: To control and lower the groundwater table by removing excess water from within the soil profile through subsurface flow.

• Applicability: Used where the primary problem is a high water table in permeable soils (sandy loams).

2.2.1 Types of Subsurface Drains

1. Horizontal Drainage:

   • Tile Drains: Perforated clay or concrete pipes laid in trenches, backfilled with gravel. Most common.

   • Mole Drains: Unlined circular channels formed in clay soil by a bullet-shaped implement. Temporary solution.

2. Vertical Drainage (Tube Wells):

   • Pumping from a network of tube wells to lower the regional water table. Effective but energy-intensive.

2.2.2 Design of a Tile Drain System (Hooghoudt's Equation)

The core design problem is to determine the spacing (S) between parallel tile drains to lower the water table to a desired depth.

1. Design Parameters:

   • $d$ = Required depth of water table below ground surface at midpoint between drains (m).

   • $D$ = Depth of impermeable layer below drain level (m). If deep, use equivalent depth (d_e).

   • $K$ = Hydraulic conductivity of the soil (m/day).

   • $q$ = Drainage coefficient: The steady-state rate of water removal required (m/day). It is the design recharge rate (from irrigation + rainfall - evapotranspiration).

2. Hooghoudt's Steady-State Equation for Spacing (S): For parallel drains resting on an impermeable layer: $S^2 = \frac{8K d_e h + 4K h^2}{q}$ For drains above the impermeable layer: $S^2 = \frac{4K}{q} (2d_e h + h^2)$ Where:

   • $h$ = Height of the water table above drain centerline at the midpoint between drains (m). $h = (water\ table\ depth\ at\ midpoint) - (drain\ depth)$.

   • $d_e$ = Equivalent depth to account for convergence of flow near the drain (from Hooghoudt's tables; $d_e ≤ D$).

3. Drain Depth and Diameter:

   • Depth: Typically 1.0 to 2.0 m below ground, depending on crop root zone and depth to impermeable layer.

   • Diameter: Based on required capacity (Q_drain). For a single drain: $Q_{drain} = q \times S \times L$, where L is drain length. Use pipe flow charts to select diameter ensuring non-silting velocity (~0.3-0.6 m/s).

2.2.3 Drainage Coefficient (q)

• The most critical and often empirically determined design input.

• Represents the rate at which water must be removed from the area.

• Typical Values:

  • Field Crops: 1-2 cm/day (0.01-0.02 m/day).

  • Horticulture: 2-3 cm/day.

  • Paddy Fields (during land preparation): 3-5 cm/day.

• It can be estimated as: $q = \frac{IR + R - ET}{Frequency}$, where IR=Irrigation, R=Rainfall, ET=Evapotranspiration.

2.3 System Layout

1. Random System: Drains laid only in wet spots. Irregular pattern.

2. Gridiron System: Laterals join the main drain at right angles. Common.

3. Herringbone System: Laterals join the main/sub-main at acute angles. Efficient for uniform sloping land.

4. Interceptor Drains: Placed upslope of an area to intercept subsurface flow.

Conclusion: Water logging is a complex land degradation issue stemming from hydrological imbalance. Its reclamation and prevention require a diagnostic approach to identify the primary cause (surface ponding vs. high water table). Surface drainage systems are designed to handle peak runoff events, while subsurface drainage systems are engineered for steady-state groundwater lowering based on soil properties and agricultural requirements. The successful implementation of an appropriate drainage system transforms unproductive, waterlogged land into fertile, arable soil, ensuring agricultural sustainability and protecting infrastructure.