Teknik Drainase 09

Agricultural Drainage Design and Management

The lecture explains how agricultural drainage is planned to prevent waterlogging and soil salinity, support crop growth, and manage excess water. It emphasizes that drainage design must account for soil and groundwater conditions, crop needs, construction and maintenance, and potential environmental effects.

Main Ideas

Why Agricultural Land Needs Drainage

• On flat land, slow infiltration can leave rainwater or irrigation water standing on the surface.
• Poor root-zone drainage can occur when water percolates through the soil more slowly than it enters.
• Excess water and salinity can harm crops, though crops differ in their tolerance of waterlogged conditions.
• Drainage can increase agricultural production and reduce some farming costs. However, poorly planned drainage can damage wetlands, alter landscapes, and lower water quality.

Improving Land and Soil Conditions

• Level and smooth the land, with a suitable, uniform slope, so excess surface water can reach drainage channels.
• Use subsoiling—loosening soil below the surface or creating channels through compacted layers—to improve soil structure, porosity, and hydraulic conductivity.
• Use subsurface drainage to help control groundwater levels, particularly where natural drainage is inadequate.

Investigations Needed Before Design

• Begin with land and hydrological surveys to identify areas prone to inundation and characterize their conditions.
• Use topographic information and, for larger projects, thematic maps and a geographic information system (GIS).
• Gather soil, hydrological, and climate data; assess environmental and socioeconomic conditions; and plan to monitor project performance.
• The lecture gives indicative survey guidance of map scales from 1:50,000 to 1:25,000 and 1–4 observations per 100 hectares. Survey density should be adjusted to the complexity of the problem.
• Soil investigations should identify landscape position, salinity, internal drainage, restrictive layers, soil stratification, and the transmissivity or resistance of relevant soil layers.

Soil Hydraulic Properties

• Important design properties include saturated hydraulic conductivity, transmissivity, and the pore space that can actually drain.
• Soil texture strongly affects drainage. Coarse materials generally drain more readily, while heavy clay has smaller drainable pores and lower conductivity.
• Hydraulic properties may be estimated from existing soil data or measured in the field. Field methods use known flow conditions and discharge observations, with calculations based on Darcy’s law.
• Because field variability is expected, designs often rely on representative or approximate values rather than exhaustive measurements.

Groundwater and Leaching

• Hydrological studies establish natural groundwater conditions and estimate how groundwater levels respond to drainage and recover after drainage stops.
• Where salts or other harmful substances accumulate, designers should estimate long-term leaching needs. Rainfall, irrigation, and plant water use are among the factors considered.
• Drainage is intended not only to remove surface water, but also to manage subsurface water and the soil’s water-and-salt balance.

Drainage Systems and Design Process

1. Choose the System Type

• Subsurface drainage may use perforated pipes to collect water from the soil.
• Systems may be single (singular) or composite, and may include pumping where gravity drainage is insufficient.
• Drainage can be arranged horizontally through pipe networks or vertically through wells.
• Mole drainage creates underground channels and is discussed as an option for heavy clay soils.
• The lecture also describes a sheet-formed pipe technique, installed by pulling equipment through the soil rather than digging a conventional trench.

2. Determine Design Parameters

Consider:

• Drain depth and spacing
• Pipe diameter and material
• Slope and outlet level
• Soil-layer thickness
• The depth of the water table and restrictive layers

Wider spacing can reduce installation costs, but may also reduce drainage performance. Spacing must therefore balance construction, operation, and maintenance costs against the required drainage rate.

The appropriate design depends on soil conductivity and transmissivity, crop tolerance of waterlogging, rainfall, and the desired groundwater level. The lecture refers to drainage coefficients and hydraulic-head conditions as inputs to design calculations. It also names different equations for layered soils, heavy clay, and barrier drainage on sloping land.

3. Plan Surface Drainage

• Surface drainage removes ponded rainfall or irrigation water.
• It requires suitable land grading and shallow channels or ditches that convey water to collectors without causing erosion or sedimentation.
• Layouts may use parallel ditches or furrows.
• The lecture gives a typical surface-ditch depth range of about 0.6–1 m, while noting that design depends on terrain, rainfall, and crop tolerance for temporary inundation.
• For new agricultural development, drainage should be planned alongside irrigation.

4. Design Collectors, Outlets, and Supporting Infrastructure

• Subsurface laterals usually discharge into collector channels or pipes, which connect to main channels and an outlet.
• Main channels may be open. Outlets may rely on gravity, tidal or water-control gates, or pumps.
• On low-lying or flood-prone land, embankments and intercepting channels can protect the agricultural area and limit incoming surface flow or seepage.
• Channel dimensions and discharge are calculated using hydraulic design principles. The lecture specifically mentions Manning’s equation for open-channel flow.
• Access roads and structures such as culverts, drops, outlets, gates, and pumps may be needed for operation and maintenance.

5. Maintain the System

• Main channels should follow natural drainage routes where practical, and existing watercourses may be adapted with minimal changes.
• Preserve natural channel configurations and vegetation where possible, and use smooth bends rather than sharp ones.
• Maintenance should involve appropriate public oversight, water-user associations, and farmers responsible for tertiary channels.

Environmental Management

Drainage water can carry salts, fertilizers, pesticides, sediment, and potentially toxic elements into surface water or groundwater. Subsurface drainage may reduce sediment and some phosphorus or organic-nitrogen losses, but can increase nitrate and salt losses.

Possible management measures include:

• Reducing pollutant inputs and improving fertilizer and pesticide efficiency.
• Improving on-farm water management and reducing unnecessary percolation.
• Reusing drainage water, potentially for salt-tolerant crops or forest plants. Mixing it with fresh water is another option.
• Treating or managing water before discharge, including considering evaporation ponds or engineered wetlands.
• Avoiding practices that could contaminate groundwater and assessing environmental impacts before and during operation.

Projects should include environmental assessment and ongoing monitoring of hydrological changes, water quality, and social and economic outcomes.

Speakers and Sources

• Speaker: One lecturer or instructor, unnamed in the subtitles.
• Materials and sources mentioned: A class chapter on agricultural drainage from the CIGR Handbook of Land and Water, and a secondary drainage textbook or book that is not identified by title.
• Technical references named in the lecture: Darcy’s law, Manning’s equation, MODFLOW, and equations attributed in the subtitles to Hooghoudt/Ernst, Boussinesq, and Glover–Dumman.