Effluent Salinity of Pipe Drains and Tube-Wells



A case study from the Indus plain
Kelleners, T.J. 2001
PhD Thesis Wageningen University
Alterra ILRI, Wageningen, 151 p
Price: No costs

This study, a case from the Indus plain in Pakistan, has been completed with the release of the PhD Thesis of Dr. T.J. Kelleners. In the thesis, numerical models based on the Darcy equation and the mass balance equation for water flow and the advection-dispersion equation for solute transport have been used to predict the effluent salinity of pipe drains and tubewells at filed level. A new modelling approach is presented that combines the one-dimensional vertical finite-difference SWAP model for the variable saturated zone with a solute impulse response function for the saturated zone and the finite-element model SUTRA is used to study the behaviour of skimming wells and pipe drains in fresh-saline groundwater systems. These model approaches are applied to experimental pipe drainage sites in Haryana, India and the Punjab, Pakistan. Results show that the effluent salinity of pipe drains and tubewell drains changes only gradually with time due to the low percolation from the irrigated fields and due to the large quantities of salts stored in the groundwater. Areas with relatively high percolation and shallow depth of the impermeable layer (the pipe drains at Sampla) still require 10 years before the effluent salinity has reduced to equilibrium levels. In contrast, desalinisation of the rootzone generally takes only 1-3 years. The implication is that farmers will benefit quickly from the installation of a drainage system. However, for the safe use and disposal of the effluent, long-term solutions are required. Furthermore, the model simulations show that pipe drains yield better effluent quality (EC of 1.2-1.3 dSm-1) compared to skimming wells (EC of ~1.7 dSm-1). This better effluent quality, however, must be evaluated against the considerably higher installation costs of pipe drains.

Project: Drainage of Marginal Lands

Alterra, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands
PS Konsultants, Kuching, Sarawak, Malaysia (ADCOPS)
University of Palangka Raya, Kalimantan, Indonesia (STRAPEAT)
Aims To develop methodologies and training modules for the drainage of so-called problem soils, i.e. peat and acid sulphate soils.
Duration 2002 – 2004

Description
The study is based on the joint projects in which ILRI is participating in the coastal lowlands of Southeast Asia and West Africa. The projects are mainly focused on the water management in areas with peat or acid sulphate soils. In peat lands controlled drainage is needed to reduce subsidence by oxidation and to reduce the release of CO2 to the atmosphere. In areas with acid sulphate soils the main aim is to reduce the acidification of the rootzone. Among others, experiences and data collected in the following projects is used in the study:

ADCOPS: Agricultural Development in Coastal Peat Swamps of Sarawak, Malaysia
STRAPEAT: Strategies for Implementing Sustainable Management of Peatlands in Borneo, Indonesia and Malaysia
Acid Sulphate Soils in the Humid Tropics, Kalimantan, Indonesia
Western Johore Integrated Agricultural Development Project: peat soil management study, Johore, Malaysia
The output of the project will include, among others, development of a model to simulate the hydrology and water management in tropical peatlands, a ILRI publication on drainage of coastal peat swamps in the humid tropics, training modules on drainage of peat and acid sulphate soils and participation in the ICID Working Group “Sustainable Development of Tidal Swamps and Estuaries ”

The ILRI staff involved in this project is Henk Ritzema (Project Leader), Frank van Berkom, Rien Bos, Rob Kselik and Jacob Vos.

ATER MEASUREMENT WITH FLUMES AND WEIRS

A.J. Clemmens, T.L. Wahl, M.G. Bos, and J.A. Replogle
ILRI Publication 58
ISBN 90 70754 55 X

To improve water management, we recommend that water-measuring capability be included in all new water projects and that existing water projects be retrofitted for water measurement as soon as possible. Usually, water measurements should be planned at all points where it can be reasonably established that knowledge of the flow rate will affect management decisions. Thus, water measurements should be provided at all bifurcations or divisions of flow within a canal distribution system, at all delivery outlets, and in the stream or river from which water is diverted.

For open-channel flow measurements we recommend the modern structures described in this book, which are all members of the family of long-throated flumes. This family of devices includes broad-crested weirs with a streamlined flow contraction. The primary advantage of using flumes and weirs to measure water flow is the theoretical predictability of their hydraulic performance. The long-throated flumes and broad-crested weirs, especially, have several major advantages over all other known weirs and flumes (e.g. Parshall flumes, cutthroat flumes, H-flumes, sharp-crested weirs, and so on). These advantages are:
Provided that critical flow occurs in the throat, it is possible to calculate a rating table with an error of less than 2% of the listed discharge. The calculation can be made for any combination of prismatic throat and arbitrarily shaped approach channel.
The throat, perpendicular to the direction of flow, can be shaped in such a way that the complete range of discharges can be measured accurately.

Minimal head loss over the weir or flume is required to ensure a unique relationship between the upstream sill-referenced head, h1, and the discharge, Q.
This head-loss requirement can be estimated with sufficient accuracy for any of these structures placed in an arbitrary channel.
Because of their gradual converging transition, these structures have little problem with floating debris.
Field observations and laboratory tests have shown that these structures can be designed to pass sediment transported by open channels with sub-critical flow.
Provided that the throat is horizontal in the direction of flow, it is possible to compute a rating table with post-construction dimensions. Thus, it is possible to produce an accurate rating table even if the flume is not constructed to the design dimensions. It is also possible to reshape the throat as needed, according to changing site conditions, and compute a new rating table with the modified dimensions.
Under similar hydraulic and other boundary conditions, long-throated flumes and broad-crested weirs are usually the most economical of all structures for accurately measuring open-channel flows, provided that conditions are such that a weir or flume is feasible.
Because of these advantages, long-throated flumes and broad-crested weirs are useful for various flow-measurement applications, particularly when the structure must have a minimal impact on existing flow conditions and water-surface elevations.

This publication integrates material from the 1984 book Flow Measuring Flumes for Open Channel Systems and the 1993 book FLUME: Design and Calibration of Long-Throated Measuring Flumes, which introduced the first interactive flume design software. The new Microsoft Windows version of FLUME (WinFlume) is presented in Chapter 8. This computer program can be used to develop the hydraulic design of long-throated flumes and broad-crested weirs to be constructed in user-specified channels, satisfying user-specified boundary conditions and design requirements. The program also determines the head versus discharge calibration (the rating) of newly designed structures and existing structures. WinFlume can be downloaded from www.ilri.nl/winflume/winflume.html.

Long-throated flumes and broad-crested weirs have evolved significantly in recent years, and several of them are now commercially available. Chapter 3 discusses new construction techniques and the structures that are on the market, and contains illustrations of many successful installations from around the world.

The range of applications of these flumes and weirs is unlimited. The authors hope that this book will contribute to the effective management of one of Earth’s most widely needed, intensively used, and extensively wasted natural resources: water.

 
 

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