Soil and Water Conservation Structures Design

Soil and water conservation is essential to tackle the global challenge of soil erosion, negatively impacting food productivity, water security, and environmental quality. This chapter traces the history of soil erosion and introduces the principles of soil and water conservation. The types of soil erosion and their causes are discussed. The chapter highlights the on-site and off-site effects of soil erosion, and the importance of soil and water conservation measures. It presents the soil and water conservation measures developed and practised under various initiatives worldwide and in India. The chapter introduces various agronomic and biological, and engineering or mechanical measures adopted for soil and water conservation. The chapter also includes the structure of the book to guide prospective readers.

Water erosion encompasses the detachment of soil particles primarily by raindrops and flowing water and their transport by runoff. Comprehending the mechanics of water erosion is essential to developing measures to control erosion. This chapter describes the principal types of water erosion, viz., raindrop splash erosion, sheet erosion, interrill erosion, rill erosion, gully erosion, tunnel erosion, and streambank erosion, and explores the mechanics of each type. The chapter also describes various agronomical and biological measures employed to control water erosion. It also introduces popular engineering or mechanical erosion control measures like terracing, bunding, vegetated waterways, and gully control structures.

The soil loss estimated using soil erosion models is vital in evaluating the existing soil conservation practices and identifying priority areas and appropriate measures to control erosion. This chapter presents various soil erosion modelling and measurement techniques for soil loss assessment. The Universal Soil Loss Equation (USLE), an empirical modelling approach, is introduced along with its factors: rainfall erosivity, soil erodibility, slope length-gradient, land cover and management, and soil conservation practice factor. Also, the Modified USLE (MUSLE), which has a runoff factor in place of the rainfall factor, and the Revised USLE (RUSLE), which includes several process-based concepts, are discussed. The chapter introduces the Water Erosion Prediction Project (WEPP) and the European Soil Erosion Model (EUROSEM), the distributed, physically-based soil erosion models that can simulate soil loss under diverse land uses and hydrologic conditions. Also, the Soil Conservation Service (SCS) curve number method and the rational method used for estimating the runoff volume and peak runoff rate are included. The chapter discusses the soil loss measurements from runoff plots. The different size plots are discussed along with commonly used devices, namely the multislot divisor and Coshocton wheel.

Terraces are the most widely practised soil erosion control measure around the world. The practice consists of earth embankments constructed across the steep slopes to intercept surface runoff and divert it at a non-erosive velocity to a safe outlet or store it to enhance soil infiltration. This chapter broadly categorises terraces into common (or normal) terraces and bench terraces. The chapter presents the design of common (or normal) terraces in terms of their spacing, grades, length, and cross-section. The design of bench terraces includes spacing, bench width, cross-section, and length, besides the volume of cut and fill or earthwork and area lost under them. The chapter also contains the terrace system planning, including its location, layout, and maintenance. The design procedures are demonstrated through solved examples.

Bunds are among the most common mechanical measures of erosion control. These consist of small embankments constructed across the land slope to reduce the slope length, runoff, and soil erosion and enhance soil infiltration. This chapter presents a broad classification of bunds. It includes the common design considerations for contour and graded bunds like storm frequency, spacing, side slopes, freeboard, and seepage through them. The chapter elaborates on the design of contour and graded bunds in terms of their height, cross-section, length, the volume of earthwork, and area lost under them. The chapter also contains the planning considerations and construction of bunds. The design procedures are demonstrated through solved examples.

Vegetated waterways are natural or constructed channels having vegetative cover to dispose of runoff safely without causing erosion. These waterways are designed using the permissible velocity approach and constructed along the natural slope. This chapter presents the preliminary design considerations for vegetated waterways and elaborates on the design processes to decide the size, shape, vegetation, permissible velocity, and roughness coefficient. Solved examples are included to demonstrate the design procedure. The chapter also contains the layout, construction, and maintenance of the waterways.

Gully control structures, i.e., the check dams, have been used since the twelfth century for soil and water conservation and more frequently over the past 150 years. These are employed in severely eroded gullies that cannot be managed with biological or vegetative erosion control measures. The temporary or permanent structures are constructed across the gully to reduce the channel gradient and stabilise the gully to prevent further erosion. This chapter presents the design principles used in designing temporary gully control structures, i.e., different check dams, preferred in areas where labour is inexpensive and the appropriate construction materials are readily available. The design includes the number of structures, spacing between structures and a spillway to handle the peak runoff due to a 10-year return period storm. Subsequently, the chapter introduces three established permanent gully control structures, i.e., the drop spillway, drop-inlet spillway and chute spillway, preferred in medium to large gullies with significantly high flows that the temporary structures cannot handle. The hydrologic, hydraulic and structural design principles of the permanent structures are introduced. The chapter also includes the prerequisites, viz., the specific energy considerations, critical flow characteristics and hydraulic jump, for designing permanent structures.

Drop spillway, one of the most widely used soil conservation structures, is primarily used for controlling and stabilising grades in a gully. The chapter focuses on the hydrologic, hydraulic and structural designs of drop spillways. The hydrologic design approaches for estimating the peak flow rate, i.e., the rational method, empirical or frequency factor method of frequency analysis and the hydrological or hydraulic modelling, are discussed. The hydraulic design of straight and box-inlet drop spillways under free and submerged flow conditions is presented. This chapter also includes the critical depth concept and its application in determining the dimensions of various components of the straight and box-inlet drop spillways. The structural design contains the analysis of the horizontal forces acting against the structure due to the hydrostatic pressure of the water column upstream and the earth pressure caused by the backfill. It also comprises the uplift pressure caused due to water seepage through the saturated foundation material. A detailed procedure to analyse the stability of the structure against overturning, sliding, piping, tension, and compression or contact pressure is demonstrated through a solved example.

Drop-inlet spillway, a widely used soil conservation structure, is preferred for sites providing substantial temporary storage above the inlet, especially in gullies having more than 3 m fall or drop. The chapter focuses on the hydraulic design of two general types of drop-inlet spillways, the first having a circular or rectangular box-type flat crest and the second having a standard or funnel-shaped crest, the latter popularly known as ‘morning glory’ or ‘glory hole’ spillway. It discusses the typical head-discharge relationships of the structure, controlled by its various components, besides the composite head-discharge relationship. The pressure distribution in various components of a drop-inlet spillway, essential for determining the hydraulic loading to ensure safety against cavitation, is discussed. The chapter mainly focuses on designing the standard-crested and the flat-crested drop-inlet spillways under specific discharge and pressure conditions. The design includes computing the water surface profile in the conduit and developing the composite head-discharge relationship. The complex computations involved in the design are demonstrated through solved examples.

A chute spillway, also called a trough spillway, is designed to dispose of surplus water from upstream to downstream through a steeply sloped open channel. The chapter describes the functions of the various components of a chute spillway and presents the hydrologic, hydraulic, and structural designs of chute spillways. The hydraulic design of the entrance or approach channel, inlet or control structure, chute channel or discharge carrier and outlet or energy dissipater is presented. The structural stability is analysed considering the weight of the structure and the uplift pressure created due to the differential head between the upstream and downstream. A detailed procedure to analyse the stability of the structure against overturning, tension, and compression is demonstrated through a solved example.

Wind erosion is a serious environmental hazard, which causes land degradation and air pollution and adversely affects human health. Dust emission generated by wind erosion is the most prominent aerosol source that directly or indirectly influences the global radiation balance. The chapter presents the factors influencing wind erosion and describes the mechanics of soil particle movement in wind erosion. The Wind Erosion Equation (WEQ), the first empirical wind erosion model for estimating the annual soil loss, and its revised version, the Revised WEQ (RWEQ), are discussed. A few popular process-based wind erosion models are introduced. The basic principles adopted for controlling wind erosion are presented. The chapter also describes the benefits of windbreaks and shelterbelts, two popular mechanical measures of wind erosion control. The design of the windbreaks and shelterbelts is discussed in terms of their height, length, continuity, density, orientation, and number of rows and plant species.

Earthen embankments are used extensively for impounding and diverting water for irrigation. Similarly, farm ponds are prevalent water harvesting structures that store excess runoff from the catchment to serve multiple purposes, like supplemental irrigation, fish culture, duck farming, and livestock consumption. Section 12.1 of this chapter presents various types of earthen embankments. The design of earthen embankments is discussed in terms of their height, side slopes, top width, freeboard, and settlement allowance. The chapter includes the procedures to determine the seepage through the body of the earthen embankments and ways to control it. The possible causes of the failure of the earthen embankments are also discussed. Section 12.2 of the chapter presents a broad classification of farm ponds into two types: embankment type and excavated farm ponds. The design of farm ponds includes the capacity, shape, and dimension of the pond, besides the inlet channel and spillway and the outlet or waste weir. The chapter also contains farm ponds construction, operation, and maintenance. Sealing of farm ponds for reducing seepage and percolation losses is discussed. The chapter includes solved examples to demonstrate the design procedures for both earthen embankments and farm ponds.

Information on the extent and spatial distribution of soil erosion is essential for planning and implementing soil conservation measures. Remote sensing (RS) and Geographic Information System (GIS) concepts and tools are widely adopted in natural resources mapping, monitoring, and modelling. This chapter introduces the basic concepts of the RS and GIS and their applications in soil conservation. RS data acquisition and GIS software are particularly emphasised. The chapter includes the RS and GIS applications in land degradation, soil erosion mapping, and parameter estimation of various soil loss estimation models. The site selection for soil conservation structures is also discussed.

Climate change and land use land cover (LULC) changes are recognised as two of the most significant causes of environmental change. Climate change and LULC changes are related to one another. Land use change may drive climate change, and a changing climate may result in land cover changes. Climate change and LULC changes are believed to influence soil erosion. This chapter analyses the impacts of climate and LULC changes on soil erosion. The causes and effects of climate change on precipitation, temperature, solar radiation, atmospheric CO₂ concentrations, and radiative forcing are discussed. The chapter includes the impacts of climate change on soil characteristics, vegetation cover, runoff, floods, and droughts and extends the impacts of these changes on water and wind erosion. The chapter explores the human alterations of LULC changes in terms of changes in the forest cover, alterations in agricultural lands, increase in urban areas, and decrease in wetland areas. The influence of the LULC changes on soil erosion and sediment production processes is discussed. Also, the combined impact of climate and LULC changes on soil erosion is explored, and mitigation strategies like sustainable land management practices and appropriate policy incentives to conserve soil are discussed.