There is a considerable choice of types of hydraulic structures, and deciding which particular one to adopt will largely depend on the uses it will be put to, and on the overall conditions of the area where it will be installed. In choosing a structure type, the on-site availability of building materials and the skills and experience of local workers should also be verified, with an eye to future maintenance requirements.
The specific characteristics of each structure type should be taken into account to select a structure that meets the demands and conditions of the particular site under consideration. Therefore, it will be useful to provide a general classification of hydraulic works before moving on to the analysis of the site selection procedure. In this chapter, use and hydraulic design are introduced as two general criteria for the classification of hydraulic works. The following chapters will mainly focus on hydraulic works that are commonly used in developing countries.
Once the type of hydraulic structure has been selected, further inquiries will allow us to decide about its feasibility and design. The investigation procedure is described in the final part of this chapter.

An extensive analysis of runoff calculation procedures does not fall within the scope of this manual, yet it will be useful to provide a general description of the hydrologic methods used to estimate a basin's runoff features. This chapter also illustrates the main procedures used to evaluate watershed features and methereologic data, required in the application of hydrologic methods.
When projecting a hydraulic work it is necessary to know the runoff features of the concerned watershed (e.g. runoff coefficient, annual flow rate and maximal runoff rate). Unless the basin has already been gauged, these runoff characteristics will be unknown, in which case we will have to refer to data collected on similar gauged basins in order to determine the runoff characteristics of the watershed. Runoff features depend on several basin features (e.g. basin shape, watershed relief and drainage pattern) and on local rainfall characteristics. The soil's nature in the drainage area will also affect the watershed's reaction in the event of a rainstorm. In hydrology, there are several ways to classify watersheds on the basis of the above mentioned characteristics. These hydrologic methods of classification are based on wide-ranging investigations done on several gauged watersheds. On the basis of data collected during runoff events in gauged basins, we can establish the relation between runoff features and various watershed characteristics (e.g. basin shape and surface, soil features...).
For these classification methods to be applied, various kinds of data on basin characteristics and rainfall features should be available beforehand. This chapter provides a general description of the watershed features required in the application of these classification methods. Some of the principal hydrologic methods used for estimating runoff features (rational, U.S. S.C.S. and Orstom) will also be briefly described. The first paragraph provides the definition and delineation of a watershed. The second paragraph describes the morphological features required to classify watersheds according to one of the methods mentioned above. Soil features inquiries are illustrated in the third paragraph. The fourth paragraph deals with the rainfall features required for hydraulic work design. The main hydrologic methods to assess runoff characteristics are briefly described in the fifth paragraph. The last two paragraphs concern other factors that interest the investigation phase, such as erosion, solid transportation and evaporation.
A GIS tool with the right kind of appliances can significantly enhance investigations on basin geomorphology. Moreover, the utilisation of satellite images will require the adoption of specific software.

The spillway is a very important component of any hydraulic work. The spillway crest's main function consists in fixing the maximum water level upstream the structure, and preventing water overflows. The spillway must be accurately dimensioned, so that it can evacuate the design flow calculated for a hydraulic work.
The main procedures used for designing the structures that generally compose a spillway (e.g. spillway channel, weirs) are briefly illustrated in this chapter, with reference to both hydraulic theory and stability computation procedures. These procedures are taken from elaborate theories, which, for the sake of simplicity, will be only briefly mentioned here. For an in-depth treatment of these theories, readers should refer to specific publications.
In natural streams, the total hydraulic energy is uniformly dissipated along the streambed. However, if a small dam or weir is built, the energy dissipation on the dam's upstream side results substantially lower than it would be in natural conditions and the potential energy level is therefore high. When this high hydraulic energy is dissipated downstream the structure, it could cause serious scour problems in the streambed, unless the rise in energy created by the structure's installation is dissipated immediately beyond the structure. This can happen naturally, if the characteristics of the streambed in question allow it, or artificially, with the creation of a 'stilling basing'. Here, the water flowing throughout the spillway must lose a portion of its total energy so as to reach a lower energy level downstream, i.e. a level equal to the one it would have had in the absence of the dam or weir.
The energy dissipation that takes place as a result of the construction of a hydraulic structure can give rise to important erosion phenomena in the streambed. Locally, this will threaten the structure's stability.
Downstream, it will scour the river's bed for a long reach. Therefore, avoiding the negative consequences of energy dissipation is one of the principal problems to be dealt with when designing a hydraulic work. A common way to solve this problem consists in concentrating the energy in a circumscribed area, called 'stilling basin'. For the importance of its function, this area should be carefully designed and realised.

There is hardly a chance that a well-designed hydraulic work resists to runoff stresses for a long time if it is not very accurately constructed. Many factors should be checked, already in the building phase, to obtain a good realisation of the structures (e.g. moisture content in the earthfill, right percentage of water, cement, gravel and sable in the concrete, proper stone size in gabions). The main phases of the realisation procedures of embankment and gabion structures are illustrated, respectively, in the second and third paragraphs of this chapter. These paragraphs contain both general and detailed recommendations: the former touch on issues such as site preparation or methods for the exploitation of borrow areas; the latter, instead, will deal with issues such as gabions building or cut-off screens realisation.
Many practical examples of structure building, accompanied by pictures or designs, are provided by way of illustration. The majority of these recommendations have been derived from the experience of the Rural Development Project of the Ader Doutchi Maggia (PDR/ADM). This Project, financed by the Italian Co-operation and the Word Food Program, and conducted by FAO, has been operative in the Keita District of the Republic of Niger, since 1984. Many small earthfill dams (40, for a total earthfill volume of about 1.500.000 m3) and gabion weirs (200 for a total volume of gabions of about 1.500.000 m3) were built in the PDR/ADM in this period.
Before addressing the various phases of hydraulic structures building, the first paragraph deals with the preliminary phases. These phases mainly concern the individuation of all the resources required for the works, the redaction of a work plan, and the preparation and organisation of the workshop.

It is very unlikely that a hydraulic structure, no matter how accurately built, will never require some kind of ordinary or extraordinary maintenance. In the first paragraph of this chapter, the methodology to follow in designing and building a hydraulic structure so as to minimise future maintenance tasks is considered. In the following paragraph, instead, we will highlight the importance of the participation and involvement of the local population, the direct beneficiaries of hydraulic structures, in the management and maintenance of the structures, and we shall consider some ways in which this involvement can be promoted. In the two following paragraphs, we will focus on the maintenance requirements of, respectively, earth dams and gabions structures. Finally, the last paragraph illustrates a selection of methods to protect a retention dam's storage volume from sedimentation.

Gabion cages can easily be assembled manually, and are therefore particularly well suited for use in developing countries. A simple method for manufacturing gabion cages is illustrated below. Selecting a zinc-coated wire, with mesh and selvedge of the same diameter, will facilitate gabion cage manual fabrication tasks.