DCOM Volume I Appendix K

APPENDIX K: Design of Dams for Water Sources

K.1 Introduction

In this Appendix, the following condensed information has been provided in order to assist design engineers to design dams that are expected to provide water for domestic use, to support livestock as well as for commercial and some industrial use. However, it should be borne in mind that the major water demand will be for domestic and livestock use. Furthermore, in Tanzania, majority of the existing dams for water sources are earthfill dams and only a few are rockfill dams. Furthermore, where a water source comes from a concrete dam, the primary purpose of that dam was for hydropower generation and hence the dam design is undertaken by the respective power generation agency. The Appendix has been broken into 20 sections that include:

• Types of Dams,
• Dam and Reservoir Pre-Surveys and Detailed Studies,
• Reconnaissance of Potential Dam Sites,
• Empirical Methods for Capacity Evaluation of Dam Construction Sites,
• Preliminary Data Collection (Pre-design),
• Designs of Earth-fill Dams,
• Height of the Dam,
• Design of the Embankment Filter
• Check of the Rip-rap Adequacy for the Safety of the Structure,
• Seepage and Stability Analysis of the Earth-fill Dams,
• Sediment Yield in the Catchment,
• Estimation of Evaporation from the Reservoir,
• Different Types of Spillways,
• Free board (Fb) Estimation,
• Estimation of Freeboard due to wave action (fwave),
• Embankment Crest Width (W),
• Embankment Slopes,
• Design Requirements for Tailings Storage Facility (TSF)
• Dam Break/Failure Analysis,
• Design Drawings.

K.2 Types of Dams

The general choice of a dam will depend to a large extent on the site selected for it. There are mainly three general types of dams that include:

• Earthfill dams which also includes rockfill dams;
• Concrete and Masonry dams;
• Sub-surface dams.

In the case of earthfill dams, the pressure exerted by their weight spreads over a much greater area by virtue of having flatter embankment slopes. A rockfill dam consists of an embankment made of stones of irregular sizes and shape faced either with concrete or masonry earth and clay. Concrete dams require foundations of much greater bearing pressure (strength) than those on which the earthfill dams can be successfully constructed. A slight settlement in the foundation of a concrete or masonry dam may give rise to fractures whereas the same can often be well accommodated in the earthfill dams. Technically, concrete dams are rigid structures while earthfill and rockfill dams are deformable structures. Because concrete dams are not common in water supply projects in Tanzania, no further discussion of these type of dams is presented in the manual.

Sub-surface dams are usually constructed by excavating a trench within the sand-bed of a river during the dry season which is then filled with impervious material that may or may not be zoned. A further design specifications of dams for water sources is focused on earthfill dams. Rock-fill and earth-fill dams are usually built in relatively broad valley sites because large-sized machines can be operated efficiently in such sites. Selection of the dam type should be carefully made, firstly basing this on materials procurable at the dam site or in the neighbourhood, and secondly in case of fill type and where this can be found. Rain and temperature greatly affects the work period available.

A flood spillway is an indispensable part of any dam structure and therefore its size, type and the natural conditions involved in its layout often proves to be a governing factor in selection of dam type. Because of the cost of spillways and the high sediment load in main watercourses, designers should always look at the possibility of off-channel storage sites on tributaries as these can often prove to be the more cost effective alternative, even though they often involve pumped storage.