Question

2.18. The flow velocity in a rectangular channel is 3 m/s at a depth of 3
m. If the channel bottom has a step rise of 0.3 m, determine the flow depth
downstream of the step assuming no losses at the step.

Answer 1

Answer:

Introduction

1. Several different kinds of structure may be used to transport water on a fish farm. The most common one is the open canal, which we will consider in detail first (Sections 8.1 to 8.6). Then we will look at other common structures, including:

simple aqueducts to transport water above ground level (section 8.8)

short pipelines to transport water above or under another structure such as a water canal or an access road (Section 8.9);

simple siphons to transport water over an obstacle such as a pond dike (Section 8.9).

8.1 Types of open water canals

1. Different types of open water canals are used on fish farms to transport water, usually by gravity*, for four main purposes:

feeder canals to supply water from the main water intake to the fish ponds. In a large farm with several diversion pond units, there is usually a main feeder canal branching into secondary and even tertiary feeder canals;

drainage canals to evacuate water from the fish ponds, for example toward an existing valley;

diversion canals to divert excess water away from barrage ponds;

protection canals to divert water runoff away from the fish ponds.

2. In this chapter you will learn about feeder, drainage and diversion canals. You will learn more about protection canals later

(see Section 11.5).

Open water canals for a small fish farm

Note: See also Section 8.7

8.2 Design of canals

1. All canals should be well designed to have the required water carrying capacity. The canals are designed using formulas that relate the carrying capacity of the canal to its shape, its effective gradient or head loss, and the roughness of the canal sides. The most commonly used formula incorporating all these factors is the Manning equation:

v = (1 ÷ n) (R2/3) (S1/2)

where

v = water velocity in the canal;

n = roughness coefficient of the canal sides;

R = hydraulic radius of the canal;

S = effective slope.

2. You will learn more about these terms below. First we will consider some basic design factors.

Planning the shape of the canal

3. Water canals can have various shapes. In theory the most efficient shape is a semi-circle, but this is impractical for earthen canals. It is therefore generally used for precast concrete flumes* or plastic flumes only.

4. It is very common for unlined fish farm canals to have a trapezoidal cross-section, defined by:

the width (b) of its horizontal bottom;

the slope ratio (z:1) of its angled sides;

the maximum water depth (h); and

the freeboard* (f) to protect against overflowing.

5. When water canals are lined with bricks or concrete, they may also have a rectangular shape(see Section 8.3).

Selecting the side slope for a trapezoidal canal

6. As you learned for pond dikes, the slope of the sides of a trapezoidal canal is usually expressed as a ratio, for example 1.5:1. This ratio is defined as the change in horizontal distance (here 1.5 m) per metre of vertical distance. The side slope can also be expressed in terms of the angle it makes with the vertical, in degrees and minutes.