# 9. POND INLET STRUCTURES

## 9.0 Introduction

### When do you need an inlet structure?

1. Inlet structures are built to control the amount of water flowing into the pond at all times. The need for an inlet structure varies with the type of water supply being used to feed the pond.

2. There is no need for an inlet structure for a pond supplied entirely by rain, surface runoff, groundwater or a spring which emerges within the pond, nor for a barrage pond built directly on the stream.

3. An inlet structure may be built for a pond supplied through a feeder canal, for example by diverted stream water, a spring outside the pond, a well or a pumped water supply.

### Different types of inlet structures

4. There are three main types of inlet structures:

• pipe inlets;
• open gutter inlets;
• canal inlets.

5. If a division box is included in the water feeder canal and it is close enough to the pond dike, the inlet structure can be built as part of the division box.

Note: if you have to feed water into two adjacent ponds, you can use a division box to regulate the water flow toward the two pond inlets. In this case, it is best to add a separate gate at each pond inlet.

### Designing inlets

6. When designing and constructing an inlet structure, you should pay particular attention to the following points:

(a) Place the inlet at the shallow end of the pond.

(b) Design its bottom level to be at the same level as the bottom of the water feeder canal and ideally at least 10 cm above the maximum level of the water in the pond.

(c) Design the inlet structure to be horizontal, with no slope.

(d) Try to arrange the structure so that water splashes and mixes as much as possible when entering the pond.

(e) Remember that you have to keep unwanted fish out of your pond. Design your pond inlet accordingly.

Example

Your pond has an area of 200 m2 and an average water depth of 0.75 m. You wish to fill it within 6 hours.

• Total water volume required: 200 m2 x 0.75 m = 150 m3 = 150000 litres
• Total time available: 6 hours = 360 minutes = 21600 seconds
• Inlet capacity required: 150000 l � 21600 s = 6.94 l/s, either = 7 l/s
• If you have this water flow available when you fill the pond, you can go ahead with this estimate and plan on filling your pond within 6 hours.

Note: design inlet for a water capacity large enough for the pond to be filled within a reasonable amount of time, from a few hours for a small pond to a few days for a large pond. The design also depends on the water flow available.

## 9.1 Pipe inlets

1. Pipe inlets can be made from various materials, depending on the water supply required and the inside diameter of the pipe. If the pipes you have are too small to provide the required water flow, you may have to use more than one pipe at each pond inlet. Usually, pipe inlets extend for about 0.60 to 1 m beyond the edge of the water surface of the pond when it is full, and they should be at least 10 cm above the final water level. Base the selection of the material of the pipe inlet on the following table:

 Pipe Inside diameter of pipe Less than 10 cm 10-15 cm Over 15 cm Bamboo yes - - Galvanized iron yes - - Plastic yes yes yes Asbestos cement - - yes Concrete - - yes

2. Remember that to estimate the water carrying capacity of a pipe you can use either Graph 1, Table 13 or a mathematical formula as explained in Section 3.8.

 3. Siphons, as flexible or stiff pipes, can also be used for filling ponds (see Section 8.9). In this case, the pond requires no proper inlet. Pumps can also be used (see Section 3.9). 4. Bamboo pipes (see Section 3.1)make cheap and good inlets whenever locally available. They can be used in several ways for filling small ponds, for example: without modification, the water flow being regulated upstream; with the inclusion of a mobile plate for flow regulation; with modification for improving water quality. Bamboo pipe inlets 5. Small galvanized iron pipes and plastic pipes are more expensive but also much more durable than bamboo pipes. They can be fitted with a mechanical valve to regulate the water flow or can be set up with a swinging arm to let water into the pond. A simple pipe sleeve can be constructed to control the water flow.

 Sliding sleeve water control Swinging arm water control Using the swinging arm water control

6. Larger plastic, asbestos cement, and concrete pipes are necessary to fill large ponds. They are generally used together with two-way division boxes, through which their water flow can be regulated.

 Concrete block two-way division box with large pipe inlet

## 9.2 Gutter inlets

1. Gutter inlets usually extend for about 1 m over the water surface when the pond is full. They can be made simply from various materials such as:

• bamboo: by cutting a bamboo culm lengthwise in half and cleaning out the partition walls. The diameter is usually limited to 10 cm or less;
• wood: by assembling three boards to form a rectangular gutter. A flow-regulating gate can easily be added;
• metal: by bending lengthwise a galvanized iron sheet into a semi-circular gutter. The flow should be regulated upstream.

2. To estimate the water carrying capacity, use a minimum head loss of 0.2 mm along a 2 m length or slope S = 0.2 � 2000 mm = 0.0001 or 0.01 percent (see also Section 8.2).

 Split bamboo four-pipe gutter Corrugated metal gutter Wooden gutter

## 9.3 Canal inlets

1. A small open canal can be built to connect the water feeder canal to the pond, usually from a division box. There are several possibilities such as:

• digging a small earthen canal, with a trapezoidal section;
• building a small lined canal, with a rectangular section and using either wood, bricks or concrete blocks. Small parallel walls are built on a light foundation along the sides of the canal. If necessary, two pairs of grooves are added to regulate the water flow with thin boards and to keep unwanted fish out with a sliding screen.

2. To estimate the water carrying capacity use a minimum slope value of S = 0.0001 as above (see also Section 8.2).

 Parts of small wooden inlet sluice Small wooden inlet sluice

 Section AA

 Section AA

 Building an inlet sluice CONCRETE BLOCK Type 1

 CONCRETE BLOCK Type 2 Note: the dimensions shown are suitable for structures for a medium-size pond system; dimensions may vary, however, depending on the size of your pond system

## 9.4 Aerating and mixing incoming water

1. Unless the pond is operating with large flows of water, the effect of aeration of inlet water on the overall pond conditions is not great; to fill the pond, or to top it up, it helps to ensure that incoming water is well aerated. Similarly, it can help to mix incoming water well with existing water. Several useful devices help to do this such as:

• inlet weir: this structure spreads out the incoming water and is particularly useful for smaller ponds where the splashing, together with the spreading, help to mix and aerate at the same time;
• inlet box: this structure can be built of wood, brick or concrete and acts simply to drive the incoming water below a sluice board, so that it enters the pond below the normal water level, thus helping to mix lower pond water levels;
• combined system: a weir can be arranged to supply aerated water, which is then forced downwards into the lower pond levels, at the same time pulling in extra air under pressure.

2. You will learn more about water aeration devices in the next manual in this series, Management, 21, Sections 2.6 and 2.7.

## 9.5 Dike protection at a pond inlet

1. Earthen pond dikes are very susceptible to erosion by running water, particularly when they are not protected by a grass cover and when they are in direct contact with falling water. It is important that you protect the part of the dike situated under the pond inlet.

2. You can do this in various ways, for example:

• by fixing a layer of brushwood along part of the dike's side and pond bottom;
• by making a simple reinforcement from wood, planks, bamboo or matting around the inlet;
• by piling rocks under the inlet pipe or gutter to break the fall of the water;
• by paving part of the wet side of the dike and of the pond bottom with a layer of rocks, gravel or wooden boards.

Note: when you start filling your pond you should be especially careful to avoid eroding your dike with running or dropping water or both.