Rainwater Harvesting with Rock Catchment

By SearNet

 
For domestic and livestock use

Introduction

In arid and semi-arid lands (ASAL) with large rock outcrops a lot of runoff is generated after rains. Through development of rock surface into a catchment, the runoff can be harvested and stored for domestic and livestock use to alleviate water shortages.

What is a rock catchment?

It is a catchment area developed from a rock outcrop to catch and concentrate rainwater runoff into a storage structure for productive use.

What do you require to develop a rock catchment?

A rock out crop with a large surface area

How do you develop a rock catchment?

Step 1: Determine a suitable site with an expansive, impermeable rock out crop

Step 2: Clear and clean the site off vegetation

Step 3: Mark out the effective catchment area of the rock surface

Step 4: Estimate the amount of runoff volume (m3) anticipated = rainfall (m) x catchment area (m2)x runoff coefficient (normally 0.9 for rock surfaces). As a guide to the design of storage structure.

Step 5: Site the water storage structure or masonry gravity dam on the outer edge of a hollow or depression on the rock surface

Step 6: Design the water storage structure or masonry gravity dam with capacity as in step 4

Step 7: Estimate the material requirements for both the rock catchment and the water storage structure or dam.

What are the other requirements in rock catchment development?

Use of locally available materials such as sand, hardcore and ballast

Skilled person/mason

Unskilled people to provide labour

Means of transporting materials, such as ox-cart or wheelbarrow

Ordinary cement-amount depends on size of catchment/storage structure

What determines the amount of runoff water that you can get from a rock catchment?

Effective catchment area enclosed by the gutters

Average seasonal rainfall

Amount of water losses through the rock surface

Shape of the catchment in relation to storage structure location

How do you store water from a rock catchment?

Masonry gravity dam, masonry/ferro-cement tank, ground or underground.

What is a stone gutter in rock catchment development?

This is a stonewall built with rough stones/hardcore, joined with mortar, around the outer edge of the rock catchment. To facilitate collection and concentration of runoff into a storage structure or reservoir.

How do you maintain a rock catchment?

Keep the catchment area clean and reservoir tidy by cleaning every season, repair cracks on the catchment and leaks on dam or reservoir.