Surface runoff rainwater harvesting: Perfect solution

In the previous post I had said that the surface runoff rainwater harvesting will solve the following problems:

• The water crisis in dry season
• The drainage problem
• The submergence problem in city area
• Reduction in flood level in rivers

To get the idea of these things we must first understand:

What is surface runoff rainwater harvesting?

Rooftop and paved surface rainwater harvesting is not enough for whole year. So it is very necessary to introduce the artificial recharge methods of rainwater harvesting. In this method the rainwater is accumulated in a small constricted area like pits, wells, trenches, shafts etc. and infiltrated under the soil through them. This is the indirect method of rainwater harvesting so there is no direct gain but in long-term this is the best method of making water available sufficient under the ground. This method of rainwater harvesting is also called recharging ground water aquifer.

There are different methods of recharging ground water aquifers. Some of them are:

i) Recharging of bore holes

ii) Recharging through wells

iii) Recharging through pits

iv) Recharging through trenches

v)  Recharging through shafts

vi) Recharging making percolation tanks

How does this work?

In this method above mentioned methods can be easily constructed. For example

Recharging through Wells

Well is constructed in such a place that maximum rainwater accumulate to that area. Or it can be chosen among the abandoned wells for the recharging. Or even wells can be constructed to recharge the drainage (storm) water i.e. connecting the drainage pipes to the wells.

But before water enters to the well it is very necessary to filter the water so that clean water gets recharged and no more sediments will settle inside the well.

For this near the well the filter media is constructed and then the filtered water is only allowed to enter into the well.

So other methods like pits, holes, trenches also work on the same process.

What factors determines the recharge rates?

Recharge rates is affected by different factors. They are:

1) Infiltration capacity of soil: It is the capacity of soil to allow water inside it. If infiltration rate of soil is high then maximum amount of water can be recharged. 

2) The location of recharge zone: The recharge zone should be in such a place that the maximum water gets collected to the place i.e. it has good catchment area.

3)  Ground Water Table: If the ground water table is so high, water will rise upward instead of recharging. So, GWT should be sufficiently low. 

How is it going to solve the problem listed?

1) Water Crisis

We extract a lot of water from different underground source of water like Well, Hand Pump, boring etc. in dry and wet season. Extracting the water isn't the problem. The problem is continuous extraction of water without recharging the aquifers. This is the main reason due to which the Ground Water Table (GWT) is going deeper and deeper. So recharging the aquifers will do two things. First, it will maintain the ground water table to a certain depth and second we can extract as much water as we can.

It is obvious that the discharge will be more than the recharge but this will reduce the rapid decrease in GWT. So we can extract the water in dry season to the greater extent if we manage to recharge the water in the same amount. This will help in solving the water crisis to some extent.

2) Drainage Problems and submergence 

Drainage problem is one of the major problems in city area. If there is continuous rain for about 3-5 hours then the whole city will be submerged and flooded. It’s because of two things. First is the paved road, pavements, streets, house compound etc. from which water cannot infiltrate and whole water turns into runoff. Second are the old and expired drainage pipes of the city area. 

So because of these two things we are having problem of drainage and submergence. So, recharge techniques will solve them by reducing the runoff volume. Recharging from one or two zone isn't enough to solve the problem. There should be enough recharge zones through which enough water can be infiltrated. This will reduce the runoff volume to great extent inside the city area.

3) Reduction of Flood

If the runoff volume is decreased then it’s obvious that the water in the river will decrease. This will reduce the flood level to some extent. Though very little amount of water will be infiltrated in comparison to the water that gets accumulated in the river, it can reduce to some extent. 

Note: In the next post I will present a case study of drainage problem at Tinkune Kathmandu realating to Surface Runoff Recharge Technique with some calculation.