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.
