How to gamble with floods

by Eng. Mahinda Panapitiya and

Eng. Wasantha Lal (PhD)

(Two residents from Attanagalu Oya Basin)

Introduction

Flooding during heavy rains and water pollution during normal time in natural streams is a common problem all over the world when human settlements are located near flood prone areas. For example, about 7-10% land area, in the US, under human settlements, are prone to flooding. In ancient cultures, flooding was perceived as a blessing in disguise because it was the main transportation method of fertilisers, free of charge, for agriculture activities in temporary submergence areas called flood plains. After moving people into flood plains because of shortage of space for settlement, floods have become a curse for humans. Deciding to settle down in flood prone area is a gamble. However, there are modern technologies called flood modelling available for us to overcome this problem.

Flood Modelling

For an example, it is now possible to simulate different flood conditions that may arise due to heavy rains, before it actually occurs, using satellite and survey data. This is called “modelling” in engineering. Any area prone to floods can be modelled and divided into zones so that land users will know in advance how deep their lands will get submerged. This type of performance-based methods also evaluates how an existing or newly introduced flood mitigation effort, performs under different flooding events.

Hidden reasons behind frequent flooding and water pollution of natural streams

* Unplanned real estate development by clearing local tree cover resulting in impervious areas (roofs, carpeted roads, etc.,) prevents water infiltrating the soil. This increases the runoff rate, causing flash floods during heavy rains. On the other hand, during droughts, all the natural tributary streams and wells in those areas dry up soon after the rain. This is very common in basin such as the Attanagalu Oya.

* The obstruction of natural stream and their tributaries due to poor maintenance. This is very common along the Kelani River basin

* Illicit encroachment causes the filling of wetlands in the flood plains. As a result, rain water has no designated place to collect before flowing out gradually. Most of the floods in Gampaha, Ja-ela and Wattala are due to this issue.

* Deposition of sediments washed down from upland areas due to lack of tree cover and also the erosion of stream banks whose reservations are encroached on either for agriculture in rural areas or for settlement in urban areas

* Inadequate flow capacity in local streams due to invasive weed growth associated with polluted water and lack of riparian tree cover. (Wattala)

* Lack of awareness among officials who manage water resources in natural streams about the role of riverine environments in flood plains which act as kidneys in our ecosystem while preventing flash floods.

How the community could face these challenges

Those who are already living in flood-prone areas or are planning to do so should be aware of the different risk levels in the areas concerned. For that, there is a need to do an exercise called Flood Hazard Zoning, This approach is very common in the developed world. This exercise will also enhance the community participation for government intervention such as canal cleaning and discouraging further encroachment on flood plains by land fillings.

Available Technologies

A sketch above extracted from a technical guideline adapted in the US shows a typical flood zoning map, which could be used by a community to decide whether they should or should not build houses in a particular location.

For example, in this map, people who are in Zone A are in a high-risk area subject to flooding. Zone C is a low risk area. A person who wants to build a house in Zone A, which is designated as “100 Year Flood Zone”, will have a 26% chance his house being submerged once in 30 years, which is the normal bank lending period of a housing loan. For the next 70 years, which is the normal lifetime of a building, the chance of being flooded is 50%. For a person who wants to build a house in Zone B designated as “500 Year Flood Zone” will have 18% chance of his residence being submerged once in 70 years. By knowing in advance through these flood zoning maps, people themselves become aware of flood danger before it occurs and, therefore, they prepare themselves for the challenges during flood situations. When there is no such initial warnings, governments will have to bear the whole responsibility.

This type of mapping would also be a useful guide for land valuation as well as for insurances against flood risks. With flood zoning, flood insurance becomes an option that adds a financial component in designing buildings to address those future risks. For example, people can build their houses at elevated levels on columns to suit predicted flood levels. Also the sewerage systems can be introduced to suit the wetland environments.

Lessons from the US

Every state in the US is required by law (water policy) to demonstrate that (a) the public is protected from floods; (b) the public has sufficient water available for drinking and farmin, etc. (d) there is enough water to support the environment. Computer models simulating the year-round hydrology are used for the purpose. Those models show how water from the rains could be saved for use during the dry season. Government agencies in the US do not use the models currently in use in Sri Lanka. They have developed their own models to simulate flooding. Models used in Sri Lanka are bought primarily from two European countries. They are normally used only to study individual flood events. The fundamental ideas used in these models have not changed since 1980s in Sri Lanka, and these models are still sold primarily to developing countries like Sri Lanka. On the other hand, teams of senior engineers are employed for developing those models used in the US, before permits are issued for new development projects. There are also Sri Lankans engineers among those teams in the US, as primary developers.

Opposite of flood

Wetlands of flood plain are the interface between aquatic and terrestrial areas. Plants in those wetlands play a very vital role in cleaning water biologically before it falls into the main streams. Wetlands are in fact the kidneys of ecosystems. Over the years, due to the so-called development, the environmental features of flood plains have undergone changes, causing not only floods during heavy rains but also malfunctioning natural water cleaning process, especially during droughts.

Note that those new technologies address not only flood situations but also help face drought situations, too, by identifying areas suitable for temporary water storages within flood plains. For example, during a previous drought situation there was a water shortage in the Attanagalu Oya basin, and the people had to purchase water from trucks, though annually the Oya releases into the sea a volume of water equal to that of the Parakrama Samudraya! Severe drought situations are even worse than floods, especially in view of the current pollution levels of natural streams bordering urban areas. To address this issue also, technologies could be used to identify naturally available water cleaning wetlands to be preserved.

King Parakramabahu’s famous quote about water conservation and utilization—“Do not release even a drop of rain water to the sea without using”—applies not only to our dry zone but also to the west zone.