Flood-proofing

 

Flood-proofing

Flood is a very common cause of destruction specially in rainy season. Located at the foot of Himalayas, Nepal is blessed with many snow-fed rivers such as the Koshi, Karnali, Narayani, Mahakali which are perennial. Originating from Himalayas, passing through the hills to the plains of Terai, these rivers accelerate, rise up and escalate during monsoon causing massive damage to the lives and properties. Those natural blessings have sometimes proven to be a curse for Nepalese.

                                                                fig: Generalized flood risk map of Nepal

(Source: Generalized flood risk map for Nepal.  | Download Scientific Diagram (researchgate.net))

Flood-proofing:

Flood-proofing refers to the methods that can be implemented to prevent buildings from the damaging effects of flood. Here we will discuss some of the measures to flood-proof a building and minimize the risk of severe flood damage.

 Flood-proofing Techniques:

1.      1. Dry Flood-proofing:

In this method, all the water approaching the buildings are prevented from entering the buildings. This can be done by building a waterproof membrane or using sealant. The waterproofing membranes may be of following types:

·         Water-proofing sheets: It may be bituminous membranes or PVC membranes. Composite membranes can also be found having a fabric base providing strength and tear resistance and a chemical that coats the fabric.

·         Liquid membranes: Waterproofing liquids are applied on surfaces by spraying or brushing which forms a seamless waterproof membrane.

                                                        Fig: Water-proofing using sheet membrane

                                                    (Source: http://wrimcowaterproofing.com.au)

1.       2. Wet Flood-proofing:

In this method, unlike dry waterproofing, water is allowed to enter the structure or area but the damage it can cause is minimized. What actually this method does is that it allows the water to enter the enclosed area of house so that the water inside the area quickly reach the flood water level outside due to which the net hydrostatic pressure (exerted by water) is greatly reduced and hence there is reduction in the loads imposed on house during the flood. As a result, the structural damage is prevented or minimized.

1.       3. Elevating Structures:

Another method for a flood-proof house is to construct it over a raised platform. The house must be elevated above the flood level allowing water to pass through below the house to prevent hydrostatic pressure on the home. Generally stilts (poles, posts or pillars) are used to elevate the house up to design flood elevation. The design flood elevation is determined on the basis of flood hazard map of the area.

The important factor to be accounted for this method is it’s foundation requirements. The foundation requirements of any structure depends upon the geotechnical characteristics of soil and the strata beneath. So, it is designed on the basis of bearing capacity of soil. The foundation obviously must be able to withstand the load from superstructure. In addition to it, the structure will be subjected to hydrodynamic and hydrostatic forces, impact force from the debris carried by floods, wind and other lateral loads. It should have adequate capacity to bear such loads and to resist flotation, collapse, and permanent lateral movement under the critical load combinations or worst conditions.

                                                    (Source: https://fontanarchitecture.com)

(Source: https://earthbound.report)

                                                         Fig: Elevated flood-proof houses

1.       4. Flood Walls:

This technique involves the construction of strong, solid and waterproof walls with watertight gates surrounding the buildings, towns or villages to protect it from flood water. Floodwalls are typically constructed of reinforced concrete or masonry. Floodwalls act as barriers against inundation, protect structures from hydrostatic and hydrodynamic forces and also deflect debris carried by flood away from the buildings.

                            (Source: http://floodquiz.savingplaces.org/permanent-floodwalls/)

Types of Floodwalls:

Ø  Cantilever Floodwalls:

Cantilever walls are generally constructed of reinforced concrete or concrete blocks embedded with steel reinforcement bars. These are the most common type of flood walls as they are economical to design and construct. Inverted T-type cantilever walls are commonly used.

The lateral forces (P) and overturning moments(M_O) is balanced partially from the weight of the soil on the heel portion of the base (W), the weight of the wall itself and the stabilizing or restoring moment (Mr) and thus stability of the wall is achieved.

 

Ø  Gravity Floodwalls:

Gravity walls are the simplest type of structures whose stability depends on its’ weight. These walls resist the overturning because of its’ dead weight of construction. They are generally constructed of concrete or masonry. These walls are made heavy so that they cannot be overturned by the lateral hydrostatic/hydrodynamic forces.

Ø  Buttress and Counterfort Walls:

Buttress/Counterfort walls are similar to cantilever walls except that these walls consist of additional transverse wall known as buttresses/counterforts. If the transverse wall is on toe side then it is called a buttress wall and if the transverse wall is on heel side then it is called a counterfort wall.

There are many factors that should be considered while deciding the flood-proofing technique to be used. Some of them are:

Ø  Optimum flood level

Ø  Flow velocity during flood peaks

Ø  Flood frequency

Ø  Duration of flood

Ø  Structural requirements

Ø  Economy

Few words at last:

Flood-proofing is not a complete solution to the flood issues. Also, it is almost impossible to entirely flood-proof a house. But these methods can surely help to reduce the devastating effects of flood.