DAMAGE FROM WATER FORCES
A.2 Hydrodynamic Forces
A house located on a floodplain where there is flowing water will be subject to forces additional to those caused by still water.
Pressures and associated forces vary because water levels are not constant when there is flow around a house. Generally the water depth increases on the upstream walls and decreases on the side and rear walls as shown in Figure 117. As long as there are sufficient openings in the walls and floors of the house, the internal water level will be relatively flat (somewhere between the external upstream and downstream levels). The increased water depths on the upstream walls result in an inward force on the wall. Similarly, the decreased water depths that normally occur on the side and rear walls result in an outward force on the wall that tends to strip the wall away from the house.
These pressures vary with house size and shape and with flow behaviour. In fact, as the depth of the flow increases and submerges the house, the pressures can drop significantly as the flow
Figure 117 Hydrodynamic forces result mainly from the afflux on the upstream wall of the house
becomes three-dimensional (i.e. in very severe floods it can then flow over the house rather than just around it).
The exact form of these pressures and forces is complex but the following provides a general description on how these forces are developed and an indication of the size of the forces involved.
The inwards force due to flowing water is mainly associated with the afflux that occurs on the upstream side of the house. The afflux is the build up of water on the upstream side of any obstruction placed in moving water. On the other hand, the outward force is similarly related to the reduction of water level. The height of the afflux is proportional to the square of the water velocity. For example, if water flowing at a certain velocity results in an afflux of 50mm, then a flow at twice the velocity will produce an afflux of around 200mm. Afflux can be calculated from the following equation:
Cdv2
Afflux = __________
2g
v = water velocity in metres/seconds g = gravitational acceleration (9.8 metres/sec2)
Cd = drag coefficient which depends on the shape of the object around which the water flows.
Table A.2A Drag Coefficients
Width to height ratio w/h Wall on ground
Drag coefficient Cd
From 1 to 12 1.25
20 1.3
32 1.4
40 1.5
80 1.75
120 1.8
160 or more 2.0
The drag coefficient, Cd, can be determined from the width to height ratio, w/h, where the width is the side perpendicular to the flow and the height is the distance from the ground to the water level.
The table above gives Cd values for different width to height ratios for water normal to the face of the structure with its base at ground level.
Where flow velocities are less than 3 metres/
second, the force of flowing water is equivalent to this increase in depth of water on the outside of the wall. This results in an unbalanced force, which applies even if the hydrostatic force is balanced.
The additional load due to afflux tends to be uniformly distributed up the wall rather than the triangular distribution associated with hydrostatic forces, (Figure 118).
REDUCING VULNERABILITY OF BUILDINGS TO FLOOD DAMAGE
121 APPENDICES
The force due to any afflux is proportional to the square of the velocity of the flow. Ignoring the hydrostatic force, the total force per metre resulting from 2.4 metre deep water on a wall perpendicular to the flow is approximately:
Table A.2B Forces on walls
Water Velocity metres per sec
Total Force on Wall Newtons per metre
0.5 290
1 1,200
2 4,900
3 10,800
These velocities and forces are only indicative and are provided merely to give an idea of the magnitude of the forces. These forces are theoretical and can vary depending on the house shape and orientation, the spacing between houses, the general subdivision layout, and flood behaviour. As a comparison against hydrostatic forces, 2.4 metre deep water has a force around 28,400 Newtons per metre of wall.
Flowing water can also cause a reduction in the
water level on other walls, principally the side and downstream walls. The resulting lower water level downstream can cause an unbalanced force on the inside of walls. These outward forces can be more damaging to a house than inward forces.
Figure 119 shows the pressures that occur around a house as determined by three-dimensional modelling of the flow around a house. These represent only the hydrodynamic pressures (i.e.
the hydrostatic component is excluded) and represent a flow with an approach velocity of 1.5 metres/sec and 2.4 metres deep (eaves level of a single-storey house).
Positive pressures represent inward pressures towards the house whilst negative represent outward pressures away from the house.
Calculating all the forces imposed on a house from flowing water is complex as it depends on a number of variables. It is important to appreciate that the water velocities around a house can be very different when the house is located in a close group of houses or on its own in an open field.
Any change in velocity can significantly change the pressures on the walls. This is discussed in more detail in Appendix B.
moving water
Figure 118 Hydrodynamic effects from moving water
H H
Hydrostatic forces balance each other
Figure 119 Pressure on walls of a house due to moving water, Water 2.4 m Deep, Pressures in Pascals
Frontal Impact
Negative pressure (suction) on sides Negative pressure (suction) on
downstream side