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Structural Damage Due to Floods
By Craig D. Rogers, P.E.
The Federal Emergency Management Agency (FEMA) reports that each year
approximately 90 percent of all disaster-related property damage results from
flooding. Over the past decade, the average flood claim in the United States has
been more than $46,000 with yearly totals averaging $3.5 billion per year. In
addition to the damage to the interior contents and finish materials, as well as the
damages to mechanical and electrical equipment, the forces associated with
floodwaters can cause damage to the structural members of a building through
several different means. Following the flood event, an engineer may be required
to determine the origin and extent of damage, and prepare the scope of repairs
required to restore the structure to its original (pre-flood) condition.
What’s a Flood?
Under the National Flood Insurance Program (NFIP), a flood is defined as “A
general and temporary condition of partial or complete inundation of two or more
acres of normally dry land area or of two or more properties (at least one of
which is the policyholder's property).” This inundation may include the overflow
of inland or tidal waters, rapid accumulation of runoff, mudflow, or the collapse of
land along a shore due to water that has exceeded anticipated cyclical levels. In
plain English,
a flood is generally described as excess water (or mud) onnormally dry land. While floods are often thought of as being limited to coastal
flooding or riverine flooding, they may also occur as a result of excess urban
runoff, dam/levee breaches, landslides, ice dams, and even bursting city water
mains. So long as the inundation that occurs meets the definition as prescribed
by NFIP, the resulting damage would typically be considered the effects of
flooding.
Coastal flooding is typically associated with storm surges. Within tropical
weather systems, the storm surge is the rising water level that is primarily the
result of wind forces pushing water towards the land. During a hurricane, the
height of the storm surge is governed by the intensity of the hurricane and the
slope of the continental shelf. Storm surges for hurricanes can exceed 25 feet,
and are typically accompanied by dangerous, battering waves on top of the
surge.
Riverine flooding is the overflow of rivers, streams, drains, and lakes due toexcessive rainfall, rapid snow melt, or ice. The flooding occurs when the flow of
runoff exceeds the capacity of the natural drainage system. The recurrence
interval of the projected flooding event defines the severity of the flood. A 100-
year flood is an event that has a 1 percent probability of occurring in a given
year, while a 500-year flood has a 0.2 percent probability of occurring in a given
year.
Urban flooding is a phenomenon that occurs where there has been man-madedevelopments within the existing floodplains or drainage areas (e.g., newresidential communities, retail establishments, commercial buildings, parking lots,etc). The changes may either increase the amount of runoff or reduce the
capacity of the natural drainage channels. The addition of impermeable surfaces
(such as asphalt or concrete pavement) increases the speed of drainage
collection, overwhelming the drainage system. Changes to the shape, slope, or
direction of the natural drainage channels to better suit development may reduce
the capacity of the channel. An aspect of urban flooding that is typically not
found in “natural flooding” is the potential of subrogation of legal damages
against developers that modified the natural or original drainage system.
How do Floods Damage Structures?
Whether the flooding at a building results from storm surge, riverine flooding, or
urban flooding,
the physical forces of the floodwaters which act on the structureare generally divided into three load cases. These load cases are hydrostaticloads, hydrodynamic loads, and impact loads. These load cases can often be
exacerbated by the effects of water scouring soil from around and below the
foundation.
The
hydrostatic loads are both lateral (pressures) and vertical (buoyant) innature. The lateral forces result from differences in interior and exterior water
surface elevations. As the floodwaters rise, the higher water on the exterior of
the building acts inward against the walls of the building. Similarly though less
common, a rapid drawdown of exterior floodwaters may
result in outwardpressures on the walls of a building as the retained indoor floodwater tried toescape.
Sufficient
lateral pressures may cause permanent deflections and damage tostructural elements within the building. Should a rapid rise or drawdown occur at
a building that is relatively tightly constructed, there may be enough elevation
difference between the interior and exterior water surfaces to damage the walls
or foundation of the building. The lateral pressure associated with floodwaters is
approximately 62 to 64 pounds per square foot for every foot of differential. So, if
the outside water were to reach two feet above the floor of a building before any
significant amount of water leaked into the building, the lower section of the wall
would experience a maximum inward pressure of 128 pounds per square foot.
However, once the water reaches the same level on the inside as the outside,
this force is eliminated. So, in a “leaky” building there would be a low potential
for damage in this manner. However, a problem sometimes is encountered
when the rapid drawdown of basement water occurs due to mechanical means
following a flood. On occasion, a local fire department or the nation guard unit,
in an effort to be helpful, has used high capacity pumps to empty basements of
floodwaters. A problem can occur if the soils near the property remained
saturated.
The lateral pressures associated with the saturated soils can possiblyexceed the structural capacity of the basement walls. The result may be aninward collapse of the basement wall itself.
The
buoyant forces are the vertical uplift of the structure due to the displacementof water, just as a boat displaces water causing it to float. These uplift forces
may be the
result of the actual building materials (the floating nature of woodproducts), or due to air on the interior of a tightly built structure. When the
buoyant forces associated with the flood exceed the weight of the building
components and the connections to the foundation system,
the structure mayfloat from its foundation. In a pier-and-beam or crawlspace foundation where
there are few or no mechanical connections between the floor system and
foundation, this would be most likely to occur. With interior-exterior water surface
differentials of as little as one foot, a typical wood-framed residence can be lifted
from its foundation. While it is unlikely that floating of a basement foundation
would occur, buoyant forces can again become a problem when the basement is
pumped and the soils remain saturated. The saturated soils below the floor of
the basement can produce buoyancy pressure that acts upward on the basement
floor system, and
may induce cracking and movement of this floor.
In addition to these hydrostatic loads, the
water flowing around the buildingduring a flood event creates hydrodynamic loads on the structure. These loads
are the frontal impact loads from the upstream flow, the drag on the sides of the
building, and the suction on the rear face of the building as the floodwaters flow
around the structure. The magnitude of the hydrodynamic loads is dependent
upon the velocity of the floodwaters and the shape of the structure. Like the
hydrostatic pressures discussed earlier, these
lateral pressures associated withthe flowing water may be capable of collapsing structural walls or floor systems.
In addition,
the net downstream force against the building may shift the buildingfrom its foundation.
Further exacerbating the
physical forces applied directly to the structure, rapidlyflowing water may also scour the soils which support the foundation. While the
rate and ease with which a soil will scour depends upon many factors, sandy and
soft silty soils will generally be more prone to scour that stiff clay soils. As the
soils are eroded from around and below a foundation system, the capacity of the
foundation is reduced. Ultimately
this may lead to a shifting of the building, apartial collapse of the structural system, or even a complete collapse of thestructure.
Impact loads during flood events may be the direct forces associated with waves,
as typically encountered during coastal flooding, or the impact of floating debris
within the floodwaters.
Impact loads can be especially destructive because theforces associated with them may be an order of magnitude higher than thehydrostatic and hydrodynamic forces during the flood event. In the FEMA
publication Building on Strong and Safe Foundations, which considered the
effects of Hurricanes Andrew, Hugo, Charley, Katrina, and Rita, it was reported
that
“post-storm damage inspections show that breaking wave loads havedestroyed virtually all wood-frame or unreinforced masonry walls below the wavecrest.” In addition, as debris travels downstream during a flood event, it exerts
impact loads on structures it may encounter. This debris may include logs,
building components, and even vehicles. Whether the impact loads against the
structure occur as a result of waves or floating debris, the effects can be
devastating as they apply large and/or concentrated loads to the structural
elements of the building.
The Role of the Engineer
Following a flood event, there may be questions regarding the origin and extent
of structural damage to a building. Often, the engineer’s role may be two-fold.
First, he/she may need to serve as a forensic engineer to evaluate the origin of
reported damage to a building. Then, he/she will likely be required to determine
the extent of damage and provide a scope of repair for the structure.
As a forensic engineer, his/her investigation should determine the cause of the
various reported instances of damage. When the results of flood damage are
severe, the cause of damage can be quite obvious. However, from time-to-time,
a building owner may identify conditions following a flood such as wall cracks,
ceiling cracks, or tilted basement walls that may or may not be related to the
flood event. Through proper forensic investigation and analysis, the engineer
should be able to determine the cause of a particular condition, and whether or
not it existed prior to the flood event. Once this portion of the investigation is
completed, the claims adjuster will be able to better determine the extent of loss
at a particular property.
From a structural standpoint, the engineer typically will determine if the
foundation and superstructure of the building have been functionally damaged by
the flood event. Functional damage occurs when the structural element is no
longer capable of performing its intended purpose or the life expectancy of the
structural element has been measurably reduced. The engineer often makes this
determination by gauging the condition of the structural element against the
appropriate performance or construction standard. For example, cracks and
deflections within a masonry basement wall may be evaluated to determine if the
wall still meets the standards and tolerances published by the Portland Cement
Association.
Once the extent of damage has been determined, it may be necessary for the
engineer to define a scope of repairs. Following a flood, these scopes of repairs
will typically include the required work to restore the structure to its original (preflood)
condition. From time-to-time, it may be necessary to note structural
improvements needed to meet current code requirements or eliminate dangerous
conditions. A claim adjuster should expect the scope of work to provide a
general outline or guideline of the repairs to be performed. The goal of the scope
of repairs is to assist the claim adjuster in determining the value of the loss. The
scope of repairs is not intended to be an “engineered repair design.” In some
cases the extent of damage or the complexity of the repair will necessitate that a
professional engineer be hired to design and oversee the repairs. However, in
most residential construction such oversight is typically not warranted.
The forensic engineer may also be helpful in identifying construction and/or
design deficiencies that caused or contributed to flooding damage at a particular
site. Examples of such deficiencies include improperly designed and constructed
levies, under-designed masonry basement walls, under-sized storm retention
systems, substandard building materials, etc. This information may be useful in
identifying all relevant parties in a particular loss, which may lead to subrogation
considerations.
Regardless of the outcome of a particular loss, the role of the forensic engineer is
to uncover the facts and provide sound, substantiated and unbiased opinions.
This information should be communicated by the engineer to others in a manner
that is readily understood. In addition, it is the forensic engineer’s duty to identify
dangerous structural conditions that exist at a particular site. Sometimes these
hazardous conditions are not obvious, and the services of a competent forensic
engineer are critical.
Craig D. Rogers, P.E. Is a Catastrophe Technical Manager, as well as a Central
Division Assistant Property Manager, for Rimkus Consulting Group, Inc. He can
be reached at cdrogers@rimkus.com or 888-474-6587. Rimkus Consulting
Group, Inc. has 30 offices in the U.S. and overseas, and can be found on the
internet at www.rimkus.com.
http://www.handr.co.uk/literature/flood%20damage%20in%20historic%20buildings.pdf
Tsunami-Induced Loading on Structures
Beyond Hollywood's Scenarios Dan Palermo, Ph.D., P.Eng., and Ioan Nistor, Ph.D., P.Eng.
Made in Hollywood: a 600 foot plunging wave breaking over a frightened couple shivering on a beach! Don’t hold your breath: it’s basically impossible, according to coastal scientists. A wave propagating towards the shoreline will break at the location where the wave height approximately equals the water depth. Hence, depending on the coastline bathymetry (underwater near-shore topography), such a wave will break, in most instances, well offshore and will continue to advance towards the shoreline as a broken, foamy wall of water. Nevertheless, the impact of a broken tsunami wave on infrastructure located near shoreline can be devastating. The December 26, 2004 Indian Ocean Tsunami is the most recent example of the tremendous forces generated by tsunami waves advancing inland.
The impact of tsunami-generated hydrodynamic forces on coastal protection structures (breakwaters, seawalls, reefs, etc.) is relatively well understood. However, knowledge of the impact on near-shoreline structures such as buildings and bridges is lagging. Further compounding the problem is the lack of guidance from building codes and understanding of tsunami-induced loading. Structural engineers are not aware of the critical conditions in the design of structures located in tsunami-prone coastal areas.
Until recently the position of structural building code officials in North America was that tsunami-induced loading is not critical. Recent events, however, demonstrate the extreme and often catastrophic consequences that arise during a tsunami event in coastal areas. Historical tsunami events of the western North American Seaboard and, to a much lesser extent, the Eastern Seaboard suggests that building codes should consider such effects. Table 1 is a list of major tsunami events on the Western Seaboard of North America over the past century, highlighting the significant tsunami wave runup, which can be defined as the maximum water elevation occurring along the shoreline after a tsunami.
Table 1: Recent Tsunamis on the Western Seaboard of North America.
Basic Mechanics of Tsunami Waves
Tsunami waves can be triggered by various geological factors: underwater earthquakes, volcanic eruptions, and submerged or aerial landslides. However, the vast majority of tsunamis are generated by a sudden vertical uplift of the ocean bottom induced by a seismic event. The vertical displacement of such an enormous volume of water generates tsunami waves that propagate at high speed over thousands of kilometres. The velocity of tsunami waves in deep ocean waters can reach several hundreds of kilometres per hour. However, as a tsunami wave advances toward the shoreline and the water depth decreases, it gets "squeezed" by the sloping ocean bottom and hence, its height increases while its speed decreases. Depending on coastal bathymetry, tsunami waves break offshore and further advance inundating low-lying coastal areas in the form of a hydraulic bore, similar to that generated by flood waves occurring in a dam break. The hydraulic bore advancing towards shoreline is similar to a foamy turbulent wall of water advancing towards the beach. In this case, the wave completely looses its shape as a result of breaking. On the other hand, tsunami inundation can also occur as a gradual rise and recession of the sea level for the case of non-breaking tsunami waves, just as a suddenly rising tide. However, this case is rare and only occurs when the near-shore beach slope is vertical, as in the case of coral atolls.
The width of the continental shelf, the initial tsunami wave shape, the beach slope and the tsunami wave length are all parameters which govern the breaking pattern of tsunami waves. A broken tsunami wave travels overland and, depending on the coastal topography, can significantly impact the infrastructure lying in its path. Low-lying coastal communities are particularly vulnerable to tsunami wave attack and subsequent coastal flooding. Moreover, the mechanisms of hydrodynamic impact induced by tsunami waves differ significantly from those generated by storm surges. The increase in water levels during coastal flooding as a result of a storm surge occurs over several hours, as opposed to seconds in the case of tsunami waves.
Tsunami-Induced Forces
Three parameters are essential for defining the magnitude and application of tsunami-induced forces: (1) inundation depth, (2) flow velocity, and (3) flow direction. These parameters mainly depend on: (a) tsunami wave height and wave period; (b) coastal topography; and (c) roughness of the coastal inland. The extent of tsunami-induced coastal flooding, and therefore the inundation depth at a specific location, can be estimated using various tsunami events with various magnitudes and directions, and modeling coastal inundation accordingly. However, the estimation of flow velocity and direction is generally more difficult. Flow velocities can vary in magnitude, while flow direction can also vary due to the local onshore topographic features, as well as soil cover and obstacles. Forces associated with tsunami bores consist of: (1) hydrostatic force, (2) hydrodynamic (drag) force, (3) buoyant force, (4) surge force and (5) debris impact.
Hydrostatic Force
The hydrostatic force is generated by still or slow-moving water acting perpendicular on planar surfaces. The point of application of the resultant hydrostatic force is located at one third from the base of the triangular hydrostatic pressure distribution. In the case of a broken tsunami wave, the hydrostatic force is significantly smaller than the drag and surge forces. However, the hydrostatic force becomes increasingly important when tsunami-induced coastal flooding is similar to a rapidly-rising tide.
Buoyant Force
The buoyant force is the vertical force acting through the center of mass of a submerged body. Its magnitude is equal to the weight of the volume of water displaced by the submerged body. The effect of buoyant forces generated by tsunami flooding was clearly evident in the areas affected following the December 2004 Indian Ocean Tsunami. Buoyant forces can generate significant damage to structural elements, such as floor slabs.
Hydrodynamic (Drag) Force
Hydrodynamic forces caused by drag occur as tsunami bore moves inland with moderate to high velocity and flows around structures. The flow is assumed to be uniform, and therefore, the resultant force will act at the centroid of the projected area in the direction of the flow. The hydrodynamic force is a function of the tsunami bore velocity and the drag coefficient, which varies depending on the shape of the structural element around which flow occurs. The formulation used to calculate the drag force is identical for the City and County of Honolulu Building Code (CCH) and FEMA 55; however, differences in the force arise due to the drag coefficient and estimated velocity. For example, drag coefficient values of 1.0 and 1.2 are recommended for circular piles by CCH and FEMA 55, respectively. For the case of rectangular piles, the drag coefficient recommended by FEMA 55 and CCH is 2.0. For walls, CCH suggests a coefficient of 1.5, whereas a range from 1.25 to 2.0, depending on the dimensions of the wall, is suggested by FEMA 55. Regarding the estimated bore velocity, there is significant disagreement. For a inundation depth of 5 meters (16.4 feet), velocities of 14 meters/second (46 ft/s) and 5 meters/second (16.4 ft/s) are assumed by FEMA 55 and CCH, respectively. Essentially, CCH estimates the velocity to be equal in magnitude to the inundation depth, while FEMA 55 estimates the velocity to be, 2√gd, where g is the gravitation constant and ds is the inundation depth.
Surge Force
The surge force is generated by the impingement of the advancing water front of a tsunami bore on a structure. The magnitude is dependent on the geometry of the structural element subjected to the impingement and the velocity of the tsunami. For example, a wall of significant length and height subjected to the impact of the advancing water front experiences significant surge (build up of water along the height of the member) relative to a column under identical flow conditions. In the case of a wall and for calculation purposes, the surge is assumed to be 9 times the hydrostatic force for the assumed inundation depth. The point of application of the resultant surge force is located at a distance h (inundation depth) above the base of the wall. For a column, the Structural Design Method of Buildings for Tsunami Resistance (SMBTR) suggests a reduced surge force, given the potential build up of water in front of the column. The magnitude of this force is 4 times the hydrostatic value and the resultant force is located at 2/3 h above the base of the column.
Debris Impact Force
A high-speed tsunami bore traveling inland carries debris such as floating automobiles, floating pieces of buildings, drift wood, boats and ships. The impact of floating debris can induce significant forces on a building, leading to structural damage or collapse. Currently the impact force is assumed to be a single concentrated load acting horizontally at the flow surface or at any point below it. The magnitude is equal to the force generated by 455 kilograms (1000 pounds) of debris traveling with the bore and acting on a 0.092 square meter (1 ft squared) surface of the structural element (FEMA 55, CCH). The impact force is to be applied to the structural element at its most critical location, as determined by the structural engineer. Relative to the other force components, the impact force is a negligible component when evaluating the global lateral force. However, it is more critical in the design of individual structural members that are subjected to debris impact.
Figure 1 shows structural damage caused by the tsunami-induced loading during the December 2004 Indian Ocean Tsunami.
Figure 1: Tsunami damage in Thailand and Indonesia (December 2004 Indian Ocean Tsunami).
Loading Combinations for Calculating Tsunami-Induced Forces
Appropriate combinations of tsunami-induced force components (hydrostatic, hydrodynamic, surge, buoyant and debris impact) should be used in calculating the total tsunami force given the location and type of structural elements. Based on current literature, tsunami loading combinations must be significantly improved and incorporated in new design codes. An example of recent improvements in loading combinations is the work of Nouri et al., (2007) as shown in Figure 2. The load combinations are separated into two scenarios: (1) Initial Impact and (2) Post Impact. The first combination occurs due to surge and debris impact forces. The second scenario considers the hydrodyamic (drag) and hydrostatic forces, simultaneously with the debris impact force. The buoyant force is omitted for calculation of the global lateral force, but should be considered in the analysis and design of flooring elements. In Figure 2, Fi , FS , Fd , FHS , and Fb are the debris impact, surge, drag, hydrostatic, and buoyant force components, respectively.
Figure 2: Loading conditions.
Design Considerations
Tsunami-induced lateral forces can be similar to or exceed seismic forces. Appropriate construction and layout of a structure located in a tsunami-prone region can reduce the hazard associated with a tsunami event. Tsunami forces increase proportionally with exposed area and non-structural elements that remain intact during the impact of the hydraulic bore. Therefore, it is prudent to orient buildings with the shorter side parallel to the shoreline. Further, structural walls should also be oriented to minimize the exposed area. Exterior elements located at lower levels should be designed with a controlled failure mechanism at the instant the tsunami impacts the structure. This concept, known as breakaway walls, reduces the amount of lateral load that is transferred to the lateral force resisting system. The use of rigid non-structural exterior components, while providing protection to the buildings from flooding, increases the lateral loading.
Current Research
Currently, the authors are conducting experimental studies on the impact of tsunami-induced forces on structural components. The research consists of imposing a simulated tsunami-induced hydraulic bore on structural models representative of full-size columns. Hydraulic bores, similar to tsunami-induced bores, are simulated in a large-scale wave flume located at the Canadian Hydraulics Centre of the National Research Council, Ottawa, Canada. The flume was modified with a swinging gate mechanism to generate a hydraulic bore. The gate swings open quickly from the base allowing retained water to flow downstream in the form of a hydraulic bore. The bore propagates down the flume and impacts the structural model. The main objective of this study involves evaluating the forces associated with a hydraulic bore, leading to a better understanding of the effects of tsunami-induced loading. Figure 3a) is a photo of the structural model, b) is a photo of the wave flume, with the installed model and measurement instrumentation, while c) shows the impact of the hydraulic bore on the structure.
Figure 3a: Column structural model.
Figure 3b: Wave flume set-up
Conclusions
Tsunami induced forces represent a serious and real threat. In spite of their rare occurrence, tsunami waves represent a significant threat for the western coastline of North America given the major recorded tsunamis in the Pacific Ocean. Considering that major cities are located along this coastline, and the lessons learned following the December 2004 Indian Ocean Tsunami, it is imperative that structural engineers became aware of the possible devastating effects of such natural phenomena on infrastructure located in coastal areas.▪
Dan Palermo, Ph.D. and P.Eng is an assistant professor of Structural Engineering at the University of Ottawa. His research interests include seismic and tsunami-induced loading on concrete structures. He can be reached at Dan.Palermo@uOttawa.ca.
Ioan Nistor, Ph.D. and P.Eng. is an assistant professor of Coastal Engineering at the University of Ottawa. His research interests include numerical and experimental modeling of tsunamis. He can be reached at inistor@uOttawa.ca.
References
Federal Emergency Management Agency. Coastal Construction Manual (3 vols.), 3rd Ed. (FEMA 55), Jessup, MD, November (2003).
Y. Nouri, I. Nistor, D. Palermo and M. Saatcioglu, "Tsunami-Induced Hydrodynamic and Devris Flow Force on Structural Elements," 9th Canadian Conference of Earthquake Engineering, Ottawa, Canada, June (2007).
Department of Planning and Permitting of Honolulu Hawaii, Chapter 16, City and County of Honolulu Building Code, Chapter 6, Article 11, (2000).
H. Yeh, I. Robertson and J. Preuss, "Development of Design Guidelines for Structures that Serve as Tsunami Vertical Evacuation Sites," Report No 2005-4, Washington Dept. of Natural Resources, (2005).
T. Okada, T. Sugano, T. Ishikawa, S. Takai and T. Tateno, "Tsunami Loads and Structural Design of Tsunami Refuge Buildings," The Building Centre of Japan, (2005).
VERNACULAR ARCHITECTURE IN FLOOD PRONE AREAS (BANGLADESH)
INTRODUCTION:
-A large proportion of the countryside as well as the majority of urban areas in
Bangladesh is flood-prone. During heavy flood, more than 60% of the land is
inundated.
-Recent floods in 2004 has destroyed many houses and about 1 million people
became homeless. To a large extent, the patterns and causes of destruction
seem to result from poor technical knowledge and wrong perceptions.
1 Technocrats do not adequately support housing projects for low-income,
flood-vulnerable communities undertaken by NGOs and the government, and
houses are mostly owner-built without proper technical guidance.
1 One of the AUDMP findings of post disaster losses of the housing stock in
Bangladesh after 2004 floods is that most of these designs are prepared by
people who are not trained as building professionals, so when implemented,
many problems emerge.
1 The usual tendency is to apply the same model irrespective of context - for
example, the same house design is built on highland and low-lying flood-
prone areas.
1 In most cases, the cost is significantly prohibitive in terms of microcredit
recovery from poor people and this high cost prevents providing subsidized
housing to a large number of people who need them.
1 There is thus a need for developing housing which is appropriate for flood-
prone areas, where the suggested solutions are ‘cost-effective’ - that is,
rationalization of economy without compromising quality.
1 Those who work in the low-income housing sector in Bangladesh in general
are still to adopt such techniques.
TRADITIONAL MATERIALS USED:
FOUNDATION: In kutcha houses with usually bamboo and sometimes timber posts embedded directly into the earthen plinth. Extremely vulnerable and get damaged even in low intensity flood, thus requiring frequent maintenance.
In moderate to high intensity flood, especially if accompanied by currents, earthen plinths tend to get completely
washed off and have to be rebuilt.
Bamboo or timber posts in saturated soil, especially during long duration or recurrent flood, get rotten at the base, thus weakening the entire structure of the buildings to damage by strong wind, differential settlement, sagging of roofing elements and doors, windows and wall elements developing cracks and losing alignment.
Frequent replacement of bamboo posts of kutcha houses is done regularly in flood-prone areas. The typical earthen
plinth in many semi-pucca houses also behaves similarly.
In semi-pucca houses, locally known as “dowa-posta”, is better at resisting erosion at the sides of a building, but the infill earth floor can experience settlement due to saturation and in prolonged flood can
become muddy, unusable and the mud can escape from below. At the same time, scouring of soil cover of the typically
shallow foundation of the perimeter brick wall can result in its instability and settlement.
WALLS:
ORGANIC/BAMBOO MATT: Typically in kutcha houses; semi-pucca houses also often have
bamboo mat walls. Organic materials (e.g. jutestick, catkin grass) have a lifespan of 2-3 years and bamboo mat 4-5 years. Decay can get accelerated in flood.
In flood of high depth and moderate duration, the damage begins in the lower part of walls and hence weakens the walls and eventually results in complete damage. Flood with strong currents can detach wall panels and wash them away, leading to partial or complete loss, especially if the connections to posts are weak.
EARTH: Used in kutcha and semi-pucca houses. Various types according to region, but not
prevalent in all areas.
In monolithic construction, flood water can cause serious damage: once the base gets affected, the entire structure is liable to collapse, often rapidly.
ROOF:
THATCH: Typically in kutcha houses, made from catkin grass, rice, wheat ormaize straw with usually bamboo and sometimes reed stalk framing. Normally has to be renewed every 2-3 years. Results in decay in houses of low height and during flood of very high depth and duration, if thatch comes into contact with flood water. In such conditions, if also
accompanied by strong current,
thatching materials can get detached and washed away.
Secondary hazard often connected to flood is heavy rainfall, which can cause damage. Strong wind can also blow away thatching materials and damage frame.
APPROPRIATE CONSTRUCTION METHODS:
FOUNDATION:
-Stabilization of the typical earthen plinth can be carried out with a mixture of earth and cement.
-The proportion of cement to be added depends on the nature of the soil which can easily be tested on site.
-For soil with more than 40% sandy-silty particles, 5% cement additive is adequate. For soil with less sandy content, sand has to be added to raise the content above 40% and may require a somewhat higher
proportion of cement additive.
-Since very little load is imposed on the wall, the footing can be constructed with brick without the need for a concrete footing.
-Minimum 1:4 cement-sand mix should be used.
-Soil cover on the foundation should be thoroughly compacted and should
preferably have plant or grassy cover to prevent scouring during flood.
1 Infill should be of cement-stabilized soil to prevent muddiness, settlement due to saturation and loss of soil from below.
-Cheapest method for protecting from dampness lower end of bamboo/ timber posts typically embedded into the ground.
-Local method known by most villagers, but not widely practiced, and thus requires promotion.
-Molten bitumen, Mobil or sump oil, or a combination of these can be used.
WALLS:
There are different ways in which the bamboo matt walls of the super structure can be protected-
1. Detachable lower panels
2. Painting with Bitumen
3. Chemical treatment of Bamboo matt walls
To Strengthen earth walls there are two techniques-
Internal framework- Internal framework is provided so that there is no compromise with the structural stability, when the flood washes away the mud walls.
Cement Stabilization-Ideal cost saving method for inside walls in buildings with brick outer walls and damp-proof
plinths.
-Processing and preparation of mix similar to that of plinth stabilization
-Walls can be built by ramming inside wooden shuttering or by making blocks with a simple
brick mold.
ROOF:
CHEMICAL TREATMENT OF THATCH: -Similar to treatment of bamboo mats and battens
-Fibrous thatching material, such as catkin grass, rice straw, palm fronds, wheat, maize or sugarcane leaves needs to be soaked in preservative solution for only 12 hours (bamboo mats and battens 24 hours)
Environmental hazard: Flooding and Its Effects on Residential
Buildings in Ilorin, Nigeria
Introduction
Flood has a devastating effect on buildings. The ultimate factor of damage is not the quantity of water but how high water is above normal restraints or embankments. Buildings may deteriorate when flooded as a result of (i) the impermeability of the rocks on the soil on which the building rests that leads to poor water penetration, (b) increase in the height of water table as a result of cumulative rainfall, which causes inability of the soil supporting building to absorb water and/or (c) obstruction of natural flow of water through drainage made to protect the buildings by debris and garbage disposed improperly by human beings
Flood is very problematic, it’s devastating effects on buildings can be categorized into three: structural, economic, and health related effects. Disasters Management Center, college of Engineering, University of Wisconsin – Madison (1995) identified the following structural effects on buildings:
•Buildings washed away due to the impact of the water under high stream velocity. Such
buildings are usually destroyed or dislocated beyond feasible reconstruction
•Floatation of buildings caused by rising water. This occurs when light–weight houses are not
securely anchored or braced.
•Damage caused by inundation of buildings: A building may remain intact and stable on its
foundation, while its material is gradually and severely damaged.
•Undercutting of building: here the velocity of flood may scour and erode the building’s foundation or the earth under the foundation. This may result in total collapse of affected buildings.
•Damage caused by debris: massive floating objects like trees and materials from other collapsed house may have impact significant enough to cause damage to the standing buildings.
Flood Resistance of the Building Envelope
by Christopher P. Jones, PE
Last updated: 11-23-2009
Introduction
Flooding is the overflow of excess water from a water body onto adjacent lands. FEMA more specifically defines a flood as a general and temporary condition of partial or complete inundation of normally dry land areas from (1) the overflow of inland or tidal waters or (2) the unusual and rapid accumulation or runoff of surface waters from any source.
One or more water bodies can contribute to flooding at a given site—a river, stream, ocean, bay, lake, pond, storm water retention/detention area, etc.—depending on local topography and hydraulic/hydrologic conditions.
Figure 1
(Left): Mississippi River Flooding, 2001. (Courtesy of FEMA)
Figure 2
(Right): Coastal Flooding during Hurricane Frederic, 1979.
(Courtesy of NOAA)
Flooding can damage buildings and their contents in many ways, but the most common flood damages arise from:
- direct damage during a flood from inundation, high velocity flow, waves, erosion, sedimentation and/or flood-borne debris,
- degradation of building materials, either during the flood or sometime after the flood, and
- contamination of the building due to flood-borne substances or mold.
The likelihood and impacts of these damages can be minimized through the use of siting, design, construction and maintenance practices appropriate to floodplain areas. In the United States, these practices are based largely on the
National Flood Insurance Program (NFIP) and its regulations governing land management and use (
44 CFR Part 60).
