Flooding & Flood Zones
Building in a flood zone is considered a special construction circumstance as it presents special problems and issues to think about. If possible, it should always be avoided. Whether or not a site gets flooded, heavily influences a site’s value. Building in a flood zone is typically done near or in urban areas that are already established with cultural value and the need outweighs the risks. Flood zones are typically around rivers and low lying elevation areas. If you have a marsh, stream, river, lake, or other water feature on/near the site, you should do more investigation into whether water will affect your building site.
- Because through the natural cycle of water, the floodplain will eventually be flooded at some point in time. That type of land should ideally be reserved for open space uses, such as recreation and agriculture. This land can provide a natural, park-like easily maintained setting.
# Year Flood
100 year flood zone is an area which has a 1% chance of having a flood.
- This process is done by taking “one” divided by the year (100/1 = 1%)
- This means there is a 1% chance of having a flood within ANY given year.
- A 100 year flood has a much larger magnitude than a 10 year flood, and inundation a lot more land than the 10 year flood. Therefore, when designing, its best to use the 100 year floodplain as a base, and build up from there.
1,000 year flood zone is an area that has a 0.1% chance of having a flood.
- (1000/1 = .1%), or a 100% chance of happening once every 1,000 years.
- Another way to think of it is that it has a theoretical 100% chance of happening sometime in 1,000 years.
While a 100 year flood technically has a 1% chance of happening any year, it’s not uncommon for it to happen twice within a couple years. The fact that weather patterns are changing has made weather patterns less predictive and more violent. It is likely that flooding will continue to get worse than previously expected.
Stormwater Management
Stormwater Management: Is the use of structural or nonstructural practices designed to reduce stormwater runoff, discharge volumes, and peak flow discharge rates
- Most states have regulations to control erosion and sediment: silt fences, sediment traps or basins, vegetated buffer strips, hay bales, and other methods
- Final constructed area should ideally minimize impervious areas, utilize natural filtration of native fauna, and capture and control excessive runoff.
- Common strategies: Pervious paving, develop wetlands, grass lined swales.
Flood Drainage and Systems Design
Just as the development of a site increases the amount of runoff, on a larger scale, urbanization has a similar effect on the hydrologic cycle. The amount and speed of runoff is increased, the runoff is warmer and contains pollutants, and the stream which eventually carries the runoff is visually and ecologically impaired as a result of pollution and erosion.
- This development has led to a drastic increase in the dangers of flooding.
Runoff
Runoff – Stormwater that is not absorbed into the ground. Consists of total precipitation, less the water infiltrated into the soil, less the water evaporates directly to the air, and less the water transpired back to the air from plants.
- Runoff and infiltration are the two main pieces of the water cycle that the site designer cares about.
- When a site is developed the amount of runoff will increase, and the amount of infiltration will decrease. Additionally the amount of transpiration will decrease
Runoff Coefficient: Fraction of total precipitation that is not absorbed into the ground.
- Value from 0 (no runoff) to 1.0 (completely waterproof)
- Calculated based on the amount of water to be drained during the most severe anticipated storm.
Detention Strategy
In general, the best approach to deal with the increased amount of water is some sort of detention strategy.
Detention Pond: A low lying area that is designed to temporarily hold a set amount of water while slowly draining to another location. They are more or less around for flood control when large amounts of rain could cause flash flooding.
Underground Water Diversion: Essentially an underground detention pond. A large area is created underground and all water is diverted into it for temporary holding until it gets filled. Only makes economical sense in areas where there is very little outdoor space for above ground detention, like urban environments.
Building Designs:
Many energy programs, codes, and other guidelines and recommending or requiring new buildings to have the ability to store water on site. Many do this within their own holding tanks, or have introduced green roof and green areas to help increase infiltration (and decrease runoff).
Barrier & Divert Strategy
Another way to prevent flooding on a smaller scale is to try and stop it from harming the building/space once it is already happening. s simply trying to stop the waters from penetrating in.
Floodproofing
Buildings are exposed to water from a variety of sources, and all designs, no matter how innovative or crazy — ultimately need to control (or resist) the effects of water on buildings. Water, along with sun, are the most damaging natural elements a building is regularly exposed to, regardless of where it is located on the globe.
There are many strategies to prevent water from infiltrating a building.
Floodproofing is a specific type of waterproofing design that is divided into two main strategies in order to deal with flood waters specifically. Generally there is an acknowledgement that if a flood can happen, it will happen. In the case of dealing with floods, you can either dry, or wet floodproof your structure.
Site Placement
The first and always best strategy is to avoid water as much as possible through the building’s site placement. The simplest way to think about this, is to simply not build in flood zones, and to always have topography of land shedding water away from the building. There are simply some places where it does not make sense for humans to try and live.
Dry FloodProofing:
Approach to keep everything dry and resist -or prevent- as much as possible any water from entering the building. This is done by utilizing water resistant materials and sealants to prevent floodwaters from entering the building and are thus effective in keeping building interiors dry.
- The structure must be designed to resist the lateral forces of the flood waters.
- Many times sealants and building systems are appropriately floodproof, however, water pressure that increases with floods causes those seals/hardware to fail. All openings need to be designed to withstand the expected pressure of water.
Wet Floodproofing:
Wet floodproofing approaches the problem by accepting that water will penetrate the building. This approach allows water to penetrate the building, and therefore when waters recede, the water would be pumped out or drained in other ways.
The benefit of this approach is that the structural system does not need to resist the pressure and forces of flood waters, as it allows the flood waters to penetrate. This strategy can include breakaway walls as well as pressure gates that allow water to come through rather than resist.
- Care and sensitivity should be given to finishes that will or will not survive water exposure. For those that cannot survive the water exposure, they should be able to be easily removed and ideally recyclable.
Mechanical Considerations
Reality is that for a single family home, the difference between dry and wet flood proofing may not be readily apparent. Most single family homes place the mechanical wherever there is room, or perhaps leftover room. In single family residences with basements, this is many times where furnaces and boilers are placed – at the lowest level possible.
In wet floodproofing, this design approach requires sensitivity to mechanical equipment as it all needs to be located well above any floodplain so that it will stay dry regardless of where the waters penetrate/touch. Typically, this places most mechanical equipment on the 2nd floor at least. In areas like Manhattan, NY – floodproofing requirements have forced all MEP equipment above the floodplain. This has structural, programming, and circulation implications.
Other Flood Measurements & Insurance
PME: Probable Maximum Flood
The most severe flood that may be reasonably possible for a particular location.
- PMEs are used to design facilities in which there must be almost no risk to flooding
SPF: Standard Projected Flood
Flood that may be expected from the most severe combination of meteorological and hydrological conditions of a particular location.
- SPFs are typically expressed as a probability frequency.
- #Year floods are an example of an SPF – such as the ‘50 year flood’ which has a 2% probability of occuring.
Government Requirements
FEMA
FEMA flood maps have been thoroughly documented over the last several decades. Floods can happen anywhere along bodies of water, and flood maps help certain agencies such as insurance determine long term risk and liability for building in certain areas.
NFIP: National Flood Insurance Program
Require that local participating governments adopt minimum floodplain management plans. Being in a floodplain, it’s obvious that any construction within the area is prone to natural disaster.
- Unfortunately, as land becomes expensive there is more pressure to build into the floodplain. Many times low density housing is placed there. This is generally considered acceptable to governments as long as the occupants know the dangers of living in a floodplain.
- Recent weather trends have increased the areas that are considered at risk, and its becoming more expensive and harder to get home insurance in these locations.
The Water Cycle
AKA Hydrologic Cycle
All the water on the earth, under the ground and in the atmosphere is part of one unified system. After water falls on the land as precipitation (rain or snowy), it generally follows three paths.
- Small amount flows off the land into streams and eventually into the ocean as runoff
- If it falls as snow, the ice/snow will hold water until it melts as more runoff
- Smaller than that quantity soaks into the ground as infiltration and is stored as groundwater
Most of the precipitation is evaporated through transpiration in a process called evapotranspiration directly into the atmosphere (action of plants).patterns less predictive and more violent. It is likely that flooding will continue to get worse than previously expected.
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