Introduction to Flood & storm water risk evaluation method

---

Evaluate home and small business risk to flood damage, using the latest FEMA flood maps available on-line at  (http://msc.fema.gov/portal/home) . Enter address , then click Search and then click Map Image icon, which will bring up an interactive flood map of your property.  If your home is located within the 100-year or 500-year flood hazard area of a nearby source of flood water, you may have a very serious problem which is likely to need professional help. Design and installation of flood protection systems is generally very costly, especially if the predicted flood water levels are well above the grade elevation near your structures. You may be only able to protect your home or small business from the 100-year elevation but the 500-year flood would enter structures and cause extensive damage without practical protection schemes. 

 If there is no flood source (river, lake, creek, etc.)  near your property and it is rather remote from major developments, there may be no FEMA flood maps available for the area. This is usually good news unless a flood source is unidentified by FEMA, which is rare.  

If your home or business is located in an area where the grade elevation changes considerably, your structures may be susceptible to flash flood damage in the event of heavy rainstorms on land that is at a higher elevation then your property. This exposure often requires professional help and is most prevalent in states that have mountains or foothills of mountains. Beware of assuming you have no exposure because you live in Michigan or some other state with little elevation change. The basement wall of a home in Houghton County, MI collapsed during a flash flood caused by an intense rainstorm on June 17, 2018. This collapse was caused by storm water flowing down a hill and impacting the "uphill facing" wall of the home's basement.  

Even if your home or small business is not near a flood zone, you should evaluate whether storm water would flow towards or away from your basement or foundation during an intense rainstorm. Ask yourself if your property is located in a low spot or if the ground slopes towards one or more sides or your home or business? In steeply sloped or relatively flat areas, this may prove to be difficult, so you may need expert assistance. In many instances of modest grade elevation changes, this may be much easier and obvious, especially if intense rainstorms have caused low areas to fill with water in your neighborhood. This is the potential you are trying to evaluate. In what low spot will the excess storm water collect if an intense rainstorm overwhelms the drainage capacity of the local storm water system. Keep in mind that the storm water may only pool for a few hours so you need to keep it out of your structures for only a short period of time.  Storm water may collect over the top of the storm drain opening at the edge of the road or it may collect around your home of small business if your structurers are lower than the road. 

Use a 4 ft. level to determine direction of slope in areas immediately around the foundation, especially at basement windows that are slightly below grade. For revaluating the slope of larger areas you may need to hire a surveyor if there is very little change in elevation. Many times a neighbors yard is considerably higher so you can assume at least some of the rain water that lands on the neighbor's yard will flow towards your property. Although each property owner is supposed to provide separate storm drainage and/or containment on their property, this building code standard has not been applied retroactively in older neighborhoods.  

Of course, the amount of solid surfaces, (including lawns, gravel, pavement and concrete), surrounding your property has a huge impact on how much water will flow across the ground and towards your property. If you live in an area which has very sandy soil (dune or beach grass) or farmland, which will absorb lots of rain, you may not have any reason to worry. If the soil is clay and has considerable pavement and lawns, then you should evaluate the potential. If you are aware of a history of water damage or near misses, then you should complete a full evaluation or have one completed.  

Keep in mind that newer structures are usually elevated above grade to minimize the chance for storm water to enter openings. If your home or small business is older (built prior to 2000), then grade level openings may be present that may allow water to enter the structure during heavy rainstorms. 

Evaluate the basement drainage system potential for backup by first, confirming floor drain or other drain openings are located in the basement. Second, determine if public storm and sanitary drainage system are separate or common. If the above two features are present, an intense rainstorm could cause storm and sanitary water to backup into your basement. 

Have any of the above events occurred or nearly happened (i.e. water surrounded foundations or approached site but did not enter home/basement.) If you have not lived in your home very long, ask neighbors if the former owners ever had water enter the basement or grade level openings. Look for newspaper articles about flood or storm water damage to neighboring structures.  If you discover a history of damage or "near misses" then  it is safe to assume your structures are at risk of water damage and improvements should be considered to mitigate the potential exposure.

A basement/crawl space may fill up if the structure is surrounded.  Is there a basement and is it finished in water-absorbent or water-resistant materials? Can carpeting and pad be replaced with ceramic tile? Can drywall be replaced with water resistant material or eliminated? Can basement be left unfinished? Can utilities be elevated to minimize damage? Can basement be kept clear of valuable furniture, tools, etc? If flood or storm water will overtop the main floor level, the damage potential is likely to be severe. 

Are basement/crawl space walls solid/poured concrete, concrete block, masonry stone? Are windows present in basement/crawl space walls at or below grade level? Are there basement/crawl space entrances (egress windows, walkout doors, etc.) and does nearby land slope away or towards these openings? Are concrete walls/floors in good condition or are they cracked and allowing water to enter. 

Are there floor drains in basement and are lines equipped with backflow prevention devices, which are in working order? If yes, then this exposure is minimal.  If no, backflow prevention devices should be installed at each leg of the sanitary system to prevent backup into the basement through the drain openings. Hopefully, you can confirmed that the backflow prevention devices are in working order by accessing them through a cleanout port.  A local plumber should be able to advise you of the presence of backflow prevention devices and check their condition.    

Does your property have storm water drainage system designed to keep water away from foundations and entrances, including capacity for eave trough down spouts and sump pump discharge? What is condition of these drainage systems (plugged with debris)? Does the existing drainage system keep water from accumulating around your structures during even intense rainstorms or does water collect in very intense rainstorms? If the latter is true, then the existing drainage system may need to be cleared, repaired or enlarged. 

Are there natural earthen berms/hills or masonry walls designed to keep water away from home or business up to a certain level? Can you easily make these barriers complete to fully surround your home or business to keep water away from foundation or basement openings?  

 Does your basement have passive storm water drains (footing drains)?  Does your basement have a sump pump in a pit that controls water level below concrete floor and around the foundation? Has your basement been retrofitted with a full perimeter trench drainage system leading to a sump pump pit? Is your sump pump provided with back up pumping system in case electrical power fails? 

Passive drainage is preferred to active that depends on electrical power to operate. A backup pumping system can be provided that uses the public water pressure to operate. Keep in mind, all pumping systems have a maximum capacity so it is still a good idea to keep storm water from approaching the perimeter of all structures on your property. 

 If the above evaluations indicate that water could enter or impact the home or business and the damage could be substantial (i.e. basement or main floor filling with water), then improvements should be considered. Be aware that it is very important to minimize the amount of storm water that flows near the outside of your basement walls/floors or around foundations. Keeping water away will minimize the moisture penetration through even poured concrete walls/floors that are in good condition. Keeping water away will minimize the potential for freeze damage to concrete walls or floors (frost heaving). 

 Keep site storm water drainage system in good operating condition by clearing down spouts, rock filled drainage areas, overflow points, etc. of any debris (leaves, seeds) or silt. If drain lines have filled with silt/sand they probably need to be replaced and drain openings should be modified to minimize soil entry. All drain openings should require water to go up over edges and through weed screens under rock basins to minimize soil entry. 

Replace standard basement/crawl space windows with sealed glass block or masonry if you can go without natural light in the basement. Storm water often enters a basement at the metal window frames that have rusted out even if poured concrete walls are in good condition. 

Install back up sump pump system (siphon using public water pressure) or back up power supply to electric motor driven sump pump. Intense rainstorms often knock out electrical power so then your basement fills with water since the pump is no longer operational. Keep in mind that many home and small business insurance policies don’t cover damage caused by loss of public power supply. If the backup system fails, the water damage will often be covered since you have provided a backup system that does not rely on the public power supply. 

Change grade level slope by landscaping modifications. If you can use gravity to direct storm water flow away from the structure's foundation, that will solve many basement water problems. "Rain gardens" can be used to absorb some rain water but in heavy rainstorms, such "low areas" are seldom large enough. This is especially true if yard areas are small as in the typical residential neighborhoods found in large  metropolitan areas.  

Install new storm drainage lines and openings to replace plugged lines or missing drains to move storm water around the house from high to low elevations. Ideally, such systems should designed based on 100-year, 60-minute rainfall intensity for all contributing lawn areas for your area of the country by going to http://hdsc.noaa.gov/hdsc/pfds_map_cont.html?bkmrk=nc). Chose Precipitation Intensity and Partial Duration and then input your address and this NOAA site will provide a table of values for various rainfall intensities. This 100-year rain storm has a 1% chance of happening in any one year, based on 1 hr. duration. In Michigan, this rainfall intensity is about 3 in./hr. for 60 minutes.  If the contributing area is large, this can lead to very large flow rates (gpm) for design of the drainage system. 

Care should be taken to slope gravity flow drain lines at a minimum uniform slope of 1/8 in. per 1 ft. and drain openings should not allow dirt/sand to enter drain lines.  Ideally, the size of the drainage system should be based on flow from neighbors property if their lots are higher and are sloped towards your property.  You will know if a neighbor's yard contributes flow towards your house during the first couple of heavy rainstorms. 

If you can't keep rain water from collecting around the perimeter of a structure, you could consider installation of solid surfaces (concrete) within 3-4 feet around foundation to prevent storm water from flowing down around basement and under floor. This is not the preferred arrangement because it is much better to drain the water away from a rock filled drainage trench around the foundation, but it can be used to hold rainwater at the surface until it drains away. 

Another approach is to install or finish earth berms or masonry walls around your home or small business. Keep in mind that movable flood barriers are available for driveways and walkways that can't be elevated (with rolled curb or ramp up/down).  Keep in mind that storm drainage will need to be allowed to flow in only one direction (out of area near building) so backflow prevention devices will be needed in these drain lines. 

Pressure inject/fill cracks in poured concrete walls if the walls have started to fail and cracks have opened up.  Keep in mind that cracks in poured concrete walls may have been caused by storm water flowing down outside of the walls and freezing as winter approaches. This is known as frost heaving when wet soil swells as ice forms. Keeping water away from the outside of poured concrete walls will also help minimize the amount of moisture that penetrates through the concrete and results in higher humidity/condensation in the basement. 

Seal coat the inside of concrete block or stone masonry walls (This is limited value if walls are in very poor condition and if you don’t keep water away from exterior of basement walls.)

Install footing drains (passive), which requires drop in elevation to street to allow the new drain line to be  connected to storm water drains at street or other storm drain discharge point (if there is room for containment basin on your property). 

Install full perimeter trench drain in basement with sump pump (active) with back-up system in case electrical power fails.  Keep in mind, all pumping systems have a maximum capacity so it is still a good idea to keep storm water from approaching the foundations or the structures on your property.  

If the water damage exposure is very high, the only option may be to change the siting by elevating or relocating the existing structure. Except in rare cases, these are not feasible options. This is the exposure when the flood or storm water heights could exceed (by several feet) the top of the concrete basement, crawl space or foundation walls. In such cases, the best option may be to complete simple, inexpensive improvements to prevent storms of low severity from causing damage. If flood waters or storm waters reach historically high levels, then your only option may be to rely on flood insurance to pay for restoration of your property. In such cases, it is still advisable to minimize damage by keeping the basement unfinished and void of valuable storages and vital utilities.  

FEMA provides essentially all flood insurance to residential property owners and they have started to buy properties that flood often. This has turned out to be cheaper for the federal government than paying for repairing the water damage multiple times at properties highly susceptible to flooding or storm water entry.