Keeping Overland Flood Water Out of Your Basement: Dry Floodproofing

Transcription

1 Keeping Overland Flood Water Out of Your Basement: Dry Floodproofing Presented at Town Hall Meeting on Flood Mitigation, Heman Park Community Center, University City, MO, December 4, 2014 By Eric Stein This presentation is based on material prepared by the author and contributed to the University City Department of Public Works and Parks for inclusion in a brochure titled Floodproofing:Cost effective ways to keep your basement dry during overland flood events. The brochure may be obtained from their office at 6801 Delmar Blvd, Disclaimer: This material describes flood mitigation steps taken by the author on his home. Flood mitigation involves risk trade-offs and it up to each homeowner to determine what is best for his/her situation. The author assumes no responsibility for mitigation steps taken by other homeowners.

2 The Problem A number of houses in U. City have basements that flood due to flash-flooding from the RDP. As water rises on the foundation, it can enter the basement through windows, outside stairwells, floor drains, and gaps around pipes passing through the foundation. This presentation describes steps I have taken to mitigate such intrusions. Basement garage entrances are a separate issue and will not be considered in this presentation.

3 My Location On the north side of the cul de sac where Chamberlain Ave dead-ends at the RDP, just north of the high school. Adjacent to channel, across from the Wilson Ave buyout properties and with comparable elevation. 100 yards upstream from the site of fatalities in the 2008 flood.

4 My Flood History At current residence since For the first 28 years, no flooding from the RDP touched my basement foundation, although it occasionally flooded the back and side yard. On September 14, 2008 as a result of rain from Hurricane Ike, flood water rose to a depth of about 30 inches on my foundation and entered my basement to a depth of seven feet. After spending the winter putting my house back together, I started looking for solutions.

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8 Floodproofing Terminology Dry floodproofing means taking steps aimed at keeping water out of a structure, which is the subject of this presentation. Wet floodproofing means taking steps to protect a structure by allowing water to enter, while mitigating damage to the building s contents.

9 Hydrostatic Pressure Standing water ( hydrostatic ) exerts pressure on all surfaces it touches. Pressure increases with depth, and the resulting forces on surfaces can be quite large. Factoid: Water four feet deep in a walkout basement stairwell exerts a force of 1500 pounds on a 36 wide basement door.

10 Cautions on dry floodproofing As water rises on the wall of a building, so does the force on the wall due to water pressure. If the depth is great enough and there is no water inside to counterbalance the external pressure, the walls may suffer structural damage. (This is when wet floodproofing is a preferable approach.)

11 Cautions on dry floodproofing FEMA recommends that dry floodproofing not be used if water depth on the building exceeds 36 inches, and that an appropriate design professional evaluate your foundation before proceeding. Important: Saturated soil around a foundation adds to the effective water depth and increases pressure.

12 Steps You Can Take #1 - Protect Windows Replace basement windows with glass block windows. These can be mortared in one block at a time or complete assemblies can be made to order and mortared in. Install in front of the old, recessed wood frame.

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15 Steps You Can Take #2 - Protect the Stairwell Build a reinforced masonry wall and removable flood gate to prevent water from entering the stairwell. This may require removing an existing railing.

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18 Gate slides in a channel and can be lifted out.

19 Channels on side are built of 2 x2 composite material and bottom is a routed composite deck board.

20 Gate is constructed from treated ¾ plywood, reinforced with 2x4s on back to prevent warping

21 Hinged cover is to keep debris & rain out. Bolts for mounting can be incorporated in brick wall as it is built.

22 Steps You Can Take #3 - Protect basement drains Overland flood water will enter the combined sewer system and may force its way into your basement via the floor drain. There are several ways to prevent such backflow :

23 Protect basement drain (con t) Install a backflow valve in the floor drain. However, these can jam due to debris in the flood water. A very effective backflow valve can be installed in your sewer lateral but cost is prohibitive, unless you qualify for MSD s sewer separation program. An inexpensive measure is to install a removable plug in the drain using a test plug.

24 Test plug. The remains of a defunct backflow valve can be seen in the drain

25 Inserted plug

26 A reinforcing column made of pipe insures the plug will not blow out from back pressure.

27 Plug reinforcing column detail

28 Also plug walkout stairwell drains. Cover over stairwell keeps rainwater from collecting.

29 Many basements have a washing machine that drains into a standpipe.

30 Keep a piece of pipe handy to extend the standpipe if a flood appears imminent (or a plug).

31 If floor drain is kept plugged, install a condensate pump at furnace to route A/C condensate to washer standpipe or a drain installed through the foundation.

32 Steps You Can Take #4 -Seal gaps in foundation Seal any points where there are gaps around pipes or wiring penetrating the foundation. Elevate any through-the-foundation sump pump discharge to above flood level. Glass block windows can be ordered with a dryer vent opening in them. Seal around vent pipe at window and make sure discharge point is above flood level.

33 Seal gaps in foundation (con t) Better yet, if there is a window in your stairwell, vent through it, since it will be protected by the flood wall and gate described earlier. If high efficiency furnace has vents and air inlets passing through the foundation, extend these to above flood level.

34 Steps You Can Take #5 - Minimize Saturation of Soil at Foundation Keep gutters cleaned out so they do not overflow. Gutter downspouts should be drained well away from the foundation (but not onto your neighbors property). Consider draining them into a rainwater tank or rain garden. If necessary, alter the grade around the house in the vicinity of the foundation.

35 Steps You Can Take A General Strategy When you realize that a flood is imminent, the less you have to do the better. Default toward keeping your defenses in place at all times. Keep drain plugs in when not using drain. If plugs are to be inserted only when needed, keep them stored near the point of insertion. Keep flood gate in except when using stairs.

36 How Effective Have These Steps Been? The real test would be another 2008 flood, which has not happened. However, less severe flood events in June 2013 and September 2014 caused water levels just above stairwell and window thresholds, and overcharged the sewer system. My basement stayed dry.

37 Debris at foot of flood gate is from September 9, 2014 flood

38 My Approximate Costs Brick flood wall - $900 labor & materials, freelance mason. A mason friend advises me that current, freelance rate for a single-course, vertical wall is around $20-25 per square foot, labor & materials. Glass block windows - $400 per window labor & materials, free-lance mason. Flood gate - $300 labor & materials, independent contractor. Stairwell cover - $50 materials, $0 labor (DIY I work cheap).

39 Costs (Con t) Test plugs Several dollars each at Lowes, Home Depot, etc. Proper gutter downspout drainage and condensate pump for A/C were already in place. Condensate pumps cost less than $100 and can be installed in an hour or so. Balance these costs against that of a flooded basement. Flood insurance pays based on actual cash value instead of replacement cost, and does not cover contents, so out-of-pocket cost is considerable.

40 Will this lower my flood insurance premiums? No. Generally the only mitigations that lower premiums are elevation, demolition and relocation. These are unaffordable without special programs, such as buyouts. But you will have a dry basement

41 Resources/Items of Interest Homeowner s Guide to Retrofitting Six Ways to Protect Your Home From Flooding, FEMA P-312, 3 rd Edition / June Can be downloaded (PDF file, 212 pages) or hard copies can be ordered for no charge. Go to the FEMA website (or simply google on FEMA P- 312) to see ordering information. This guide is not an easy read it contains a significant amount of jargon (but with definitions), and the bureaucracy inherent in a large federal agency is on display. Nor is it intended as a practical do-it-yourself guide. However, it does contain a wealth of information regarding FEMA s view of floodproofing. It is very clear that their preference is for elevation, demolition or relocation of a residence, with much less enthusiasm for dry floodproofing, even though the first three are not affordable to most homeowners without special federal programs, such as buyouts. Floodproofing Non-Residential Buildings. FEMA P-936, Contains more practical information on dry floodproofing than P-312 above, but is not intended for residential use. FEMA will certify dry floodproofing for some non-residential applications, but not residential. Flood gates and barriers. A wide variety of these are available from special suppliers, including temporary flood gates to keep water from entering the doors of a residence and barriers for garage doors, as well as large-scale barriers for protecting commercial buildings. There are too many sites to list here.simply google on flood barriers to find them. National Flood Insurance Program (NFIP). Homeowners in the floodplain need to be aware that the NFIP is undergoing a massive revision, which will have consequences. The great flood of 1993, Katrina, and super-storm Sandy have left the program awash in red ink. In response, the Biggert-Waters Flood Insurance Act of 2012 was enacted to reform the program and pass much more of the cost onto to those affected by flooding. The steep increase in NFIP rates and the addition of many properties to flood zones or more severe flood zones resulted in enough public outcry to cause the 2014 congress to actually do something, in the form of the Homeowners Flood Insurance Affordability Act of 2014, signed into law in March. FEMA has one year to implement the details. While delaying and softening some of the impact of the of the 2012 act, flood insurance rules & rates will change (increase) significantly over the next few years. NFIP Flood Maps. Along with NFIP reform, new flood maps are coming. St. Louis county has an excellent on-line digital map that integrates county parcel information with the FEMA flood maps, allowing you to see your property s status. The site shows existing floodplains, as well as preliminary designations of new zones ( FEMA layer ), and can be panned and zoomed. Layers can be turned on and off for better views. To find it, go to maps.stlouisco.com/femamaps, or google on Saint Louis county FEMA map.