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When The Retention Pond Fails

In July of 1996, the Fox River in Illinois flooded at an alarming rate after almost 17 inches of rain.  In April of 2013, we are again seeing major flooding in Illinois, this time affecting a broader area, but still causing the same kind of damage and chaos.  There are no rules about when and how flooding can happen – it happens anywhere it rains.  However as developed areas grow, they can become more susceptible to flooding due to outdated drainage and growth that exceeds the planned need for retention or has too much area covered with hard surfaces preventing natural drainage.

Factors for Flooding: When designing, we don’t design for events greater than the 100 year storm.  The recent flooding in the Chicagoland area would be considered an extraordinary event where much of the flooding was not due to design failure, but rather current conditions.  There are several factors to consider when flooding happens to determine what, if any improvements can or should be made based on cost and potential risk.  These factors can include things like soil saturation, current pond and river water levels [is there room for more water?], rate at which the water comes down and flows into the current retention system and whether or not the drainage systems are maintained correctly.  [Is there a pile of leaves or other junk obstructing the path of the water?]

100 Year Storm:  As news crews report the flooding that happens the phrase ‘100 year flood’ or ‘100 year storm’ can be used to describe this extraordinary event.  This can be confusing… since this doesn’t truly reference how often such flooding occurs, but rather the likelihood of it happening.  The term “100-year flood” is used in an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year. Likewise, the term “100-year storm” is used to define a rainfall event that statistically has this same 1-percent chance of occurring. In other words, over the course of 1 million years, these events would be expected to occur 10,000 times. But, just because it rained 10 inches in one day last year doesn’t mean it can’t rain 10 inches in one day again this year.

Stormwater Runoff:  Impervious surfaces like driveways, sidewalks, and streets prevent stormwater runoff from naturally soaking into the ground.  As precipitation from rain or snowmelt flows over the ground it can pick up debris, chemicals, dirt, and other pollutants and flow into a storm sewer system or directly to a lake, stream, river, wetland, or coastal water. Anything that enters a storm sewer system is discharged untreated into the water bodies used for swimming, fishing and providing drinking water.   During times of flooding that same runoff can quickly become a dangerous moving body of water causing damage and gathering ever more pollutants as it travels over areas not typically covered in water.

Poorly managed stormwater causes three big problems:  Pollution contaminating water, Damaging Floods, and oddly enough – Water Shortages especially in developed areas with more impervious surfaces.  These surfaces can keep rainfall from soaking into the ground and replenishing groundwater and streams used for drinking water or fish habitat.

Retention Ponds:  Retention ponds are one of the most common forms of stormwater management.  Retention ponds or “wet ponds” are ponds constructed to manage stormwater runoff, prevent flooding, limit downstream erosion, replace tree absorption due to development, and improve water quality in adjacent bodies of water. Retention ponds are permanent pools of standing water, many times with plantings and sometimes even walking paths to make them more enjoyable or even creating a ‘feature’ to a developed area.  These ponds provide a buffer allowing the stormwater to be ‘treated’ by allowing the water to go thru the natural cleaning process of sedimentation and nutrient uptake.  As with any stormwater management strategy, some maintenance is required.  Regular inspections for pests and erosion are recommended and the areas around the pond maintained.

A detention basin, commonly called a ‘dry pond’ is an area that temporarily stores water after a storm, but is not meant to stay wet and eventually empties out at a controlled rate into a body of water.  An infiltration Basin is similar to detention areas, but instead of going to a body of water, it is designed to direct stormwater through a permeable area to groundwater.

Flood Plains:  A flood plain is an area of low-lying ground adjacent to a river, formed mainly of river sediments and subject to flooding.  In the real estate market, a home in a legally defined flood plain is eligible for purchase of federal flood insurance. In this case, the broad definition of flood plain, also known as a flood zone, becomes more specific and detailed.  Lenders use the process of flood zone determination to evaluate the property and structures that secure mortgages. Federal banking regulations require certain flood zone properties to carry flood insurance as a condition of extending the loan.

The National Flood Insurance Program was established in 1968 to reduce the costs of emergency assistance in flooded areas. By the law, lenders had to require that buyers purchase this insurance on properties that fall within a Special Flood Hazard Area.  A Special Flood Hazard Area, also known as the 100-year floodplain, is a zone that has a 1 percent chance each year of experiencing a greater than normal flood. These zones are shown in detail on the National Flood Insurance Program map.  Owners or buyers whose property falls within a Special Flood Hazard Area may contest this determination by applying for a Letter of Map Amendment, Letter of Map Revision or Letter of Determination Review. The forms needed are offered for free on the Federal Emergency Management Assistance website. Having the designation removed allows the buyer to purchase the property without the legal requirement of federal flood insurance, though a lender may still require the insurance by its own guidelines.

When the Retention Pond Fails…

So what happens after the floods come and the damage is done?  That’s when municipalities take a look at their flood plans, ordinances are reviewed and Civil Engineers get to work continuing to make improvements and look for ways to control the water so that next time maybe the damage won’t be quite so bad.  Do you have flooding issues?  Need to get it looked at?  MeritCorp can help.

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