Flooding Basics

A Brief History of Flood Control

How Watershed Development Affects Flooding

Preventing & Reducing Development Caused Flooding

How CEDS Can Help

 

 

 

 

 

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Flooding & Watershed Development

From 2000 to 2014 flood damages totaled $129 billion throughout the United States and caused more than a thousand fatalities. The figure below indicates that the East Coast had suffered the greatest flood damages from 1993 to 2008.

flood damages 1993-2005These damages resulted from three forms of flooding: upland runoff damaging homes, streams and rivers swelling over banks to inundate nearby structures, and storm driven surges along oceans, bays, and other tidal waters.  If not properly managed new development can worsen flooding damages.  However, with the right stormwater measures and land use controls damages can be kept in check or even reduced.  Unfortunately, far too few localities require the right stormwater practices or manage land use in the most flood-damage reducing ways.

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Flooding Basics

Floods occur when the volume of runoff causes water levels to rise above elevations normally inundated.  With regard to watershed development, there are three forms of flooding of concern: upland, riverine and coastal. 

Upland and riverine flooding result from rainfall intensity exceeding the rate at which water can soak into the earth.  The excess water becomes surface runoff which creates problems when the volume exceeds the capacity of the swales, ditches or channels that carry it downstream. Coastal flooding is usually a result of high winds causing tidal waters to rise to higher than normal elevations.  The worst coastal flooding usually occurs when riverine flooding combines with high winds along with unusually high tides.

downspoutUpland Flooding:  A wet basement is the most common problem caused by upland flooding.  It usually results from runoff originating near a home or other structure.  Wet basements can also be caused by the ground water table rising closer to the land surface.  Frequently, downspouts discharging too close to a foundation are the cause of wet basements due to upland flooding. 

Riverine Flooding:  A stream or river typically flows in a defined channel.  Except in mountainous or headwater areas, a flat area known as a floodplain usually borders the channel.  Through the mysteries of geomorphology most channels erode until it is wide and deep enough to contain floods that normally recur every one- to two-years.  Over a very long period there will be a flood once every 100 years which doubles the depth of the channel.  If the depth from the bank tops to bed is 6 feet then the 100-year floodwater surface will be about 12 feet above the channel bed.

floodplain

Coastal Flooding:  As with riverine flooding, coastal flooding reaches predictable depths at what were fairly regular intervals.  The figure below shows the area subject to coastal flooding once every 100 years over a long period of time.  The VE zone is subject to violent exposure to waves.  The A zones have the potential for breaking waves and erosion.  To determine the extent of flood vulnerable lands in your area go to the FEMA Flood Map Service Center.

coastal flooding

Climate Change & Flooding: An analysis of National Weather Service data shows that high flood damage years are becoming more common.  Annual U.S. flood damages are projected to increase 30% by the end of this century.  Worldwide increased flooding caused by sea-level rise and climate change could cause damages costing $1 trillion annually by the year 2050.  The increase in flooding is due to changes in precipitation patterns and sea-level rise in coastal areas. 

The figure below shows how precipitation may change throughout the continental U.S. by the year 2100.

future preciptation changes map

And this figure shows areas of the continental U.S. vulnerable to sea-level rise.

us sea level rise map

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A Brief History of Flood Control

From 1928 to 2000, the U.S. Army Corps of Engineers constructed $122 billion worth of  dams, levees and other flood control structures.  Despite this investment flood damages in the U.S. have increased

leveeLevees have been used for flood control for more than 3,000 years. As shown to the right, levees usually consist of earthen ridges placed along waterways to reduce flood damage.  In the U.S. levees were first built 150 years ago,  Today there are 100,000 miles of levees nationwide. 

Los Angeles RiverKnown as channelization, many miles of waterways were also enlarged, straightened and smoothed in hopes of reducing flooding.  Channelization was abandoned by the 1970s due to the tremendous ecological damage it caused along with the evolution of more effective flood control measures. 

Also in the 1970s, construction of flood control dams became more widespread.  Restrictions on placing new homes in floodplains were adopted by many states.  In many areas programs were initiated to purchase then relocate or demolish existing homes located in flood prone areas.  A number of flood vulnerable homes benefitted from various flood proofing measures. 

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How Watershed Development Affects Flooding

New homes, shopping centers and other forms of development can exacerbate flooding in two ways. First, by increasing the volume of floodwaters inundating downstream properties. Second, by placing more homes and other structures in flood prone areas.  However, the right stormwater management practices can prevent new development from aggravating flooding and even bring about a reduction when applied to existing developed areas.

Impervious Surfaces Increase Flood Volume
Impervious surfaces prevent rainfall and snowmelt from soaking into the earth.  Streets, parking lots, buildings and sidewalks are the most common impervious surfaces.  It can take 1.5 inches of rain (in 24-hours) to produce runoff from a lawn but just 0.2 inches causes runoff from impervious surfaces.  By the time runoff begins from your lawn, the roof off your home has probably produced 1200 gallons of runoff.  Multiply that by all the other impervious surfaces draining to a waterway and you get a tremendous volume of floodwaters.

 

Smoother, Straighter Drainage Paths Add To Floodingforest floor
When runoff begins in a forest it first flows along a very rough leaf and twig covered surface.  These rough surfaces retard flow velocity which keeps runoff from forming large floodwaves.

street runoffRunoff travels about 50 times faster along an asphalt or concrete surface like a street, gutter or storm drain pipe.  The faster flow velocity causes floodwaves to reach far greater volumes and inundate structures located at higher elevations above the normal water surface.

forest channelSmoothing stream and river banks by removing trees and shrubs has the same flood-magnifying effect.  This is one of the reasons why channelization was abandoned in favor of more effective flood control measures.

Combined Effects of Impervious Surfaces & Modified Drainage Paths
To appreciate the combined effect of increased runoff volume and smoother drainage paths, consider that transforming a forest-covered watershed to homes on quarter-acre lots could increase the frequency and severity of flooding a hundred-fold.  Put another way, floodwater depths only seen once a century before development may recur annually afterwards.  However, with the right stormwater management and channel-floodplain protections watershed development can occur with minimal increase in flooding. 

Diminishing Effect Of Impervious Surfaces On Floodingsaturated soil
Major floods tend to occur after a week or so of rain saturates the soil surface.  Saturated soil cannot absorb more water so subsequent rainfall all becomes runoff.  If a major storm occurs when the soil is saturated or frozen then severe flooding may result.  When soil is saturated it acts like a parking lot or any other impervious surface by producing runoff almost immediately after rainfall begins. 

Diminishing Effect Of Development On Larger Storms
The map below shows the inches of rain falling in a 24-hour period once every 100 years.  Storms recurring annually drop about 2- to 3-inches of rain compared to the 4- to 14-inches shown below.  During a one-year storm the 2- to 3-inches of rain saturates lawns, forests and other pervious lands by the end of the storm.  The initial 2- to 3-inches of a 100-year storm also saturates the soil, then another 1- to 11-inches also falls resulting in a tremendous volume runoff then floodwaters.  It is for this reason that development tends to cause a much greater increase in runoff from frequent storms compared to the much larger, but less common events.  

100 year rainfall

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Preventing & Reducing Development Caused Flooding

In the 1980s use of ponds and other stormwater management practices become more common on newly developed lands.  A USEPA summary shows that all 50 states now have stormwater management requirements.

The Problem With Ponds
Ponds have traditionally been the practice of choice for managing stormwater and reducing downstream flooding.  Typically an engineer would compute the volume of existing and post-development runoff then design the pond to store the difference.  The pond outlet (spillway) would be designed to release the stored runoff at the same rate (cubic feet/second) that it flowed from the site prior to development.

The problem with ponds is that they manage the rate at which runoff is discharged, but do nothing to reduce the volume of increased runoff.  As a result:

Low-Impact Development: A Better Way
By the year 2005, many localities and states began modifying stormwater regulations and guidance to reduce volume through the use of measures that infiltrate runoff into the soil while reducing the impervious surfaces created by new development.  Many also require that when a site is redeveloped, such as when an old shopping center is converted to a new use, that both remaining and new impervious surfaces drain to highly-effective measures.  This redevelopment requirement makes it possible to gradually reverse the effects of existing development on flooding, channel erosion and aquatic ecosystems.  For every acre of existing impervious area draining to highly-effective measures, 75- to 500-feet of degraded downstream waters are restored to a healthier condition. 

Bioretention:  This practice is emerging as one of the most frequently used and effective low-impact development measures.  As shown in the illustration below, a pit three- to four-feet deep is excavated at a point to intercept runoff from a parking lot, house or other impervious surface.  bioretentionA Perforated pipe is placed on the bottom of the pit then covered with a layer of gravel.  Pea-gravel is placed on top of the larger stone to prevent the next layer from sifting down.  The next layer is two- to three-feet of a mixture of sand-organic matter (planting soil).  The surface is either planted in grass or covered with several inches of wood mulch.  Attractive plantings are frequently added.  The surface of the bioretention facility is depressed up to 12 inches to store the first inch of runoff from the impervious surfaces it treats.  The perforated pipe is there to draw off treated runoff should the bottom of the facility ever clog.

For every 10-square feet of impervious areas you need about 1-square foot of bioretention area.  Bioretention and related measures can be placed in locations that are normally lawn or landscaped.  This eliminates the need to set aside a large amount of developable land for stormwater management.   In many projects the use of these low impact methods can increase profit for the development company by allowing more houses or commercial space.

Highly-Effective Flood, Erosion & Ecosystem Protection:  By reducing the total volume of runoff, facilities like bioretention are more effective than ponds in preventing or correcting downstream flooding and channel erosion.  However, there are some situations where a pond may still be needed for flood management.  As illustrated by the graph below, bioretention and other infiltration practices are far more effective than ponds in keeping nutrients out of downstream waters.  The same is true for the many other pollutants commonly entrained in runoff from impervious surfaces. 

bmp pollutant removal

Maintenance, The Achilles Heel:  All stormwater practices require maintenance.  Ponds fill with sediment and spillways can be damaged by floodwaters.  Bioretention and other infiltration facilities can clog which prevents runoff from percolating into underlying soils to provide the recharge needed to maintain wells and dry-weather stream flow.  To ensure that practices are maintained, local government must regularly inspect each facility.  It appears that one full-time inspector can oversee about 600 to 1,000 facilities a year.  CEDS assessments have shown a stormwater practice failure rate as high as 50%.  This extremely high failure rate was directly attributed to a lack of inspections.  But when citizen advocacy groups lobbied for correction, inspection staffing was increased and the failure rate was cut by a third within three years.  The graph below shows the results of the 2015 Greater Baltimore Survey of nearly 400 existing stormwater practices.  The report for the 2015 survey provides all the detail you would need to initiate a similar effort in your area.

greater baltimore survey 

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How CEDS Can Help

If you are concerned that growth in your area may cause flooding, channel erosion or pollution then feel free to send us project plans for a quick, no-cost assessment of control effectiveness.  We can help you determine if the project will cause either floor-related impact and, if so, then recommend steps that have resolved similar issues in other areas.  We can also help you launch a Greater Baltimore type survey in your area to ensure stormwater practices are well maintained. 

If you don't have plans but just have a question contact us at 410-654-3021 or Help@ceds.org. Advice by phone is always provided free to those seeking to protect communities or the environmental.  However, if your question requires research we may need to change a fee.  Of course we'll request your approval before initiating the research. 

For further detail on this and related topics visit our the following CEDS webpages:

CEDS exists solely to help citizens win land development issues. Because of this specialization, we can pull together a top-notch team of leading experts to quickly analyze your situation and develop the easiest, least expensive strategy for success. Our strategies employ an aggressive approach on multiple fronts: legal, technical, and political. Because of this unique approach our clients win 90% of their cases vs. the much lower success rate (5%-10%)typical of more conventional campaign strategies. And our victories come at a fraction of the cost.  Our approach to winning land use and environmental cases is described in a 300-page book which can be downloaded free by clicking on the following title: How To Win Land Development Issues: A Citizens Guide To Preserving & Enhancing Quality of Life in Developing Areas.

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