LAND USE AND FLOODING

Johnson County has been one of the fastest-growing, or sprawling, counties in Indiana in recent years. As every landowner knows, when somebody builds upstream, the increased runoff heads their way. Local politicians, such as the County Commissioners, have seen very few development plans that they didn't like, to the point that former Commissioner RJ McConnell actually complained that county government couldn't keep up with the case load! The fact is, government's purpose is to watch out for the best interests of the PEOPLE, not developers. They are beholden to the former, not the latter. Or they're supposed to be, that is. However, the squeaky wheel gets the "grease" as they say. Which is why developers move through that revolving door of local government rubberstamping, blueprints in hand, so effortlessly.

"There is a great deal of concern throughout Johnson County about the rate of development, the type of development, and the effects of development. Many residents believe that the character of current development is affecting their quality of life... The scattered, unsystematic growth that has occurred throughout the county illustrates the need for a more proactive planning approach. Uncoordinated development in White River Township exemplifies the type of growth that can result from a lack of up-to-date master planning. There is a need to manage and organize new growth so that it has a minimal impact on the surrounding uses... A strong plan is needed to ensure orderly growth in Johnson County. Growth is bound to occur; therefore, the county needs to be proactive. The county and the incorporated areas need to work together to minimize the negative effects of sprawl." - Comprehensive Plan Johnson County, Indiana. Adopted: April 1997

Johnson County land use map from county gov't's GIS department, with outline of the Youngs Creek watershed overlaid. Runoff from development in the northern part would flow into Youngs Creek, via various courses, and through Franklin, then south towards Edinburgh.

Image clips from a study shown in the Purdue forestry exhibit at the Indiana State Fair.

Translation: The "IN" is apparently over part of Indy, with Greenwood & Center Grove in red below the "N" - Franklin below that, joined at the hip with Shelbyville; below Franklin is Edinburg-Columbus-Seymour. So, 46131 appears to be in the upper category for Parking Lot Footprint per square kilometer.

EXCERPTS FROM:

The Water Cycle: Surface Runoff
U.S. Department of the Interior | U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu/watercyclerunoff.html

Meteorological factors affecting runoff:

Type of precipitation (rain, snow, sleet, etc.)
Rainfall intensity
Rainfall amount
Rainfall duration
Distribution of rainfall over the drainage basin
Direction of storm movement
Precipitation that occurred earlier and resulting soil moisture
Other meteorological and climatic conditions that affect evapotranspiration, such as temperature, wind, relative humidity, and season

Physical characteristics affecting runoff:

Land use
Vegetation
Soil type
Drainage area
Basin shape
Elevation
Topography, especially the slope of the land
Drainage network patterns
Ponds, lakes, reservoirs, sinks, etc. in the basin, which prevent or delay runoff from continuing downstream

When rain hits saturated or impervious ground it begins to flow overland downhill. It is easy to see if it flows down your driveway to the curb and into a storm sewer, but it is harder to notice it flowing overland in a natural setting. During a heavy rain you might notice small rivulets of water flowing downhill. Water will flow along channels as it moves into larger creeks, streams, and rivers.

Human activities can affect runoff:

Picture showing erosion caused by surface runoff. As more and more people inhabit the Earth, and as more development and urbanization occur, more of the natural landscape is replaced by impervious surfaces, such as roads, houses, parking lots, and buildings that reduce infiltration of water into the ground and accelerate runoff to ditches and streams. In addition to increasing imperviousness, removal of vegetation and soil, grading the land surface, and constructing drainage networks increase runoff volumes and shorten runoff time into streams from rainfall and snowmelt. As a result, the peak discharge, volume, and frequency of floods increase in nearby streams.

Urbanization can have a great effect on hydrologic processes, such as surface-runoff patterns. Imagine it this way: in a natural environment, think of the land in the watershed alongside a stream as a sponge (more precisely, as layers of sponges of different porosities) sloping uphill away from the stream. When it rains some water is absorbed into the sponge (infiltration) and some runs off the surface of the sponge into the stream (runoff). Assume a storm lasting one hour occurs and one-half of the rainfall enters the stream and the rest is absorbed by the sponges. Now, gravity is still at play here, so the water in the sponges will start moving in a general downward direction, with most of it seeping out and into the streambanks during the next day or two.

Next, imagine that roads and buildings have replaced most of the watershed surface. When that one inch of rainfall occurs, it can't infiltrate these impervious surfaces and will runoff directly into the stream, and very quickly, too! The result is a very quick and short-lived urban flood, rather than a gradual rise and fall in the river. Still, a flood lasting even 10 short minutes is enough to ruin your basement.

This concept is illustrated by this hydrograph [below] of a rural (Newaukum Creek - blue line) and an urban (Mercer Creek - green line) creek in Washington State. If you measured the area under both curves (the total volume of water that flowed by the measurement location for the time period shown on the X axis) in the chart, they might be the same. But in the urban stream, the water at the measurement site rose at a much higher rate and reached a much higher stage (height) than the rural stream did. The tall, steep curve of Mercer Creek showed that much higher streamflows occurred in the urban stream. The urban stream stage fell back towards baseflow much quicker, too, indicating that it wasn't receiving much seepage from ground water. The rural stream rose much slower and reached a lower peak, meaning it may not have flooded at all. It took longer to fall back to baseflow as ground water slowly seeped into the streambanks over the next week.

Streamflow in Mercer Creek, an urban stream in western Washington, increases more quickly, reaches a higher peak discharge, and has a larger volume during a one-day storm on February 1, 2000, than streamflow in Newaukum Creek, a nearby rural stream. Streamflow during the following week, however, was greater in Newaukum Creek.

U.S. GEOLOGICAL SURVEY
Fact Sheet 076-03
Effects of Urban Development on Floods
URL: http://pubs.water.usgs.gov/fs07603

By C. P. Konrad

Over the past century, the United States has become an increasingly urban society. The changes in land use associated with urban development affect flooding in many ways. Removing vegetation and soil, grading the land surface, and constructing drainage networks increase runoff to streams from rainfall and snowmelt. As a result, the peak discharge, volume, and frequency of floods increase in nearby streams. Changes to stream channels during urban development can limit their capacity to convey floodwaters. Roads and buildings constructed in flood-prone areas are exposed to increased flood hazards, including inundation and erosion, as new development continues. Information about streamflow and how it is affected by land use can help communities reduce their current and future vulnerability to floods.

HYDROLOGIC EFFECTS OF URBAN DEVELOPMENT

Streams are fed by runoff from rainfall and snowmelt moving as overland or subsurface flow. Floods occur when large volumes of runoff flow quickly into streams and rivers. The peak discharge of a flood is influenced by many factors, including the intensity and duration of storms and snowmelt, the topography and geology of stream basins, vegetation, and the hydrologic conditions preceding storm and snowmelt events.

Land use and other human activities also influence the peak discharge of floods by modifying how rainfall and snowmelt are stored on and run off the land surface into streams. In undeveloped areas such as forests and grasslands, rainfall and snowmelt collect and are stored on vegetation, in the soil column, or in surface depressions. When this storage capacity is filled, runoff flows slowly through soil as subsurface flow. In contrast, urban areas, where much of the land surface is covered by roads and buildings, have less capacity to store rainfall and snowmelt. Construction of roads and buildings often involves removing vegetation, soil, and depressions from the land surface. The permeable soil is replaced by impermeable surfaces such as roads, roofs, parking lots, and sidewalks that store little water, reduce infiltration of water into the ground, and accelerate runoff to ditches and streams. Even in suburban areas, where lawns and other permeable landscaping may be common, rainfall and snowmelt can saturate thin soils and produce overland flow, which runs off quickly. Dense networks of ditches and culverts in cities reduce the distance that runoff must travel overland or through subsurface flow paths to reach streams and rivers. Once water enters a drainage network, it flows faster than either overland or subsurface flow.

The hydrologic effects of urban development often are greatest in small stream basins where, prior to development, much of the precipitation falling on the basin would have become subsurface flow, recharging aquifers or discharging to the stream network further downstream. Moreover, urban development can completely transform the landscape in a small stream basin, unlike in larger river basins where areas with natural vegetation and soil are likely to be retained.

REDUCING FLOOD HAZARDS IN URBAN AREAS

There are many approaches for reducing flood hazards in basins under development. Areas identified as flood-prone have been used for parks and playgrounds that can tolerate occasional flooding. Buildings and bridges have been elevated, protected with floodwalls and levees, or designed to withstand temporary inundation. Drainage systems have been expanded to increase their capacity for detaining and conveying high streamflows; for example, by using rooftops and parking lots to store water. Techniques that promote infiltration and storage of water in the soil column, such as infiltration trenches, permeable pavements, soil amendments, and reducing impermeable surfaces have also been incorporated into new and existing residential and commercial developments to reduce runoff from these areas. Wet-season runoff from a neighborhood in Seattle, Washington, was reduced by 98 percent by reducing the width of the street and incorporating vegetated swales and native plants in the street right-of-way.

In response to frequent flooding along the Napa River in California, the local community integrated many of these approaches into a single plan for flood protection that is expected to reduce flood damage while helping to restore the river ecosystem. The plan involves bridge reconstruction, levee setbacks, a floodwall, moving of vulnerable structures, detention basins, larger stormwater conveyances, and a high-flow bypass channel.

Download the full report (PDF, 123 KB)

Open Space Planning
Purdue University Cooperative Extension Service

The Basis for Open Space Planning

All too often land use decisions are not made by communities, but rather by developers. The piecemeal, mostly unplanned, development of our landscape has led to a steady decline in the amount of open space. The farmland and natural areas that contribute to Indiana’s traditional character are being lost. When communities lack definitive plans for preserving these lands, they are often at a loss when developers come with plans for new commercial areas or residential neighborhoods.

Comprehensive open space planning puts communities in the driver’s seat when it comes to growth. Open space planning, or land use planning, centers around the concept of communities directing their own development. When communities develop their own strategy for development, they get to decide what is important in their particular situation. Through open space planning, stakeholders can identify areas that they feel are important to protect from development and direct growth to areas that can sustain it. Open space planning can be used to develop ordinances that incorporate conservation features, or areas, into subdivisions. Farms, woodlands, and streams can be protected, and development projects can be fully supported by the community.