The following features should be considered when designing a retention ponds. 1. Maximize flow length between inlet and outlet. A suggested minimum length-to-width ratio is 3:1. If it can not be achieved, baffles can be used to break up flow. 2. The pond should expand gradually in the direction of flow. 3. Pool depth should be between 4 and 8 ft. A level safety bench at least 10 ft wide by 1 ft deep should be provided around the perimeter of the pond.

2.Reduce the dimensions of parking spaces and aisle widths to reflect more accurately the size of today’s automobiles.
Today many automobiles can be easily accommodated in a 9 ft
Detention facilities

Detention facilities, or dry basins, are used as a means of controlling peak discharge rates through the temporary storage of storm runoff.
Retention ponds

Retention ponds are basins that contain a permanent pool of water.
Benefits: increased property values, recreational opportunities, and the creation of wildlife habitat.
Hydrologic Cycle
Hydrologic Changes

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Peak discharges can be as much as two to five time the predevelopment rate, generally increasing the frequency and severity of flooding.
The design features of detention basins are similar to those of retention facilities in many ways, however, additional features are required to increase potential use.
this was accomplished by conveying runoff by storm sewers, swales and
channels to the nearest water body.

The most recent philosophy with regard to storm water management is to develop a comprehensive, integrated approach that address water quality in addition to the volume and rate of runoff.
Principles and Techniques

Measures that have been developed to control, store, and treat storm water are referred to as best management practices (BMPs).

4. The use of a complex and organic geometry for the shape of and the topography within the pond can improve both the feature’s treatment of storm and its habitat value.

2.Identify, assess and prioritize problems 3.Establish management objectives for the watershed and to identify the nonstructural objectives, such as land acquisition, impervious cover restrictions.
requires approval from appropriate governing agencies.


Cluster development
Concentrate residential development while preserving a certain percentage of the site in open space.
Storm Water Management Strategies


Watershed-based storm water management
1.Characterize the watershed in terms of flooding, water quality, surface cover and land use.

Disadvantages: safety problems, algae bloom, offensive odors,
mosquitoes, or the need for maintenance and sediment removal.
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Flood elevations are raised. The quality of storm water is degraded.
Management Philosophy

Historically, the primary concern in dealing with storm water runoff was to remove it as quickly as possible from a developed site. Traditionally,
Storm Water Management
Storm Runoff

Storm or surface runoff is storm water that moves on the ground by gravity and flows into streams, rivers, ponds, lakes and oceans.

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1. The length-to-width ratio is not as critical for a detention basin as retention basin, but an elongated form is still preferred.

Higher runoff velocities reduce the time it takes the peak discharge to reach a
stream or drainage channel, and provide less opportunity for infiltration.

4.Implement a long-term monitoring and enforcement program.
Reducing imperviousness


Parking lots
e realistic parking demand ratios for a particular land use or development project.

Streams are widened due to increased volume and velocity, which results in increased stream bank erosion.

Eroded material is deposited in stream channels as sediment, which reduces stream flow capacity.

Subsurface runoff is storm water that infiltrates and moves through the soil both horizontally and vertically.

The rate of movement is influenced by soil permeability and usually occurs at a much slower rate than surface runoff.

2.The side slopes of the basin should not be steeper than 3:1(H:V). The floor of the basin should have a 2% minimum slope toward the outlet to ensure positive drainage. An access way at least 10 ft wide with a slope of 5:1 or less should be provided for maintenance equipment.