7.4.2 Manmade Site Conditions

The site plan provides the locations of the manmade development that will occur on site.  It establishes the spatial relationships, as well as the relationships between manmade and existing natural features.  Manmade site conditions include all development on the installation to include buildings, roadways, parking lots, walkways, walls, fences, utilities, and other facilities.  Buildings, roadways, parking lots, and above ground utilities are the primary manmade visual determinants. 

7.4.2.1 Low Impact Development (LID)

LID is a sustainable site design strategy with the goal of maintaining or replicating the predevelopment hydrologic regime through the use of design techniques that create a functionally equivalent hydrologic landscape.  Hydrologic functions of storage, infiltration, and ground water recharge, as well as the volume and frequency of discharges, are maintained through the use of integrated and distributed micro-scale stormwater retention and detention areas, reduction of impervious surfaces, and the lengthening of flow paths and runoff time (Coffman, 2000).  Other strategies include the preservation and protection of environmentally sensitive site features such as riparian buffers, wetlands, steep slopes, valuable (mature) trees, floodplains, woodlands, and highly permeable soils (EPA 2000).

The approach used in LID designs is not a new one.  LID borrows its basic hydrologic principles from nature - uniform distribution of micro-management controls.  In a natural setting, stormwater is controlled by a variety of mechanisms (interception by vegetation, small depression storage, channel storage, infiltration, and evaporation) uniformly distributed throughout the landscape.  LID mimics these mechanisms by uniformly distributing small infiltration, storage, and retention and detention measures throughout the developed landscape.  Each development feature (green space, landscaping, grading, streetscapes, roads, and parking lots) can be designed to provide some type of beneficial hydrologic function (Coffman, 2000).

Specific design principles and stormwater management techniques for implementing LID methods at Department of Defense facilities are available in the Unified Facilities Criteria Manual (UFC) 3-210-10 (http://www.wbdg.org/ccb/DOD/UFC/ufc_3_210_10.pdf.)

7.4.2.1.1 Low Impact Design Guidance

Reference UFC 3-210-10, Chapter 9 - Site Planning Process.

The primary design goal of LID is to maintain historic hydraulic functions of all drainage basins.  Eliminate any increase or alteration of the stormwater runoff flows or volumes from Fort Carson and PCMS and ensure all discharges, on and off post, meet or exceed environmental water quality standards.

LID site design reduces the hydrologic impact of development and incorporates techniques that maintain or restores a site’s hydrologic and hydraulic functions.  The optimal LID site design will minimize runoff volume and preserve existing flow paths to minimize infrastructural requirements.  LID site design strategies should achieve all the basic objectives of site design while also minimizing the generation of runoff.  LID site design considerations include:

• Grade to encourage sheet flow and lengthen flow paths.

• Maintain natural drainage divides to keep flow paths dispersed.

• Disconnect impervious areas, such as pavement and roofs, from the storm drain network, allowing runoff to be conveyed over pervious areas instead.

• Preserve the naturally vegetated areas and soil types that slow runoff, filter out pollutants, and facilitate infiltration.

• Direct runoff into or across vegetated areas to help filter runoff and encourage recharge.

• Provide small-scale distributed features and devices that help meet regulatory and resource objectives.

• Treat pollutant loads where they are generated, or prevent their generation.

• Consider channel geomorphology, sediment supply and transport; and utilize natural channel design principles.

LID site design strategies can be achieved through the use of specific Integrated Management Practices (IMP).   Below is a basic list of IMPs that should be considered to reduce stormwater impacts and the effects of development/redevelopment.  More detailed descriptions of these individual practices are presented in Chapter 8 of UFC 3-210-10.

Bioretention Area

• Bioretention:  Vegetated depressions that collect runoff and facilitate its infiltration into the ground.

• Dry Wells:  Gravel, or stone-filled pits that are located to catch water from roof downspouts or paved areas.

• Filter Strips:  Bands of dense vegetation planted immediately downstream of a runoff source designed to filter runoff before entering a receiving structure or water body.

• Grassed Swales:  Shallow channels lined with grass and used to convey and temporarily store runoff.

• Infiltration Trenches:  Trenches filled with porous media, such as bioretention material, sand, or aggregate that collect runoff and exfiltrate it into the ground.

Permeable Pavement Cross Section

• Inlet Pollution Removal Devices:  Small stormwater treatment systems that are installed below grade at the edge of paved areas and trap or filter pollutants in runoff before it enters the storm drain.

• Permeable Pavement:  Asphalt or concrete rendered porous by the aggregate structure.

• Permeable Pavers:  Manufactured paving stones containing spaces where water can penetrate into the porous media placed underneath.

• Soil Amendments:  Minerals and organic material added to soil to increase its capacity for absorbing moisture and sustaining vegetation.

• Tree Box Filters:  Curbside containers placed below grade, covered with a grate, filled with filter media, and planted with a tree in the center.

• Vegetated Buffers:  Natural or manmade vegetated areas adjacent to a water body, providing erosion control, filtering capability, and habitat.

• Vegetated Roofs:  Impermeable roof membranes overlaid with a lightweight planting mix with a high infiltration rate and vegetated with plants tolerant of heat, drought, and periodic inundation.

When selecting sites for IMPs, consideration should be given to how best to use the site’s hydrologic soil groups and site topography to reduce and control runoff.  These considerations should include:

• Maintain natural drainage patterns, topography, and depressions.

• Preserve as much existing vegetation as possible in pervious soils.

• Locate IMPs in pervious soils.

• Where feasible, construct impervious areas on less pervious soil.

• Disconnect impervious surfaces.

• Direct and disburse runoff to areas with pervious soils.

• Flatten slopes within cleared areas to facilitate on lot storage and infiltration.

• Quickly re-vegetate cleared and graded areas.

7.4.2.2 Criteria for Visual and Spatial Review of the Installation

temperate region

hot arid region

hot humid region

cool region

• Cluster buildings to reduce impact on the natural environment and reduce roadways and utility corridors needed to serve the development.  At the same time, give full consideration to antiterrorism and force protection requirements.

• Locate large buildings in relatively flat areas to reduce the cut and fill and preserve the natural vegetation and drainage and orient to topography.

• Use landform/topography to inform and guide architectural features, building design and energy efficiency alternatives.

• Provide accessibility and amenities such as benches and trees from transit stop locations to the facility.

• Minimize solar heat gain for cooling and maximize solar heat gain and retention for heating.

• Proper building orientation alone can reduce energy use up to 20%.  Rectangular buildings should be oriented with the long access running east-west.  Well planned and optimally orientated building relates well to its site and the climate.  This maximizes opportunities for passive solar heating, solar gain avoidance, natural ventilation, and day lighting opportunities.

• Site buildings with consideration for the microclimate conditions of the site that result in variances in wind or light because of adjacent land forms, structures, or trees.

• Orient outdoor pedestrian areas for most comfortable exposure.

• Utilize lighter color on building surfaces exposed to the sun and darker colors on recessed surfaces to absorb radiation.

• Orient windows according to impact of climatic conditions.

• Locate development on leeward side of hills.

• Design and locate roads to provide a hierarchy of traffic carrying capacities.

• Locate roads to blend with topography and vegetation.

• Design and locate parking lots to minimize visual impact of broad expanses of pavement and vehicles.  Consider incorporating LID techniques in parking lot design.

• Design and locate pedestrian walkways and bicycle paths to fit the physical environment and to provide a comfortable pedestrian experience, limiting conflicts with vehicular traffic.  Make pedestrian and bicycle access to the building convenient and otherwise encourage these modes as preferred transportation alternatives.

• Locate trees and shrubs to buffer harsh natural conditions.

• Deciduous materials provide for sun in the winter and shade in the summer.  Evergreen materials provide windbreaks for cold north winds.

• Design and locate site elements to blend with and enhance the physical environmental.

• Force protection requirements should be designed and located to blend with the physical environment.

• See Sections 9-12 for more site planning requirements and criteria.

Site Planning Design Criteria