Study Guide Ch. 5 Opportunities and Constraints Flashcards
Watershed
Land area that contributes surface water to given location and is defined by surface topography
Composed of subwatersheds (typically ranging in size from a few to several square miles)
Watershed boundaries
Occur along ridges and water flows from these high points into valleys and other low points (such as rivers), with ridges dividing watersheds or areas within a watershed. This process defines the watershed boundary
Subwatersheds
Composed of a group of catchment areas (usually measured in acres)
Typically ranging in size from a few to several square miles
Time of Concentration
Amount of time needed for water to flow from the most remote point in a watershed to the watershed outlet (or a given point inside a watershed)
Note that the most remote point in the watershed is not necessarily determined by distance, since overland and channel flow time is dependent on slope, surface, and channel characteristics
Evapotranspiration
Evaporation occurs on land and water surfaces
Transpiration from plants also returns water to the atmosphere
May recycle as much as 50 percent of precipitation back to the atmosphere and the rate of evapotranspiration depends upon numerous factors, most notably climate
(i.e., evapotranspiration occurs at a much higher rate in dry, hot conditions than it would in a cold humid climate)
Surface runoff
Precipitation that runs off the land’s surface and flows downhill.
Precipitation, soil type, slope, and vegetation all influence the amount of surface runoff.
Example: areas with low-permeability soils, steep slopes, and large areas of impermeable paving will have more surface runoff than areas with highly permeable soils, gentle slopes, and little paving
Note that runoff will flow from the site’s high point to the site low point following the prevailing site topography
Water always flows perpendicular to the direction of the contour line
Recharge
Water that infiltrates the lands’ surface and percolates downward to the underlying water table, the upper surface of groundwater
Areas with high-permeability soils, minimal slope, and sparse vegetation have the highest recharge rates.
Groundwater
Subsurface water flow that discharges to streams, lakes, wetlands, or the ocean.
Under normal conditions, this discharge is called “base flow”
Riparian Corridor
A riparian zone or riparian area is the interface between land and a river or stream, and the riparian corridor is the area that encompasses a river or stream and the land adjacent to it, with some sources defining the outer extent of the corridor as the area within the streams mean flow during the raining season or during normal precipitation events.
Note that erosion in riparian systems occurs on the outside bank of the river, whereas the sediment deposition occurs on the inside bank of the river.
Water Table Depth
Varies between sites and surface water features (e.g., lakes) form in those situations where the water table is higher than the land below it.
High water tables constrain development in several ways:
- prevent adequate site drainage, including groundwater recharge
- preclude the use of septic systems (in areas not connected to a sewer system)
- complicate subsurface excavation
- require waterproofing for building foundations and subsurface structures (e.g., basements)
Floodplains
The area of land adjoining a body of water that has been or may be covered by floodwater. Flooding occurs within the floodplain.
Areas that accommodate floodwaters in excess of channel capacity by storing floodwater, thereby reducing peak flows downstream.
Are a “pressure-relief valve” for the channel by regulating the increase in stream energy in the channel
Floodplains, and flood risk, are delineated
By the frequency with which they are likely to flood
100-year floodplain: an area of land that has a 1% chance to flood in any given year.
The extents of floodplains can be determined by looking at four key variables:
- topography
- soils
- vegetation types
- extent of past flood flows
Flood Insurance Rate Maps
Produced by FEMA
document floodplains and special hazard areas throughout the US
Upstream development impacts floodplains how
Reduces the amount of previous surface that can absorb precipitation, more water is forced downstream during weather events, and a x-year flood can become a x-year flood with correspondingly higher chances
50-year 2%
25-year 4%
Floodplains are composed of three areas
- channel
- floodway
- flood fringe
Channel
The portion of the floodplain where a stream/river flows under normal conditions
Floodway
Portion of the floodplain that is used to convey floodwaters during a 100-year flood
Flood fringe
The portion of the floodplain outside of the floodway that does not convey floodwaters and usually contains slow-moving or standing water
Base Flood Elevation (BFE)
Whole-foot elevations of the 100-year floodplain that have been studied in detail at selected intervals.
In areas where building has occurred within the 100-year floodplain, BFE calculations are often used to determine the height to which living spaces must be constructed to be safe from a 100-year flood.
Freeboard
And portion of the flow in excess of the base flood elevation (measured in feet)
Building within 100-year floodplain according to LARE
Buildings should not be placed inside the boundaries of the floodplain
You should choose uses that do not require erecting any structures on site (e.g., golf courses, recreational uses, gardens)
Flood hazards are exacerbated by the presence of
“Hard” engineering structures such as levees and riprap
They simply redirect flood energy further downstream and often result in increased stream bank erosion
Flood hazards are best mitigated by:
- expanding opportunities for stormwater infiltration
- minimizing the uses of impervious surfaces
- decreasing the volume of runoff during storm events
- through restricting development to areas outside of floodplains
- replacing “hard” engineering structures with “green” infrastructure (e.g., live plant/willow cuttings) that slows water velocities along the channel
Soils well-suited for stormwater infiltration
Highly permeable soils
Containing large ratios of sand and/or larger material such as gravel
Soils poorly suited for infiltration
Clay soils known for their low permeability
Used to elucidate specific characteristics on a site
Vía a soils test
Existing uses (e.g., industrial contamination)
Vegetation (e.g., presence of acid-loving plants)
Hydrology (e.g., erosion patterns, ponding/slow drainage)
Siting rural developments and soil percolation
Sites with a slow rate of percolation cannot accommodate septic systems, and these areas often preclude the development of housing or other uses that might require a septic system (where a municipal sewer system is not present)
Permeability
Rate at which water moves through soil
Infiltration rate
The rate of speed at which water flows into soil through small pores
Percolation
The downward movement of water in a soil
Expansive Soils
Soils that swell when exposed to large amounts of water and shrink when the water evaporates
Typically contain large ratios of clay
Best method is to find alternative construction site
Ground heave
Upward movement of the ground usually associated with the expansion of clay soils that swell when wet
As the soil generally cannot expand downwards or sideways, the result is that the exposed upper surface of the soil rises up (and is therefore the opposite of subsidence)
Liquefaction
Takes place when loosely packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking (e.g., earthquakes)
Makes solid soils behave like liquids
The greater the soil density, the lower the liquefaction risk, therefore dense, clay soils tend to have a much lower risk of liquefaction
Differential Subsidence
Subsidence occurs when the soil beneath a building is unstable and sinks downward
Differential occurs when two points settle at different rates
Several causes but within LARE be aware that placing a single structure across two separate soil profiles could cause differential subsidence to occur
Settlement
Downward movement of the ground caused by the weight of a structure
Landslides
Caused by a number of factors including heavy rainfall, unstable soils, extreme topography, loss of vegetation and construction activity
Regulations typically prevent construction on sites that are vulnerable to landslides, and geotechnical engineers are often employed during the design process to determine landslide risk on a site, as well as any necessary mitigation measures (e.g., catch walls, diversion ditches)
Note that roads and major circulation pathways should never be placed in areas prone to landslides and areas with highly erosive soils
Damage by earthquakes is exacerbated by
- building height (i.e., taller buildings are more susceptible to earthquake damage)
- hillsides, ridges and steep topography
- proximity to major fault lines
- placing structures parallel to the anticipated direction of seismic waves
Most important factor in determining a site design
Site topography
Affected by the topographical features on a site
- building location
- road alignments
- pedestrian circulation and safety
- stormwater management
Site planning and design should follow or otherwise relate to existing landforms to
Respect context
Grading causes significant site disturbance and is costly
Site elevations impact:
Drainage patterns
Visibility
Elevation both on a site and in the surrounding landscape determines
Size and spatial configuration of local viewsheds
Elevation data is generally shown in two different ways on topographic maps
Spot elevations
Contour lines
Spot elevations
Highly accurate readings shown for specific points
Often areas of importance to designer (e.g., finished floor elevation of structure, top or bottom of wall) or to the understanding of the landform (e.g., high point of hill, summit of mountain)
More accurate than information provided by contour lines
Contour lines
Lines on a topographic map that establish the elevation at any given point along that line
Contour intervals (vertical distance between each contour line) will vary according to the accuracy of a map, but the elevational information provided by the contour line is generally considered accurate to one half the contour interval given
If a map provides 2-foot contour intervals, that map would be considered accurate to one foot
Topographical information can be used to
Determine the intensity of a slope
Closely spaced contour lines
Indicate steep slope
Contour lines that are widely spaced indicate
A gentle slope
Contour line can also indicate the “signature” of a landform
Ridges are identified by contour lines that point downhill
Valleys/Swales are identified by contour lines that point uphill
Character of slopes
0-3% nearly level
3-7% gently sloping
7-12% moderately sloping
12-25% strongly sloping
25-40% steeply sloping
40-70% very steeply sloping
70%+ extremely sloping
Most sources identify 2% as the minimum slope necessary for a site to shed water and have proper drainage
Slope Analysis
When mapped onto a site, slope percentages can be used to conduct a slope analysis
Used to identify steep and unbuildable slopes and to identify the possible location for building sites and access (e.g., roads, walkways, etc.) as well as for stormwater management
Usually a graphic representation of slope shown in classes or ranges, with these ranges corresponding to (or precluding specific site uses)
Slope Aspect/Orientation
Direction that the slope faces relative to the sun
Aspect is typically described by compass direction (e.g., a northerly aspect) and - along with slope - impacts the amount of solar radiation received at a given location on a site
For example, orientation toward the sun may influence a building’s energy use and performance, and it helps determine what types of vegetation can be planted in specific areas (e.g., sun, part sun, shade)
Military Crest
Refers to the point on a hill just below the top of the hill that offers greatest visibility of the slope below
Relief
The relative difference in elevation between two or more points
Microclimate
Site-scale variations in climate due to topography, vegetation, and orientation, among other variables
Micro-climatic conditions comprise the vast majority of climate-related subject matter on the Section 3 exam
Southern slope receive
The most sun in winter months
Southeastern slopes
Offer the most desirable microclimates
North-facing slopes are colder
Than south-facing slopes
Western slopes are
Hottest in the summer
Cold winter winds
Blow from the northwest
Cooling summer breezes
Blow from the southwest
Water features (e.g., lakes) mitigate
Climate extremes (i.e., they act as a heat sink in winter and provide cooling effect during summer)
Hot, arid
- avoid heat-absorbing materials (e.g., metal) and large exposed glass surfaces
- use walls to create favorable microclimates
- deflect hot,dry winds with earthworks, screening and walls
- prevent heat build-up in structures through the use of thick walls
- use a drought-tolerant plant palette
- use pergola and trellis structures on south and southwest walls
- provide shade in outdoor areas, but position them such that they allow
for winter sun
Hot, Humid
- avoid using solid walls and earthworks that block cooling winds
- use high canopy trees and open planting patterns to maximize evaporation and cooling winds
- provide shade structures with high ceilings/venting
Temperate
- promote shade and evaporative cooling during summer months
- block winter winds without disrupting summer wind patterns
- locate uses and structures along a southern orientation (from SSE to SSW) to promote solar gain in winter
Cold
- locate uses and structures along a southern orientation (from SSE to SSW) to promote solar gain in winter
- avoid northern entrances to site and buildings
- block winter winds with mixed deciduous and evergreen plants
- use earthworks, walls or structures to divert cold NE/NW winter winds
- promote cooling summer winds
- provide afternoon shade during summer months using deciduous trees
Urban heat island effect
Occurs when natural land cover is replaced by dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat
This effect increases energy costs (e.g., air conditioning), air pollution levels, and heat-related illness and mortality
Note that surface albedo (the ability of surfaces to reflect sun energy) plays an important role in urban heat islands, with dark surfaces absorbing and storing heat and light surfaces reflecting heat energy back into the air
Key urban heat island points
- dark roofs are significant contributors to the urban heat island effect
- dark asphalts absorb more heat energy than concrete
- green roofs can be used to mitigate urban heat islands (and roofs can be painted white to reflect solar energy)
- vegetation and water can be used to mitigate urban heat islands
- tree canopy cover can play a significant role in reducing surface temperatures in urban environments
Note that although this phenomenon is considered to be negative
The urban heat island effect does provide a beneficial warming effect during winter months in extremely cold climates
Urban-wildland interface
Areas where human development abuts undeveloped land
Present the highest potential for wildfires because urban uses introduce a variety of fire catalysts (e.g., cigarettes, grills, etc.) and are exposed to any baseline fire danger that already exists in these “wild” areas
Fire danger is determined by the following three variables
- existing fuel load (the amount of flammable organic material in an ecosystem)
- weather
- topography
Best fire management practices entail
Proactive removal of fuel and creating a landscape that provides a fire-resistant buffer surrounding all structures
Fire resistant buffer divided into
1. Building protection zone
2. Building buffer zone
3. Natural area fuel reduction zone
Building Protection Zone
Located 30 ft from all sides of a structure
Either be free from vegetation or contain only low-growing, fire-resistant plant species
Site furnishings should be constructed of nonflammable material (e.g., metal, stone) and gravel, recycled glass, or other nonflammable material should be used instead of bark mulch.
Fire departments will often provide a list of plants suitable to be planted within this “defensible space” and they advise against planting trees near houses because trees can create a “fuel ladder” effect in which fire climbs up the tree (from the ground), thereby igniting the structure. Landscape maintenance should reduce the structure’s vulnerability to fire dangers and allow easy access for fire suppression equipment if an emergency arises
Building Buffer Zone
Extends from the edge of the building protection zone to all areas within 100 ft. of a structure
Well-spaced and pruned trees are allowed within the building buffer zone. Trees and shrubs should be spaced at least 10 ft. from one another, as measured from the edge of crown. Plants should be removed in density is too high, although pruning can also be used to maintain spacing in certain instances. Any tree branches that are within 6 ft. of the ground or one-third of live crown height (whichever is greater) from the ground should be removed to discourage the “fuel ladder” effect. Landscape maintenance should be conducted as often as necessary to maintain the spacing and branch structure described above.
Natural Area Fuel Reduction Zone
Extends indefinitely beyond the building buffer zone, with the goal of reducing the available fuel load by removing debris, brush and dead branches that accumulate over time.
Although fire management maintenance in this zone is conducted less frequently than in the other two zones, many of the same principles still apply. As in the building buffer zone, trees in the natural area fuel reduction zone should be pruned to prevent “ladder fuel” formation, and selective pruning should be used to address unsafe conditions.
Carrying capacity
Measure of the type and density of development that can be supported without detrimental effects to society, the economy, or the environment and without decreasing the capacity of the environment to sustain these uses into the future.
Habitat fragmentation
The breaking up of a continuous habitat into smaller patches with diminished ecosystem function and less biodiversity
These changes affect the movement of the organisms (especially migration) and can alter ecosystem characteristics to the extent that they no longer support native vegetation and wildlife
Patches
Fragmented habitats
Corridors
Areas that connect patches
Gaps
Areas between patches that are not connected by a corridor
Typically possess little ecosystem value
Size of patch and the distance between patches correlate to ecosystem impact
Larger patches and those located closer to one another tend to support greater biodiversity and improved ecosystem function
The best way to preserve biodiversity is to
Avoid activities that cause habitat fragmentation
Large, contiguous natural areas - especially riparian corridors and wetland areas - should be given the
Highest priority for protection from development
Existing corridors require protection to help
maintain biodiversity and ecosystem connectivity
Habitat restoration is most effective when
It addresses gaps in existing corridors
Many animal species need more than one habitat type for different life cycle stages, such as
Reproduction and migration
Wetlands
Fragile ecosystems
Unnecessary excavation and grading can disrupt groundwater flows that feed into them, causing irreparable harm
Heavily reliant on watershed management and are exceptionally sensitive to “upstream” changes in a hydrological system
For LARE wetlands
No construction should ever occur inside wetlands (or - if forced to choose - the proposed use should be the least disruptive) and stormwater should not be drained into wetlands without being sufficiently treated for quantity/volume and quality
A wetland will confer the following benefits to a site and its users
- groundwater recharge and discharge
- sediment stabilization
- flood attenuation
- water quality maintenance
- wildlife habitat
- climate moderation
- shoreline protection
Wetlands are defined by their
Hydrology
Soils
Presence of specific vegetation (namely, hydrophytes)
US Fish and Wildlife Wetland Classification System defines five major wetland types, and they are as follows:
1. Marine (open ocean and its associated coastline)
2. Estuarine (tidal waters of coastal rivers, salty tidal marshes, mangrove swamps, and tidal flats)
3. Riverine (rivers and streams)
4. Lacustrine (lakes, reservoirs, and large ponds)
5. Palustrine (marshes, wet meadows, fens, bogs, swamps)
Note that salt marshes are generally considered to be the most important, productive and diverse of all ecosystem types. Situated at the interface between land and sea, and salt and freshwater ecosystems, salt marshes offer habitat and food to a wide variety of terrestrial and aquatic life
Bog
Type of wetland found in northern climates that is characterized by acidic soils, rich deposits of organic material (such as peat) and a diversity of vegetation types.
Note that bogs and fens are identical with the exception that bogs are fed by rainwater, whereas fens are fed by groundwater
Estuary
A semi-enclosed coastal body of water connected with the open sea. Estuaries are strongly affected by tidal action and contain brackish water (seawater is mixed with fresh water from land drainage)
Littoral Shelf
A shallow area in a water body that is planted with native aquatic vegetation normally located by an outflow or at the waterline/shoreline of a constructed wetland or stormwater management structure. Plantings located in the littoral shelf are used to filter out the nutrients and improve water quality.
Marshes
Type of wetland characterized by herbaceous vegetation no taller than 6’
Swamps
Type of wetland dominated by woody vegetation
Acidification
Occurs in water due to excessive carbon dioxide or the presence of sulfur and nitrogen compounds that dissolve in the water.
Occurs due to air pollution or as a large-scale process related to global climate change
Eutrophication
Is the over abundance of nutrients (e.g., nitrogen, phosphorous) in water, leading to excessive plant growth (typically algae)
Often results from fertilizer runoff from agricultural lands or residential yards
Noise
Generally travels by line of sight and dissipates over distance
Comprises the volume/intensity of the noise, the nature of the noise source, the height of the noise source in relation to the elevation/height of the site, the material characteristics of the surrounding area (i.e., vegetated and soft surfaces tend to absorb noise, whereas hard, paved surfaces tend to reflect noise) and the presence of (or lack thereof) sound shielding or barriers
Noise-sensitive uses should be built as far from the noise source as possible, and, if possible, noise-reducing barriers should be placed between the noise source and the site in question
If conducting a noise analysis on a site, the landscape architect should record three key items:
- source and type of the noise
- direction and distance that noise travels from the source
- duration and intensity of the noise
The effectiveness of any noise barrier will depend upon five factors
1. Distance
2. Height
3. Continuity
4. Length
5. Mass
Distance to the noise source
Noise barriers should be constructed as close as possible to either the noise source or to the receiving location in order to maximize noise diffraction
Height of the barrier in relation to the noise source
At minimum, barriers should be built to a height such that they block the line of sight between source and receiver
Continuity of a barrier
A single, continuous barrier is more effective than multiple, fragmented barriers because noise will penetrate any gaps in a barrier
Length of a barrier
The length of a noise barrier should generally be 1-2 times the distance between the source and the barrier to minimize sound diffraction that would otherwise travel around the barrier to the receiver
Mass of the barrier
Barriers with greater mass have a greater impact on noise mitigation.
For example, a 6’ tall earthwork is a better noise barrier than a 6’ tall brick wall because the earthwork has considerably more mass than the brick wall
Most important considerations when designing a noise control feature
Acoustic effectiveness
Cost
Visual attractiveness
Surface texture impacts noise diffraction
A smooth surface reflects sound more than a textured surface
Vegetation makes a relatively poor barrier to noise
If plantings must be used as a noise barrier, the density of the plant (densely branched plants are better at blocking noise than plants with an open branch structure) and its foliage type (evergreen foliage will dissipate noise throughout the year, whereas deciduous plants would only offer seasonal benefit)
