How To Draw A Watershed Boundary
A watershed is an area of land that drains to an associated water resource such every bit a wetland, river or lake. Depending on the size of the expanse and the topography, a watershed tin can incorporate numerous tributaries, such as streams and ditches, and ponding areas such as detention structures, natural ponds and wetlands.
Rainwater and snowmelt that does not evaporate or infiltrate into the soil runs off into a nearby tributary or ponding area, and and so flows to the primary wetland, river or lake within the watershed. It is through this linkage that the upper portions of a watershed can effect downstream areas. Thus, the quality of a wetland, stream or lake often reflects the land utilize and other activities beingness conducted in the upstream areas.
Because the relationship of cause and effect tin can extend for distances throughout the entire watershed, it is of import to address ecology direction bug from a watershed perspective.
Stormwater Management
One such ecology issue which challenges many local and municipal officials today is stormwater management. Stormwater is created when rain falls on impervious surfaces and runs off the surface. This runoff tin accept negative effects on h2o quality and quantity within the watershed.
Water Quality Concerns: As development and urbanization increase within the watershed, various pollutants (eastward.thousand., nutrients, leaner, pesticides, heavy metals, sediments, organic matter, trash) volition be deposited. Urban surfaces too act as heat collectors. When stormwater runs off the urban landscape, it carries the pollutants with information technology into the receiving waters. Likewise, the temperature of the stormwater increases every bit it passes over these surfaces. Every bit a result, the quality of the water resources tends to degrade within the immediate vicinity of the urbanized area, as well equally in areas further downstream. The degradation of water quality can crusade immediate (acute) and long term (chronic) negative furnishings on aquatic and terrestrial found and animal species and their habitat, while also degrading drinking water supplies, and limiting other uses of the h2o resources such as swimming and recreation. In add-on to these water quality concerns from increased urbanization, water quality as well degrades when stream banks erode due to changes in water quantity. The sediment from stream bank erosion can alter stream bed characteristics, thereby affecting benthic habitat, and reducing light transmission.
H2o Quantity Concerns: When agronomical and natural areas get urbanized, vegetation (grasses, copse, etc) and base of operations soils are replaced with impermeable materials such as asphalt and physical. Stormwater that once soaked into the soil now runs off in greater quantities and at college velocities to the smaller tributaries and ponding areas, which in turn convey the water to the wetland, river or lake of that watershed. Without proper stormwater management, urbanization affects the water quantity, velocity, pinnacle menstruum rates, and menstruation frequency in the watershed's tributaries and river system. This can cause flooding, erosion and scouring problems downstream.
Stormwater Management from a Watershed Perspective
Effective stormwater management to protect and preserve nearby and downstream water resources can be conducted on a watershed level. To begin this process, one of the primary tools required is a map of the watershed.
Such a map can exist used to determine the cumulative furnishings of land development or country employ changes on both water quantity and water quality. Afterwards a watershed is delineated, useful determinations can be made that tin aid in the land apply planning process such as:
- Visualizing the effects that a future development may have on downstream areas, also equally the influence of additional or existing developments upstream.
- Measuring the acreage of the watershed to predict soil loss and estimate pre-development and post-development stormwater runoff volumes.
- Measuring the acreage of proposed projects to estimate pollutant loads.
- Determining advisable sites for regional stormwater detention facilities to control peak volume runoff and sedimentation.
In addition, watershed maps tin can:
- Prove natural resource assets like woodlands, prime farmland, and habitat areas.
- Target areas susceptible to soil loss and erosion.
- Place wetland areas and potential flood zones.
Watershed Delineation: A common method of locating and delineating the boundaries of watersheds is to use topographic maps post-obit the bones principle that h2o runs downhill. A topographic map represents the physical features of the land such as hills, valleys, basins, ridges, and channels. The mapping technique used is based on elevation datum (usually mean sea level) and profile intervals commonly of x feet.
Watershed Delineation Steps
i. Use a topographic map(s) to locate the river, lake, stream, wetland, or other waterbodies of interest.
2. Trace the watercourse from its source to its mouth, including all tributaries. This step identifies the general outset and ending boundaries.
3. Examine the dark-brown lines on the topographic map that are virtually the watercourse. These are referred to every bit "profile" lines. They connect all points of equal elevation above or below a known reference elevation.
The dark chocolate-brown profile lines (thick lines) will have a number associated with them, indicating the pinnacle. The light brown contour lines (thin lines) are usually mapped at 10-ft intervals, and the dark brown (thick) lines normally are mapped at l-ft intervals. To determine the final elevation of a location, just add or decrease the appropriate contour interval for every low-cal brownish (sparse) line, or the appropriate interval for every dark chocolate-brown (thick) line.
Contour lines spaced far apart point that the landscape is more than level and gently sloping. Contour lines spaced very close together indicate dramatic changes (rise or autumn) in summit over a brusk horizontal distance.
4. Check the gradient of the mural by locating two adjacent contour lines and determine their respective elevations. The slope is calculated every bit the modify in superlative divided by the altitude.
A depressed area (valley, ravine, swale) is represented by a series of contour lines "pointing" toward the highest peak.
A higher area (ridge, hill) is represented by a series of contour lines "pointing" towards the everyman elevation.
5. Determine the direction of drainage in the surface area of the water body by drawing arrows perpendicular to a series of profile lines that subtract in elevation. Stormwater runoff seeks the path of least resistance as information technology travels downward the slope. The "path" is the shortest distance between contours, hence a perpendicular route.
6. Marking the suspension points surrounding the h2o trunk. The "break points" are the highest elevations where one-half of the runoff drains toward one trunk of water, and the other half drains toward another body of h2o.
vii. Connect the break points with a line following the highest elevations in the expanse. The completed line represents the purlieus of the watershed.
Editor's Note: This article, edited to Water Online's and Public Works Online's Journalistic fashion, was prepared originally by the Watershed Direction Unit-Water Sectionalization of the U. S. Environmental Protection Agency'due south Region Five, the U.Due south. Department of Agriculture Soil Conservation Service (Illinois), the U.Southward. Fish and Wildlife Service Chicago Metro Wetlands Office, and Terrene Plant. Tetra Tech, Inc. assisted with the publication of the paper. For more data or copies of this and other articles in the serial, contact Terrene Institute at 1717 K Street, Washington, D.C. 20006; Tel. 202-833-8317.
Source: https://www.wateronline.com/doc/delineating-watershedsa-first-step-toward-eff-0001
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