Lare Section 4 - Definitions

Front 1

Fiber Rolls

Back 1

Prefabricated tubes consisting of biodegradable materials such as coconut fiber or rice and wheat straw.

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Filter Strip

Back 2

A vegetated buffer zone for removing sediments and pollutants before runoff reaches ponds, waterways, or other drainage facilities.

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Free water

Back 3

Soil water that moves by gravity, in contrast to capillary and hydroscopic water.

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Grassed waterway

Back 4

A natural or constructed channel, usually broad and shallow, covered with erosion-resistant vegetation, used to conduct surface runoff.

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Hydraulic radius

Back 5

The cross-sectional area of flow of a pipe or channel divided by the wetted perimeter.

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Hydrologic condition

Back 6

Describes the vegetative cover, residue, and surface roughness of a soil as it may affect potential runoff.

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Hydrologic cycle

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The concept of a closed system, involving the transformation of water from the vapor phase to the liquid (and solid) phase and back to the vapor phase, and the movement of that water.

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Hydrologic Soil Group or HSG

Back 8

A soil classification system based on infiltration and potential runoff chracteristics.

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Initial abstraction

Back 9

Losses before runoff begins, including infiltration, evaporation, interception by vegetation, and water retained in surface depressions.

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Intercepting ditch

Back 10

An open drain to prevent surface water from flowing down a slope by conducting it around a slope. See also diversion.

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Manning's equation

Back 11

A formula for calculating the velocity of flow in a channel as a function of relative roughness, cross-sectional configuration, and gradient.

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Maximum potential retention or S.

Back 12

The greatest proportion of precipitation that could possibly be retained by a specific soil and land use combination.

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Modified Rational Method or MRM.

Back 13

An extension of the Rational method for calculating the rate of runoff from a drainage area. It includes provisions for antecedent precipitation and for developing hydrographs.

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Moisture content

Back 14

The percentage, by weight, of water contained in soil or other material, usually based on dry weight.

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Pedology

Back 15

The study of the soil as a natural body, including its origin, characteristics, classification, and description.

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Pile

Back 16

Vertical structural member made of concrete or steel that is driven into the ground by a pile driver until there is significant resistance or bedrock is reached to support the intended load.

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Principal spillway

Back 17

A component of retention or detention ponds, generally constructed of permanent materials. It is designed to regulate the normal water level, provide flood protection, and/or reduce the frequency of operation of the emergency spillway.

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Rational Method

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A formula for calculating the peak runoff rate from a drainage area based on land use, soils, land slope, rainfall intensity and drainage area.

Q equals c.i.A.

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Rising limb (of a hydrograph)

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The portion of a hydrograph preceding the peak when the flow rate increases.

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roughness coefficient (N)

Back 20

A factor in the Manning formula representing the effect of channel or conduit roughness on energy losses in the flowing water.

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Shallow concentrated flow

Back 21

Flow in shallow rills

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Shear street of a channel

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Force per unit area exerted on the wetted area of a channel, acting in the direction of flow.

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Slide

Back 23

Movement of soil on a slope resulting in a reduced angle of repose, usually occurring as a result of rainfall, high water, or thaw.

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Soil bioengineering

Back 24

Use of live, woody vegetative cuttings to repair slope failures and increase slope stability.

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Subdrain

Back 25

A pervious backfilled trench containing a pipe with perforations or open joints for the purpose of intercepting groundwater or seepage.

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Superelevation

Back 26

The rise of the outer edge of the pavement relative to the inner edge of a curve in the highway; expressed in feet per foot, intended to overcome the tendency of speeding vehicles to overturn when rounding a curve.

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Tangent

Back 27

A straight road segment connecting two curves.

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Time of concentration (Tc)

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The time for water to flow from the hydraulically most remote point in a drainage area to the point of interest.

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Time of recession (Trec)

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The period of time from the peak of a hydrograph until it reaches the beginning flow rate.

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Time of rise (of a hydrograph) (Trise)

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The period of time from the beginning flow rate until the peak flow is reached.

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Weephole

Back 31

A small hole, as in a retaining wall, to drain water to the outside.

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Weir

Back 32

An opening in the crest of a dam or an embankment to discharge excess water; also used for measuring the rate of discharge.

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Wetted perimeter

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The length of the wetted contact between the water and the containing conduit, measured along a plane that is perpendicular to the conduit.

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What is T.R.- 55?

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Technical Release 55 was released by the NRCS in 1975. It presents simplified procedures for estimating runoff and peak discharges in small watersheds. Applies to any small watershed in which certain limitations are met.

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What are some common effects of urbanization?

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Changes the watershed's response to precipitation.

These include:

reduced infiltration and decreased travel time, which significantly increase peak discharges and runoff.

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Who is runoff determined?

Back 36

The amount of precipitation and infiltration characteristics related to:

soil type

soil moisture

antecedent rainfall

cover type

impervious surfaces

surface retention

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How is travel time determined?

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slope

length of flow path

depth of flow path

roughness of flow surfaces

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How are peak discharges determined?

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Based on the relationship of both travel time and runoff and on the total drainage area of the watershed, location of the development, effect of any flood control works or other natural or manmade storage and the time of distribution of rainfall during a given storm event.

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How does T.R. 55 work?

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The model begins with a rainfall uniformly imposed on the watershed over a specified time distribution. Mass rainfall is converted to mass runoff by using a runoff curve number or C.N.

C.N. is based on soils, plant cover, the amount of impervious areas, interception and surface storage.

Runoff is then transformed into a hydrograph.

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What is a hydrologic condition?

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Indicates the effects of cover type and treatment on infiltration and runoff and is generally estimated from the density of plant and residue cover on sample areas.

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What are hydrologic soil groups?

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Due to the variability of infiltration rates and by subsurface permeability, as well as surface intake rates, soils are classified into four hydrologic soil groups. A, B, C, D. Based on minimum infiltration rate, which is obtained for bare soil after prolonged wetting.

Any disturbance of a soil profile can significantly change its infiltration characteristics. With urbanization, native soil profiles may be mixed or removed or fill material from other areas may be introduced.

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What are factors affecting time of concentration and travel time?

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Surface roughness

Channel shape and flow patterns

Slope

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What is sheet flow?

Back 43

Flow over plan surfaces. Usually occurs in the headwater of streams.

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What is shallow concentrated flow?

Back 44

Occurs after sheet flow of 300 feet. The sum of the consecutive travel times through a watershed.

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There are two Manning's equations. What are they?

Back 45

Manning's roughness coefficient for sheet flow.

Manning's roughness coefficient for open channels.