Section 3-Site Planning and Management Section 4-Predicting Soil Loss Flashcards
Resource Planning
on site/off site resources to protect and avoid during development
Erosion and sediment hazards
- large increase of soil exposed to erosion
- increased water runoff, soil movement, sediment accumulation and peak flows caused by:
a. removal of plant cover
b. decrease in area of soil with addition of impervious surface
c. change in drainage areas through grading, diversions, streets
d. change in volume and duration by altering steepness, distance, surface roughness
e. soil compaction
f. prolonged exposure of unprotected sites
- altering groundwater regime affecting drainage systems, slope stability, survival of existing vegetation and plant establishment
- exposing subsurface materials too rocky, acid, alkaline for plant establishment
- obstructing flow with new buildings, dikes, and fills
- improper timing and sequence of construction and development
- abandonment of site before completion
Factors that influence erosion- Soil erodibility
vulnerability of a soil to different erosion processes. Soil texture, structure, and percentage of organic matter. Most erodible contain silt and very fine sand. Clay and organic matter decreases erodibility. Clays are cohesive binding soil particles. Organic matter acts as an aggregate.
Factors that influence erosion-Vegetative Cover
protects soils from raindrop impact and runoff scour. Top growth shields soil while root mass holds soil in place. Moderates freeze/thaw, shrink/swell that causes detachment of bare soil and increases available transport by overland flow. Grass can act as sediment filter and slow velocity increasing infiltration capacity. Most important factor in minimizing erosion during development
Factors that influence erosion-Topography
slope length and steepness impact volume and velocity. Long slopes=more runoff to base of slopes. Steep slopes=increased runoff velocity. Slope roughness reduces both impacts
Factors that influence erosion-Climate
frequency, intensity, duration directly influence amount of runoff. Increased frequency, intensity, long duration=less chance to drain. Saturated soils increase runoff potential.
Factors that influence erosion-Season
temperature and rainfall. Soil thaw at surface with subsurface freeze creates runoff. Seasons of higher intensity, higher frequency storms=higher runoff.
Land Development Plans-Geography
land use, size, topo, drainage, geology, hydrology, soils, veg
Land Development Plans-Study of Soils
- descriptions, erodibility, limitations, capabilities
- engineering properties-grain size, plasticity index
- suitability for topsoil
- structural tolerances-buildings, roads, dams, veg
Erosion and sediment control plan-shows
existing topo, how and when it will be altered. Shows erosion and sediment control measures, how/when implemented/maintained. Phased schedule showing practices during construction
The Planning Process-Plan development that fits site
how it can be developed to minimize environmental damage through grading, avoiding sensitive areas, offsite impacts
The Planning Process-Determine limits of grading and clearing
minimize when possible. Staged clearing vs. mass clearing
The Planning Process-Divide site into natural drainage areas
define watersheds. Limit erosion at source rather than control sediment at perimeter
The Planning Process-Design Erosion and Sediment Control Practices
structural vs. vegetative. State/local handbooks to be used.
Design Erosion and Sediment Control Practices-Vegetative controls
best way. Temp seeding or mulching. Permanent stabilization as soon as possible. Control plans must contain provisions for permanent stabilization. Seed type, amendments, bed prep, and mulching should be described. Permanent veg include the following for each plant species:
a. establishment requirements
b. adaptability to site conditions
c. aesthetic and resource value
d. maintenance requirements
Design Erosion and Sediment Control Practices-Structural controls
when vegetative control cannot be promptly used
The Planning Process-Plan map details
- map of existing topo/grading
- Provisions for erosion/sediment control
- schedule of construction and erosion/sediment control activities
- maintenance phasing
The Planning Process-Implementation of Erosion/sediment control
good construction management,
- clear only what where under construction. Phase large projects.
- restablize asap
- divert off site run on and convey to stable areas
- physical demarcation of limits of disturbance
- educate workers
- establish person of responsible charge
- implement daily inspection program
The Planning Process-Planning Assistance
- streets at right angles create excessive grades
- utility plans include sediment control
- quality improved when open space, parks, etc incorporated into plan. Base upon soils, veg, topo, wildlife, aesthetics
- plant materials/mulch retain existing vegetation
- control measures installed asap after construction starts
- maintenance/installation of permanent BMPs promptly
Construction Sequences scheduling -Preconstruction actions
evaluate, mark, protect important trees, unique areas to be preserved, onsite septic fields, filter strip suitable veg
Construction Access
Entrances, routes, parking
Sediment barriers and traps types
basins, traps, silt fences, inlet protection
Runoff Control types
diversions, perimeter dikes, water bars, outlet protection. Install after sediment traps but before grading
Runoff conveyance system types
stabilized stream banks, storm drains, inlet and outlet protection, channels. Stabilize stream banks, install principle conveyance system. Install remainder after grading
Land clearing and grading mitigation
grading, drains, sediment traps, barriers, diversions, surface roughening
Surface Stabilization types
temporary and permanent seeding, mulching, sod, rip rap
Landscaping and final stabilization types
top soiling, plant materials, seeding, mulching, sodding, rip rap-remove temporary control measures and stabilize
Revised Universal Soil-Loss Equation (RUSLE)
Soil loss equations were developed to extrapolate limited erosion data that have not been directly represented in research
Erosion and sedimentation by water involve the processes of
detachment, transport and deposition of soil particles. Major forces are raindrop impact and water flowing over land surface.
Sediment Yield and erosion are/ are not the same
are not
Sediment yield
amount of eroded soil that is delivered to a point in the watershed that is remote from the original of the detached soil particles. It includes erosion from slopes, channels, and mass wasting, minus the sediment deposited after it is eroded but before it reaches the point of interest.
RUSLE does/ does not estimate sediment yield
does not estimate sediment yield. It computes the average annual erosion expected on field slopes. Designed to predict the longtime average annual soil loss (A) carried by runoff from specific slopes in specified management conditions
RUSLE Equation
A=R x K x L x S x C x P
A=
computed spatial average soil loss and temporal average soil loss per unit of area, expressed in the units selected for K and the period selected for R. In practice, these are usually selected so that A is expressed in tons per acre per year.
R=
Rainfall-runoff erosivity factor-the rainfall erosion index plus a factor for any significant runoff from snowmelt.
K=
Erodibility Factor-the soil-loss rate per erosion index unit for a specified soil.
L=
slope length factor-the ratio of soil loss from the field slope length to soil loss from a 72.6’ length under identical conditions
S=
Slope Steepness Factor-the ratio of soil loss from the field slope gradient to soil loss from a 9% slope under otherwise identical conditions.
C=
management factor-the ratio of soil loss from an area with specified cover and management to soil loss from an identical area in a bare condition.
P=
Erosion Control Practice Factor-the ratio of soil loss with certain conservation practice to that of no practice
Soil loss at various parts of the slope do/ do not differ greatly from one area of the slope to another
do
Soil loss at top is much more then soil loss from the bottom T/F
F-Soil loss at top is much less then soil loss from the bottom
variable steepness on a slope can causes higher variation T/F
T-Areas experiencing higher losses in that given area can be seriously excessive in comparison to the rest of the slope
RUSLE is a tool meant to determine specific events. T/F
F-RUSLE is a tool meant to determine averages over long periods of time, not specific events.
C factor will account for compaction T/F
T
LS factor accounts for
slope steepness effects on soil loss from disturbed lands. Research on land disturbance, i.e. construction is not extensive as with most other applications. LS chart available rather then figuring L and S seperately
Rainfall-runoff Erosivity factor (R)-
differs greatly from one area of the slope to another. Observations are that major erosion only happens in a few severe storm events at peak intensities. Data collection debunks this. It accounts for the effect of raindrop impact and the amount of runoff likely to be associated with rain. The factor can be determined by maps.
Soil Erodibility Factor (K)-
rate of soil loss per rainfall erosion index unit [Tons x Acres x h(hundreds of acres x ft-tons x in)^-1] as measured by a unit plot. The unit plot is 72.6ft long, has a 9% slope and is continuously clean-tilled fallow condition with tillage upslop eand downslope. Complex-ease with which soil is detached by splash during rainfall, by surface flow, or both. In the RUSLE it accounts for the influence of soil properties on soil loss during storm events on upland areas.
Soil Erodibility factors 3 values to account for
for soil loss on construction sites because these layers are exposed during site disturbance.
Surface soil (A layer)-topsoil 0-10in thick Surface soil (B layer)-subsoil between A and parent material. 10-26 in thick Substratum (C layer)-upper part of parent material. 26-60 in thick
Soil survey reports give the depths. K factor can be calculated using a provided nomograph or provided K from soil type table. Soil info required to use the nemograph includes grain size distribution, soil structure, permeability, percent organic matter.
Slope Length Factor (L)
the effect of topography on erosion is accounted for by the LS factor. Erosion increases as slope length increases, represented by slope length factor (L).
Slope length is defined as the horizontal distance from the origin of overland flow to the point where either:
- slope gradient decreases through enough that deposition occurs
- runoff becomes concentrated in a defined channel
Surface runoff usually concentrates at 400 ft-practical slop-length limit in many situations. Slope lengths over 1,000 ft sometimes found.
Unless carefully graded into furrows and ridges, slopes exceeding ? ft should not be used in the RUSLE.
1000
___________ is the best method of measurement because topo maps don’t usually have enough detail to show concentrated flow areas defining RUSLE lengths
pacing off
Erosion prediction usually evaluate Length and steepness together (LS) T/F
T
slope length factor (L) long average erosion for
a slope length λ (in feet) USE FOR UNIFORM SLOPES
Slope Steepness Factor (S)
reflects the influence of slope gradient on erosion. Steepness is estimated in the field by use of clinometer, abney level or similar device. Can be determined with 2’ contour interval with considerable care.
Soil loss increases less with slope steepness then slope length T/F
F- Soil loss increases more with slope steepness then slope length
Shape of slopes affects the soil loss and loss along the slope. Concave has significantly more while convex has significantly less. T/F
F Shape of slopes affects the soil loss and loss along the slope. Concave has significantly less while convex has significantly more.
Max erosion at concave is about ___ down the slope.
1/3
Sediment Yield from ith segment from top of slope
Ei=R Ki Ci Pi Si [((λi)m+1)-(( λi-1) m+1)]/(72.6) m
Ei = sediment yield from ith segment from top of slope
R = rainfall and runoff factor
Ki = soil erodibility for an ith segment
Ci = crop-management factor for the ith segment
Pi = erosion-control practice factor for the ith segment
Si = slope-steepness factor for the ith segment
λi = length (f) from top of slope to lower end of ith segment
m=β/(1+ β) - a variable slope-length exponent
β =(sin (θ)/0.0896)/ (3.0 sin (θ)0.8+0.56) - rill erosion (caused by flow) of inter-rill erosion (principally caused by raindrop impact)
θ = slope angle
Slope length derivation-upside, walk upslope perpendicular to contours, until
origin of overland flow is reached
Slope length derivation-Lower end-walk downslope until
broad area of deposition or natural waterway is reached
Usually starts to deposit when decreased to about
5%
Calculation of where deposition ends
- Calculate the ratio of slope steepness at the end to the slope steepness where deposition begins.
- Subtract that ratio from 1.0
- Multiply that difference by the distance from where deposition begins to the end of the slope.
- Add that product to the distance where the deposition begins.
Ex 400’ slope 2% slope at the end Deposition begins at 250’ w/ slope at 5% Slope steepness ratio is 0.40 Depositions begins to the end of slope is 150’
250+(1.0-0.40)(150)=340’
Cover Management Factor (C)-
used by USLE and RUSLE, reflects the effect of management practices on erosion rates. Used most often to compare impacts based upon cover management practice options
C factor is the deviation of the standard. T/F
T
SLR-Soil loss ratio-
estimate of the ratio of soil loss under actual conditions to losses experienced under the reference conditions.
Soil loss ratio shows
impacts under previous cover management vs. protection under vegetative cover vs. reduction in erosion due to surface cover and roughness Factors are found in a table by NRCS
Support Practice Factor (P)-
ratio of soil loss with a specific support practice to the corresponding loss with upslope and downslope tillage. Effect erosion by modifying flow pattern, grade or direction of surface runoff and reducing amount and rate of runoff. Found in a table provided by USDA-NRCS
RUSLE 2-uses empirically based factor valuesinstead of daily predictions T/F
F-RUSLE 2-uses daily predictions instead of empirically based factor values. Flexibility by season, temperature, at two week increments
Estimating Gross Erosion2
used when estimating all types annual gross erosion. Effects of management practices. Before/after management practices. Includes wind and water
Sheet and Rill Erosion
If uniform soil, slope, conditions, factors can be used directly in the RUSEL to determine erosion rates. If broken up into more then one area, calculate and add together to get total
Total Erosion
total estimated erosion sum of calculated sheet and rill erosion plus other erosion. Gross erosion before and after planned BMPs. Stated in tons per acre by dividing total tons reduced by acreage of the area. Caution: estimated soil loss to be utilized for comparison purposes only. Figures in whole tons. Value A may be modified by a factor M to estimate.
Adjustment of (M) for
estimating monthly and portions of annual soil loss- based on distribution of rainfall intensity through year.-
Wind Erosion- occurs when
not adequately protected against high velocity winds. Bare, loose, dry soils begin eroding hen wind reaches 13 MPH at 1’ above the surface.
Types of Wind Erosion-Suspension
refers to very fine (<0.05 mm) soil, silt size and smaller, are carried high into air. Small percentage of overall.
Types of Wind Erosion-Saltation
movement of medium size (0.05-0.5 mm) partiles, very fine, fine, and medium sand, lifted a short distance into air. Breaks down, destroys stable surface features, accounting for over half of total soil movement
Types of Wind Erosion-surface creep
movement of large particles (0.5 mm and larger) dislodges and wind rolls them since too heavy to lift.
Countermeasures for wind erosion inculde ridging, turning over soil, irrigating, growing vegetating. T/F
T Countermeasures: ridging, turning over soil, irrigating, growing vegetating.
wind erosion varys with (3 items)
particle size (s), moisture (p), and wind speed (f).
Control systems to counder wind erosion on piles (3 items)
- cover
- windscreen
- changing shape of pile
forming new, less erodible surface for wind (3 itmes)
- spraying water
- chemical dust palitive
- establish veg
