Draft Calculation Slides

1
Draft Calculation Slides

• for SWPPP Preparation

2
Calculating the area of disturbance

• Formulas for calculating area
• Rectangle length x width
• Triangle ½ width (base) x length (height)
• Trapezoid height/2 x (b1 b2)
• Parallelogram base x height
• Measure distances in feet!
• Result will be in square feet
• One acre 43,560 sq ft
• Break site down into squares,
• rectangles, triangles, etc.
• and add up each area total

Use the same formulas to calculate the area of
each drainage zone
3
Calculating the amount of seed and mulch needed
for a given area

• Use area formulas to calculate total area
• Measure in feet, result square feet
• Divide total sq ft by 43,560
• Result will be in acres
• Multiply number of total acres by pounds of seed
  recommended, or tons of mulch
• Example 15.2 acres x 65 lb seed/acre 988 lb
• 4.3 acres x 2 tons of straw/acre 8.6 tons total
• Pay attention to units! Tons vs Pounds, etc.

4
Calculating fertilizer or lime needed

• Fertilizer bag numbers show nitrogen, phosphorus,
  potassium content
• Number is percent of each nutrient
• Multiply percent times weight of bag to get total
  pounds of each element
• Example 50 lb bag x 0.10 N 5 lb of nitrogen in
  this bag

5
Calculating fertilizer or lime needed

• Example for fertilizer
• Soil test calls for 20 lb of phosphorus per acre
• Site area is calculated at 3.7 acres
• 3.7 acres x 20 lb 74 lb of phosphorus needed
• Youre buying 0-46-0 fertilizer (46 P) in 50 lb
  bags
• 0.46 x 50 lb 23 lb of P in each bag of
  fertilizer
• 74 lb divided by 23 lb of P per bag 3.2 bags
• If using 5-10-5 fertilizer 0.10 x 50 lb of P /
  bag
• 5 lb of P per bag 74 divided by 5 14.8 bags
• For lime, multiply tons needed per acre x total
  acres to get total amount required

6
Calculating slope angles

• Slope can be expressed several ways
• Percent slope Number of feet rise or fall
  (vertical) per 100 ft run (horizontal)
• 6 slope has 6 ft of vertical change per 100 ft
  run
• Ratio Number of horizontal ft run per vertical
  ft change (rise or fall)
• 31 slope has 3 ft of horizontal run for each one
  foot rise or fall
• Degrees Angle of the slope vs level (horizontal)
  ground

7
Calculating slope

• Locate slope to be measured
• Measure 100 ft on slope section, with tape pulled
  tight hold uphill end on ground
• Make sure tape is LEVEL!
• With second tape, measure distance from downhill
  end of 100 ft tape to ground
• Measure in FEET this will be the percent slope

8
To change percent slope to ratio, divide 100 by
slope percent numberExample 33 slope becomes
100 divided by 33 3H1V slopeIn steep
terrain, pull 50 ft double the vertical
changeor pull 25 ft and quadruple the vertical
change
100 ft tape measure held tight level
Use second tape to measure distance to the
ground, in feet. This is the percent slope.
9
Calculating slope from topo map

• Note topographical intervals
• Note map scale
• Count number of intervals, multiply by interval
  distance to get vertical change
• Measure distance on map and multiply by scale to
  get horizontal distance
• Result will be horizontal vertical
• Divide vertical into horizontal to get ratio

10
Topo map example

• Topo lines are at 10 ft intervals (vertical
  elevation change)
• Scale is 124,000 (1 2000 ft)
• Horizontal distance shown is 0.1 inch, or 200 ft,
  and 5 topo lines
• 5 topo lines x 10 ft 50 ft elevation change,
  over 200 ft
• 200 ft / 50 ft 4H1V slope (25)

11
Approximate slope conversions
12
Calculating the spacing required for slopes with
multiple silt fences

• Use slope calculation method to find ratio
• Identify predominant soil type on slope
• Use table below to find proper spacing

13
Using the Universal Soil Loss Equation to
quantify ESC practices

• USLE can be used to estimate average annual
  construction site soil erosion
• It can also be used to identify practices that
  reduce soil loss to acceptable levels
• Practices include soil cover (seed or mulch),
  sediment basins/traps, and diversions

14
Using USLE

• Handout See Appendix C in the Kentucky
  Technical Specifications Manual
• Here is the problem
• What will the annual soil loss be for this site
• Site is in Robertson County, with mostly
  Faywood-Cynthiana soils. Average slope of the
  site is 30H1V. Slope length is 80 ft. Temporary
  cover will be straw mulch at 2 tons per acre. A
  small onsite sediment basin will receive runoff
  from the entire site.

15
Solution to USLE problem

• Annual average soil loss per acre is expressed as
• A R x K x (LS) x C x P
• A 175 x 0.28 x 0.297 x 0.25 x 0.20
• A 0.728 tons per acre per yr of erosion from
  site
• What if we had left the site bare, compact and
  smooth, scraped with a dozer up and down the hill?

16
Solution to problem

• Use the same formula
• A R x K x (LS) x C x P
• Replace C factor with 1.3
• A 175 x 0.28 x 0.297 x 1.3 x 0.20
• A 3.78 tons per acre of erosion from site
• Now, see what happens if we eliminate the
  sediment basin (leave off the 0.20 in the
  equation . . . . 18.92 tons per acre!)

17
Calculating sediment trap volume

• Calculate directly, using excavated volume or
  volume impounded by dam or berm
• Calculate using simple spreadsheet tools
• Calculate using SEDCAD or other computer assisted
  design aid

18
Direct calculation of trap volume

• Calculate as a triangular pyramid
• Volume Area of the triangular water surface (at
  the outfall elevation) x 1/3 the deepest depth
  (usually in the middle, at the base of the dam or
  berm)
• Calculate as a trapezoidal or rectangular prism
• LOTS of volume calculators on the web
• Example http//grapevine.abe.msstate.edu/fto/to
  ols/vol/index.html

19
Example triangular trap design
Length
Width
Depth
Surface Area ½ Width x Length Volume Surface
Area x 1/3 Depth
20
Width 42 ft Length 71 ft Depth 4
ft Volume ½ x (42 x 71) x 1/3 x 4 Volume
1988 cubic ft One acre requires 3600 cu ft
storage space Trap would treat 1988/3600 0.55
acres
Height
Base
Depth
21
Other approaches

• Calculate volume as a trapezoidal prism
• ½(AB) x Y x Z
• Calculate volume as a triangular prism
• B x ½H x L

22
Spreadsheet tools

• Simplified methods
• Easy data entry
• Several available

23
SEDCAD

• Routes flow thru selected BMPs
• Includes silt fences, ditches with checks, traps,
  ponds
• Evaluates BMP effectiveness
• Predicts effluent sediment concentration

24
Calculating required volumes for concrete truck
washout facilities

• Each mixer truck holds about 10 yards
• Wash water and waste volume per truck is
  estimated at 0.1 cu yd on the average
• Initial volume required is one cubic yard of
  volume for every 10 concrete truckloads.
• Volume calculation for rectangular prism
  containment structure
• Volume Length x Width x Height (cubic units)
• Use ft or yds (remember 27 cu ft 1 cu yd)
• Require that washout structure be maintained and
  enlarged or replaced as needed