SP 4B DESIGN OF STORMWATER EROSION CONTROL BASIN

You are working for a grading contractor in the process of grading a large site for a

new subdivision. The Regional Water Quality Control Agency requires the grading

contractor to develop a Storm Water Pollution Prevention Plan (SWPPP or ("Swip") which

will include a sedimentation basin so as to remove 100% of a particle with a 0.1 mm

diameter. The specific gravity of erodible materials ranges from 1.4 to 1.8. For this project

we will use a specific gravity of 1.65. The design flow for the site has been determined

using the rational method to be 0.15 m³/s. Use a factor of safety of 1.5 on the settling zone

length to account for the inlet and outlet zones.

The basin length shall be 5 times the width. The depth shall be 2 m. Assume a water

temperature of 10° C.

a) Using the spreadsheet developed for settling velocity, determine settling velocity of the

particle that is to be removed 100% of the time

b) Determine the length and width of the basin settling zone. Then, determine the total length

of the basin including inlet and outlet zones, assuming the 1.5 safety factor is applied to the

length of the settling zone to account for the inlet and outlet zones; round up to the nearest

0.5 m.

c) Determine the Hydraulic Retention Time at the design peak flow rate, minutes. Base it on

the actual, final, rounded dimensions.

d) If a particle has a settling velocity of 0.001 m/s, will it be removed and if it will, determine

the % removal. When making the calculations, use the settling zone dimensions.

e) If a particle has a settling velocity of 0.1 m/s, will it be removed and if it will, determine the

% removal. When making the calculations, use the settling zone dimensions./nSP 4A--DISCRETE SETTLING

Part 1

Develop an excel spreadsheet or Matlab program to calculate the settling velocity of a particle based on Stokes

Law (Eq 10-8 in the text, i.e., the "general equation"). Set it up so you can enter the specific gravity of the

particle, water temperature, water specific gravity, water viscosity, particle diameter. The spreadsheet should

be able to calculate Reynolds Number, Cd, etc. In excel, set it up to use "solver."

Use the spreadsheet to calculate the settling velocity of a particle under the following conditions:

Specific gravity of particle

2.65

12 °C

Water Temperature

Particle diameter

0.000075 m (corresponds to # 200 sieve material)

Part 2

From the spreadsheet or Matlab program develop a program to calculate the particle diameter, given the

settling velocity. Use the spreadsheet to calculate the diameter of a particle whose settling velocity is 0.01 m/s

Note: we will be using this spreadsheet/program several times during the course of the semester, so document

it well.