During a heavy rainstorm, a river experiences increased flowrates and runoff from the surrounding fields. The runoff carries with it solids of size A and B and dissolved organic matter (DOM). Ten kilometersdownstream the river runs past a town. Given the list of provided data, answer the following questions about the river.

Assumptions: Assume complete DO saturation before runoff, assume ideal plug flow conditions for the river.

Givens:

River width: 3m

River slope: 0.005

River depth after rainfall: 2m

Natural channel roughness coefficient: 0.3

Particle size A radius: 10^-3 cm

Particle size B radius: 10^-2 cm

Density of organic matter: 1.2 g/cm

Biodegradation rate of DOM (k): = 0.057/d

O2 Reaeration Coefficient (kr) = 0.21/d

DO saturation = 10.2 mg/L

a. What is the velocity of the river after the rainstorm event?

b. If the initial concentration of DOM in the river as a result of runoff is 30mg/L, what will be the concentration in the river by the time it reaches the town 10km downstream?

c. What is the minimum distance particles sized A and B will travel down the river before beginning to settle? Will either particle make it to the town 10km downstream?

d. A sample from the river just after the runoff was collected and was found to have an ultimate BOD of 25mg/L and a BOD5 of 9.5mg/L.

i. Use the BOD5 to estimate the deoxygenation rate (kd) in the river.

ii. Use an equation from the slides to find the time (days) and distance (km) downstream where the lowest dissolved oxygen concentration is expected to occur.

iii. What will be the minimum oxygen level(mg/L) in the river as a result of the runoff?