ABE 527
Computer Models in Environmental and Natural Resources Engineering
Rabi H. Mohtar
Estimating Water Detention Time in a Pond

What is detention time?
Time that a volume of water stays in the pond

Why is this important?
Biological, chemical, physical separation and processes

Target detention time?
For a certain inflow of mean particle diameter what is the time needed to settle
this size of a particle?
This is estimated using Stocke’s Law

Actual detention time is a function of velocity of flow, pond depth, and flow
length
 there is no one-size-fits-all criteria for a desired detention time to ensure high
trapping efficiency.
Guidelines for detention time criteria:
1.
Check the target range of particle sizes to be removed
2.
Set acceptable % particles to be removed
3.
Set the settling velocity for a given particle size
Dead storage zones:
Pond shape:
 Ponds with larger
Length
ratios will settle particles more efficiently
Width
L
Area
, where WE  effective width =
WE
Length
L:W ratio of 2 as a minimum.
if L:W=2 can not be achieved, then introduce baffles to improve efficiency
Estimating detention time
1.
Minimum drain time. Time required to completely drain water starting from
a given water surface.
2.
If criteria was such that minimum drain time = 24 hrs, pond will pass criteria.
However, if the criteria is to detain 50% in 24 hrs pond will fail.
First flush volume
First flush is high in pollutant and sediments. Some areas require separate offline detention for the treatment of first flush volume. Water exits these ponds
only through:
3.
4.
5.
 Evaporation
 Infiltration
 Filtration
Since these are slow processes detention times are large. You need to make
sure that the pond has enough volume before next significant event. Minimum
drain time can be used to estimate the detention drain time for first flush
volume.
VFF  DFF  AP
VFF = Volume of first flush water draining into the pond
DFF = depth of initial runoff containing high percentage of pollution
Ap = runoff area contributing to pond
Peak to peak
The longer peak-to-peak, the longer the detention time. Very inaccurate – it
does not take into account the shape of the hydrograph and its distribution
Centroid to Centroid. More accurate than peak-to-peak since it takes the
shape of the hydrograph into consideration.
Average plug flow – first come first out