Lesson 17: Water Quality
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Lesson 17:
Water Quality
Objective
In this lesson we will answer the following questions:
How is the type of water quality BMP chosen?
How is the size of a water quality BMP determined?
Reading Assignment
Along with the online lecture, read section 3.06 and sections 3.09 through 3.15 in chapter
3 of your text.
Lecture
Choosing and Sizing a Water Quality BMP
Introduction
The final aspect of stormwater management which we must consider is water quality.
Urban development typically results in a variety of pollutants which are carried from
impervious surfaces by stormwater runoff and enter the local watershed. Impervious
surfaces include roofs, roads, parking lots, and other structures which do not allow rain
to soak into the soil.
The pollutants which are found in runoff from impervious surfaces include nutrients,
sediments, bacteria, oils, trace metals, and others. In addition, stormwater is often
warmed by running over hot pavement, and this thermal pollution can cause as much
damage to the local streams as the other pollutants do. The purpose of a water quality
BMP is to remove all of these pollutants from runoff before the water enters local
streams.
Is a Water Quality BMP Necessary?
The amount of impervious cover which will be added to a watershed will determine
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whether a water quality BMP is required. In general, if the development will not raise the
percent impervious cover in the watershed above 16%, then a water quality BMP is not
required. (16% is the average land cover, the average amount of impervious surface
within a watershed.)
In many cases, the percent of impervious cover in the entire watershed before
development is not known. This value can be assumed to be 16% if there are other
developed areas within the watershed. As a result, if the percent of impervious cover to
be created in the development is greater than 16% of the development area, then a water
quality BMP will be required.
Choosing a Water Quality BMP
Once you determine whether your development will require a water quality BMP, the
next step is choosing a BMP for the water quality portion of your stormwater
management plan. As you can see in Table 14, a wide variety of BMP's are suitable for
water quality control. You should be reading about each BMP in Chapter 3 of your text.
The choice of the appropriate BMP for your development will depend on the percent of
impervious cover in the watershed. Since impervious cover generates the largest quantity
of pollutants, a greater percentage of impervious cover will require a greater pollutant
removal capacity. Table 16 lists which BMP's are appropriate for each percentage of
impervious cover.
For example, if your development will have 50% of the watershed covered with parking
lots and roofs, then the appropriate water quality BMP's include a bioretention basin, a
bioretention filter, an enhanced extended detention basin, a retention basin II, and an
infiltration basin with a volume of 1 !WQV. You should be aware that the BMP's rated
for a higher percentage impervious cover (sand filter, infiltration basin (2 ! WQV), and
retention basin III, in this case) can also be used. However, BMP's rated for a lower
percentage impervious cover are inappropriate for your development.
Sizing Your BMP
The next step is to choose the size of your water quality BMP. The size requirements for
a BMP depend on the mechanism which the BMP uses to remove pollutants. These
mechanisms include settling (in which the pollutants gravitationally settle out of the
water), biological removal (in which bacteria and other living organisms "eat" the
nutrients in the water), and filtering (in which pollutants are physically filtered out of the
water.) Table 17 lists water quality BMP's based on their pollutant removal mechanism.
Both gravitational settling and biological removal require the polluted water to spend a
certain length of time within the water quality BMP. As a result, these types of BMP
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should be built to retain a certain volume of water --- the water quality volume, which
will be discussed in the next section.
Filtering BMP's strain stormwater through some sort of filter and are limited to small
drainage areas (less than 5 acres) since all of the stormwater to be treated must pass
through the filter. The size of this type of BMP is based on the flow rate of the water.
Individual standards can be found in the sections devoted to individual types of BMP's in
Chapter 3. In this lesson, we will limit our consideration to the other two types of water
quality BMP's for which a more general standard has been developed.
Water Quality Volume
The water quality volume, or WQV, is a measure of the amount of water treated in a
water quality BMP. This amount is usually assumed to be the first 1/2" of runoff. This
volume of water contains the greatest concentration of pollutants since it washes
impervious areas relatively clean.
For this course, water quality BMP's are required to treat two times the water quality
volume for the development unless a larger volume is specified in the BMP's design
criteria.
Calculations
Overview of Design Procedure
This section summarizes the procedure which should be used to choose and design an
adequate water quality BMP.
Step 1: Calculate the percent impervious cover in the developed area after development.
Step 2: Determine whether a water quality BMP is required.
A water quality BMP is required if the percent impervious cover from Step 1 is
more than 16%.
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Lesson 17: Water Quality
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Step 3: Choose a water quality BMP from Table 16. Use Table 17 to ensure that the
BMP uses settling or biological removal processes. (Filtering BMP's may be used, but a
different procedure is used to determine the size of those BMP's.)
Step 4: Calculate the water quality volume.
WQV = Impervious area ! (0.5 in.) / (12 in./ft.)
Step 5: Design the BMP using standards in Chapter 3 of your text and the sizing
criterion of 2 ! WQV.
Example
Given: A developer is planning to disturb 11.9 acres of a 25 acre watershed. Of this
disturbed area, 9.28 acres will be impervious cover after development.
Find: Determine whether a water quality BMP is required. If so, choose and size the
BMP.
Step 1:
Step 2: Based on the percent cover calculated in Step 1, a water quality BMP is required.
Step 3: Based on the 78% impervious cover in the disturbed area after development, the
developer has a choice between a sand filter, an infiltration basin with 2 ! WQV, and a
retention basin with 4 ! WQV and an aquatic bench. Since this lesson will be concerned
only with BMP's using settling and biological processes as the pollutant removal
mechanisms, we are limited to the retention basin.
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Step 4: The water quality volume can be calculated as follows:
WQV = (9.28 acres) ! (0.5 in.) / (12 in./ft.)
WQV = 0.39 acre-feet
Step 5: Although most water quality BMP's should be sized at 2 ! WQV, in this case
Table 16 has specified the volume to be 4 ! WQV. Four times the WQV is 1.56
acre-feet. If the BMP is also to be used for stream erosion control and/or flooding
control, then a larger volume may be necessary.
Review
The following procedure is used to choose and size a water quality BMP which uses
settling or biological methods to remove pollutants:
1.
2.
3.
4.
5.
Calculate percent impervious cover after development.
Determine whether a water quality BMP is required.
Choose a water quality BMP.
Calculate the WQV.
Design the BMP so that it holds 2 ! WQV (unless otherwise specified.)
Water quality BMP's which use filtration have standards based on flow rate, so steps 4
and 5 differ for this type of BMP.
New Formulas Used
To calculate the percent of impervious cover in the development:
To calculate the water quality volume:
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Lesson 17: Water Quality
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WQV = Impervious area ! (0.5 in.) / (12 in./ft.)
New Tables and Diagrams Used
Table 16: Choosing a Water Quality BMP
Table 17: Pollutant Removal Mechanisms of Water Quality BMP's
Assignments
1. Complete the lesson quiz.
2. Using the procedure from this lesson, choose and size a water quality BMP for
your development. The area which you plan to develop will end up with about 35%
impervious cover.
Quiz
Answer the questions in the Lesson quiz . When you have gotten all the answers correct,
print the page and either mail or fax it to the instructor. You may also take the quiz online
and directly submit it into the database for a grade.
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