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Post-construction Plan Review

Minimum Measure: Post-Construction Stormwater Management in New Development and Redevelopment

Subcategory: Municipal Program Elements

An example of conservation design principles.
Photo Caption: Conceptual plan using conservation design principles (Arendt, Conservation Design for Subdivisions)

Description

Development can alter landscapes by increasing imperviousness (i.e. roofs, driveways, parking lots) and changing drainage patterns, thereby increasing the volume and velocity of runoff from the site. Increased volume leads to degradation of receiving waters and increases in the occurrence of flooding. Stormwater from developed impervious areas can also contain a variety of pollutants that are detrimental to water quality, such as sediment, nutrients, road salts, heavy metals, pathogenic bacteria, and petroleum hydrocarbons.

Considering water quality impacts early in the design process can provide long-term water quality benefits. For example, a project designed with reduced impervious surfaces and increased use of biofiltration practices will result in significant reductions in stormwater runoff volume from the site. New development projects on undeveloped land offer many opportunities to reduce stormwater runoff from the site. Redevelopment projects, which replace an existing development and are typically in more urban areas, usually have less land area available for stormwater controls.

Phase II regulated municipalities are required to develop ordinances which require property owners and operators to include a combination of structural and non-structural BMPs and ensure adequate long-term operation and maintenance of BMPs. To do this, municipalities need to thoroughly review development plans to ensure that they minimize water quality impacts from the site after construction is complete.

Post-construction plan review process

Each municipality has its own process to review and approve development plans. However, the following process has been successful in ensuring that water quality is addressed early in the development plan review process:

  1. Education and training on low impact development and post-construction BMPs
  2. Pre-submittal meetings with developers/engineers
  3. Review of conceptual and design plans

1. Education and training on low impact development and post-construction BMPs

One of the most critical aspects in reviewing post-construction plans is the training of plan review staff. Staff must be familiar with the stormwater standards, site design techniques, and BMPs to ensure projects meet all applicable requirements and effectively control stormwater volume, velocity, and pollutants.

In addition, developers and engineers must be educated on the post-construction requirements and options available to meet those requirements. This is an opportunity to educate developers and engineers on low impact development practices and how to choose effective post-construction BMPs. For example, the Puget Sound Water Quality Action Team has developed the publication Natural Approaches to Stormwater Management: Low Impact Development in Puget Sound [PDF - 1.13 MB - 64 pp] Exit EPA Site as a way to educate engineers and developers on innovative approaches to land development and stormwater management.

2. Pre-submittal meetings with developers/engineers

Early interaction with developers and engineers is important to ensure that stormwater is addressed at the beginning states of project planning. Pre-submittal meetings are encouraged to discuss post-construction requirements and issues; green development, better site design; locations of sensitive areas, and other issues critical to stormwater. These meeting are an opportunity for the municipality to encourage more natural practices that reduce the volume, velocity and pollutant load of stormwater runoff.

3. Review of conceptual and design plans

At any new development site, the design engineer should prepare both a "conceptual" plan and a final stormwater management design plan. The conceptual plan identifies basic site information, locations of proposed development features, and preliminary locations and sizing of stormwater treatment practices. The final plan provides more detailed design information for stormwater practices and includes much greater detail in terms of hydrologic conditions and site features. Post-construction stormwater runoff should be addressed in both the conceptual and final design plan stages. Municipal staff should ensure that, to the extent practical, conceptual plans address preservation of natural areas, minimization of impervious surfaces, low impact development, and promotion of vegetative practices and on-site infiltration. This will ensure that these practices are addressed as early in the design process as possible. The North Central Texas Council of Governments has developed an "integrated Stormwater Management" or "iSWM" Design Manual for Site Development. Appendix E of the iSWM Design Manual Exit EPA Site has checklists with issues municipal staff should consider when reviewing conceptual plans and preliminary/final plans.

The post-construction plan review process will likely occur at the same time as the construction phase plan review.

Elements to look for in an effective post-construction plan

An effective post-construction plan will:

  1. Include a pre-and post-development hydrologic analysis
  2. Identify low impact development opportunities
  3. Identify pollutants of concern
  4. Identify pollution prevention measures
  5. Identify controls that provide treatment and reduce stormwater volume and velocity
  6. Provide for long-term operation and maintenance of controls.

1. Include a pre- and post-development hydrologic analysis

The Phase II Rule sets the goal that BMPs chosen should "attempt to maintain pre-development runoff conditions." A hydrologic analysis or drainage report that considers both pre- and post-development runoff usually is conducted to address the following storm sizes:

  • The 1-year or 2-year storm, also known as the "water quality storm," to protect natural channels from erosion
  • The 10-year or 25-year storm to size storm drainage infrastructure
  • The 100-year storm to address flooding.

2. Identification of low impact development opportunities to reduce stormwater volume and velocity

Project proponents should attempt to mimic a site's natural hydrology after development by following better site design principles including minimizing the project's impervious footprint, conserving natural areas, and minimizing directly connected impervious areas. These types of practices encourage infiltration and reduce the volume of stormwater discharged from the site. Some of the practices that can be used by project proponents to follow these principles include:

3. Identification of pollutants of concern

Municipal staff should ensure that project proponents identify the pollutants of concern expected at the site after development so that the most appropriate BMPs to address those pollutants are selected. The most common pollutants from new development projects include:

  • Sediments
  • Nutrients
  • Metals
  • Organic compounds
  • Trash & debris
  • Oxygen-demanding substances
  • Oil and grease
  • Bacteria and viruses
  • Pesticides

Table 1. Potential Pollutants for Various Land Uses (adapted from San Diego Co-permittees, 2002)

Category

Sediments

Nutrients

Heavy Metals

Trash & Debris

Oil & Grease

Bacteria & Viruses

Pesticides

Residential Development

X

X

X

X

X

X

Commercial Development

X

X

X

X

X

X

Auto Repair Shops

X

X

X

Restaurants

X

X

X

Parking Lots

X

X

X

X

X

Streets and Highways

X

X

X

X

X = pollutant can potentially be expected at that project type.

4. Identification of pollution prevention measures

Municipalities should review projects to ensure that the initial design includes practices to minimize and control the sources of pollutants. These pollution prevention measures are usually the most cost-effective methods of controlling stormwater. Some examples of pollution prevention measures include:

  • Providing storm drain stenciling and signage
  • Designing outdoor material storage areas to minimize exposure to stormwater
  • Designing trash storage areas to minimize exposure to stormwater
  • Using efficient irrigation systems and landscape design
  • Designing vehicle and equipment wash areas to minimize discharges to the storm drain
  • Designing fueling areas to prevent spills and exposure to stormwater

Washington State's Department of Ecology's Stormwater Management Manual for Western Washington: Volume IV - Source Control BMPs Exit EPA Site includes more specific information on source controls.

5. Identification of controls that provide treatment and reduce stormwater volume and velocity

Treatment controls may need to be installed where site design and source controls are not adequate to minimize stormwater pollutants. Treatment controls should be designed and sized to control runoff from a specific storm size appropriate for the area. Treatment controls can fall into several categories:

  • Biofilters
  • Detention basins
  • Infiltration basins
  • Wet ponds or wetlands
  • Drainage inserts
  • Filtration
  • Hydrodynamic separation devices

Treatment controls should be selected based on their effectiveness in reducing the identified pollutants of concern at the project. The municipality should discourage selecting BMPs with relatively low removal efficiency for a pollutant when other BMPs with a higher removal efficiency are available. The table below presents a summary of pollutant removal effectiveness information for various BMPs.


Effectiveness of management practices for runoff control (adapted from Caraco and Winer, 2000, as cited in USEPA, 2005).

Runoff Treatment
or Control Practice

Category or Type

Median Pollutant Removal (Percent)

No. of studies

TSS

TP

OP

TN

NOx

Cu

Zn

Quality Control Pond

3

3

19

N/A

5

9

10

5

Dry Extended Detention Pond

6

61

20

N/A

31

-2

29

29

Dry Ponds

9

47

19

N/A

25

3.5

26

26

Wet Extended Detention Pond

14

80

55

69

35

63

44

69

Multiple-Pond System

1

91

76

N/A

N/A

87

N/A

N/A

Wet Pond

28

79

49

39

32

36

58

65

Wet Ponds

43

80

51

65

33

43

57

66

Shallow Marsh

20

83

43

66

26

73

33

42

Extended Detention Wetland

4

69

39

59

56

35

N/A

-74

Pond/Wetland System

10

71

56

37

19

40

58

56

Submerged Gravel Wetland

2

83

64

14

19

81

21

55

Wetlands

36

76

49

48

30

67

40

44

Organic Filter

7

88

61

30

41

-15

66

89

Perimeter Sand Filter

3

79

41

68

47

-53

25

69

Surface Sand Filter

7

87

59

N/A

31.5

-13

49

80

Vertical Sand Filter

2

58

45

21

15

-87

32

56

Bioretention

1

N/A

65

N/A

49

16

97

95

Filtering Practices a

18

86

59

57

38

-14

49

88

Infiltration Trench

3

100

42

100

42

82

N/A

N/A

Porous Pavement

3

95

65

10

83

N/A

N/A

99

Ditches b

9

31

-16

N/A

-9

24

14

0

Grass Channel

3

68

29

32

N/A

-25

42

45

Dry Swale

4

93

83

70

92

90

70

86

Wet Swale

2

74

28

-31

40

31

11

33

Open Channel Practices

9

81

34

1.0

84

31

51

71

Oil-Grit Separator

1

-8

-41

40

N/A

47

-11

17

Shaded rows show data for groups of practices (i.e., dry ponds include quality control ponds and dry extended detention ponds).
Numbers in italics are based on fewer than five data points.
aExcludes vertical sand filters
bRefers to open channel practices not designed for water quality.
TSS=total suspended solids, TP=total phosphorus, OP=ortho-phosphorus, TN=total nitrogen, NOx=nitrate and nitrite nitrogen, Cu=copper, Zn=zinc.

6. Provide proof of on-going maintenance

BMPs are not effective unless properly maintained. The plan approved by the municipality should address who will be responsible for on-going maintenance of the stormwater BMPs identified. Maintenance can be conducted by the municipality, by the property owner, or by an association or special district. The proof of maintenance should be contained in a maintenance agreement that is recorded with the property. This maintenance agreement should contain the following information:

  • A description of the routine maintenance that will need to be performed
  • Schedules for maintenance
  • Inspection requirements
  • Provisions for the municipality to access BMPs
  • Penalties for failure to maintain BMPs
  • A provision to legally record the maintenance agreement.
  • Maintenance of these BMPs will need to be periodically assessed by the municipality, as described in the BMP inspection and maintenance fact sheet.
References

San Diego Co-permittees. 2002. Model Standard Urban Stormwater Mitigation Plan for San Diego County, Port of San Diego, and cities in San Diego County. [http://www.projectcleanwater.org/pdf/wurmp_final_model_10-17-02.pdf [PDF - 2.74 MB - 51 pp] Exit EPA Site].

Caraco, D., and R. Winer. 2000. Updated National Pollution Removal Performance Database for Stormwater Treatment Practices. Center for Watershed Protection, Ellicott City, MD. As cited in USEPA. 2005. National Management Measures to Control Nonpoint Source Pollution in Urban Areas. EPA 841-B-05-004. U.S. Environmental Protection Agency, Washington, DC.

 

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Last updated on October 27, 2008
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