1. BMP 5.4.3: Protect/Utilize Natural Flow Pathways in Overall Stormwater
  2. Planning and Design
      1. Potential Applications
      2. Key Design Elements
      3. Stormwater Functions
      4. Water Quality Functions
    1. Description
    2. Variations
    3. Applications
      1. • Use buffers to treat stormwater runoff.
    4. Design Considerations
      1. Detailed Stormwater Functions
      2. Peak Rate Mitigation Calculations
      3. Water Quality Improvement
    5. Construction Issues
    6. Maintenance Issues
    7. Cost Issues
      1. Specifications

BMP 5.4.3: Protect/Utilize Natural Flow Pathways in Overall Stormwater

Back to top


Planning and Design
Identify, protect, and utilize the site’s natural drainage
features as part of the stormwater management system.
Potential Applications
Residential:
Commercial:
Ultra Urban:
Industrial:
Retrofit:
Highway/Road:
Yes
Yes
No
Yes
Yes
Yes
Key Design Elements
Stormwater Functions
Volume Reduction:
Recharge:
Peak Rate Control:
Water Quality:
Low/Med.
Low
Med./High
Medium
Water Quality Functions
TSS:
TP:
NO3:
30%
20%
0%
.
Identify and map natural drainage features (swales, channels,
ephemeral streams, depressions, etc.)
.
Use natural drainage features to guide site design
.
Minimize filling, clearing, or other disturbance of drainage
features
.
Utilize drainage features instead of engineered systems
whenever possible
.
Distribute non-erosive surface flow to natural drainage features
.
Keep non-erosive channel flow within drainage pathways
.
Plant native vegetative buffers around drainage features
363-0300-002 / December 30, 2006
Page 21 of 98

Description
Most natural sites have identifiable drainage features such as swales, depressions, watercourses,
ephemeral streams, etc. which serve to effectively manage any stormwater that is generated on the
site. By identifying, protecting, and utilizing these features a development can minimize its stormwater
impacts. Instead of ignoring or replacing natural drainage features with engineered systems that
rapidly convey runoff downstream, designers can use these features to reduce or eliminate the need for
structural drainage systems. Naturally vegetated drainage features tend to slow runoff and thereby
reduce peak discharges, improve water quality through filtration, and allow some infiltration and
evapotranspiration to occur. Protecting natural drainage features can provide for significant open
space and wildlife habitat, improve site aesthetics and property values, and reduce the generation of
stormwater runoff. If protected and used properly, natural drainage features generally require very little
maintenance and can function effectively for many years.
Figure 5.3-1 Protect natural drainage features
Variations
Natural drainage features can also be made more effective through the design process. Examples
include constructing slight earthen berms around natural depressions or other features to create
additional storage, installing check dams within drainage pathways to slow runoff, and planting
additional native vegetation.
363-0300-002 / December 30, 2006
Page 22 of 98

Applications
Use buffers to treat stormwater runoff.
Figure 5.3-2 Section of buffer utilization
Figure 5.3-3 Section of buffer utilization
Use natural drainage pathways instead of structural drainage systems
Figure 5.3-4 The natural surface can provide stormwater
drainage pathways
363-0300-002 / December 30, 2006
Page 23 of 98

Use natural drainage features to guide site design
Figure 5.3-5 Natural drainage features can guide the design
Others…
Figure 5.3-6
Natural surface depressions can temporarily store
stormwater.
Design Considerations
1. IDENTIFICATION OF NATURAL DRAINAGE FEATURES.
Identifying and mapping natural
drainage features is generally done as part of a comprehensive site analysis. This process is an
integral part of site design and is the first step for many of the non-structural BMPs described in this
Chapter.
2. NATURAL DRAINAGE FEATURES GUIDE SITE DESIGN.
Instead of imposing a two-dimensional
‘paper’ design on a particular site, designers can use natural drainage features to steer the site layout.
Drainage features can be used to define contiguous open space/undisturbed areas as well as road
alignment and building placement. The design should minimize disturbance to natural drainage
features and crossings of them. Drainage features that are to be protected should be clearly shown on
363-0300-002 / December 30, 2006
Page 24 of 98

all construction plans. Methods for protection, such as signage and fencing, should also be noted on
applicable plans.
3. UTILIZE NATURAL DRAINAGE FEATURES.
Natural drainage features should be used in place of
engineered stormwater conveyance systems wherever possible. Site designs should use and/or
improve natural drainage pathways to reduce or eliminate the need for stormwater pipe networks. This
can reduce costs, maintenance burdens, disturbance/earthwork related to pipe installation, and the size
of other stormwater management facilities. Natural drainage features should be protected from any
increased runoff volumes and rates due to development. The design should prevent the erosion and
degradation of natural drainage features through the use of upstream volume and rate control BMPs.
Level spreaders, erosion control matting, re-vegetation, outlet stabilization and check dams can also be
used to protect natural drainage features, where appropriate.
4. NATIVE VEGETATION.
Natural drainage pathways should be provided with native vegetative
buffers and the features themselves should include native vegetation where applicable. If drainage
features have been previously disturbed, they can be restored with native vegetation and buffers.
Detailed Stormwater Functions
Volume Reduction Calculations
Protecting/utilizing natural drainage features can reduce the volume of runoff in several ways.
Reducing disturbance and maintaining a natural cover can significantly reduce the volume of runoff
through infiltration and evapotranspiration. This will be self-crediting in site stormwater calculations
through lower runoff coefficients and/or higher infiltration rates. Utilizing natural drainage features can
reduce runoff volumes because natural drainage pathways allow infiltration to occur, especially during
smaller storm events. Encouraging infiltration in natural depressions also reduces stormwater
volumes. Employing strategies that direct non-erosive sheet flow onto naturally vegetated areas can
allow considerable infiltration. See Chapter 8 for volume reduction calculation methodologies.
Peak Rate Mitigation Calculations
Protecting/utilizing natural drainage features can reduce the anticipated peak rate of runoff in several
ways. Reducing disturbance and maintaining a natural cover can significantly reduce the runoff rate.
This will be self-crediting in site stormwater calculations through lower runoff coefficients, higher
infiltration rates, and longer times of travel. Using natural drainage features can lower discharge rates
significantly by slowing runoff and increasing on-site storage.
Water Quality Improvement
Protecting/utilizing natural drainage features can improve water quality through filtration, infiltration,
sedimentation, and thermal mitigation. See Chapter 8 for Water Quality Improvement methodologies.
Construction Issues
1. At the start of construction, natural drainage features to be protected should be flagged/fenced
with signage as shown on the construction drawings.
2. Non-disturbance and minimal disturbance zones should be strictly enforced.
3. Natural drainage features must be protected from excessive sediment and stormwater loads
while their drainage areas remain in a disturbed state.
363-0300-002 / December 30, 2006
Page 25 of 98

Maintenance Issues
Natural drainage features that are properly protected/utilized as part of site development should require
very little maintenance. However, periodic inspections and maintenance actions (if necessary) are
important. Inspections should assess erosion, bank stability, sediment/debris accumulation, and
vegetative conditions including the presence of invasive species. Problems should be corrected in a
timely manner. If native vegetation is being established it may require some support – watering,
weeding, mulching, replanting, etc. – during the first few years. Undesirable species should be
removed and desirable replacements planted if necessary.
Protected drainage features on private property should have an easement, deed restriction, or other
legal measure to prevent future disturbance or neglect. DEP has worked with the Pennsylvania Land
Trust Association (PALTA) to develop an easement template with guiding commentary for permanently
protecting forest riparian buffers. The model is tailored to protect a relatively narrow ribbon of land
along a waterway or lake. Presumably, the riparian buffers will most often comprise lands of severely
limited development potential and the landowner will not be seeking a charitable federal income tax
deduction.
In preparing the model, it was also assumed that landowners would be receiving no more than a
nominal sum for placing the restrictive covenants on their land. To promote landowner donation, the
model was drafted to be as brief as possible while providing core protections to forest riparian buffers.
The model with guiding commentary is available at http://conserveland.org/model_documents/#riparian
PALTA is now offering landowners who use this model a grant of up to $6000 to cover associated costs
such as attorney’s fees.
Cost Issues
Protecting/utilizing natural drainage features generally results in a significant construction cost savings.
Protecting these features results in less disturbance, clearing, earthwork, etc. and requires less re-
vegetation. Utilizing natural drainage features can reduce the need and size of costly, engineered
stormwater conveyance systems. Together, protecting and utilizing drainage features can reduce or
eliminate the need for stormwater management facilities (structural BMPs), lowering costs even more.
Design costs may increase slightly due to a more thoughtful, site-specific design.
Specifications
Not applicable
363-0300-002 / December 30, 2006
Page 26 of 98

Back to top