Water Runs Downhill:

Managing Runoff on Steep Slopes

G. Frantz, Cornell University

2021
Guidance prepared by
Southern Tier Central Regional Planning & Development Board

With funding from the Great Lakes Commission

 Steep Slope Guidance

Why Are Steep Slopes Important?

Green hills with steep slopes contribute to the natural beauty of the Finger Lakes region. However, when
intense rain falls on these hills, water rushes down the slopes and causes flash flooding, washouts, and water
pollution. Although some amount of flooding and erosion is natural, there are ways to reduce the costly
damage to property and roads.
Development on slopes with a grade of
When rain falls on natural slopes covered by healthy vegetation,
15% or greater should be avoided, if
the plants and soil absorb and slow down some of this water. possible, to limit soil loss, erosion,
However, development on steep slopes often disrupts these excessive stormwater runoff and the
natural functions and results in more runoff, faster runoff, and degradation of surface water. On slopes
less protection for the soil. Concentrated high energy flow greater than 25%, no development, re-
causes erosion, washouts, and landslides. Increased peak flows grading, or stripping of vegetation
cause flood damage to homes and property. In most cases, should even be considered.
homeowners insurance policies do not cover damage due to (Source: New York State Stormwater
flooding, landslides, or other “earth movement events.” The Management Design Manual, January
quality of streams and lakes is impaired by the soil and debris 2015;
that washes off the hillsides. Damage to roads is costly for the https://www.dec.ny.gov/chemical/2907
2.html.)
entire community.

No Adverse Impact is the principle that it is not ok to make changes to your land that adversely impact
others by causing erosion, flooding, or water quality impairment.

Developing on steep slopes can cause a myriad of issues. As slopes become steeper, runoff becomes more
erosive and stabilizing a slope becomes more challenging. This guide outlines strategies for managing the
energy of rain and snow melt and thus reducing the potential for damage. The objective of these techniques
can be summarized as: Slow it down. Spread it out. Soak it in.

Steep slopes present challenges

because of the difficulties of stabilizing steep soils
and safely managing runoff.
Sustainable Keuka Lake Municipal Handbook
USDA Natural Resources Conservation Service

Mecklenburg Fire Department

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 Steep Slope Guidance

Good Practices for Managing Slopes

Plan the Project: Evaluate the Site

Because steep slopes are prone to drainage and erosion problems, land use activities should be designed to fit
the landscape and protect natural features. The first step is to evaluate the existing conditions.

• Measure the slope. How steep is the

slope? Are there less steep areas that
are more suitable for development? A
standard way to describe the steepness
of a slope is as the percent of the

Saskatchewan, CA
vertical change relative to horizontal
change. This can be measured in the
field or on a topographic map. A slope
with 1-foot vertical change for each
horizontal foot is a 100% grade. For
development, a 15% grade is
considered steep and requires
extra attention to slope stability

University. of Tennessee Extension

and drainage issues (if the
elevation change is more than 1 ½
feet over a 10-foot horizontal
distance). A 25% grade is
challenging and should be left
undisturbed if possible.

If the elevation changes 14 inches over

the length of an 8-foot board, the
grade is 15%. A 2-foot elevation
change over 8 feet is a 25% grade.
GeoSTAC

On a topographic map, the contour lines

are closer together in steep areas and
farther apart on gentler slopes. If the
horizontal distance between 10-foot
contour lines is less than 65 feet, the slope
is greater than 15% and is considered
steep. 40 feet between 10-foot contour
lines indicates a 25% grade, which should
be protected.

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 Steep Slope Guidance

• Determine soil characteristics. Soil survey maps contain a wealth of information about soil
characteristics and can be accessed on the Web Soil Survey online application
(https://websoilsurvey.sc.egov.usda.gov). This can assist with selecting plant species and designing
erosion protection techniques.
• Observe Drainage Patterns. Valuable information can be obtained by observing the slope under heavy
rainfall events or persistently wet conditions. Note the direction and speed of runoff. How slowly or
quickly does water soak into the soil? Are there springs or seeps? Is drainage concentrated into
channels or rivulets that increase the potential for erosion and washouts? Are there catch basins,
ditches, or other practices intended to manage runoff? Look for opportunities to slow down or
disperse concentrated flow.
• Identify Existing Vegetation. What types of plants are growing on the site? Is the vegetation healthy? Is
the plant diversity and density sufficient to protect the soil?
• Characterize any erosion problems. Identify any areas where exposed soil is vulnerable to erosion. Are
there gullies, rills, or other signs of soil erosion? If so, is the eroded sediment captured before it
washes off the site? If there is a stream or drainage ditch, are the banks stable?
• Identify special features. A hillside often offers vistas, wildlife habitat, or other features that contribute
to its charm. Identify those special places or characteristics that you want to protect or enhance.
• Prepare a map of existing conditions. Document the existing water bodies and natural features on a
site map. Also show any existing structures, access roads, or other development.

Plan the Project: Develop a Site Plan

Some sites are too steep to be developed safely. Other steep slopes present development challenges that can
be managed with careful site planning and project design. The site planning process involves balancing the
existing features and limitations of a site with the desired use. The objective is to design a project that fits the
landscape—building on its assets and avoiding problems. Natural features and drainage patterns should be
retained to the extent possible. Plan the development to fit the site.

The objective of site planning is to anticipate and avoid problems.

• Protect existing natural features.

Whenever possible preserve and maintain
healthy vegetation, existing drainage
Keuka Lake Scenic Viewshed Protection Plan

patterns, and natural features, such as

streams and wetlands, as well as
vegetated buffers along all waterbodies. If
a sloped area is currently stable and well
vegetated, the best management strategy
is usually to protect the existing
vegetation and not disturb the soil. Use
the existing topography wherever possible
to minimize grading.

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 Steep Slope Guidance

• Correct existing problems. If the site

has drainage or erosion problems,

Sustainable Keuka Lake Municipal

additional effort will be needed to
stabilize the slope. If gullies or
washouts have occurred, it may be
necessary to re-grade the severely
eroded areas, add topsoil, and install
erosion control practices to prevent

Handbook
additional erosion.
• Locate development away from steep
Cluster subdivision design enables protection of steep
areas. Whenever possible locate
slopes or other sensitive areas by allowing denser
buildings, driveways, walkways,
development in less sensitive areas without changing the
utilities, and other development on
number of developable sites.
gentler slopes and away from the top
or the toe of steep areas. Use steep
parts of the site as natural areas and
wildlife habitat.
• Minimize disturbance of vegetation
and soil. Planning for both the final
site design and construction activities
should strive to retain the benefits of
existing soils and vegetation. Avoid

City of Durango, CO
removal of or damage to plants
whenever possible. Minimize cutting,
filling, and grading, as well as removal
or compaction of topsoil.
• Minimize hard surfaces that shed
If you must build on a sloped site, minimize the amount of
water. Rain that falls on buildings,
cutting and filling by stepping the foundation and the
pavement, and compacted soil runs
building with the slope.
off quickly and is unable to soak into
the ground, increasing the amount of
runoff that needs to be managed. Southern Tier Central Regional Planning &

• Determine proposed drainage

patterns. Water flow paths must allow
safe conveyance down the slope
without causing gullies or washouts.
Development Board

Moderate flows can be directed onto

soils with stable vegetation.
Concentrated flows should not be
allowed to rush straight down the
hillside and may require
Driveways should not funnel water down the hill. A
professionally designed management
recommended solution is to create a series of shallow
practices. Retain or restore natural
channels across the drive to intercept runoff and disperse
drainage patterns wherever possible.
water into established vegetation.

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 Steep Slope Guidance

• Use best management practices for timber harvesting. If trees are removed from steep areas, either as
part of a logging operation or to develop a site, this should be done during dry weather (when the soils
are dry) or when the ground is frozen and covered with snow. Winch logs off steep slopes, where
possible. If logging roads and skid trails are needed, hire a consulting forester to ensure that the
project is properly planned and executed.

Water Conservation District

Chemung County Soil and
Roads and driveways should follow the natural grade whenever possible.

• Anticipate future changes. If additional utilities or access routes may be desired in the future, include
this in the initial site plan. Consider the full-grown size of plants so that they don’t block the view or
interfere with other activities.
• Develop an implementation plan. Plan for phased construction that minimizes the amount of soil or
vegetation that is disturbed at any time. Stabilize one area before disturbing the next. Use temporary
seeding or mulching if permanent stabilization must be delayed.

It is recommended that an Erosion and Sediment Control Plan

be developed for any site with a slope greater than 15%.

Get professional assistance. If development must be located on areas with slopes greater than 15%, it is
recommended that you obtain professional assistance with project planning, particularly if there are
existing drainage or erosion problems. An erosion and sediment control specialist or professional engineer
can help with selecting and designing management practices for erosion control and runoff management.

Cultivate Healthy Soil

Good soil structure reduces its susceptibility to erosion and supports healthy vegetation, which provides
additional erosion protection. The soil should be loose enough to absorb and store water, but not so soft that
it erodes easily. A deep, well-drained soil that’s rich in organic matter is able to absorb rainwater (which helps
prevent flooding and soil erosion), filters out water pollutants, and also promotes vigorous plant growth that
will require less irrigation, pesticides, and fertilizer.

• Preserve existing soil. Healthy soils have taken years to develop. Protect this resource by avoiding
grading, shaping, and topsoil removal whenever possible.

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 Steep Slope Guidance

Soil preparation (topsoil, tilling, raking, and amendment)

is critical for establishing healthy vegetation.

• Restore topsoil. It may be necessary to replace the topsoil if it has been washed away by erosion or
stripped during construction (leaving an infertile, compacted soil surface). Loosen the subsoil to a
depth of at least 12 inches and then apply topsoil to provide a good growth medium for vegetation.
Fresh topsoil should be compacted to avoid erosion and slippage, preferably with hand-held tools.
• Avoid over-compaction of soil. Although topsoil should not be too soft, the use of heavy equipment
can cause severe compaction, which limits the ability of soil to absorb water and to support
vegetation. If soil compaction cannot be avoided, it can be loosened or de-compacted before planting
using a tiller or other equipment.
• Add compost. Compost enriches the soil and enables it to hold more moisture. Compost can be tilled
into the subsoil, incorporated into topsoil, or applied to the surface as a blanket.
• Fertilize. Fertilizer can be incorporated into the top two to four inches of topsoil. The fertilizer mix and
application rates should meet the nutritional needs of the specific site, as determined by a soil test. Do
not over-fertilize, as the unused nutrients can
pollute nearby waterbodies. (All application

NYS Dept. of Environmental Conservation

of fertilizer must be in accordance with the
New York State Nutrient Runoff Law – ECL
Article 17, Title 21.)
• Roughen the surface of exposed soil. Prepare
bare soil for seeding or planting by creating
horizontal grooves or steps across the slope.
This can be accomplished with stair-step
grading, tracking (driving equipment with
cleats up and down the slope), or other Cutting furrows along the contour helps to
techniques. These irregularities in the soil prevent erosion before vegetation is
surface catch rain, reduce erosion, and help established. After the slope is roughened, it
to establish seeds. should be seeded and mulched as soon as
possible.

Protect and Restore Vegetation

Healthy vegetation is the best practice for both managing runoff and preventing erosion on steep slopes.
Plants draw water out of the soil and return it to the atmosphere. The foliage intercepts rain and shields the
soil surface from raindrop impacts, which would otherwise strike the surface at 25-30 feet per second and
cause splash erosion of soil particles. Root systems add structure to the soil and increase its strength (like
woody rebar). Overland flow is slowed and dispersed by the stems and leaf litter, reducing the concentrated
energy that can scour out soil.

Establishing and maintaining vegetation are the most

important factors for protecting steep slopes.

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 Steep Slope Guidance

• Protect existing vegetation. Because exposed soil is

vulnerable to erosion, care is needed to minimize
damage to existing protective vegetation. Physically

Chemung County Soil and Water Conservation District

mark the limits of land disturbance on the site.
• Establish permanent vegetative cover. The site should be
stabilized with permanent vegetation as soon as
possible. If this is not possible, protect the soil surface
with temporary seeding or mulching.
• Select a suitable mixture of plants. A mixture of trees,
shrubs, vines, and ground cover can provide deep and
complex root structures for holding the soil in place.
Select plant species that are suited to the local site
conditions, including soil type, drainage, and degree of
shading.
• Plant native species. Native species are more likely to Minimize disturbance of existing
thrive and require less maintenance. forest cover on steep slopes.

• Mulch. Use mulch to protect exposed soil and conserve

moisture while vegetation is becoming established. This
is a temporary practice. Long-term protection is

USDA Natural Resources

provided by vegetative cover.

Conservation Service
• Irrigate. Water may be essential to establish new plants.
• Use biotechnical soil stabilization practices if needed. A
variety of practices integrate plants with woody
material, mats, or other products to stabilize the soil
and give plants time to become established. Mulch bare soils with plant residues.

Growing plants on a steep slope is not just landscaping.

The plants are intended to prevent erosion, so technical assistance may be needed
to select suitable species and get them established.

Divert or Slow Down Water

The erosive force of water increases with velocity and steep slopes encourage faster runoff. Managing water
flow paths can reduce the potential for erosion, washouts, and flash flood damage. The most feasible practices
for use on steep slopes are generally those that replicate natural systems (called green infrastructure).

• Divert flow away from steep areas. A swale or berm along the top of a slope or diagonally across a
slope can be used to divert runoff to a more stable area and prevent erosive flow down the slope.
• Slow it down. Fast-moving water erodes soil and quickly finds its way into rivers and streams, with
much of this water arriving at the same time and contributing to high peak flows. Vegetation, low
spots, uneven surfaces, and gentle (less steep) flow paths can slow down and hold back water.

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 Steep Slope Guidance

Acton Wakefield Watersheds Alliance

A series of water bars can be constructed diagonally across a trail or other
slope to deflect water into a stable vegetated area. The water bar can be an
earthen ridge at a 30-degree angle across the slope. Or it can be made by
placing a log, rocks, or other material in a trench and backfilled on the downhill
side.

• Spread it out. Allowing runoff to spread out, rather than concentrating flows in ditches or channels,
slows the water and absorbs its energy. This reduces erosion and peak flows.
• Soak it in. Water that soaks into the soil is water that does not contribute to washouts or flooding.

It is always more efficient and cost-effective to prevent erosion

than it is to repair damage after the fact.

Protect Waterbodies

Streams and lakes are dynamic systems that naturally overflow their banks during periods of high water.
Streams adapt to changing conditions by adjust their shape or location. Streambanks and the shorelines of
lakes are often threatened by erosion. Allowing space for these natural processes protects the natural systems
and keeps vulnerable development away from high-risk areas.
Keuka Lake Land Use
Leadership Alliance

• Locate development a safe

distance from waterbodies.
To protect both development
and natural systems,
buildings and other intense
uses should be set back 50 or
100 feet from the banks of If buildings and development that are setback from potentially
most streams, rivers, eroding banks, a vegetated buffer can provide bank stability,
wetlands, and lakeshores. protect water quality, and enhance habitat.

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 Steep Slope Guidance

• Allow floodplains to flood.

The floodplain is an

Amy Reges, Upper Susquehanna Coalition

important part of a stream
system because it provides a
place for water to spread out
and slow down during floods.
This temporary storage
reduces downstream
flooding. In addition, the
slower moving water has less
energy for eroding
streambanks. Berms, levees,
Whenever possible, buildings and other development should be
or high banks can disconnect
located outside of mapped floodplains and any unmapped
a stream from its floodplain
areas that are prone to flooding. Any development within the
and cause downstream
floodplain should be protected from flood damage.
problems.
• Maintain a vegetated buffer along streams and lakeshores. Whenever possible, preserve natural
vegetation near a stream or lake. The root systems hold the soil in place—preventing bank erosion. In
addition, the plants also slow down water, both surface runoff flowing down the bank and out-of-bank
flows during a flood—reducing the potential for erosion and flooding. Avoid mowing near the bank
because grass generally lacks adequate root systems to stabilize the soil. A mixture of trees, shrubs,
and groundcover generally provides the best erosion protection and habitat along streams and
lakeshores.
• Do not disturb the channel or banks of

Water Conservation District

a stream. Leaves, branches, and even

Chemung County Soil and

logs are a natural part of healthy
stream systems, providing nutrients
and improving in-stream habitat.
However, if the stream contains
manmade debris or if diverted flow
threatens development it can be
removed—but this should be done in a Stream crossing structures should be located and
way that avoids altering the stream constructed to minimize disturbance of the banks
channel or banks, including vegetation. and bottoms of streams.
The channel and banks should not be
altered without assistance from a
stream professional.

Retaining or restoring a variety of plants along the bank of a stream or lake

is the easiest and most effective way to protect the bank,
while also providing habitat and protecting water quality.

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 Steep Slope Guidance

Monitor and Maintain the Site

Maintenance may be needed to ensure that vegetation is established, remains healthy, and continues to
stabilize the slope. Water and fertilize plants as needed. If erosion problems develop, the site should be
repaired in a timely manner—preferably before the next major storm event.

Regulations, Standards, and Guidance

Development on slopes—as in other locations—is subject to various building, environmental, and land use
regulations. Even if no permit is required, the design standards developed by New York State provide valuable
guidance for designing and implementing good practices on steep slopes. The following resources can help with
stabilizing and safely using steep land.

• New York State Uniform Code. The state Building and Residential Codes require that foundation drains
and surface runoff from a building site be discharged to a drainage system approved by the building
official. 1 Buildings must be set back from both the top and the toe of slopes steeper than one unit
vertical in three units horizontal (33.3% slope).2

• New York State Stormwater Construction Permit. A state permit is required for construction activity
that disturbs one or more acres. This permit requires development of a Stormwater Pollution
Prevention Plan (SWPPP) that addresses erosion and sediment control during construction. For some
projects, the SWPPP must also address the long-term impacts on water quantity and water quality.
Construction activities must minimize the disturbance of steep slopes that are greater than 25% (based
on the soil type). If the steep slope drains to a drinking water source (including Seneca and Keuka
Lakes), the construction project may be ineligible for coverage under the general permit (and
therefore requires an individual permit with a more stringent design and review process).3
• New York State Design Standards for Erosion and Sediment Control. The “Blue Book” provides
minimum design standards and specifications for the selection, design, and implementation of erosion
and sediment control practices. It includes detailed guidance for (1) the site planning process, (2)
runoff control, (3) soil stabilization, and (4) sediment control. This manual recommends that an Erosion

1
2020 Residential Code of New York State, Sections R401.3 and R405; 2020 Building Code of New York State, Section
1805.4.3; and 2020 Plumbing Code of NYS, Section 1101.2.
2
2020 Residential Code of New York State, Section R403.1.7; 2020 Building Code of New York State, Section 1808.7.
3
New York State Department of Environmental Conservation, SPDES General Permit for Stormwater Discharges from
Construction Activity, Permit No. GP-0-20-001, Effective Date: January 29, 2020;
https://www.dec.ny.gov/chemical/43133.html.

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 Steep Slope Guidance

and Sediment Control Plan be prepared for all land development and construction activities on slopes
that exceed 15% (regardless of regulatory requirements).4
• New York State Stormwater Design Manual. The peak flows from a site should not be any greater after
development than they were before it was developed. To achieve this, the design manual spells out a
6-step process for site planning and practice selection. It recommends that steep slopes be kept in an
undisturbed natural condition to limit soil loss, erosion, excessive stormwater runoff, and the
degradation of surface water. Development on slopes with a grade of 15% or greater should be
avoided, if possible. On slopes greater than 25%, no development, re-grading, or stripping of
vegetation should be considered.5
• Municipal land use management. Some local governments use their land use authorities to regulate
development on steep slopes and to assist with developing a site plan that fits the site and protects
the natural features. Although local requirements vary, the objective is generally to encourage
development on flatter sites, limit or prohibit development on steep slopes, and minimize risks from
activities that do occur on steep slopes. Working with local officials early in the development process
helps to ensure compliance with these local regulations and may improve the overall outcomes for the
completed project.
• Best Management Practices During Timber Harvesting Operations. Timber harvesting on steep slopes
can result in the rapid development of gullies and should be avoided where possible. When slopes are
logged, water that collects on forest roads and skid trails must be controlled and dispersed. Guidance
for forest management and BMP implementation has been developed by the Chemung County Soil
and Water Conservation District.6
• Critical Area Treatment Handbook. Healthy plants are critical for stabilizing steep slopes. To assist with
site planning, selecting appropriate plant materials, and techniques for seeding and planting, USDA
Natural Resources Conservation Service has developed a handbook for conservation plantings in the
northeast.7
BE A GOOD NEIGHBOR
The small steps that you take to manage runoff from your
property will combine with the efforts of your neighbors to
reduce flooding and erosion damage in your community.

G. Frantz, Cornell University

4
New York State Standards and Specifications for Erosion and Sediment Control, November 2016;
https://www.dec.ny.gov/chemical/29066.html.
5
New York State Stormwater Design Manual, January 2015; https://www.dec.ny.gov/chemical/29072.html.
6
Best Management Practices during Timber Harvesting Operations, 2020, Chemung County Soil and Water Conservation
District; http://www.u-s-c.org/forestrybmps.pdf.
7
A Guide to Conservation Plantings on Critical Areas for the Northeast, 1991, USDA Natural Resources Conservation
Service; https://www.nrcs.usda.gov/Internet/FSE_PLANTMATERIALS/publications/nypmspu11417.pdf.

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