Fact Sheet A5 – Ditch Retrofit Strategies
Overview
Factors to Consider
Ditch Retrofit
Strategies
Fact Sheet A5
Retrofitting ditches provides an opportunity to improve water quality treatment and/or increase conveyance
capacity within existing ditches. In areas that have already been developed, especially where ditches are
underperforming, retrofitting existing infrastructure may provide the most cost-effective opportunity to remove
pollutants prior to discharge to surface waters. Ditch retrofit strategies described in this fact sheet include:
Ditch retrofits may not always be possible or advised due to existing site conditions and feasibility limitations.
Limitations may include:
Certain retrofit strategies may be more appropriate for water quality improvements,
while others are better suited for increasing conveyance capacity.
Lack of right-of-way space may prevent flow redirection, ditch modifications, or adjacent enhancements
Established plants, which should not be disturbed if they provide water quality treatment and ditch
stabilization
Steep slopes that prevent planting improvements or other strategies for regulating ditch velocity
Enhance Ditch
Enhance ditch vegetation
Enhance filtration media
Add ditch buffer
Enhance ditch buffer
Add an underdrain elbow
Add an upturned elbow
Convert/Eliminate Ditch
Replace ditch with a water quality treatment and flow control BMP
Replace ditch with a water quality treatment or flow control BMP
Convert ditch to a closed system with water quality treatment
Regulate Ditch Velocity
Modify Ditch Shape/Grade
Modify Inlet/Outlet
Scour at ditch inlet
Scour at ditch outlet
Redirect ditch outfall
Refer to the potential retrofit strategies listed below to determine which strategies are most appropriate for
addressing various ditch issues.
February 7, 2019
Fact Sheet A5 – Ditch Retrofit Strategies
Intended
Outcome
Issue
Potential Retrofit Strategy
Ditch could benefit from and
has suitable conditions to slow
water, reduce erosion, and
increase plant uptake.
Plant ditch vegetation with wetland vegetation
(see Ditch Conversion/ Elimination) or meadow habitat vegetation.
Pollutant build-up in soil media
Amend the soil media in portions of the ditch.
Prior to installation of soil amendment, confirm that nutrients of
concern will not be exported by the amended soil media.
Note: Several soil amendments are under development.
Replace existing soil media with filtration media
See Ditch Conversion/ Elimination on page 3 of this fact sheet
Install nutrient removal check dams or in-ditch bioreactors.
Products are under development to target specific nutrients (e.g.,
woodchip socks).
Note: In-ditch nutrient removal may cause low dissolved oxygen
in ditch flows. Water quality treatment systems should be paired
with rock splash pads.
Larger-scale off-line bioreactors for denitrification have been
used in agricultural ditch networks; smaller bioreactors are under
development.
Pollutants entering ditch from
adjacent property
Add a narrow-width vegetative buffer (15 to 30 feet), such as a filter
strip or grass, between the adjacent property and the ditch.
Fill slope roadway
embankment is long and wide
and could provide additional
water quality treatment.
Amend roadside embankment with compost similar to a compost
amended vegetated filter strip.
Roadway flooding in areas
with low permeability soils,
sensitive side slopes, and/or
low ditch slopes
Install a buried conduit (e.g., underdrain or French drain) to collect
water.
Receiving water is sensitive to
nitrogen
Use upturned elbow for internal water storage to increase retention
time for denitrification
Additional Resources:
Chesapeake Bay Roadside Ditch Management Team, 2017
Chesapeake Stormwater Network, 2018
Needelman et al., 2007
Issues and Potential Retrofit Strategies
Enhance Ditch
Fact Sheet A5 – Ditch Retrofit Strategies
Intended
Outcome
Issue
Potential Retrofit Strategy
Stormwater runoff entering
ditch could benefit from
infiltration or water quality
treatment.
Replace ditch with a water quality treatment and/or flow control BMP:
Dispersion
Wetland
Biofiltration swale
Wet biofiltration swale
Bioretention area
Sand filter
Ditch segment is located
along urban roadway with
limited space for water quality
treatment BMP.
Convert ditch to closed system with water quality treatment:
Tree box filters
Modular sand filters
Infiltration trench
Boxed treatment can also be added at existing piped segments
in urban ditch networks.
Ditch erosion, high-velocity
ditches, and high sediment
loads from rural ditches
This practice involves reconfiguring roadside drainage to promote
sheet flow into adjacent forested buffers. Techniques include:
Raising the road profile
Remove roadside berms
Out-sloping the road
If steep slopes are present, consider ditch elimination only on the
down-slope side; cross pipes may be installed to convey drainage
across the road.
Additional Resources:
Chesapeake Bay Roadside Ditch Management Team, 2017
Chesapeake Stormwater Network, 2018
Examples of ditch enhancement Photo Source: Kitsap County
Enhance Ditch
Convert/Eliminate Ditch
Fact Sheet A5 – Ditch Retrofit Strategies
Intended
Outcome
Issue
Potential Retrofit Strategy
Steep longitudinal slope with
a high potential for erosion
Reduce ditch slope-length by adding:
Cross-pipes (culverts)
Turnouts (bleeders or lead-off ditches)
Broad-based dips or grade breaks
Single inlet with high velocity
resulting in scour and erosion
Add a flow spreader to distribute the water across the ditch surface
area:
Flow dispersion trench
Alternative flow dispersion trench
Tee type energy dissipater
Approved engineered flow spreading BMP
Channelized, incised flow in
ditches with steep slopes and
uneven vegetation
Add internal energy dissipation mechanisms (e.g., check dams).
Guidance from Kitsap County (2012) recommends:
Soft (e.g., straw wattle, coir log, coir netting) or rock check
dams for 2.5-10% slope
Rock-lined ditch with hard check dams for 10-15% slope
Cascade check dam > 15% slope
Additional Resources:
Chesapeake Bay Roadside Ditch Management Team, 2017
Kitsap County, 2012
Steep longitudinal slopes and high velocities flowing through ditches can lead to numerous issues, but they
may be easily addressed through a ditch retrofit.
Adding check dam to regulate ditch velocity
Photo Source: Kitsap County
Cascade Weir
Regulate Ditch Velocity
Fact Sheet A5 – Ditch Retrofit Strategies
This section summarizes major modifications to the ditch shape and/or grade. For corrective maintenance
strategies for excavating minor amounts of accumulated sediment in order to restore original ditch slope and/
or grade line, refer to the Field Guide for Roadside Ditch Maintenance in Western Washington.
Intended
Outcome
Issue
Potential Retrofit Strategy
High flow rate in a V-shaped
ditch is resulting in erosion
Reshape to U-shape or trapezoidal ditch with wider bottom than
the original V-shape.
Reshape to a natural channel (2-stage ditch with a low-flow channel
and vegetated floodplain bench).
Storage capacity has been
substantially reduced by
accumulated sediment
Excavate/remove sediment to restore to original proposed grade.
Refer to the Field Guide for Roadside Ditch Maintenance in
Western Washington for specific ditch cleaning and reshaping/
regrading methods.
Additional Resources:
University of Minnesota, 2014
Stabilizing ditch during reshaping
Photo source: Kitsap County
Ditch sediment removal prior to stabilization
Photo source: King County
Modify Ditch Shape/Grade
Fact Sheet A5 – Ditch Retrofit Strategies
Intended
Outcome
Issue
Potential Retrofit Strategy
Scour at inlets and/or outlets
Realign culverts or provide headwalls and endwalls at crossings.
Outfall from ditch discharges
directly to a stream
Disconnect direct outfalls to streams by:
Redirect ditch outlets away from streams
Redirect ditch outlets through vegetated area
Unfiltered ditch flow
discharges to receiving water
through cross-culvert under
road
Install structure to intercept and treat flow at ditch outlet.
Examples:
Tree-box filter (for urban settings)
Type II Catch Basin with sump and internal water quality
treatment component
Additional Resources:
Kitsap County, 2012
This section summarizes major modifications to the inlet and/or outlet structures. For corrective maintenance
strategies for inlets and outlets, such as installing or repairing riprap, aprons, and/or rock plunge pools, refer to
the Field Guide for Roadside Ditch Maintenance in Western Washington.
Additional Resources
Chesapeake Bay Roadside
Ditch Management Team
(2017)
Draft Options for Crediting Pollutant Reduction from Roadside Ditch Management
Practices (RDM) in the Chesapeake Bay Watershed.
Chesapeake Stormwater
Network (2018)
Roadside Ditch Management Guidance Webcast, Part 1 and Part 2
Kitsap County (2012)
Roadside Ditch and Shoulder Water Quality Enhancement Plan
Needelman et al. (2007)
Improved Management of Agricultural Drainage Ditches for Water Quality Protection:
An Overview
Schneider and Boomer (2016)
Re-plumbing the Chesapeake Watershed: Improving Roadside Ditch Management to
Meet TMDL Water Quality Goals, STAC Workshop Report
Strock et al. (2010)
Drainage Water Management for Water Quality Protection
University of Minnesota (2014) Field Guide for Maintaining Rural Roadside Ditches: Protecting Lakes and Streams
through Proper Ditch Maintenance
Modify Inlet/Outlet
This fact sheet was developed by Herrera Environmental Consultants with input from King County and the Regional Operations and Maintenance
Program (ROADMAP). Project funding was provided by a National Estuary Program grant from the Washington State Department of Ecology.