ROADSIDE DITCH
MAINTENANCE IN
WESTERN WASHINGTON
FIELD GUIDE
February 7, 2019
Field Guide for Roadside Ditch Maintenance in Western Washington: Routine Maintenance
Ditch maintenance strategies fall into two categories: routine maintenance and corrective
maintenance. Some maintenance strategies can fall into both categories.
Different ditch types and different existing conditions require different
maintenance strategies, and appropriate maintenance strategies may
be limited by cost, space availability, and environmental regulations.
Some existing ditches may be so degraded that they require more
complicated and expensive source control or retrofit strategies.
This field guide focuses on the implementation of routine and
corrective maintenance strategies by municipal maintenance staff in
roadside ditches that convey stormwater.
Introduction
The reference bar at the bottom of most of the maintenance strategy pages refers to a set
of 8 fact sheets that were also developed as part of this project:
Fact Sheet A1 – Ditch Mapping
Recommendations
Fact Sheet A2 – Prioritizing Ditches for
Inspection and Maintenance
Fact Sheet A3 – Permit Requirements for
Ditch Maintenance
Fact Sheet A4 – Source Control Strategies
for Ditches
Fact Sheet A5 – Ditch Retrofit Strategies
Fact Sheet F1 – Considerations for Field
Evaluation of Roadside Ditches
Fact Sheet F2 – Ditch Cleaning Strategies
Fact Sheet F3 – Maintaining Ditches that
Convey Natural Flow
This guide is for reference only. All maintenance strategies must comply with local codes and regulations.
Maintenance strategies in ditches that convey natural flow require special considerations.
This field guide 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.
Fact sheets are boxed and shown in brighter text if they are directly applicable to the maintenance
strategy described on that page of the field guide.
Introduction
Field Guide for Roadside Ditch Maintenance in Western Washington: Routine Maintenance
What is ditch vegetation management?
Vegetation management includes the establishment and maintenance of healthy,
beneficial vegetation in ditches and the control of excess or unwanted vegetation.
Why is ditch vegetation management important?
Ditches are designed to transport water away from the roadway to prevent roadway
flooding. Vegetation can promote this essential ditch function, but can also inhibit
the capacity of a ditch to convey water. Vegetation can also capture pollutants from
stormwater runoff.
Routine maintenance strategies for vegetation management should ensure that vegetation
is healthy, but does not encroach on the adjacent roadway or cause ditch capacity issues.
Mowing (page 3)
Routine Maintenance
Weed Control
(page 6)
(page 8)
(page 9)
Invasive Species and
Noxious Weed Removal
(page 7)
Brush Cutting (or
Brushing)
(page 4)
Minor Reseeding/
Replanting
(page 5)
Ditch Vegetation
Management
Nuisance Animal/
Insect Control
Inlet/Outlet
Cleaning
Ditch Vegetation Management
In general, routine maintenance can be completed by municipal maintenance staff
without major analysis, engineering, or permits. Ideally, ditches that are in good
condition do not require corrective maintenance and can be maintained through
routine inspections and maintenance.
The routine maintenance strategies outlined in this field guide include:
Field Guide for Roadside Ditch Maintenance in Western Washington: Routine Maintenance
Ditch Vegetation Management Strategies
Mowing
Description
Cutting vegetation to a reasonable height to allow for proper water flow and aesthetics
Condition
Tall vegetation is impeding flow of water through the ditch or causing line of sight issues
Ditch Surface Types
Vegetated
Water Flow
No flow
Common Equipment
and Materials
Mower
Specialized mower for steep slopes
Brooms, scoops, shovels, and/or handheld blowers
Additional Equipment
and Materials for
Natural Flow
Reflective markers (“fish sticks”)
Considerations Prior to
Implementation
Set the mowing height at the highest acceptable level. For standard turf grass, mowing
only the top 1/3 of the grass blade height is recommended. Where vegetation growth is
excessive, additional mowing may be necessary.
Avoid operating mowers in wet areas or rough terrain to minimize scalping and rutting.
Strategize mowing direction to minimize spreading of cut material onto adjacent paved
surfaces.
If vegetation is providing flow control or treatment, too much removal or trimming
could reduce these functions.
If possible, retain vegetation on the south or west sides of the ditch to provide shading
of the ditch and reduce water temperature.
If there is an opportunity to re-seed, low growing grass seed mix is recommended to
reduce mowing frequency and cost.
Steep Slope
Considerations
Use a specialized mower when steep slopes (≥ 15 percent) are present.
Disposal
Considerations
Turf grass may be mulch mowed and left in place; however, large quantities of turf grass
clippings may lead to outlet clogging and nutrient loading in downstream water bodies.
Fact Sheet References
Additional References
Refer to Invasive Species and Noxious Weed
Removal (page 7) for additional recommendations
regarding mowing.
Refer to local codes and regulations for additional
requirements.
Photo source: King County
Field Guide for Roadside Ditch Maintenance in Western Washington: Routine Maintenance
Ditch Vegetation Management Strategies
Brush Cutting or “Brushing”
Description
Trimming woody vegetation to remove overgrown and/or excessive vegetation to allow for
proper water flow and to restore sight distance
Condition
Overgrown/ excessive vegetation impeding flow or storage of water and sediments
Safety or structural integrity of the roadway is jeopardized
Ditch Surface Types
Vegetated
Water Flow
No flow
Common Equipment
and Materials
Brush cutters
Power saws
Axes and/or machetes
Pruning shears, loppers, and/or clippers
Brooms, scoops, shovels, and/or rakes
Truck cover (for securing load during transport)
Considerations Prior
to Implementation
If vegetation is providing flow control or treatment, too much removal or trimming
could reduce these functions.
If possible, retain vegetation on the south or west sides of the ditch to provide
shading of the ditch and reduce water temperature.
Disposal
Considerations
Remove cut branches/other vegetative debris after brushing to reduce outlet
clogging and spreading invasive species.
Compost or stockpile vegetative matter in a clean green stockpile at your
maintenance facility, if possible.
Vegetation management demonstration
Photo source: Kitsap County
Additional References
Refer to local codes and regulations
for additional requirements.
Field Guide for Roadside Ditch Maintenance in Western Washington: Routine Maintenance
Ditch Vegetation Management Strategies
Minor Reseeding/Replanting
Description
Adding seed and/or plants to stabilize exposed soils. Applies to conditions affecting a small
section of a ditch that can be addressed as part of routine maintenance (see Additional
References below).
Condition
Sparse vegetation/ eroded patches on ditch bottom
Poor grass growth
Ditch Surface Types
Vegetated
Bare Soil
Water Flow
No flow
Common Equipment
and Materials
Seed mix
Hydroseeder
Post-seeding erosion control BMPs (e.g., straw mulch, biodegradable nets and
blankets, coir mats)
Considerations Prior
to Implementation
Ensure that erosion control BMPs are installed properly to avoid blockages.
Low growing grass seed mix is recommended to reduce mowing frequency and cost.
Confirm that there are no weed seeds or invasive plant seeds in the seed mixes.
Seed and/or plant during the following optimum windows:
Late spring (April 1 through June 30)
Early fall (September 1 through October 1)
Establishment of vegetation may not be feasible in coarse grained or mineral soils.
Fact Sheet References
Examples of sparse or absent vegetation that could be classified as poor (left),
moderate (center), and good (right) vegetation coverage.
Additional References
When major reseeding/replanting is
necessary to correct poor conditions, refer
to Major Reseeding/Replanting (page 16)
in the Corrective Maintenance section of
this field guide.
Refer to local codes and regulations for
additional requirements.
Field Guide for Roadside Ditch Maintenance in Western Washington: Routine Maintenance
Ditch Vegetation Management Strategies
Weed Control
Description
Control of weeds through biological, physical, mechanical, chemical, or cultural methods
Condition
Weeds are present in a ditch
Ditch Surface Types
Vegetated
Bare Soil
Rock
Water Flow
No flow
Standing water
Low flow
Common Equipment
and Materials
Required permits (keep documentation on-site during work)
Weeding tools
Weed burner
Brooms, scoops, shovels, and/or rakes
Considerations Prior
to Implementation
Verify that required permits have been obtained prior to beginning work.
Review preferred implementation strategies documented in an Integrated Pest
Management (IPM) plan.
Ensure that herbicide applications are performed by licensed, qualified staff.
Use physical and/or mechanical methods of vegetation removal rather than
applying herbicides, where practical.
Disposal Conditions
Compost or stockpile vegetative matter in a clean green stockpile at your maintenance
facility, if possible, and only if invasive species and noxious weeds are not present.
Fact Sheet References
Reed canarygrass, Photo source: WA State Noxious Weed
Control Board
Additional References
Refer to Invasive Species and Noxious Weed
Removal (page 7) for disposal considerations
when invasive species and noxious weeds are
present.
Refer to your local IPM plan, local codes and
regulations for additional requirements.
Field Guide for Roadside Ditch Maintenance in Western Washington: Routine Maintenance
Ditch Vegetation Management Strategies
Invasive Species and Noxious Weed Removal
Description
Control of invasive species and noxious weeds through biological, physical, mechanical,
chemical, or cultural methods
Condition
Invasive species present Noxious weeds present
Ditch Surface Types
Vegetated Bare Soil Rock
Water Flow
No flow Standing water Low flow
Common Equipment
and Materials
Required permits (keep documentation on-site during work)
Weeding tools
Weed wrench
Targeted herbicide applicator (woody painter/ herbicide wand)
Garbage bags (to prevent seed development and dispersal)
Protective clothing and eye protection (for toxic, noxious weeds)
Considerations Prior to
Implementation
Verify that required permits have been obtained prior to beginning work.
Invasive species may need to be removed by hand.
Pull plants when soils are moist and before seeds are produced.
Identify invasive species that can and should be controlled or reduced by mowing.
Ensure that herbicide applications are performed by licensed, qualified staff.
Prior to mowing, implement the following for small populations of invasive plants:
Use herbicides early in the summer.
Physically remove (cut) flower or seed heads.
Physically remove rootstock (mechanically excavate).
Implement the following for large, mature invasive plants:
Control large purple loosestrife plant populations with biocontrol beetles
(Hylobias sp. or Galrucella sp.) prior to mowing.
Mow plants prior to seed maturation, allow the plants to regrow to a height of 2 to
4 feet and then treat with foliar herbicide.
If mowing occurs after seed maturation, hand clean the upper parts of the mowing.
equipment with a brush or broom prior to moving to a new location.
Disposal Conditions
Ensure proper disposal per the Washington State Noxious Weed Control Board.
Bag cut flowers and seed heads.
Fact Sheet References
Common invasive species that thrive in ditches include reed canarygrass and
Himalayan blackberry. These species can make ditches difficult to access and
inspect and can reduce ditch flow capacity.
Additional References
Refer to the Washington State Noxious
Weed Control Board (www.nwcb.wa.gov) for
weed identification, removal, and disposal
considerations.
Refer to your local IPM plan, local codes and
regulations for additional requirements.
Field Guide for Roadside Ditch Maintenance in Western Washington: Routine Maintenance
Inlet/Outlet Cleaning
Description
Clean accumulated sediment and blockages from inlets and outlets
Condition
Accumulated sediment or blockage impeding flow (≥ 50% blockage) at inlet/outlet pipe
Ditch Surface Types
Vegetated Bare Soil Rock
Paved Metal
Water Flow
No flow Standing water
Common Equipment
and Materials
Rake, hoe, or shovel
Wheelbarrow or buckets
Steep Slope
Considerations
Consider adjusting threshold to ≥ 30% blockage when steep slopes (≥ 15 percent) are
present.
Disposal Conditions
Use or disposal options for the sediment removed from the inlet and/or outlet will depend
on the characterization of the waste (see Fact Sheet A3 – Permit Requirements for Ditch
Maintenance).
Fact Sheet References
Culvert outlet is partially buried in sediment (left). Inlet blocked by structure (right).
Additional References
Refer to local codes and regulations for
additional requirements.
Field Guide for Roadside Ditch Maintenance in Western Washington: Routine Maintenance
Nuisance Animal/Insect Control
Description
Control nuisance animals/insects through mechanical, manual, or chemical methods. Examples
of nuisance animals and insects include:
Beavers: May block ditch capacity with dams
Bees: Could pose a hazard to crews maintaining the ditch
Moles: Contribute to erosion by burrowing holes
Mosquitoes: May result from stagnant flow in ditch; nuisance and public health
hazard
Nutria: Contribute to erosion by destroying the banks of ditches
Condition
Nuisance animals/insects present
Ditch Surface Types
Vegetated
Bare Soil
Rock
Paved
Metal
Water Flow
No flow
Standing water
Common Equipment
and Materials
Animal guards (e.g., rods, flap gates, and finger-type flap gates) for outlet pipes
Traps (if allowed)
Considerations Prior
to Implementation
Preferred implementation strategies should be documented in an Integrated Pest
Management (IPM) plan.
Fact Sheet References
Evidence of moles in a roadside ditch.
Additional References
Refer to your local IPM plan, local codes and regulations
for additional requirements.
Field Guide for Roadside Ditch Maintenance in Western Washington: Corrective Maintenance
Corrective Maintenance
Corrective maintenance may require analysis, engineering, or permitting prior to implementation. The
corrective maintenance strategies outlined in this field guide include:
Trees of Concern
(page 17)
Ditch Cleaning
Minor Ditch Reshaping/
Regrading (page 15)
Ditch Stabilization
(page 14)
Major Replanting/
Reseeding (page 16)
Hand Ditching (page 11)
Bucket Ditching (page 12)
Shoulder Ditching (page 13)
Minor Inlet/Outlet Repair
(page18)
What is ditch cleaning?
Ditch cleaning is a…