4A. Ditches - Negative Slopes
Public Ditches (with 16.5’ buffer standard)
This common alternative practice may be applicable in locations where:
- the primary land slope is away from the top of the channel of a public drainage ditch or there are existing high areas or berms next to the channel which prevent flow from uniformly entering the water body; and
- flow typically enters the public ditch through areas of open channel concentrated flows; or
- flow typically enters the public ditch through an installed pipe or conduit.
Note: Alternative practices installed per this guidance should be coordinated with the public drainage authority to ensure consistency with the acquisition and establishment of permanent strips of perennial vegetation in accordance with Chapter 103E.
- SWCD should verify by site visit that perennial vegetation is installed from the top of the channel to the top of the constructed berm/spoil bank. This perennial vegetation helps provide bank stability and serves as a tillage setback to prevent sedimentation or direct application of fertilizer or pesticides below the top of bank or normal water level.
- In addition to the perennial vegetation, treatment of all discharge which flows into the waterbody via concentrated flow or other conduit may be treated using a combination of situations 1-3 below.
- Site-specific diagrams or pictures should be used to document validations.
Situation 1: Runoff approaches as concentrated flow and discharges via a conduit, structure or vegetated pathway that meets NRCS Design standards.
- SWCD staff should determine that the structure(s) will be/is functioning as specified to provide the as designed water quality functions.
Situation 2: Runoff approaches as concentrated flow and discharges via a conduit, structure or vegetated pathway that does not meet NRCS Design standards.
- SWCD staff should confirm that outfall locations are stable and functioning as intended to prevent erosion, reduce flow and trap sediment.
- Develop a drainage area map(s) for all outfall locations to determine depth/length of a critical area planting where all areas of concentrated flow enter through a structure or channel which is not based on a NRCS design standard. For consistency and predictability purposes, some suggested dimensions have been provided in the table to the right for low slope sites. When using the table, SWCD staff should consider site specific factors such as topography, soil type and tillage management to assure comparable water quality benefit.
- Use NRCS Code 342 (Critical Area Planting) or 327 (Conservation Cover) specifications to determine proper seed mixture. Seed tags and invoices to show that it was seeded to Critical Area Planting or Conservation Cover specifications may be needed for SWCD validation.
|Area draining to site (acres)
||Width (ft) Perennial Veg.
||Length (ft) Perennial Veg.
Situation 3: Water flows overland to open tile intake(s).
- Establish perennial vegetation around all open tile intakes within the parcel adjacent to the waterbody, following the NRCS 393 practice standard. (Typically 15’-60’ radius.)
- A DNR public waters work permit may be required for course, current or cross-section alterations below the Ordinary High Water Level of a Public Water. Landowners should contact their DNR Area Hydrologist before starting a project to determine if permitting requirements may apply.
- In areas where perennial vegetation is established to treat runoff, occasional maintenance and re-shaping and re-seeding to maintain flow and remove and distribute vegetative mass may be useful. Side inlets or similar structural practices will also require occasional maintenance to function as designed.
This common alternative practice provides treatment of runoff along the entire frontage of the waterbody. In instances where all land flows away from the top edge of the constructed channel, it provides a tillage setback. The perennial vegetation helps provide bank stability and serves as a tillage setback to prevent direct application of fertilizer or pesticides below the top of bank or normal water level. It also helps prevent direct disturbance of the banks so soil is not deposited directly onto the banks or into the water body due to tillage practices. The concentrated inflow measures treat the water before it enters the waterbody more than one-rod of perennial vegetation would, as the runoff would not flow through the buffer uniformly towards the water body. Use of the combination of perennial vegetation and discharge structures or other vegetated areas provide comparable or greater water quality benefit.
Figure 3: Cross Section view A, Ditches - Negative Slopes (not to scale)
Figure 4: Cross Section view B, Ditches - Negative Slopes (not to scale)
Figure 5: Bird’s Eye view, Ditches - Negative Slopes (not to scale)