4B. Ditches – Flat Land
Public Ditches (with 16.5’ buffer standard)
This common alternative practice may be applicable on Public Ditches in locations where:
- the primary surface runoff contributing area is approximately 0.1% slope; and
- the channel and banks are fully vegetated and stable; and
- surface water typically enters the public ditch through open channel concentrated flows; or
- surface flow directly discharging over the channel bank into the public ditch is minimal and infrequent.
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 staff should verify that perennial vegetation is clearly established outward beyond the top of the channel to provide a sufficient setback to protect the top of the bank from tillage disturbance, direct deposition of sediment, and direct as well as indirect application of fertilizer, pesticides & herbicides into the channel.
- In addition to the perennial vegetative setback, treatment of all runoff water into the ditch via concentrated flow or other conduit may be treated using a combination of situations 1-4 below.
- Site-specific diagrams or pictures should be used to document validation of compliance.
Situation 1: Surface 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) are functioning as specified to provide the as- designed water quality functions.
Situation 2: Surface runoff approaches the ditch as concentrated flow and discharges via a conduit, structure or vegetated pathway that does not meet NRCS design standards.
- SWCD staff should confirm that conduit outlets or graded outfalls are stable and not causing bank erosion.
- Develop a drainage area map(s) for all outfall locations to determine depth/length and width 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: Surface runoff flows overland to open tile intake.
- Establish perennial vegetation around all open tile intakes within the parcel adjacent to the ditch, following the NRCS 393 practice standard. (Typically 15’-60’ radius.)
Situation 4: Surface runoff does not directly discharge to the ditch via concentrated flow.
- Provide a minimum 16.5’-50’ strip of no-till/strip till (NRCS 329 standard) or cover crops (NRCS 340 standard) in addition to the vegetated tillage setback.
- 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 where it enters the ditch. In instances where there is minimal flow over the top edge of the constructed channel, this alternative practice provides bank stability and serves as a tillage setback to prevent direct deposition of soil into the channel associated with tillage practices or application of fertilizer or pesticides below the top of bank or normal water level. The concentrated inflow measures treat the water before it enters the ditch more than one-rod of perennial vegetation would, as the runoff would not typically flow through the buffer towards the ditch. Use of the combination of perennial vegetation and discharge structures or other vegetated areas provide comparable or greater water quality benefit.
Figure 6: Cross Section view A, Ditches – Flat Land (not to scale)
Figure 7: Cross Section view B, Ditches – Flat Land (not to scale)
Figure 8: Bird's Eye view, Ditches – Flat Land (not to scale)