Ditches and Drainage Plan Criteria

Roadside drainage ditch with rocks

This guideline applies to unincorporated area residential subdivisions with lots larger than 5 acres.

In a subdivision with lots smaller than 5 acres APWA Section 5600 applies.

All required drainage calculations and improved channels must comply with APWA Section 5600 criteria.


I. Natural Drainage Channel Areas

General: Natural drainage channel areas are allowed where no channel improvements are anticipated by the developer, which is usually the case in large lot rural subdivisions. The intent of the natural drainage channel area is to delineate the area that floods so that buildings are not located near flood prone areas. The natural drainage channel areas do not apply to FEMA designated flood plains or flood areas designated by the watershed studies.

Width: Per APWA 5600 the width of the natural drainage area should be large enough to enclose the 100 year flood plus one foot of freeboard, plus 10 ' measured horizontally on each side. The minimum width of the drainage area will be 30’ where calculations are provided. As an option for under 30 acre drainage areas calculations are not required if width shown on the plat is 140 ft. and generally centered on the natural channel.

Plats: Delineate natural drainage channel areas on preliminary and final plat. A note should be shown on the plat that prohibits building construction in the natural drainage channel area and prohibits low opening of any structure to be lower than the adjacent boundary of the natural drainage channel area.

Location: On drainage courses, the natural drainage channel area shall begin where drainage area accumulates to five acres.

Flood Elevations: Base flood elevations and bench marks are required on natural drainage channel areas where the drainage area exceeds 300 acres and watershed study results are not available.

II. Drainage Calculations

Use watershed study information where available. If available calculations are only required upstream of studied areas.

Small Drainage Areas: For natural drainage areas where the drainage area is less than 30 acres calculations are not required if a minimum width of 140 feet is shown. For a width of less than 140 ft. calculations are required to justify the reduced width.

Hydrology: Use APWA section 5600. Assume fully developed upstream and compute 100 year flow rate. Use FEMA or watershed data if appropriate.

Open Channel Hydraulics: Use APWA Section 5600 or HEC-2. HEC-2 is required in FEMA detail study areas, and floodway delineation in FEMA approximate areas. Compute flood limits upstream of watershed studies and FEMA areas to a point where drainage is less than 5 acres or to 30 acres if 140 ft. width is proposed upstream of 30 acres. Elevations may be required if near FEMA area.

Headwater at crossroad culverts: If land is upstream from a crossroad culvert compute head water elevation at crossroad culverts for 100 year storm (usually inlet control).

Existing Dams: Large lot subdivisions do not increase impervious area enough to affect dam hydraulics. However, they do affect flood elevations and limits of natural drainage channel area. The natural drainage area upstream of dams should include the area below the top of dam elevation plus 2 feet, unless flood routing calculations are performed. On dams with more than 30 acres drainage the limits of natural drainage area should be calculated for the emergency spillway.

Delineation: On preliminary and final plat, show base flood elevation data and flood plain limits along FEMA designated flood plains, watershed study areas and other areas where drainage area exceeds 300 acres.


Detailed study area:

  • Show floodway as determined by FEMA.
  • Show flood limits and elevations determined by FEMA.
  • If applicable, show limits of additional flooding using FEMA elevations and more current contours and recent watershed studies.

FEMA Approximate study area:

  • Use watershed study information if available, if not follow steps listed below.
  • Calculate 100 year runoff using fully developed conditions upstream.
  • Calculate flood limits and elevations.
  • Delineate floodway if work or development is proposed in flood plain.
  • On preliminary plat, indicate FEMA flood plain, calculated floodplain. Show floodway if work is proposed in flood plain.
  • Show base flood elevation data and flood plain limits on preliminary and final plat.

IV. Required Field Surveys

Surveyed channel cross sections are required to calculate flood elevations.

  • A cross section is required at front building line if channel parallels side lot line.

If development is upstream from existing road.

  • Profile road center line with field survey.
  • Measure crossroad structures and determine flow line elevation.

Profile top of dam including spillway.

  • Cross section of emergency spillway.
  • Bench marks are required when base flood elevation must be determined due to being in a FEMA area or drainage area exceeding 300 acres.

Not less than two permanent sea level bench marks shall be inside or immediately adjacent to each subdivision or development.

  • Bench marks must be described on the preliminary plat or development plan.
  • Survey of dams is required to determine limits of natural drainage area.
  • Bench Marks