Good surface drainage practice

Following are the points to be remembered for good drainage:

  • (i) Gravity is the primary vehicle for carrying runoff away. There must be a continuous minimum fall in the ground level to assure drainage, and a minimum slope of 1% to 5% for grass swales. 1% minimum slope for smooth interior pipe is a general guideline for pipe conveying run off water to a discharge point
  • (ii) Large amounts of water should not cross a sidewalk to reach the street storm drain. Use drains or install piping to cross walks or other pedestrian walkways to prevent hazards.. Consult an engineer or architect for minimum slope in critical applications,
  • (iii) Break up one large drain to several smaller drains to:
  • (a) Prevent erosion on steep landscapes by intercepting water before it accumulates too much volume and velocity.
  • (b) Provide a safety factor. If a drain inlet clogs, other surface drains may pick up water
  • (c) Improve aesthetics. Several smaller drains will be less obvious than one large drain.
  • (d) Spacing smaller drain inlets will give surface runoff a better chance of reaching the drain. Water will have farther to travel to reach one large drain inlet.
  • (iv) Erosion is a big problem in drainage - slopes must be carefully calculated to ensure continuous flow, yet not steep enough to erode,
  • (v) Slow moving water will create a bog, while water moving too fast will cause erosion, form gullies and weaken foundations. Design a drainage system that will eliminate both extremes.
  • (vi) Design paved areas so they are graded almost level - avoid wildly sloping paved areas or dramatic changes in slope,
  • (vii) Runoff water must never be directed purposefully from one property onto another property. It is acceptable for water that flows naturally from one property to the other to continue, but you must never increase this flow artificially through grading and piping,
  • (viii) Check local code requirements and their applications
  • (ix) When designing a system, work from the discharge point towards the highest elevations,
  • (x) Design a secondary drain route to allow for overflow conditions during severe rainfall or in case the primary drain system fails,
  • (xi) Many systems require a grate or "clean out" fitting every 50 to 100 feet or at alignment changes of 45 degrees or greater to clean out the pipeline. Clean outs are normally constructed at grade,
  • (xii) Keep it simple. Over-design in storm water systems is expensive.
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