Design and application of drainage engineering in combination of ditch with ponds

Abstract: Drainage engineering technology combined with ditch and pond was proposed and its design method were developed to meet the challenge of increasing design discharge that had been caused by the increase of extreme rainfall events and variations of basin underlying surfaces. Considering drainage patterns and data shortage at small ungauged watershed, the design method consisted of the determinations of drainage criteria and drainage hydrograph, and the design of pond and weir sizes. Design storm amount was determined by the statistical analysis of long-term daily precipitation and local design return period, and the design storm duration, usually 24 h, was discretized based on storm temporal distribution (hourly) in local hydrology manual. Given that drainage hydrograph can be approximated by a triangular hydrograph at every discrete time interval, drainage hydrograph was synthetized each triangular hydrograph calculated with the improved SCS (soil conservation service-curve number) model and concentration equations every discrete time interval. Pond and weir sizes were estimated by the trial and error solution of the stage-storage function, weir equation and continuity equation, as the pond outflow rate was close to design drainage rate of the receiving ditch downstream. In the lowland of the northern Huaihe River Plain, the pond of 800 m2 was designed for combining with field ditch and installed with the weir of 0.6 m wide and weir coefficient of 1.28, which would be used to drain the runoff from the area of 0.06 km2. The pond of 26000 m2 was designed for combining with collector ditch and installed with the weir of 2.0 m wide and weir coefficient of 1.28, which would be used to drain the runoff from the area of 1.36 km2. The drainage engineering combined with the ditch and pond extended the time of concentration of 0.5 h for field ditch district and1 h for collector ditch district, respectively, under the return periods of 24 h storm of 3-year, 5-year and 10-year. The drainage engineering combined with the ditch and pond reduced peak storm flows of more than 25% under the return periods of 24 h storm of 3-year, 5-year and 10-year. Combined with the pond weir of 0.6 m wide and 0.7 or 0.85 m high, the drainage criteria of the field ditch could be increased from the return periods of 3-year or 5-year to the return periods of 5-year or 10-year, respectively. Combined with the pond weir of 2.0 m wide and 1.8 or 2.3 m high, the drainage criteria of the collector ditch could be increased from the return periods of 3-year or 5-year to the return periods of 5-year or 10-year, respectively. Drainage engineering technology combined with ditch and ponds could be an option to mitigate the flood damage of the drainage district and downstream. However, drainage engineering technology combined with ditch and ponds should be assessed for different weather and soil conditions, and the design parameters of ponds and weir should be evaluated to adapt different ditches and weather conditions. The effects of drought, flood and environment should be considered as the combined drainage engineering technology is used to control flood.