Flow duration curves

Article Review

Fan, C., Li, James (2004) “A Modelling Analysis of Urban Stormwater Flow Regimes and their Implication for Stream Erosion” Water Quality Re. Journal Canada, Vol. 39, No. 4.

The objective of this paper is to analyze flow-duration characteristics of a stream, with and without control measures applied during and after urbanization and its potential for erosion.

“Flow duration curve is the complement of the cumulative distribution function of stream flow. It represents the relationship between the magnitude and frequency of daily, weekly, monthly or even longer period of time of stream flow for a particular watershed.” The flow duration curve is the relationship between exccedance probability and discharge.

According to the authors, stream erosion is a continuous process of removal and deposition of sediments. Urbanization can alters drastically this balance due alteration of the flow regime. Since measuring both discharge of solid and liquid is something difficult to perform, especially in the planning process of development, the hypothesis of this work is that hydrological models can be used to estimate the flow regimes alteration by the flow-duration curve and evaluate the implication on stream erosion.

A period of 3 months of measure data were used to calibrate the model ISWMS (Integrated Storm Water Management Simulator). The watershed was discretized in 9 sub catchments. The calibrated parameters were base flow and horton’s equation for infiltration. Twenty-seven years of hourly rainfall data was used in the long-term simulations.

Different degrees of development were tested by changing the percent of imperviousness (20, 30 and 40 %), of all catchment, with exception of catchment #5 which represents a wetland area. It was noted that urbanized duration curves are shifted from the original curve as more imperviousness is used, indicating that for the same discharge, the frequency is higher.

To analyze the impact of flow control measures, three scenarios were tested: (1) an extended detention basin was added to each subcatchment except subcatchment 5 (the wetland); (2) two extended detention basins were added, one more upstream and other downstream; and (3) an extended detention basin was added downstream of all the subcatchments. The basins were sized considering the criteria of capturing 5 mm of rainfall and the outlet orifices were sized for 24 and 48 hours.

The first analysis compared the use of detention basin in the outlet of the watershed (scenario 3). The results indicate that both 24 hrs and 48 hrs extended basins control the runoff greater than 0.1 m³/s. For lower flow than 0.05 m³/s, 24 hrs basins performed slightly better than 48 hrs basins, and the opposite for flows varying from 0.1 to 0.05 m³/s.

When other locations are analyzed, it was found that regional control (scenario 2) provide better control performance than scenario 3 for flows greater than 0.07 m³/s. Local control (scenario 1) generally performs best when the flow is less than 0.09 m³/s. The authors conclude that the optimal location of detention basis is dependent on the flow regime.

In summary, the paper concludes that urbanization can change significantly the hydrological regime of a watershed and that extended detention basins with 48 hours of detention time provide better control of flow-duration characteristics than 24 hours of detention for higher flows. For lower flows, the 24 hour basin perform better. This make the authors suggest that the detention basins should be sized based on the range of flows which determine the stability of a stream. Finally, the paper states that if a watershed is urbanized beyond 40 % of imperviousness, it may be impossible to restore the flow-duration curve to its original position.

Discussion

This paper seems valid in order to illustrate the impact of urbanization in the flow regime. It has used as a tool of analysis the flow duration curve, analyses that can be found in more literature (see last post). Some comments however I would like to do:

(1) The paper doesn’t specify the area of the watershed that is been analyzed. The findings of the work may not be extrapolate to other watershed with different scales;
(2) The period of calibration is very small (just 3 months). A period of verification of the modeling was not performed;
(3) The limit of 40 % of imperviousness indicating that it may be impossible to restore the flow-duration curve seems very specific for this particular watershed. Different hydrological characteristics may turn this number lower or even higher;
(4) The results shown in the work prove good control over the high flow portion of the flow duration curve. For lower flows (and high frequent event) none of the scenarios seems to replicate the predevelopment conditions (see last post);
(5) The original hypothesis of the work was partially proved. The hydrological alterations were demonstrated, but no further analysis was done to evaluate the implication of the alterations on stream erosion.