Infiltration swales: Quantitative performance on an urban catchment scale

by  H.R. Geerling

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Introduction

Worldwide urban areas are getting larger. Existing rural areas will be replaced by impervious areas like roofs or roads. The increasing urbanization influence the hydrology of the urban area and therefore the stress on urban drainage systems. More runoff will be generated as less water is ’lost’ to the environment.

Research

My research focused on the quantitative performance of infiltration swales on an urban catchment scale. A swale in the city of Utrecht was monitored for 5.5 weeks. Groundwater levels, inflow and outflow were measured. The measured data was used for the validation of a swale model in MetaSwap-Modflow developed for this research. An Infoworks model is used to study the performance of infiltration swales on an urban catchment scale.

 The following aspects were studied:

  1. Quantitative performance of individual infiltration swale
  2. Influence of initial soil moisture conditions
  3. Influence of the inflow characteristics of the swale (head vs. lateral inflow)
  4.  Quantitative performance of swales on an urban catchment scale

Results

  1. Fourteen events were used for the analysis for this study. Four events were excluded because of inconsistencies in the observed data.
  2. The median value for  the volume reduction  was this study is 7%. The median value is 41% taken into account only the events with a positive volume reduction.
  3. In 85% of the events more than 40% of the inflow peak is ’lost’.
  4. The peak delay is shorter than one hour for more than 60% of the events
  5. The subsurface part of the infiltration swale was never completely dry. The top 40 cm of the swale was dry within 24 hours.
  6. A swale with a lateral inflow performs a little better or at least equal to the head inflow swale. The increase in performance can be explained by the fact that less water is ponding at the head of the swale at the beginning of the event.
  7. Heads and peaks in drain discharge are not much influenced by initial soil moisture conditions.
  8.  Higher heads prior to the start of a precipitation event (-0.20 m NAP vs. +0.40m NAP) will result in a smaller volume reduction (10% smaller) and a shorter peak delay (3 hours shorter).
  9. Volume reduction on an urban catchment scale is consistent with the performance of an individual swales and the peak reduction is 4%-88% depending on the area percentage that is connected to a swale . Design storm 8 (T=2 yr.) was used for this analysis.
  10. For design storm 8 it was sufficient to connect 30% of the area to  swales to prevent water on the streets.

Conclusions and recommendations

Infiltration swales are beneficial on an urban catchment scale. A cost benefit analysis should be done to determine if swales are cost effective as well. 

Student:       H.R. Geerling

Committee:    Prof. dr. ir. N. C. van de Giesen (TU Delft), Dr. ir. F. H. M. van de Ven (TU Delft/Deltares), Dr. ir. J. G. Langeveld (TU Delft) and Ing. M. Rijsdijk (Gemeente Utrecht)

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