URBAN HYDROLOGY MONITORING TECHNIQUES: FROM PARKING LOTS TO STREAMS (Invited Presentation)

Hydrology in urban systems is complex, with hidden pathways that make monitoring difficult. However, simple monitoring techniques can reveal some of these pathways if comparisons can be made between inlets and outlets or before and after construction of stormwater control measures. More complex monitoring is required for process level understanding and water budgets. Several examples will be presented to evaluate whether stormwater control measures are effective. Water level monitoring of storm inlets provided a simple evaluation of storm capture for a parking lot infiltration gallery. The type and size of the storm was an important factor, and only 1 cm or greater storms were captured by the infiltration gallery. Temperature monitoring was used to evaluate back flow from street storm drains, and no evidence of backflow was observed. Water level monitoring before and after construction of rain gardens and infiltration trenches in an urban neighborhood showed that there was insufficient capture area to change discharge to a nearby stream. Water levels were monitored for a year before and a year after construction in a railroad constriction to avoid changes in bank overflow. Time series analysis (cross correlation of water level and rain) showed no difference in timing or amount of storm discharge. The effect of stream restoration structures on extent of the hyporheic zone was monitored using electrical resistivity tomography. This geophysical technique was more expensive, but provided process level understanding. Stream depth had an important influence on data inversion. Time lapse resistivity of a stream tracer test revealed sediment played a bigger role in the location of hyporheic zone storage than hydraulic steps. This study suggests that the hyporheic zone can be dynamic in urban streams as stormflow moves sediment both before and after restoration structures are installed. Monitoring urban streams and stormwater structures reveals that control measures do not always achieve the desired aims of increasing infiltration and stabilizing streams. Problems occur due to insufficient capture areas, unexpected water rerouting, and stream sediment mobility. Multi-year monitoring helps reveal these patterns.