Geoscience Reference
In-Depth Information
5.3 Hydrological Measurement
5.3.1 Rain Measurements
Rainfall data were taken from theMeteorological Service measurement station and the
Erosion Research Stations for the years 2006-2009. Automatic tipping-bucket rainfall
recorders with a resolution of 0.1 mm were installed at three stations (Fig
.
5.1
)to
accurately represent the spatial and temporal distribution of precipitation in the city.
For each rain event, the starting and ending times were defined on the basis of
hydrograph (the overall rain from near the start of the flow event until the flow
ended was taken into account). The calculations of the rain intensity was made for
10 min interval and shown in mm/h units for the various stations. During the first year
the quantities of rain were calculated by averaging the values of the rain volume from a
number of stations. During the last year, due to mishaps and damage to the measure-
ment instruments from extraneous causes, data was taken fromoutside the city as well.
5.3.2 Runoff Measurements
Five runoff stations were installed at the outlets of five stormwater drains (four in
Herzliya and one in Ra'anana). In 2007/2008, one station was moved from a school
basin to a road basin (Fig.
5.1
).
Each station included a pressure transducer (Diver with a resolution of 10 mm
and data logger) to rescored water head in the channel. The velocities and storm
discharges were calculated by using the Manning formula for open channel flow.
The area draining into each of the stations was estimated using an ESRI GIS
(ArcInfo 9) aerial Ortho-photo map, topographic and infrastructure data and drain-
age network provided by the local municipalities. Because the drains channeled
water from areas of different sizes and land uses, the impact of these characteristics
on the quantity and quality of storm water could be evaluated.
5.3.3 Water Sampling
Storm water samples were collected both manually and automatically. Automated
samplers were installed next to the storm water gauges at the three stations draining
relatively large areas (Fig.
5.1
), whereas samples were collected manually from the
other drains. The runoff gauges provided information about the discharge and cumu-
lative runoff that could be matched for each sample. The sampler was programmed to
start sampling at the beginning of each flow event, then at 10-min intervals during the
first hour, followed by 30-min sampling intervals thereafter. Water samples for
analysis of major ions were collected in a 0.5 l pre washed plastic bottle and kept
under refrigeration. Water samples for trace element analysis were collected in 20 ml
plastic bottles after being passed through a 45
