subject: Using Water Screens To Control Flow [print this page] In addition to quality control of water its also important to consider the effects of water flow especially in the context of storm water control. But what are the types of water flow?
Dual Flow
Inserted parallel to the water flow the Dual Flow type typically offers more efficient screening as it utilizes both sides of the screening media and completely eliminates the possibility of debris carry over.
Thru-Flow
Thru Flow is the most common type of Water Screen. They are placed into the intake at right angles to the flow. The water passes through the rotating baskets. Only one side of the unit is utilized for screening thereby reducing the effectiveness of this design.
It is recommended that prior to the implementation of a sampling program some
information pertaining to runoff characteristics be obtained and studied (King and
Harmel, 2003). Automated storm water sampling generally requires setting both a
minimum flow threshold and an appropriate sampling interval. The minimum flow
threshold for this study was based on recommendations from previous research
(Harmel et al., 2002).
Traveling Water Screens have been screening raw water intakes since it became necessary to have clean, debris free water. Simplistic in design the basic Traveling Water Screen has not changed much in over 100 years. Except for some minor design variations, there are two basic screens in use worldwide today: Thru Flow Water Intake Screen and Dual Flow Water Intake Screen.
The appropriate sampling interval was estimated according to a
site-specific rainfall-runoff ratio. The rainfall-runoff ratio was then used to back calculate
a runoff curve number (CN). Once the CN value was obtained, the runoff depths for a
range of storms were calculated according the Soil Conservation Service (SCS) Runoff
56 Curve Number method (Cronshey, 1986), also referred to as the NRCS method.
Although standard coefficient values can be applied to simple models to estimate
runoff, site-specific rainfall and runoff data is almost always more effective in
determining runoff characteristics of a specific study area. The following methodology
illustrates a procedure used in estimating the appropriate sampling interval for
collecting storm water samples from small sites.
Minimum Flow Threshold Prior to determining a sampling interval, automated storm water sampling requires setting a minimum flow threshold for beginning and ending sampling. This is a crucial number, since minimum flow thresholds set too low will enable sampling for small storms in which no significant pollutant loads will be transported.
However, if flow thresholds are set too high, an entire event or a substantial portion of an event will be missed. Increasing flow thresholds has been shown to result in increased error of the true or total pollutant load (Harmel et al., 2002). For small watersheds, errors were shown to be substantial even for small increases in minimum thresholds.
Flow Interval Sampling
Once the minimum flow threshold has been established the next important
consideration is whether to sample based on a time or flow-interval. Flow-interval
sampling has been shown to better represent storm loads because more samples are
taken at higher flow rates (Harmel, King, and Slade, 2003). In addition, the Event Mean
Concentration (EMC) of a storm event can be easily averaged from flow-interval
