Sampling sites must be chosen according to study objectives, but hydraulic conditions and constraints necessary to the adopted procedures should be given attention. The sampling site must be located at a section downstream of the study site, i.e. corresponding to well-known sewer systems, land use types, special activities, etc. It is recommended that highly turbulent sections with well mixed flow be sampled. However, for the study of sediment transport deposit, these conditions may not be suitable, as suspended solids in the highly turbulent section may be scattered.
For monitoring in-stream impacts, the area of interest should be bracketed by upstream and downstream stations. A control station on a hydro logically similar but undisturbed watershed can be used to determine baseline conditions.
Two types of monitoring stations are employed for nonpoint source surveys:
1. Small catchment stations ranging from 12 to 125 acres (5 to 50 ha) in size, are used to gather data on specific land uses or special areas. They are usually found on storm sewers, drainage ditches or small tributaries.
2. Another type of station is built to monitor larger basins of greater than 125 acres (50 ha), and measure nonpoint source pollution loads impacting a receiving body, such as stream channels or rivers.
There are cases where the final choice must be made from a group of catchments. In such cases, the technique of weighted suitability ratings, as developed for land use, is recommended (Alley 1977). Assignment of suitability values is perhaps the most subjective part of the schedule.
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