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Exposed field tile after sediment removal

Sediment from two primary sources impact agricultural waterways: alluvial deposits and floodplain deposits.

Alluvial deposits reduce the capacity of a waterway by raising the bottom of the channel. Raising the bottom of the channel also raises the static groundwater level under the adjoining fields. This is especially evident where field tiles are present and the alluvial deposits cover the outlet of the field tiles. 

Waterways in floodplains also accumulate sediment as the result of periodic flooding.  Unlike alluvial deposits, flooding sediment is usually fine grain silts and clays.  Flooding sediment can cause the same problems as alluvial deposits, although the impacts usually take much longer to materialize because only a little sediment is deposited with each flood.

For questions about the Agricultural Drainage Assistance Program or ADAP, please contact Brian Sleight, Supervising Engineer, King County Stormwater Services Section.