By Dennis E. Peyton P.E.
Civil Engineer
July 19, 2001 — Fines are being significantly controlled at the City of Fresno's Leaky Acres recharge basins using the naturally occurring forces of gravity, wave action and water currents.
Easily constructed slope sided and flat bottomed 'furrows' have proven unequaled in maintaining a relatively 'clean' basin floor without routine scraping or discing.
The furrows are quickly constructed using a motor grader after the basin floor has been excavated and ripped or after a 'cleaning' (normal scraping). The furrowed basin floors require substantially less maintenance than a conventional flat bottomed floor while consistently providing 'fine free' bottom area through which recharge efficiencies are maximized.
Background and commonly occurring conditions
Leaky Acres is a 200+ acre groundwater recharge facility located within the City of Fresno, California near the western edge of the Sierra Nevada range. The alluvial strata at Leaky Acres contain clayey aquitards at approximately 30 and 60 foot depths below ground surface (BGS).
The unperched groundwater is normally at about 105 feet BGS. The aquitards limit the percolation of the surface waters applied to between 0.3 and 0.5 feet per day. All of Leaky Acres' surface soils are sandy to silty sand whose permeability is significantly greater, up to 10 times, than that of the aquitards. Often with a minimum of 10% of the basin bottoms being clean/open, the facility's infiltration rates are maximized. The typical challenge has been to develop a low maintenance facility without compromising effectiveness.
The City has used discing, ripping, scraping and combinations thereof to control and/or remove the 'clogging fine layer' which would develop each recharge season with varying degrees of success. Discing without routine drying cycles proved to be ineffective as the near surface became clogged with anaerobic microbes.
Drying cycles are difficult to achieve since only the uppermost 1 inch of material becomes visibly dry. The anaerobes were supported primarily by the remains of the single cell algae often present in the influent water and sustained by available sunlight energy in the shallow basins (2 to 4 feet deep). Not discing was also ineffective as the surface sealed with fines.
The surface clogging was effected by the several sources of fines, i.e. single cell and filamentous algae, silts and clays in the irrigation/recharge water and generated by interbasin erosion (filling and levy erosion). Scraping and subsequent ripping was effective but costly and required at least every three years.
The Fines Control System - flat bottomed furrows with sloping sides successfully use wave action
Through observation and development, furrows and ridges, to direct the fines to 1/3 of the basin floor area, were constructed in a twenty acre basin. The furrows are spaced on 25 foot centers to allow a paddle wheel scraper to remove the collected fines when necessary. The slopes should exceed 4% as fines will tend to migrate off greater than 2% slopes. The soils at Leaky Acres sustain side slopes ranging between 4% and 30%.
Providing a brief 'dewatering or cleaning cycle' by lowering the water level and exposing the side slopes to wave action is an effective mechanism to ensure maximum utility. The fines are directed to the flat furrow 'bottom' where they will be removed in many years to come, perhaps decades.
This system was 'acid tested' in March, 2001 when the furrowed and ridged basins at Leaky Acres was intentionally dewatered, after 2 years of operation without a 'washing', during the spring windy period and the ridges washed by wave action. The results were phenomenal. The infiltration rate was returned to the average maximum of 60 ac. ft./day.
Much more information, including a paper on this subject and photos of recent related work visit http://communities.msn.com/RechargeRidges/home.htm.
The relatively clean basin bottoms promote the even greater infiltration rates as basins are constructed and operated with deeper water depths. They are so effective at providing a 'hands off' basin bottom that nearly all 23 Leaky Acre basins now incorporate furrowed bottoms.
Studies of several ridge orientations, with respect the prevailing winds, have concluded that maximum 'cleaning' is achieved with the ridges 'running' perpendicular to the prevailing winds.
This is an abstract prepared by the author for WaterWorld Magazine, based upon his paper published for the 2001 Tuscon Groundwater Recharge Symposium June 7-9.
