Recommendations

    The team recommends the following:

  • Drywells CAN handle on-site stormwater at the CAL Ranch Site, although:
    • The use of drywells in a retention basin is cost prohibitive at the CAL Ranch site
    • A traditional detention basin would be most cost effective for handing on-site stormwater
  • Drywells would pose no threat to groundwater quality in the Flagstaff area
  • Other areas in town (e.g. Cinder Hills) should be evaluated for the feasibility of drywells
  • In places where groundwater recharge is a priority AND infiltration rates are high (>4in/hr) may consider the use of drywells

Conclusion

From the hydrologic analysis, it was found that the peak on-site runoff inflow was 5.61 cfs. The discharge through infiltration from the retention basin and drywells given a maximum drain time of 36 hours was 0.437 cfs. Due to the low infiltration discharge relative to the runoff inflow, the optimal retention/drywell system yielded a retention volume of 1.3 acre-feet (56,321 cubic feet) and 10 drywells with a radius of 3 feet and depth of 10 feet. The estimated cost for this retention/drywell system was $114,209 The estimated cost for a detention-only system was estimated to be $35,490, resulting in a savings of $78,719. Due to the high cost of the drywell/retention system, and lack of need for groundwater recharge in the area, it is concluded that a drywell stormwater management system is not feasible at the CAL Ranch site.