Waller Creek Tunnel LA-Qual (v 9.05) Dissolved Oxygen Model for Recirculation Operations

This short report details the function and expected results of a Waller Creek tunnel designed to move sediment and regulate dissolved oxygen within the system.The City of Austin is constructing a stormwater bypass tunnel in lower Waller Creek. Water must be recirculated from Lady Bird Lake through the tunnel during non-storm conditions to prevent water in the tunnel from becoming anoxic. LA-Qual (version 9.05) steady-state models were used to identify optimum tunnel recirculation flow rates under three seasonal conditions and during summer months immediately following storm events in order to maintain dissolved oxygen in the tunnel near 4 mg/L. Model predictions were applied to measured Waller Creek flow data from 1994 to 2009 to estimate the total recirculation withdrawal from Lady Bird Lake for comparison to the maximum allowed under a Firm Water Contract with the Lower Colorado River Authority. Predicted Lady Bird Lake withdrawal for recirculation varies from 1 to 4 ft3 /s during non-storm conditions depending on season, with 4th Street and 8th Street side inlet tunnel flows less than 0.5 ft3 /s. Post-storm recirculation rates are predicted to optimally be 35 ft3 /s, although the actual maximum pump capacity is 28 ft3 /s. Operating the Waller Creek immediately following storm events at 28 ft3/s will not maintain DO in the tunnel above 4 mg/L, but should be sufficient with cascade aeration prior to discharge to Waller Creek to maintain aquatic life use standards. Operation of the Waller Creek Tunnel under these recirculation flow conditions are not anticipated to exceed the annual maximum withdrawal from Lady Bird Lake allowed by the Lower Colorado River Authority.Waller Creek Working Grou