Flow Split Management
Presently, the culverts governing flow from Landa Lake into the Old Channel are inoperable. As a result, a constant level of springflow proceeds through the culverts and into the Old Channel. Over time, this has led to the scouring of preferred native vegetation types for fountain darters, and the establishment and eventual dominance by non-native non-preferred aquatic vegetation. Flow-split management is intended to complement the ecological restoration of aquatic vegetation in the Old Channel, by reducing long-duration high flows and allowing for more seasonal variability to be maintained, mimicking a more natural flow pattern.
To minimize and mitigate the impacts of low flows, the City of New Braunfels staff will manipulate at least once monthly the valves and culverts to the Old Channel and New Channel of the Comal River for the protection of existing and restored native aquatic vegetation in the river, based on EAA’s real-time flow gauges in these channels and as often as appropriate for the maintenance of a beneficial hydrologic condition of the Old Channel habitat. Prior to this, the City of New Braunfels will replace and repair existing gates and control water flow per the flow split strategy and the prevention of sustained high flows in the Old Channel that resulted in scouring.
A second objective is to maximize the quality of habitat in the Old Channel. This will be accomplished by: (1) providing an appropriate level of flow variability during average to high flow conditions; and (2) allowing proportionally more water to flow through the Old Channel versus the New Channel during periods of critically low-flow with the ultimate goal of preserving high quality fountain darter habitat with the Old Channel as long as possible.
A detailed description of flow-split management is described in BIO-WEST(2011c). Based on the analysis conducted to date, the desired goal for maximizing fountain darter habitat in upper portions of the Old Channel at all times is to maintain 40-80 cfs. Extremely uniform suitable habitat is present in the New Channel under modeled (10-300 cfs) flows (Hardy 2011). During average to high flow conditions the focus is on a seasonal flow split in order to optimize habitat conditions in the Old Channel over time. Slightly higher flows during the fall and winter will provide some channel maintenance benefit while not hindering overall fountain darter habitat. Optimal habitat conditions are proposed for spring and summer to provide the best opportunity for fountain darter recruitment.
When total Comal springflow flows drop to 150 cfs, the flow split will be shifted to protecting the maximum amount of the habitat within the Old Channel year-round, while continuing in the New Channel at all times. Additionally, when total Comal springflow drops below 100 cfs, if necessary, the City of New Braunfels staff will manipulate the valves and culverts more frequently to maintain the flow split ratio.
As discussed in Hardy (2011), 20 cfs in the Old Channel will provide approximately 75 percent of the maximum available fountain darter habitat in the Old Channel from a physical habitat perspective. In addition to physical habitat, four checkpoint temperature ranges have been identified as critical to the foundation darter life cycle: at and above 77 and 79 degrees Fahrenheit there is reduction in fountain darter larval production; between 79 degrees and 82 degrees and above there is a reduction in fountain darter larval production, and at approximately 91 degrees and 94 degrees larval and adult thermal death can be expected based on laboratory studies. At 20 cfs, under the extreme ambient temperature conditions modeled in Hardy (2011), the Old Channel area between Landa Lake and Golf Course Road is projected to maintain water temperature below three of the four temperature threshold ranges at all times. Reduced larval production (up to 63%) has the potential to occur for portions of the day based on laboratory results from McDonald et al. (2007). Hardy (2011) shows the lower portion of the next modeled segment downstream is projected to have water temperatures high enough during portions of the day to cause reduction in egg production as well. All subsequent downstream Old Channel segments also are projected to exceed levels necessary for adult or juvenile survival.
Additionally, it should be noted that the City of New Braunfels is in the process of restoring the functionality of the Landa Lake Spillway and Landa Lake Dam. This repair and restoration project will protect the Old Channel.