The Dallas Southern Gateway Park offers visitors opportunities to enjoy picnic areas, walking trail, and 8-acre lake with a fishing pier. However, when the lake levels began to drop in 2013 and then when the lake eventually went dry in 2014, the City of Dallas wanted to investigate the loss of such a beloved amenity.
Located near the Trinity River, the loss of the lake water had impacted the area’s ecological system of the park as well as loss of recreational opportunities. The City of Dallas reached out to Terracon to evaluate what factors were contributing to the water loss and to provide solutions to maintain the lake water level Collaboration between Terracon and the City of Dallas staff resulted in a successful partnership and project for the City.
Terracon used a combination of environmental and geotechnical conditions to investigate the fluctuation of water elevation in the lake. The causes focused on possible water level influences which included:
Terracon began by researching the origin of the lake through review of historic aerial photographs dating back to 1942. The photographic evidence indicated the lake was excavated to provide soils for nearby highway construction. The lake had no creeks entering for water inflow and there was limited drainage area for water to exit the site. The investigation then became more technical as Terracon looked into the conditions surrounding the lake. The lake water loss volume due to evaporation was estimated based on Class A pan evaporation rates from three nearby lakes. The seasonal variations of lake water losses were used to estimate the water quantities needed to maintain the lake level.
Losses from the lake due to water seeping through the earth were estimated based on permeability tests, groundwater levels, and soil stratigraphy. The lake’s total estimated seepage loss at normal pool levels was computed to be about 3,700 gpd (gallons per day).The water loss due to seepage was found to be a minor contributing factor compared to the water loss due to evaporation.
Rainfall recharge of the lake was estimated based on data from a nearby City of Dallas site. The rainfall data was used to compute the estimated amount of water inflow from the surrounding 21-acre watershed and directly into the lake. The quantity of water from precipitation into the lake and watershed was found not to be sufficient to sustain the water level in the lake.
Flood events on the Trinity River were correlated with aerial photographs of the lake dating from 2001 to 2015 which allowed the team to understand what flood stage elevation resulted in filling of the lake. This comparison indicated that the flood stage river level would need to be above Elevation 380 feet to begin filling the lake. When there were prolonged periods when the river did not flood, the lake was observed to approach dry conditions. During the drying stages of the lake, the river did not crest above Elevation 380 feet during a flood stage.
Total water loss versus time during periods without flooding was estimated using the evaporation and rainfall data collected for this study. The predicted rate of water loss appeared to be consistent with lake level observations.
Based upon predicted evaporation and rainfall data the lake water loss was found to be consistent with lake level conditions in years without flooding.
Terracon concluded that any water gains were mainly due to Trinity River flood events, with limited quantities of water entering the lake from direct precipitation and run off. Water losses from the lake were determined to be largely a function of evaporation, with a comparatively small losses due to seepage. The water loss calculations were used to develop a series of solutions for maintaining water levels during prolonged dry periods.
Working in close conjunction with the City of Dallas, Terracon proposed several alternatives for refilling the lake and maintaining the lake water level at a desirable pool elevation. The alternatives included pumping water from the nearby Trinity River, re-use of water purged from nearby municipal water lines during routine flushing, installing a moderate-yield water well into the deep aquifer, and a series of shallow, low-yield water wells advanced into the underlying alluvium. Combinations of the above alternatives were presented along with seasonal water demands, cost, schedule, sustainability, and permitting implications so that the city could make informed decisions on how to proceed.
The Class A Evaporation Pan has diameter of 47.5 inches and depth of 10 inches and is used to estimate evaporation rates from water bodies.In May 2015, as the recommendations were being provided, North Texas experienced prolonged rainfall causing the Trinity River to flood. The threshold river stage of Elevation 380 feet was reached on May 14 and which started to refill the lake as Terracon had predicted. The lake was completely filled less than a month later. The knowledge gained on the hydrology of the lake and the options presented for maintaining the water levels going forward allow the City of Dallas to effectively manage this ecological and recreational resource for the enjoyment of their citizens. One or more of the options discussed in this article will be implemented for future draught events.