by: William L. Merrill, P.E. Merrill Engineering, Inc.
August, 2023
This article is intended as an overview of how to use topographic and groundwater conditions as a guide to predicting the location and outflow potential of springs in the Pensacola area. In short, high output springs require large upland areas that are needed to absorb the rainwater that eventually discharges as a spring. This data can also be used to define likely areas of historic camps and settlements. Humans and their animals were dependent upon available freshwater sources for their survival in historic settings.
Areas such as downtown Pensacola and Bayou Chico are down gradient from large receiving areas such as Brownsville, North Hill and East Hill. It is therefore predictable that springs would emerge along the slopes of hills leading to the bay. In contrast, areas such as Emanuel Point and East Pensacola Heights would be less conducive to high volume springs since the upland receiving area is comparably smaller. Peninsulas such as East Pensacola Heights are particularly restrictive to high volume spring formation because the upland absorption area is small and what groundwater does accumulate, has multiple directions to flow out to receiving waters instead of concentrating along the base of one hill or bluff.
When engineers develop a site, whether it be into a shopping center or a new neighborhood, one of the first things they do is determine the site’s stormwater runoff rates for both pre-development and post-development conditions. Depending on soil, terrain and groundwater conditions, undeveloped land can absorb as much as, if not more than, 80-percent of the rainfall that falls on it.
This rainfall, at least initially, remains close to the surface as part of the unconfined water table. Some of this groundwater percolates down to deep aquifers but depending on soil conditions the bulk of the water remains relatively shallow, thus, moving underground toward an outfall such as a river, bayou, or bay.
Groundwater elevations follow what engineers call a hydraulic grade line. This hydraulic grade line is not flat as is a waterbody, rather it decreases in elevation as it flows toward its outfall, much like the surface elevation of a stream or river
decreases along its path. The angle or steepness of this hydraulic grade line is dependent upon soil types, underlying geology, and rainfall. The elevation of the ground’s surface is unrelated to the hydraulic grade line and thus the water table’s depth varies.
In areas where the ground surface descends at a slope steeper than the groundwater’s hydraulic grade line, those two lines can intersect resulting in groundwater emerging from the soil as a spring.
Springs originating from surficial groundwater as the result of grade change are the only type of spring in the immediate Pensacola area. Prior to industrialization, these springs would have been a safe and reliable source of potable water. Post-industrialization, surficial groundwater in many parts of Pensacola has been polluted from a variety of sources and is no longer an acceptable source of potable water. Utilities now supply drinking water by pumping groundwater from deeper aquifers.
Pensacola and its surrounding area are particularly conducive surficial groundwater springs because large areas north of downtown are at higher elevations and able to be charged from rainfall. Groundwater travels through the sandy soil, generally southward toward the bay and bayous. In areas where the surface elevation decreases faster than the groundwater’s hydraulic line, springs emerge along that slope. According to the USGS National Topographic map, the intersection of Spring Street and Belmont Street is at approximate elevation 30 while the elevation just two blocks north is at elevation 70 or more. This ground elevation change in a short distance creates a surface slope steeper than the hydraulic grade line. Where the water table meets the surface springs emerge. Prior to urbanization, this emerging groundwater just north of downtown formed the headwaters of creeks such as Washerwoman Creek and Cadet Creek that once flowed on the west and east side of downtown, respectfully. Modern stormwater infrastructure currently keeps these once flowing surface waters underground, discharging to the bay via pipes.
Other areas, such as along the north side of Bayou Chico, also have similar conditions and are therefore conducive to spring formation. While Bayou Chico is near sea level, areas immediately to the north of it, such as the Brownsville area, are in the range of 70 to 75 feet above sea level. Rainfall recharges this large, high area and that groundwater migrates toward Bayou Chico. Where the surface elevation slopes down at rates greater than the hydraulic grade line and the two lines meet, springs emerge and form creeks. Groundwater can also push out through the soil below the waterline of Bayou Chico, forming a sub-aquatic discharge.
The springs in the Pensacola area are not the picturesque types that bubble out in a clear pool. Karst springs, common in other areas of Florida, have a confining layer of limestone above the water table, with large, clear pools forming at openings in the limestone. Artesian springs are a type that flow upward out of a confining layer with water pressure greater than atmospheric pressure. Because the water table in the Pensacola area is not confined, its springs typically dribble out of the soil, oftentimes along a slope or line near the base of the slope. As all of this water collects and forms a creek, without having an obvious point-source. Springs can also emerge in depressions in the soil and form a pool or pond at the headwater of the creek.
Groundwater flows in the direction of least resistance. The soil itself provides resistance to flow and the further the groundwater has to flow toward an outfall, the greater the resistance for a given soil. The direction to the closest downgradient waterbody usually provides least resistance assuming soil uniformity. Areas such as narrow peninsulas are not conducive to springs with significant output because the groundwater has multiple directions to reach its closest waterbody and the recharging area is smaller. because the groundwater has multiple directions to reach its closest waterbody and the recharging area is smaller.
Areas immediately north of Bayou Chico and downtown Pensacola have large upland areas that collect and absorb groundwater, have a surface topography that slopes down relatively steep and have a singular flow direction, southerly toward Pensacola Bay. Formations of springs would be expected along these areas and the flow rate of those springs would correlate with the size of the upland absorption area. In areas where one or more of those conditions are not met, spring formation would either not occur, or will be limited.
Areas immediately north of Bayou Chico and immediately north of downtown Pensacola have large upland areas that collect and absorb groundwater. They also have a surface topography that slopes down relatively steep and have a singular southerly flow in the direction toward Pensacola Bay. Formations of springs would be expected along these areas and the flow rate of those springs would correlate with the size of the upland absorption area. It is likely that the water requirements for large numbers of people and their animals in historic campsites and colonies are easier met in Bayou Chico and Downtown Pensacola than in East Pensacola Heights.
Figure 2: A view of a spring fed lake at the headwaters of Bayou Chico demonstrating the immensity of the springs emulating from the base of the hills to the north of Bayou Chico. The lake was created by a commercial company excavating sand for sale. The springs then filled their excavations. Further evidence of the numerous springs at the headwaters of Bayou Chico is that the Spanish fleet of 40 ships led by General Galvez off-loaded some 3,500 men during the 1781 attack on British held Pensacola. It takes a lot of water to satiate that number of people and their animals. The springs on Bayou Chico met their needs. That is a fact. Yet more evidence of the freshwater availability on Bayou Chico is when the American army led by General Jackson arrived on the bayou in 1814 with 4,000 men to oust the Spanish from Pensacola. Again, it took a lot of freshwater to maintain that many men and their animals. Bayou Chico provided that freshwater. That is a fact. Is it possible that the 1558 Spanish Luna Colony comprised of approximately 1,500 people and their animals decided on Bayou Chico as the location of their colony to satisfy their water needs?