Office of Planning, Environment, & Realty (HEP)
Planning • Environment • Real Estate
Transportation planners and engineers rely on streamflow properties measured at specific stream gage stations to estimate the magnitude and frequency of floods. Flooding is an important factor to consider in the design of bridges, culverts, highway embankments, dams, and other hydraulic structures near streams.1 Flooding affects a number of human systems, and can have detrimental impacts on transportation infrastructure, disrupting and damaging transportation systems. In some cases, increased runoff and discharge may also be associated with increased scour of bridge piers, which can potentially make bridges unsafe for use.
This section presents the methodology and key findings for several analyses related to observed and projected changes in streamflow. This section discusses environmental variables that directly inform present-day decisions of transportation planners and engineers in Mobile.
This section describes the methodology used to analyze observed streamflow data in the Mobile region. For more detailed information, see Appendix C.8.
Five USGS stream gage sites located in and around Mobile County were selected and analyzed for this study. Stream gages provide localized data that transportation planners and engineers can use in the design of bridges, culverts, dams, and other transportation infrastructure.3
Data came from the USGS Surface-Water database of stream gage data. This database provides streamflow properties at over 24,000 sites across the United States.4 Though the USGS stream gage database provides important local information and useful flood frequency statistics, very few sites provide a long enough record to investigate long-term trends (i.e., how peak streamflow has changed over the twentieth century).
Definition of Streamflow Characteristics
Surface runoff is the portion of precipitation that doesn't infiltrate the ground or evaporate, but flows across land surfaces into surface streams or water bodies (mm).
Discharge is defined as outflow, and can be applied to a variety of locations, ranging from discharge from a pipe to discharge across a section of a stream. Discharge is typically expressed as a volume rate (e.g., cubic feet per second).
Streamflow is a specific type of discharge that occurs in a natural channel (cubic feet per second).
Flooding occurs when streamflow gets high enough to overtop stream banks.
Sources: USGS, 2011f; USGS, 2011g
Stream gages from within this database were selected to provide a representative range of basin characteristics and stream sizes. The selection criteria are outlined in Figure 42. The selection criteria identified three stream gage sites for the analysis: Chickasaw Creek, Crooked Creek, and Hamilton Creek. Two sites did not meet the criteria but were included due to their unique location, size, and basin characteristics: Mobile River and Fowl River. Table 17 summarizes the streamflow and discharge data available for the five selected sites. Mean stream discharge data was provided by USGS using daily mean time-series at each gage site. This historical data is available on a daily, monthly, and annual basis. For the purpose of this analysis, monthly and annual data were utilized to provide monthly and annual present-day conditions.
Figure 42: Flowchart Describing the Stream Gage Selection Process
| Site | Site# | Characteristics | Annual Peak Streamflow | Monthly Mean Discharge | Annual Mean Discharge | |||
|---|---|---|---|---|---|---|---|---|
| Start | End | Start | End | Monthly | Annual | |||
| Chickasaw Creek5 | 02471001 | Large stream; 125 mi2 drainage area |
5/1952 | 5/2010 | 10/1951 | 9/2010 | 1952 | 2010 |
| Mobile River6 | 02470630 | Large river; 44,000 mi2 drainage area | 4/1951 | 2/2004 | X | X | X | X |
| Fowl River7 | 02471078 | Urban stream; 16.5 mi2 drainage area | 4/1995 | 1/2010 | 3/1995 | 9/2010 | 1995 | 2010 |
| Crooked Creek8 | 02479980 | Small rural stream; 8 mi2 drainage area | 1/1991 | 1/2010 | 6/1990 | 9/2010 | 1990 | 2010 |
| Hamilton Creek9 | 02480002 | Small urban stream; 8 mi2 drainage area | 5/1991 | 1/2010 | 6/1990 | 9/2010 | 1990 | 2010 |
Figure 43: Selected Stream Gage Sites in the Mobile Region
USGS data from these five stream gage sites were used to analyze annual peak streamflow, as well as annual and monthly mean stream discharge. The Mann Kendall trend analysis was used to investigate whether there was a statistically significant positive or negative trend in the Chickasaw Creek annual peak streamflow (the only stream gage site with a long-term period of record). In addition, a series of USGS reports published between 2004 and 2010 provided an analysis of regional flood frequency of urban and rural streams in Alabama.10
Key Findings for Historical Streamflow and Flooding
Streamflow is heavily influenced by physical and topographical factors, including elevation and slope, drainage patterns and barriers (natural or man-made), and reservoirs that prevent runoff from continuing downstream. In Mobile County, flooding caused by heavy precipitation events is influenced by the flow and channel changes in the Mobile River, which drains almost two-thirds of Alabama, and is fed by the Alabama River to the east and Tombigbee to the west (see Figure 44). The elevation of land across Alabama tends to slope to the south and west.11 The southern portion of Alabama, including Mobile County, is a coastal plain.12
Figure 44: Map of the Mobile-Alabama-Coosa River
Streamflow in Mobile County is also affected by storm surge. As freshwater in the river basin travels downstream to the coast, it can collide with a surge of saltwater traveling up the estuary, causing the river to back up. This event can be caused by the natural fluctuations of the tides or exacerbated by a storm.
Factors Affecting Streamflow
Streamflow is a function of the volume of water in the stream, the speed at which water flows, and the size of the stream channel. It is also dependent on runoff. The amount, intensity, duration, and distribution of precipitation events all impact streamflow. Soil saturation from earlier precipitation events also plays a role. For example, if the ground is still wet from a previous rain event, the soil will be less able to absorb excess water and more likely to cause runoff.
| Meteorological Factors Affecting Runoff/Streamflow | Physical Characteristics Affecting Runoff/Streamflow |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Sources: USGS, 2011f; USGS, 2011g
Over the 20th century, much of the United States has not experienced a significant change in high levels of peak streamflow.13 This is consistent with the findings for Mobile. This section presents key findings for Mobile based on observed data at the five selected stream gage sites. The findings are presented in three categories:
Flood frequencies for Alabama provide transportation planners with vital information for designing transportation infrastructure. The accuracy of these frequencies is dependent on the amount of stream gage data available, the accuracy of those data, changes in land-use that impact the river drainage area, climate, and how well the theoretical distribution fits the stream gage data.14 A series of USGS reports published between 2004 and 2010 provide regional flood frequency analyses of urban and rural streams in Alabama.15
Table 18 describes flood magnitudes based on annual peak streamflows for several different recurrence intervals. For example, a "Q2" recurrence interval has a 50% probability of occurring in any given year (i.e., a 2 year event). The "Q500" recurrence interval is an extreme flood that has a 0.2% probability of occurring in any given year (i.e., a 500-year event).
Site |
Years of Data |
Peak discharge for recurrence interval in years (cfs) |
|||||||
|---|---|---|---|---|---|---|---|---|---|
Q2 |
Q5 |
Q10 |
Q25 |
Q50 |
Q100 |
Q200 |
Q500 |
||
Chickasaw Creek |
51 |
4,450 |
8,840 |
12,900 |
19,600 |
25,800 |
33,400 |
42,300 |
56,800 |
Mobile River |
52 |
278,000 |
352,000 |
397,000 |
450,000 |
487,000 |
522,000 |
556,000 |
600,000 |
Fowl River |
13 |
1,590 |
3,600 |
5,540 |
8,770 |
11,800 |
15,500 |
19,800 |
26,700 |
Hamilton Creek |
17 |
930 |
1,910 |
2,720 |
3,940 |
4,960 |
6,070 |
7,280 |
9,040 |
Crooked Creek |
13 |
685 |
1,180 |
1,590 |
2,200 |
2,740 |
3,350 |
4,040 |
5,090 |
Figure 45 illustrates annual peak streamflow at the five selected stream gage sites in Mobile County from 1990 to 2010. This figure demonstrates how often peak streamflow has reached or exceeded levels representative of floods of varying recurrence intervals. Over the past 20 years, all sites have surpassed the Q5 and Q10 recurrence intervals. Chickasaw Creek, Crooked Creek, and Mobile River have reached an annual peak streamflow representative of the Q25 recurrence interval, while no sites have exceeded this threshold.
The Mobile River site demonstrates the greatest annual peak streamflow with an average of about 300,000 cubic feet (9,000 cubic meters) per second. The Chickasaw Creek site reports the next highest average of 6,600 cubic feet (198 cubic meters) per second. The remaining three sites had average annual peak streamflows ranging from about 800 to 2,400 cubic feet (24 to 72 cubic meters) per second. All sites, except Mobile River, show similar patterns of peak streamflow with high levels in the mid to late 1990s, and unusually low levels in 2006. Hurricane Georges, which struck the southeast in 1998, is responsible for peak flows at Chickasaw Creek and Hamilton Creek. Overall, the sites demonstrate large variability in year-to-year peak streamflow events.
The Chickasaw Creek site provides historical context with a longer period of record (58 years), beginning in 1952. The average annual peak streamflow based on its full record is slightly lower than the average from 1990 to 2010, at 6,287 cubic feet (189 cubic meters) per second. A trend analysis suggests a positive (increasing), but statistically insignificant, trend.16
Figure 45: Annual Peak Streamflow (cfs) Measured at Stream Gage Sites in Mobile County, AL, 1990-2010
The horizontal lines represent the Q5, Q10, and Q25 recurrence intervals specific to each stream gage site.
Annual and monthly mean stream discharge indicate the overall conditions for each stream, important for understanding the general conditions in the Mobile region (e.g., is there significant variability from year-to-year or seasons with high mean discharge?). These conditions impact transportation planners when considering assets or operations sensitive to overall regional "sogginess" or drought.
Annual mean stream discharge data for four sites is illustrated in Figure 46 (this information was not available for the stream gage located at Mobile River). Over the past 20 years, all stream gage locations show a similar pattern of annual mean discharge. This pattern includes higher annual mean discharge in 1998 and 2005 and lower annual mean discharge from 2000 to 2002 and 2006 to 2007.
Unlike the peak streamflow comparison, the average annual mean stream discharge at Chickasaw Creek from 1952 to 2010 is approximately the same as the average from 1990 to 2010. This suggests that the general characteristics affecting annual discharge (e.g., annual precipitation, land use) have not changed over time. The trend analysis also found no statistically significant change.17
Figure 46: Annual Mean Discharge (cfs) Measured at Four Stream Gage Sites in Mobile County, 1990-2010
The red line indicates the overall mean across the entire time series, while the shaded gray area represents one standard deviation above and below the mean.
Figure 47 illustrates the monthly mean discharge averaged from 1990 to 2009 for four sites, as data were available. Though precipitation is greatest during the summer, the monthly discharge data patterns show that streamflow tends to be highest from February to April and lowest from October to November.18
The high monthly discharge data from February to April corresponds to Alabama's "flash flood" season, which can occur during late winter or early spring when vegetation is dormant, the ground is cold (sometimes frozen), and cooler temperatures reduce evaporation rates.19 During this time, a heavy precipitation event can induce flooding. The flood waters tend to begin in northern Alabama and flow southerly over several days until reaching the Gulf of Mexico.20
Figure 47: Average Monthly Mean Discharge (cfs) Measured at Four Stream Gage Sites across Mobile County for Data Available from 1990 to 2009
Due to the significantly higher range in values for the Chickasaw stream gage, the Chickasaw data is on a secondary axis.
Soil types in Mobile County impact how quickly precipitation runs off the surface or is absorbed by the soil. Figure 48 provides a description of the soils found in Mobile
Figure 48: Soil Map for Mobile County, Alabama
This section describes the analysis of projected monthly streamflow based on a monthly water balance model driven by Mobile-specific information. These projections inform how monthly hydrological properties may change in the future, which could affect transportation stressors such as wetland performance; however, they will not provide information on impacts influenced by peak flow rates.
Monthly projections were developed for an artificial basin using the USGS' modified Thornwaite monthly water balance model (WBM) driven by Mobile-specific information.21 This model estimates monthly runoff, evapotranspiration, and soil moisture within a basin or sub-basin using user-provided monthly precipitation and temperature data. The monthly runoff projections were translated to monthly discharge projections using the basin area for each stream gage. The model assumes:
Optimum values for the user-defined parameters were determined for Mobile using runoff data from three stream gage sites and meteorological data from the Coden and Mobile observations stations (see Appendix C.8 for detailed discussion of the methodology and calibration results).
Once calibrated for Mobile, the WBM was run with the climate model baseline simulations and compared against the stream gage monthly mean discharge values. These runs show that the model underestimates monthly discharge compared to stream gage monthly discharge. In addition, the WMB does not capture the extreme daily peaks of discharge and has the most difficulty accurately portraying fall monthly runoff. The test runs indicate that the projected streamflow may best represent changes at Chickasaw Creek.
Key Findings for Streamflow Projections
This section presents the results of the projected streamflow analysis, including a discussion of:
The results of the projected streamflow analysis for Chickasaw Creek are presented here (see Figure 49). Appendix C.9 contains the results for Hamilton Creek and Crooked Creek, which demonstrate similar patterns in projected monthly stream discharge.
Monthly stream discharge is projected to increase across much of the winter and early spring months, regardless of emission scenario and time period. However, there are noticeable differences in monthly discharge projections by emission scenario. By end-of-century, January discharge at Chickasaw Creek is projected to increase by as much as 23.9 cubic feet (0.7 cubic meters) per second under the high (A1FI) emission scenario and 160.5 cubic feet (4.8 cubic meters) per second under the low (B1) emission scenario.
Under the moderately-high (A2) and high (A1FI) emission scenarios, monthly discharge during much of the year (April through December) is projected to decrease substantially compared to baseline. This projected decrease in discharge is coupled with a projected increase in monthly evapotranspiration (i.e., evaporation from ground surfaces and plants), which is reflective of the projected warmer temperatures. The combination of these two projections suggests soil moisture may be drier over much of the year compared to baseline conditions.
Under the low (B1) emission scenario, monthly discharge and evapotranspiration are projected to increase compared to baseline across all months.
Figure 49: Modeled Baseline and Projected Monthly Streamflow Discharge (ft3/sec) for Chickasaw Creek and Actual Evapotranspiration (mm) by Time Period and Emission Scenario
Soil moisture is affected by changes in both monthly streamflow and evapotranspiration, and is defined as the amount of water stored in the soil (mm).22 Projections of soil moisture suggest that summer months will become increasingly dry under the moderately-high (A2) and high (A1FI) emission scenarios over time (see Figure 50). Drier conditions traditionally experienced during the summer months are projected to extend into late spring and through the fall. The low (B1) emission scenario does not demonstrate large differences from simulated baseline conditions.
These results suggest large changes in summertime soil moisture under the moderately-high (A2) emission scenario by end-of-century and under the high (A1FI) emission scenario by both mid-century and end-of-century. In the upcoming vulnerability assessment under Task 3, soil moisture capacity projected by the WBM may be used to drive hydrologic modeling to estimate event-driven changes in projected streamflow and to establish changes in long-term soil conditions.
Figure 50: Modeled Soil Moisture (mm) by Time Period and Emission Scenario
The transportation implications of changing streamflow patterns are similar to those resulting from severe precipitation events, discussed above. Streamflow changes are likely to have the most significant effects on roadways, but may also impact rail lines; landside operations at ports; and facilities at airports, bus stations and train terminals. Changes in seasonal and monthly hydrology could require consideration of wetland performance. Erosion patterns may also be affected, necessitating more frequent maintenance and changes in vegetation management. Under the next task of this project (Task 3: Vulnerability Screen and Assessment), additional analysis may be done to consider the effects on peak flow events, which can affect performance of culverts, ditches, and water runoff collection and treatment systems.
The implications of the streamflow findings detailed in this report on transportation assets and services in Mobile will be investigated in the next task of this study (Task 3: Vulnerability Screen and Assessment).
1 USGS, 2007
2 USGS Surface-Water database, available athttp://waterdata.usgs.gov/usa/nwis/sw
3USGS, 2004; USGS, 2010a
4 Changes in extreme peak streamflow will be investigated during the risk assessment on an as needed basis.
5 USGS, 2011a
6 USGS, 2011d
7 USGS, 2011b
8 USGS, 2011c
9 USGS, 2011e
10 USGS, 2004; USGS, 2010. These reports fit a Pearson Type III distribution to the logarithm of annual peak streamflow to obtain flood magnitudes.
11 USGS, 2004
12 This coastal plain ranges in elevation from sea level to 1,000 ft above National Geodetic Vertical Datum (NGVD) of 1929.
13 Pielke et al., 1999
14 USGS, 2010b [General Information Product 106, available at http://pubs.usgs.gov/gip/106/pdf/100-year-flood-handout-042610.pdf)
15 USGS, 2004; USGS, 2010a
16 The Mann Kendall trend analysis was used to investigate whether a statistically significant positive or negative trend occurred in the Chickasaw Creek annual peak streamflow. The analysis found no significant trend (i.e., tau correlation coefficient of approximately 0.05 with a p value of approximately 0.5).
17 The Mann Kendall trend analysis was used to investigate whether a statistically significant positive (wetter) or negative (drier) trend was noticed in the Chickasaw Creek annual mean discharge period of record from 1952 to 2010. The analysis found no significant trend (i.e., tau correlation coefficient is -0.007 with a p value of 0.94)).
18 This is consistent with the runoff discussion published in USGS (2010a).
19 Evans, 2009
20 Evans, 2009. Also noted that some but not all Alabama river systems have flood controls or reservoirs.
21 http://wi.water.usgs.gov/Soil_Water_Balance/index.html
22 The monthly water balance model assumes as the soil becomes drier, water is increasingly difficult to remove from the soil and less is available for actual evapotranspiration.
