Watershed Management


Advisory Council
The Watershed Advisory Council Group meets on a quarterly basis to discuss items relating to development, regulation, and pollution prevention. Please contact the Stormwater Department at 770-229-6424 to learn more about this group and how to participate.
Watershed Committee Members


Advisory Council Minutes
March 2007
December 2006
September 2006
July 2006
April 2006
November 2005


Hydrologic & Hydraulic Study - (Hydrology)
Hydrologic modeling for the City of Griffin was accomplished using the Regression Equations developed by the United States Geological Survey (USGS) for the State of Georgia (“Flood-Frequency Relations for Urban Streams in Georgia – 1994 Update”, Water-Resources Investigations Report 95-4017, U.S. Geological Survey). Using the regression relations, the peak flows for various storm events can be estimated for urban basins. Hydrologic characteristics such as drainage area and impervious area were input into the equations to determine the peak flow rates for each drainage basin within the city.

Individual drainage basins within the each of the three watersheds were divided into sub-basins. Delineation of sub-basins was based primarily on major road crossings and confluences of sub-tributaries within the watershed. Other divisions were made to account for variations in land use and the existing drainage.

Soil data for each watershed were compiled from the Soil Survey of Spalding County, Georgia (SCS, 1964). Soils within the City of Griffin are predominantly of the Cecil-Madison association, which is described as well drained and having a red sandy clay loam or clay loam subsoil. The majority of the soils (slightly less than 91%) included in the three watersheds are Group B soils. Group C and D soils are found mainly within the floodplains of the creeks.

Land use in the City of Griffin is defined by 9 land use categories, which were developed from TR-55, “Urban Hydrology for Small Watersheds” (SCS, 1986). These categories range from open space and wooded areas to commercial and industrial developments. Residential areas are further broken down into eight classes based lot size and average impervious area. A runoff curve number was assigned to each land use category and for the hydrologic soil group, and the average impervious area was estimated for each category. City of Griffin Tax Parcel data was used for the modeling of land use within the project area. Each tax parcel was assigned a runoff curve number and impervious area based on actual land use. Using the Geoprocessing extension within ArcView, the soil and land use information was combined to calculate the weighted runoff curve number and total impervious area for each sub-basin.


Hydrologic & Hydraulic Study - (Hydraulics)
Water surface profiles for the all tributaries were modeled using the US Army Corps of Engineers Hydrologic Engineering Center’s River Analysis System (HEC-RAS). The HEC-RAS model is an open channel steady state hydraulic simulation program. It uses the standard step backwater calculations to compute changes in water surface elevation between adjacent cross sections on the basis of energy losses. HEC-RAS also has the ability to compute water surface elevations at structures such as bridges and culverts.

HEC-RAS can be set to compute the water surface profiles using a sub-critical flow regime, a super-critical flow regime, or a mixed flow regime. Although some of the streams under study exhibit super-critical flow at times (especially during the more common events such as the 2-year discharge), considering super-critical results presents concerns for engineers, such as blockage of the channel and/or structures by debris. Debris blockage is impossible to predict, but given the vegetative cover of the watersheds debris blockage can be expected. If a stream is in a super-critical flow regime, and debris blockage occurs, then the flow regime will revert to the sub-critical state resulting in an increase in the water surface elevation. Therefore, floodplain analysis and delineation used only the results from the sub-critical water surface profiles.

The physical characteristics of each stream (i.e. cross section geometry, channel geometry, reach lengths and flow paths) where processed within ArcView. Cross Sections were surveyed approximately every 200 – 400 feet as well as upstream and downstream of every major structure. When physical parameters of the tributaries had been determined, the HEC-GeoRas extension within ArcView was used to process the data in 3-D and export it to a HEC-RAS input file.
The HEC-GeoRas geometry data was then imported into HEC-RAS for analysis. Parameters such as Manning’s roughness coefficients (“n” values) and expansion/contraction coefficients were entered into the hydraulic model. Bridge and culvert data were input using data from the field survey and ineffective flow areas were defined. Water surface elevations for the 2-, 10-, 25-, 50-, and 100-year frequency floods at the first cross section of the HEC-RAS Model were typically started at normal depth based on slope-area computations of convergent profiles.


H&H Study - (Results)
The results from the hydrologic and hydraulic studies have been used as a tool to identify flooding problems at streams, culverts and bridges under the existing land use conditions in the City. The types of problems identified include structural flooding, non-structural flooding, areas that could potentially erode, undersized or deteriorated drainage infrastructure, and roadway overtopping by floodwaters. The results of the existing drainage system evaluation will be used to develop comprehensive stormwater management priorities. Capital Improvement Project locations were identified and prioritized based on the level of service for stream crossings in the watershed.
H&H Study Results


H&H Study- (Floodplain Delineation)
After the hydraulic modeling was complete, the water surface profile data was exported from HEC-RAS back into ArcView. Using the post-processing features of HEC-GeoRas, the floodplains for each tributary within the city were delineated and floodplain maps created. Using the Geoprocessing extension in ArcView, buildings and parcels that lay within the floodplains were identified and mapped. Structural as well as non-structural flooding problems were identified as well. Using the velocities for the hydraulic analysis, stream exhibiting erosion potential were mapped.


H&H Study - (Level of Service Ratings)
Each stream crossing structure within the corporate boundaries of the City of Griffin has been assigned a Level of Service rating (LOS) based on the return period of the event the structure conveys. A service rating of 10-years means that the 10-year event does not overtop the roadway but the 25-year event does. If a road is overtopped by the 2-year flood event the service rating is 0-years. The degree of roadway overtopping at each discharge was also quantified.


H&H Study - (Capital Improvement Program Recommendations)
After the LOS was determined for stream crossings the potential Capital Improvement Program (CIP) sites were evaluated and prioritized based on their current service rating. Primary consideration for selecting CIP is the ability for emergency service access during floods. The criterion used in CIP site selection is to prioritize the potential CIP’s into three classes. Class 1 sites have the highest priority for future CIP’s or have an immediate need for improvement based on the following factors: a high degree of roadway overtopping at frequent rain return periods and/or a large urban areas isolated by roadway overtopping. Class 2 sites have the next level of priority for future CIP’s based on the following factors: roadway overtopping at frequent to rare rain return periods and/or urban areas isolated by roadway overtopping. Class 3 sites have lowest priority for CIP’s based of the following factors: minimal roadway overtopping at rare rain return periods and/or little if any urban areas isolated by roadway overtopping.

To assist the City in planning budgets for future CIP’s, preliminary cost estimates were prepared for Class 1 sites. The remedy for each Class 1 site is assumed without defining the specific improvement to the LOS or the effect on upstream or downstream flooding. When the project is programmed for improvement, a detailed evaluation of the project area will be performed to increase the LOS to current LOS standards.

Stream Flow Gauging
The City of Griffin is currently performing rainfall gauging at selected locations throughout the City. Rainfall and flow gauging provides important historical rainfall information as well as provides a calibration tool to help create the models used in the stormwater modeling efforts of the entire community watershed. The City will use utilize this information to evaluate the characteristics of the existing stormwater systems. The initial sites selected for rainfall gauging are as follows:

· Honey Bee Creek at Hill Street
· Potato Creek at Maple Drive
· Cabin Creek at Colonial Pipeline
· Shoal Creek at Lyndon Avenue

Additional sites for rainfall and flow gauging stations will be selected and monitored within the watershed until all six basins have been assessed and baseline data created.

Stormwater Flow Monitoring - The City of Griffin has positioned several flow monitor systems in select drainage structures throughout the City. The flow monitoring provides historical data concerning the amount of stormwater within the watershed. This information can then be compared to the rainfall data collected through rain gauging to calibrate the stormwater master plan. Stormwater flow monitoring will be conducted at the following locations and other locations as necessary:

· Heads Creek at Oakview Drive (2 channels monitored)
· Heads Creek at Mobley Street (1 channel monitored)
· Shoal Creek at Pine Hill Road (3 channels monitored)
· Honey Bee Creek at Hill Street (1 channel monitored)
· Potato Creek at Maple Drive (2 channels monitored)
· Potato Creek at Grandview Drive (3 channels monitored)
· Cabin Creek at Colonial Pipeline (1 channel monitored)
· Shoal Creek at North Griffin Regional Detention Pond (2 channels monitored)
· Shoal Creek at Lyndon Avenue (3 channels monitored)

Watershed Descriptions
As with many cities and towns of Georgia, Griffin sits astride a major watershed boundary. Typically, railroad development has dictated town locations and the railroad has historically been routed where bridge requirements are minimized. This means the railroad routes have generally followed drainage divides.

Originally situated around a railroad junction, Griffin has developed along the divide between the Flint River system to the west and the Ocmulgee River system to the east.

Six watersheds within the City of Griffin are contained within these two river systems. Draining eventually to the Ocmulgee is the Cabin Creek Watershed. Draining eventually to the Flint are the Heads Creek, Shoal Creek, Wasp Creek, Honey Bee Creek, and the Potato Creek watersheds. The City of Griffin lies atop the headwaters of all these watersheds.


Cabin Creek
This is the only Watershed within the City of Griffin that drains to the Ocmulgee River. As with the Heads Creek Watershed, the Cabin Creek Watershed is separated into two areas. A tiny 26-acre portion lies adjacent to the Heads Creek and Shoal Creek Watersheds in the Experiment area. This area is bounded by Collier Street to the north, Old Atlanta Road to the west, Highway 92 to the southwest, and the Norfolk Southern rail line to the east.

The lion’s share of the Watershed, approximately 1,169 acres, lies in the North Griffin area. Extending generally north of the downtown east-west rail lines, the watershed is predominately residential or rural. A small sliver of the Watershed extends southward between the Shoal Creek and Potato Creek Watersheds to Poplar Street in the downtown area between Hill Street and 9th Street. The Watershed encompasses the entire North Griffin area east of the Experiment area, north of the downtown rail lines and extends to the eastern boundaries of Griffin north of the Norfolk Southern rail line. In the eastern area there is an industrial warehouse area near Searcy Avenue south of East Broadway Street. Cabin Creek is a tributary of the Towaliga River which flows to the Ocmulgee.
Cabin Creek Watershed Assessment Vol I | Cabin Creek Watershed Assessment Vol II


Heads Creek
The approximately 263 acres of the Watershed are divided within the City of Griffin into two separate areas. To the west, approximately 243 acres of the Watershed is found in the area bounded by West McIntosh to the south, Old Atlanta Road to the east, Lucky Street and Ridgewood Drive to the north and Rosewood Drive and Hallmark Drive to the west. The By-Pass separates this area to the east and west into two sub-watersheds. Included in the watershed to the west of the By-Pass is the North Griffin Square shopping center.

There is also a smaller 20-acre portion of the Heads Creek Watershed to the south of Ellis Road in the Westchester Drive residential area. This small area drains to the north and Heads Creek rather than to the south and Shoal Creek.

Heads Creek forms the Heads Creek Reservoir and then flows into Wildcat Creek in western Spalding County which then flows directly to the Flint River.


Honey Bee Creek
Only the upper fringes of the Honey Bee Creek watershed, approximately 364 acres, are found within the City of Griffin. These fringe areas are found along the southwest boundary of the Potato Creek Watershed. Maddox Road generally forms the divide between Honey Bee and Potato in the southern portion of the Honey Bee Creek Watershed. Areas to the southwest of Maddox Road are contained within the Honey Bee Creek Watershed. In the northern portion, the NACOM facility and the immediate surrounding area is within the watershed with the NACOM Lake serving as the source of Honey Bee Creek. Honey Bee Creek is a tributary of Potato Creek, joining it in Lamar County.


Potato Creek
The Potato Creek watershed is the largest covering approximately 3,639 acres in the southeast portion of the City. The watershed extends generally south from the downtown rail lines covering most of the downtown area. The watershed extends to the southeast from downtown encompassing most of the older residential neighborhoods within the City such as those along College Street and Maple Drive. There is some industrial development on the east side of the watershed including the area around the State Patrol Barracks and the Griffin Daily News. On the west side of the watershed is the Griffin-Spalding County Airport which is wholly contained in the Potato Creek Watershed. Also within the watershed are the Library, Griffin City Park, Oak Hill Cemetery, and the Spalding Regional Hospital.

Ironically, Potato Creek itself is not found within the City; the watershed within the City consisting of two major tributaries, Ison’s Branch and Grape Creek. These two streams merge just outside the City east of the Four Oaks and Maplewood areas to form Potato Creek. Potato Creek then flows through Lamar and Upson Counties eventually emptying to the Flint River.
Potato Creek Watershed Assessment | Potato Creek Watershed Management Plan


Shoal Creek
The Shoal Creek Watershed is the second largest within the City covering approximately 3,127 acres on the western side of Griffin. The watershed encompasses the complete collection of land uses found in the City of Griffin. Extending west from the downtown area, the far east of the watershed begins near 9th Street. To the south the boundary generally follows Poplar Street separating the Potato Creek from the Shoal Creek watersheds. To the north the boundary follows along the Norfolk Southern rail line heading toward Experiment. The watershed rapidly widens from north to south as it extends northwest along the rail line and southwest along Meriwether Street. It eventually stretches from West McIntosh Road in the north to the intersection of the Highway 19 By-Pass and the Norfolk Southern rail line in the south. The Georgia Experiment Station, Griffin Tech Institute, The Griffin Water Works and Griffin High School are within the watershed in the areas along the By-Pass.

Found near Ellis Road is the engineering design award winning North Griffin Regional Detention Pond. This facility is improving water quality in the Shoal Creek Watershed by filtering stormwater through a wetland system in a tributary of Shoal Creek.

West of the by-pass the Watershed encompasses portions of the State Experimental Farms outside the City boundaries between Pine Hill Road and the By-Pass. West of Pine Hill Road the Griffin Country Club and the surrounding residential area are found within the Shoal Creek Watershed.

From the Griffin Country Club Lake, Shoal Creek flows into Wildcat Creek in western Spalding County which then flows to the Flint River.
Shoal Creek Watershed Assessment Vol I | Shoal Creek Watershed Assessment Vol II
Shoal Creek - Sediment Evaluation | Shoal Creek - Stream Channel Assessment - Final Report


Wasp Creek
Only a tiny 69-acre portion of the Wasp Creek Watershed is found within the City of Griffin, directly south of the Shoal Creek Watershed. This industrial area to the southwest is generally bounded by Carver Road to the west, Industrial Drive to the south, the Highway 19 By-Pass to the east and the Norfolk Southern rail line to the north. Wasp Creek is a tributary of Turnpike Creek in Pike County, which in turn is a tributary of Potato Creek.