Channel Stability Assessment

Stream Channel Stability Assessment of the Shoal Creek Watershed. Griffin, Georgia.

PREPARED BY FOR Tetra Tech, Inc. City of Griffin 2110 Powers Ferry Rd., Public Works and Suite 202 Stormwater Department Atlanta, GA 30339

Shoal Creek, Griffin, Georgia 1 July, 2004 Channel Stability Assessment

Table of Contents

Goals ...... 4 Methods...... 4 Overall State of the Stream Channels of the Shoal Creek Watershed ...... 4 Summary of Channel Erosion Hot Spots by Tributary...... 8 Sedimentation of Ponds in the Shoal Creek Watershed...... 11 Future Steps to Reduce Sediment supplied to Shoal Creek...... 11 Estimation of Quantities of Sediment Delivered by Different Sources...... 12 Channel Sediment Sources ...... 12 Upland Sediment Sources...... 12 References...... 17

Tables

Table 1. Erosion hotspots in the Shoal Creek watershed. June 2004...... 8 Table 2. Estimate of sediment eroded and delivered to Griffin Country Club Lake based on land use...... 13 Table 3. Main Stem and Headwaters channel stability assessment data...... 18 Table 4. Tributaries 3 and 4 channel stability assessment data...... 19 Table 5. Tributaries 5, 6, 7 and 8 channel stability assessment data...... 20 Table 6. Tributaries 9, 11, 12, 13, 14 channel stability assessment data and upland site data. ... 21

Maps

Map 1. Watershed and stream map...... 22 Map 2. Stream reaches assessd for channel stability...... 23 Map 3. Channel erosion hot spots...... 24

Shoal Creek, Griffin, Georgia 2 July, 2004 Channel Stability Assessment

Figures

Figure 1. Old stumps at base of bank are remnants of clearing and snagging that typically took place during channelization (Site 52, Trib 4)...... 6 Figure 2. Leaning tree indicates that previously the banks were high enough to undergo slumping. Presently both the bed and floodplain are aggrading. (Site 47, Trib 3)...... 6 Figure 3. Headcut below pond. Rate of migration is not known. Note fencepost dangling by barbed wire. Scale is 1.3m long. (Site 43, Trib 3)...... 7 Figure 4. Dense vegetation on stream banks helps to limit erosion (Site 21, Main Stem)...... 7 Figure 5. Mass wasting of stream banks (Site 85, Main Stem)...... 8 Figure 6. Pasture land at the University of Georgia Experiment Station is maintained with a heavy grass cover as a soil erosion BMP (Site 42)...... 15 Figure 7. Although poorly vegetated, a topographic depression at the Super Walmart shopping plaza creates sink trapping eroded sediment before it can wash into Shoal Creek (Site 75 )...... 15 Figure 8. A small scale redevelopment, at Broad and 17th Streets, of a previously developed area creates a potential high soil erosion source (Site 105)...... 16 Figure 9. Yards kept in bare soil, such as this one at 17th and Drewery, may be long term contributors of sediment (Site 120)...... 16

Assessment Site Photographs

Digital format on attached CD.

Shoal Creek, Griffin, Georgia 3 July, 2004 Channel Stability Assessment

Goals

The project goals were to assess the geomorphic state of streams in the Shoal Creek watershed above the Griffin Country Club Lake (Map 1) and locate sediment source hotspots, then provide descriptions via a written report, maps, and photographs.

Methods

During the week of 21-24 June, 2004, the assessment was carried out by a fluvial geomorphologist walking on the stream bed and conducting Rapid Geomorphic Assessments (RGAs). RGAs were conducted at each major change in channel morphology, bed and bank lithology, and riparian vegetation. Tetra Tech staff walked in or along the entirety of the main channel and each tributary except for reaches with dense vegetation, which were typically located along the smaller tributaries, and fenced off reaches. The reaches walked are highlighted on Map 2. Data was collected at the assessment points shown on Map 2. RGA data collected included assessment point descriptions including location, stage of channel evolution (per the channel evolution model described in the Watershed Assessment report, Tetra Tech, 2004), bank heights, bed and bank materials, stability, vegetation, present stability state, future stability issues, and photos. The data is compiled in Table 1 and the photos are compiled into a photo log. Channel erosion hotspots were selected from the data set based on the type and degree of recent channel erosion activity. The channel erosion hotspots are shown on Map 3 and are listed in Table 1. The tributary numbering system is identical to that used in the Watershed Assessment (Tetra Tech, 2004)

Overall State of the Stream Channels of the Shoal Creek Watershed

The geomorphology of the Shoal Creek watershed indicates that it has undergone changes typical of urban watersheds throughout most of the eastern United States. The stream and its tributaries have been directly affected by channelization and indirectly affected by changes in surrounding landuse. Channelization typically steepens channel gradients and removes bank vegetation. Steeper gradients result in increased stream power which increases the energy available to erode stream banks. The loss of bank vegetation results in a reduction of the stabilizing effect that roots have on the streambank soil. Changes in landuse affect the channel by changing the timing and quantity of runoff delivered to the channel and the quantity of sediment delivered from the uplands to the channel. Historically, as watersheds change from forested to agricultural and urban landuses, the amount of runoff increases, thereby increasing the height and frequency of flood peaks. This increases the time that the channel is subjected to erosive flows, and therefore increases the potential for erosion by bed incision and bank failure. However, during recent times, probable changes in hydrology are taken into account during the development process and provisions, such as the North Griffin Regional Stormwater Detention Ponds system at the Super Walmart plaza, are constructed in order to reduce flow peaks. Typically soil erosion increases during land use change and is higher for all landuses other than forest. This means that stream channels will begin aggrading if there is not enough flow to transport the excess sediment. Sedimentation rates in reservoirs thus typically increase during periods of land use change in a watershed.

Shoal Creek, Griffin, Georgia 4 July, 2004 Channel Stability Assessment

The current condition of the Shoal Creek Watershed is most certainly in a vastly different state than during pre-settlement times. Throughout the watershed are obvious signs of historical channelization (Figure 1), periods of channel incision, and later periods of aggradation of sediment in channels and on floodplains (Figure 2. The scale used is all photos is divided into 10cm intervals. It’s total length is 1.3m).

The full potential of effects due to the historical channelization of Shoal Creek and its tributaries are mitigated by frequent natural bedrock grade controls above North Pine Hill Road throughout the watershed and man made controls at road crossings most commonly found above highway 19/41. These grade controls restrict the distance that a headcut can migrate. Only one headcut was noted in the watershed and is located on Tributary 3 immediately below the pond (Figure 3, site 43). However this headcut is significant both in size and location. It has about an 8 foot fall and given enough time will cause the upstream dam to fail. Otherwise below highway 19/41 the stream bank material is a generally cohesive and erosion resistant soil, and bank vegetation is dense thereby enhancing bank stability via dense root masses (Figure 4). Above highway 19/41 the banks are slightly higher than downstream, and there is a lack of deep rooted vegetation to support the banks. The result is a greater occurrence of mass wasting stream banks (Figure 5), and thus most of the channels are classified, per the channel evolution model, as stage V.

It is noteworthy that the stream bank heights of the upstream reaches were typically as great as main channel bank heights below highway 19/41. Bank heights are 2.5 and 2 m respectively for the Headwaters channel at 15th Street (site 104) and Tributary 7 at Hillwood Street (site 106). Progressing upstream, channels should become smaller as they are carrying less flow. However, with the exceptions of Tributaries 6a, 6b, and 8 above site 91, the urban reaches of Shoal Creek and its tributaries tend to have bank heights greater than expected. Without knowing the history of stream channel management within the urbanized part of the watershed, it is not clear if the bank heights are the result of deepening through channelization, natural incision, or filling of floodplains. In any case the now higher banks are more susceptible to mass wasting. The increased runoff from the urban area further exacerbates bank failure through more frequent high flows.

Sewage pipes were noted parallel to the main channel from just below Tributary 3 to above 17th Street, and along Tributaries 3, 4, and 9. In several locations the stream has migrated close to the sewage line necessitating hardening the stream bank with rip-rap to protect the pipe. This form of erosion and bank protection will very likely be an ongoing concern until either the banks are sufficiently hardened to resist erosive flows, or the channel gradient is sufficiently reduced by lengthening through meandering.

Shoal Creek, Griffin, Georgia 5 July, 2004 Channel Stability Assessment

Figure 1. Old stumps at base of bank are remnants of clearing and snagging that typically took place during channelization (Site 52, Trib 4).

Figure 2. Leaning tree indicates that previously the banks were high enough to undergo slumping. Presently both the bed and floodplain are aggrading. (Site 47, Trib 3).

Shoal Creek, Griffin, Georgia 6 July, 2004 Channel Stability Assessment

Figure 3. Headcut below pond. Rate of migration is not known. Note fencepost dangling by barbed wire. Scale is 1.3m long. (Site 43, Trib 3).

Figure 4. Dense vegetation on stream banks helps to limit erosion (Site 21, Main Stem).

Shoal Creek, Griffin, Georgia 7 July, 2004 Channel Stability Assessment

Figure 5. Mass wasting of stream banks (Site 85, Main Stem).

Summary of Channel Erosion Hot Spots by Tributary

Channel erosion hot spots are summarized in Table 1. The reaches presently undergoing the most severe erosion are considered to be erosion hotspots and are highlighted pink. Reaches presently undergoing minor erosion or those that show a potential for significant future erosion are considered to be sensitive and are highlighted yellow. The data for sites assessed are listed in Tables 3 through 6.

Table 1. Erosion hotspots in the Shoal Creek watershed. June 2004.

Channel and Assessment Point Location Channel Condition Main Stem 4 fairway Widening via bank sloughing 6 fairway Widening via bank sloughing Hard clay banks scouring, not mass 14 - wasting. Hard clay banks scouring, not mass 15 - wasting.

Shoal Creek, Griffin, Georgia 8 July, 2004 Channel Stability Assessment

16 - Mass wasting by block failure. 27 - Mass wasting caused by debris jam 36 above hwy 19/41 culvert Active mass wasting on both banks 84 above Atlanta Rd. Active mass wasting on both banks 85 Below Melrose Street Active mass wasting on both banks 89 south of Lyndon Ave Active mass wasting on both banks

Headwaters 94 S. of basketball ct S.W. of 18th St. Mass wasting by block failure. 97 below 17th St. Mass wasting 103 between 17th and Ray St. Mass wasting Mass wasting outside bends. Stable inside 104 below 15th St. bends. 104 +70m above 15th St. Mass wasting outside bends.

Trib 1 Outside of watershed.

Trib 2 Outside of watershed.

Trib 3 Headcut above and scourpool and intense scour immedialty below. Cohesive bank 43 headcut below pond material prevents mass wasting

Trib 3a No hotspots

Trib 4 Channelized, incised, good bank veg, several slumps. Coke can w/pull tab and 52 - UPC symbol buried 0.7 m below bank top.

Trib 4a Air photos indicate unstable reach above air photos above neighborhood neighborhood in 1999

Trib 5 No hotspots

Trib 5a No hotspots

Trib 5b No hotspots

Trib 6

Shoal Creek, Griffin, Georgia 9 July, 2004 Channel Stability Assessment

Rip-rap on eroding cut banks. Short reach so total quantity of sediment available may 98 between Garrett and Moody Sts. not be as great as other reaches. 101 - Minor bank sloughing.

Trib 6a No hotspots on reaches assessed

Trib 6b No hotspots on reaches assessed

Trib 7 Bank failures present, however veg is 87 holding banks. Scour caused by debris caught on fence. 88 above fence Banks slumping above fence. Minor bank failures. Bars forming in 107 above Pleasant St. channel. 106 below Hillwood St. Active mass wasting as recent as 29 June

Trib 8 91 between factory driveways Active mass wasting

Trib 9 68 adjacent to silt fence Alternate bars.

Mass wasting on left bank. Left riparian zone is very boggy along sewage easement. Mass wasting possibly due to saturated soil conditions caused by groundwater seeping from steep slope 67 adjacent to silt fence adjacent to sewage easement. fallen tree from left bank. Silt 66 fence Channel encroaches on sewage easement

Trib 9a Not Assessed

Trib 10 Not Assessed

Trib 11 No hotspots on reaches assessed

Trib 11a No hotspots on reaches assessed

Trib 12 No hotspots on reaches assessed

Trib 13 No hotspots on reaches assessed

Shoal Creek, Griffin, Georgia 10 July, 2004 Channel Stability Assessment

Trib 14 No hotspots on reaches assessed

Uplands 105 17th and Broad Streets ~1 acre bare soil 115 apartments NE of 19/41 and 16 ~1 acre bare soil at apartment complex Essentally bare soil. Grass recently planted but grass has not yet developed sufficient 119 Griffin Institute N of Varsity Rd. coverage to reduce soil erosion.

Sedimentation of Ponds in the Shoal Creek Watershed

Numerous ponds throughout the watershed serve as sediment traps thereby reducing the load of course particles (> .062mm dia) transported from the tributaries into Shoal Creek. Given enough time all these ponds will become filled with sediment.

Natural ponds, such as the one created by a beaver dam on Tributary 9, typically will trap sediment for several years to perhaps one or two decades before a major flood will wash out the dam. The sediment trapped behind the dam will then begin to erode and be transported downstream, however it may be several centuries before all of the originally trapped sediment is eroded.

Future Steps to Reduce Sediment supplied to Shoal Creek

In order to reduce the quantity of sediment supplied to Shoal Creek several steps can be taken: Reduce peak flows, modify channel forms to handle existing flows, and/or apply revetments to banks to handle existing flows.

• Methods for reducing peak flows include constructing ponds for stormwater detention and increasing pervious surface area to improve infiltration.

• Two methods for modifying the channel form include creating meanders, and/or adding a floodplain. One method of urban stream restoration entails creating a low-flow channel meandering between high-flow flood terraces. This sort of technique could conceivably work along reaches with plenty of space to create a flood plain, such as the area south of Lyndon Ave.

• A wide variety of revetments are available. They fall into three broad categories of solid (concete), hard and flexible (rip-rap), or living (willow posts). The choice is dependent upon the funds available, the aesthetics desired, as well as the long term maintenance plan.

A combination of these techniques may work well to immediately stabilize the banks above highway 19/41. The banks crossing the golf course fairways may be more difficult to manage due to the large amount of sediment being deposited in the channel. If the bed keeps aggrading,

Shoal Creek, Griffin, Georgia 11 July, 2004 Channel Stability Assessment

then during floods, the flow may leave the existing channel and cut a new one through the golf course in order to pass the high water and sediment load. If the sediment load is reduced then either living or hard bank protection will probably prevent any further erosion.

Estimation of Quantities of Sediment Delivered by Different Sources

Estimates of sediment loadings were done to compare the relative amounts of sediment delivered from channel sources versus upland sources. This is was done as a quick check to see if either channel or upland sediment sources dominate the sediment loading to Shoal Creek.

Channel Sediment Sources

Sediment supplied from channel erosion can most accurately be estimated by surveying a stream channel at several cross sections, then repeating the surveys over time in order to determine the change in cross sectional area. Other methods include estimating the volume of material eroded from sloughed banks. This can be done by estimating the length of the failed reach, the present cross sectional profile and an assumed pre-failure bank profile. If we assume an average of 1 meter high banks for the 16 kilometers of Shoal Creek and tributaries, the bulk density of bank material equal to 1.3 tons/m3, and an average bank retreat rate 1 to 10 cm per year, the result is approximately 400 to 4000 tons eroded per year. The quantity of sediment delivered to downstream reservoirs is difficult to estimate due to the variability in transportation rates of the various eroded particle sizes. Gravels may take decades traveling from one riffle to the next with each high flow event. Sand will move during high flows at maximum of about ¾ of the water velocity. Data from the Cabin Creek stream gage shows that it has a flashy runoff response with peaks lasting 2 to 3 hours. Therefore, annually there is perhaps 20 to 30 hours of high flows in Shoal Creek. Most of the sand eroded from the banks is probably deposited on the bed or on the floodplains. Sand on the bed can be mobilized during the next high flow event and will be carried further downstream for deposition on either the floodplains or streambed. Ponds on the tributaries will trap sand and coarser particles. Sand in the main stem of Shoal Creek and in the undammed tributaries will eventually be carried to the Griffin Country Club Lake. Sand deposited on the floodplains will remain there indefinitely. Typically floodplain deposits are only re-mobilized when a stream’s meander bends erode across the floodplain. Silt, like sand, will become trapped in ponds and on floodplains. However silts can be carried at velocities nearer to that of the water and will quickly reach the downstream reservoir. Clay size particles typically are fine enough to be carried through small ponds during high flows all the way to the Griffin Country Club Lake, and possibly all the way through the lake. In summary, it with detailed measurements, accurate estimates of sediment eroded from the banks can be made. However estimating the amount delivered to the lake is more difficult.

Upland Sediment Sources

During the assessment of stream channels several, observations were made concerning upland sediment sources. A description of general upland erosion processes and processes specific to the Shoal Creek watershed are included to provide a more complete picture of potential sediment sources.

Shoal Creek, Griffin, Georgia 12 July, 2004 Channel Stability Assessment

The amount of sediment generated in any given watershed is related to several factors including land use, soil type (erodability), precipitation (intensity and total quantity), and topography (steepness of slopes). However, of these factors, typically only land use can be significantly altered through human impacts. Therefore, assuming soil type, precipitation, and the topography of the Shoal Creek watershed remain constant over time, we can do a soil erosion estimation based solely on land use. The amount of soil eroded from various land uses has been well established though numerous studies. There is a several order of magnitude range which makes it possible to do a quick estimation with a high degree of confidence for determining the dominant sediment source. The following ranges for soil erosion rates are considered typical for regions, such as Georgia, where soils are moderate in erodability, precipitation is sufficient to promote the growth of full vegetative cover, precipitation occasionally occurs as intense thunderstorms, and the topography is moderate ranging between flat plains and steep mountains.

Present land use in the watershed can be estimated from the land cover conditions figure from the Watershed Assessment. During the stream walk it was noted that land use has changed over several large areas including the Super Walmart and Lowe’s. It was also noted that much of the commercial land is well grassed, in particular the University of Georgia property north of Ellis Road and east of highway 19/41 and the Griffin High School athletic fields.

The estimate based on landuse weighted by area, assuming typical ranges for soil erosion rates, and assuming sediment delivery ratios typical for small watersheds (Dunne and Leopold, 1978; Morris and Fan, 1997), suggests that agricultural and construction site land uses will be the dominant upland sediment sources (Table 2).

Sediment delivery ratios, the amount of sediment that actually makes it to the Griffin Country Club Lake vs the amount eroded, have accepted ranges from 30% to 10% with 30% typical for watersheds less than a few square miles and 10% typical for watersheds over a few thousand square miles. For Shoal Creek a delivery ratio of 20% for the 2728 acre watershed above the Griffin Country Club Lake would be reasonable.

Table 2. Estimate of sediment eroded and delivered to Griffin Country Club Lake based on land use. Sediment Estimated Portion of Erosion Delivered to Lake Portion of Soil Eroded Land Use Watershed Rate at 20% delivery Watershed (t/y) (ac) (t/ac/y) ratio (percent) (t/y) 0.01 to Forest 25 690 7 to 70 1.4 to 14 0.1 Lawn (any well grassed 40 1070 0.1 to 1 110 to 1100 22 to 220 and undisturbed area) Agricultural (pasture to 25 690 1 to 10 700 to 7000 140 to 1400 row crop) Bare soil (construction 1 28 10 to 100 280 to 2800 56 to 560 sites) Impervious (rooftops and 9 250 0 0 0 pavements) ~1000 to Total 100 2728 - ~200 to 2000 10000

Shoal Creek, Griffin, Georgia 13 July, 2004 Channel Stability Assessment

The total quantities shown in Table 2 are rough estimates and should not be used for engineering designs. However they can be used to indicate the dominant upland sources of sediment.

Three points can be made from the results in Table 2. 1. Areas of bare soil contribute approximately 1/4 of the upland sediment supplied to Shoal Creek yet take up only 1% of the watershed. 2. Areas of agricultural land use may contribute approximately 2/3 of the upland sediment supplied to Shoal Creek. 3. The sediment contributions of areas permanently covered by vegetation (forests, grassed, residential lawns, parks, athletic fields, etc) range from negligible to perhaps 10% of the total.

An estimate of the background sediment loading came out to ~50 to 500 tons of sediment delivered per year to the Griffin Country Club Lake. This is based on the assumption of the watershed being fully built out as residential land (0.1 to 1 t/ac/y erosion rate and 20% delivery ratio).

Without more detailed information on land use it is not possible to make a more specific determination on where in the uplands sediment may be coming from. The aerial photo data set and ground truthing during the stream walk indicate that historically the agricultural lands (Univeristy of Georgia Experiment Station) may have supplied large amounts of sediment to Shoal Creek. However soil erosion Best Management Practices (BMPs) are presently well implemented on land not in use for studies. The upland slopes are either forested or kept in grass (Figure 6) which should limit the amount of sediment eroded to the low end of the 1 to 10 t/ac/y range given in Table 2.

Sediment generated from land under development is more variable due as the amount of land unvegetated changes as projects are started and completed, and due to the differences in the effectiveness of erosion BMPs. For example, Lowes, Super Walmart, and the park-n-ride were all started and completed between the shooting of the air photos in 1999 and the writing of this report in 2004 (Figure 7). Land disturbed by these projects equaled about 2.5% (~70 acres) of the watershed and thus may have been the dominant sources of sediment during that period. Presently no major development projects were noted, however this does not mean none exist. The goal of this project was to investigate stream channel erosion and this section on upland erosion is included because upland erosion processes were noted during the investigation. Minor projects, such as the one at Broad and 17th Streets (Figure 8) and residential yards kept perpetually in bare soil (Figure 9) could potentially be the dominant sediment sources if soil erosion BMPs are poorly implemented.

The two estimates, although crude, indicate that neither channel nor upland sources provide the overwhelming majority of sediment to the system. Also it should be noted that although the individual portions of sediment delivered from each of the channel and upland sources may be difficult to accurately estimate, the total sediment load of Shoal Creek can be accurately measured in the amount of material removed from the sediment trap at the lake’s headwaters.

Shoal Creek, Griffin, Georgia 14 July, 2004 Channel Stability Assessment

Figure 6. Pasture land at the University of Georgia Experiment Station is maintained with a heavy grass cover as a soil erosion BMP (Site 42).

Figure 7. Although poorly vegetated, a topographic depression at the Super Walmart shopping plaza creates sink trapping eroded sediment before it can wash into Shoal Creek (Site 75 ).

Shoal Creek, Griffin, Georgia 15 July, 2004 Channel Stability Assessment

Figure 8. A small scale redevelopment, at Broad and 17th Streets, of a previously developed area creates a potential high soil erosion source (Site 105).

Figure 9. Yards kept in bare soil, such as this one at 17th and Drewery, may be long term contributors of sediment (Site 120).

Shoal Creek, Griffin, Georgia 16 July, 2004 Channel Stability Assessment

References

Tetra Tech, 2004. Watershed assessment. Section 206 ecosystem restoration report for Shoal Creek, Griffin, Georgia.

Dunn, T., and L.B. Leopold, 1978. Water in environmental planning. W.H. Freeman and Company.

Morris, L., and J. Fan, 1997. Reservoir sedimentation handbook. McGraw-Hill.

Shoal Creek, Griffin, Georgia 17 July, 2004 Channel Stability Assessment

Table 3. Main Stem and Headwaters channel stability assessment data.

Shoal Creek, Griffin, Georgia 18 July, 2004 Channel Stability Assessment

Table 4. Tributaries 3 and 4 channel stability assessment data.

Shoal Creek, Griffin, Georgia 19 July, 2004 Channel Stability Assessment

Table 5. Tributaries 5, 6, 7 and 8 channel stability assessment data.

Shoal Creek, Griffin, Georgia 20 July, 2004 Channel Stability Assessment

Table 6. Tributaries 9, 11, 12, 13, 14 channel stability assessment data and upland site data.

Shoal Creek, Griffin, Georgia 21 July, 2004 Channel Stability Assessment

Map 1. Watershed and stream map.

Shoal Creek, Griffin, Georgia 22 July, 2004 Channel Stability Assessment

Map 2. Stream reaches assessd for channel stability.

Shoal Creek, Griffin, Georgia 23 July, 2004 Channel Stability Assessment

Map 3. Channel erosion hot spots.

Shoal Creek, Griffin, Georgia 24 July, 2004