Revegetation of a Detention Basin in Harris County, Texas: Lessons Learned

Transcription

1 Revegetation of a Detention Basin in Harris County, Texas: Lessons Learned Sherri Dunlap Harris County Flood Control District, 9900 Northwest Freeway, Houston, Texas (phone), (fax, sherri.dunlap@hcfcd.org Carolyn White Harris County Flood Control District, 9900 Northwest Freeway, Houston, Texas (phone), (fax), carolyn.white@hcfcd.org Sherri Dunlap is the Manager of Applied Technology and New Products for the Harris County Flood Control District Operations Division. Sherri manages research projects and aids HCFCD project managers and their consultants in the selection and design with appropriate products for erosion and sediment control on Capital Improvement Projects. She reviews and approves plans from consultants and writes new specifications for non-standard products for project manuals. Dr. Dunlap has a BS Chemical Engineering from Drexel University, BS Horticulture from Texas A&M University, MBA in Future Studies from University of Houston Clear Lake, and Doctorate in Engineering (Geotechnical) from Texas A&M University. Carolyn White is a Project Manager for the Harris County Flood Control District Environmental Services Department. She manages projects under the water quality and revegetation programs including stormwater regulatory compliance, water quality monitoring, capital project reforestation and habitat restoration. Projects include: ongoing water quality monitoring, wetland planting plans for water quality enhancement, detention basin layout, and preparation of tree planting plans. Ms. White holds a Master in Landscape Architecture degree from UC Berkeley and a BA in geology from Carleton College. Abstract Post-construction establishment of native vegetative cover on side slopes of a multi-use stormwater multipool detention basin in Harris County, Texas proved to be a challenge for the Harris County Flood Control District (HCFCD). Harris County lies on the humid, sub-tropical, gulf coastal plain and receives 1.22 m (48 inches) of annual rainfall. Created for flood damage reduction and water quality enhancement, the 108 hectare-m (880 acre-foot) capacity detention basin in the Sims Bayou watershed was excavated to a depth of approximately 10.4 m (34 feet) below existing ground elevation. The basin side slopes and central "hill" were created from the excavated basin clay soils. Insufficient quality topsoil from the site s grassland/fallow farmland was available for application to slopes as specified in the contract documents. The basin slopes were, therefore, formed from subsurface clays and silty clays. Dry application seeding of native grasses and forbs, performed over an 8-month period from fall 2005 to spring 2006, was unsuccessful in stabilizing slopes. Tree planting and installation of a subsurface irrigation system produced further unchecked soil disturbances. Sediment filled the backslope drain system concentrating overland flows and causing gullies to form in the basin side slopes. Attempts to repair erosion prior to reseeding activities were impeded in the areas where native trees were planted. After approximately 2 years of repeated site stabilization attempts, actions to stabilize the site and to repair gullies over 3.05 m (10 feet) deep will begin in The site will become a demonstration site for products and techniques not commonly used on HCFCD projects. First, the hill slopes and upland slopes will be stabilized, then the swales drainage function will be restored and the site will be stabilized to further prevent erosion. As enhancements trees, irrigation systems, native grasses and forbs, etc. are added. Timing of activities and communication between HCFCD divisions and careful oversight of the re-vegetation activities at the site are critical to successful establishment of the long-term goal of native vegetative cover on HCFCD detention basins. Keywords and Phrases: erosion and sediment control, detention basin construction sequencing, planting sequence, vegetative soil stabilization. 1

2 1.0 INTRODUCTION As part of its mission to provide flood damage reduction projects, Harris County Flood Control District (HCFCD) develops large scale regional detention basins to mitigate potential impacts from flooding within Harris County. Located adjacent to Sims Bayou in south Houston, Harris County, Texas the Hill at Sims Detention Basin (referred to as HCFCD Unit No. C ) was designed to relieve flood waters from Sims Bayou during storm events (Figure 1). A side weir was constructed to reduce downstream flood hazard on Sims Bayou and maintain a 1% flood within its banks under future anticipated development conditions. Like many basins within the County, C has a permanent pool, which will support naturalized and transplanted emergent wetlands. This basin is unique to Harris County because of its topographic relief and complexity. A large central hill and perimeter hills were created on the site using excavated materials (Figure 2). Since its construction, a combination of factors has contributed to severe erosion, including: sterile soil conditions, unsuccessful attempts to establish native grassland vegetation, revegetation activities disturbing the slopes, disturbances due to tree planting and associated irrigation system installation, inadequate maintenance road-way stabilization and grading, compromised drainage swales, and ATV activity. These effects have contributed to erosion on the upper slopes, filling drainage swales, concentrating flows down the slopes and forming deeply incised gullies at locations throughout the basin. This paper outlines the site s history, its current condition, and lessons learned with regard to site stabilization. Preliminary recommendations for site repairs are also provided. 2.0 BACKGROUND 2.1 Harris County Flood Control District The HCFCD or the District is a special purpose district created by the Texas Legislature in 1937 in response to devastating floods that struck the region in 1929 and The District's jurisdictional boundaries are set to coincide with Harris County, a community of more than 3.7 million people that includes the City of Houston. The other boundaries in which the HCFCD operates those provided by nature - are of the 22 primary watersheds within Harris County's 4,548 km 2 (1,756 square miles). Each has its own flooding problems. The District's drainage and flood control infrastructure is extensive, including more than 1,500 channels totaling about 4,023 km (2,500 miles) in length. Nature also challenges Harris County with flat terrain, clay soils that do not absorb water very well and an average annual rainfall of 1.22 m (48 inches). The flooding problems in the community are severe. Several hundred thousand homes and businesses are in the identified floodplain (not all flooding areas are mapped), and projects to reduce the risk of flooding are estimated in the billions of dollars. Almost all of the man-made and improved channels were built prior to establishing the criteria of the 1% (100-year) flood. The county doesn't flood with normal type rainfall. However, under extreme rainfall conditions when rainfall exceeds several inches per hour for several hours, many areas of the county are susceptible to flooding. After all, flooding is Harris County's natural disaster. Today Harris County "drains" very well, but it still "floods" a lot. So, although the belief early in the last century was that man could control nature (even the District's name implies that control is the goal), the fact is that, reducing risk is possible - eliminating risk is not. What HCFCD does is cope with the natural flooding potential in order to take advantage of everything else this region has to offer. Harris County bayous and waterways are an integral part of the local landscape. Houston is widely known as the Bayou City, and the rest of Harris County is much the same. In many places throughout the county, bayous offer distinctive vistas, whether in their original pristine condition, or sculpted by modernization. Balancing the use of land with its ability to store and convey floodwaters is a 2

3 continual challenge in what is now the nation's third most populous county, encompassing the nation's fourth-largest city. All of the District's capital improvement projects are implemented efficiently, and with appropriate regard for community and natural values. 2.2 Sims Bayou Watershed The Sims Bayou watershed is located in southern Harris County. Most of the 245 km 2 (94 sq miles) watershed is within the City of Houston. However, the upper reach of the watershed drains the City of Missouri City and the lower reach of the watershed drains the cities of South Houston and Pasadena. There are about 195 km (121 miles) of open streams within the watershed, including the primary streams, Sims and Berry Bayous, and tributary channels. The estimated population within the Sims Bayou watershed (Harris County portion) is just over 231,000. Structural flooding has occurred numerous times along Sims Bayou and its tributaries. The majority of the structures that are flood prone were built prior to the existence of detailed floodplain maps and prior to floodplain management regulations. The watershed is almost fully developed, with the exception of the middle reaches around SH 288 where there are large undeveloped areas. Development is expected to continue at a fairly steady pace. The District is currently partnering with the Corps of Engineers on the Sims Bayou Federal Project. The District has also constructed two regional stormwater detention basins in the upper reaches of the watershed that cover a total of about 170 hectares (418 acres). These projects will significantly reduce the risk of flooding along Sims Bayou. There are other sub-regional basins for specific projects that mitigate impacts from new development. 2.3 The Hill at Sims Project The Hill at Sims is a large stormwater detention basin, located adjacent to Sims Bayou near Scott Street and West Orem Drive. It is capable of holding almost 1.2 mega liters (325 million gallons) of excess stormwater, greatly reducing the risk of flooding for area residents. It is a model stormwater detention basin and primary example of how flood control projects can be more than simply functional. Unlike other HCFCD detention basins, this basin features a hill that is almost 18 m (60 feet) tall. The hill feature alone required almost 0.4 mega m 3 (0.5 million cubic yards) of soil, utilized from on-site excavation total detention excavation was more than 1.6 mega m 3 (2.1 million cubic yards) of soil. Standing on top of the hill offers spectacular views of the Downtown Houston skyline. The District hopes to partner with another entity, such as Harris County or the City of Houston, to create park amenities and multi-use features for area residents. 3.0 PHYSICAL SETTING The 0.44-km 2 (108-acre) detention basin was excavated in previously fallow farmland/grasslands with woody shrub and tree growth. Clay excavated from the basin was used on site to create a large central hill and perimeter hills. A secondary hill was created through excavation of the basin (Figure 3). Figure 4 shows two generalized cross sections through the basin, including both the central and secondary hills and the basin side slopes with relation to the perimeter hills and natural ground elevation. Hills ranging from 3-6 m (10-20 feet) above the natural ground elevation were created around the perimeter of the basin at the property line. Slopes on these perimeter hills are in the range of 4.5:1 (h:v). A swale drainage system is located around the entire property perimeter and on the basin side of the hills to collect runoff from the slopes and divert it to the permanent pool. The basin side slopes are typically 4.5:1 (h:v) slopes with a maintenance shelf generally bisecting the m ( foot) slope lengths. The central hill and secondary hill are approximately 22 m (72 feet) and 10 m (32 feet) above the basin s permanent pool, respectively. The central hill was designed with a maximum 3:1 (h:v) slope, flattening to a 4.5:1 (h:v) slope at its base. The drainage swale system is located along the base of the central hill. Native vegetation was retained within an approximate 15-m (50-foot) buffer surrounding the hill. The basin s 4.5:1 (h:v) side slopes are below that buffer zone. The secondary hill was designed as an 3

4 island within the basin. Native vegetation is found at the top of the hill at the natural ground elevation. The hill slopes range from 4.5:1 to 11:1 (h:v). Although the construction contract required topsoil be stockpiled and placed back on the slopes, insufficient amounts of quality topsoil were available for use on this Project. As a result the slopes topsoil is mostly sterile sandy clays and other geologic sediment. 3.1 Soils The basin area stratigraphy varies from clay dominated in the northern portion to more stratified clay-sand in the south. Within the northern portions clays, including fat clay, sandy fat clay, lean clay, lean clay with sand and sandy lean clay were encountered from the ground surface to approximately 4.6 to -3 m (15 to -10 feet). The clay stratum typically terminated between elevations 2.4 to 0.3 m (8 to 1 foot). The clays were underlain by poorly graded sand with silt, poorly graded sand with clay, silty clayey sand, poorly graded sand, and silty sand to the borehole termination depth at approximate elevation -6 to -6.7 m (-20 to-22 feet). In the southern portion the profile generally consisted of a more stratified clay-sand profile. Fat clay, sandy fat clay, and fat clay with sand were encountered from the ground surface to approximate elevation 6.4 to 5.6 m (21 to 18.5 feet). Silty clays, sandy silt, silt with sand, and silty sands were encountered below the clay to approximate elevations 4.3 to 2.4 m (14 to 8 feet). Fat clays were encountered to approximate elevations -4 to -5.2 m (-13 to -17 feet) and were underlain with poorly graded sand and poorly graded sand with silt to the borehole termination depth at approximate elevations of -6 to -6.4 m (-20 to -21 feet). Once the clays were excavated to create the basins and build the hills, the remaining soils were extremely erodible, as evidenced by the quick forming gullies. Mass wasting of gully side slopes is commonly observed at the site. 3.2 Ground Water Ground water levels within the western portion of this tract appear related to Sims Bayou water levels. Ground water in the southeastern portion of this tract was observed in silty sands and silts encountered below approximate elevation 6.7 to 5.6 m (22 and 18.5 feet). This ground water does not appear to be related to Sims Bayou. Wellpoints to lower the ground water elevation were necessary to prevent bottom instability during the detention basin excavation. Active groundwater seeps into the basin are located in the lower portion of its side slopes. 3.3 Swale Drainage System Drainage swales were constructed around the basin and surrounding the central hill. The drainage swale around the central hill is approximately 915 m (3,000 feet) long with three urban interceptor structures. The drainage divide shown in Figure 5 as Area 7 is coincident with the largest gully with the highest erosion rate. Six additional interceptor structures are located within the swale drainage system that drains the remainder of the property, including the perimeter ditches. Spacing of these drains is approximately m (800-1,000 feet) with swale high points spread evenly, per HCFCD design criteria. While these interceptor structures perform well in low-sediment conditions, their capacity was readily compromised in a sediment rich environment. 3.4 Roads Maintenance access to the site is through a driveway off of Scott Street located on the eastern boundary of the basin. A 6-m (20-foot) maintenance road circles the basin at the top of the side slopes. This road is generally adjacent to and downslope of the backslope swales. A secondary maintenance road was cut into the side slopes approximately one-half to two-thirds of the way down. All maintenance roads were roughly graded from on-site soils. No roadside drainage or cross drainage was provided and no additional material was added as fill or stabilizer. 4

5 4.0 TIMELINE OF ACTIVITIES 4.1 Site Construction Construction at the site was generally complete in fall As was previously discussed, the basin was excavated into geologic subsurface soils; topsoil had not been retained at the site for postconstruction application on the slopes. Subsequent activities relating to site revegetation are outlined in Figure 6 and discussed below. 4.2 Turf Establishment Post construction HCFCD Infrastructure Facilities Maintenance Department proceeded to establish turf and plant trees at the basin. Dry application seeding began in October 2005 and continued through February 2006 on portions of the site. Native grasses and wildflowers were seeded in prairie habitat zones specified in the planting plan at a rate of 45 kg per hectare (40 lbs per acre). Methods used in dry seeding include spreading of fertilizer, incorporation of fertilizer by disking, raking and/or harrowing the seedbed, seed distribution, rolling with culti-packer, straw mulching, and securing with guar gum tackifier. Concurrent with initial dry seeding activities in January and February 2006, erosion repairs (disking the rills) were made on the central hill. Approximately 4 hectares (10 acres) of repaired slopes were overseeded. Overseeding methods include mowing the site, lightly disking or harrowing soil surface to no more than 2.5 cm (1 inch) deep, seeding, and culti-packing or lightly raking area to cover seed. Over 19,000 m 2 (16,000 sq yards) of block sod was applied to select areas to help control erosion. Tree planting activities were also concurrent with dry seeding and overseeding in January and February More than 9,500 trees were planted over approximately 0.2 km 2 (45 acres) at light to moderate density of trees per hectare (100 to 200 trees per acre). The planting plan provided for variable habitats based on elevation and aspect. Trees were planted in rows and evenly spaced in the habitat areas to allow continued mowing between each tree. By April 2006, maintenance and revegetation activities at the site were in a cycle of dry seeding, overseeding, and mowing. These activities continued into August 2006 when additional erosion repairs were made at select locations. In August/September 2006 a subsurface irrigation system was installed to water the transplanted trees. Trenching was done both parallel and perpendicular to the slopes to install irrigation pipes and control valves. In an attempt to further stabilize the site, due to slow germination and low success of native seed establishment, common Bermudagrass was seeded on the slopes in September The site was added to regular HCFCD cyclical mowing schedule after the Bermudagrass was established in November Mowing is done three times during the growing season (April October). HCFCD mowing specifications stipulate that grass on sideslopes and maintenance berms is cut at 15 cm (6 inches). Mowing of sideslopes and maintenance berms is accomplished using a combination of gang mowers and hand work. Gang mowers are generally used between rows of trees, parallel to the waterline of the permanent pool. Hand work using gas powered weed eaters is generally done between individual trees. Gang mowers are attached to tractors and typically cut swaths 4.5 m (15 feet) wide. The hydraulic batwing mower has the ability to modulate width by raising one or both sides during cutting. Tractors are set to cut at the specified 15 cm (6 in) height, however on uneven surfaces the blades can often cut longer or shorter. There is a potential for significant disturbance of surface soils as a result of mowing. At times mowing is done under wet conditions increasing potential for heavy tractors to pump water in the sandy soils, lessening the soil strength as water rises in the soil profile. This leads to extensive rutting. 4.3 Gully Formation Challenges in establishing vegetative cover in the sterile soils resulted in rilling across most of the upper slopes above the swale drainage system. This erosion occurred on the central hill and on the perimeter hills (Figures 7 and 8). Soil lost from these areas filled the swale drainage system and clogged the interceptor structures. With the drainage system compromised, concentrated overbank flows led to formation of substantial gullies in certain areas. The major gullies are coincident with 1) compromised 5

6 drainage swales, 2) overwhelmed drainage divides within the swales, and 3) the gap in the drainage swale system. As shown in Figure 8, the large gully forming on the left is just downhill from an interceptor structure that is covered with approximately 0.6 m (2 feet) of sediment. The entire drainage swale in that reach is full of sediment. The pair of gullies shown on the right of the photo is directly below the area where no backslope drain was installed. This gap in the drainage swale that circles the central hill concentrates overland flow and places stress on the lower side slopes of the basin. Down cutting and head cutting of these gullies has occurred at an accelerated rate. Presently these gullies have coalesced into one large feature as shown in the 7 month time series of photos in Figure 9. Four gullies were measured from June 2007 to August 2007 (see Figure 5). Total displaced soil from the largest gully (D) increased from 270 m 3 (355 cubic yards) on June 25 to 400 m 3 (518 cubic yards) on August 3. Houston experienced high precipitation rates over that time period with 32.4 cm (12.75 inches) recorded at the site. Maximum erosion rate of 3.85 m 3 /cm (12.78 cubic yards/inch) of rainfall occurred at Gully D over that time. 5.0 LESSONS LEARNED Both physical and institutional factors have contributed to the severe erosion problems at C Physical factors include: Insufficient quality top soil available with no soil biology rehabilitation included in revegetation efforts; Lack of stabilization for maintenance roads; Unsuccessful repeated seeding with native grasses and wildflowers; Repeated soil disturbance during tree planting, irrigation system installation, and mowing; Unabated erosion filled drainage swales; Swale drainage divides overwhelmed allowing concentration of overland flow. Institutional factors included: Misunderstanding or lack of appreciation for geotechnical report that anticipated severe erosion once the overlying clay was removed; report discussed the fine, non-cohesive soils below were highly erodible. Erosion expert not fully integrated into Design team or Plan review; this oversight has been corrected by involving more in-house experts in Design and Plan review and by providing more complete design information to Plan reviewers. No provisions were made to quickly assess and stabilize the site during construction. Failed to understand and accommodate the destabilizing effects of irrigation system installation and mitigate those effects. Soil displacement resulting from long-term site instability during attempts to establish native vegetation was unmitigated. Turf Establishment activities did not include cross-linked polymer soil stabilizer as specified in HCFCD Standard specifications. 6.0 PRELIMINARY RECOMMENDATIONS A number of recommendations for stabilizing and repairing erosion at C are possible. At this time, HCFCD has considered the following. 6.1 Project Team Create new CIP project multidisciplinary team including appropriate external experts to assess site and develop plan to restore the site and achieve project s original goals. 6.2 Assess Change in Basin Capacity Very large quantities of sediment have flowed into the site s detention basin. A priority task is to determine the potential loss of storage capacity. Any sediment removal activities need to occur before any other site stabilization work. All sediment removed should be disposed of off-site, per regulations. Use aerial photo documentation to establish condition of site prior to restoration will assist in future monitoring. 6

7 6.3 Stabilization of Hill Work should proceed by stabilizing the highest areas elevations first and working down slope. Regrading of the perimeter hills, central hill and portions of the side slopes may be necessary to remove rills. Increase vegetation density by enhancing soil biology and overseeding with Bermudagrass. Provide winter cover vegetation, such as annual ryegrass, clovers, or other legumes. Mow ryegrass in spring to allow Bermudagrass to germinate and grow. All areas of highly erodible soils should be stabilized by using cross-linked polymer application rate to last 6 months added to hydromulch seeding application. Areas may require augmentation of soil stabilizer with bonded fiber matrix products and erosion control blankets. Repeated applications or applications of different products may be necessary to address all the problems in these areas. Establishment of health biology will also help bind soils and mitigate erosion. 6.5 Stabilize Roads Regrade and stabilize roads with surfacing materials and/or heavy application of cross-linked polymer soil stabilizer or repeated light applications. Provide level spreaders to prevent concentrated flow down the hill. Discourage ATV usage by limiting access to site, barbed-wire fencing, no trespassing signs and increased police patrols. 6.4 Fix Drainage System Clear out existing drainage swales. Re-evaluate type, location, and number of swale interceptor inlets and modify as required. Design new drainage system and regrade the drainage swales to ensure proper function. Once complete sod all swales with Bermudagrass. Ensure drainage swale system works as designed before proceeding to repair downslope gullies formed by concentrated flow. Add double sod strip rows, as needed, to diminish runoff velocity downslope of swale drainage system. 6.5 Gully Repair As the gullies have become so deeply incised and wide, no simple, cheap fix will suffice. Once the concentrated flows upslope have been diverted, remove transplanted trees and irrigation system near gullies and excavate to common depth to establish level working floor. Gullies can be either reconstructed to grade using imported fill or side slopes can be laid back as a swale and stabilized. 6.6 Revegetation/Reforestation Complete intended planting plan by increasing density of tree areas to eliminate mowing within mulched reforestation zones. For areas of sparse tree planting, only after the site has achieved 75-80% vegetative cover with no active rills or gully formation, can the irrigation system and trees be restored. All areas disturbed by the irrigation system repairs must be immediately sodded to prevent severe erosion from compromising the site. If irrigation system is not functional during gully repairs, water trucks will be necessary to irrigate trees during summer months. 6.7 Operations and Maintenance Create special oversight team to frequently monitor especially fragile or high visibility sites. Establish annually-funded Capital Improvement Program (CIP) to address problems on this site with nonstandard solutions. Utilized CIP on-call contracts to supplement and quickly implement activities not currently in HCFCD Maintenance contracts. 7

8 Figure 1. Location Map Figure 2. Aerial Photo of Hill at Sims Detention Basin 8

9 Figure 3. Topographic Map Figure 4. Generalized Cross Sections 9

10 Figure 5. Backslope Drainage Swale System ID 1 Construction Task Name Q4 05 Q1 06 Q2 06 Q3 06 Q4 06 Q1 07 Q2 07 Q3 07 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep 2 Turf Establishment Native Seeding 3 Tree Planting 4 Repair and Overseeding 5 Irrigation Installation 6 Turf Establishment Bermudagrass 7 HCFCD Cyclical Mowing Figure 6. General Timeline of Activities 10

11 Figure 7. Photograph of erosion on the Central Hill looking west from eastern side of the basin Figure 8. Photograph of erosion on the perimeter hill and side slopes, looking west from southeastern corner of the basin 11

12 Photo 1. Photo 2. Photo 3. Figure 9. Time Series photographs of Gully D, as labeled on Figure 5. Photo 1 from 2/20/07, Photo 2 5/23/07, Photo 3 9/28/07 (7 months later). 12