DIAMONDHEAD LAKE RURAL IMPROVEMENT ZONE ENGINEERING REPORT

Transcription

1 H R G R E E N. C O M DIAMONDHEAD LAKE RURAL IMPROVEMENT ZONE ENGINEERING REPORT June 2, 2017 HR Green Project No: Prepared For: Diamondhead Lake Rural Improvement Zone I hereby certify that this engineering document was prepared by me or under my direct personal supervision and that I am a duly licensed Professional Engineer under the laws of the State of Iowa. CHAD MASON, P.E. License No My renewal date is December 31, 2017 Date: June 2, 2017 Pages or sheets covered by this seal: Entire document

2 Diamondhead Lake- Engineering Report Table of Contents Introduction...4 Rural Improvement Zone Questions (IA HF 615) Surface Area of Diamondhead Lake and 3. Contributing Watershed Acres and Characteristics Watershed Land Use Characteristics Historical Annual Silt Accumulation Present Silt Accumulation Anticipated Silt Accumulation Estimate of Storage Space Available for Dredged Materials Estimated Storage Space Required for Dredge Materials Current Water Quality Assessment Assessment of the Current Need for Preventative Practices to Improve Water Quality Impact of Preventative Practices Cost Estimate for Erosion Control and Water Quality Recommendations Appendix TABLES Table 1. Diamondhead Lake watershed soils....4 Table 2. Diamondhead Lake watershed land use summary....5 Table 3. Estimated sediment within Diamondhead Lake....7 Table 4. Preventative practice recommendations and cost estimates Table 5. Dredge management recommendations and cost estimates Table 6. Appendix. NRCS best management practices to address watershed sources of sediment to Diamondhead Lake Table 7. Appendix. NRCS best management practices to address stream channel sources of sediment to Diamondhead Lake Table 8. Appendix. Costs of various NRCS practices (adapted from the Red Lake River One Watershed, One Plan, Red Lake River Planning Partnership) FIGURES Figure 1. Historic and projected enrolment of lands within CRP (USDA Agricultural Projections to 2026, USDA)...8 Figure 2. Various crop projections in the United States (USDA Agricultural Projections to 2026, USDA)....8 Figure 3. Appendix. Diamondhead Lake sub-basins and main tributaries Figure 4. Appendix. Diamondhead Lake streams and gullies Figure 5. Appendix. Diamondhead Lake potential upland source contributions to sedimentation Figure 6. Appendix. Gully and streambank erosion locations (IDNR) Figure 7. Appendix. Watershed sediment basin locations (IDNR) Figure 8. Appendix. Pre-project estimated sediment delivery (IDNR) Table of Contents

3 Figure 9. Appendix. Dredging site locations and lake perimeter sediment basins Figure 10. Appendix. North Diamondhead Lake sediment thickness Figure 11. Appendix. South Diamondhead Lake sediment thickness Figure 12. Appendix. Sediment basin 1 sediment thickness Figure 13. Appendix. Sediment basin 2 sediment thickness Figure 14. Appendix. Sediment basin 3 sediment thickness Figure 15. Appendix. Sediment basin 4 sediment thickness Figure 16. Appendix. Sediment basin 5 sediment thickness Figure 17. Appendix. Sediment basin 6 sediment thickness Figure 18. Appendix. Horseshoe Bend dredging area.-appendix. Horseshoe Bend dredge area Figure 19. Appendix. Horseshoe Bend sedimentation basin Figure 20. Appendix. Long Branch Creek basin, dredging area and dredge material storage area Table of Contents

4 Introduction HR Green was hired to conduct an engineering study of Diamondhead Lake near Dexter, Iowa (Guthrie County). Diamondhead Lake is located within a Rural Improvement Zone (RIZ), which is under consideration for renewal. This engineering report contains information that addresses the 13 questions set forth in Sec. 4, Section 357H.2, subsection 1, paragraph a, Code 2015 of the Iowa Administrative Code (IAC) as amended by Iowa House File 615, An Act Relating to the Establishment, Operation, and Dissolution of Rural Improvement Zones. HR Green previously completed the Diamondhead Lake Master Plan (July 2007; hereafter referred to as the 2007 Plan). The 2007 Plan provided a detailed analysis of the watershed, including results of an Iowa Department of Natural Resources (IDNR) watershed model and a watershed review by the Natural Resources Conservation Service (NRCS), the Guthrie County Soil and Water Conservation District (GSWCD) and the Diamondhead Lake Rural Improvement Zone board (DLRIZ). The IDNR watershed model provided estimates of annual sediment deposition within the Lake. The NRCS-GSWCD-DLRIZ review identified cost share programs, such as Environmental Quality Incentive Program (EQIP), to treat pastureland erosion problems. The 2007 Plan made recommendations for measures to improve drainage and reduce sediment delivery to the lake including specific sites for construction of sediment ponds throughout the watershed. This engineering report updates and expands upon the 2007 Plan related to the watershed and lake. This report addresses watershed characteristics and land use implications for runoff and sediment sources and transport. This report evaluates past, present and expected sediment accumulation within Diamondhead Lake, and estimates storage space requirements for dredged materials. This report discusses water quality implications of sedimentation and describes watershed, channel and in-lake management strategies and their effects. Finally, an estimate of implementation costs is presented. Rural Improvement Zone Questions (IA HF 615) 1. Surface Area of Diamondhead Lake Diamondhead Lake is a man-made earthen embankment lake with a surface area of approximately 105 acres (Figure 3). 2. and 3. Contributing Watershed Acres and Characteristics 8700 acres drain to Diamondhead Lake Soils in the watershed are summarized in the following table Table 1. Diamondhead Lake watershed soils. Hydrologic Soil Group Acres Percentage of Area A % C 7, % D % Page 4

5 Many physical attributes of Diamondhead Lake s watershed contribute to high runoff volume and rate, as well as sediment transport to the Lake. The contributing watershed draining to Diamondhead Lake is 8,700 acres with over 220 vertical feet of fall in the five miles of the main tributary, Long Branch Creek. Long Branch Creek contributes approximately 8000 acres of the 8700 acre watershed. The remainder of the watershed drains to the west side of the lake via Horseshoe Bend. The watershed s rolling hills and steep topography, paired with predominantly agricultural land use (see Watershed Land Use Characteristics ), likely increases the sediment source-transport process. The gently- to steeply-rolling topography produces many smaller streams and gullies into the landscape that are susceptible to erosion (Figure 4). Watersheds that are long and narrow hasten the transport of sediment from field to stream. Many of the subwatersheds within Diamondhead Lake s watershed exhibit this morphology. Diamondhead Lake s watershed-to-lake ratio is 83:1. This large contributing drainage area transports a disproportional amount of runoff and sediment relative to the size of the lake. Generally watershed-to-lake ratios ranging from 10:1 to 30:1 are preferred for lake planning and design. Soil types also affect runoff rates and volumes due to their varying capacity to retain rainfall. Sandier soils typically absorb a greater proportion of rainfall before discharging runoff to nearby ditches and streams. Diamondhead Lake s watershed is predominantly comprised of siltier soils (Type C) with lower capacity for infiltrating rainfall. These soils are more likely to convert precipitation to surface runoff at a higher volume and rate. Stream channels adjust to their watershed s hydrology and sediment supply. As a watershed is converted from natural conditions, its hydrology and sediment supply are altered. Conversion to agriculture and urban systems seen within this watershed have increased runoff rates and volumes as well as sediment supply, driving stream channel shifts in position, depth and, ultimately, stream-sourced sediment to the Lake. This is evidenced through observations of bank erosion along Long Branch Creek. 4. Watershed Land Use Characteristics Land uses within the Diamondhead Lake watershed are summarized in the table below. Table 2. Diamondhead Lake watershed land use summary. Land Use Acres Percent Barren % Developed/Low Intensity % Developed/Medium Intensity % Pasture/Hay % Row Crops % Woodland/Shrubland % Land use greatly affects runoff rate and volume as well as sediment supply to conveyance channels in the Diamondhead Lake watershed (Figure 5). A review of current aerial imagery suggests the Diamondhead Lake watershed land use is predominantly agricultural in nature (Table 2). The watershed s agricultural lands are a mixture of pasture and cropland rotations of corn and soybeans. At the upper southwest portion of the watershed is the community of Stuart with a high concentration of impervious area. Impervious surfaces nearly completely convert rainfall and snowmelt to runoff that is conveyed via storm sewer networks to streams and, along with it, sediment that has built up between precipitation Page 5

6 events. The majority of the town s storm sewer runoff, pervious areas (open spaces) and the community s wastewater treatment plant discharge drains to Long Branch Creek and then to Diamondhead Lake. The watershed s land use is dominated by agriculture (68%). Conversion of natural land cover to pasture and row crops produces more erodible conditions in the watershed. However, implementation of best management practices (BMP) within and alongside these land uses has partially mitigated this effect. The 2007 Plan identified fields enrolled within the Conservation Reserve Program (CRP) and suggests adoption of additional BMPs (terracing and grassed waterways) to address sheet and rill erosion. Sheet erosion is the removal of a thin uniform layer of soil from raindrops or local water runoff. Sheet erosion can occur in any exposed soil condition. As runoff flows across a field, within a short distance it is concentrated into small channel flow conditions that lead to rill formation and erosion. As several rills merge and associated runoff from and greater contributing area increases, more pronounced channelization occurs, increasing the erosive potential. Deeply cut land surfaces occur producing heavy soil loss. Channelized flow and increased sediment export is conveyed to streams that are unable to maintain stability and bank erosion occurs. Specific locations along the Long Branch Creek corridor show signs of channel erosion. Figure 6 outlines individual gully and stream bank erosion areas needing land use/bmp treatment that can mitigate for this effect. Treatments for channel cutting and bank erosion are site specific, but the general principle is to reduce channel velocities and provide toe protection to vertical bank areas. The watershed contains approximately 80 small ponds and impoundments that are effective in trapping coarse sediment (Figure 7). These ponds also assist in restoring the watershed hydrology that formed the channel geometry of streams in this watershed, though it is unclear to what extent they have been designed to do so. In some cases, in similar watersheds, over-use of detention ponds has led to reduced channel flows and sedimentation of stream beds which can similarly stress banks and lead to bank erosion and flooding. Currently, no known detailed fluvial geomorphic and hydrologic and hydraulic analyses have been performed for this system and, as such, no definitive conclusions can be made in this regard. 5. Historical Annual Silt Accumulation Annual sediment loads were modeled by the IDNR at approximately 3,500 tons. This would equate to 70,000 tons of sediment deposition over the past 20 years ( ). The following analysis validates this estimate. The annual silt accumulation for the lake during the 20 years immediately preceding this report ( ) was estimated within the 2007 Plan. The average annual sediment load was estimated at approximately 3,500 tons per year during the 41 year life of the lake. The IDNR analysis reported that watershed-sources, including 2006 BMPs in place, yielded average annual sediment delivery rates of 0.41 tons/acre/year. The IDNR estimate includes runoff from the entire watershed, including nonagricultural areas (Figure 8). Several tributaries fed by these lands empty to constructed sedimentation basins before emptying to the lake, though it is unclear if the IDNR modeling has taken these into account in relation to treatment efficacy (Figure 9). Recent conversations with NRCS specialists suggest that similar agricultural land cover can yield between 3 to 7 tons per acre of sediment each year (based on Universal Soil Loss Equation methodology validated by field investigation and calibration). Page 6

7 A relative check was made of the IDNR model using the NRCS relative estimates as well as through use of a simple urban stormwater model (P8 Urban Catchment Model) and published delivery ratios. The expected range of un-managed sediment export from the field to a conveyance channel as expected from NRCS findings for similar watersheds in the region is 17,091 39,879 tons per year. The P8 Urban Catchment model predicts approximately 47 tons of unmanaged annual sediment export (0.05 tons/acre/yr). For this rapid check, no estimate of sediment export from woodland or stream banks was made. Once sediment is exported to a conveyance channel (ditch or stream channel) it is moved downstream. The further the distance of the channel from source to receiving water body, the more opportunities exist for a portion of the sediment load to be removed through assimilation into pits within the channel and floodplain. Published sediment delivery ratios suggest such abstractions as coefficients applied to field export estimates relative to watershed size. Using an average SDR of 0.16, the total potential sediment yield to the Lake is estimated at 2,735 6,380 tons per year ( tons/acre/yr) from agricultural and developed areas without BMPs. The IDNR estimate of 3,500 tons of sediment per year fits within this range. Considering that there are 80 sedimentation ponds, CRP and other field BMPs in place within the watershed, it is likely that the actual delivery of sediment to the Lake is most likely close to IDNR estimate and very reasonable. The IDNR took into account these practices within their watershed model (1996). A review of aerial photography from suggests only minor changes in land use within the Diamondhead Lake watershed. No apparent conversion of forest lands to agriculture were detected and urbanization of agricultural lands were insignificant. Though the 1997 Plan included maps of CRP acres, current NRCS policy prohibited inclusion of such imagery for this report. However, discussions with NRCS and Farm Service Agency (FSA) suggest the following: Current CRP o o Approximately a 5.5% increase in CRP lands in the lands immediately surrounding the Lake. In 2007, there were approximately 215 acres in CRP and as of 5/25/2017 there are 227 acres in CRP. Approximately a 13% decrease in CRP in the greater watershed. There were approximately 508 acres in CRP in 2007 and there is approximately 440 acres in CRP as of 5/25/2017. Tile density o Increased commodity prices from 2008 to 2013 may have led to an increase in drain tiling in the watershed. However, no data are currently available to confirm this within the Diamondhead Lake watershed. 6. Present Silt Accumulation Current silt amounts within the Lake measured in 2014 (Figure Appendix; Table 3). Table 3. Estimated sediment within Diamondhead Lake. Location Sediment Max Depth (ft) Avg Depth (ft) Volume (ac-ft) Acres North Diamondhead Lake Area South Diamondhead Lake Area Totals Page 7

8 7. Anticipated Silt Accumulation It is estimated that future average annual silt accumulation rates will remain similar to present-day rates (i.e., 3,500 tons per year). Future silt accumulation will depend greatly on changes in the applicability of new land being suitable for various BMPs and programs, adoption of BMPs throughout the watershed and land cover conversion from natural to agricultural or developed land uses. Given this uncertainty, it is expected that the annual future sediment loading to the lake will average 3,500 tons per year, or 70,000 tons over the next 20 years. There is limited land suitable for additional CRP opportunities remaining within the Diamondhead Lake. Some lands are limited or unable to be enrolled in the program. Commodity prices are driven by local, federal and international trends and influence land use choices in agricultural landscapes. National participation in the CRP program has fluctuated through time and is expected to remain constant through 2026 (Figure 1). Figure 1. Historic and projected enrolment of lands within CRP (USDA Agricultural Projections to 2026, USDA). Commodity pricing has declined in recent years, though projected real prices are expected to remain above the average levels seen in the decade prior to 2017 (Figure 2). This is in part due to global economic growth, which helps to bolster crop demand (USDA Agricultural Projections to 2026, USDA). An overall declining trend in planted acreage is expected, nationally, though specific crops vary. Figure 2. Various crop projections in the United States (USDA Agricultural Projections to 2026, USDA). Page 8

9 The USDA data suggests the possibility of either a future slight decline in cropped acreage with potential increase in planted soybeans over corn and/or wheat if the watershed follows national trends. Soybean and corn fields generate higher rates of runoff and erosion that small grains given their rowed nature, limited cover and shallow root systems when compared to small grains. Opportunities still exist for in-field and edge of field BMPs, however, it is extremely difficult to predict future enrolment in EQIP trends, though the CRP data can serve as a surrogate. Given the historic and expected CRP enrolment potential within the Diamondhead Lake watershed and national trends in production, the watershed is likely to remain close to its current sediment export estimates. 8. Estimate of Storage Space Available for Dredged Materials Currently, dredged materials are stored on the south side of Diamondhead Lake (Figure 9).The main storage site has approximately 26.2 ac-ft of storage available below the average berm height. An additional ac-ft of piling up to 20 feet in height is likely available at this location. 9. Estimated Storage Space Required for Dredge Materials A total of ac-ft of sediment is reported to exist within the Lake as of 2014 (Table 3). Once the current site is filled, a remaining ac-ft of dredge material in addition to annual watershed inputs remain (30 ac-ft over 20 years). Therefore, in order to restore the original bathymetry of the Lake during the next 20 years, a total of approximately 300 ac-ft of storage is required assuming the current dredge management/storage area remains filled. 10. Current Water Quality Assessment To-date, no known data exist to describe water quality trends within Diamondhead Lake. From a qualitative standpoint, the lake s primary quality concerns likely include algal blooms, periodic ebbs in dissolved oxygen content, and relatively high turbidity. To make any robust interpretation of Diamondhead Lakes past, current or future water quality, a robust monitoring effort would be required. In lake assessment of water quality, and for estimating total maximum daily loads, clarity, total phosphorus, total nitrogen and additional parameters are usually monitored during the extent of the growing season for two to three years by first sampling temperature at one-meter intervals from the lake s surface to its bottom to identify the location of the lakes water quality transition from the upper warm later and its cooler bottom layer. Several water quality samples are then taken from each layer to assist in assessing the lake s water quality. This is repeated at least monthly during the growing season for a minimum of two years to adequately describe water quality and to ensure that false interpretations from fewer, random samples (i.e., opportunistic grab samples are insufficient in determining a lake s water quality or trend). State and Federal thresholds for impairment are then referenced to designate a lake as impaired or unimpaired for either recreational uses or aquatic life, generally. Diamondhead Lake is a private, artificial lake system that is not currently monitored by the IDNR, nor has any record of water quality sampling and analysis been found. Though no defensible, quantitative description of the Lakes water quality can be made at this time, some qualitative statements can be made. The Lake s watershed to lake area ratio is dramatically disproportionate and it is expected that the long-term transport of excessive sediments and nutrients through time. Land use conversion to agriculture across the majority of watershed has also likely accelerated this process. Phosphorus is carried from the watershed by sediment. Once in the Lake, it is only removed by dredging or filtration as phosphorus has no gas phase which would otherwise provide a means of partial losses of total in-lake phosphorus to the atmosphere. As such, the combination of legacy and current sediment and phosphorus watershed supply (termed external loading ) have very likely led Page 9

10 to what is termed internal loading events: releases of lake bottom sediment-bound phosphorus to the water column. These somewhat regular pulses of nutrients fuel algal blooms, decreases in dissolved oxygen and poor water clarity. 11. Assessment of the Current Need for Preventative Practices to Improve Water Quality There is significant need for erosion-preventative practices within the Diamondhead Lake watershed to supplement existing sedimentation basins and improve water quality within the Lake. Though the balance of CRP enrollment is somewhat stable, little opportunity exists for additional CRP acreage. CRP acreage alone will not meet the demands for improved water quality in this large watershed. Existing agricultural ponds and lake inlet sedimentation basins also are likely overtaxed. Infield, edge of field and in-channel practices would supplement existing practices by treating the sources of sediment and associated nutrients that lead to poor water quality. Preventative practices to reduce sedimentation and improve water quality in Diamondhead Lake may be implemented directly within the lake, within the contributing watershed, or within the contributing stream channel. Though there is a slight decline in participation within the CRP, locally and nationally, there are existing and future practices that can reduce sediment transport to Diamondhead Lake. There are approximately 80 ponds that have been constructed in the watershed for livestock water supply and for recreational value (Figure 3). There are also seven sedimentation basins located around the perimeter of the Lake designed to intercept sediments being conveyed from the streams before entering the Lake (Figure 9). Guthrie County, led by the local NRCS office and the Guthrie County Soil and Water Conservation District, participates in EQIP. EQIP is a voluntary conservation program of the NRCS that promotes agricultural production and environmental quality. The program is available to farmers and offers financial and technical assistance to install or implement structural and management practices on eligible agricultural land. This type of financial assistance and technical assistance for cropped and pastureland treatment can directly aid in deriving solutions for limiting livestock access to stream banks. Watershed erosion control practices involve the cooperation of the landowner and ongoing discussions with landowners for conservation improvements are needed. Continued work with the GSWCD and the NRCS will be beneficial in identifying funding sources and sponsorship criteria. Additional measures recommended for grazing lands are to fence off access to channel areas by cattle. Crossings and access to water should be limited so as to reduce bank erosion and loss of vegetation from eroded, vertical bank areas. Generally, where access is desirable for animal or vehicle crossings, the banks are sloped back and the crossing is layered with crushed stone. Immediate upstream and downstream access to the channel is limited by fencing. For watering needs, pools are created adjacent to the rocked crossing, or by pooling water over the rock crossing. Within the Contributing Watershed There are many commonly used NRCS Practices that may be applied to the watershed to limit sediment export and mediate for altered hydrology to limit sedimentation within Diamondhead Lake (Table 6- Appendix). Implementation locations and sizes of these practices vary primarily related to contributing drainage area, hillslopes, existing infrastructure and public participation. The IDNR identified source areas Page 10

11 as well as potential locations for a few of the following practices, though mainly focused on grass waterways. Within the Stream Channel Similar to watershed best management practices, there are many commonly used NRCS Practices that may be applied to the stream channels upland of the Lake to limit sediment export to limit sedimentation within Diamondhead Lake (Table 7-Appendix). Implementation locations and sizes of these practices vary primarily related to contributing stream belt width limitations, streamflow access to the floodplain, drainage area, channel slope, bad and bank mineral composition, existing infrastructure and public participation. 12. Impact of Preventative Practices Preventative practices within the Lake, watershed and channels will reduce sediment deposition with the Lake, improve water quality, improve habitat and improve recreational use quality. No comprehensive quantification of the effects versus costs of potential implementation strategies has been made to date as it will require systemic modeling beyond the scope of this report. Watershed and channel BMP implementation (e.g., CRP, EQIP BMPs) can significantly reduce the total load of sediment delivered to Diamondhead Lake. Though the previous IDNR watershed model and report identified critical sources areas of sediment export as well as bank erosion risk locations, to-date no iterative scenario modeling to precisely target, measure and prioritize implementation strategies and their effects has been made. However, general statements can be made as to the effects of BMPs on sediment transport to the Lake. Within Diamondhead Lake Diamondhead Lake has been the recipient of increased watershed and channel sediments and nutrients, as well as herbicides and pesticides, since the initiation of land development in the watershed. Both agricultural and urbanized land uses have contributes in various ways, though there is a much more significant contribution of these originating from the historical and current agricultural landscapes (see Watershed Land Use Characteristics, Historical Annual Silt Accumulation and Present Silt Accumulation). The dramatic disparity between the size of the watershed and the lake area have teamed with land uses to result in dramatic siltation of the lake. Siltation of the Lake has several detrimental effects including reduced water depths, over-nutrification, decreased water clarity and quality, losses to macroinvertebrate (i.e., beneficial insects) and fish habitats and, ultimately, impacts to recreational opportunities and overall aesthetics. Dredging of sediments has been employed within Diamondhead Lake to help mitigate for these resulting effects. Dredging not only helps restore the original bathymetric shape (i.e., depth contours) of the lake, but it also harvests phosphorus and other pollutants that drive water quality and lake clarity. Dredging can have temporary negative effects as it is disruptive to easily mobilized silt and other sediments and nutrients. Given the coarse nature of the machinery required to dredge lake bottoms, dredging is not highly precise and can disrupt or impact existing habitat. Care needs to be taken to prevent this. Gauging volume and depth removal requirements for specific locations, as well as confirming successful removal targets, is also challenging. The use of modern sonar technology can greatly improve dredge management planning and implementation compliance, thereby reducing risks; both environmental and financial. Page 11

12 Dredging temporarily stirs loose sediments and impacts submerged vegetation, which often leads to decreased water clarity and quality. It is not uncommon for a decline in beneficial submerged aquatic vegetation. Submerged aquatic vegetation serves critical functions for water quality and habitat. Submerged vegetation root structure holds sediment-bound nutrients in place. When it is removed, sediments are easily suspended in the water column during wind events leading to algal blooms. Increased sediment suspension and algal blooms limit light penetration further inhibiting plant growth. Submerged aquatic vegetation provides oxygen to the water column as a byproduct of photosynthesis. Oxygen is required by fish to proliferate as well as to allow iron found in the lake to successfully bind organically-available phosphorus which would otherwise be used by algae to bloom at un-natural rates. Submerged aquatic vegetation also provides critical habitat for macroinvertebrates to filter algae from the water as well as for fish cover. A healthy community of submerged, native aquatic vegetation maintains water quality from the bottom-up of the lake ecosystem though several pathways from nutrient management to fish community support. Within the Contributing Watershed and Channels Watershed BMPs are designed to slow water and stabilize soils. Slowing water runoff from the landscape reduces its erosive power on exposed soils as well as provides time for suspended sediments to fall out of flows into pits or pools within the landscape before entering the stream channel. This is accomplished through various functions such as rainfall interception via cover, infiltration via organic-rich, un-compacted and densely rooted topsoils, filtration of sheet, rill and small channel flows via high plant stem density and prolonged ponding to allow sediment to fall from suspension. BMPs are best used in combination to provide a treatment train effect whose sum is greater than its parts in terms of total sediment loss reduction. Various in-field and edge-of-field practices (see Preventative Practices Within the Contributing Watershed) have many benefits to not only Diamondhead Lake but to crop production and investments in the fields. For instance, nutrient management plans identify strategies to focus nutrient supplementation to specific, low-risk time periods, optimize application rates, and identify optimal locations for manure storage and application timing. No-till practices increase surface roughness during bare soil conditions to limit runoff and field sediment and nutrient losses. Cool season cover crops provide rain interception and root structure to limit and filter runoff and erosion. Inter-rill nurse crops provide this same function for corn and soybean fields and can improve soil fertility. Vegetated swales provide soil stabilization and runoff attenuation through interception of rainfall, filtration of runoff and soil stabilization. Terracing and contour farming reduces slope and retain localized runoff and soil losses. Waters and sediment control basins can detain runoff allowing for sediment containment and reductions in bank erosion. Vegetated buffer strip along ditches and streams filter runoff of sediment and assist in reducing bank erosion and land losses. Saturated buffer strips also serve this function but with added nutrient and chemical capturing capacity. Cattle crossings and barriers along streams, paired with off-channel watering/detention ponds limits significant impacts to channel banks and subsequent sediment export and land losses. Conservation subsurface drainage systems can be designed to optimize soil moisture and nutrient losses to ditches and streams. When thoughtfully targeted, designed and managed, the various options for in-field and edge-offield BMPs can serve to reduce soil, nutrient and soil moisture in the fields as well as significantly reduce sediment transport to Diamondhead Lake. Page 12

13 13. Cost Estimate for Erosion Control and Water Quality Recommendations The following recommends implementation strategies and associated costs for preventative practices and in-lake dredge management to improve water quality within Diamondhead Lake. As no iterative strategy modeling has been performed to measure the effects and extent requirements of implementation, this is recommended as a 20-year implementation plan to be re-assessed on the subsequent RIZ application submittal. Table 4. Preventative practice recommendations and cost estimates. Strategy Estimated Cost Re-enroll and re-establish cover in lost CRP acreage areas (68 acres). $612,000 Address streambank and gulley erosion at areas identified by the IDNR (Figure 6; Figure 8; Table 7; Table 8). It is recommended that gulley erosion be addressed via grade stabilization and grassed waterways. It is recommended that streambank erosion be addressed via bed stabilization along the extent of the channel, reshaping channels into 2-stage design to promote floodplain connectivity (energy dissipation), the use of bioengineering methods along banks and cattle exclosure and controlled crossings for long - term sustainability. Estimated at 74,000 linear feet Gully erosion (54,000 linear ft) combination of grade stabilization and grassed waterways. Streambank erosion (20,000 linear feet) combination of bed stabilization, 2-stage channel; bioengineering. Develop and implement a robust lake and inlet water quality sampling protocol. Follow State protocol and use LBMC volunteers for data collec tion. Model watershed and lake response model, calibrated to monitoring data, to target and prioritize in-field and edge-of-field practices (Table 6; Table 8) and their measurable effects for optimal implementation strategies. Implement optimal in-field and edge-of-field strategy. $480,000 $2,070,000 $20,000 (two years of lab fees) $60,000 - $75,000. To be determined by participation level. Table 5. Dredge management recommendations and cost estimates. Strategy Develop updated Dredge Management Plan to alternative uses and storage options for dredge material. Dredging Either as single large project, or ongoing annual dredging. (Single large project recommended.) Estimated Cost $25,000 $2,000,000 - $3,000,000 Page 13

14 Estimating the 20-year costs associated with managing sediments in the Diamondhead Lake Watershed and lake bottom depends on past practice adoption and implementation as well as an assessment of probability of future implementation adoption. Preceding sections of this document discuss past and current adoption of various watershed implementation strategies, local reports and studies and communications from the NRCS to help build an opinion of probable costs. It also uses national projections to assist in forming an estimate of future land use and EQIP and CRP enrolment. Estimations of costs associated with watershed and in-channel practices used unit EQIP reimbursement values as well as input from Soil and Water Conservation District Managers and additional resources (Table 8). The Diamondhead Lake RIZ website also provides information on the costs of dredging, to-date as well as projections of future dredging budgets. Adoption of various watershed and stream channel BMPs (Table 6; Table 7) relies heavily on volunteer participation in the CRP and EQIP programs provided by the NRCS as well as coordination through the GSWCD. Though some adoption of practices is evident in the Diamondhead Lake watershed, it is also apparent that a much greater level of sediment and streambank control implementation is required to significantly reduce siltation of the lake. To-date, no analysis of the terrain to target specific practices has been made, nor any modeling to iteratively assess or measure the performance and associated costs of various implementation scenarios that would lead to a prioritized implementation plan that identifies the greatest return on investment. Such an analysis would greatly assist the GSWCD and NRCS by identifying priority land owners to begin discussions with. Though local NRCS and GSWCD efforts have likely been terrific and successful in accomplishing strides toward conservation goals, historical trends and forecasts suggest that the next twenty years will likely remain similar in local adoption of BMPs. Challenges to local adoption of BMP strategies include implementation funding limitations, land suitability and public understanding and acceptance of these conservation strategies. Page 14

15 Appendix Figure 3. Appendix. Diamondhead Lake sub-basins and main tributaries. Page 15

16 Diamondhead Lake-RIZ Renewal Engineering Report Figure 4. Appendix. Diamondhead Lake streams and gullies. Page 16

17 Figure 5. Appendix. Diamondhead Lake potential upland source contributions to sedimentation. Page 17

18 Figure 6. Appendix. Gully and streambank erosion locations (IDNR). Page 18

19 Figure 7. Appendix. Watershed sediment basin locations (IDNR). Page 19

20 Figure 8. Appendix. Pre-project estimated sediment delivery (IDNR). Page 20

21 Figure 9. Appendix. Dredging site locations and lake perimeter sediment basins Page 21

22 Figure 10. Appendix. North Diamondhead Lake sediment thickness. Page 22

23 Figure 11. Appendix. South Diamondhead Lake sediment thickness. Page 23

24 Figure 12. Appendix. Sediment basin 1 sediment thickness. Page 24

25 Figure 13. Appendix. Sediment basin 2 sediment thickness. Page 25

26 Figure 14. Appendix. Sediment basin 3 sediment thickness. Page 26

27 Figure 15. Appendix. Sediment basin 4 sediment thickness. Page 27

28 Figure 16. Appendix. Sediment basin 5 sediment thickness. Page 28

29 Figure 17. Appendix. Sediment basin 6 sediment thickness. Page 29

30 Figure 18. Appendix. Horseshoe Bend dredging area.-appendix. Horseshoe Bend dredge area. Page 30

31 Figure 19. Appendix. Horseshoe Bend sedimentation basin. Page 31

32 Figure 20. Appendix. Long Branch Creek basin, dredging area and dredge material storage area. Page 32

33 Table 6. Appendix. NRCS best management practices to address watershed sources of sediment to Diamondhead Lake. NRCS Practice Code Definition Purpose / Benefit Conservation Cover 327 Establishing and maintaining permanent vegetative cover (1) Reduce soil erosion and sedimentation. (2) Improve water quality. (3) Improve air quality (4) Enhance wildlife habitat. (5) Improve soil quality (6) Manage plant pests Conservation Crop Rotation 328 A planned sequence of crops grown on the same ground over a period of time (i.e. the rotation cycle). Constructed Wetland 656 An artificial ecosystem with hydrophytic vegetation for water treatment. Contour Buffer Strips 332 Narrow strips of permanent, herbaceous vegetative cover established around the hill slope, and alternated down the slope with wider cropped strips that are farmed on the contour. Contour Farming 330 Aligning ridges, furrows, and roughness formed by tillage, planting and other operations to alter velocity and/or direction of water flow to around the hillslope. Cover Crop 340 Grasses, legumes, and forbs planted for seasonal vegetative cover. Critical Area Planting 342 Establishing permanent vegetation on sites that have, or are expected to have, high erosion rates, and on sites that have physical, chemical or biological conditions that prevent the establishment of vegetation with normal practices. (1) Reduce sheet, rill and wind erosion. (2) Maintain or increase soil health and organic matter content. (3) Reduce water quality degradation due to excess nutrients. (4) Improve soil moisture efficiency. (5) Reduce plant pest pressures. (6) Provide feed and forage for domestic livestock. (7) Provide food and cover habitat for wildlife, including pollinator forage, and nesting. (1) For treatment of wastewater and contaminated runoff from agricultural processing, livestock, and aquaculture facilities, or (2) For improving the quality of storm water runoff or other water flows lacking specific water quality discharge criteria. (1) Reduce sheet and rill erosion. (2) Reduce water quality degradation from the transport of sediment and other water-borne contaminants downslope. (3) Improve soil moisture management through increased water infiltration. (4) Reduce water quality degradation from the transport of nutrients downslope. (1) Reduce sheet and rill erosion (2) Reduce transport of sediment, other solids and the contaminants attached to them - Resource Concern (3) Reduce transport of contaminants found in solution runoff (4) Increase water infiltration (1) Reduce erosion from wind and water. (2) Maintain or increase soil health and organic matter content. (3) Reduce water quality degradation by utilizing excessive soil nutrients. (4) Suppress excessive weed pressures and break pest cycles. (5) Improve soil moisture use efficiency. (6) Minimize soil compaction. (1) Stabilize stream and channel banks, pond and other shorelines (2) Stabilize areas with existing or expected high rates of soil erosion by wind or water (3) Stabilize areas, such as sand dunes and riparian areas Page 33

34 NRCS Practice Code Definition Purpose / Benefit Cross Wind Trap Strips 589C Herbaceous cover established in one or more strips typically perpendicular to the most erosive wind events (1) Reduce soil erosion from wind and windborne sediment deposition. (2) Induce snow deposition to improve soil moisture management. (3) Improve plant health by protecting the growing crops from damage by wind-borne soil particles. (4) Improve air quality by reducing the generation of airborne particulate matter. Dam 402 An artificial barrier that can impound water for one or more beneficial purposes. Dam, Diversion 348 A structure built to divert all or part of the water from a waterway or a stream. (1) Reduce downstream flood damage. (2) Provide permanent water storage for one or more beneficial uses such as irrigation or livestock supply, fire control, municipal or industrial uses, develop renewable energy systems, or recreational uses. (3) Create or improve habitat for fish and wildlife (1) To divert all or part of the water from a waterway in such a manner that it can be controlled and used beneficially such as irrigation or livestock supply, fire control, municipal or industrial uses, develop renewable energy systems, or recreation. (2) To divert periodic damaging flows from one watercourse to another watercourse thereby reducing the damage potential of the flows. Drainage Water Management Early Successional Habitat Development/Management 554 The process of managing water discharges from surface and/or subsurface agricultural drainage systems. 647 Management for early plant succession to benefit desired wildlife or natural communities. (1) Reduce nutrient, pathogen, and/or pesticide loading from drainage systems into downstream receiving waters (2) Improve productivity, health, and vigor of plants (3) Reduce oxidation of organic matter in soils (4) Reduce wind erosion or particulate matter (dust) emissions (5) Provide seasonal wildlife habitat (1) Increase plant community species and structural diversity.(2) Provide wildlife habitat for those species that use early successional stage vegetative habitat. (3) Provide habitat for declining species. Fence 382 A constructed barrier to animals or people. Field Border 386 A strip of permanent vegetation established at the edge or around the perimeter of a field. Filter Strip 393 A strip or area of herbaceous vegetation that removes contaminants from overland flow. (1) This practice facilitates the accomplishment of conservation objectives by providing a means to control movement of animals and people, including vehicles. (1) Reduce erosion from wind and water(2) Protect soil and water quality (3) Provide wildlife food and cover and pollinator or other beneficial organism habitat (4) Increase carbon storage (5) Improve air quality (1) Reduce suspended solids and associated contaminants in runoff. (2) Reduce dissolved contaminant loadings in runoff. (3) Reduce suspended solids and associated contaminants in irrigation tailwater. Page 34

35 NRCS Practice Code Definition Purpose / Benefit Forest Stand Improvement 666 To manipulate species composition and stocking by cutting or killing selected trees and understory vegetation. Grade Stabilization Structure 410 A structure used to control the channel grade in natural or constructed watercourses. Grassed Waterway 412 A shaped or graded channel that is established with suitable vegetation to convey surface water at a nonerosive velocity using a broad and shallow cross section to a stable outlet. Pond 378 A water impoundment made by constructing an embankment or by excavating a pit or dugout. Precision Land Forming 462 Precision Land Forming is reshaping the surface of land to planned grades. (1) Increase the quantity and quality of forest products by manipulating stand density and structure. (2) To facilitate forest stand regeneration. (3) To improve understory aesthetics, wildlife habitat, or recreation. (1) To stabilize grade, reduce gully erosion, and/or improve water quality (1) To convey runoff from terraces, diversions, or other water concentrations without causing erosion or flooding. (2) To prevent gully formation. (3) To protect/improve water quality. (1) A pond stores water for livestock, fish and wildlife, recreation, fire control, erosion control, flow detention, and other uses such as improving water quality. (1) This practice improves surface drainage and controls erosion. Prescribed Grazing 528 Managing the harvest of vegetation with grazing and/or browsing animals. Residue and Tillage Management, No Till Residue and Tillage Management, Reduced Till 329 Limiting soil disturbance to manage the amount, orientation and distribution of crop and plant residue on the soil surface year around. 345 Managing the amount, orientation and distribution of crop and other plant residue on the soil surface year round while limiting the soildisturbing activities used to grow and harvest crops in systems where the field surface is tilled prior to planting. (1) Improve or maintain desired species composition and vigor of plant communities. (2) Improve or maintain quantity and quality of forage for grazing and browsing animals health and productivity. (3) Improve or maintain surface and/or subsurface water quality and quantity. (4) Improve or maintain riparian and watershed function. (5) Reduce accelerated soil erosion, and maintain or improve soil condition. (6) Improve or maintain the quantity and quality of food and/or cover available for wildlife. (7) Manage fine fuel loads to achieve desired conditions. (1) Reduce sheet, rill and wind erosion (2) Reduce tillage-induced particulate emissions (3) Maintain or increase soil quality and organic matter content (4) Reduce energy use (5) Increase plant-available moisture (6) Provide food and escape cover for wildlife (1) Reduce sheet, rill and wind erosion (2) Reduce tillage-induced particulate emissions (3) Maintain or increase soil quality and organic matter content (4) Reduce energy use (5) Increase plant-available moisture Page 35

36 NRCS Practice Code Definition Purpose / Benefit Riparian Forest Buffer 391 An area predominantly trees and/or shrubs located adjacent to and upgradient from watercourses or water bodies. (1) Create shade to lower or maintain water temperatures to improve habitat for aquatic organisms. (2) Create or improve riparian habitat and provide a source of detritus and large woody debris. (3) Reduce excess amounts of sediment, organic material, nutrients and pesticides in surface runoff and reduce excess nutrients and other chemicals in shallow ground water flow. (4) Reduce pesticide drift entering the water body. (5) Restore riparian plant communities. (6) Increase carbon storage in plant biomass and soils. Riparian Herbaceous Cover 390 Grasses, sedges, rushes, ferns, legumes, and forbs tolerant of intermittent flooding or saturated soils, established or managed as the dominant vegetation in the transitional zone between upland and aquatic habitats. Saturated Buffer 604 A subsurface, perforated distribution pipe is used to divert and spread drainage system discharge to a vegetated area to increase soil saturation. Sediment Basin 350 A basin constructed with an engineered outlet, formed by an embankment or excavation or a combination of the two. Stormwater Runoff Control 570 Controlling the quantity and quality of stormwater runoff. Strip Cropping 585 Growing planned rotations of erosion-resistant and erosion susceptible crops or fallow in a systematic arrangement of strips across a field Structure for Water Control 587 A structure in a water management system that conveys water, controls the direction or rate of flow, maintains a desired water surface elevation or measures water. Terrace 600 An earth embankment, or a combination ridge and channel, constructed across the field slope. (1) Provide or improve food and cover for fish, wildlife and livestock, (2) Improve and maintain water quality. (3) Establish and maintain habitat corridors. (4) Increase water storage on floodplains. (5) Reduce erosion and improve stability to stream banks and shorelines. (6) Increase net carbon storage in the biomass and soil. (7) Enhance pollen, nectar, and nesting habitat for pollinators. (8) Restore, improve or maintain the desired plant communities. (9) Dissipate stream energy and trap sediment. (10) Enhance stream bank protection as part of stream bank soil bioengineering practices. (1) To reduce nitrate loading to surface water from subsurface drain outlets. (2) To enhance or restore saturated soil conditions in riverine, lacustrine fringe, slope, or depression hydrogeomorphic landscape classes. (1) To capture and detain sediment laden runoff, or other debris for a sufficient length of time to allow it to settle out in the basin. (1) Minimize erosion and sedimentation during and following construction activities. (2) Reduce the quantity of stormwater leaving developing or developed sites. (3) Improve the quality of stormwater leaving developing or developed sites. (1) Reduce water erosion (2) Reduce wind erosion (3) Reduce the transport of sediment and other water and wind borne contaminants (4) Protect growing crops from damage by wind-borne soil particles (1) The practice may be applied as a management component of a water management system to control the stage, discharge, distribution, delivery or direction of water flow. (1) Reduce erosion and trap sediment (2) Retain runoff for moisture conservation Page 36