- Cropland is expected to be the largest source of nitrogen and phosphorus loads. 7x Runoff volume increase in suburban residential areas compared to pre-settlement conditions.
- Gully and streambank erosion is expected during a one-year storm event (2.67" in 24 hrs) to be a large source of sediment load. 43x Rate of flow increase for same conditions From 2001-2011, construction sites made up 97% Reduction in peak outflow rates from developing areas for the one-year event, using 2-3% of this subwatershed. This small portion new stormwater design methods outlined in of this landscape is estimated to contribute: the Iowa Stormwater Management Manual.
- 61% Sediment load
(compared to current methods)
- 17% Nitrogen
- 26% Phosphorus
1/2 Restoring healthy topsoil layers to open space areas can reduce stormwater runoff by 1/2. 25% Construction sites likely contribute more than Modifying key pond outlet structures to manage small storms could reduce: 25% of the total sediment load in the Walnut
• One-year outflow rates for the area served by more than 40% Creek watershed.
• Phosphorus and sediment loads downstream by 10%
Subwatershed Case Studies
The Walnut Creek watershed covers an area of nearly 83 square miles.
It would take significant investments within an area of this scale to notice measurable improvements in water quality. This is the primary reason that certain subwatersheds have been selected for more intense study. Focusing efforts in these "case study" areas allows monitoring to better measure changes in water quality that result from localized improvements. This provides the opportunity to review results and make strategic adjustments which can be applied to improvements in other subwatershed areas. A secondary benefit of this approach is more precise modeling of the subject area, using information about land use, streambank conditions, gully formation and existing management practices at a higher level of detail than is practical to collect at the larger watershed scale.
One subwatershed was selected to represent a typical rural setting, another in a developed area and one in an area which is expected to experience rapid urban growth in the next few years. Four candidate subareas of each of these types were presented to the Walnut Creek WMA board for review, to establish a consensus on which ones were to be designated as case study subwatersheds. For each selected subwatershed, a specific plan has been developed to target expected sources of key pollutants (see map on page 136).
A more detailed review of each case study is included within an appendix to this plan.
Rural Case Study—Subwatershed
Location This area is located in the headwaters of Walnut Creek. This 6.5-square-mile area is generally located between Dallas Center and Grimes, with Highway 44 running east-west through the center of the area. This subwatershed has been divided into 18 smaller areas, or microwatersheds, for analysis.
Pollutant Sources More than 80% of this subwatershed is used for row-crop agricultural production. Over the past two years, these areas were primarily farmed either in a rotation of corn and soybeans, or planted as corn in each year. Modeling results indicate that
Water Quality Improvement Plan
The following strategies are recommended to improve water quality within this subwatershed area and develop and evaluate a template for future action within other rural agricultural areas. Subwatershed Strategy #1—Employ best management practices (BMPs) which are identified in the Nutrient Reduction Strategy document or other resources, with a goal of reducing nutrient loads from this subwatershed area. Loading reduction targets are 41% for nitrogen and 29% for phosphorus by 2025.
This chapter outlines a “model plan,” which is one possible set of improvements that collectively would reach these goals. Many other combinations are possible.
Staff and resources from local and regional groups such as the Heartland Co-op, County Soil and Water Conservation Districts (SWCDs), IDNR, NRCS and Iowa Soybean Association should work with local farmers and landowners to expand knowledge about these practices and find the right fit for practices throughout the landscape.
The model plan focuses on several key practices to meet the desired load reductions. Brief descriptions of these practices are included in Chapter 15 of this plan. Six of these practices were projected to be applied broadly across this subwatershed area. Expected load reductions are typically based on values from the 2014 edition of the Nutrient Reduction Strategy.
The “model plan” also identifies practices recommended to be installed within certain smaller microwatershed areas.
Land Retirement to CRP—Convert some steep-slope (slopes > 5%) cropland areas to grasslands through use of CRP or by dedication to permanent conservation easements. The model assumes that 5% of the cropland in area 411.01, 2% of the cropland in 411.04 and 1% of the cropland in 411.05 would be converted in this way. (The Raccoon River Water Quality Improvement Plan identified this as a strategy to address nutrient losses on steeper, more erodible lands.) – Total land affected = 20 acres. – Expected reductions of 85% nitrogen and 75% phosphorus loading from groundwater and surface runoff from the affected areas.
Saturated Buffers—Intercept tile drainage systems and divert most subsurface drainage through a saturated buffer strip adjacent to the stream. The model included 50% of the land area within subarea 411.04 and 35% of the land area within subarea 411.05 being managed using this method. – Total land area treated = 442 acres.
Bioreactors—Intercept tile drainage systems for smaller areas (less than 100 acres) and divert most subsurface drainage through a bioreactor system. The model included 30% of the land area within subareas 411.02, 411.03, 411.11, 411.31, 411.32, 411.33 and 411.41 being treated in this manner. – Total land area treated = 311 acres. – Expected reduction of 43% nitrogen loading from groundwater from the treated area.
Grass waterways—Create or enhance grass waterways to maintain a minimum 33-foot width, or wider as dictated by current design guidelines or as needed to protect the five-year flood plain. The model included installing such waterways (where they don’t yet exist) along 90% of the “zero order” streams mapped as part of this plan located within subareas 411.12, 411.21, 411.32, 411.33, 411.42, 411.51, 411.52, 411.61 and 411.71. Installing such waterways would impact 26 acres of cropland area. – Total land area treated = 1,632 acres. Length = 34,200 feet. – Expected reduction of 50% phosphorus loading from surface runoff from the treated area.
Wetlands—Construct wetland features in areas where productivity is most commonly lost due to standing water. Flow paths immediately upstream of road crossings are also good candidate locations. Wetlands should be designed with multi-stage outlet structures to maximize reduction of peak flow rates from small and moderate storm events. (Provide drawdown of a one-year return period storm over a period of 24–48 hours). Such outlet modifications would allow the wetlands to serve two key purposes: nutrient reduction and stormwater peak flow reduction for storms of approximately 3” or less. For this model, it is assumed that wetlands could intercept runoff from 30% of subarea 411.05 and 100% of subarea 411.06. Total wetland new area expected to be 30 acres in size. – Total land area treated = 633 acres. – Expected reduction of 52% phosphorus loading from surface runoff from the treated area.
Two-Stage Ditch—Although these features have not been included in the Nutrient Reduction Strategy, several studies have demonstrated that these features have been very effective at removing nitrogen and phosphorus from streams with larger drainage areas. They are best implemented in areas without adequate buffer widths, where the stream is narrow or where the streambanks or channel bottom are unstable. This practice allows for expansion of the channel cross-section, slowing flow velocities and allowing for increased filtration of runoff. One key section of channel extending through parts of subarea 411.04 and 411.05 appears best suited for this practice. It would treat not only runoff from this subarea, but all areas located upstream. Installation of this practice would likely affect only two acres of current row-crop production. – Total land area treated = 2,244 acres. – Expected reduction of 10% nitrogen and 15% of phosphorus loading from both surface and groundwater runoff from the upstream treated area. Subwatershed Strategy #2—Address key areas of gully and streambank erosion.
Streambank stabilization and restoration—Target efforts to a one-mile stretch of stream within subarea 411.01 and a half-mile segment within subarea 411.02. These improvements have the potential to reduce the annual rate of erosion by 265 tons.
Two-stage ditch—Conversion of a section of stream within parts of subareas 411.04 and 411.05 to a two-stage ditch would also reduce the annual rate of erosion by up to 52 tons.
The “model plan” includes the two improvements listed above. There are also some other gully areas in subareas 411.01 and 411.11 which could be addressed that could reduce annual erosion rates by up to 170 tons. Such repairs have not been included in the model calculations.
Subwatershed Strategy #3—Look for opportunities to reduce the peak rates of flow caused by small to moderate storm events. Where practices are constructed that detain or retain water (i.e. wetlands, sediment basins, ponds, etc.) use multi-stage outlet designs that provide temporary stormwater storage for extended detention of small and moderate storm events. A one-year return period, 24-hour storm event in this area is 2.67” of rainfall. Such controls could reduce runoff peak rates by over 95%. The multi-stage design would not necessarily be designed to fully detain runoff from larger storms; however, the runoff from the one-year event is approximately 40% of the flow volume of a 100- year return period event. This would be captured and slowly released by managing runoff from the more commonly occurring smaller storms. Therefore, such outlet structures would provide downstream benefits during both small and large storm events.
Urban Case Study—Subwatershed 213
Location This subwatershed includes areas which are tributary to South Walnut Creek, which flows into Walnut Creek just south of Hickman Road, west of 128th Street in Clive. Most of this area drains through Country Club Lake in Clive. This 4.5-square-mile area is almost completely developed at this point.
More than 80% of this subwatershed is now developed into suburban land uses, and as such modeling indicates that a majority of nutrient loadings are expected to be sourced from these land uses. Cropland makes up less than 3% of the watershed, but is expected to be the source of over 13% of nitrogen and 7% of phosphorus loading. As these areas continue to be developed, the loading attributed to cropland is expected to decrease. Overall, nutrient loading from this subarea is expected to be generally lower than the Walnut Creek Watershed averages. However, within this subwatershed there are several areas with pollutant loadings that are expected to be much higher than the watershed average, based on completed modeling.