Subwatershed Case Studies
Because of the large area of the Walnut Creek Watershed, 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 and 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.
Rural Case Study—Subwatershed 411
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.
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 cropland areas are the most significant sources of nutrient loadings. Row-crop areas also produce the majority of the sediment loading from this subwatershed, although streambank and gully erosion are also significant contributors of this pollutant. There are several areas within this subwatershed with pollutant loadings that are expected to be much higher than the Walnut Creek Watershed averages, based on completed modeling.
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.
- Subwatershed Strategy #2—Address key areas of gully and streambank erosion.
- Subwatershed Strategy #3—Look for opportunities to reduce the peak rates of flow caused by small to moderate storm events.
Urban Case Study—Subwatershed 213
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.
These modeling results represent pollutant loads expected at the downstream end of this subwatershed, where it enters Walnut Creek. Loadings of phosphorus and sediment might be significantly higher if they were measured upstream of one of the many wet ponds located in this area. Many of these ponds are projected to trap about half of the sediment and phosphorus loadings that pass through them, based on their current design. Identifying opportunities to address these pollutants in upstream areas would likely result in water quality improvements within the ponds themselves.
Sediment loading from the subwatershed over the recent past is expected to be primarily attributed to construction site and streambank erosion. Between 2001 and 2011, about 650 acres within this area was converted from agricultural to suburban land uses. If each construction site takes one to two years to construct (from initial construction to final home site development), then at any given time between 65 to 130 acres of land may have been in some stage of site construction. From modeling results, these sites, making up only 2-5% of the area within this subwatershed, may have generated more than 60% of the sediment load.
Streambank erosion is the next largest generator at nearly 24% of the expected load. A large share of loading due to streambank erosion is expected to come from microwatershed 213.01. This area is located downstream of Country Club Lake and features a heavily eroded segment of South Walnut Creek. This area bypasses all the ponds and other features within the subwatershed, so there is little opportunity to capture generated sediment before it enters Walnut Creek.
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 urban developed areas.
- Subwatershed Strategy #1—Review opportunities to develop public-private partnerships to modify outlets of existing ponds or entrances to existing culverts to provide better management of small storm events. Primary and secondary opportunities for these improvements are noted on the improvement plan map for this subwatershed. Other locations for such modifications may need to be evaluated.
- Subwatershed Strategy #2—Complete streambank stabilization and restoration projects in key identified areas, as identified on the improvement plan map for this subwatershed.
- Subwatershed Strategy #3—For developing and redeveloping areas, require compliance with policy initiatives as identified within Chapter 9 of this plan.
Developing Case Study—Subwatershed 601
This area is located within the watershed of Little Walnut Creek and is expected to see rapid urban growth over the next decade. This 960-acre area is generally located on both sides of Little Walnut Creek between Warrior Lane and NW 170th Street. This subwatershed has been divided into 12 smaller microsheds for analysis.
This case study is different from the rural and urban subwatersheds, in that the land uses of concern do not yet exist. The purpose of this analysis is to evaluate stormwater management techniques to prepare a plan for management which mitigates the impact to receiving streams caused by increased runoff rates and volumes as land uses change within this area. Nutrient, pathogen and sediment loading reductions are expected to be complementary benefits to this approach.
To establish management strategies and to better explain their reasoning, the following outcomes need to be achieved by this study:
- Understand predicted runoff volumes and rates within this area for four conditions:
- Pre-settlement conditions: Primarily tall-grass prairie prior to pioneer settlement.
- Future condition with soil quality restoration: Similar to the conditions described in “c,” except that it is assumed that techniques such as topsoil respreads or soil amendments are used to create a healthy layer of topsoil on the disturbed landscape.
- Future conditions: Mainly low- to medium-density residential growth with some commercial development. This case assumes that normal mass grading practices are used for development construction and minimal topsoil is replaced on the disturbed landscape.
- Existing conditions: Primarily agricultural row-crop uses with some areas reserved within a flood plain buffer.
- Locate and size potential “regional” stormwater management practices (wet detention ponds). Determine the storage volume and outflow rates for these systems for the following conditions:
- Traditional stormwater management, with principal focus on management of larger storm events (5-year storms or larger).
- Use of the Unified Sizing Criteria from the Iowa Stormwater Management Manual to manage both small and large storm events. For this case, assume no soil quality restoration is completed.
- Use the Unified Sizing Criteria and implement soil quality restoration throughout this area. Predict pollutant loads under existing and future conditions.
- Determine expected load reductions that could be provided by regional stormwater management practices for conditions listed under “2a” through “2c” above.