A Happy Medium
There are a variety of drainage and conservation techniques that can be implimented to improve the water quality within a watershed, and the Seven Mile Creek Watershed Project is attempting to educate local farmers in these practices. The efficacy of all of the techniques addressed below hinge on the idea that if flow velocities are slowed and residence time in the fields is increased, erosion rates and contamination levels will be reduced. By increasing residence time, effectively slowing flow velocity, sediment and attached phosphorus is able to settle out and will not continue downstream. Increasing residence time also has the effect of subjecting contaminated water to biological processes for a longer period of time, whereby aquatic plants and organisms have the opportunity to consume a greater proportion of dissolved contaminants.
Buffer Zones refer to areas of land that are intended to prevent sediment and contaminants from entering waterways. Buffer strips can be used to line cultivated fields, line the banks of open ditches and stream, and border wetlands. Buffer zones are strips of grass or other naturally occurring plants, whose root systems hold the soil together to prevent erosion while the stalks trap sediment. By constructing such a barrier, water passing through these strips will move slowly enough that many of the potentially harmful chemicals would be filtered out, decomposed, or consumed by the buffer vegetation.
A compound channel is a naturally occurring feature of any river or stream. The word compound refers to the channel's ability to handle two stages of flow: normal flow and flood flow. Normal flows are confined to the meandering banks of the channel, while flood flows are associated with the more expansive, flat, straighter channel of the floodplain. Taking a page out of mother nature's book, open drainage ditches can be constructed to mimic compound stream channels. By widening the ditch beyond the need of normal flow, the stream will be able to trace out natural meanders. Most of the time a portion of the channel will not be innundated, but that area remains avaiable to handle the high flows of spring thaw and storm events. Three environmental benefits can be achieved by constructing compound channel ditches. As explained previously, meanders decrease flow velocity. By dissipating the water's energy around every bend, flow will not be able to build up the energy to erode vigorously downstream. Secondly, this slower moving water will not be able to carry as much in sediment load. Therefore rather than washing all the sediment from the upland down into Seven Mile Creek, a portion of it will be deposited throughout the meanders on accretionary points bars. In addition, the floodplain provides another surface onto which sediment can be deposited during time of flood flows subsidence. Finally, in dissipating flow velocity throughout meanders, the residence time of potentially contaminated runoff in the ditches will be increased.
Closed Intakes and Rock Inlets
These both are techniques used to reduce the amount of upland erosion directly contributing to the contamination of Seven Mile Creek. A closed intake (left) is a structure that fits over the mouth of a tile line intake located in the middle of a cultivated field. The tile line is identical to the one mentioned in an open intake structure, but a closed intake offers a better means of trapping sediment. The orange column has holes big enough to allow water to pass through at a sufficient rate during a flooding event, but it does not allows a pit to develop that soil can pour down into. It also has a greater ability to retain debris (you can see that the bottom holes are clogged) than the orange basket mounted on the open intake. A rock inlet (right) is another structure located in the field that provides a filter on top of a tile intake. As opposed to the tile line emerging at the surface, it is burried beneath a pile of gravel. Sediment caught up in overland flow makes its way across the field and encounters the rock inlet. Water is able to percolate down through the gravel and into the tile line, but soil, phosphorus, and other debris gets caught up.
Grassed waterways are strips of uncultivated grass running through a cultivated field. They are natural or constructed channels intended to transport water without erosion. They are resistant to erosion because of their grass cover and associated root structure. While grassed waterways serve the same function as ditches for drainage purposes, there are environmental benefits associated with the grassed channels. In addition to trapping phophorus and sediment, grassed waterways provide food, cover, and nesting sites to a variety of bird species. These birds thrive on the plants that spring up when the waterway is dry and they feed on insects, helping to control pest populations for the farmer.
Grassed waterways provide cover, food, and nesting sites to a variety of bird species.
Farmers can control the amount of water diverted to a grassed waterway.
This is an aerial photograph of a cultivated plot in the upland region of SMC Watershed. This photo is one in a series that was taken after a period of heavy rain. It portrays a portion of one farmer's field that has flooded, following the recent storm event. The collection of photographs taken throughout the watershed was used to map out areas in the upland region that are naturally susceptible to flooding. The Farmable Wetlands Project (FWP) has been created to provide an incetive to farmers to join in an effort to re-establish wetlands in the watershed. As seen in the aerial photo, the locations to be targeted identify themselves as suitable spots for a wetland, as naturally controlled by topography and hydrology. Farmers choosing to enroll in the program receive monetary subsidies for the acreage they take out of cultivation and turn into a wetland. To prepare a location, a buffer zone is planted around the perimeter and tile lines are routed into the basin. An example of the resulting wetland is shown here.
The ideal location for a wetland project like this is in the corner of a field, so it is practical for the farmer to continue farming around the area, and near an open ditch, to ensure that the wetland is down gradient from most of the field so that tile lines can be routed toward it. This photo shows a nearly perfect location. Though ideally, the innundated area would be in the upper righthand corner of the field, closest to the public ditch.
The greatest benefit of wetlands lies in their ability to provide the residence time needed to naturally clean contaminated water. Water that is high in dissolved nitrates, as most all agricultural runoff in this area is, is cleansed through biological processes before it seeps out of the wetland and enters groundwater or stream flow. Wetlands also remove phosphorus and sediment from the water cycle. Slow flow through the wetland causes these solids to settle out of suspension. The advection of fecal coliform through the wetland environment is slowed sufficiently to allow microbial processes to break down the harmful bacteria. Water exits the constructed wetland basin in two ways. It is routed out through tile lines into ditches, and it seeps naturally into the ground below.
Now, let's travel down out of the upland region and into the park.