Mountain stormwater management in Jefferson County

By Michelle DeLaria, Jefferson County stormwater coordinator

Most stormwater management discussions focus on urbanized land development and its effect on water quality.

We know that land development alters natural hydrology by blocking the surface with buildings and pavement. Connecting these impervious areas – such as draining roofs onto parking areas and into storm sewer pipes – maximizes runoff and removes water from both small and large storms.

Even though land development in the mountains appears less “urbanized," the effects on mountain land and waterways are similar to the effects in the Plains:  Concentrated flows from impervious areas cause erosion and sedimentation, and degrade natural waterways. 

It is important to understand that in mountain development, roads (even dirt roads) are the major source of impervious area and concentrated runoff. For this reason, large-lot mountain development is not protective of watershed health, and clustered development that includes open space is still recommended for watershed protection.

Michelle DeLaria stands near a culvert outlet pipe from a commercial development and parking lot.  The erosion was caused by concentrated flows discharged from a pipe to a slope that previously received sheetflow. Sheetflow is the movement of water across a surface in a sheet-like manner and has less erosive power than concentrated flow that occurs within streams or discharges from pipes. (Enlarge)

This erosion channel formed when a roadside ditch was directed to a mountain slope.  The natural gulch in the bottom of the photo is where sheetflow drainage would normally collect.  (Enlarge)

Low-impact development techniques that absorb runoff, such as landscape buffers and porous pavement, are used in the Front Range to help restore predevelopment hydrology and reduce damage to our waterways.  These techniques can also be applied in mountainous terrain to reduce the damage to mountain property and waterways. 

Jefferson County uses the following low-impact development techniques in the foothills that seek to mimic predevelopment hydrology by slowing and infiltrating stormwater, and restoring sheetflow:

• Rock check dams - roadside
• Flow diffusers
• Level spreaders
• Rock rings
• Rock check structures - gulch

Concentrated flow in a roadside ditch is detained in a series of rock check dams. The rock check dams should be keyed into the sides of the ditch. (Enlarge)	Sediment builds behind the rock check dams and stabilizes with vegetation, creating a permanent stair-stepped ditch.  If there is too much sediment transport for this to occur, the sediment can easily be removed.  Sediment levels must remain below the road surface or water will leave the ditch line, flow around the rock check dam and into the roadway. (Enlarge)

Concentrated flow from a culvert is directed to a flow diffuser.  The void space between the large fractured rocks around the cistern absorbs and infiltrates small storm flows.  (Enlarge)	A rendering of a level spreader in combination with a flow diffuser.  It is anticipated that this technique will be used in Jefferson County in 2007.  A level spreader can be added to a flow diffuser or used as a single structure to restore sheetflow across a mountain meadow.  (Enlarge)

In areas where the culvert discharges on a gentle slope, a large rock ring may be installed. (Enlarge)	The ponding area can fill with sediment, stabilize with vegetation and will function as a flow diffuser and infiltration structure. (Enlarge)

When concentrated runoff is directed to natural gulches, a series of check dams can be added.  The design was provided by Jehn Engineering, Inc. (Enlarge)	These large check dam structures will reduce erosion in the gulch and allow stormwater to infiltrate into the fractures. (Enlarge)

Stormwater management in the Rocky Mountains should emphasize infiltration over conveyance, and flows from impervious area should be directed immediately to infiltration areas or to structures that slow the runoff.  Diverting runoff from the conveyance path keeps the runoff volume low. It also reduces the costs associated with managing large volumes of runoff, such as needing larger pipes and culverts, additional outlet protection, and increased trenching in difficult terrain.

All of these techniques can be combined to reduce and slow runoff volume.  For example, a section of mountain road could be stabilized with permanent rock check dams.  This will reduce the flow volume, velocity and sediment entering a culvert.  In turn, the culvert can discharge to a rock ring or flow diffuser.

Stormwater structures and management techniques that reduce runoff and promote infiltration and sheetflow help retain water in the watershed, recharge groundwater, reduce erosion, protect riparian areas and create a more environmentally and financially sustainable site. 

To learn more about mountain stormwater Best Management Practices, logon http://jeffco.us/highways/highways_T48_R25.htm#Mountain. The Web site includes informational flyers about mountain stormwater management, the concept paper “Mountain Stormwater BMPs,” and a PowerPoint presentation with additional photos of stormwater techniques and structures.

For more information contact Michelle DeLaria at mdelaria@jeffco.us or 303-271-8454.

All images provided by Michelle DeLaria.