by Bob Neal, P.E., Metro Wastewater Reclamation District, Denver, Colorado
and Ted Johnson, P.E., Camp Dresser & McKee, Denver, Colorado
Background and Objectives
Segment 15 of the South Platte River extends 26 miles north of Denver from the Burlington Ditch headgate to its confluence with Big Dry Creek near Fort Lupton as shown in Figure 1. The Segment 15 flow regime is frequently dominated by the discharge from Metro Wastewater Reclamation District’s (Metro District) 185-mgd Central Treatment Plant. Low dissolved-oxygen levels have occurred at nighttime during the low-flow warm weather months of July through October. This has been partially attributed to the instream conversion of ammonia in the Central Treatment Plant effluent to nitrate.
In 1990, the Metro District completed construction of nitrification facilities for the plant’s 70-mgd North Complex, which significantly improved Segment 15 water quality. However, the dissolved oxygen standards, based on 24-hour average and instantaneous national criteria, were not being attained even though field studies showed that the segment supports a diverse and abundant aquatic community. Water-quality modeling indicated that even with nitrifying the plant’s 115mgd South Complex, about 65 percent of the segment would not meet the dissolved oxygen standard. As a result, the Metro District conducted a series of studies to develop a proposed site-specific dissolved oxygen standard and a plan to meet the standard.
The plan features a series of six low-head drop structures and modifications to two irrigation diversions dams that will passively reaerate the stream. Phase I, consisting of a low-head drop structure and 4,000 feet of channel improvements located upstream of 88th Avenue, was completed in 1996 and dramatically improved dissolved oxygen in this reach of the river. In April 1997, the Colorado Water Quality Commission adopted the Metro District’s proposed site-specific dissolved oxygen standard and a phased program to construct additional instream improvements. Phase II calls for construction of instream improvements extending from 88th Avenue to the Brantner Ditch diversion structure to be completed by December 2000. Phase III will include improvements from the Brantner Ditch diversion to the Brighton Ditch diversion and Phase IV will address improvements from the Brighton Ditch diversion to the end of Segment 15 just upstream of Fort Lupton.
Phase II Reach Characteristics
Studies have recently been conducted to characterize water quality, geomorphology, hydraulics, and environmental issues in the Phase II reach. The Segment 15 Water Quality Model, the primary tool used to site instream improvements, was updated and recalibrated with new data to reestablish baseline dissolved oxygen predictions of regulatory interest. For the critical July average condition, baseline dissolved oxygen levels upstream of the Brantner Ditch diversion structure were predicted to increase while those for reaches downstream of Brantner were predicted to decrease. These changes were largely due to adjustments in community metabolism rates and river reaeration rates.
From 88th Avenue to 104th Avenue, the river traverses a suburban setting and is generally channelized within the 10-year frequency floodplain. Downstream of 104th Avenue the river flows through a rural setting, meandering through a wide floodplain with relative freedom to move laterally. In addition to localized channel restrictions at highway crossings and irrigation ditch diversion structures, grade control structures have been installed at various locations along the Phase II reach to mitigate channel bed degradation. River bed sediments are generally characterized as gravelly sands with some silts. Gravel mining continues to be active within the river corridor with gravel pits reclaimed for water storage purposes.
Since the 1977 Flood Hazard Area Delineation (FHAD) was developed, the South Platte River channel and floodplain has undergone significant changes including channelization, natural lateral migration, changes in gradient and invert elevations, and development of the adjacent overbank. The 1977 FHAD hydraulic model, which is the basis for the regulatory floodplain, was updated based on new topography, channel cross-sections, vertical datum adjustments, and model refinements to improve water surface elevation predictions at bridge crossings and other structures. Because of these changes, the existing 100-year water surface profile is predicted to be less than or equal to the regulatory profile.
Natural vegetation in the Phase II corridor consists of shortgrass prairie. Wetlands occur primarily along the river’s edge, in backwater areas and along bars. Wetlands of regulatory interest are located on the west bank between 88th Avenue and Cooley Drop 2, approximately 2,500 feet downstream of 88th Avenue, along Bull Seep, eastern tributary to the South Platte that runs along the floodplain terrace, between McKay Road and the Fulton Ditch, and below 104th Avenue on the east bank at the north end of the first bar. Two threatened species are of concern in the Phase II reach: the Ute ladies’ tresses orchid (Spiranthes diluvialis) and the Preble’s meadow jumping mouse. No suitable Preble’s habitat was found due to the riparian corridor being disrupted by anthropogenic activities. Potential Spiranthes habitat was identified in the Bull Seep wetlands between McKay Road and the Fulton Ditch.
Plan Update and Alternatives Considered
The Segment 15 Water Quality Model, based on a spreadsheet version of the STREAMDO IV and Colorado Ammonia models, was used to predict the dissolved oxygen performance of a variety of alternative instream improvements. Alternatives considered various combinations of reaeration structure locations and weir heights, and modifications to the Fulton Ditch Diversion structure. Also, the Metro District was interested in assessing the reaeration performance of placed-rock sloping chute grade control structures (sloping chutes) and the effects of backwater pools upstream of the Fulton and Brantner Ditch diversion structures on dissolved oxygen.
Water quality modeling suggested that two new low-head reaeration structures, identified as Reaeration Structures 2 (RS2) and 3 (RS3), located as shown on Figure 2, would meet the July chronic dissolved oxygen standard. However, at the Brantner pool, dissolved oxygen is predicted to fall below standard about 0.2 miles upstream of the Brantner Ditch diversion structure (Figure 2). Efforts to move RS3 closer to the diversion structure would cause submergence of the weir, impacting reaeration performance. It was decided to retain the proposed RS3 location with the understanding that modifications may be needed to the Brantner Ditch diversion structure or the backwater pool to eliminate this potential shortfall in dissolved oxygen.
Hydraulic analyses indicated that the selected plan would not increase the size of the regulatory floodplain, or regulatory flood events, thus allowing Adams County to issue a floodplain use permit. Sediment transport analyses indicates that during years of extended high flows, the Phase II reach is relatively stable and that the proposed instream improvements may cause a small amount of aggregation immediately downstream of the structures. Environmental investigations conducted at the RS2 and RS3 sites revealed that there was no potential threat to existing populations of endangered species. However, jurisdictional wetlands were delineated along both banks of the river as well as overbank locations that may be impacted by construction and require mitigation.
The Metro District also evaluated the feasibility of installing fish screening devices on the Fulton Ditch diversion. An inclined screen design concept was selected because it is expected to have minimal adverse impacts on fish, eggs, and larvae; requires no energy to operate; and has low maintenance requirements. Field tests in the Fulton Ditch revealed that debris build-up on the screen can occur under low flow conditions when all flow is captured through the screen. Because of the hydraulic configuration of the Fulton diversion and ditch, a low head screen configuration is necessary requiring about 250 feet of screen to handle the 250 cfs Fulton maximum ditch flow rate. The Metro District has engaged the U. S. Bureau of Reclamation and Colorado State University of conduct laboratory tests to further define screen hydraulic performance criteria and assess biological impacts on fish.
Phase II Design
Design criteria developed for the Phase I reaeration structure (RS1) are applied to the Phase II reaeration structures with some modifications. Reaeration criteria include:
- A uniform unit flow over a vertical, broad-crested weir;
- A nappe flow over the weir that is aerated and separated from the face of the weir, and;
- A tailwater depth that is two-thirds of the water surface drop height.
Figure 3 provides a schematic view of a typical reaeration weir. Reaeration weir length was established based on a hydraulic loading of 1 cfs/foot to maximize reaeration efficiency. The 30-day average low-flow condition for the month of July was used to set total weir length for each structure. This flow rate has been the standard approach utilized by regulatory agencies in setting permit limits.
The Phase I reaeration structure (RS1) design consisted of a U-shaped configuration with the boat chute located at the apex of the structure. RS1 exhibits a symmetry about the channel centerline that created an angular and uniform appearance. Although the structure has performed well in providing reaeration, it was the objective of the Phase II design to provide better visual integration of RS2 and RS3 into the river channel environment. This was accomplished using a curvilinear design concept that mimics a cross-channel riffle as shown in Figure 4. The uniformity of the weir crest will be broken by installing boulder clusters upstream and downstream to create the appearance of intermittent cascades through the structure. The boat chute will be placed adjacent to the channel embankment to improve boater egress should they choose not to enter the boat chute. Construction of RS2 and RS3 will impact jurisdictional wetlands generally located along the river’s edge and sandbars. It is estimated that approximately 0.3 acres of wetlands will be disturbed during the construction of RS2. Restoration will include creating backwater areas and increasing the irregularity of the river edge which will result in more extensive wetlands of higher functional value. Wetlands are more extensive at RS3, totaling 2.1 acres. However, impacts can be minimized by excluding construction traffic and activities that would otherwise disrupt the wetlands in these areas. Aquatic habitats will be diversified with boat chutes at RS2 and RS3 providing pools and slackwater areas. Boat chutes will have an average gradient of less than 10 percent to accommodate fish passage. In addition to construction of RS2 and RS3, improvements to the boats chutes at RS1 and 88th Avenue are proposed. These are intended to increase the depth of flow through the boat chutes, and to eliminate a hydraulic jump that has formed at 88th Avenue, thereby enhancing boater safety. The 88th Avenue modifications include removal of a concrete sill at the base of the boat chute, replacement of loose rip rap with grouted boulders, and placement of additional boulders to minimize lateral flow into the boat chute and provide slack water areas for resting.