How the 2015 Storms Filled John Redmond Reservoir with Sediment

The Upper Neosho and Cottonwood watersheds above John Redmond Reservoir, with the upstream and downstream USGS monitoring sites. Base: 2011 National Land Cover Dataset. USGS.

A reservoir filling with sediment

The Upper Neosho and Cottonwood rivers drain about 3,015 square miles of east-central Kansas into John Redmond Reservoir.

• Faster than planned: sedimentation from 1964 to 2014 ran almost 90 percent above the design rate of 404 acre-feet a year, costing about 40 percent of the conservation (multipurpose) pool.
• Estimates of storage loss range from 765 acre-feet a year (1964–2014) to 492 acre-feet a year (2007–14).
• Countermeasures: the Kansas Water Office has carried out more than two dozen streambank erosion control projects and started dredging to restore nearly 2,000 acre-feet of storage near the dam for downstream water users.

The 2015 storms

From May through July 2015, heavy rain sent large volumes of runoff into the reservoir. Because of flooding downstream in Oklahoma, water was held in the flood pool until late May, then released for the rest of the study. Those changes in how long sediment-laden water stayed in the reservoir directly affected how much sediment settled.

How it was measured: the USGS, with the Kansas Water Office and the State Water Plan Fund, runs streamflow and continuous water-quality sites above and below the reservoir. Regression models using one upstream and one downstream site computed sediment loads in and out, plus trapping efficiency and the effect on storage.

Water in and out

• In: about 966,000 acre-feet; out: about 1.07 million (the gap reflects uncertainty in the ungaged part of the watershed and unmeasured factors such as evaporation).
• May: fifty-five percent of all inflow (535,000 acre-feet) arrived, and was mostly held — releases were 72 percent less (150,000 acre-feet).
• From May 28: releases rose to as much as 25,400 acre-feet a day, the reservoir's maximum. June: 520,000 out, 225,000 in. July: 396,000 out, 206,000 in.

Inflow (blue), outflow (black) and storage (green), May–July 2015. USGS.

Sediment in and out

• Totals: about 872,000 tons of suspended sediment entered; 57,000 tons left.
• May alone brought 559,000 tons — almost twice June (142,000) and July (171,000) combined (313,000). May's load exceeded both the reservoir's annual design sedimentation rate and its estimated annual rate for 2007–2014.
• Out: about 2.5 times more left in June (24,500 tons) and July (16,500) combined (41,000) than in May (16,000).
• Ratio: loads entering were 5 to 34 times those leaving, even at peak releases.

Sediment loads above and below the reservoir: A, the whole study period; B, by month. USGS.

Trapping efficiency

The reservoir trapped 93 percent of incoming sediment on average — 97 percent in May, when water was held, and 83 percent in June, the month of largest releases. A 2007–8 study found 91 percent. Trapping was lowest when releases were highest, so the link between releases, residence time and trapping could be used to manage sedimentation.

Sediment trapped versus released, by month and overall. USGS.

Storage lost

With a mean sediment bulk density of 38.2 pounds per cubic foot and 815,000 tons retained, the storms cost about 980 acre-feet of storage — assuming all of it settled in the multipurpose pool — or about 1.6 percent of the conservation pool in three months, slightly more than 1 percent in May alone. (Some sediment likely settled in flooded land upstream instead, an amount this analysis can't measure.)

Like other Kansas reservoir studies, this shows that large amounts of sediment arrive in short bursts — hours or days — and big storms combined with minimal releases can take a substantial bite out of storage.

Sources

Based on May through July 2015 Storm Event Effects on Suspended-Sediment Loads, Sediment Trapping Efficiency, and Storage Capacity of John Redmond Reservoir, East-Central Kansas, by Guy M. Foster and Lindsey R. King, USGS Fact Sheet 2016–3035, prepared with the Kansas Water Office, U.S. Geological Survey (DOI), summarizing Foster (2016) and citing the Kansas Water Office (2015), Jakubauskas and others (2014), Lee and others (2008) and Juracek (2010); a work of the United States government in the public domain. The figures come from the fact sheet's PDF. The sheet dates the 765 acre-feet-a-year figure to 1964–2014 in one place and 1964–2006 in another.