This article should help to explain it. The key points are that raising and lowering the lake when at nornal levels have little impact on capacity. Also, at maximum inflow (like they are currently experiencing) the original 11.5% of lake would fill in 12-18 hours.......
Note: Date of article is March 8 2010
Polk County Iowa: Army Corps Alters Saylorville Lake Operations in Anticipation of Spring Flooding | Facebook
ROCK ISLAND, Ill. – In preparation for the National Weather Service’s high probability forecast for spring 2010 flooding due to snowmelt and rainfall runoff, the U.S. Army Corps of Engineers has approved a temporary, minor deviation from the authorized Saylorville Lake Regulation Plan.
The deviation, as requested by the City of Des Moines, Iowa, assists in conserving flood storage capacity in the lake. The deviation permits operating the lake to control for a stage of 26 feet at the S.E. 6th Street gage in Des Moines, an increase of two feet above the authorized control stage of 24 feet. As conditions permit, the deviation allows higher outflows between March 8 and April 20 to keep the lake at its lowest possible level without causing additional flood damages downstream.
The deviation will increase Lake Red Rock inflows and may impact outflows; however, Lake Red Rock releases will not be increased above the regulated, non-damaging, maximum outflows. If necessary, maintaining the tandem operation of Saylorville and Red Rock lakes’ pools will supersede the temporary deviation.
Saylorville Lake’s pool elevation is being maintained at the normal conservation pool level of 836’ mean sea level (msl). The 836’-msl conservation pool occupies approximately 11.5 % of Saylorville Lake's total storage capacity. The conservation pool consists of approximately 73,600 acre feet of water (23.9 billion U.S. gallons) that must be maintained for authorized project purposes to include providing state-contracted water supply.
Flood storage above the conservation pool to the full 890’ flood storage pool elevation occupies 88.5% of Saylorville Lake’s total storage capacity. The flood storage pool consists of approximately 567,400 acre feet of water (184.8 billion U.S. gallons).
Reducing the lake’s conservation pool does not provide significant flood storage capacity. The 73,600 acre feet in the 836’-msl conservation pool would fill in 12 to 18 hours at the maximum inflow rates experienced in 2008. Additionally, drastically lowering the pool in the spring would result in bank sloughing and increase the risk for fish kills and significant ice jams at the controlling works which could prevent the efficient release of flood waters and cause the pool to rise more rapidly.
This article should help to explain it. The key points are that raising and lowering the lake when at nornal levels have little impact on capacity. Also, at maximum inflow (like they are currently experiencing) the original 11.5% of lake would fill in 12-18 hours.......