Flood peak attenuation caused by storage of flood water on overbank surfaces effectively reduces the magnitude of peak discharges in some, but not all watersheds. Several geomorphic factors that affect the storage and conveyance of flood water were investigated to assess their quantitative influence on downstream peak discharges. The MIKE11 rainfall-runoff and hydrodynamic models were calibrated for the Grant River watershed, southwestern Wisconsin. Alternative geomorphic conditions were modeled and compared to the original case. Results indicate that channel-fioodplain-terrace morphology, valley width, stream slope, and hydraulic roughness each influence peak discharges, especially for moderate magnitude (5-to 50year recurrence interval) floods. Peak discharges varied by as much as 49% between simulations depending on geomorphic conditions. Watersheds that effectively attenuate produce peak discharges that are strongly correlated with total runoff. Watersheds that attenuate little produce peak discharges that exhibit greater variance due to storm intensity and duration. floodplain flow velocities are typically much lower than channel flow velocities, and the floodplain acts as a storage reservoir [Bhowmik and Stall, 1979; Gendel'man, 198!; Wong and Laurenson, 1983; Bates and Pilgrim, 1983]. This storage and later release of a. portion of the total flood volume produces flood hydrographs that are low and broad compared to those produced in similar watersheds that lack floodplain storage (e.g., gullies or mountain streams). For high-magnitude, low-frequency floods, peak discharges are only minimally attenuated as the total discharge is dominated by overbank flow, with the entire floodplain delivering flood water much like a channel. This general explanation is well understood. Some research has considered the attenuation process in more detail, including map and field analysis of the hydraulic geometry and carrying capacity of floodplains [Bhowmik, 1984; Bhowmik and Demissie, !982; Bhowmik and Stall, 1979], geomorprologic studies of floodplain inundation and drainage [Zwolinski, 1992; Hughes, 1980; Lewin and Manton, 1975], and flume studies of the interaction of channel and floodplain flows [Smith, 1978; Rajaratnam and Ahmadi, 1979; Knight and Demetriou, 1983; Knight, 1989]. Quantitative data on flood peak attenuation are limited to comparison of upstream and downstream measurements of discharge and relatively simple computer modeling studies. Archer [1989] cites gage data that show up to 30% attenua-Paper number 94WR00323. 0043-1397/94/94WR-00323 $05.00 tion of peak discharges between two gages separated by 34.6 km on the River Tees, England. Diehi [1990], using a diffusive wave model of Black Earth Creek, Wisconsin, found that an increase in floodplain roughness from 0.053 to 0.100 (Manning n) can reduce peak discharges of moderate floods (4-to 50-year recurrence interval) by up to 27%. Wolff's [1983] computer simulations of an idealized system consisting of a prismatic channel and simplified floodplain produced...

Woltemade, Christopher J., 2010. Impact of Residential Soil Disturbance on Infiltration Rate and Stormwater Runoff. Journal of the American Water Resources Association (JAWRA) 46(4): 700‐711. DOI: 10.1111/j.1752‐1688.2010.00442.x Abstract:  Soil disturbances such as excavation and compaction in residential developments affect lawn infiltration rates and stormwater runoff. These effects were investigated via measuring saturated infiltration rates at 108 residential sites and 18 agricultural sites near Shippensburg, south‐central Pennsylvania, using a double‐ring infiltrometer. Residential sites included four neighborhoods distributed across three soil series classified as hydrologic soil group (HSG) B. Additional parcel data included date of house construction, percentage impervious area, lawn condition, and woody vegetation condition. Measured infiltration rates ranged from 0 to >40 cm/hour. Analysis of variance indicated significantly different mean infiltration rates (p < 0.001) for lots constructed pre‐2000 (9.0 cm/hour) and those constructed post‐2000 (2.8 cm/hour). Test results were used to determine a “field‐tested” HSG for each site, representing disturbed soil conditions. Stormwater runoff was estimated from residential lots for a range of 24‐hour design storms using the TR‐55 model and several alternative methods of determining curve numbers, including five different representations of soil conditions. Curve numbers and stormwater runoff were substantially higher when based on field‐tested HSGs for lots constructed post‐2000 compared with lots built pre‐2000 and when based on the HSG for undisturbed soils, documenting the magnitude of possible error in stormwater runoff models that neglect soil disturbance.

No abstract

Stream temperatures are critically important to aquatic ecology, especially cold-water fish such as salmonids. Stream temperatures are influenced by multiple factors, including local climate, solar radiation on the stream channel, stream discharge volume and groundwater contributions. The Heat Source hydrodynamic and thermodynamic numerical model was used to evaluate temperatures in three stream reaches in the Navarro River watershed, California, USA. The model was calibrated and validated for summer 2015 conditions and then applied to scenarios that address changes in air temperatures, riparian forest cover and stream discharge. Modelling results indicate that stream temperatures are sensitive to changes in air temperatures and riparian forest cover and that higher discharge volume mitigates those impacts. Modelled stream maximum weekly average temperatures (MWAT) increased by 1.5-2.3°C in response to an air temperature increases of 3.5°C under low flow conditions (drought) but by only 0.9-2.0°C under moderate flow. Complete removal of riparian forest in a large-scale forest fire would increase MWAT by 2.2-5.9°C in low discharges and by 1.0-4.4°C under moderate discharge. Riparian zone reforestation would decrease MWATs by less than 0.8°C, a modest change reflecting high existing shade on the modelled stream reaches. Comparison of identical climate and land cover change scenarios under low and moderate discharge conditions reveals that efforts to conserve stream discharge volume could be an effective mechanism to mitigate stream temperature increases.

No abstract