Canals and tunnels in hydropower plants must be able to receive high shock-like flows without damaging either the dam or the rock foundation. Although the canals often consist of rock, erosion can occur when water is released. The natural riverbeds and lakes in Sweden usually run along large faults and other zones of weakness in the rock. This is because the water could more easily erode its way along these weakness zones. Spillways of hydropower dams are generally unlined thereby exposing the bedrock to erosion during floods.This study focuses on block erosion mechanisms and characteristics in unlined spillway canals that comprises hard rock mass systems. Two hydropower dam spillway canals were investigated as case studies; identified as Dam1 and Dam 2. The spillway canals of these two dams have uniquely different bed rock characteristics. At Dam 1 the rock mass is very blocky with visually estimated GSI classification in the range of 50 to 70, while Dam 2 is composed of massive rock mass with visually assessed GSI classification of 70 to 90.The erosion characteristics observed in these two spillway canals are uniquely different. The rock mass is obviously the principal factor contributing to these observations. However, there are also other factors, namely the hydraulic factors, as well as the geometrical factors of the canals. In this report these factors have been described in detail.  Three main mechanisms of block erosion were observed, (i) removal or plucking of rock blocks, (ii) fracturing of intact rock blocks and (iii) abrasion. At Dam 1 spillway canal all three mechanisms were observed to be significantly evident. At Dam 2, abrasion is the dominant mechanism of erosion. Hydraulic parameters, water pressure and velocity, affect the criticality of the erosion.Numerical simulations of the spillway canals were conducted using 3DEC. These simulations show that block displacements greater than 10 m are experienced within 1 to 2 minutes of flow. This observation is consistent with observations made during an actual discharge from a dam. Numerical simulations indicated that blocks with sizes less than 1 m3 would easily be plucked and transported downstream. If they are intact and with unfavourable geometry, they can be easily fractured by the spill water loads. Field investigations support these observations.Remedial measures would first require classification of a spillway canal into erosion domains based on erosion vulnerability. For example, the upstream sections of the channels are typically vulnerable to high intensity erosion. Hydraulic jumps, plunge pools, stilling basins, etc, have been typically used to break up the energy before the water flows downstream. However, erosion still occurs further down since the energy is still very large. Reinforcing the bedrock with artificial supports such as rock bolting, widening and levelling of canals, diverting the flow to less vulnerable areas of the canal, etc, have been some means to reduce block erosion. This study concludes that, remedial measures must start with identifying the mechanisms of block erosion, three of which have been described above. Domaining of the channels into erosion critically domains may also assist in monitoring and application of remedial measures. Empirical methods, such as Pells (2016) can be applied in each domain to identify their erosion potential. This study also concludes that the hydraulic pressure and displacements that occur around a rock block needs to be further investigated, either by field measurements in a spillway or by using physical models. In this way, it will be possible to better understand the conditions around blocks in a spillway and erosion mechanisms during a discharge.